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

Abstract

AbstractThe factors controlling the spatiotemporally varying deformation patterns in Tibet, a prolonged period (∼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 ∼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.