Building the Zagros collisional orogen: Timing, strain distribution and the dynamics of Arabia/Eurasia plate convergence
Résumé
he Zagros Mountains are the result of the Arabia/Eurasia collision initiated at ~ 35 Ma as the rifted Arabian lithosphere was underthrusted beneath the Iranian plate due to its negative buoyancy. The onset of crustal thickening started at ~ 25 Ma, as recorded by the hinterland exhumation and foreland clastic deposition. Deformation throughout the Arabia/Eurasia collision zone and the uplift of the Iranian plateau occurred after 15–12 Ma, as a result of shortening/thickening of the thin Iranian crust. We emphasize that only 42% of the post-35 Ma convergence is partitioned by shortening within central Iran. Tomographic constraints show ongoing slab steepening or breakoff in the NW Zagros, whereas underthrusting of the Arabian plate is observed beneath central Zagros. The current subduction dynamics can be explained by the original lateral difference in the buoyancy of the distal margin that promoted slab sinking in NW Zagros and underthrusting in central Zagros. Critical wedge approach applied to the Zagros favors the hypothesis of strong brittle crust detached above a viscous lower crust. In contrast, the weak sedimentary cover deforms by buckling of a thick multilayered cover. Thrust faulting associated with folding occurs in the competent layers and is responsible for most of the earthquakes. There is evidence that the role of the slab pull force in driving the Arabian plate motion was reduced after ~ 12 Ma. Large-scale mantle flow induced by mantle upwelling at the Afar plume appears to be the main driver of the Arabia plate motion. We stress that the main kinematic change in the Zagros region occurred at 15–12 Ma as the Zagros uplifted, before the Arabian slab detached. The Zagros appears key to investigate coupling between continental rheology, plate driving forces and mountain building, in which the role of rift inheritance appears to be central.