Geochemistry, Geophysics, Geosystems

Argus, Donald F.; Gordon, Richard G.; DeMets, Charles

doi: 10.1029/2011GC003751pmid: N/A

NNR‐MORVEL56, which is a set of angular velocities of 56 plates relative to the unique reference frame in which there is no net rotation of the lithosphere, is determined. The relative angular velocities of 25 plates constitute the MORVEL set of geologically current relative plate angular velocities; the relative angular velocities of the other 31 plates are adapted from Bird (2003). NNR‐MORVEL, a set of angular velocities of the 25 MORVEL plates relative to the no‐net rotation reference frame, is also determined. Incorporating the 31 plates from Bird (2003), which constitute 2.8% of Earth's surface, changes the angular velocities of the MORVEL plates in the no‐net‐rotation frame only insignificantly, but provides a more complete description of globally distributed deformation and strain rate. NNR‐MORVEL56 differs significantly from, and improves upon, NNR‐NUVEL1A, our prior set of angular velocities of the plates relative to the no‐net‐rotation reference frame, partly due to differences in angular velocity at two essential links of the MORVEL plate circuit, Antarctica‐Pacific and Nubia‐Antarctica, and partly due to differences in the angular velocities of the Philippine Sea, Nazca, and Cocos plates relative to the Pacific plate. For example, the NNR‐MORVEL56 Pacific angular velocity differs from the NNR‐NUVEL1A angular velocity by a vector of length 0.039 ± 0.011° a−1 (95% confidence limits), resulting in a root‐mean‐square difference in velocity of 2.8 mm a−1. All 56 plates in NNR‐MORVEL56 move significantly relative to the no‐net‐rotation reference frame with rotation rates ranging from 0.107° a−1 to 51.569° a−1.

Honda, S.

doi: 10.1029/2011GC003827pmid: N/A

We have proposed an existence of small‐scale convection in the mantle wedge in order to explain the alignment of group of volcanoes in the NE Honshu subduction zone, Japan. The roll type thermal convection whose axis is normal to the plate boundaries and “flip‐flopping” of rolls (warmer and colder parts exchange their positions, as time passes) are reported. However, the subsequent study shows an existence of non‐flip‐flopping rolls, also. Thus, in this study, I investigate the possible causes of these differences systematically. I found that along‐arc wavelength of small‐scale convection is controlled by two length scales which may be associated with the inclined nature of the bottom of the low viscosity mantle wedge where the small‐scale convection emerges. When the convection is in early stage of evolvement or the speed of subducting slab is small, the long‐wavelength rolls become prominent. As the convection evolves or the speed of subduction increases, short‐wavelength rolls tend to take over the long‐wavelength rolls. The transition from the long‐wavelength to the short‐wavelength rolls occurs in a several way. It may occur through the splitting of rolls, adjustment of roll patterns and flip‐flopping. If the pattern becomes short‐wavelength roll, it becomes stable. The range of existence of flip‐flopping is limited and the existence of viscosity jump in the top thermal boundary layer may be required.

Sagnotti, Leonardo; Macrì, Patrizia; Lucchi, Renata; Rebesco, Michele; Camerlenghi, Angelo

doi: 10.1029/2011GC003810pmid: N/A

A high‐resolution paleomagnetic and rock magnetic study has been carried out on sediment cores collected in glaciomarine silty‐clay sequences from the continental shelf and slope of the southern Storfjorden trough‐mouth fan, on the northwestern Barents Sea continental margin. The Storfjorden sedimentary system was investigated during the SVAIS and EGLACOM cruises, when 10 gravity cores, with a variable length from 1.03 m to 6.41 m, were retrieved. Accelerator mass spectrometry (AMS) 14C analyses on 24 samples indicate that the cores span a time interval that includes the Holocene, the last deglaciation phase and in some cores the last glacial maximum. The sediments carry a well‐defined characteristic remanent magnetization and have a valuable potential to reconstruct the paleosecular variation (PSV) of the geomagnetic field, including relative paleointensity (RPI) variations. The paleomagnetic data allow reconstruction of past dynamics and amplitude of the geomagnetic field variations at high northern latitudes (75°–76° N). At the same time, the rock magnetic and paleomagnetic data allow a high‐resolution correlation of the sedimentary sequences and a refinement of their preliminary age models. The Holocene PSV and RPI records appear particularly sound, since they are consistent between cores and they can be correlated to the closest regional stacking curves (UK PSV, FENNOSTACK and FENNORPIS) and global geomagnetic model for the last 7 ka (CALS7k.2). The computed amplitude of secular variation is lower than that outlined by some geomagnetic field models, suggesting that it has been almost independent from latitude during the Holocene.

