Forward Modeling of Post-Jurassic Shortening in the Southeast Pamir Mountains, Tajikistan
Bunge, Eric M 1982-
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One outstanding issue in understanding the Cenozoic evolution of the Himalayan-Tibetan orogen is determining variations in the amount of crustal shortening along the length of the orogen. Current estimates assign ~50%-80% more Cenozoic intra-Asia shortening to the western portion as compared to the central and eastern portions of the orogen, while the eastern portion has accommodated ~25% more convergence overall. These estimates for the western end of the orogen rely on old compilations of shortening, which have since been recognized to have significant flaws, requiring re-evaluation to better understand the nature of along strike variation. Cenozoic internal shortening in the South Pamir has been estimated to be ~240 km. However, ~110 km of this was attributed to a nappe structure that has since been reinterpreted as an extensional detachment fault related to exhumation of the Shakhdara and Alichur gneiss domes. In this study, I forward model the deformation in Jurassic marine carbonates of the fold and thrust region of the Southeast Pamir. This model constrains the magnitude of post-Jurassic deformation in this area in order to re-evaluate the previous estimate of 50km of Cenozoic shortening. In order to recreate the interpreted surface structures, my forward modeling suggests: 1) A dual vergent system, with northeast-directed structures in the south- and southwest-directed structures in the north. 2) Three main detachment levels; A) lower levels including basement involved thrusting and at the base of the Permian to Triassic section, B) a detachment in the upper Permian to Triassic section, and C) a detachment at the base of the Jurassic section on the lower Jurassic unconformity. 3) The detachments in the Permian to Triassic section suggests reactivation of structures formed during pre-Jurassic deformation. My forward model yields 7.5 km of northeast-southwest shortening across the Southeast Pamir, significantly less than the previous estimate of 50 km. This reduces the previous estimate of ~240 km of internal shortening in the Southern Pamir to ~90 km. These results suggest that the magnitude of Asian internal shortening is more consistent along the strike of the orogen than previously recognized. This revised shortening estimate also requires crustal underthrusting of the Pamir in order to account for the current crustal thickness.