"Geochemistry, Geophysics, Geosystems"

Eijsink, A. M.; Ikari, M. J.

doi: 10.1029/2022gc010369pmid: N/A

The northern Hikurangi subduction margin hosts slow slip events (SSEs), which are exceptionally shallow (<15 km). The sedimentary sequence on the incoming plate is therefore representative of the shallow fault material where the SSEs will take place once they enter the subduction zone. Knowledge about the frictional behavior of these sediments is required to know which lithologies are capable of hosting SSEs, and what mechanisms are causing them. Frictional behavior is material specific and depends on sliding velocity, but it is unknown how these natural sediments behave at plate‐rate velocities. We performed laboratory shearing experiments testing the major lithologies sampled during International Ocean Discovery Program (IODP) Expedition 375, at velocities ranging from the plate convergence rate at the Hikurangi margin (5 cm/year), up to those characteristics of the shallow SSEs (160 and 530 cm/year), under simulated in‐situ as well as standardized conditions. We find that the calcite‐rich pelagic sediments are relatively strong and display the velocity‐weakening frictional behavior required for slip events, whereas other lithologies are weaker and show velocity‐neutral to velocity‐strengthening friction. We observe spontaneous laboratory SSEs in the calcareous pelagic sediments, which show partial locking in between sliding events, consistent with the interpretation of SSEs within the spectrum of slow to fast earthquakes. For the Hikurangi margin, our results suggest that SSE occurrence requires the stronger carbonate‐rich unit to be incorporated into the plate‐boundary fault zone, which we suggest occurs because the rough incoming plate introduces geometrical complexity into the fault zone.

King, Michael T.; Welford, J. Kim

doi: 10.1029/2022gc010372pmid: N/A

Deformable plate tectonic models have been demonstrated to be a useful technique for quantifying temporal variations in strain rate and crustal thickness within recent plate kinematic studies. Using the GPlates software, deformable plate models offer an approach to visualize and assess the interplay of plate kinematics and deformation. However, several assumptions are imposed in previous studies that limit their ability to explain the crustal evolution of various tectonic regimes. Examples of these assumptions include, but are not limited to, the rigid nature of continental blocks and boundaries used to define deformable regions, and uniform crustal thickness assumptions at model start times. In this study, we address these assumptions with newly presented applications using the interplay of GPlates and its python programming library, pyGPlates. In particular, we demonstrate the ability to create deformable continental blocks, reconstruct present day crustal thickness estimates back through time, and how the landward extent of present day crustal thickness estimates can be used to define the limits of deformable plate models and rift domain boundaries a priori. To demonstrate their application and validity, these concepts are evaluated using a previously published deformable plate model of the southern North Atlantic that is tested using 4 modeling scenarios herein to assess the impact of variable model inputs. These models provide insight regarding the pre‐Jurassic (200 Ma) crustal thickness template of the southern North Atlantic, the evolution of continental blocks during rift‐related deformation, and the potential impact of ancient orogenic terranes during subsequent rifting within the North Atlantic.

King, Michael T.; Welford, J. Kim

doi: 10.1029/2022gc010373pmid: N/A

The offshore rifted margins of the North Atlantic have a spatially complex crustal structure comprised of variable crustal morphologies, continental blocks, and inherited structures. Recently, deformable plate tectonic models have permitted the interplay of plate kinematics and deformation to be assessed throughout the North Atlantic, and elsewhere. In particular, the ability to calculate temporal variations in crustal thickness has provided insight into the kinematic role of continental blocks and their interplay with large and micro‐tectonic plates during the formation of the North Atlantic offshore rifted margins. In this study, the deformable plate modeling workflow introduced in the companion contribution of this study (Part 1) is used to investigate previously published and newly presented deformable plate models of the Newfoundland, Irish, and West Iberian margins. This approach permits the deformation and subsequent crustal thickness evolution within previously recognized continental blocks and sedimentary basins throughout the southern North Atlantic Ocean to be visualized and assessed from 200 Ma to present day. The segmentation of early rift crustal thicknesses calculated by deformable plate models demonstrate strong correlations with the offshore extension of Appalachian and Caledonian terrane boundaries. Thus, our observations suggest that inherited orogenic boundaries potentially play a key role in the early rift crustal structure of sedimentary basins and the partitioning of deformation around and within continental blocks.

