Deep crustal seismic reflection images from the Dharwar craton, Southern India—evidence for the Neoarchean subduction

Deep crustal seismic reflection images from the Dharwar craton, Southern India—evidence for the... Abstract Major part of the Earth's continental crust is evolved during the Archean, however, the mechanism for its formation is controversial. It could have formed either through horizontal accretion similar to the modern plate tectonic processes or by vertical accretion by plume activity. Here, we present the results of a new deep crustal seismic reflection Profiling, the DHARSEIS experiment conducted along a 200 km long Perur-Chikmagalur profile across the Archean Dharwar craton, to understand the crustal evolutionary processes during the Neoarchean. The data were processed using the Common Reflection Surface (CRS) stack method. Seismic images show distinctly different reflectivity patterns in the Mesoarchean Western and Neoarchean Eastern Dharwar Cratons (WDC and EDC), the two crustal blocks of the composite Dharwar craton. The WDC consists of a simple structure with a major part of the crust from 6 to 28 km displaying a gently dipping reflection fabric and a subhorizontal reflection fabric from 28 to 40 km except beneath the Chitradurga schist belt. On the other hand the EDC displays a complex reflectivity pattern, contrary to the simple crustal structure suggested by various other studies. A dipping Moho, oppositely dipping reflection fabric and a thrust fault are the major crustal features in the EDC. The present seismic study imaged a west-dipping reflection fabric extending from 34 to 43 km in the EDC, which is interpreted to represent an upper-mantle subduction zone. During this process the EDC was thrusted obliquely against the pre-existing proto-continent WDC and accreted to it. Oppositely dipping reflection fabrics with a crustal root at the convergence boundary suggest accretion of WDC and EDC during the Neoarchean orogeny. The collisional boundary coincides with the location of ∼2.5 Ga Closepet granite. Seismic images suggest the Moho as a detachment boundary. The present study reveals a likely two-stage subduction-accretion process for the evolution of continental crust during the Neoarchean. Plate tectonic processes were responsible for the Neoarchean crustal growth in the region. Composition and structure of the continental crust, Continental imaging, Continental margins: convergent, Continental tectonics: strike-slip faulting, Cratons, Subduction zone processes 1 INTRODUCTION Configuration of the present day continental crust is a manifestation of protracted history of lithospheric evolution involving complex crust–mantle interaction processes. Growth of the continental crust took place either by lateral accretion of arc complexes at continental margins or by vertical accretion of the mafic material at the base of the crust from upwelling upper mantle zones (Stein & Hofmann 1994). On the other hand, continents are progressively destroyed at the surface by physico-chemical processes and at its base by processing involving delamination (Condie 1989). Thus, a large part of the continental crust might have gone through recycling process of the perpetually dynamic Earth since its formation in the Hadean to the early Archean (Kamber 2015). But, so far no consensus has emerged concerning the extent to which modern plate tectonic processes can be extrapolated. So, understanding the structure, dynamics, origin and evolution of the continental crust is fundamental to the Earth science. The Archean period represents one-third of the Earth's history during which as much as 70 per cent of present cratons were formed and stabilized with major tectonic assembly. So, the study of Archean crust is important to understand the Earth's geodynamics. Amongst the Archean blocks, the Precambrian Indian Shield possesses certain unique geotectonic and geodynamical characteristics, which provide an opportunity to understand its multistage crustal evolution both in space and time. The Precambrian Indian shield is a mosaic of several cratons and mobile belts. The late Archean Dharwar craton with 3.4 Ga Earth's history (Nutman et al. 1992; Ramakrishnan & Vaidyanadhan 2008; Jayananda et al. 2013) is an important part of the collage of Precambrian terrain in the Indian shield and an ideal natural laboratory to test the hypotheses concerning the Archean crustal evolution. The Dharwar craton of the Indian shield represents one of the largest and oldest cratonic blocks (3.4 Ga) available on the Earth, similar to the Superior Province, the Yilgaran craton, the Karelian craton and the Kaapval craton. It is flanked by the high grade granulitic terrains to the south and east and by the cover of late Cretaceous Deccan flood basalt to the north (Fig. 1a). Almost the entire craton is covered by the Tonalitic Trondjhemitic Gneisses (TTG) with several northwest–southeast trending greenstone belts of volcano-sedimentary formations ranging in age between 3.4 and 2.7 Ga and intrusive granite (Nutman et al.1992; Bhaskar Rao et al .1992; Ramakrishnan & Vaidyanadhan 2008; Jayananda et al.2013). Three prominent tectono-thermal events that occurred between 3400 and 2500 Ma in the region provided important constraints to understand the crustal evolution during the Archean. The Dharwar craton comprises two distinctly different crustal blocks, namely, the meso-Neoarchean western and the predominantly Neoarchean eastern blocks (Nutman et al.1992; Bhaskar Rao et al.2008; Jayananda et al.2008), which are popularly referred to as the Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC). Figure 1. View largeDownload slide (a) Geological map of the Southern India along with the Perur-Chikmagalur seismic reflection profile marked over it. Numbers indicate the Shot Points locations used in reflection data acquisition. WDC: Western Dharwar Craton; EDC: Eastern Dharwar Craton; SGT: Southern Granulite Terrain; PCSZ: Palghat Cauvery Shear zone (modified after GSI & ISRO 1994). XY indicates the profile along which geological cross-section is provided in Fig. 2(a). (b) Shear zones are shown as continuous lines in blue colour and Proterozoic dykes in small black lines over the geological map of the Dharwar craton. Note that shear zones are trending towards NW–SE direction (modified after Vijaya Rao et al.2015b). Figure 1. View largeDownload slide (a) Geological map of the Southern India along with the Perur-Chikmagalur seismic reflection profile marked over it. Numbers indicate the Shot Points locations used in reflection data acquisition. WDC: Western Dharwar Craton; EDC: Eastern Dharwar Craton; SGT: Southern Granulite Terrain; PCSZ: Palghat Cauvery Shear zone (modified after GSI & ISRO 1994). XY indicates the profile along which geological cross-section is provided in Fig. 2(a). (b) Shear zones are shown as continuous lines in blue colour and Proterozoic dykes in small black lines over the geological map of the Dharwar craton. Note that shear zones are trending towards NW–SE direction (modified after Vijaya Rao et al.2015b). A detailed deep image of the subsurface is essential to understand the dynamics, origin and evolution of the continental crust of any region. In this context, deep continental crustal structure is a result of continental lithospheric dynamics. Analysis of deep crustal seismic data across the globe suggests that deep seismic reflection profiling has proven to be a power full tool that provides high resolution images of the subsurface and reveals the structural information necessary for recognition of individual terrains and boundaries, especially when dealing with Precambrian shield areas (Mooney & Meissner 1992; Cook et al.1999; Goleby et al. 2004). These studies are fundamental for developing geodynamic models on the evolutionary history of the Earth and to understand the juxtaposition of the crustal blocks in the geologic past as well as continental dynamics. In this context, a large number of geological and geochronological studies were carried out to resolve the issues related to deep crustal configuration and tectonics of the Dharwar craton (e.g. Naqvi 1985; Radhakrishna & Naqvi 1986; Chadwick et al. 2000; Chardon et al.2008). So far only two deep seismic refraction/wide-angle reflection data have been acquired out across the Dharwar craton to delineate its crustal velocity structure (Kaila et al.1979; Sarkar et al.2001; Mall et al.2012; Vijaya Rao et al.2015a,b). However, the results based on refraction/wide-angle reflections that primarily provide velocity information of the subsurface are insufficient to explain the tectonic settings and evolutionary history of the Dharwar craton. Seismological data along with receiver function analysis suggest only velocity information and variation of crustal thickness in the Dharwar craton without any precise subsurface structural details, due to inherent limitations of those methods (Rai et al.2003; Borah et al.2014). Even though surface geology was well studied, the deep crustal structure and tectonics of the Dharwar craton remained inadequately constrained. Still there exists a big debate regarding the relationship between the eastern and western blocks, subduction related tectonics and its evolutionary history. On this background, to bridge the knowledge gap in the region, a coincident refraction/wide-angle reflection and narrow-angle deep seismic reflection study was carried out along the ENE–WSW trending Perur-Chikmagalur profile (Fig. 1a) across the Dharwar Craton as a part of DHARSEIS (Dharwar seismic) experiment during 2010–2012 (Vijaya Rao et al.2015a,b). Deep seismic reflection data that provide the highest resolution information on the structure and physical properties of the continental crust and the Moho boundary are acquired for the first time over the Dharwar craton. The present paper deals with this dataset with primary objective to understand the crustal structure and Moho configuration, both in the eastern and western Dharwar cratons and determine their accretionary boundary using high-resolution seismic images. Further, it is used to derive a mechanism for the Neoarchean accretion and crustal growth process by developing an evolutionary model for the Dharwar craton. 2 GEO-TECTONIC FRAMEWORK 2.1 Geology The Dharwar craton is divided into two distinct tectonic blocks, for example, the Mesoarchean Western Dharwar Craton (WDC) and the Neoarchean Eastern Dharwar Craton (EDC) with different crustal evolution patterns based on the differences in lithotectonic association, nature and abundance of greenstone belts, age distribution, deformation, metamorphic pattern and degree of melting (Chadwick et al. 2000; Ramakrishnan & Vaidyanadhan 2008). Prominent shear zone known as the Chitradurga Eastern Boundary Shear Zone (CEBSZ) or Chitradurga Thrust located all along the eastern margin of the NW–SE trending Chitradurga schist belt is generally considered as the boundary between the EDC and WDC (Chadwick et al.2000; Reddy et al.2000; Jayananda et al.2006; Chardon et al.2008, 2011). Geochronological data suggest progression of the Dharwar craton from young to old age as we move from east to west. The geological map of the Dharwar Craton is shown in Fig. 1(a) and different stratigraphic units compiled from available literature (Bhaskar Rao et al.2008; Jayananda et al.2008, 2013) are listed in Table 1. Table 1. Geological framework of the Dharwar craton. Western Dharwar Craton (WDC) Peninsular Gneisses (Old) 2.9–3.5 Ga Komatiite volcanism 3.35 Ga Greenstone belts (WDC) Sargur Group >3.0 Ga Dharwar Supergroup 2.9–2.6 Ga    Bababudan Group 2.9 Ga    Chitradurga Group 2.75 Ga Potassic K-Granites 2.61 Ga Predominance of sedimentary rocks over volcanic rocks. Kimberlites are absent. Eastern Dharwar Craton (EDC) Peninsular Gneisses (Young) 2.7–2.9 Ga Greenstone belts 2.7–2.55 Ga Calc-alkaline to Potassic granites 2.5 Ga Volcanic rocks are predominant and sedimentary rocks are sparse. Komatiites are rare. Island arc basalts and other volcanic rocks (boninites & adakites) are predominant. Kimberlites are predominant Western Dharwar Craton (WDC) Peninsular Gneisses (Old) 2.9–3.5 Ga Komatiite volcanism 3.35 Ga Greenstone belts (WDC) Sargur Group >3.0 Ga Dharwar Supergroup 2.9–2.6 Ga    Bababudan Group 2.9 Ga    Chitradurga Group 2.75 Ga Potassic K-Granites 2.61 Ga Predominance of sedimentary rocks over volcanic rocks. Kimberlites are absent. Eastern Dharwar Craton (EDC) Peninsular Gneisses (Young) 2.7–2.9 Ga Greenstone belts 2.7–2.55 Ga Calc-alkaline to Potassic granites 2.5 Ga Volcanic rocks are predominant and sedimentary rocks are sparse. Komatiites are rare. Island arc basalts and other volcanic rocks (boninites & adakites) are predominant. Kimberlites are predominant View Large Table 1. Geological framework of the Dharwar craton. Western Dharwar Craton (WDC) Peninsular Gneisses (Old) 2.9–3.5 Ga Komatiite volcanism 3.35 Ga Greenstone belts (WDC) Sargur Group >3.0 Ga Dharwar Supergroup 2.9–2.6 Ga    Bababudan Group 2.9 Ga    Chitradurga Group 2.75 Ga Potassic K-Granites 2.61 Ga Predominance of sedimentary rocks over volcanic rocks. Kimberlites are absent. Eastern Dharwar Craton (EDC) Peninsular Gneisses (Young) 2.7–2.9 Ga Greenstone belts 2.7–2.55 Ga Calc-alkaline to Potassic granites 2.5 Ga Volcanic rocks are predominant and sedimentary rocks are sparse. Komatiites are rare. Island arc basalts and other volcanic rocks (boninites & adakites) are predominant. Kimberlites are predominant Western Dharwar Craton (WDC) Peninsular Gneisses (Old) 2.9–3.5 Ga Komatiite volcanism 3.35 Ga Greenstone belts (WDC) Sargur Group >3.0 Ga Dharwar Supergroup 2.9–2.6 Ga    Bababudan Group 2.9 Ga    Chitradurga Group 2.75 Ga Potassic K-Granites 2.61 Ga Predominance of sedimentary rocks over volcanic rocks. Kimberlites are absent. Eastern Dharwar Craton (EDC) Peninsular Gneisses (Young) 2.7–2.9 Ga Greenstone belts 2.7–2.55 Ga Calc-alkaline to Potassic granites 2.5 Ga Volcanic rocks are predominant and sedimentary rocks are sparse. Komatiites are rare. Island arc basalts and other volcanic rocks (boninites & adakites) are predominant. Kimberlites are predominant View Large A major part of the WDC is covered largely by unclassified gneissic granitoids referred as ‘older gneissic complex’. The complex is mainly composed of polyphase TTG gneisses of 3.4–2.9 Ga age and K–granites. During the 3.4–3.0 Ga periods the WDC experienced multiple cycles of juvenile crustal addition and recycling and ∼2.60 Ga K-granites mark the end of plutonism (Jayananda et al. 2006). The WDC contains two types of supracrustal sequences: the Sargur and the Dharwar type schist belts. The Sargur type schist belt hosts the older sequence (3.4–3.0 Ga) comprising highly deformed ambhibolite to granulite facies rocks of shelf region (Ramakrishnan & Vaidyanadhan 2008). The gneisses and Sargur rocks are unconformably overlain by 2.9–2.6 Ga greenstone belts (Bhaskar Rao et al.1992) in the WDC. The later greenstone belt, known as the Dharwar schist belt, has larger dimension of greenstone terranes (Fig. 1a) that contains dominant greenschist facies metasedimentary components with predominant metabasalts along with komatiite–tholeiite association and minor bimodal volcanic rocks. The Dharwar schist belt contains iron and manganese ore deposits. The Dharwar greenstone belt is further divided into older Badabudan (2.9 Ga) and younger Chitradurga group (2.8 Ga). The litho-units of the Chitradurga group suggest continental margin to deep marine environments for their deposition (Chadwick et al.2000). The WDC is characterized mostly by medium pressure and low-high temperature rocks. In the present paper greenstone belt and schist belt are used synonymously to represent the supracrustal sequences of the region. The EDC is mainly consists of linear N–S to NNW trending belts of gneissic granitoids referred as ‘younger gneissic complex’. The complex is mainly composed of the Neoarchean TTG gneisses (2.7–2.55 Ga) with remnants of 3.0 Ga rocks and calc-alkaline to potassic-granites (2.56–2.50 Ga). They are interleaved with narrow 2.7–2.55 Ga greenstone belts mostly made up of greenschist to ambhibolite facies rocks (Chadwick et al.2000,; Ramakrishnan & Vaidyanadhan 2008). Some of these greenstone belts host gold mineralization. Major type of granite suite is the Closepet granite (2.52 Ga) and its equivalents. A considerable part of the EDC represents a juvenile addition to the crust from the mantle dominantly during 2.7–2.5 Ga (Peucat et al.1993). Geochemical investigation on the volcanic of Neoarchean greenstone belts of the EDC carried out by various researchers reveals the presence of subduction related high-Mg andesites, bonintes, Nb-enriched basalts and adakites (Naqvi et al.2006; Manikyamba & Kerrich 2012). Bimodal volcanic rocks are more prevalent with some komatiites in the EDC. 2.2 Tectonics of the region Absence of proper geophysical data, especially high-resolution deep crustal seismic structure, resulted in a number of evolutionary models for the region based mainly on surface geological data. Thus, the geodynamic evolution of the Archean Dharwar craton is highly controversial. The Neoarchean crustal evolution can be mainly divided into two types: subduction-accretion and mantle plume tectonics. Plate tectonic models suggest that the EDC had evolved by successive accretion of arcs against a foreland (the WDC) in a convergent setting during the Neoarchean (Chadwick et al.2000). The Neoarchean schist belts of EDC are interpreted as intra-arc basins, whereas those of the WDC are backarc basins (Dey 2013). Contrarily, the plume model suggested that a Neoarchean mantle plume supplied heat, softened the crust and induced inverse diaprism, metamorphism, partial melting of the crust and granitic magmatism (Jayanandan et al.2000). Some other models with mixed-mode operation invoke initial subduction, followed by closure of oceanic crust and arc-continent collision with subsequent mantle plume activity causing crustal reworking and metamorphism (Moyen et al.2001). The N–S to NNW trending structural fabric of the entire Dharwar craton could be a result of transcurrent shear deformation due to crustal-scale shortening in a convergent setting during the Neoarchean (2.56–2.51 Ga). Fig. 1(b) shows the shear zones and Proterozoic dykes in the Dharwar craton. Further, the EDC is considered as a hot orogen characterized by soft, buoyant crust due to magmatic accretion in an arc setting with ridge subduction and opening of a slab window during the Neoarchean. During convergence, the lower crust of EDC underwent pervasive 3-D flow process (mass distribution) called as lateral constructional flow, which includes orogen-normal shortening, lateral constrictional stretching and transtension. This resulted in high-temperature–low-pressure metamorphism in the EDC (Chardon et al.2008; Jayananda et al.2013). Another model suggests that the volcano-sedimentary facies of the Chitradurga schist belt, located between the WDC and EDC, may represent closure of a basin located between these two continental crustal blocks with some form of plate separation and subsequent collision (Naqvi 1985). The eastern margin of the Chitradurga schist belt marked by a series of N–S trending shear zones made up of 1–1.5 km wide mylonites (Chadwick et al.2000). The N–S orientation of tectonic fabrics in the schist belt and the magmatic and tectonic fabrics in the plutonic rock, together with other structural geometry, indicate an E–W compressive regime of Meso-Neoarchean period (Chadwick et al.2007). In the given litho-tectonic framework, the faulted contact zone between the WDC and EDC has been interpreted as a cryptic suture (Naqvi 1985). A geological cross section across the WDC and EDC derived from surface geological mapping and tectonics is shown in Fig. 2. It would emphasise the need for deep seismic reflection study as the reflection image enhance/change the understanding of the crustal structure in the region similar to the IBERSEIS deep seismic profiling (Simancas et al. 2003). Figure 2. View largeDownload slide (a) Geological cross-section along the profile XY shown in Fig. 1(a) derived from the surface geology and tectonics (after Chadwick et al.2007). (b) Geological model derived from geological constraints (Jayananda et al.2006): MHZ, Medikeripura High-strain Zone; Gg, Guntakal granodiorite. Figure 2. View largeDownload slide (a) Geological cross-section along the profile XY shown in Fig. 1(a) derived from the surface geology and tectonics (after Chadwick et al.2007). (b) Geological model derived from geological constraints (Jayananda et al.2006): MHZ, Medikeripura High-strain Zone; Gg, Guntakal granodiorite. 