Modelling of crustal composition and Moho depths and their Implications toward seismogenesis in the Kumaon–Garhwal Himalaya

We image the lateral variations in the Moho depths and average crustal composition across the Kumaon–Garhwal (KG) Himalaya, through the H–K stacking of 1400 radial PRFs from 42 three-component broadband stations. The modelled Moho depth, average crustal Vp/Vs, and Poisson’s ratio estimates vary from 28.3 to 52.9 km, 1.59 to 2.13 and 0.17 to 0.36, respectively, in the KG Himalaya. We map three NS to NNE trending transverse zones of significant thinning of mafic crust, which are interspaced by zones of thickening of felsic crust. These mapped transverse zones bend toward the north to form a NE dipping zone of maximum changes in Moho depths, below the region between Munsiari and Vaikrita thrusts. The 1991 Mw6.6 Uttarakashi and 1999 Mw6.4 Chamoli earthquakes have occurred on the main Himalayan thrust (MHT), lying just above the mapped zone of maximum changes in Moho depths. Modelled large values of average crustal Vp/Vs (> 1.85) could be attributed to the high fluid (metamorphic fluids) pressure associated with the mid-crustal MHT. Additionally, the serpentinization of the lowermost crust resulted from the continent–continent Himalayan collision process could also contribute to the increase of the average crustal Vp/Vs ratio in the region.

Estimating compositions of the deep continental crust

<p>The deep continental crust's chemical makeup is central to the debate of crustal formation, evolution, strength, and bulk composition. The impenetrable depths and pressures of the deep (roughly > 10 km) crust force geoscientists to rely on indirect sampling methods, studying medium- to high-grade metamorphic terrains and xenoliths to ascertain the composition of the middle and lower continental crust. Analyzing the deep crust in situ requires geophysical data, such as seismic velocities: Vp, Vs, and the Vp/Vs ratio. Each method provides a different perspective on deep crustal composition, but alone, neither is definitive. </p><p>To address the nonuniqueness in crust composition modeling, we use thermodynamic modeling software (i.e. Perple_X) to relate observed seismic velocities to bulk compositions and mineralogies. We present a multidisciplinary model for the composition of Earth's deep crust, using geochemical and geophysical data. Through a Monte Carlo modeling approach, we determine the best-fit geochemical model for bulk middle and lower crustal compositions. For 12 different tectonic regimes, we quantify uncertainties in crustal composition, temperature, and seismic velocity while recognizing our own scientific biases. We present a global model of deep crustal composition conclude that regional scale geological variations benefit from a higher resolution model. Overall, our model forecasts 77% of the deepest continental crust has 45 to 55 wt.% SiO<sub>2</sub>; 15% 55 to 65 wt.% SiO<sub>2</sub>; 8% may have > 65 wt.% SiO<sub>2</sub>. Of perhaps equal or greater importance, however, we present a scalable, modular program that can be altered to incorporate additional petrological and geophysical constraints, allowing geoscientists to more easily compare different scenarios for the deep crust.</p>

Seismic Evidence for Proterozoic Collisional Episodes along Two Geosutures within the Southern Granulite Province of India

The Southern Granulite Province of India had witnessed episodes of multiple tectonic activities, leading to sparsely preserved surface geological features. The present study is focused on unraveling the geodynamic evolution of this terrain through measurement of Moho depth and Vp/Vs ratio using data from a large number of broadband seismic stations. These results unambiguously establish three domains distinct in Moho depth and crustal composition. An intermediate to felsic crust with a 7–10 km step-in-Moho is delineated across the Moyar–Bhavani region. Anomalously high felsic crust with abrupt jump in Moho (~8–10 km) together with a dipping feature at deeper level characterizes the transition from eastern to southern segments of the Jhavadi–Kambam–Trichur region. By contrast, the central zone hosting the Palghat–Cauvery shear zone records uniform felsic crust and flat Moho. Drawing analogy from similar results in different parts of the globe, juxtaposition of petrologically dissimilar crustal blocks characterized by varied depths to the Moho is argued to point towards unambiguous presence of two distinct geosutures in the study area: one along the Moyar–Bhavani region and the other across the Jhavadi–Kambam–Trichur. This inference is corroborated by the presence of layered meta-anorthosite, related rock suites, and mafic-ultramafic bodies, supporting the view of a suprasubduction setting in the Moyar–Bhavani region. The Jhavadi–Kambam–Trichur area is marked by operation of the Wilson cycle by way of sparsely preserved geological features such as the presence of ophirags (ophiolite fragments), alkali syenites, and carbonatites. Geochronological results suggest that the suturing along Moyar–Bhavani took place during the Paleoproterozoic and that along Jhavadi–Kambam–Trichur was during the late Neoproterozoic.