Zhao, Lingfeng; Li, Lun; Liao, Jie; Dong, Shixian; Liang, Yanling; Gao, Rui

doi: 10.1029/2021gc010263pmid: N/A

The Himalayas is currently rising due to the collision of the Indian and Asian plates and hosts frequent earthquakes, some of which are devastating, such as the 2015 Mw7.8 Gorkha earthquake. Despite the importance of deep dynamic processes to understand the uplift of the Himalayas and the occurrence of large earthquakes, it remains limitedly constrained due to the lack of a detailed three‐dimensional subsurface image under this region. Here, we construct new models of shear‐wave velocity and radial anisotropy down to the 150 km depth from Rayleigh‐ and Love‐wave tomography in the Nepal Himalayas. We find that the 2015 Gorkha earthquake and its main aftershock occurred in a velocity contrast that is presumably interpreted as Main Himalayan Thrust (MHT). A duplex structure, imaged as relatively high velocities, is inferred to exist above MHT under the Lesser Himalayas. This duplex shows heterogeneous features along the strike of the Himalayas that may control the rupture behavior during the occurrence of a large earthquake. Additionally, a low‐velocity anomaly is observed at depths from Moho to 100 km under the Lhasa Terrane and north of the Himalayan Terrane between 85° and 88°E. We interpret this low‐velocity anomaly to be likely caused by mantle upwelling resulting from either possible Indian slab tearing, or northward subduction of the Indian plate. If this is the case, the north‐south trending rifts that situate within the dispersal of the low‐velocity anomaly are probably associated with the mantle upwelling. This study provides a new independent constraint on the geometry of the MHT system and deep dynamic processes occurring in the Nepal Himalaya.

Arns, A. I.; Evans, D.; Schiebel, R.; Fink, L.; Mezger, M.; Alig, E.; Linckens, J.; Jochum, K. P.; Schmidt, M. U.; Jantschke, A.; Haug, G. H.

doi: 10.1029/2022gc010445pmid: N/A

Calcareous foraminifer shells (tests) represent one of the most important archives for paleoenvironmental and paleoclimatic reconstruction. To develop a mechanistic understanding of the relationship between environmental parameters and proxy signals, knowledge of the fundamental processes operating during foraminiferal biomineralization is essential. Here, we apply microscopic and diffraction‐based methods to address the crystallographic and hierarchical structure of the test wall of different hyaline foraminifer species. Our results show that the tests are constructed from micrometer‐scale oriented mesocrystals built of nanometer‐scale entities. Based on these observations, we propose a mechanistic extension to the biomineralization model for hyaline foraminifers, centered on the formation and assembly of units of metastable carbonate phases to the final mesocrystal via a non‐classical particle attachment process, possibly facilitated by organic matter. This implies the presence of metastable precursors such as vaterite or amorphous calcium carbonate, along with phase transitions to calcite, which is relevant for the mechanistic understanding of proxy incorporation in the hyaline foraminifers.

Das, Archana; Sodhi, Aashima; Vedpathak, Chintan D.; Prizomwala, S. P.; Agnihotri, Rajesh; Makwana, Nisarg; Joseph, Jaquilin; Patel, Nikhil; Chopra, Sumer; Ravi Kumar, M.

doi: 10.1029/2021gc010264pmid: N/A

The transformation of mature (urbanized) phase of the ancient Indus civilisation between ∼4200 and 3800 years Before Present (yr BP) overlaps with the beginning of the Meghalayan Age (∼4200 ± 100 yr BP). Though exact cause(s) for decline of urbanized Indus phase are not yet clear, researchers continue to debate whether monsoonal dryness was the sole cause or several other regional factors manifested in a compounding manner. Here, we show a regional relative sea level fall in the downstream area of Indus habitation (south‐western Gujarat region) which initiated at 4150 ± 230 and continued up to 3625 ± 200 yr BP. We provide a multi‐proxy (chronological, sedimentological, mineralogical, isotopic and elemental abundance) data set from a well‐dated vertical sediment trench from Lothal (ancient dockyard area of Indus era) to support this inference. Chief proxies used for inferring the relative sea level fall were bulk sediment carbon and sulfur contents along with their stable isotopes (δ13C and δ34S) and foraminiferal assemblage. The conspicuous shifts in majority of proxies hint at a lowering of sea stand at the regional level that likely dried this ancient Harappan dockyard (used for sea trade). Findings of our study possess implications for Holocene climate changes and their plausible impact(s) on Harappan trade and culture. Additionally, it invites evidences for large scale geological changes at ∼4200 yr BP distinct to the Meghalayan era.

doi: 10.1002/ggge.22532pmid: N/A

No abstract is available for this article.