3 SEISMIC DATA ACQUISITION The DHARSEIS deep seismic reflection profiling data were acquired in an end-on configuration with 100/200 m shot and 100 m receiver spacing using a 150 channel Eagle-88 RF telemetry system. The data were acquired using 4 ms sampling interval and recorded to a length of 24 s. A theoretical fold of 37 was expected from the above source-receiver geometry. But, the fold varies between 10 and 33 with an average value of ∼20 along the profile because of various logistics in the field. The data were acquired using 4.5 Hz geophones that provide a broad frequency bandwidth of 5–80 Hz. The topography varies from 550–1000 m along the seismic profile. More details on data acquisition are provided in Table 2. Table 2. Data acquisition parameters. Length of the profile 240 km Type of source Explosives Shot hole depth 25–28 m Charge size/hole 50–75 kg No. of shots 900 No. of channels 150 Shot point spacing 200 m Receiver spacing 100 m Source–receiver offset 100 m (nearest), 15,000 m (farthest) Spread length 15 km Theoretical fold 37 Type of spread End-on Record length 24 s Sampling interval 4 ms Type of magnetic tape IBM 3490 cartridge Type of data SEG-D, Demultiplexed format Frequency range 3.5–250 Hz Uphole recording Yes Instrument Eagle-88 RF Telemetry system Geophones 4.5 Hz, 10 phone string, Bunching Length of the profile 240 km Type of source Explosives Shot hole depth 25–28 m Charge size/hole 50–75 kg No. of shots 900 No. of channels 150 Shot point spacing 200 m Receiver spacing 100 m Source–receiver offset 100 m (nearest), 15,000 m (farthest) Spread length 15 km Theoretical fold 37 Type of spread End-on Record length 24 s Sampling interval 4 ms Type of magnetic tape IBM 3490 cartridge Type of data SEG-D, Demultiplexed format Frequency range 3.5–250 Hz Uphole recording Yes Instrument Eagle-88 RF Telemetry system Geophones 4.5 Hz, 10 phone string, Bunching View Large Table 2. Data acquisition parameters. Length of the profile 240 km Type of source Explosives Shot hole depth 25–28 m Charge size/hole 50–75 kg No. of shots 900 No. of channels 150 Shot point spacing 200 m Receiver spacing 100 m Source–receiver offset 100 m (nearest), 15,000 m (farthest) Spread length 15 km Theoretical fold 37 Type of spread End-on Record length 24 s Sampling interval 4 ms Type of magnetic tape IBM 3490 cartridge Type of data SEG-D, Demultiplexed format Frequency range 3.5–250 Hz Uphole recording Yes Instrument Eagle-88 RF Telemetry system Geophones 4.5 Hz, 10 phone string, Bunching Length of the profile 240 km Type of source Explosives Shot hole depth 25–28 m Charge size/hole 50–75 kg No. of shots 900 No. of channels 150 Shot point spacing 200 m Receiver spacing 100 m Source–receiver offset 100 m (nearest), 15,000 m (farthest) Spread length 15 km Theoretical fold 37 Type of spread End-on Record length 24 s Sampling interval 4 ms Type of magnetic tape IBM 3490 cartridge Type of data SEG-D, Demultiplexed format Frequency range 3.5–250 Hz Uphole recording Yes Instrument Eagle-88 RF Telemetry system Geophones 4.5 Hz, 10 phone string, Bunching View Large 4 COMMON REFLECTION SURFACE (CRS) METHOD-SEISMIC REFLECTION DATA In the present study CRS stack approach is used to process the multifold seismic reflection data. Initially, a brief description of the methodology is provided and subsequently the processing strategy. 4.1 CRS methodology The conventional Common-Midpoint (CMP) method requires a velocity model that is used to apply for Normal Move Out (NMO) correction to remove offset effect before stacking the data. The stacking velocity plays a major role in stacking the multichannel seismic data in the CMP method. However, deriving an accurate velocity model becomes very difficult due to inhomogeneity of the subsurface and lateral velocity variations due to complex structures of the Earth, especially in the Archean terrains. Also, small source-receiver offset compared with the target depth in deep crustal seismic reflection data puts another limitation in velocity analysis (Yoon et al. 2009; Mandal et al. 2013). Additionally, the NMO equation is valid only for homogeneous and layered Earth and may not be appropriate for multilayered inhomogeneous media with curved interfaces (Mann 2002). These limitations may lead to poor stack sections of deep seismic reflection data from shield regions that exhibit low signal-to-noise (S/N) ratio, weak reflection coefficients, faulted and folded structures due to long geological history. Improving the quality of CMP stack section is in focus of intense research. In this context, a new multifocusing approach to reflection data is found an alternative to CMP method. In this method each zero-offset trace is constructed by stacking traces which need not belong to same CMP gather, but whose sources and receivers are in certain vicinity of the central point (Mann et al.1999; Jäger et al.2001; Zhang et al.2001; Menyoli et al.2004). The subsurface area covering them is called as CRS (Fig. 3). Since the traces being stacked no longer belong to the same CMP gather, such a procedure requires a more general moveout correction than used in conventional CMP stacking. The CRS/multifocusing approach consists of stacking seismic data with an arbitrary source-receiver distribution, according to a new paraxial moveout correction that is based on a local spherical approximation of the reflection wave fronts in the vicinity of an observation surface (Schleicher et al.1993). The moveout correction parameters are the emergence angle (β) and radii of curvature (RN and RNIP) of two fundamental wave fronts corresponding to the normal wave and normal-incident-point wave (Hubral 1983; Höcht et al.1999; Jäger et al.2001; Menyoli et al.2004). Figure 3. View largeDownload slide CRS stacking surface. The CRS stack sums all the data along the green surface for all finite-offset and purple for zero-offset reflections related to red arc around Common Reflection Point (CRP) and assign the stacked result to the CRP (after Vieth 2001). Figure 3. View largeDownload slide CRS stacking surface. The CRS stack sums all the data along the green surface for all finite-offset and purple for zero-offset reflections related to red arc around Common Reflection Point (CRP) and assign the stacked result to the CRP (after Vieth 2001). In this approach a super-gather, usually referred as the CRS gather is prepared which consists of traces from a number of CMP gathers and stacking them increases the fold by an order of magnitude. This is beneficial for data with low fold and low S/N ratio. In the CRS approach for each t0 on each zero-offset trace, three stacking parameters (β, RN and RNIP) are required instead of a single parameter (stacking velocity) in conventional NMO stack. The stacking parameters (CRS attributes) that best fit to an actual reflection event in the seismic data are determined for each sample (t0, x0). This is achieved by correlation measures using coherency analysis by measuring semblance along the stacking operator (Taner & Koehler 1969; Neidell & Taner 1971). The cost of calculating the correlation measure for all possible combinations of three stacking parameters over a CRS-gather is prohibitively high. Further, displaying and picking maxima of the correlation measure as a function of four variables (t0, β, RN and RNIP) may be a difficult task. Thus, automatic stacking procedure has come into existence in the CRS approach (Mann et al.1999; Jager et al.2001). Maximization of semblance in coherency analysis is achieved by a nonlinear global optimization method. Superiority of the CRS stack over the conventional CMP stack in deep seismic reflection data analysis has been reported by several researchers (e.g. Menyoli et al.2004; Luis et al.2006; Yoon et al.2009; Mandal et al. 2013, 2014). 4.2 CRS data processing In general, the data exhibits moderate S/N ratio and broad frequency bandwidth (4–80 Hz) required for the deep crustal study. The data are contaminated with low-frequency, high amplitude ground roll. Deep seismic reflection data are processed using the Promax software and CRS stack approach (Mann et al.1999; Jager et al.2001; Zhang et al.2001) The processing sequences are shown in Table 3. The CRS processing can be divided into three steps. Step-I involves raw field data of a CMP gather, which is same as CMP processing. Step-II replaces velocity analysis, deriving macrovelocity model and NMO and DMO of CMP processing with generation of CRS gathers and determination of three CRS attributes. Finally, stacking of traces is done in respective domains. Step-III, the post-stack processing steps are same in both CMP and CRS domains. Table 3. CRS data processing sequence. View Large Table 3. CRS data processing sequence. View Large Deep seismic reflection data were processed with straight line geometry using the demultiplexed field data. Initially, source–receiver recording geometry was incorporated on the field seismic data followed by editing of bad traces and correction of polarity reversals. Static corrections for elevation differences were performed with a datum plane located at 500 m above mean sea level with a replacement velocity of 5000 m s−1. Refracted first arrivals were muted. Spherical divergence correction is applied using an inverse gain function estimated from the raw field data. A 50 Hz notch and 4–8 to 30–40 Hz bandpasss filter were applied. The parameters were chosen after frequency analysis of field data (Fig. 4). A spiking deconvolution with an operator length of 200 ms and a prediction gap of 24 ms is applied to increase the lateral resolution. Automatic Gain Control (AGC) with a time window of 2000 ms was applied to increase the amplitude of weak signal, especially at deeper levels and to balance amplitudes laterally and vertically. Trace mixing of immediate adjacent trace from either side to the central trace is performed to increase the S/N ratio. Data were sorted to CMP domain and CMP gather is prepared (Table 3). Quality check is performed over the processed data and shown in Fig. 5. Figure 4. View largeDownload slide Raw field record along with the frequency spectrum of the seismic signal. Figure 4. View largeDownload slide Raw field record along with the frequency spectrum of the seismic signal. Figure 5. View largeDownload slide Representative raw shot gathers, SP485 and SP1043 (a,c), with end-on configuration are shown up to 5 s twt. The shot gathers are dominated by source generated and random noise. Same shot gathers after processing with first arrival mute, bandpass filtering, spherical divergences correction, F-K filtering, predictive deconvolution and AGC (b,d). Note: source generated (ground roll) and random noise is reduced and signal-to-noise ratio is increased after processing. Reflection events (R1, R2, R3 and R4) brought out by processing are shown with red arrows. Sw, Shear wave; Gr, Ground roll; SP, Shot point. Figure 5. View largeDownload slide Representative raw shot gathers, SP485 and SP1043 (a,c), with end-on configuration are shown up to 5 s twt. The shot gathers are dominated by source generated and random noise. Same shot gathers after processing with first arrival mute, bandpass filtering, spherical divergences correction, F-K filtering, predictive deconvolution and AGC (b,d). Note: source generated (ground roll) and random noise is reduced and signal-to-noise ratio is increased after processing. Reflection events (R1, R2, R3 and R4) brought out by processing are shown with red arrows. Sw, Shear wave; Gr, Ground roll; SP, Shot point. Subsequently, three CRS stacking parameters are determined one after another, that is, each time one parametric search is made by means of coherency analysis and measuring semblance (Taner & Koehler 1969; Neidell & Taner 1971). In the first search coherency analysis is performed to derive a best coherent section and an automatic CMP stack is derived as a preliminary stack section in the CRS search to estimate all three attributes (Table 3). This automatic stack section is similar to the CMP stack section in the conventional processing chain, where optimum stacking velocity (VNMO) is determined from the CMP gather in this search using the CRS attributes. The VNMO is automatically determined based on the best coherency fitting to the seismic data. Next, in the second search, the automatic CMP stack is utilised to determine emergence angle (β) by assuming RN = ∞, as a first-order approximation, which implies plane normal waves emerging at the surface. Finally, a third search was made using the known value of β, with hyperbolic second-order representation of stacking operator and RN is determined. Knowing the values of β and VNMO, RNIP is calculated (for more details see Mann et al.1999; Jäger et al. 2001).These initial CRS attributes are applied in the form of a 3-D search and optimization procedure to the original dataset making use of CRS gathers. 3-D optimization makes use of the spatial CRS stacking surface in (xm–h–t) space (Fig. 3) using flexible polyhedron search using the CRS attributes (Nelder & Mead 1965; Jäger et al. 2001). In order to minimize computational time, a range of stacking velocities between 5000 and 8500 m s−1 with an average velocity of 6600 m s−1 are provided as guide lines to perform the CRS stack. Finally, the CRS stack section is obtained by summing multifold data using the CRS gather along the 2-D stacking operator using the optimized CRS attributes and presented in Fig. 6. Whenever the coherency value exceeds a threshold, the three-parameters are optimised again to maintain the accuracy. F–X deconvolution and coherency filtering is applied to the stack. Finally, depth migration is carried out. Figure 6. View largeDownload slide CRS stack section of the Perur-Chikmagalur profile. CT, Chitradurga Thrust; BGB, Bababudan Greenstone Belt; CGB, Chitradurga Greenstone Belt; CG, Closepet Granite; YG, Younger Gneisses (2.7–2.9 Ga); OG, Older Gneisses (2.9–3.5 Ga); WDC, Western Dharwar Craton; EDC, Eastern Dharwar Craton. Figure 6. View largeDownload slide CRS stack section of the Perur-Chikmagalur profile. CT, Chitradurga Thrust; BGB, Bababudan Greenstone Belt; CGB, Chitradurga Greenstone Belt; CG, Closepet Granite; YG, Younger Gneisses (2.7–2.9 Ga); OG, Older Gneisses (2.9–3.5 Ga); WDC, Western Dharwar Craton; EDC, Eastern Dharwar Craton. Knowledge of near-surface velocity (V0) is essential in the CRS search that is measured from the refracted first arrivals. The V0 acts like a scale factor connecting the travel time derivatives along the reflection events with their geometrical interpretation in terms of emergence angles and wave front curvatures, that is, in terms of the CRS attributes. Near-surface velocity (approximately the RMS velocity) of 5000 m s−1 for the most shallow stable reflection event is chosen here. During automatic CRS stacking, two important stacking apertures, namely, the half-offset (h) in the time-offset (t–h) plane (CMP aperture) and zero-offset in the time-midpoint (t–xm) plane (ZO aperture), are to be specified with care (Fig. 3). Increasing aperture length increases S/N ratio. But, large aperture causes smearing effect beyond certain value and small aperture causes low coherency. A zero-aperture is the conventional CMP stack. Proper adjustment of these parameters is necessary for high spatial resolution of the CRS stack section and its image quality. The aperture size depends on the data quality, that is, on the S/N ratio of the seismic data. Larger apertures are generally used in poor S/N condition (Hertweck et al. 2007). In the present study, the CRS stack is performed with three sets of different ZO apertures. To fix initial value of the ZO aperture, width of the first projected Fresnel zone at the targeted depth was chosen. Finally, minimum and maximum values of ZO aperture were fixed at 100 m (0.2 s twt) and 2000 m (5.0 s twt) and for CMP aperture at 5 m (0.2 s twt) and 15 000 m (5.0 s twt) to obtain the best coherency section. Similar values are being used for several data sets from different geological provinces (e.g. Mann 2002; Yoon et al. 2009). Generally, it is assumed that seismic data processing provides the structure beneath the profile and energy comes entirely from within the plane of the section. However, the out-of-plane topography of a reflector (3-D structure) can be imaged as an inplane reflector at an appropriate depth depending on the off-line distance (Hobbs et al.2006). During the stacking, the out-of-plane energy can be suppressed up to some extent if it has a lower-velocity compared with in-plane reflectors at same depths. Invariably, the Earth is 3-D in nature and the effects of out-of-plane energy are not considered when data are processed and interpreted in 2-D seismic imaging techniques as in the present case and elsewhere also (e.g. Australia, Canada) as they are normally less problematic at later reflection times (Cook 2002; Kennett & Saygin 2015). To the best of our knowledge there is no proper mechanism to understand fully, the effects of the 3-D structure in 2-D section (Hobbs et al.2006). 3-D effects of the Earth can be resolved by acquiring 3-D data or recording a number of parallel or cross profiles. This is prohibitively costly for deep crustal studies. 