Kinematic evolution of a continental collision: Constraining the Himalayan-Tibetan orogen via bulk strain rates

We investigated temporal variations of the bulk strain rate across the Himalayan-Tibetan orogen, which impact the magnitudes, rates, and distribution of deformation across the orogen. The present-day strain rate estimated from geodesy is similar to 5-6x10(-16) s(-1) and estimation of past rates depends on the orogenic shortening rate and effective orogen width. Plate-circuit reconstructions provide constraints on time-varying India-Asia convergence rates, showing a marked deceleration since initial collision at ca. 58 Ma. Geologic evidence suggests that most of the Tibetan crust started deforming shortly after initial collision, which simplifies estimation of the initial orogen width. We examined several kinematic models: (1) Greater Indian and Tibetan crust started deforming immediately after collision, (2) only Tibetan crust deformed initially, (3) plate convergence was decoupled from crustal shortening, with Tibetan crust deforming slower than plate rates, or (4) a hard India-Asia collision at ca. 45 Ma following closure of the Xigaze backarc. Models 1, 3, and 4 yield early Cenozoic bulk strain rates that match present-day rates (5-6x10(-16) s(-1)), whereas Model 2 yields faster rates (1.2x10(-15) s(-1)) that must have decreased through time to avoid shortening exceeding total plate convergence. Using these bounds, we constrain crustal shortening across the entire orogen and along its southern and northern margins, defined by the Himalaya and Qilian Shan, respectively. Application of these models to Tibetan crustal thickening suggests that the plateau reached its present-day value (similar to 70 km) by ca. 15 Ma, which corresponds to the onset of lateral deformation (e.g., strike-slip and normal faulting). Lateral deformation in the Himalayan-Tibetan orogen may have resulted from progressive crustal thickening and reorientation of the intermediate principal stress axis. At the thrust-belt scale, our modeling yields shortening rates and magnitudes that match geologic observations. This approach provides testable external kinematic constraints to guide future geological and geophysical investigations.