Palma, José L.; Blake, Stephen; Calder, Eliza S.

doi: 10.1029/2011GC003715pmid: N/A

Variations in gas emissions of open‐vent volcanoes are investigated using a model of magma convection in narrow conduits. Laboratory experiments with both vertical and inclined conduits and dimensional analysis show that for Grashof numbers lower than 100 the volumetric rate of magma ascent is a simple function of equivalent conduit radius, density difference between the magmas, and viscosity of the degassed magma that descends back to the reservoir. The rate of magma ascent depends on the flux coefficient, estimated as 0.1 and 0.2 for vertical and inclined conduits, respectively. The equivalent radius parameter accounts for the dimensions of the conduit(s) regardless of its geometry, thus extending the treatment by previous models that used flow in pipes. The volume flow rate of convection increases with higher density difference and conduit size, but is also highly influenced by the large variations in viscosity of the degassed magma as volatile content and crystallinity change. The model presented here can be used to constrain the degassing and ascent rates of volatile‐rich magma when combined with petrologic data on magmatic volatile content. Application of the model to Villarrica volcano (Chile) reveals that the background degassing levels observed (∼3 kg s−1 SO2) are associated with convective ascent of a relatively degassed magma (0.04 wt% S, ∼0.5 wt% H2O), while episodes of higher SO2 emissions (measurements up to 15 kg s−1) can be explained by the ascent of magma with higher volatile content (up to 0.09 wt% S, ∼1.5 wt% H2O).

Smith, Deborah K.; Schouten, Hans; Zhu, Wen‐lu; Montési, Laurent G. J.; Cann, Johnson R.

doi: 10.1029/2011GC003689pmid: N/A

The Galapagos triple junction is not a simple ridge‐ridge‐ridge (RRR) triple junction. The Cocos‐Nazca Rift (C‐N Rift) tip does not meet the East Pacific Rise (EPR). Instead, two secondary rifts form the link: Incipient Rift at 2°40′N and Dietz Deep volcanic ridge, the southern boundary of the Galapagos microplate (GMP), at 1°10′N. Recently collected bathymetry data are used to investigate the regional tectonics prior to the establishment of the GMP (∼1.5 Ma). South of C‐N Rift a band of northeast‐trending cracks cuts EPR‐generated abyssal hills. It is a mirror image of a band of cracks previously identified north of C‐N Rift on the same age crust. In both areas, the western ends of the cracks terminate against intact abyssal hills suggesting that each crack initiated at the EPR spreading center and cut eastward into pre‐existing topography. Each crack formed a short‐lived triple junction until it was abandoned and a new crack and triple junction initiated nearby. Between 2.5 and 1.5 Ma, the pattern of cracking is remarkably symmetric about C‐N Rift providing support for a crack interaction model in which crack initiation at the EPR axis is controlled by stresses associated with the tip of the westward‐propagating C‐N Rift. The model also shows that offsets of the EPR axis may explain times when cracking is not symmetric. South of C‐N Rift, cracks are observed on seafloor as old as 10.5 Ma suggesting that this triple junction has not been a simple RRR triple junction during that time.

Liu‐Zeng, Jing; Wen, Li; Oskin, Michael; Zeng, Lingsen

doi: 10.1029/2011GC003652pmid: N/A

We use river sediment load data to map the pattern of modern denudation across the Longmen Shan margin of the Tibetan Plateau. Suspended sediment load, with corrections of bed load and solute load contributions, is used to calculate watershed‐averaged denudation rates. Decadal erosion is spatially heterogeneous, and seasonally modulated by monsoon flows, which account for 80–90% of the sediment load. Enhanced denudation occurs in a ∼50 km wide band on the hanging wall of the Longmen Shan and Huya fault zones, reaching 0.5–0.8 mm/yr. These rates are similar to kyr‐scale rates deduced from cosmogenic 10Be and to Myr‐scale rates from low‐temperature thermochronology. The sediment flux‐derived erosion rates decrease with increasing distance plateauward, to less than 0.05 mm/yr at a distance ∼200 km northwest of the foot of the Longmen Shan. The gradient in precipitation across this margin alone cannot explain the one order of magnitude spatial difference in erosion. Rather, the river sediment load data delineates a zone of relatively rapid denudation around active faults that carry the Longmen Shan in their hanging wall. From the similarity of denudation rates measured over Myr, kyr, and decadal time scales, we propose that erosion of the Longmen Shan margin has approached a flux steady state. The erosional efflux is balanced by advection of rock toward the Longmen margin above the ∼20°NW dipping ramp of the margin‐bounding fault. Our results suggest that high amounts of landslide material mobilized by earthquakes such as the Mw 7.9 2008 Wenchuan event are gradually removed by rivers, smoothing sediment flux over time. Our results also suggest that caution should be exercised when interpreting young cooling ages as evidence of the initiation of plateau uplift. Advection of an already high plateau into the belt of higher erosion rate at the Longmen Shan could also give rise to an abrupt cooling history.

