THERMOCHRONOLOGIC ANALYSIS OF EXHUMATION IN THE ZAGROS MOUNTAINS: CONSTRAINTS ON THE TIMING OF THE ARABIA-EURASIA CONTINENTAL COLLISION

The active continental collision between Arabia and Eurasia has resulted in the formation of the Zagros mountain belt in Iran. A fundamental problem concerning the Zagros is the precise timing of initial continental collision. To date, thermochronologic cooling ages have been used to quantify the timing of exhumation associated with deformation in the Zagros. The dating method uses apatite (U-Th)/He thermochronometry on Cambrian sandstones sampled along thrust sheets, salt plugs and within Neogene growth strata south of the Zagros suture. The data suggests major exhumation of the Zagros commenced in middle Miocene time, with some locations recording early Miocene cooling. It is therefore proposed, that initial continental collision occurred during early Miocene time (25-20 Ma). The helium ages from the High Zagros Fault (HZF), Kurang Fault and associated salt plugs show a broad range from 20 to 8 Ma, consistent with an observed pattern of progressively younger exhumation from west to east along the fold-thrust belt and suggests diachronous collision of the Arabian margin with Eurasia. There are additional evidence for Pliocene (<5Ma) and Pleistocene (.79-.38 Ma) cooling, relating to recent exhumation of the salt plugs along the HZF. The analysis of growth stratal units from the Bakhtyari Fm. are furthermore consistent with early Miocene exhumation in the hinterland regions of the fold-thrust belt, and middle Miocene exhumation proximal to the High Zagros Fault.