ASSESING PRE-CENOZOIC SHORTENING IN THE PAMIR

The Central and South Pamir are Gondwanaland fragments that accreted to the southern margin of Asia during the Cimmerian orogeny (~200 Ma). Within the Central and South Pamir, it is commonly interpreted that: 1) crustal deformation did not occur until the Cenozoic Indo-Asian collision, and 2) ~300 km of internal shortening took place during the Cenozoic. However, as the Pamir was part of a retro-arc tectonic environment during the Cretaceous (145 Ma - 66 Ma), it is more likely that much of the observed crustal deformation occurred before the Indo-Asian collision.

To address the magnitude and timing of internal shortening within the Pamir, we conducted geologic field mapping and cross-sectional analysis of a post-Cimmerian Jurassic carbonate (Gurmudi Group) within the Central and South Pamir, coupled with detrital zircon U-Pb geochronologic analyses of syn-tectonic red beds. Within the South Pamir, the Gurmudi group is deformed by broad open folds and limited thrusting, suggesting that internal shortening is less than the 50 km previously reported to be accommodated by this region. Detrital zircon analyses on associated syn-tectonic terrigenous red beds yield middle Cretaceous (~100 Ma) maximum depositional ages. Farther north, terrigenous red beds in the footwall of the suture zones that bound the Central Pamir yield a maximum depositional age of Late Jurassic (160 Ma) to Early Cretaceous (137 Ma). When comparing the ages of the red beds to zircon analyses of river drainages within the Pamir, with prominent ages of ~45 Ma and ~100 Ma, our data suggest that deformation began in the Early to middle Cretaceous and likely concluded by the Paleocene. Further, our results indicate that upper crustal internal shortening in the Pamir is likely significantly less than previously proposed and occurred as a retroarc deformation zone prior to the Cenozoic.