3-D Dynamic Models of the India-Eurasia Collision Zone and Implications for Vertical Viscosity Substructure

Sarah Bischoff and Lucy Flesch
EAPS, Purdue University

The India-Eurasia collision zone is the largest zone of continental deformation on the Earth’s surface. The proliferation of geodetic, seismic, and geologic data across the zone provides a unique opportunity for constraining geodynamic models and increasing our understanding of mountain building and plateau growth. We present a 3-D geodynamic, Stokes flow, finite element model of the India-Eurasia collision with seismic data constraints on the lithospheric volume, velocity boundary conditions taken from continuous surface velocities inferred from GPS and Quaternary fault slip data, estimates of laterally-variant viscosity from Flesch et al. (2001), and a positive viscosity gradient between the Indian plate and Eurasia. The model volume extends laterally north from the southern tip of India to the Tian Shan, and east from the Pamir Mountains to the South China block. Vertically the lithosphere volume extends to a depth of 100 km, and is divided into three layers: upper crust, lower crust, and upper mantle. We use COMSOL Multiphysics (www.comsol.com) to investigate the role of vertical viscosity variation on surface deformation by holding the dynamics constant, adjusting the viscosity substructure, and determining the resultant stress and velocity fields. Model surface velocities are then compared to the observed surface velocities inferred from GPS and Quaternary fault slip rates. A 2-layer model employing laterally-variant viscosity estimates throughout the crust and mantle is ineffective at replicating the observed force balance. The weak crustal viscosities necessary for attaining the observed clockwise rotation around the eastern Himalayan syntaxis also results in erroneous southward velocities in southern Tibet driven by excessive gravitational collapse. Strengthening crustal viscosities balances the boundary/body forces and allows for accommodation of India plate motion across Tibet, but no longer produces clockwise rotation around the eastern syntaxis. The best-fit velocity magnitude and rotation solution is achieved by a full three layer model incorporating an intermediate strength upper crust, a weaker lower crust, and a stronger upper mantle. Our three-layer model achieves rotation around the indenter without excessive gravitational collapse. Model and observed velocities diverge slightly in the Tarim Basin, the southern Gobi, and the northern South China block. Model velocities in the Tarim Basin are shifted in an easterly direction; possibly indicating a weaker than previously assumed Altyn Tagh fault, while Gobi and South China model velocities are shifted to the north; suggesting the presence of an additional level of complexity.

AAPG Search and Discovery Article #90181©2013 AAPG/SEG Rocky Mountain Rendezvous, University of Wyoming, Laramie, Wyoming, September 27-30, 2013