4.3 Post-stack depth migration Migration moves the dipping reflectors into their true subsurface locations and collapses diffractions, thereby delineating detailed subsurface structure more accurately. So, migrated sections are preferred over stack sections and very essential for proper interpretation. But, migration of deep seismic reflection data may produce artefacts, which appear as circular arcs or smiles due to discontinuous nature of reflectors, low S/N ratio, truncation of reflections on stack sections due to poor signal penetration, changes in orientation of profiles and existence of strong lateral velocity variations (Warner 1987). As lateral velocity variations are more significant in the present study region because of its long geological history, time migration may not produce true subsurface picture and we preferred depth migration. Post-stack Kirchhoff depth migration is performed over the CRS stack section using a velocity model derived from coincident refraction/wide-angle reflection data (Vijaya Rao et al.2015b) and shown in Fig. 7(a). It looks similar to a geological cross section. We used migrated section for interpretation as it brought out the horizontal and true geometrical nature of the Moho and other structures more clearly compared with stack section. Small portions at both ends of the migrated section are not used in interpretation because of edge-effects aroused during migration process. Figure 7. View largeDownload slide (a) Post-stack CRS depth-migrated seismic section of the Perur-Chikmagalur profile without any interpretation. (b) Post-stack CRS depth-migrated seismic section of the Perur-Chikmagalur profile with interpretation. A bright subhorizontal reflection band at 40 km depth represents as the Moho in WDC. Note the differences in Moho signature in WDC and EDC. Abbreviations are same as in Fig. 3. SW-dipping reflection band is marked as ‘A’ and NE-dipping reflection bands as ‘B and C’. (c) Line drawing derived from prominent reflections from the CRS seismic depth-migrated section, along with the velocity model derived from coincident refraction/wide-angle reflection data (1-D velocity model is after Vijaya Rao et al. 2015b). The lower-crustal subhorizontal reflection band (orange colour) coincides with the high-velocity (7.1 km s−1) lower-crustal layer. Steeply dipping faults (F1–F3) are inferred based on the differences in reflectivity on either side. They coincide with the tectonic boundaries of the region. Abbreviations are same as in Fig. 3. Yellow colour represents the geometry of the Closepet granite and red colour corresponds to major thrust fault of the region. More details are provided in the text. Figure 7. View largeDownload slide (a) Post-stack CRS depth-migrated seismic section of the Perur-Chikmagalur profile without any interpretation. (b) Post-stack CRS depth-migrated seismic section of the Perur-Chikmagalur profile with interpretation. A bright subhorizontal reflection band at 40 km depth represents as the Moho in WDC. Note the differences in Moho signature in WDC and EDC. Abbreviations are same as in Fig. 3. SW-dipping reflection band is marked as ‘A’ and NE-dipping reflection bands as ‘B and C’. (c) Line drawing derived from prominent reflections from the CRS seismic depth-migrated section, along with the velocity model derived from coincident refraction/wide-angle reflection data (1-D velocity model is after Vijaya Rao et al. 2015b). The lower-crustal subhorizontal reflection band (orange colour) coincides with the high-velocity (7.1 km s−1) lower-crustal layer. Steeply dipping faults (F1–F3) are inferred based on the differences in reflectivity on either side. They coincide with the tectonic boundaries of the region. Abbreviations are same as in Fig. 3. Yellow colour represents the geometry of the Closepet granite and red colour corresponds to major thrust fault of the region. More details are provided in the text. 5 RESULTS OF THE DHARSEIS SEISMIC REFLECTION IMAGE The seismic image is presented in the form of CRS stack section in Fig. 6 and depth migrated section in Figs 7(a) and (b), without and with interpretation. A line drawing derived from prominent reflections of the migrated section is shown in Fig. 7(c). A detailed image (depth migrated) of the eastern part of the profile is shown in Fig. 8 and prominent structural features from different segments of the profile in Figs 9(a)–(d). Figs 8 and 9 are the detailed segments of depth migrated section (Fig. 7). Figure 8. View largeDownload slide Enlarged version of the eastern part of the CRS depth migrated section. The prominent SW-dipping reflection fabric (A-reflection band, Fig. 7b) in the Eastern Dharwar Craton extends from 34 to 43 km depth between SP1 and SP500. Abbreviations are same as in Fig. 3. Figure 8. View largeDownload slide Enlarged version of the eastern part of the CRS depth migrated section. The prominent SW-dipping reflection fabric (A-reflection band, Fig. 7b) in the Eastern Dharwar Craton extends from 34 to 43 km depth between SP1 and SP500. Abbreviations are same as in Fig. 3. Figure 9. View largeDownload slide (a) Enlarged version of the seismic section from 28 to 40 km depth of the WDC indicating the subhorizontal Moho n fabric. (b) Enlarged version of the NE-dipping reflection band from 6 to 28 km depth from SP900 to SP500 indicating the CEBSZ, the boundary between the WDC and EDC. (c) Enlarged version of the seismic section from 6 to 28 km depth from SP2200 to SP1950 indicating NE-dipping reflection band in WDC. (d) Enlarged version of the seismic section showing gently SW-dipping reflection bands in EDC. WDC, Western Dharwar Craton; EDC, Eastern Dharwar Craton; BGB, Bababudan Greenstone belt (WDC). Figure 9. View largeDownload slide (a) Enlarged version of the seismic section from 28 to 40 km depth of the WDC indicating the subhorizontal Moho n fabric. (b) Enlarged version of the NE-dipping reflection band from 6 to 28 km depth from SP900 to SP500 indicating the CEBSZ, the boundary between the WDC and EDC. (c) Enlarged version of the seismic section from 6 to 28 km depth from SP2200 to SP1950 indicating NE-dipping reflection band in WDC. (d) Enlarged version of the seismic section showing gently SW-dipping reflection bands in EDC. WDC, Western Dharwar Craton; EDC, Eastern Dharwar Craton; BGB, Bababudan Greenstone belt (WDC). Deep seismic reflection images delineated in the present study exhibit complex and variable patterns of crustal reflectivity with prominent seismic reflections at many crustal levels. In general, the crust is more reflective in the eastern part of the profile compared with the western part. Our interpretation is based on the CRS post-stack depth migrated section and the line drawing derived from it (Fig. 7). Significant structures of the seismic sections are as follows: A change in reflectivity pattern is observed at 6 km depth throughout the seismic section—a less reflective/transparent zone is observed from the surface to a depth of 6 km followed by a reflective zone further deep. Distinct reflectivity patterns mainly consisting of dipping events are observed throughout the study area from 6 to 40 km depth in the seismic image (Figs 6–9). Strong NE-dipping reflection band (C) are observed from 6 to 28 km depth in the western part of the profile between Shot Point (SP) 2100 and SP1085 (Fig. 9c). These events almost terminate at the CEBSZ/Chitradurga Thrust. A strong steep NE-dipping reflection band (B) is observed from 4 to 30 km depth between SP 950 and SP 500 between the CEBSZ and the Closepet granite (Fig. 9b). An SW-dipping reflection band (A) is observed between SP300 and SP10 from 20 to 43 km depth down to the base of the reflective zone in the eastern part of the seismic section (Figs 8 and 9). Prominent among the SW-dipping reflection bands is the bright reflection band extending from 34 to 43 km from SP 1 to SP 500 (Fig. 7). The SW and NE dipping reflection bands A and B mark a change of reflection character in the crust. These reflection bands constitute a divergent reflection fabric and are observed nearer to the Closepet granite. Bright subhorizontal to gently updoming reflection bands are observed from 4 to 25 km depth between SP 300 and SP 500 in the eastern part of the seismic section beneath the Closepet granite (Figs 7b and c). A thick subhorizontal high-amplitude lower crustal reflection band is observed from 28 to 40 km depth with a sharp decrease in reflectivity further deep between SP 2300 and SP 1450 in the WDC (Fig. 9a). A deepest lower crustal NE-dipping reflection band terminated by a transparency further deep is observed from 34 to 42 km depth between SP 1100 to SP 500. Contrarily, an SW-dipping reflection band is observed from 34 to 43 km between SP1 and SP500 (Figs 7 and 8). The reflectivity beneath the Chitradurga greenstone belt (SP1450- SP1100) is poor in spite of higher fold compared with other regions of the profile (Figs 1 and 7). Poor reflectivity is also observed in the lower-crust of Bababudan greenstone belt (SP2406- SP2000). 6 INTERPRETATION Based on the reflectivity patterns the crust is divided into three parts. The upper non-reflective part extending upto a depth of ∼6 km and represents the thickness of greenstone belts in the region. Global comparison of deep crustal seismic reflection data suggests a transparent upper crust for many Precambrian terrains, except for sedimentary basins (Mooney & Brocher 1987). It is generally observed that reflection profiling in shield regions are unable to image the brittle upper crust properly, which may be due to small-scale heterogeneities associated with complex structures, folds and steep faults and partly to the signal contamination from source-generated noise (Milkereit et al.1994; Reddy & Vijaya Rao 2013). It can also depend on the source-receiver geometry, receiver spacing and the frequencies used in the study. It is observed that shallow structures of the tectonically undisturbed sedimentary basins are well resolved, but not the shallow image of the disturbed areas with same acquisition parameters. The shallow structure (0–6 km depth) derived from the coincident refraction/wide-angle reflection data using tomographic approach (Vijay Rao et al.2015a) can be used to understand the upper crustal features that could not be imaged properly here. The upper crust is separated from the middle one by gentle-moderately dipping (in WDC) and complex reflectivity patterns (in EDC) from 6 to 28 km depth. The lower crust is demarcated from the middle portion with bright subhorizontal reflection fabric from 28 to 40 km depth in WDC (Fig. 7). The reflectivity beneath the Chitradurga greenstone belt is poor in spite of higher fold compared with other regions of the profile (Figs 3 and 5). Poor reflectivity is also observed in the lower-crust of Bababudan greenstone belt. There are various reasons for absence of reflectivity, important among them are poor energy transmission, scattering due to heterogeneities of the crust and poor acoustic impedance contrast between rocks at various depths. 2800 Ma long geological history of the region with different tectonic domains, granitic intrusions and presence of steep-angle shear zones might have disrupted the continuity of reflectors at some places. Important reflectivity patterns in the middle crust are the NE-dipping reflection bands (B and C) and the SW-dipping reflection band (A). Prominent among the SW-dipping reflection bands is the bright reflection band extending from 34 to 43 km from SP1 to SP500 (Fig. 4). This deepest SW-dipping reflection band terminated by transparency further deep is interpreted as the Moho. The dipping reflection bands (B and C) observed throughout the crust is terminated at the Moho, thereby indicating the Moho as a possible detachment boundary. Based on the nature of seismic images derived in the present study and the geological data, we interpret the deepest SW-dipping reflection band (A) representing the Moho as the relict low-angle Neoarchean subduction zone (Fig. 7). High thermal regime of the Archean terrain might be responsible for different rheological properties between the crust and mantle and decoupled them, thereby allowing only the mantle to subduct. Geochemical studies on the volcanic of several Neoarchean schist belts of the EDC have also revealed the presence of subduction-related high-Mg andesites, boninites, Nb-enriched basalts and adakites (Naqvi et al.2006; Manikyamba & Kerrich 2012). Melting of subduction slab resulted in the formation and widespread occurrence of TTG gneisses in the Archean Dharwar craton. Lower-crustal/mantle subduction is found to be a common phenomenon during the Archean as observed from several Lithoprobe deep crustal seismic reflection profiling data from the Canadian Shield (Cook 2002; van der Veldon & Cook 2005). These studies suggest that the direction of subduction zone reflections were commonly found to dip toward the craton as observed in the present study. Mantle reflections represent relict subduction zones, which in turn represent accretionary boundaries as observed from different parts of the world (McBride & Brown 1986; Warner et al.1996; Cook 2002; van der Veldon & Cook 2005). Continuous subduction has resulted in a collision between the western and eastern blocks of the Dharwar craton and developed thrust-fold structures. The NE and SW dipping reflection bands extending from SP1100 to SP500 and SP1 to SP500 respectively constitute a divergent reflection fabric that extends up to the Moho and becomes listric. It represents a zone of convergence between two crustal blocks, namely the EDC and WDC during the Neoarchean orogeny. Two oppositely dipping (SW and NE) deformation patterns formed during convergence represent a collision signature formed above the location of subduction zone (Fig. 7) and are consistent with geodynamical modelling studies (Beaumont & Quinlan 1994). The two dipping reflection bands correspond two distinct crustal blocks of independent evolutionary history, namely the WDC and EDC that are involved in collision in a transpressional tectonic regime. We infer that the Closepet granitic marks the location of collision zone and the crust thickens here to a depth of ∼43 km compared to 40 km and 34 km on either side. Crustal thickness of 42 km and 38 km for the WDC and EDC respectively was identified based on the coincident refraction/wide-angle reflection study (Vijaya Rao et al.2015b). The resolution limit of this study in the deeper part of the crust is ±3 km vertically. The difference in crustal thicknesses derived from near-vertical and wide-angle reflection studies could be either within the error limits of both methods or due to the presence of anisotropic crust in the region. High-grade metamorphic rocks like schists, shear zones and collisional boundaries present in the region could be exhibiting anisotropy. Anisotropy is identified from deep crustal reflection and refraction/wide-angle reflection studies from different parts of the world (Carbonell & Smithson 1991; Jones et al.1996). Passive seismic studies from Dharwar craton have identified anisotropy in the region (Saikia et al.2010). Convergence of crustal blocks generated tectonically imbricated crust, now represented by dipping reflection fabric and such a crustal feature is also observed in several Precambrian orogenic areas of the world (e.g. BABEL Working Group 1990; Calvert et al.1995,; Cook et al.1999; Golbey et al.2004). We interpret that the relict Neoarchean subduction zone identified in the present study represents accretionary boundary and marks the mantle suture, as the Moho of EDC is underthrusted in to the upper mantle. Chadwick et al. (2000, 2007), using the geological data have suggested a Neoarchean accretion of the EDC to the WDC similar to the Phanerozoic-type subduction environment, where an oceanic plate subducted below a Mesoarchean continental margin represented by the WDC. Identification of the location of the subduction zone places an important constraint on the interpretation of an orogen. Among the NE-dipping reflections, the ‘B’ reflection band dipping from 4 to 30 km depth from SP 950, nearer to the eastern boundary of Chitradurga greenstone belt (Figs 1a and 7) is an important thrust fault of the region, referred here as CEBSZ. It is a retro-shear/back-thrust interpreted to have formed during westward thrusting of EDC over the WDC during the Neoarchean orogeny. It is located between the top of the lower crust (30 km depth) to a depth of 4 km. Its extension to the surface could not be imaged properly. But, if extended it may reach the surface at SP1085, the boundary between WDC and EDC. The thrust fault acted as a conduit and carried the slab melts upward to the surface, which formed as younger gneisses (Fig. 7). We interpret that the CEBSZ may represent the surface expression of the suture and located to the west of mantle suture/subduction zone. After the crust-mantle detachment the upper mantle of EDC subducts beneath the WDC and its crust is thrusted over the WDC crust. We interpret that many gently eastward dipping reflection bands observed in major parts of WDC may be a result of thrusting of the EDC crust over the WDC. The structural details imaged in the present study are similar to those derived from the geodynamic model studies (Quinlan et al. 1993; Beaumont & Quinlan 1994) as shown in Fig. 10, where the mantle is underthrusted and its crust moves upward into the deforming orogen. Similar structures are observed in most of the orogens, including Archean, Proterozoic and Phanerozoic period (BABEL Working Group 1990; Quinlan et al.1993; Calvert et al.1995; Hall et al.1995; Cook et al.1999; White et al.2003; van der Veldon & Cook 2005) suggesting that detachment and underthrusting may be a common feature of compressional orogens. In general, the Archean cratons were assembled above shallowly dipping detachments. Earlier seismic refraction and magnetotelluric studies (Kaila et al.1979; Gokaran et al.2004) suggested the presence of an easterly dipping thrust fault in this part of the region. However, those studies didn’t provide much details regarding mechanism of its formation and its relationships with other structures. Figure 10. View largeDownload slide Schematic diagram showing the Moho as a detachment boundary and the oppositely dipping reflection fabric as the signature of a collision, derived from geodynamic modelling studies (after Beaumont & Quinlan 1994). Figure 10. View largeDownload slide Schematic diagram showing the Moho as a detachment boundary and the oppositely dipping reflection fabric as the signature of a collision, derived from geodynamic modelling studies (after Beaumont & Quinlan 1994). The characteristic features of the Moho, crustal structure and its thickness are different for the EDC and WDC. In general the WDC consists of simple structure with gently dipping reflection fabric from 6 to 28 km depth with a subhorizontal reflection fabric from 28 to 40 km depth (Fig. 7). On the other hand, the EDC display complex reflectivity patterns consisting of dipping Moho from 34 to 43 km depth, collision signature, a thrust fault and a crustal root along with an updoming-to-horizontal structure from 4 to 25 km depth at the Closepet granite (SP600-SP800). Seismic reflection images of the present study identified such differences for the first time in the region. It is in contrasts with the simple structure suggested for EDC by earlier studies. The differences in crustal structure, age, nature of schist belts, other geological and geochemical features of the EDC and WDC provide support to the accretion of two independent crustal blocks. During the E–W/NE–SW convergence, the structures of greenstone belts, granites and gneiss were aligned in the N–S/NW–SE direction extending to a maximum distance of ∼450 km, namely, the Closepet granite and Chitradurga greenstone belt (Figs 1a and b). These linear belts suggest the operation of plate tectonics during the Neoarchean in the region. The dipping reflection fabric observed throughout the crust correlate well with the craton-wide imbricate fold-thrust belt mapped with the surface geological data (Chadwick et al.2007). It indicates that much of high-amplitude reflectivity is associated with the shear zones mapped at the surface. The fold-thrust structure observed in the Dharwar craton is attributed to the oblique convergence of two crustal blocks (Chadwick et al. 2000). Prior to the availability of deep seismic images in the region, it is suggested that the thrust-ramp structure become listric at a subhorizontal detachment zone located at a depth of 20 km. However, the deep crustal seismic images of the present study identified the detachment zone at the crust-mantle boundary located at a depth of 40 km. Just above the deep SW-dipping reflection band (A), bright subhorizontal to gently updoming reflection bands are observed from 8 to 25 km depth between SP300 and SP500 just beneath the 2.5 Ga Closepet granite (Figs 1a and 7). The 450 km long and 40 km wide Closepet granite is probably represented by these reflection bands at depth. Terrane accretion and subduction slab break-off resulted in hot mantle asthenospheric upwelling and possibly triggered melting of the mantle wedge that resulted in the formation of various calc-alkaline to K-rich granites. The Closepet granite being located just above the inferred subduction zone (Fig. 7) could have evolved due to slab melting at the subduction zone as suggested by the geochemical analysis (Moyen et al.2001). Geochemical investigations shows increase of εNd values along with decrease of LREE and LILE contents to the east of Closepet granite (Jayananda et al.2000) that is consistent with the direction of subduction to the west/southwest. 6.1 Moho configuration in the region Moho is identified with the brightest lower crustal reflection band terminated with a transparent upper mantle as normally observed elsewhere. Based on the geometry of reflections at the Moho boundary, more than one type of reflection Moho is identified along the profile. Type-I, subhorizontal reflection fabric, type-II, no distinct Moho reflections or absence of clear Moho reflections and type-III, reflections that project from the crust into the mantle. Similar Moho patterns were observed along various LITHOPROBE transects across the Canadian Shield and also in various other parts of the world (BABEL Working Group 1990; Cook 2002; Carbonell et al.2013; Kennett & Saygin 2015). 