THE COINCIDENCE OF SOUTH CASPIAN SUBSIDENCE AND RAPID UPLIFT OF THE ALBORZ MOUNTAINS, NORTHERN IRAN

The Alborz Mountains lie between the South Caspian basin and the Arabian-Eurasian suture zone. Folded and faulted Phanerozoic strata and two plutons in the Alam Kuh area provide important timing constraints on uplift and deformation. U/Pb zircon, ⁴⁰Ar/³⁹Ar biotite and K-feldspar, and (U-Th)/He apatite ages constrain the thermal and structural evolution of the west-central Alborz Mountains. The Akapol suite intruded at 56±2 Ma, coeval with Neo-Tethyan subduction and Paleocene-Eocene arc volcanism. Similar ⁴⁰Ar/³⁹Ar ages suggest emplacement at ambient temperature <350°C. K-feldspar thermal history modeling indicates rapid cooling from ~300°C to ~150°C between ~50 to ~40 Ma with isothermal conditions near 150° persisting until at least 25 Ma. Initial cooling may reflect exhumation and crustal deformation related to Eocene unconformities in central Iran. The isothermal period suggests tectonic quiescence, as does stable marine sedimentation to the south. The Alam Kuh granite intruded at 6.8±0.1 Ma and K-feldspar thermal history indicates rapid cooling to ambient temperatures of 125-175°C by 6 Ma. Elevation-correlated (U-Th)/He ages from Akapol apatites yield ~0.7 km/m.y. exhumation rates between 6 and 4 Ma, similar to South Caspian sedimentation rates since ~5 Ma. Onset of late stage exhumation is nearly synchronous with acceleration of South Caspian subsidence and with initial coarse clastic sedimentation in the Zagros fold belt, suggesting that continued collision between Arabia and Eurasia caused simultaneous widespread vertical motions. Dextral faults offset the Akapol granodiorite, but the Alam Kuh granite intrudes similar dextral faults. In contrast, seismogenic slip occurs on sinistral faults parallel to these dextral faults. Dextral deformation occurred between at least ~56 and 6.8 Ma. Late Neogene synorogenic conglomerates nearby are intruded by dikes showing subsequent sinistral offset. We infer that the switch from dextral to sinistral transpression was related to initiation of eastward extrusion of Iranian microplates shortly after? widespread late Miocene vertical movements and was synchronous with onset of South Caspian folding ~3.4 Ma.