Pre-Eocene Arabia-Eurasia collision: New constraints from the Zagros Mountains (Amiran Basin, Iran)

The timing of continental collision between Arabia and Eurasia is a highly controversial issue, on which new constraints are here provided from the Amiran Basin (Zagros Mountains, Iran). Upper Cretaceous carbonate ramps grown along the Arabian northern margin are overlain by the siliciclastic deep-water Amiran and shallow-water Kashkan Formations, dated biostratigraphically as 64−60 Ma (Paleocene) and 56−52 Ma (earliest Eocene), respectively. Abundant ophioliticlastics, detrital Cr-spinel geochemistry, and detrital zircons with positive εHf(t) values dated as 110−80 Ma, 180−160 Ma, and 260−200 Ma indicate that the Amiran Formation was derived from the obducted Kermanshah ophiolite and Sanandaj-Sirjan zone. Besides sharing similar composition and zircon-age spectra, the overlying Kashkan Formation contains recycled detritus and one new zircon-age component with negative εHf(t) values dated as 250−200 Ma, suggesting supply from additional sources in Central Iran. The Amiran Formation thus indicates that the Kermanshah ophiolite, obducted in the Late Cretaceous, was subaerially exposed to erosion in the Paleocene. The Kashkan Formation testifies to the establishment of a new fluvial system, sourced from Central Iran and flowing across the Zagros suture zone. This implies that continental collision between Arabia and Eurasia took place before the beginning of the Eocene (56 Ma) in the Lorestan region (Iran).