Cenozoic exhumation of the internal Zagros: First constraints from low-temperature thermochronology and implications for the build-up of the Iranian plateau

The Iranian plateau is a flat ~. 1.5-2. km high plateau thought to result from the collision between the Arabian and Eurasian plates since ~. 30 ± 5. Ma, and may represent a young analogue to the so far better studied Tibetan plateau. In order to constrain the exhumation history of the internal Zagros and 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-Dokhtar magmatic arc (UDMA), Central Iran and Kopet Dagh. Thermochronologic data show that the SSZ was exhumed early in the collision process (essentially before 25-20. Ma), with a likely acceleration of cooling during the late Eocene, from 0.04 to 0.3. mm/year. Results suggest that cooling of the internal Zagros migrated from the SSZ to the UDMA during a more mature stage of the continental collision, after ~. 17. Ma (i.e., coeval with the outward propagation of deformation and topography fronts in the external Zagros). Constant exhumation rates in the UDMA (~. 0.3. mm/year) suggest that no significant variation of erosion rates occurred since the onset of continental collision. In Central Iran, the overlap of ZFT, AFT and AHe ages from g. neissic samples points to rapid cooling during the late Eocene (~. 42. °C/Myr), which is consistent with previous reports on the formation of Eocene metamorphic core-complexes.