6.1.1 Type-I The Moho is identified with the termination of deepest bright subhorizontal reflection band, extending from 28 to 40 km depth with a transparent upper mantle in major part of the WDC (Figs 7 and 9a). The subhorizontal reflection band might have developed due to post-collisional extentional process by reworking of lower-crust by mafic underplating (12 km thick) or melting resulting in mafic restites. Thus, the Moho is a younger feature that may not be related to rocks at the surface. It explains the high-velocity (7.0 km s−1, Fig. 7c) observed in the lower-crust and may represent mafic granulites (Vijaya Rao et al. 2015b). During this modification some of the deeper reflections might have obliterated. The evidence for magmatic underplating is derived from the presence of craton-wide mafic dyke swarms (Fig. 1b) and other later tectonic activities of the region (Kumar et al.2012; Vijaya Rao et al.2015b). The Moho boundary (at 40 km depth) may be acting as a shear zone created in response to the shortening and imbrications of the crust above a more rigid upper mantle. It acts as a prominent rheological boundary that may localize zone of detachment during subsequent deformation. The receiver function studies from Australia suggest similar crustal thickness with high-velocity lower-crust in the Neoarchean cratons (Yuan 2015). 6.1.2 Type-II No Moho reflections are observed beneath the Chitradurga greenstone belt. It could be due to various reasons, like insufficient contrast in velocity and density at the crust-mantle boundary may be due to the presence of mafic granulites or garnet-bearing granulites in the lower crust, which have velocity and density similar to mantle values; gradual change in velocity and density, which may not produce reflections; the boundary is too complex to produce coherent reflection energy; the complex nature of the crust, like low-velocity, high attenuation intrinsic structural complexity, etc., of the near-surface region, which forbids clear imaging of the crust below; multiple scattering in the crust may also responsible significantly to the reflection character and absence of reflections (Holliger et al.1994). It is not possible to establish which of these processes is most likely responsible for the observed Moho characteristics in this part. 6.1.3 Type-III A bright SW-dipping reflection band projects from the crust at 34 km depth into the transparent upper mantle to a depth of 43 km from SP1 to SP500 in the EDC. It is interpreted as the Moho. The Moho from SP500 to SP1000 is represented by a reflection band extending from 42 to 34 km depth. The entire Moho band in the EDC from SP1 to SP1000 shows thinning of the crust at both ends with a thickest crust at the subduction-accretion boundary at SP500, in the middle (Fig. 7). Similar dipping reflections from the crust to the mantle are observed to a depth of 100 km beneath the Slave craton, 60 km in the eastern part of Superior province, 46 km in the western Superior province and to a depth ∼70 km in the Baltic shield (Cook 2002; White et al.2003; BABEL Working Group 1990). New continental crust is accreted at the convergent margins by complex geological processes. Similarly, the reflectivity pattern at the Moho, the crust-mantle boundary, in these regions is also complex as observed in the EDC. Temperature near a subduction zone is generally cold and increases away from it. Therefore, the crust and mantle structures adjacent to a subduction zone likely to be frozen as observed in EDC and other regions of the world (BABEL Working Group 1990; Cook et al.2002; Kennett & Saygin 2015). They remain intact as long as no thermal activity over prints them. From the above discussion we conclude that the crustal structure and character of the Moho in EDC are related to the ages of the rocks at the near surface. In the present study, the depth of the Moho and its characteristics are changing across the tectonic boundaries. Whereas, the reflection Moho is at a constant depth even beneath the regions with different ages (Archean-Phanerozoic) and tectonic histories along major part of the Canadian LITHOPROBE transects (Cook 2002). It shows that the Moho is a reworked younger feature in those regions. The diverse character of the reflection Moho observed in the present study region suggests that it does not arise from a single process, but rather reflects the action of a geological history of the region. Juxtaposition of horizontal Moho in the WDC with its absence in the adjacent Chitradurga greenstone belt indicates the processes that alter the physical properties of the Moho and may not necessarily change the structural geometry of the surrounding rocks. 7 DISCUSSION AND IMPLICATIONS High temperatures of the Archean period promoted hot orogenic model for the growth of Archean terrains (Chardon et al.2011). The model suggests that during the Archean orogenic activity, low-viscosity, presence of partial melts, high-buoyancy of the juvenile crust and very low strength of mantle lithosphere did not allow the crust to sustain thickening. But, crustal shortening is accommodated by pervasive 3-D flow of the viscous lower crust that combines orogen-normal shortening, lateral constrictional stretching and transtension. Thus, the hot orogenic model predicts mechanical and thermal homogenization of the lower crust. We find differences in the lower-crustal structure between the WDC and EDC. Subhorizontal lower-crustal reflection fabric is observed in major part of WDC, except beneath the Chitradurga greenstone belt, whereas earlier dipping reflection fabric related to the orogenic activity is imaged in EDC with a preserved 43 km thick crust at the collision zone (Fig. 4). Such a structural variation suggests that the post-orogenic extension or high temperatures of Archean period modified the lower-crust of WDC, whereas it neither disturbed the crustal fabric formed during accretion nor homogenized the lower-crust of EDC. The melts of granitic magma formed during extension ascended to the surface through the fault pattern, which acted as conduits leaving behind the lower crust enriched with mafic material. The Closepet and other granites are evolved by such process. The ascending fluids must have made good acoustic impedance contrast with host rocks that resulted in bright seismic reflectivity in the EDC. Granitoid emplacement studies in the Cordellera emphasized the important role played by fault-system (Petford et al.2000). It is observed that post-collisional extension activities have preserved earlier crustal structures in many parts of the world (Calvert & Ludden 1999; van der Veldon et al.2004; Yuan 2015). The hot orogenic model can be applied to a part of Mesoarchean WDC that exhibits thermal homogenization of the lower-crust and may not appropriate to the Neoarchean EDC, where the crustal root (43 km thick crust, Fig. 7) and accretionary fabric with dipping reflections are still preserved at various crustal depths, including lower-crust. An interpretative line drawing, Fig. 7(c), superposed with the 1-D velocity model derived from the coincident refraction data (Vijaya Rao et al.2015b) shows coincidence of the reworked subhorizontal reflection fabric with the high-velocity (7.1 km s−1) lower-crustal layer and complements it. A small discrepancy in crustal thickness derived from refraction and reflection data could be due to differences in methodology adopted in data acquisition and processing. Steeply dipping/subvertical faults F1–F3 are inferred based on the differences in reflectivity on either side and marked over the line drawing and we interpreted them as strike-slip faults. They demarcate the different tectonic boundaries of the region and coincide with the geologically identified faults/shear zones, located parallel to the orogeny (Figs 1a and b and 2b). They must have formed either during the transpressional tectonic regime of the Neoarchean oblique collisional orogeny or during the post-collisional period. The Great Glen fault, the Makarovo fault, Norumbega fault, etc., are some of the strike-faults developed in a compressional regime and identified using seismic signature (McBride & Brown 1986; Stern & McBride 1998; Mandal et al.2013). Such steeply dipping faults are also identified from the deep seismic reflection studies of other Archean terrains of different regions, for example, the Canadian Shield (van der Veldon et al.2006). These faults were found to have been formed during post-orogenic period and played little role in thickening of the crust. Small and rapid movement of plates, high-temperatures, and longer ocean ridge lengths are regarded as the tectonic environment of Archean terrains that may be responsible for the low-angle subduction observed in the Dharwar craton. Deep seismic images have identified similar low-angle subduction zones to further east in EDC and also from the Baltic and Canadian shields (BABEL Working Group 1990; Calvert et al.1995; White et al.2003; Vijaya Rao et al.2006). The dipping reflection fabric and style of reflective lower-crust observed in the present study are similar to those found in the Phanerozoic and Proterozoic orogens with some minor changes, indicating operation of plate tectonics-like processes during the Neoarchean. Dominantly linear architecture of the Dharwar craton (Fig. 1a) supports accretionary orogenic interpretation. The present study suggests difference in Moho depth between east (34 km) and west (40 km) of the Chitradurga greenstone belt (Fig. 7), indicating a thicker crust for the WDC and thinner one for the EDC. The coincident refraction/wide-angle reflection study along with other geophysical and geological data identified this zone as a west-dipping subduction zone (Vijaya Rao et al.2015b). This mantle subduction zone derived from refraction data coincides with the dashed-line marked to join the Moho boundary in the reflection data and shown in Fig. 7(c). The geochronological data identified two distinct episodes of greenstone volcanism, mafic and felsic, respectively at 2.7–2.65 Ga and 2.58–2.55 Ga (Jayananda et al.2013). These two episodes of volcanism with the associated plutonism correspond to two crustal accretionary events and a two stage growth of the Dharwar craton. Seismic reflection, coincident refraction and other geological data suggests that that the region has undergone two-stage subduction process with westerly dip. The first stage of subduction-accretion (2.7–2.65 Ga) was at the eastern margin of Mesoarchean WDC and the second event (2.58–2.52 Ga) is identified at the eastern boundary of Closepet granite. The present study resolves the ambiguity associated with the EDC–WDC boundary, the location being CEBSZ or the Closepet granite (Kaila et al.1979; Naqvi & Rogers 1987; Griffin et al.2009) with a two-stage subduction-accretion process. The 2.62 Ga potassic plutons observed at the eastern margin of the WDC (Ramakrishnan & Vaidyanadhan 2008) are due to melting of the lower-crust during earlier event, whereas the TTG gneisses, calc-alkaline plutons and felsic volcanism throughout the EDC are due to melting of subducted slab and mantle wedge during the second accretionary event (Figs 1 and 3b). Two-stage subduction process is also observed in the Archean Superior Province of Canada (Calvert & Ludden 1999; White et al.2003). Further, the Archean Superior Province was assembled through five separate accretionary orogenic events over a period of ∼40 Ma instead of a single Neoarchean orogeny (Percival et al.2006) and crustal growth was the product of subduction-accretion tectonics (Wyman et al.2002). The evolutionary model of the region is schematically shown in Fig. 11. Figure 11. View largeDownload slide Schematic derived from seismic data constrained from geological and geochemical data shows the evolutionary model of the Dharwar craton. (a) The Mesoarchean continental crust (WDC) is located on the southwest and the oceanic crust to the northeast with initial subduction (2.7 Ga) at eastern margin of WDC. (b) Accretion of two crustal blocks and formation of CEBSZ. (c) Second stage of subduction (at 2.5 Ga) farther to the northeast. Subduction-1 slab break off. (d) Accretion of WDC and EDC with the formation of Closepet and other granites and represents present day configuration. Figure 11. View largeDownload slide Schematic derived from seismic data constrained from geological and geochemical data shows the evolutionary model of the Dharwar craton. (a) The Mesoarchean continental crust (WDC) is located on the southwest and the oceanic crust to the northeast with initial subduction (2.7 Ga) at eastern margin of WDC. (b) Accretion of two crustal blocks and formation of CEBSZ. (c) Second stage of subduction (at 2.5 Ga) farther to the northeast. Subduction-1 slab break off. (d) Accretion of WDC and EDC with the formation of Closepet and other granites and represents present day configuration. Conventionally, it is believed that the accretionary boundary between the Mesoarchean WDC and Neoarchean EDC is sharp and located at the CEBSZ. But, the recent geological and Nd isotopic studies identified the boundary as a ∼200 km wide diffused zone (Chadwick et al.2007) with the older Mesoarchean ages of the WDC extending further east of the CEBSZ up to a distance of 150 km (Dey 2013). The diffused zone also referred as the Central Dharwar Craton, is associated with juvenile Neoarchean plutonic rocks to its east compared with mixed juvenile and crustally derived granites to the west (Peucat et al.2013). Accidentally, the boundaries of the Central Dharwar craton coincide with two subduction zones identified in the present study and supports two-stage subduction process for the region. 7.1 Geodynamic evolution of the Dharwar craton The deep crustal structure derived in the present study along with the available geological and geochemical data suggests that an Andean-type environment was prevailing during the Neoarchean. 3.4 Ga continental crust represented by the WDC was located in the west and an oceanic crust to the east in the region. The denser oceanic plate located to the east (EDC) subducted under the continental plate located to the west (WDC), which is manifested by west dipping Moho (Fig. 7). The ocean was closed due to continuous subduction process and finally the EDC (accretionary complex) with juvenile crustal material made up of series of arcs, intra-arc basins and batholiths of 2.7 Ga age (Balakrishnan et al.1999) was accreted against the continental margin of the WDC and responsible for the formation of Neoarchean orogeny in the region. The westward thrusting of the EDC over the WDC during the orogeny resulted in the formation of a back-thrust/retro-shear at the boundary of WDC, referred as the CEBSZ. The orogeny is manifested in the form of widespread syn- to post-orogenic (2.56–2.50 Ga) granitic intrusion, Proterozoic dykes, sills, strike-faults and shear zones on the surface (Fig. 1b). At some places high-Mg andesites, boninites, Nb-enriched basalts, rhyolites and adakites are identified (Manikyamba & Kerrich 2012). During the E–W/ENE–WSW convergence, the structure of the greenstone belts, granites and gneisses were aligned in the N–S/NW–SE direction extending to a maximum distance of 450 km (e.g. Closepet granite, Fig. 1a). These linear belts suggest the operation of plate tectonics during the Neoarchean in the region. The processes were likely dominated by a two-stage subduction-accretion process. Initial subduction was at the eastern margin of WDC and the later one was at the eastern boundary of the Closepet granite (Central Dharwar Craton) (Fig. 11). It is suggested that granite-greenstone terrains represent middle to upper crustal remnants of Archean orogenic belts, which were once parts of much larger orogenic systems (de Wit & Ashwal 1997). Others believe that greenstone belts are backarc basins developed in an arc-subduction environment because of their association to volcanic composition, while others consider that the greenstone belts represent marginal basins associated with subduction zones. It is interpreted that the Granite-greenstone belts in the EDC are evolved by such a process. Increased geothermal gradient in the Archean, with more efficient horizontal heat transfer and high-temperature–low-pressure tectono-metamorphic episode, leads to pervasive melting and production of calc-alkaline to K-rich juvenile granitic magma, namely, the Closepet and a large number of other granites in the EDC that cover a large area (Fig. 1a). Terrane accretion, the subduction slab breaks-off, resulted in upwelling of hot mantle asthenosphere and probably triggered melting of the mantle wedge. As a result the Closepet and other granites were formed and intruded into the crust as suggested by Dey (2013) from the geochemical studies. All these granites exhibit a narrow linear trend and extend to a distance of 350 km in the NW–SE direction. These evidences give credence to the operation of plate tectonics with subduction and collision, inferred from the present seismic study. Using geochemical studies from the Dharwar craton, Krogstad et al. (1989) suggested operation of plate tectonics in the region during Neoarchean. Thus, the crustal evolution of Archean Dharwar craton can be seen as a wide-orogenic domain of amalgamated cordillera-type terrains, which closely resemble the Archean crustal evolution in different parts of the globe as suggested by van der Veldon et al (2006). 8 CONCLUSIONS The DHARSEIS deep seismic reflection profile across the Archean Dharwar craton provides crustal structure with variable reflectivity patterns at many crustal levels. A simple structure with subhorizontal Moho of the WDC separates from complex reflectivity pattern consisting of dipping Moho, collision signature, a thrust fault and a crustal root of the EDC. The crustal thickness is not uniform along the profile as demonstrated by the depth and characteristics of the Moho. The diverse characteristics of the Moho suggest that it does not arise from a single process and reflects action of geological history of the region. Seismic crustal structure derived in the present study is consistent with convergence, subduction and accretion of crustal blocks. Deep seismic reflection images complemented with geology indicate that the Dharwar craton records a Neoarchean orogeny involving the Mesoarchean rocks on the west and a Neoarchean juvenile arc system to the east. The orogeny is partially over-printed by wide-spread syn-to-post-orogenic granitic intrusions. They are responsible for the crustal evolution of the Dharwar craton during the Neoarchean. The deep crustal structure of the Neoarchean Dharwar craton that is broadly analogues to the Phanerozoic accretionary terrains provides evidence for the operation of plate tectonics during the Neoarchean over the Indian shield. The seismic image suggests a two-stage subduction—accretion process for the crustal evolution in the Neoarchean Dharwar Craton with a crust-mantle detachment. The dipping seismic reflections from the lower-crust extending into the mantle are imaged for the first time, which we interpret to represent a relict 2.5 Ga old suture associated with mantle subduction. Acknowledgements The deep seismic reflection experiment was funded by the Department of Science & Technology (DST), Government of India. We thank DST for the research grant under DST-DCS programme. The digital data were collected using the SN 388 RFT system and the 4.5 Hz (HGS made) geophones belonging to the National Geophysical Research Institute (NGRI). These organisations can be approached to access the data if required. BM is grateful to F. Wenzel and J. Mann for providing the CRS code during the DAAD Fellowship at Kalrsruhe Institute of Technology, Germany. Director, CSIR-NGRI is duly acknowledged for all support and encouragement. We thank Prakash Kumar for valuable suggestions. We are also thankful to P. Sai Vijay Kumar and N. Damodara for their technical support. This paper is dedicated to Late V. Sridhar, who helped in data acquisition and processing. Reviews by Ewald Luschen, an anonymous reviewer and Editor Gabi Laske helped to improve the manuscript substantially and are much appreciated. REFERENCES BABEL Working Group , 1990 . Evidence for early Proterozoic plate tectonics from seismic reflection profiles in the Baltic Shield , Nature , 348 , 34 – 38 . Crossref Search ADS Balakrishnan S. , Rajamani V. , Hanson G.N. , 1999 . 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Published by Oxford University Press on behalf of The Royal Astronomical Society. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geophysical Journal International Oxford University Press

Deep crustal seismic reflection images from the Dharwar craton, Southern India—evidence for the Neoarchean subduction

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Oxford University Press
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© The Authors 2017. Published by Oxford University Press on behalf of The Royal Astronomical Society.
