Normal faulting and viscous buckling in the Tibetan Plateau induced by a weak lower crust

Abstract

Flow of weak lower crust has been invoked to reconcile observed topographic gradients, uniform elevations, slow seismic velocity, and high conductivity measured in the Tibetan Plateau, with viscosity estimates of 1016–1021 Pa·s. Here we investigate the dynamic response resulting from a range of lower crust viscosities in a 3-D lithospheric-scale geodynamic simulation of the India–Eurasia collision zone to determine bounds of physically viable lower crustal strengths. We show that thickening of the plateau is accommodated through viscous buckling of the upper crust in response to lower crustal flow for a lower crustal viscosity on the order of 1020 Pa·s. This generates two east–west trending bands of surface subsidence and dilatation consistent with observed normal faulting and estimates of vertical velocity. These results suggest viscous buckling of the upper crust, induced by lower crustal flow from gravitational pressure gradients due to high topography, is responsible for the observed extension in Tibet.

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

“…Regardless of slab strength, upper crust‐mantle strength contrasts, or boundary conditions, all cases with depth‐partitioned strength and a weak lower crust produce two east‐west trending bands of subsidence separated by a band of uplift across the Tibetan Plateau (Figures and d–f), which we attribute to the presence of weak lower crust (Figure b; Bischoff & Flesch, ). While present‐day rates of vertical velocity are highly uncertain, GPS‐determined dilatation rates, or the divergence of surface velocity, provide insight on expected vertical motions.…”

“…As in case 2, however, roughly southwestward‐simulated surface motion just east of the Burma Arc exhibits a systematic misfit to observed southward motion. The majority of the features of case‐9 vertical surface motion (Figure b) are identical to those produced by cases 5, 7, and 8, particularly the alternating east‐west bands of uplift and subsidence across the Tibetan Plateau (Bischoff & Flesch, ). This pattern of uplift and subsidence across the plateau, which we earlier attribute to inclusion of a weak lower crust beneath Tibet, we now further postulate results from gravitational collapse of the lithosphere with a weak lower crust and is invariant to convergent forces.…”

Episodic Lithospheric Deformation in Eastern Tibet Inferred From Seismic Anisotropy

“…To date, however, there is a lack of consensus on the tectonic mechanisms for plateau uplift and deformation. Several competing models have been proposed, including stepwise plateau growth accompanied by lateral extrusion of coherent terranes and oblique subduction of lithospheric mantle (Tapponnier et al, 2001); thickening of the whole lithosphere as a thin viscous sheet (England & Houseman, 1986); and channel flow in the middle-lower crust (Bird, 1991;Bischoff & Flesch, 2018;Clark & Royden, 2000;Royden et al, 2008). In view of significant regional seismic hazards and its location along the expansion front of the plateau, the Eastern Tibet region has been a central focus of this debate (Figure 1).…”

Current Deformation in the Tibetan Plateau: A Stress Gauge in the India‐Asia Collision Tectonics

“…Although here the “balanced topography assumption” (Warners‐Ruckstuhl et al, ) is followed, implying that the topographic load is balance to the first order (plate‐scale) by stress due to plate boundary forces (Lyon‐Caen & Molnar, ). The role of crustal flow (Clark & Royden, ; England & McKenzie, ) and gravitational spreading has been addressed by several works focusing on a reduced Asian area (Bischoff & Flesch, ; Clark, ; Clark & Royden, ; Copley et al, ), or with a reduced set of boundary forces (Copley et al, ; Flesch et al, ; Tunini et al, ; Warners‐Ruckstuhl et al, ), that is neglecting the Asian subducting margins. Additional complexities in the Tibetan domain, such as extension, might be explained by the combined action of topography, the southeast Asian margin, as proposed here, and the Pacific subduction (Schellart et al, ).…”

“…Although here the "balanced topography assumption" (Warners-Ruckstuhl et al, 2012) is followed, implying that the topographic load is balance to the first order (plate-scale) by stress due to plate boundary forces (Lyon-Caen & Molnar, 1985). The role of crustal flow (Clark & Royden, 2000;England & McKenzie, 1982) and gravitational spreading has been addressed by several works focusing on a reduced Asian area (Bischoff & Flesch, 2018;Clark, 2012;Clark & Royden, 2000;Copley et al, 2010) (Schellart et al, 2019).…”