Quantifying the impact of mechanical layering and underthrusting on the dynamics of the modern IndiaAsia collisional system with 3D numerical models

The impact of mechanical layering and the strength of the Indian lower crust on the dynamics of the modern India-Asia collisional system are studied using 3-D thermomechanical modeling. The model includes an Indian oceanic domain, Indian continental domain, and an Asian continental domain. Each domain consists of four layers: upper/lower crust, and upper/lower lithospheric mantle. The Tarim and Sichuan Basins are modeled as effectively rigid blocks and the Quetta-Chaman and Sagaing strike-slip faults as vertical weak zones. The geometry, densities, and viscosities are constrained by geophysical data sets (CRUST2.0, gravity, and seismology). Both static (no horizontal movement of model boundaries) and dynamic scenarios (indentation) are modeled. It is demonstrated that 3-D viscosity distributions resulting from typical creep flow laws and temperature fields generate realistic surface velocities. Lateral variations in the gravitational potential energy cause locally significant tectonic overpressure (i.e., difference between pressure and lithostatic pressure) in a mechanically strong Indian lower crust (up to ~500MPa for the static scenario and ~800MPa for the dynamic scenario). Different density distributions in the lithosphere as well as different viscosities (3 orders ofmagnitude) in the Indian lower crust cause only minor differences in the surface velocity field. This result suggests that surface velocities alone are insufficient to infer the state of mechanical coupling of the lithosphere. Model results are in agreement with GPS velocities for Indian lower crustal viscosities of 10–10 Pa s, for a strong Quetta-Chaman Fault (10 Pa s) and a weak Sagaing Fault (10 Pa s).