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0956-540X
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Abstract

Abstract Major part of the Earth's continental crust is evolved during the Archean, however, the mechanism for its formation is controversial. It could have formed either through horizontal accretion similar to the modern plate tectonic processes or by vertical accretion by plume activity. Here, we present the results of a new deep crustal seismic reflection Profiling, the DHARSEIS experiment conducted along a 200 km long Perur-Chikmagalur profile across the Archean Dharwar craton, to understand the crustal evolutionary processes during the Neoarchean. The data were processed using the Common Reflection Surface (CRS) stack method. Seismic images show distinctly different reflectivity patterns in the Mesoarchean Western and Neoarchean Eastern Dharwar Cratons (WDC and EDC), the two crustal blocks of the composite Dharwar craton. The WDC consists of a simple structure with a major part of the crust from 6 to 28 km displaying a gently dipping reflection fabric and a subhorizontal reflection fabric from 28 to 40 km except beneath the Chitradurga schist belt. On the other hand the EDC displays a complex reflectivity pattern, contrary to the simple crustal structure suggested by various other studies. A dipping Moho, oppositely dipping reflection fabric and a thrust fault are the major crustal features in the EDC. The present seismic study imaged a west-dipping reflection fabric extending from 34 to 43 km in the EDC, which is interpreted to represent an upper-mantle subduction zone. During this process the EDC was thrusted obliquely against the pre-existing proto-continent WDC and accreted to it. Oppositely dipping reflection fabrics with a crustal root at the convergence boundary suggest accretion of WDC and EDC during the Neoarchean orogeny. The collisional boundary coincides with the location of ∼2.5 Ga Closepet granite. Seismic images suggest the Moho as a detachment boundary. The present study reveals a likely two-stage subduction-accretion process for the evolution of continental crust during the Neoarchean. Plate tectonic processes were responsible for the Neoarchean crustal growth in the region. Composition and structure of the continental crust, Continental imaging, Continental margins: convergent, Continental tectonics: strike-slip faulting, Cratons, Subduction zone processes 1 INTRODUCTION Configuration of the present day continental crust is a manifestation of protracted history of lithospheric evolution involving complex crust–mantle interaction processes. Growth of the continental crust took place either by lateral accretion of arc complexes at continental margins or by vertical accretion of the mafic material at the base of the crust from upwelling upper mantle zones (Stein & Hofmann 1994). On the other hand, continents are progressively destroyed at the surface by physico-chemical processes and at its base by processing involving delamination (Condie 1989). Thus, a large part of the continental crust might have gone through recycling process of the perpetually dynamic Earth since its formation in the Hadean to the early Archean (Kamber 2015). But, so far no consensus has emerged concerning the extent to which modern plate tectonic processes can be extrapolated. So, understanding the structure, dynamics, origin and evolution of the continental crust is fundamental to the Earth science. The Archean period represents one-third of the Earth's history during which as much as 70 per cent of present cratons were formed and stabilized with major tectonic assembly. So, the study of Archean crust is important to understand the Earth's geodynamics. Amongst the Archean blocks, the Precambrian Indian Shield possesses certain unique geotectonic and geodynamical characteristics, which provide an opportunity to understand its multistage crustal evolution both in space and time. The Precambrian Indian shield is a mosaic of several cratons and mobile belts. The late Archean Dharwar craton with 3.4 Ga Earth's history (Nutman et al. 1992; Ramakrishnan & Vaidyanadhan 2008; Jayananda et al. 2013) is an important part of the collage of Precambrian terrain in the Indian shield and an ideal natural laboratory to test the hypotheses concerning the Archean crustal evolution. The Dharwar craton of the Indian shield represents one of the largest and oldest cratonic blocks (3.4 Ga) available on the Earth, similar to the Superior Province, the Yilgaran craton, the Karelian craton and the Kaapval craton. It is flanked by the high grade granulitic terrains to the south and east and by the cover of late Cretaceous Deccan flood basalt to the north (Fig. 1a). Almost the entire craton is covered by the Tonalitic Trondjhemitic Gneisses (TTG) with several northwest–southeast trending greenstone belts of volcano-sedimentary formations ranging in age between 3.4 and 2.7 Ga and intrusive granite (Nutman et al.1992; Bhaskar Rao et al .1992; Ramakrishnan & Vaidyanadhan 2008; Jayananda et al.2013). Three prominent tectono-thermal events that occurred between 3400 and 2500 Ma in the region provided important constraints to understand the crustal evolution during the Archean. The Dharwar craton comprises two distinctly different crustal blocks, namely, the meso-Neoarchean western and the predominantly Neoarchean eastern blocks (Nutman et al.1992; Bhaskar Rao et al.2008; Jayananda et al.2008), which are popularly referred to as the Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC). Figure 1. View largeDownload slide (a) Geological map of the Southern India along with the Perur-Chikmagalur seismic reflection profile marked over it. Numbers indicate the Shot Points locations used in reflection data acquisition. WDC: Western Dharwar Craton; EDC: Eastern Dharwar Craton; SGT: Southern Granulite Terrain; PCSZ: Palghat Cauvery Shear zone (modified after GSI & ISRO 1994). XY indicates the profile along which geological cross-section is provided in Fig. 2(a). (b) Shear zones are shown as continuous lines in blue colour and Proterozoic dykes in small black lines over the geological map of the Dharwar craton. Note that shear zones are trending towards NW–SE direction (modified after Vijaya Rao et al.2015b). Figure 1. View largeDownload slide (a) Geological map of the Southern India along with the Perur-Chikmagalur seismic reflection profile marked over it. Numbers indicate the Shot Points locations used in reflection data acquisition. WDC: Western Dharwar Craton; EDC: Eastern Dharwar Craton; SGT: Southern Granulite Terrain; PCSZ: Palghat Cauvery Shear zone (modified after GSI & ISRO 1994). XY indicates the profile along which geological cross-section is provided in Fig. 2(a). (b) Shear zones are shown as continuous lines in blue colour and Proterozoic dykes in small black lines over the geological map of the Dharwar craton. Note that shear zones are trending towards NW–SE direction (modified after Vijaya Rao et al.2015b). A detailed deep image of the subsurface is essential to understand the dynamics, origin and evolution of the continental crust of any region. In this context, deep continental crustal structure is a result of continental lithospheric dynamics. Analysis of deep crustal seismic data across the globe suggests that deep seismic reflection profiling has proven to be a power full tool that provides high resolution images of the subsurface and reveals the structural information necessary for recognition of individual terrains and boundaries, especially when dealing with Precambrian shield areas (Mooney & Meissner 1992; Cook et al.1999; Goleby et al. 2004). These studies are fundamental for developing geodynamic models on the evolutionary history of the Earth and to understand the juxtaposition of the crustal blocks in the geologic past as well as continental dynamics. In this context, a large number of geological and geochronological studies were carried out to resolve the issues related to deep crustal configuration and tectonics of the Dharwar craton (e.g. Naqvi 1985; Radhakrishna & Naqvi 1986; Chadwick et al. 2000; Chardon et al.2008). So far only two deep seismic refraction/wide-angle reflection data have been acquired out across the Dharwar craton to delineate its crustal velocity structure (Kaila et al.1979; Sarkar et al.2001; Mall et al.2012; Vijaya Rao et al.2015a,b). However, the results based on refraction/wide-angle reflections that primarily provide velocity information of the subsurface are insufficient to explain the tectonic settings and evolutionary history of the Dharwar craton. Seismological data along with receiver function analysis suggest only velocity information and variation of crustal thickness in the Dharwar craton without any precise subsurface structural details, due to inherent limitations of those methods (Rai et al.2003; Borah et al.2014). Even though surface geology was well studied, the deep crustal structure and tectonics of the Dharwar craton remained inadequately constrained. Still there exists a big debate regarding the relationship between the eastern and western blocks, subduction related tectonics and its evolutionary history. On this background, to bridge the knowledge gap in the region, a coincident refraction/wide-angle reflection and narrow-angle deep seismic reflection study was carried out along the ENE–WSW trending Perur-Chikmagalur profile (Fig. 1a) across the Dharwar Craton as a part of DHARSEIS (Dharwar seismic) experiment during 2010–2012 (Vijaya Rao et al.2015a,b). Deep seismic reflection data that provide the highest resolution information on the structure and physical properties of the continental crust and the Moho boundary are acquired for the first time over the Dharwar craton. The present paper deals with this dataset with primary objective to understand the crustal structure and Moho configuration, both in the eastern and western Dharwar cratons and determine their accretionary boundary using high-resolution seismic images. Further, it is used to derive a mechanism for the Neoarchean accretion and crustal growth process by developing an evolutionary model for the Dharwar craton. 2 GEO-TECTONIC FRAMEWORK 2.1 Geology The Dharwar craton is divided into two distinct tectonic blocks, for example, the Mesoarchean Western Dharwar Craton (WDC) and the Neoarchean Eastern Dharwar Craton (EDC) with different crustal evolution patterns based on the differences in lithotectonic association, nature and abundance of greenstone belts, age distribution, deformation, metamorphic pattern and degree of melting (Chadwick et al. 2000; Ramakrishnan & Vaidyanadhan 2008). Prominent shear zone known as the Chitradurga Eastern Boundary Shear Zone (CEBSZ) or Chitradurga Thrust located all along the eastern margin of the NW–SE trending Chitradurga schist belt is generally considered as the boundary between the EDC and WDC (Chadwick et al.2000; Reddy et al.2000; Jayananda et al.2006; Chardon et al.2008, 2011). Geochronological data suggest progression of the Dharwar craton from young to old age as we move from east to west. The geological map of the Dharwar Craton is shown in Fig. 1(a) and different stratigraphic units compiled from available literature (Bhaskar Rao et al.2008; Jayananda et al.2008, 2013) are listed in Table 1. Table 1. Geological framework of the Dharwar craton. Western Dharwar Craton (WDC) Peninsular Gneisses (Old) 2.9–3.5 Ga Komatiite volcanism 3.35 Ga Greenstone belts (WDC) Sargur Group >3.0 Ga Dharwar Supergroup 2.9–2.6 Ga    Bababudan Group 2.9 Ga    Chitradurga Group 2.75 Ga Potassic K-Granites 2.61 Ga Predominance of sedimentary rocks over volcanic rocks. Kimberlites are absent. Eastern Dharwar Craton (EDC) Peninsular Gneisses (Young) 2.7–2.9 Ga Greenstone belts 2.7–2.55 Ga Calc-alkaline to Potassic granites 2.5 Ga Volcanic rocks are predominant and sedimentary rocks are sparse. Komatiites are rare. Island arc basalts and other volcanic rocks (boninites & adakites) are predominant. Kimberlites are predominant Western Dharwar Craton (WDC) Peninsular Gneisses (Old) 2.9–3.5 Ga Komatiite volcanism 3.35 Ga Greenstone belts (WDC) Sargur Group >3.0 Ga Dharwar Supergroup 2.9–2.6 Ga    Bababudan Group 2.9 Ga    Chitradurga Group 2.75 Ga Potassic K-Granites 2.61 Ga Predominance of sedimentary rocks over volcanic rocks. Kimberlites are absent. Eastern Dharwar Craton (EDC) Peninsular Gneisses (Young) 2.7–2.9 Ga Greenstone belts 2.7–2.55 Ga Calc-alkaline to Potassic granites 2.5 Ga Volcanic rocks are predominant and sedimentary rocks are sparse. Komatiites are rare. Island arc basalts and other volcanic rocks (boninites & adakites) are predominant. Kimberlites are predominant View Large Table 1. Geological framework of the Dharwar craton. Western Dharwar Craton (WDC) Peninsular Gneisses (Old) 2.9–3.5 Ga Komatiite volcanism 3.35 Ga Greenstone belts (WDC) Sargur Group >3.0 Ga Dharwar Supergroup 2.9–2.6 Ga    Bababudan Group 2.9 Ga    Chitradurga Group 2.75 Ga Potassic K-Granites 2.61 Ga Predominance of sedimentary rocks over volcanic rocks. Kimberlites are absent. Eastern Dharwar Craton (EDC) Peninsular Gneisses (Young) 2.7–2.9 Ga Greenstone belts 2.7–2.55 Ga Calc-alkaline to Potassic granites 2.5 Ga Volcanic rocks are predominant and sedimentary rocks are sparse. Komatiites are rare. Island arc basalts and other volcanic rocks (boninites & adakites) are predominant. Kimberlites are predominant Western Dharwar Craton (WDC) Peninsular Gneisses (Old) 2.9–3.5 Ga Komatiite volcanism 3.35 Ga Greenstone belts (WDC) Sargur Group >3.0 Ga Dharwar Supergroup 2.9–2.6 Ga    Bababudan Group 2.9 Ga    Chitradurga Group 2.75 Ga Potassic K-Granites 2.61 Ga Predominance of sedimentary rocks over volcanic rocks. Kimberlites are absent. Eastern Dharwar Craton (EDC) Peninsular Gneisses (Young) 2.7–2.9 Ga Greenstone belts 2.7–2.55 Ga Calc-alkaline to Potassic granites 2.5 Ga Volcanic rocks are predominant and sedimentary rocks are sparse. Komatiites are rare. Island arc basalts and other volcanic rocks (boninites & adakites) are predominant. Kimberlites are predominant View Large A major part of the WDC is covered largely by unclassified gneissic granitoids referred as ‘older gneissic complex’. The complex is mainly composed of polyphase TTG gneisses of 3.4–2.9 Ga age and K–granites. During the 3.4–3.0 Ga periods the WDC experienced multiple cycles of juvenile crustal addition and recycling and ∼2.60 Ga K-granites mark the end of plutonism (Jayananda et al. 2006). The WDC contains two types of supracrustal sequences: the Sargur and the Dharwar type schist belts. The Sargur type schist belt hosts the older sequence (3.4–3.0 Ga) comprising highly deformed ambhibolite to granulite facies rocks of shelf region (Ramakrishnan & Vaidyanadhan 2008). The gneisses and Sargur rocks are unconformably overlain by 2.9–2.6 Ga greenstone belts (Bhaskar Rao et al.1992) in the WDC. The later greenstone belt, known as the Dharwar schist belt, has larger dimension of greenstone terranes (Fig. 1a) that contains dominant greenschist facies metasedimentary components with predominant metabasalts along with komatiite–tholeiite association and minor bimodal volcanic rocks. The Dharwar schist belt contains iron and manganese ore deposits. The Dharwar greenstone belt is further divided into older Badabudan (2.9 Ga) and younger Chitradurga group (2.8 Ga). The litho-units of the Chitradurga group suggest continental margin to deep marine environments for their deposition (Chadwick et al.2000). The WDC is characterized mostly by medium pressure and low-high temperature rocks. In the present paper greenstone belt and schist belt are used synonymously to represent the supracrustal sequences of the region. The EDC is mainly consists of linear N–S to NNW trending belts of gneissic granitoids referred as ‘younger gneissic complex’. The complex is mainly composed of the Neoarchean TTG gneisses (2.7–2.55 Ga) with remnants of 3.0 Ga rocks and calc-alkaline to potassic-granites (2.56–2.50 Ga). They are interleaved with narrow 2.7–2.55 Ga greenstone belts mostly made up of greenschist to ambhibolite facies rocks (Chadwick et al.2000,; Ramakrishnan & Vaidyanadhan 2008). Some of these greenstone belts host gold mineralization. Major type of granite suite is the Closepet granite (2.52 Ga) and its equivalents. A considerable part of the EDC represents a juvenile addition to the crust from the mantle dominantly during 2.7–2.5 Ga (Peucat et al.1993). Geochemical investigation on the volcanic of Neoarchean greenstone belts of the EDC carried out by various researchers reveals the presence of subduction related high-Mg andesites, bonintes, Nb-enriched basalts and adakites (Naqvi et al.2006; Manikyamba & Kerrich 2012). Bimodal volcanic rocks are more prevalent with some komatiites in the EDC. 2.2 Tectonics of the region Absence of proper geophysical data, especially high-resolution deep crustal seismic structure, resulted in a number of evolutionary models for the region based mainly on surface geological data. Thus, the geodynamic evolution of the Archean Dharwar craton is highly controversial. The Neoarchean crustal evolution can be mainly divided into two types: subduction-accretion and mantle plume tectonics. Plate tectonic models suggest that the EDC had evolved by successive accretion of arcs against a foreland (the WDC) in a convergent setting during the Neoarchean (Chadwick et al.2000). The Neoarchean schist belts of EDC are interpreted as intra-arc basins, whereas those of the WDC are backarc basins (Dey 2013). Contrarily, the plume model suggested that a Neoarchean mantle plume supplied heat, softened the crust and induced inverse diaprism, metamorphism, partial melting of the crust and granitic magmatism (Jayanandan et al.2000). Some other models with mixed-mode operation invoke initial subduction, followed by closure of oceanic crust and arc-continent collision with subsequent mantle plume activity causing crustal reworking and metamorphism (Moyen et al.2001). The N–S to NNW trending structural fabric of the entire Dharwar craton could be a result of transcurrent shear deformation due to crustal-scale shortening in a convergent setting during the Neoarchean (2.56–2.51 Ga). Fig. 1(b) shows the shear zones and Proterozoic dykes in the Dharwar craton. Further, the EDC is considered as a hot orogen characterized by soft, buoyant crust due to magmatic accretion in an arc setting with ridge subduction and opening of a slab window during the Neoarchean. During convergence, the lower crust of EDC underwent pervasive 3-D flow process (mass distribution) called as lateral constructional flow, which includes orogen-normal shortening, lateral constrictional stretching and transtension. This resulted in high-temperature–low-pressure metamorphism in the EDC (Chardon et al.2008; Jayananda et al.2013). Another model suggests that the volcano-sedimentary facies of the Chitradurga schist belt, located between the WDC and EDC, may represent closure of a basin located between these two continental crustal blocks with some form of plate separation and subsequent collision (Naqvi 1985). The eastern margin of the Chitradurga schist belt marked by a series of N–S trending shear zones made up of 1–1.5 km wide mylonites (Chadwick et al.2000). The N–S orientation of tectonic fabrics in the schist belt and the magmatic and tectonic fabrics in the plutonic rock, together with other structural geometry, indicate an E–W compressive regime of Meso-Neoarchean period (Chadwick et al.2007). In the given litho-tectonic framework, the faulted contact zone between the WDC and EDC has been interpreted as a cryptic suture (Naqvi 1985). A geological cross section across the WDC and EDC derived from surface geological mapping and tectonics is shown in Fig. 2. It would emphasise the need for deep seismic reflection study as the reflection image enhance/change the understanding of the crustal structure in the region similar to the IBERSEIS deep seismic profiling (Simancas et al. 2003). Figure 2. View largeDownload slide (a) Geological cross-section along the profile XY shown in Fig. 1(a) derived from the surface geology and tectonics (after Chadwick et al.2007). (b) Geological model derived from geological constraints (Jayananda et al.2006): MHZ, Medikeripura High-strain Zone; Gg, Guntakal granodiorite. Figure 2. View largeDownload slide (a) Geological cross-section along the profile XY shown in Fig. 1(a) derived from the surface geology and tectonics (after Chadwick et al.2007). (b) Geological model derived from geological constraints (Jayananda et al.2006): MHZ, Medikeripura High-strain Zone; Gg, Guntakal granodiorite. 