Constraining crustal silica on ancient Earth

Accurately quantifying the composition of continental crust on Hadean and Archean Earth is critical to our understanding of the physiography, tectonics, and climate of our planet at the dawn of life. One longstanding paradigm involves the growth of a relatively mafic planetary crust over the first 1 to 2 billion years of Earth history, implying a lack of modern plate tectonics and a paucity of subaerial crust, and consequently lacking an efficient mechanism to regulate climate. Others have proposed a more uniformitarian view in which Archean and Hadean continents were only slightly more mafic than at present. Apart from complications in assessing early crustal composition introduced by crustal preservation and sampling biases, effects such as the secular cooling of Earth’s mantle and the biologically driven oxidation of Earth’s atmosphere have not been fully investigated. We find that the former complicates efforts to infer crustal silica from compatible or incompatible element abundances, while the latter undermines estimates of crustal silica content inferred from terrigenous sediments. Accounting for these complications, we find that the data are most parsimoniously explained by a model with nearly constant crustal silica since at least the early Archean.

The effects of hydrodynamic sorting on the Ti isotope composition of sediments

<p>Detrital sediments provide a useful tool to investigate the composition of the continental crust through time. Mass-dependent (“stable”) isotope variations in Archaean to present-day sediments (shales, diamictites) have recently received much attention and Ti, in particular, holds significant promise as a novel tracer of crustal composition [1, 2, 3]. This approach is based on i) the contrasting Ti isotope composition of mafic versus felsic rocks as a result of the removal of isotopically light oxides during igneous differentiation; and ii) the chemical behaviour of Ti, a refractory and biologically inert element that should not fractionate during weathering and sedimentation. Hence, current interpretations of the Ti isotope detrital sediment record rely heavily on the assumption that it reflects the integrated composition of the source(s), and thus provides a record of the proportion of felsic to mafic rocks in that source.</p><p>A potential caveat, however, is the hydrodynamic sorting of dense minerals in coarse, more proximal sediments [4]. This effect is well-known for zircon; coarser sediments tend to have higher Zr/Al<sub>2</sub>O<sub>3</sub> and a less radiogenic Hf isotope composition due to the concentration of zircon grains [e.g., 5, 6]. Shales form the complementary zircon-depleted reservoir characterised by lower Zr/Al<sub>2</sub>O<sub>3</sub> and a more radiogenic Hf isotope composition relative to the source. Common Ti-rich phases such as ilmenite and rutile are also resistant against physical and chemical weathering and could be concentrated together with zircon in coarse sediments.</p><p>We examined a suite of Eastern Mediterranean passive margin sediments with well-constrained provenance [7] and found that Ti indeed behaves like Zr. Fine-grained samples have lower TiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> compared to coarser, proximal deposits of identical provenance. The removal of Ti-rich phases with a light Ti isotope composition into coarse-grained sediments could thus bias the Ti isotope composition of shales towards isotopically heavier values. We will report on the δ<sup>49/47</sup>Ti isotope composition of these sediment samples, but a TiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> mass balance suggests that a bias of more than 0.05 ‰ in the δ<sup>49/47</sup>Ti of shales is possible. Understanding the consequences of hydrodynamic sorting for Ti isotopes in sediments is crucial for their use as a quantitative proxy of crustal composition and for reconciling the shale and diamictite Ti isotope records.</p><p>[1] Greber <em>et al.</em> (2017) <em>Science</em> <strong>357</strong> 1271-1274; [2] Deng <em>et al.</em> (2019) <em>PNAS</em> <strong>116-4</strong> 1132-1135; [3] Saji <em>et al.</em> (2019) <em>Goldschmidt abstract</em> <strong>2929</strong>; [4] Greber & Dauphas (2019) <em>GCA</em> <strong>255</strong> 247-264; [5] Patchett <em>et al.</em> (1984) <em>EPSL</em> <strong>69</strong> 365-378; [6] Carpentier <em>et al.</em> (2009) <em>EPSL</em> <strong>287</strong> 86-99; [7] Klaver <em>et al.</em> (2015) <em>GCA</em> <strong>153</strong> 149-168.</p>

Linking subduction dynamics to back-arc deformation

<p> <span>The morphology of back-arc basins shows how complex their formation is and how pre-existing lithospheric structures, rifting and spreading processes, and subduction dynamics all have a role in shaping them. </span><span>Often, back-arc basins present multiple spreading centres that form one after the other (e.g. Mariana subduction zone), propagate and rotate (e.g., Lau Basin) following trench retreat. Episodes of fast and slow trench retreat can cause rift jumps, migration of magmatism, and pulses of higher crustal production (e.g., Tyrrhenian Basin). The evolution of a back-arc basin is not only tightly linked to subduction dynamics, but it is likely that the composition and the pre-existing structure of the lithosphere play a role in shaping the basin too. </span><span>In this work, we investigate the interplay between these features with numerical models of lithospheric extension with a visco-plastic rheology. We use the finite element code ASPECT to model the rifting of continental and oceanic lithosphere with boundary conditions that simulate the asymmetric type of extension caused by the trench retreat. We perform a parametric study in which we systematically change key parameters such as crustal composition and thickness, initial thermal structure and rheology of the lithosphere, and rate of extension. These models aim at understanding how pre-existing lithospheric structures affect back-arc rifting and spreading and what processes control spreading centres jumps in back-arc settings. Preliminary results show that time-dependent boundary conditions that simulate episodes of fast trench retreat, thus fast extension, play an important role into the style of lithospheric back-arc deformation. Finally, we will compare our model results with the location and timing of back-arc rifting and spreading in different active and inactive back-arc basins.</span></p>