Impact of Decelerating India-Asia Convergence on the Crustal Flow Kinematics in Tibet: An Insight From Scaled Laboratory Modeling

The factors controlling the spatiotemporally varying deformation patterns in Tibet, a prolonged period (similar to 50 to 19 +/- 3 Ma) of NNE-SSW shortening, accompanied by eastward flow and orogen-parallel extension in a later stage (19 +/- 3 Ma to present-day), are still poorly constrained. Using viscous models, we performed scaled laboratory experiments with steady and unsteady state collision kinematics to address this issue. Our model Tibet under steady-state collision, irrespective of high (5.5 cm/yr) or low (3.5 cm/yr) indentation rates fails to produce the present-day crustal velocity fields and the deformation patterns, reported from GPS observations. An unsteady-state collision with decelerating convergence rates (5.5-3.5 cm/yr) is found to be a necessary condition for the initiation of eastward flow and ESE-WNW extensional deformations. The model results also suggest that the mechanical resistance offered by the rigid Tarim block resulted in crustal uplift at faster rates in western Tibet, setting a west to east topographic gradient, existing till present-day. This topographic gradient eventually polarized the gravity-controlled flow in the east direction when the convergence velocity decelerated to similar to 3.5 cm/yr at around 19 +/- 3 Ma. Our model shows the present-day eastward flow in central Tibet follows nearly a Poiseuille type velocity profile, bounded by the Himalaya in the south and the Tarim basin in northern Tibet. This flow kinematics allows us to explain the preferential locations of crustal-scale dextral and sinistral faults in southern and northern Tibet, respectively. Finally, the present-day model crustal-flow velocity, strain-rates, and topographic variations are validated with GPS and geological field data.