3 SEISMIC DATA ACQUISITION The DHARSEIS deep seismic reflection profiling data were acquired in an end-on configuration with 100/200 m shot and 100 m receiver spacing using a 150 channel Eagle-88 RF telemetry system. The data were acquired using 4 ms sampling interval and recorded to a length of 24 s. A theoretical fold of 37 was expected from the above source-receiver geometry. But, the fold varies between 10 and 33 with an average value of ∼20 along the profile because of various logistics in the field. The data were acquired using 4.5 Hz geophones that provide a broad frequency bandwidth of 5–80 Hz. The topography varies from 550–1000 m along the seismic profile. More details on data acquisition are provided in Table 2. Table 2. Data acquisition parameters. Length of the profile 240 km Type of source Explosives Shot hole depth 25–28 m Charge size/hole 50–75 kg No. of shots 900 No. of channels 150 Shot point spacing 200 m Receiver spacing 100 m Source–receiver offset 100 m (nearest), 15,000 m (farthest) Spread length 15 km Theoretical fold 37 Type of spread End-on Record length 24 s Sampling interval 4 ms Type of magnetic tape IBM 3490 cartridge Type of data SEG-D, Demultiplexed format Frequency range 3.5–250 Hz Uphole recording Yes Instrument Eagle-88 RF Telemetry system Geophones 4.5 Hz, 10 phone string, Bunching Length of the profile 240 km Type of source Explosives Shot hole depth 25–28 m Charge size/hole 50–75 kg No. of shots 900 No. of channels 150 Shot point spacing 200 m Receiver spacing 100 m Source–receiver offset 100 m (nearest), 15,000 m (farthest) Spread length 15 km Theoretical fold 37 Type of spread End-on Record length 24 s Sampling interval 4 ms Type of magnetic tape IBM 3490 cartridge Type of data SEG-D, Demultiplexed format Frequency range 3.5–250 Hz Uphole recording Yes Instrument Eagle-88 RF Telemetry system Geophones 4.5 Hz, 10 phone string, Bunching View Large Table 2. Data acquisition parameters. Length of the profile 240 km Type of source Explosives Shot hole depth 25–28 m Charge size/hole 50–75 kg No. of shots 900 No. of channels 150 Shot point spacing 200 m Receiver spacing 100 m Source–receiver offset 100 m (nearest), 15,000 m (farthest) Spread length 15 km Theoretical fold 37 Type of spread End-on Record length 24 s Sampling interval 4 ms Type of magnetic tape IBM 3490 cartridge Type of data SEG-D, Demultiplexed format Frequency range 3.5–250 Hz Uphole recording Yes Instrument Eagle-88 RF Telemetry system Geophones 4.5 Hz, 10 phone string, Bunching Length of the profile 240 km Type of source Explosives Shot hole depth 25–28 m Charge size/hole 50–75 kg No. of shots 900 No. of channels 150 Shot point spacing 200 m Receiver spacing 100 m Source–receiver offset 100 m (nearest), 15,000 m (farthest) Spread length 15 km Theoretical fold 37 Type of spread End-on Record length 24 s Sampling interval 4 ms Type of magnetic tape IBM 3490 cartridge Type of data SEG-D, Demultiplexed format Frequency range 3.5–250 Hz Uphole recording Yes Instrument Eagle-88 RF Telemetry system Geophones 4.5 Hz, 10 phone string, Bunching View Large 4 COMMON REFLECTION SURFACE (CRS) METHOD-SEISMIC REFLECTION DATA In the present study CRS stack approach is used to process the multifold seismic reflection data. Initially, a brief description of the methodology is provided and subsequently the processing strategy. 4.1 CRS methodology The conventional Common-Midpoint (CMP) method requires a velocity model that is used to apply for Normal Move Out (NMO) correction to remove offset effect before stacking the data. The stacking velocity plays a major role in stacking the multichannel seismic data in the CMP method. However, deriving an accurate velocity model becomes very difficult due to inhomogeneity of the subsurface and lateral velocity variations due to complex structures of the Earth, especially in the Archean terrains. Also, small source-receiver offset compared with the target depth in deep crustal seismic reflection data puts another limitation in velocity analysis (Yoon et al. 2009; Mandal et al. 2013). Additionally, the NMO equation is valid only for homogeneous and layered Earth and may not be appropriate for multilayered inhomogeneous media with curved interfaces (Mann 2002). These limitations may lead to poor stack sections of deep seismic reflection data from shield regions that exhibit low signal-to-noise (S/N) ratio, weak reflection coefficients, faulted and folded structures due to long geological history. Improving the quality of CMP stack section is in focus of intense research. In this context, a new multifocusing approach to reflection data is found an alternative to CMP method. In this method each zero-offset trace is constructed by stacking traces which need not belong to same CMP gather, but whose sources and receivers are in certain vicinity of the central point (Mann et al.1999; Jäger et al.2001; Zhang et al.2001; Menyoli et al.2004). The subsurface area covering them is called as CRS (Fig. 3). Since the traces being stacked no longer belong to the same CMP gather, such a procedure requires a more general moveout correction than used in conventional CMP stacking. The CRS/multifocusing approach consists of stacking seismic data with an arbitrary source-receiver distribution, according to a new paraxial moveout correction that is based on a local spherical approximation of the reflection wave fronts in the vicinity of an observation surface (Schleicher et al.1993). The moveout correction parameters are the emergence angle (β) and radii of curvature (RN and RNIP) of two fundamental wave fronts corresponding to the normal wave and normal-incident-point wave (Hubral 1983; Höcht et al.1999; Jäger et al.2001; Menyoli et al.2004). Figure 3. View largeDownload slide CRS stacking surface. The CRS stack sums all the data along the green surface for all finite-offset and purple for zero-offset reflections related to red arc around Common Reflection Point (CRP) and assign the stacked result to the CRP (after Vieth 2001). Figure 3. View largeDownload slide CRS stacking surface. The CRS stack sums all the data along the green surface for all finite-offset and purple for zero-offset reflections related to red arc around Common Reflection Point (CRP) and assign the stacked result to the CRP (after Vieth 2001). In this approach a super-gather, usually referred as the CRS gather is prepared which consists of traces from a number of CMP gathers and stacking them increases the fold by an order of magnitude. This is beneficial for data with low fold and low S/N ratio. In the CRS approach for each t0 on each zero-offset trace, three stacking parameters (β, RN and RNIP) are required instead of a single parameter (stacking velocity) in conventional NMO stack. The stacking parameters (CRS attributes) that best fit to an actual reflection event in the seismic data are determined for each sample (t0, x0). This is achieved by correlation measures using coherency analysis by measuring semblance along the stacking operator (Taner & Koehler 1969; Neidell & Taner 1971). The cost of calculating the correlation measure for all possible combinations of three stacking parameters over a CRS-gather is prohibitively high. Further, displaying and picking maxima of the correlation measure as a function of four variables (t0, β, RN and RNIP) may be a difficult task. Thus, automatic stacking procedure has come into existence in the CRS approach (Mann et al.1999; Jager et al.2001). Maximization of semblance in coherency analysis is achieved by a nonlinear global optimization method. Superiority of the CRS stack over the conventional CMP stack in deep seismic reflection data analysis has been reported by several researchers (e.g. Menyoli et al.2004; Luis et al.2006; Yoon et al.2009; Mandal et al. 2013, 2014). 4.2 CRS data processing In general, the data exhibits moderate S/N ratio and broad frequency bandwidth (4–80 Hz) required for the deep crustal study. The data are contaminated with low-frequency, high amplitude ground roll. Deep seismic reflection data are processed using the Promax software and CRS stack approach (Mann et al.1999; Jager et al.2001; Zhang et al.2001) The processing sequences are shown in Table 3. The CRS processing can be divided into three steps. Step-I involves raw field data of a CMP gather, which is same as CMP processing. Step-II replaces velocity analysis, deriving macrovelocity model and NMO and DMO of CMP processing with generation of CRS gathers and determination of three CRS attributes. Finally, stacking of traces is done in respective domains. Step-III, the post-stack processing steps are same in both CMP and CRS domains. Table 3. CRS data processing sequence. View Large Table 3. CRS data processing sequence. View Large Deep seismic reflection data were processed with straight line geometry using the demultiplexed field data. Initially, source–receiver recording geometry was incorporated on the field seismic data followed by editing of bad traces and correction of polarity reversals. Static corrections for elevation differences were performed with a datum plane located at 500 m above mean sea level with a replacement velocity of 5000 m s−1. Refracted first arrivals were muted. Spherical divergence correction is applied using an inverse gain function estimated from the raw field data. A 50 Hz notch and 4–8 to 30–40 Hz bandpasss filter were applied. The parameters were chosen after frequency analysis of field data (Fig. 4). A spiking deconvolution with an operator length of 200 ms and a prediction gap of 24 ms is applied to increase the lateral resolution. Automatic Gain Control (AGC) with a time window of 2000 ms was applied to increase the amplitude of weak signal, especially at deeper levels and to balance amplitudes laterally and vertically. Trace mixing of immediate adjacent trace from either side to the central trace is performed to increase the S/N ratio. Data were sorted to CMP domain and CMP gather is prepared (Table 3). Quality check is performed over the processed data and shown in Fig. 5. Figure 4. View largeDownload slide Raw field record along with the frequency spectrum of the seismic signal. Figure 4. View largeDownload slide Raw field record along with the frequency spectrum of the seismic signal. Figure 5. View largeDownload slide Representative raw shot gathers, SP485 and SP1043 (a,c), with end-on configuration are shown up to 5 s twt. The shot gathers are dominated by source generated and random noise. Same shot gathers after processing with first arrival mute, bandpass filtering, spherical divergences correction, F-K filtering, predictive deconvolution and AGC (b,d). Note: source generated (ground roll) and random noise is reduced and signal-to-noise ratio is increased after processing. Reflection events (R1, R2, R3 and R4) brought out by processing are shown with red arrows. Sw, Shear wave; Gr, Ground roll; SP, Shot point. Figure 5. View largeDownload slide Representative raw shot gathers, SP485 and SP1043 (a,c), with end-on configuration are shown up to 5 s twt. The shot gathers are dominated by source generated and random noise. Same shot gathers after processing with first arrival mute, bandpass filtering, spherical divergences correction, F-K filtering, predictive deconvolution and AGC (b,d). Note: source generated (ground roll) and random noise is reduced and signal-to-noise ratio is increased after processing. Reflection events (R1, R2, R3 and R4) brought out by processing are shown with red arrows. Sw, Shear wave; Gr, Ground roll; SP, Shot point. Subsequently, three CRS stacking parameters are determined one after another, that is, each time one parametric search is made by means of coherency analysis and measuring semblance (Taner & Koehler 1969; Neidell & Taner 1971). In the first search coherency analysis is performed to derive a best coherent section and an automatic CMP stack is derived as a preliminary stack section in the CRS search to estimate all three attributes (Table 3). This automatic stack section is similar to the CMP stack section in the conventional processing chain, where optimum stacking velocity (VNMO) is determined from the CMP gather in this search using the CRS attributes. The VNMO is automatically determined based on the best coherency fitting to the seismic data. Next, in the second search, the automatic CMP stack is utilised to determine emergence angle (β) by assuming RN = ∞, as a first-order approximation, which implies plane normal waves emerging at the surface. Finally, a third search was made using the known value of β, with hyperbolic second-order representation of stacking operator and RN is determined. Knowing the values of β and VNMO, RNIP is calculated (for more details see Mann et al.1999; Jäger et al. 2001).These initial CRS attributes are applied in the form of a 3-D search and optimization procedure to the original dataset making use of CRS gathers. 3-D optimization makes use of the spatial CRS stacking surface in (xm–h–t) space (Fig. 3) using flexible polyhedron search using the CRS attributes (Nelder & Mead 1965; Jäger et al. 2001). In order to minimize computational time, a range of stacking velocities between 5000 and 8500 m s−1 with an average velocity of 6600 m s−1 are provided as guide lines to perform the CRS stack. Finally, the CRS stack section is obtained by summing multifold data using the CRS gather along the 2-D stacking operator using the optimized CRS attributes and presented in Fig. 6. Whenever the coherency value exceeds a threshold, the three-parameters are optimised again to maintain the accuracy. F–X deconvolution and coherency filtering is applied to the stack. Finally, depth migration is carried out. Figure 6. View largeDownload slide CRS stack section of the Perur-Chikmagalur profile. CT, Chitradurga Thrust; BGB, Bababudan Greenstone Belt; CGB, Chitradurga Greenstone Belt; CG, Closepet Granite; YG, Younger Gneisses (2.7–2.9 Ga); OG, Older Gneisses (2.9–3.5 Ga); WDC, Western Dharwar Craton; EDC, Eastern Dharwar Craton. Figure 6. View largeDownload slide CRS stack section of the Perur-Chikmagalur profile. CT, Chitradurga Thrust; BGB, Bababudan Greenstone Belt; CGB, Chitradurga Greenstone Belt; CG, Closepet Granite; YG, Younger Gneisses (2.7–2.9 Ga); OG, Older Gneisses (2.9–3.5 Ga); WDC, Western Dharwar Craton; EDC, Eastern Dharwar Craton. Knowledge of near-surface velocity (V0) is essential in the CRS search that is measured from the refracted first arrivals. The V0 acts like a scale factor connecting the travel time derivatives along the reflection events with their geometrical interpretation in terms of emergence angles and wave front curvatures, that is, in terms of the CRS attributes. Near-surface velocity (approximately the RMS velocity) of 5000 m s−1 for the most shallow stable reflection event is chosen here. During automatic CRS stacking, two important stacking apertures, namely, the half-offset (h) in the time-offset (t–h) plane (CMP aperture) and zero-offset in the time-midpoint (t–xm) plane (ZO aperture), are to be specified with care (Fig. 3). Increasing aperture length increases S/N ratio. But, large aperture causes smearing effect beyond certain value and small aperture causes low coherency. A zero-aperture is the conventional CMP stack. Proper adjustment of these parameters is necessary for high spatial resolution of the CRS stack section and its image quality. The aperture size depends on the data quality, that is, on the S/N ratio of the seismic data. Larger apertures are generally used in poor S/N condition (Hertweck et al. 2007). In the present study, the CRS stack is performed with three sets of different ZO apertures. To fix initial value of the ZO aperture, width of the first projected Fresnel zone at the targeted depth was chosen. Finally, minimum and maximum values of ZO aperture were fixed at 100 m (0.2 s twt) and 2000 m (5.0 s twt) and for CMP aperture at 5 m (0.2 s twt) and 15 000 m (5.0 s twt) to obtain the best coherency section. Similar values are being used for several data sets from different geological provinces (e.g. Mann 2002; Yoon et al. 2009). Generally, it is assumed that seismic data processing provides the structure beneath the profile and energy comes entirely from within the plane of the section. However, the out-of-plane topography of a reflector (3-D structure) can be imaged as an inplane reflector at an appropriate depth depending on the off-line distance (Hobbs et al.2006). During the stacking, the out-of-plane energy can be suppressed up to some extent if it has a lower-velocity compared with in-plane reflectors at same depths. Invariably, the Earth is 3-D in nature and the effects of out-of-plane energy are not considered when data are processed and interpreted in 2-D seismic imaging techniques as in the present case and elsewhere also (e.g. Australia, Canada) as they are normally less problematic at later reflection times (Cook 2002; Kennett & Saygin 2015). To the best of our knowledge there is no proper mechanism to understand fully, the effects of the 3-D structure in 2-D section (Hobbs et al.2006). 3-D effects of the Earth can be resolved by acquiring 3-D data or recording a number of parallel or cross profiles. This is prohibitively costly for deep crustal studies. 