Cenozoic exhumation and topographic build-up of the Iranian plateau: first constraints from low-T thermochronology

The Iranian plateau is a smooth ~1.5-2 km high topography at the rear of the Zagros orogen. It is thought to result from the collision between the Arabian and Eurasian plates since ~30 × 5 Ma following long-standing subduction, and may represent a young analogue to the so far better studied Tibetan plateau. In order to constrain the topographic build-up of the Iranian plateau, we herein present apatite (U-Th)/He (AHe) and apatite (AFT) and zircon fission-track (ZFT) data on plutonic rocks from the Sanandaj-Sirjan Zone (SSZ), Urumieh-Doktar magmatic arc (UDMA), Central Iran and Kopet Dagh. Data show that the SSZ was exhumed early in the collision process (essentially before 20 Ma), with a likely acceleration in exhumation rate during the late Eocene (i.e., 39.6 × 6.5 Myr), from 0.04 to 0.3 mm/yr. Constant exhumation rates along the UDMA (~0.3 mm/yr) indicate that no significant variations of erosion rates occurred since the onset of continental collision. In Central Iran, the overlap of ZFT, AFT and AHe ages from gneissic samples points to rapid cooling during the upper Eocene (~42°C/Myr), which is consistent with previous reports of Eocene metamorphic core-complexes. These results suggest that 1) topographic build-up of the internal Zagros first occurred in the SSZ and migrated to the UDMA during continental collision after ~20 Ma (i.e., coevally to the outward propagation of deformation and topography in the external Zagros) and that 2) the uplift of the Iranian plateau was essentially a constant, steady process over the last 20 Ma.