MESOZOIC SHORTENING AND SYN-TECTONIC SEDIMENTATION IN THE SOUTHERN PAMIR: IMPLICATIONS FOR GNEISS-DOME METAMORPHISM AND CRUSTAL THICKENING

The Pamir plateau hosts a suite of gneiss domes that were exhumed during the Miocene from mid-crustal depths. Prograde metamorphism occurred during the late Paleogene and is commonly attributed to crustal thickening as a result of shortening in a retro-arc setting, analogous to metamorphic core complexes in the North American Cordillera. To evaluate this model, we examined the Southern Pamir Thrust Belt (SPTB), located down-plunge along strike to the east and in the hangingwall of the Shakhdara-Alichur gneiss-dome complex, the largest in the Pamir. A regional angular unconformity at the Triassic-Jurassic boundary facilitates quantification of post-Cimmerian deformation. The SPTB is south-vergent, thin-skinned, and has a decollement in the Permian-Triassic section that roots into the Rushan-Pshart suture zone at depth to the north. The magnitude of shortening recorded in the thrust belt is relatively low, ~10 km over the ~90 km width of the SPTB (~10% shortening), with shortening decreasing southward from the suture zone. Detrital zircon U-Pb geochronology suggests that syn-tectonic sedimentary strata preserved in the footwalls of thrust sheets in the SPTB are Cretaceous in age. Both the low amount of shortening and Cretaceous age of the syn-tectonic sedimentary rocks suggest that shortening cannot account for Paleogene metamorphism in the Southern Pamir gneiss domes. Alternative explanations may include crustal heating or decoupled thickening between the lower and upper crust.