4.3 Post-stack depth migration Migration moves the dipping reflectors into their true subsurface locations and collapses diffractions, thereby delineating detailed subsurface structure more accurately. So, migrated sections are preferred over stack sections and very essential for proper interpretation. But, migration of deep seismic reflection data may produce artefacts, which appear as circular arcs or smiles due to discontinuous nature of reflectors, low S/N ratio, truncation of reflections on stack sections due to poor signal penetration, changes in orientation of profiles and existence of strong lateral velocity variations (Warner 1987). As lateral velocity variations are more significant in the present study region because of its long geological history, time migration may not produce true subsurface picture and we preferred depth migration. Post-stack Kirchhoff depth migration is performed over the CRS stack section using a velocity model derived from coincident refraction/wide-angle reflection data (Vijaya Rao et al.2015b) and shown in Fig. 7(a). It looks similar to a geological cross section. We used migrated section for interpretation as it brought out the horizontal and true geometrical nature of the Moho and other structures more clearly compared with stack section. Small portions at both ends of the migrated section are not used in interpretation because of edge-effects aroused during migration process. Figure 7. View largeDownload slide (a) Post-stack CRS depth-migrated seismic section of the Perur-Chikmagalur profile without any interpretation. (b) Post-stack CRS depth-migrated seismic section of the Perur-Chikmagalur profile with interpretation. A bright subhorizontal reflection band at 40 km depth represents as the Moho in WDC. Note the differences in Moho signature in WDC and EDC. Abbreviations are same as in Fig. 3. SW-dipping reflection band is marked as ‘A’ and NE-dipping reflection bands as ‘B and C’. (c) Line drawing derived from prominent reflections from the CRS seismic depth-migrated section, along with the velocity model derived from coincident refraction/wide-angle reflection data (1-D velocity model is after Vijaya Rao et al. 2015b). The lower-crustal subhorizontal reflection band (orange colour) coincides with the high-velocity (7.1 km s−1) lower-crustal layer. Steeply dipping faults (F1–F3) are inferred based on the differences in reflectivity on either side. They coincide with the tectonic boundaries of the region. Abbreviations are same as in Fig. 3. Yellow colour represents the geometry of the Closepet granite and red colour corresponds to major thrust fault of the region. More details are provided in the text. Figure 7. View largeDownload slide (a) Post-stack CRS depth-migrated seismic section of the Perur-Chikmagalur profile without any interpretation. (b) Post-stack CRS depth-migrated seismic section of the Perur-Chikmagalur profile with interpretation. A bright subhorizontal reflection band at 40 km depth represents as the Moho in WDC. Note the differences in Moho signature in WDC and EDC. Abbreviations are same as in Fig. 3. SW-dipping reflection band is marked as ‘A’ and NE-dipping reflection bands as ‘B and C’. (c) Line drawing derived from prominent reflections from the CRS seismic depth-migrated section, along with the velocity model derived from coincident refraction/wide-angle reflection data (1-D velocity model is after Vijaya Rao et al. 2015b). The lower-crustal subhorizontal reflection band (orange colour) coincides with the high-velocity (7.1 km s−1) lower-crustal layer. Steeply dipping faults (F1–F3) are inferred based on the differences in reflectivity on either side. They coincide with the tectonic boundaries of the region. Abbreviations are same as in Fig. 3. Yellow colour represents the geometry of the Closepet granite and red colour corresponds to major thrust fault of the region. More details are provided in the text. 5 RESULTS OF THE DHARSEIS SEISMIC REFLECTION IMAGE The seismic image is presented in the form of CRS stack section in Fig. 6 and depth migrated section in Figs 7(a) and (b), without and with interpretation. A line drawing derived from prominent reflections of the migrated section is shown in Fig. 7(c). A detailed image (depth migrated) of the eastern part of the profile is shown in Fig. 8 and prominent structural features from different segments of the profile in Figs 9(a)–(d). Figs 8 and 9 are the detailed segments of depth migrated section (Fig. 7). Figure 8. View largeDownload slide Enlarged version of the eastern part of the CRS depth migrated section. The prominent SW-dipping reflection fabric (A-reflection band, Fig. 7b) in the Eastern Dharwar Craton extends from 34 to 43 km depth between SP1 and SP500. Abbreviations are same as in Fig. 3. Figure 8. View largeDownload slide Enlarged version of the eastern part of the CRS depth migrated section. The prominent SW-dipping reflection fabric (A-reflection band, Fig. 7b) in the Eastern Dharwar Craton extends from 34 to 43 km depth between SP1 and SP500. Abbreviations are same as in Fig. 3. Figure 9. View largeDownload slide (a) Enlarged version of the seismic section from 28 to 40 km depth of the WDC indicating the subhorizontal Moho n fabric. (b) Enlarged version of the NE-dipping reflection band from 6 to 28 km depth from SP900 to SP500 indicating the CEBSZ, the boundary between the WDC and EDC. (c) Enlarged version of the seismic section from 6 to 28 km depth from SP2200 to SP1950 indicating NE-dipping reflection band in WDC. (d) Enlarged version of the seismic section showing gently SW-dipping reflection bands in EDC. WDC, Western Dharwar Craton; EDC, Eastern Dharwar Craton; BGB, Bababudan Greenstone belt (WDC). Figure 9. View largeDownload slide (a) Enlarged version of the seismic section from 28 to 40 km depth of the WDC indicating the subhorizontal Moho n fabric. (b) Enlarged version of the NE-dipping reflection band from 6 to 28 km depth from SP900 to SP500 indicating the CEBSZ, the boundary between the WDC and EDC. (c) Enlarged version of the seismic section from 6 to 28 km depth from SP2200 to SP1950 indicating NE-dipping reflection band in WDC. (d) Enlarged version of the seismic section showing gently SW-dipping reflection bands in EDC. WDC, Western Dharwar Craton; EDC, Eastern Dharwar Craton; BGB, Bababudan Greenstone belt (WDC). Deep seismic reflection images delineated in the present study exhibit complex and variable patterns of crustal reflectivity with prominent seismic reflections at many crustal levels. In general, the crust is more reflective in the eastern part of the profile compared with the western part. Our interpretation is based on the CRS post-stack depth migrated section and the line drawing derived from it (Fig. 7). Significant structures of the seismic sections are as follows: A change in reflectivity pattern is observed at 6 km depth throughout the seismic section—a less reflective/transparent zone is observed from the surface to a depth of 6 km followed by a reflective zone further deep. Distinct reflectivity patterns mainly consisting of dipping events are observed throughout the study area from 6 to 40 km depth in the seismic image (Figs 6–9). Strong NE-dipping reflection band (C) are observed from 6 to 28 km depth in the western part of the profile between Shot Point (SP) 2100 and SP1085 (Fig. 9c). These events almost terminate at the CEBSZ/Chitradurga Thrust. A strong steep NE-dipping reflection band (B) is observed from 4 to 30 km depth between SP 950 and SP 500 between the CEBSZ and the Closepet granite (Fig. 9b). An SW-dipping reflection band (A) is observed between SP300 and SP10 from 20 to 43 km depth down to the base of the reflective zone in the eastern part of the seismic section (Figs 8 and 9). Prominent among the SW-dipping reflection bands is the bright reflection band extending from 34 to 43 km from SP 1 to SP 500 (Fig. 7). The SW and NE dipping reflection bands A and B mark a change of reflection character in the crust. These reflection bands constitute a divergent reflection fabric and are observed nearer to the Closepet granite. Bright subhorizontal to gently updoming reflection bands are observed from 4 to 25 km depth between SP 300 and SP 500 in the eastern part of the seismic section beneath the Closepet granite (Figs 7b and c). A thick subhorizontal high-amplitude lower crustal reflection band is observed from 28 to 40 km depth with a sharp decrease in reflectivity further deep between SP 2300 and SP 1450 in the WDC (Fig. 9a). A deepest lower crustal NE-dipping reflection band terminated by a transparency further deep is observed from 34 to 42 km depth between SP 1100 to SP 500. Contrarily, an SW-dipping reflection band is observed from 34 to 43 km between SP1 and SP500 (Figs 7 and 8). The reflectivity beneath the Chitradurga greenstone belt (SP1450- SP1100) is poor in spite of higher fold compared with other regions of the profile (Figs 1 and 7). Poor reflectivity is also observed in the lower-crust of Bababudan greenstone belt (SP2406- SP2000). 6 INTERPRETATION Based on the reflectivity patterns the crust is divided into three parts. The upper non-reflective part extending upto a depth of ∼6 km and represents the thickness of greenstone belts in the region. Global comparison of deep crustal seismic reflection data suggests a transparent upper crust for many Precambrian terrains, except for sedimentary basins (Mooney & Brocher 1987). It is generally observed that reflection profiling in shield regions are unable to image the brittle upper crust properly, which may be due to small-scale heterogeneities associated with complex structures, folds and steep faults and partly to the signal contamination from source-generated noise (Milkereit et al.1994; Reddy & Vijaya Rao 2013). It can also depend on the source-receiver geometry, receiver spacing and the frequencies used in the study. It is observed that shallow structures of the tectonically undisturbed sedimentary basins are well resolved, but not the shallow image of the disturbed areas with same acquisition parameters. The shallow structure (0–6 km depth) derived from the coincident refraction/wide-angle reflection data using tomographic approach (Vijay Rao et al.2015a) can be used to understand the upper crustal features that could not be imaged properly here. The upper crust is separated from the middle one by gentle-moderately dipping (in WDC) and complex reflectivity patterns (in EDC) from 6 to 28 km depth. The lower crust is demarcated from the middle portion with bright subhorizontal reflection fabric from 28 to 40 km depth in WDC (Fig. 7). The reflectivity beneath the Chitradurga greenstone belt is poor in spite of higher fold compared with other regions of the profile (Figs 3 and 5). Poor reflectivity is also observed in the lower-crust of Bababudan greenstone belt. There are various reasons for absence of reflectivity, important among them are poor energy transmission, scattering due to heterogeneities of the crust and poor acoustic impedance contrast between rocks at various depths. 2800 Ma long geological history of the region with different tectonic domains, granitic intrusions and presence of steep-angle shear zones might have disrupted the continuity of reflectors at some places. Important reflectivity patterns in the middle crust are the NE-dipping reflection bands (B and C) and the SW-dipping reflection band (A). Prominent among the SW-dipping reflection bands is the bright reflection band extending from 34 to 43 km from SP1 to SP500 (Fig. 4). This deepest SW-dipping reflection band terminated by transparency further deep is interpreted as the Moho. The dipping reflection bands (B and C) observed throughout the crust is terminated at the Moho, thereby indicating the Moho as a possible detachment boundary. Based on the nature of seismic images derived in the present study and the geological data, we interpret the deepest SW-dipping reflection band (A) representing the Moho as the relict low-angle Neoarchean subduction zone (Fig. 7). High thermal regime of the Archean terrain might be responsible for different rheological properties between the crust and mantle and decoupled them, thereby allowing only the mantle to subduct. Geochemical studies on the volcanic of several Neoarchean schist belts of the EDC have also revealed the presence of subduction-related high-Mg andesites, boninites, Nb-enriched basalts and adakites (Naqvi et al.2006; Manikyamba & Kerrich 2012). Melting of subduction slab resulted in the formation and widespread occurrence of TTG gneisses in the Archean Dharwar craton. Lower-crustal/mantle subduction is found to be a common phenomenon during the Archean as observed from several Lithoprobe deep crustal seismic reflection profiling data from the Canadian Shield (Cook 2002; van der Veldon & Cook 2005). These studies suggest that the direction of subduction zone reflections were commonly found to dip toward the craton as observed in the present study. Mantle reflections represent relict subduction zones, which in turn represent accretionary boundaries as observed from different parts of the world (McBride & Brown 1986; Warner et al.1996; Cook 2002; van der Veldon & Cook 2005). Continuous subduction has resulted in a collision between the western and eastern blocks of the Dharwar craton and developed thrust-fold structures. The NE and SW dipping reflection bands extending from SP1100 to SP500 and SP1 to SP500 respectively constitute a divergent reflection fabric that extends up to the Moho and becomes listric. It represents a zone of convergence between two crustal blocks, namely the EDC and WDC during the Neoarchean orogeny. Two oppositely dipping (SW and NE) deformation patterns formed during convergence represent a collision signature formed above the location of subduction zone (Fig. 7) and are consistent with geodynamical modelling studies (Beaumont & Quinlan 1994). The two dipping reflection bands correspond two distinct crustal blocks of independent evolutionary history, namely the WDC and EDC that are involved in collision in a transpressional tectonic regime. We infer that the Closepet granitic marks the location of collision zone and the crust thickens here to a depth of ∼43 km compared to 40 km and 34 km on either side. Crustal thickness of 42 km and 38 km for the WDC and EDC respectively was identified based on the coincident refraction/wide-angle reflection study (Vijaya Rao et al.2015b). The resolution limit of this study in the deeper part of the crust is ±3 km vertically. The difference in crustal thicknesses derived from near-vertical and wide-angle reflection studies could be either within the error limits of both methods or due to the presence of anisotropic crust in the region. High-grade metamorphic rocks like schists, shear zones and collisional boundaries present in the region could be exhibiting anisotropy. Anisotropy is identified from deep crustal reflection and refraction/wide-angle reflection studies from different parts of the world (Carbonell & Smithson 1991; Jones et al.1996). Passive seismic studies from Dharwar craton have identified anisotropy in the region (Saikia et al.2010). Convergence of crustal blocks generated tectonically imbricated crust, now represented by dipping reflection fabric and such a crustal feature is also observed in several Precambrian orogenic areas of the world (e.g. BABEL Working Group 1990; Calvert et al.1995,; Cook et al.1999; Golbey et al.2004). We interpret that the relict Neoarchean subduction zone identified in the present study represents accretionary boundary and marks the mantle suture, as the Moho of EDC is underthrusted in to the upper mantle. Chadwick et al. (2000, 2007), using the geological data have suggested a Neoarchean accretion of the EDC to the WDC similar to the Phanerozoic-type subduction environment, where an oceanic plate subducted below a Mesoarchean continental margin represented by the WDC. Identification of the location of the subduction zone places an important constraint on the interpretation of an orogen. Among the NE-dipping reflections, the ‘B’ reflection band dipping from 4 to 30 km depth from SP 950, nearer to the eastern boundary of Chitradurga greenstone belt (Figs 1a and 7) is an important thrust fault of the region, referred here as CEBSZ. It is a retro-shear/back-thrust interpreted to have formed during westward thrusting of EDC over the WDC during the Neoarchean orogeny. It is located between the top of the lower crust (30 km depth) to a depth of 4 km. Its extension to the surface could not be imaged properly. But, if extended it may reach the surface at SP1085, the boundary between WDC and EDC. The thrust fault acted as a conduit and carried the slab melts upward to the surface, which formed as younger gneisses (Fig. 7). We interpret that the CEBSZ may represent the surface expression of the suture and located to the west of mantle suture/subduction zone. After the crust-mantle detachment the upper mantle of EDC subducts beneath the WDC and its crust is thrusted over the WDC crust. We interpret that many gently eastward dipping reflection bands observed in major parts of WDC may be a result of thrusting of the EDC crust over the WDC. The structural details imaged in the present study are similar to those derived from the geodynamic model studies (Quinlan et al. 1993; Beaumont & Quinlan 1994) as shown in Fig. 10, where the mantle is underthrusted and its crust moves upward into the deforming orogen. Similar structures are observed in most of the orogens, including Archean, Proterozoic and Phanerozoic period (BABEL Working Group 1990; Quinlan et al.1993; Calvert et al.1995; Hall et al.1995; Cook et al.1999; White et al.2003; van der Veldon & Cook 2005) suggesting that detachment and underthrusting may be a common feature of compressional orogens. In general, the Archean cratons were assembled above shallowly dipping detachments. Earlier seismic refraction and magnetotelluric studies (Kaila et al.1979; Gokaran et al.2004) suggested the presence of an easterly dipping thrust fault in this part of the region. However, those studies didn’t provide much details regarding mechanism of its formation and its relationships with other structures. Figure 10. View largeDownload slide Schematic diagram showing the Moho as a detachment boundary and the oppositely dipping reflection fabric as the signature of a collision, derived from geodynamic modelling studies (after Beaumont & Quinlan 1994). Figure 10. View largeDownload slide Schematic diagram showing the Moho as a detachment boundary and the oppositely dipping reflection fabric as the signature of a collision, derived from geodynamic modelling studies (after Beaumont & Quinlan 1994). The characteristic features of the Moho, crustal structure and its thickness are different for the EDC and WDC. In general the WDC consists of simple structure with gently dipping reflection fabric from 6 to 28 km depth with a subhorizontal reflection fabric from 28 to 40 km depth (Fig. 7). On the other hand, the EDC display complex reflectivity patterns consisting of dipping Moho from 34 to 43 km depth, collision signature, a thrust fault and a crustal root along with an updoming-to-horizontal structure from 4 to 25 km depth at the Closepet granite (SP600-SP800). Seismic reflection images of the present study identified such differences for the first time in the region. It is in contrasts with the simple structure suggested for EDC by earlier studies. The differences in crustal structure, age, nature of schist belts, other geological and geochemical features of the EDC and WDC provide support to the accretion of two independent crustal blocks. During the E–W/NE–SW convergence, the structures of greenstone belts, granites and gneiss were aligned in the N–S/NW–SE direction extending to a maximum distance of ∼450 km, namely, the Closepet granite and Chitradurga greenstone belt (Figs 1a and b). These linear belts suggest the operation of plate tectonics during the Neoarchean in the region. The dipping reflection fabric observed throughout the crust correlate well with the craton-wide imbricate fold-thrust belt mapped with the surface geological data (Chadwick et al.2007). It indicates that much of high-amplitude reflectivity is associated with the shear zones mapped at the surface. The fold-thrust structure observed in the Dharwar craton is attributed to the oblique convergence of two crustal blocks (Chadwick et al. 2000). Prior to the availability of deep seismic images in the region, it is suggested that the thrust-ramp structure become listric at a subhorizontal detachment zone located at a depth of 20 km. However, the deep crustal seismic images of the present study identified the detachment zone at the crust-mantle boundary located at a depth of 40 km. Just above the deep SW-dipping reflection band (A), bright subhorizontal to gently updoming reflection bands are observed from 8 to 25 km depth between SP300 and SP500 just beneath the 2.5 Ga Closepet granite (Figs 1a and 7). The 450 km long and 40 km wide Closepet granite is probably represented by these reflection bands at depth. Terrane accretion and subduction slab break-off resulted in hot mantle asthenospheric upwelling and possibly triggered melting of the mantle wedge that resulted in the formation of various calc-alkaline to K-rich granites. The Closepet granite being located just above the inferred subduction zone (Fig. 7) could have evolved due to slab melting at the subduction zone as suggested by the geochemical analysis (Moyen et al.2001). Geochemical investigations shows increase of εNd values along with decrease of LREE and LILE contents to the east of Closepet granite (Jayananda et al.2000) that is consistent with the direction of subduction to the west/southwest. 6.1 Moho configuration in the region Moho is identified with the brightest lower crustal reflection band terminated with a transparent upper mantle as normally observed elsewhere. Based on the geometry of reflections at the Moho boundary, more than one type of reflection Moho is identified along the profile. Type-I, subhorizontal reflection fabric, type-II, no distinct Moho reflections or absence of clear Moho reflections and type-III, reflections that project from the crust into the mantle. Similar Moho patterns were observed along various LITHOPROBE transects across the Canadian Shield and also in various other parts of the world (BABEL Working Group 1990; Cook 2002; Carbonell et al.2013; Kennett & Saygin 2015). 