Impact of Lithospheric Strength Distribution on India‐Eurasia Deformation From 3‐D Geodynamic Models

Abstract

Models for continental lithospheric strength are not well resolved due to a lack of direct measurement; however, numerical simulations provide means for evaluating the physicality of endmember cases. We simulate the 3-D dynamics of continental lithosphere within the India-Eurasia collision zone and compare results to observed deformation. Three-dimensional lithospheric deformation is approximated with creeping flow in a layered spherical cap. We partition strength with depth according to endmember models, using laterally varying estimates of vertically averaged effective viscosity to constrain the absolute values of 3-D viscosity in the upper crustal and mantle layers. Comparisons between dynamic solutions and observed geophysical data indicate: (1) Deformation within the subducted Indian slab is required to simulate uplift along the entire Himalayan front with 10 22 Pa·s representing an upper bound for slab strength.(2) Along-strike variations in crustal strength are necessary to match surface observations. (3) Balance of gravitational forces acting on Tibetan lithosphere with a weak lower crust are able to replicate observed vertical deformation patterns. A west-to-east decrease in the lateral extent of the underthrust Indian slab is required for reproducing observed surface motions in Tibet. Geodynamic simulations yield subsidence in southern Qiangtang and Southeast Asia consistent with global positioning system-derived dilatation. We note a trade-off between upper crustal strength and surface velocity rotation around the Eastern Himalayan syntaxis. Simulations with stronger upper crust replicate velocities in western Tibet but not the east, while those with weaker upper crust produce observed rotation in eastern Tibet but overpredict magnitudes to the west.

High‐Resolution 3‐D Shear Wave Velocity Model of the Tibetan Plateau: Implications for Crustal Deformation and Porphyry Cu Deposit Formation

“…In the “flow” region, because of the weak mechanical strength of the weakened middle‐lower crust, decoupled movements of surface motion and middle crust material may have developed; therefore, eastward “flow” may not be significantly manifested in surface motions. Recent numerical simulations that considered an entire weakened Tibetan lower crust supported this idea (Bischoff & Flesch, 2019). In the best fit model, west of 95°E, the velocity at a depth of 30 km shows a clockwise rotation compared to the surface velocity.…”

Lateral Variations in Lower Crustal Strength Control the Temporal Evolution of Mountain Ranges: Examples From South‐East Tibet

“…Previous studies incorporating vertical gradients of horizontal velocity have focused on reproducing geologically instantaneous deformation in south‐east Tibet (e.g., Bischoff & Flesch, 2019; Copley, 2008; Lechmann et al., 2014). These studies have demonstrated that key features of the instantaneous earthquake‐ and GPS‐derived velocity field can be explained by lateral viscosity contrasts between cratonic blocks and the surrounding mountain ranges.…”

“…Houseman and England, 1986;England and Houseman, 1986), so incorporating temporal evolution is an important extension to models considering the geologically-instantaneous effects of strength contrasts (e.g. Copley, 2008;Bischoff and Flesch, 2019). This paper concerns the physical controls on mountain building, and the constraints which recently-published stable-isotope palaeoaltimetry observations can provide on lithosphere rheology.…”

“…A key outstanding question about the effect of lateral strength contrasts is how regions with strong lower crust, and the flow of less viscous material over and around them, affect the evolution of mountain ranges over tens of millions of years. Previous studies of continental deformation demonstrate that models which are able to reproduce instantaneous strain rates do not necessarily lead to the formation of the observed topography over time (e.g., England & Houseman, 1986; Houseman & England, 1986), so incorporating temporal evolution is an important extension to models considering the geologically instantaneous effects of strength contrasts (e.g., Bischoff & Flesch, 2019; Copley, 2008). This study concerns the physical controls on mountain building, and the constraints which recently published stable‐isotope palaeoaltimetry observations can provide on lithosphere rheology.…”

Lower Crustal Rheology Controls the Development of Large Offset Strike‐Slip Faults During the Himalayan‐Tibetan Orogeny

“…However, if the lower crust is too strong (>10 22 Pa·s), the whole crust undergoes pure shear thickening which results in coherent deformation and less developed shear zones. Thus, the absent SZ2 may be potentially explained by different deformation styles due to along‐strike rheological heterogeneities between the east‐west (Bischoff & Flesch, 2019; Chen et al, 2017; Zheng et al, 2020) and the north‐south (Huangfu et al, 2018) Tibetan terranes. Notably, the effective viscosities of upper and lower crust in our reference model are consistent with previous models which account for normal faulting and viscous buckling (Bischoff & Flesch, 2018; Flesch et al, 2001) in Tibet.…”