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 (∼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. Plain Language Summary The Himalaya‐Tibet Mountain system is believed to have formed due to crustal shortening and thickening in the course of India‐Asia collision over the last 50 million years (Ma). The geological records, however, reveal a complex tectonic history of Tibet, showing dominantly north‐south contractional deformations in the first phase (50 to 19 Ma), but replaced by eastward crustal flows and east‐west extensional deformations after 19 Ma. The possible factors behind such tectonic transition in Tibet are not well known. This gap motivates us to explore them with the help of scaled laboratory modeling. Our model experiments allow us to rule out the possibility of steady state collision, either at fast or slow rates in setting the p