Shorttle, O.; Maclennan, J.

doi: 10.1029/2011GC003748pmid: N/A

Lithological variations in the mantle source regions under mid‐ocean ridges and ocean islands have been proposed to play a key role in controlling melt generation and basalt composition. Here we combine compositional observations from Icelandic basalts and modeling of melting of a bilithologic peridotite‐pyroxenite mantle to demonstrate that, while short–length scale major element variation is present in the mantle under Iceland, source heterogeneity does not make an important contribution to excess melt production. By identifying the major element characteristics of end‐member Icelandic melts, we find enriched melts to be characterized by low SiO2 and CaO, but high FeO. We quantitatively compare end‐member compositions to experimental partial melts generated from a range of lithologies, pressures and melt fractions. This comparison indicates that a single source composition cannot account for all the major element variation; depleted Icelandic melts can be produced by depleted peridotite melting, but the major element composition of enriched melts is best matched by melting of mantle sources that have been refertilized by the addition of up to 40% mid‐ocean ridge basalt. The enriched source beneath Iceland is more fusible than the source of depleted melts, and as such will be overrepresented in accumulated melts compared with its abundance in the source. Modeling of peridotite‐pyroxenite melting, combined with our observational constraints on the composition of the Icelandic mantle, indicates that crustal thickness variations in the North Atlantic must be primarily due to mantle temperature and flow field variations.

Clague, David A.; Paduan, Jennifer B.; Caress, David W.; Thomas, Hans; Chadwick, William W.; Merle, Susan G.

doi: 10.1029/2011GC003791pmid: N/A

High‐resolution (1.5 m) mapping from the autonomous underwater vehicle (AUV) D. Allan B. of West Mata Volcano in the northern Lau Basin is used to identify the processes that construct and modify the volcano. The surface consists largely of volcaniclastic debris that forms smooth slopes to the NW and SE, with smaller lava flows forming gently sloping plateaus concentrated along the ENE and WSW rift zones, and more elongate flows radiating from the summit. Two active volcanic vents, Prometheus and Hades, are located ∼50 and ∼150 m WSW of the 1159 m summit, respectively, and are slightly NW of the ridgeline so the most abundant clastic deposits are emplaced on the NW flank. This eruptive activity and the location of vents appears to have been persistent for more than a decade, based on comparison of ship‐based bathymetric surveys in 1996 and 2008–2010, which show positive depth changes up to 96 m on the summit and north flank of the volcano. The widespread distribution of clastic deposits downslope from the rift zones, as well as from the current vents, suggests that pyroclastic activity occurs at least as deep as 2200 m. The similar morphology of additional nearby volcanoes suggests that they too have abundant pyroclastic deposits.

Coblentz, D.; Karlstrom, K. E.

doi: 10.1029/2011GC003579pmid: N/A

The topography of the western United States provides a classic field laboratory for investigations of the relationship between surface features and sub‐crustal dynamic processes. The interpretation of recently collected, high‐resolution seismic images of the upper mantle beneath the central Colorado Rocky Mountains substantiates the notion that much of the high elevation coincides with thin or attenuated continental crust (with respect to predicted Airy crustal thicknesses), necessitating topographic support by anomalously buoyant mantle. This is highly suggestive that broad‐scale topographic features may be correlated with buoyancy variations in the upper mantle. In an attempt to sharpen our understanding of the underlying geodynamics, we evaluate the correlation between the surface topographic character and data sets that provide information about density variations indicative of buoyancy in the upper mantle, including the lithospheric geoid, upper mantle seismic velocity anomalies, and crustal (Lg) Q. Our general conclusion is that mantle buoyancy is driving differential surface uplift throughout the western United States and this driver of topography is manifested by measureable anomalies in the topographic roughness at short wavelengths (tens of kilometer) and elevated spectral power in the topography at longer (several hundred kilometers) wavelengths. A provocative conclusion is that the long‐recognized physiographic provinces of the Colorado Plateau, Rocky Mountains, and Rio Grande rift are also neotectonic provinces that are related to convective processes and related buoyancy in the upper mantle.

Philibosian, Belle; Simons, Mark

doi: 10.1029/2011GC003775pmid: N/A

Of the hundreds of volcanic centers throughout the Indonesian archipelago, few are adequately monitored for pre‐eruptive activity due to socioeconomic and logistical barriers, with the result that volcanic hazards in the region are not well quantified. The advent of satellite‐borne L‐band synthetic aperture radar provides an opportunity for detection and measurement of volcanic deformation over broad regions in heavily vegetated tropical island arcs. We use data from the PALSAR instrument on the Japanese ALOS satellite to conduct a comprehensive survey of volcanic deformation on the Indonesian island of Java, over a time period of two years (2007–2008). To obtain the most complete, temporally continuous record of ground deformation, we use a temporally overlapping set of short‐time‐interval radar image pairs to produce a deformation time series. Consistent with previous results from other regions, our survey suggests that volcanoes experiencing small eruptions are typically fed by magma bodies too small and/or too shallow or deep to produce a recognizable InSAR signal. However, we identified a deformation event at Lamongan volcano which is likely linked to a magmatic intrusion at several kilometers' depth, and a second one at Slamet volcano at a shallower depth that may have been related to a subsequent eruption. This initial test of a broad application of L‐band data allowed us to better define the satellite imaging criteria required for successful observation, as well as developing a useful methodology for monitoring deformation over a wide region.