6.1.1 Type-I The Moho is identified with the termination of deepest bright subhorizontal reflection band, extending from 28 to 40 km depth with a transparent upper mantle in major part of the WDC (Figs 7 and 9a). The subhorizontal reflection band might have developed due to post-collisional extentional process by reworking of lower-crust by mafic underplating (12 km thick) or melting resulting in mafic restites. Thus, the Moho is a younger feature that may not be related to rocks at the surface. It explains the high-velocity (7.0 km s−1, Fig. 7c) observed in the lower-crust and may represent mafic granulites (Vijaya Rao et al. 2015b). During this modification some of the deeper reflections might have obliterated. The evidence for magmatic underplating is derived from the presence of craton-wide mafic dyke swarms (Fig. 1b) and other later tectonic activities of the region (Kumar et al.2012; Vijaya Rao et al.2015b). The Moho boundary (at 40 km depth) may be acting as a shear zone created in response to the shortening and imbrications of the crust above a more rigid upper mantle. It acts as a prominent rheological boundary that may localize zone of detachment during subsequent deformation. The receiver function studies from Australia suggest similar crustal thickness with high-velocity lower-crust in the Neoarchean cratons (Yuan 2015). 6.1.2 Type-II No Moho reflections are observed beneath the Chitradurga greenstone belt. It could be due to various reasons, like insufficient contrast in velocity and density at the crust-mantle boundary may be due to the presence of mafic granulites or garnet-bearing granulites in the lower crust, which have velocity and density similar to mantle values; gradual change in velocity and density, which may not produce reflections; the boundary is too complex to produce coherent reflection energy; the complex nature of the crust, like low-velocity, high attenuation intrinsic structural complexity, etc., of the near-surface region, which forbids clear imaging of the crust below; multiple scattering in the crust may also responsible significantly to the reflection character and absence of reflections (Holliger et al.1994). It is not possible to establish which of these processes is most likely responsible for the observed Moho characteristics in this part. 6.1.3 Type-III A bright SW-dipping reflection band projects from the crust at 34 km depth into the transparent upper mantle to a depth of 43 km from SP1 to SP500 in the EDC. It is interpreted as the Moho. The Moho from SP500 to SP1000 is represented by a reflection band extending from 42 to 34 km depth. The entire Moho band in the EDC from SP1 to SP1000 shows thinning of the crust at both ends with a thickest crust at the subduction-accretion boundary at SP500, in the middle (Fig. 7). Similar dipping reflections from the crust to the mantle are observed to a depth of 100 km beneath the Slave craton, 60 km in the eastern part of Superior province, 46 km in the western Superior province and to a depth ∼70 km in the Baltic shield (Cook 2002; White et al.2003; BABEL Working Group 1990). New continental crust is accreted at the convergent margins by complex geological processes. Similarly, the reflectivity pattern at the Moho, the crust-mantle boundary, in these regions is also complex as observed in the EDC. Temperature near a subduction zone is generally cold and increases away from it. Therefore, the crust and mantle structures adjacent to a subduction zone likely to be frozen as observed in EDC and other regions of the world (BABEL Working Group 1990; Cook et al.2002; Kennett & Saygin 2015). They remain intact as long as no thermal activity over prints them. From the above discussion we conclude that the crustal structure and character of the Moho in EDC are related to the ages of the rocks at the near surface. In the present study, the depth of the Moho and its characteristics are changing across the tectonic boundaries. Whereas, the reflection Moho is at a constant depth even beneath the regions with different ages (Archean-Phanerozoic) and tectonic histories along major part of the Canadian LITHOPROBE transects (Cook 2002). It shows that the Moho is a reworked younger feature in those regions. The diverse character of the reflection Moho observed in the present study region suggests that it does not arise from a single process, but rather reflects the action of a geological history of the region. Juxtaposition of horizontal Moho in the WDC with its absence in the adjacent Chitradurga greenstone belt indicates the processes that alter the physical properties of the Moho and may not necessarily change the structural geometry of the surrounding rocks. 7 DISCUSSION AND IMPLICATIONS High temperatures of the Archean period promoted hot orogenic model for the growth of Archean terrains (Chardon et al.2011). The model suggests that during the Archean orogenic activity, low-viscosity, presence of partial melts, high-buoyancy of the juvenile crust and very low strength of mantle lithosphere did not allow the crust to sustain thickening. But, crustal shortening is accommodated by pervasive 3-D flow of the viscous lower crust that combines orogen-normal shortening, lateral constrictional stretching and transtension. Thus, the hot orogenic model predicts mechanical and thermal homogenization of the lower crust. We find differences in the lower-crustal structure between the WDC and EDC. Subhorizontal lower-crustal reflection fabric is observed in major part of WDC, except beneath the Chitradurga greenstone belt, whereas earlier dipping reflection fabric related to the orogenic activity is imaged in EDC with a preserved 43 km thick crust at the collision zone (Fig. 4). Such a structural variation suggests that the post-orogenic extension or high temperatures of Archean period modified the lower-crust of WDC, whereas it neither disturbed the crustal fabric formed during accretion nor homogenized the lower-crust of EDC. The melts of granitic magma formed during extension ascended to the surface through the fault pattern, which acted as conduits leaving behind the lower crust enriched with mafic material. The Closepet and other granites are evolved by such process. The ascending fluids must have made good acoustic impedance contrast with host rocks that resulted in bright seismic reflectivity in the EDC. Granitoid emplacement studies in the Cordellera emphasized the important role played by fault-system (Petford et al.2000). It is observed that post-collisional extension activities have preserved earlier crustal structures in many parts of the world (Calvert & Ludden 1999; van der Veldon et al.2004; Yuan 2015). The hot orogenic model can be applied to a part of Mesoarchean WDC that exhibits thermal homogenization of the lower-crust and may not appropriate to the Neoarchean EDC, where the crustal root (43 km thick crust, Fig. 7) and accretionary fabric with dipping reflections are still preserved at various crustal depths, including lower-crust. An interpretative line drawing, Fig. 7(c), superposed with the 1-D velocity model derived from the coincident refraction data (Vijaya Rao et al.2015b) shows coincidence of the reworked subhorizontal reflection fabric with the high-velocity (7.1 km s−1) lower-crustal layer and complements it. A small discrepancy in crustal thickness derived from refraction and reflection data could be due to differences in methodology adopted in data acquisition and processing. Steeply dipping/subvertical faults F1–F3 are inferred based on the differences in reflectivity on either side and marked over the line drawing and we interpreted them as strike-slip faults. They demarcate the different tectonic boundaries of the region and coincide with the geologically identified faults/shear zones, located parallel to the orogeny (Figs 1a and b and 2b). They must have formed either during the transpressional tectonic regime of the Neoarchean oblique collisional orogeny or during the post-collisional period. The Great Glen fault, the Makarovo fault, Norumbega fault, etc., are some of the strike-faults developed in a compressional regime and identified using seismic signature (McBride & Brown 1986; Stern & McBride 1998; Mandal et al.2013). Such steeply dipping faults are also identified from the deep seismic reflection studies of other Archean terrains of different regions, for example, the Canadian Shield (van der Veldon et al.2006). These faults were found to have been formed during post-orogenic period and played little role in thickening of the crust. Small and rapid movement of plates, high-temperatures, and longer ocean ridge lengths are regarded as the tectonic environment of Archean terrains that may be responsible for the low-angle subduction observed in the Dharwar craton. Deep seismic images have identified similar low-angle subduction zones to further east in EDC and also from the Baltic and Canadian shields (BABEL Working Group 1990; Calvert et al.1995; White et al.2003; Vijaya Rao et al.2006). The dipping reflection fabric and style of reflective lower-crust observed in the present study are similar to those found in the Phanerozoic and Proterozoic orogens with some minor changes, indicating operation of plate tectonics-like processes during the Neoarchean. Dominantly linear architecture of the Dharwar craton (Fig. 1a) supports accretionary orogenic interpretation. The present study suggests difference in Moho depth between east (34 km) and west (40 km) of the Chitradurga greenstone belt (Fig. 7), indicating a thicker crust for the WDC and thinner one for the EDC. The coincident refraction/wide-angle reflection study along with other geophysical and geological data identified this zone as a west-dipping subduction zone (Vijaya Rao et al.2015b). This mantle subduction zone derived from refraction data coincides with the dashed-line marked to join the Moho boundary in the reflection data and shown in Fig. 7(c). The geochronological data identified two distinct episodes of greenstone volcanism, mafic and felsic, respectively at 2.7–2.65 Ga and 2.58–2.55 Ga (Jayananda et al.2013). These two episodes of volcanism with the associated plutonism correspond to two crustal accretionary events and a two stage growth of the Dharwar craton. Seismic reflection, coincident refraction and other geological data suggests that that the region has undergone two-stage subduction process with westerly dip. The first stage of subduction-accretion (2.7–2.65 Ga) was at the eastern margin of Mesoarchean WDC and the second event (2.58–2.52 Ga) is identified at the eastern boundary of Closepet granite. The present study resolves the ambiguity associated with the EDC–WDC boundary, the location being CEBSZ or the Closepet granite (Kaila et al.1979; Naqvi & Rogers 1987; Griffin et al.2009) with a two-stage subduction-accretion process. The 2.62 Ga potassic plutons observed at the eastern margin of the WDC (Ramakrishnan & Vaidyanadhan 2008) are due to melting of the lower-crust during earlier event, whereas the TTG gneisses, calc-alkaline plutons and felsic volcanism throughout the EDC are due to melting of subducted slab and mantle wedge during the second accretionary event (Figs 1 and 3b). Two-stage subduction process is also observed in the Archean Superior Province of Canada (Calvert & Ludden 1999; White et al.2003). Further, the Archean Superior Province was assembled through five separate accretionary orogenic events over a period of ∼40 Ma instead of a single Neoarchean orogeny (Percival et al.2006) and crustal growth was the product of subduction-accretion tectonics (Wyman et al.2002). The evolutionary model of the region is schematically shown in Fig. 11. Figure 11. View largeDownload slide Schematic derived from seismic data constrained from geological and geochemical data shows the evolutionary model of the Dharwar craton. (a) The Mesoarchean continental crust (WDC) is located on the southwest and the oceanic crust to the northeast with initial subduction (2.7 Ga) at eastern margin of WDC. (b) Accretion of two crustal blocks and formation of CEBSZ. (c) Second stage of subduction (at 2.5 Ga) farther to the northeast. Subduction-1 slab break off. (d) Accretion of WDC and EDC with the formation of Closepet and other granites and represents present day configuration. Figure 11. View largeDownload slide Schematic derived from seismic data constrained from geological and geochemical data shows the evolutionary model of the Dharwar craton. (a) The Mesoarchean continental crust (WDC) is located on the southwest and the oceanic crust to the northeast with initial subduction (2.7 Ga) at eastern margin of WDC. (b) Accretion of two crustal blocks and formation of CEBSZ. (c) Second stage of subduction (at 2.5 Ga) farther to the northeast. Subduction-1 slab break off. (d) Accretion of WDC and EDC with the formation of Closepet and other granites and represents present day configuration. Conventionally, it is believed that the accretionary boundary between the Mesoarchean WDC and Neoarchean EDC is sharp and located at the CEBSZ. But, the recent geological and Nd isotopic studies identified the boundary as a ∼200 km wide diffused zone (Chadwick et al.2007) with the older Mesoarchean ages of the WDC extending further east of the CEBSZ up to a distance of 150 km (Dey 2013). The diffused zone also referred as the Central Dharwar Craton, is associated with juvenile Neoarchean plutonic rocks to its east compared with mixed juvenile and crustally derived granites to the west (Peucat et al.2013). Accidentally, the boundaries of the Central Dharwar craton coincide with two subduction zones identified in the present study and supports two-stage subduction process for the region. 7.1 Geodynamic evolution of the Dharwar craton The deep crustal structure derived in the present study along with the available geological and geochemical data suggests that an Andean-type environment was prevailing during the Neoarchean. 3.4 Ga continental crust represented by the WDC was located in the west and an oceanic crust to the east in the region. The denser oceanic plate located to the east (EDC) subducted under the continental plate located to the west (WDC), which is manifested by west dipping Moho (Fig. 7). The ocean was closed due to continuous subduction process and finally the EDC (accretionary complex) with juvenile crustal material made up of series of arcs, intra-arc basins and batholiths of 2.7 Ga age (Balakrishnan et al.1999) was accreted against the continental margin of the WDC and responsible for the formation of Neoarchean orogeny in the region. The westward thrusting of the EDC over the WDC during the orogeny resulted in the formation of a back-thrust/retro-shear at the boundary of WDC, referred as the CEBSZ. The orogeny is manifested in the form of widespread syn- to post-orogenic (2.56–2.50 Ga) granitic intrusion, Proterozoic dykes, sills, strike-faults and shear zones on the surface (Fig. 1b). At some places high-Mg andesites, boninites, Nb-enriched basalts, rhyolites and adakites are identified (Manikyamba & Kerrich 2012). During the E–W/ENE–WSW convergence, the structure of the greenstone belts, granites and gneisses were aligned in the N–S/NW–SE direction extending to a maximum distance of 450 km (e.g. Closepet granite, Fig. 1a). These linear belts suggest the operation of plate tectonics during the Neoarchean in the region. The processes were likely dominated by a two-stage subduction-accretion process. Initial subduction was at the eastern margin of WDC and the later one was at the eastern boundary of the Closepet granite (Central Dharwar Craton) (Fig. 11). It is suggested that granite-greenstone terrains represent middle to upper crustal remnants of Archean orogenic belts, which were once parts of much larger orogenic systems (de Wit & Ashwal 1997). Others believe that greenstone belts are backarc basins developed in an arc-subduction environment because of their association to volcanic composition, while others consider that the greenstone belts represent marginal basins associated with subduction zones. It is interpreted that the Granite-greenstone belts in the EDC are evolved by such a process. Increased geothermal gradient in the Archean, with more efficient horizontal heat transfer and high-temperature–low-pressure tectono-metamorphic episode, leads to pervasive melting and production of calc-alkaline to K-rich juvenile granitic magma, namely, the Closepet and a large number of other granites in the EDC that cover a large area (Fig. 1a). Terrane accretion, the subduction slab breaks-off, resulted in upwelling of hot mantle asthenosphere and probably triggered melting of the mantle wedge. As a result the Closepet and other granites were formed and intruded into the crust as suggested by Dey (2013) from the geochemical studies. All these granites exhibit a narrow linear trend and extend to a distance of 350 km in the NW–SE direction. These evidences give credence to the operation of plate tectonics with subduction and collision, inferred from the present seismic study. Using geochemical studies from the Dharwar craton, Krogstad et al. (1989) suggested operation of plate tectonics in the region during Neoarchean. Thus, the crustal evolution of Archean Dharwar craton can be seen as a wide-orogenic domain of amalgamated cordillera-type terrains, which closely resemble the Archean crustal evolution in different parts of the globe as suggested by van der Veldon et al (2006). 8 CONCLUSIONS The DHARSEIS deep seismic reflection profile across the Archean Dharwar craton provides crustal structure with variable reflectivity patterns at many crustal levels. A simple structure with subhorizontal Moho of the WDC separates from complex reflectivity pattern consisting of dipping Moho, collision signature, a thrust fault and a crustal root of the EDC. The crustal thickness is not uniform along the profile as demonstrated by the depth and characteristics of the Moho. The diverse characteristics of the Moho suggest that it does not arise from a single process and reflects action of geological history of the region. Seismic crustal structure derived in the present study is consistent with convergence, subduction and accretion of crustal blocks. Deep seismic reflection images complemented with geology indicate that the Dharwar craton records a Neoarchean orogeny involving the Mesoarchean rocks on the west and a Neoarchean juvenile arc system to the east. The orogeny is partially over-printed by wide-spread syn-to-post-orogenic granitic intrusions. They are responsible for the crustal evolution of the Dharwar craton during the Neoarchean. The deep crustal structure of the Neoarchean Dharwar craton that is broadly analogues to the Phanerozoic accretionary terrains provides evidence for the operation of plate tectonics during the Neoarchean over the Indian shield. The seismic image suggests a two-stage subduction—accretion process for the crustal evolution in the Neoarchean Dharwar Craton with a crust-mantle detachment. The dipping seismic reflections from the lower-crust extending into the mantle are imaged for the first time, which we interpret to represent a relict 2.5 Ga old suture associated with mantle subduction. Acknowledgements The deep seismic reflection experiment was funded by the Department of Science & Technology (DST), Government of India. We thank DST for the research grant under DST-DCS programme. The digital data were collected using the SN 388 RFT system and the 4.5 Hz (HGS made) geophones belonging to the National Geophysical Research Institute (NGRI). These organisations can be approached to access the data if required. BM is grateful to F. Wenzel and J. Mann for providing the CRS code during the DAAD Fellowship at Kalrsruhe Institute of Technology, Germany. Director, CSIR-NGRI is duly acknowledged for all support and encouragement. We thank Prakash Kumar for valuable suggestions. We are also thankful to P. Sai Vijay Kumar and N. Damodara for their technical support. This paper is dedicated to Late V. Sridhar, who helped in data acquisition and processing. Reviews by Ewald Luschen, an anonymous reviewer and Editor Gabi Laske helped to improve the manuscript substantially and are much appreciated. REFERENCES BABEL Working Group , 1990 . Evidence for early Proterozoic plate tectonics from seismic reflection profiles in the Baltic Shield , Nature , 348 , 34 – 38 . Crossref Search ADS Balakrishnan S. , Rajamani V. , Hanson G.N. , 1999 . 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Geophysical Journal InternationalOxford University Press

Published: Feb 1, 2018

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