Spang, A.; Baumann, T. S.; Kaus, B. J. P.

doi: 10.1029/2021gc010265pmid: N/A

Geodynamic codes have become fast and efficient enough to facilitate sensitivity analysis of rheological parameters. With sufficient data, they can even be inverted for. Yet, the geodynamic inverse problem is often regularized by assuming a constant geometry of the geological setting (e.g., shape, location and size of salt diapirs or magma bodies) or approximating irregular bodies with simple shapes like boxes, spheres or ellipsoids to reduce the parameter space. Here, we present a simple and intuitive method to parameterize complex 3D bodies and incorporate them into geodynamic inverse problems. The approach can automatically create an entire ensemble of initial geometries, enabling us to account for uncertainties in imaging data. Furthermore, it allows us to investigate the sensitivity of the model results to geometrical properties and facilitates inverting for them. We demonstrate the method with two examples. A salt diapir in an extending regime and free subduction of an oceanic plate under a continent. In both cases, small differences in the model's initial geometry lead to vastly different results. Be it the formation of faults or the velocity of plates. Using the salt diapir example, we demonstrate that, given an additional geophysical observation, we are able to invert for uncertain geometric properties. This highlights that geodynamic studies should investigate the sensitivity of their models to the initial geometry and include it in their inversion framework. We make our method available as part of the open‐source software geomIO.

Huber, K.; Vrijmoed, J. C.; John, T.

doi: 10.1029/2021gc010267pmid: N/A

Many exposed high‐pressure meta‐serpentinites comprise a channelized network of olivine‐rich veins that formed during dehydration at depth and allowed the fluid to escape from the dehydrating rock. While previous studies have shown that chemical heterogeneities in rocks can control the formation of olivine‐enriched vein‐like interconnected porosity networks on the sub‐millimeter scale, it is still unclear how these networks evolve toward larger scales and develop nearly pure olivine veins. To explore this, we study the effect of reactive fluid flow on a dehydrating serpentinite. We use thermodynamic equilibrium calculations to investigate the effect of variations in the bulk silica content in serpentinites on the dehydration reaction of antigorite + brucite = olivine + fluid and the silica content of this fluid phase. Further, we develop a numerical model that combines the effects of intrinsic chemical heterogeneities with reactive transport with dissolved silica as metasomatic agent. Our model shows how reactive transport can lead to vein widening and olivine enrichment within a vein in an antigorite‐rich matrix, such as observed in the veins of the Erro Tobbio meta‐serpentinites. This is a critical step in the evolution toward larger‐scale vein systems and in the evolution of dynamic porosity, as this step helps account for the chemical feedback between the dehydrating rock and the liberated fluid.

Zhang, Ping; Miller, Meghan S.; Schulte‐Pelkum, Vera

doi: 10.1029/2021gc010262pmid: N/A

The convergent plate boundary in eastern Indonesia and Timor‐Leste captures an active oblique collision between the Banda Arc and the Australian plate. We analyzed ∼5 years' worth (2014–2019) of radial and tangential teleseismic Ps receiver functions (RFs) observed at 30 temporary broadband seismic stations across the area. Azimuthal variations in RF arrivals are observed throughout the region, indicative of the presence of oriented tectonic fabrics (dipping contrasts or plunging axis anisotropy) from a variety of crustal depths. The two main strikes of these fabrics are roughly parallel to the orogen and the plate convergence across the outer arc islands, likely associated with orogenic and strike‐slip structures. We observe distinct double polarity‐reversal arrivals with opposite polarity that reflect an anisotropic layer with orogen‐parallel strikes in the shallow crust beneath Timor and Savu, interpreted as metamorphic rocks. Fabrics oriented E‐W are imaged beneath the Flores and Lomblen that host active volcanoes, where we find interesting correlations with magmatic structures. NNW‐SSE striking fabric is imaged at ∼13 km depth beneath central Flores, which relates to a connected dike magmatic system that feeds the aligned cinder cones exposed on the surface. Finally, we identify convergence‐parallel fabrics on the volcano‐extinct islands of Alor and Atauro, consistent with one main fabric orientation imaged in Timor. We suggest all convergence‐parallel fabrics might accommodate strike‐slip motion generated by the overall NNE convergence of the Australian plate with respect to Eurasian plate and contribute to strain partitioning between the trough and backarc resulting from the collision.