THE ONSET OF RAPID COOLING AND LATE CENOZOIC SHORTENING IN THE ALBORZ MOUNTAINS OF NORTHERN IRAN: ARE SHORTENING RATES CONSTANT OVER TIME?

New thermochronological and geochronological data from several sample transects spread across the western and central Alborz (also Elburz, Elburs) are combined with geologic data to better constrain the late Cenozoic tectonic evolution of northern Iran in the context of the Arabia-Eurasia collision. We present U/Pb, 40Ar/39Ar, and (U-Th)/He data obtained from analysis of zircon, K-feldspar and apatite. The Lahijan pluton located near the Caspian shore, intruded at 552 +/- 7 Ma, cooled to ~270°C by 190 Ma and remained at or near this temperature until 80 Ma after which time it cooled to ~150°C by 50 Ma. (U-Th)/He ages from the Lahijan pluton indicate slow cooling through 75°C from 50 to ~15 Ma. The Nusha pluton intruded at 98 +/- 1 Ma and cooled to ~200°C by 20Ma. Elevation correlated (U-Th)/He ages from Nusha zircons and apatites indicate a ~2°C /m.y. cooling rate between 35 and 13 Ma (zircon) and a ~12°C /m.y. cooling rate between 7 and 2 Ma (apatite). (U-Th)/He ages of 2 Ma and 13 Ma for apatite and zircon, respectively, from the same Nusha sample, indicate a ~10°C /m.y. cooling rate from 13 to 2 Ma assuming closure temperatures of 75°C and 180°C for apatite and zircon respectively. Thus, rapid late Miocene cooling probably began at ~13 Ma. Assuming a 25°C /km geothermal gradient, we predict ~8-10 km of Alborz exhumation since Middle to Late Miocene time. GPS data from the Alborz indicate a ~ 5 mm/year shortening rate across the range. Assuming a constant shortening rate since 13 Ma predicts predicts ~65 km of Miocene to recent shortening. Balanced cross-sections from the west-central Alborz indicate that post Eocene shortening varies laterally along the range from ~15 km to ~28 km. This suggests that in the west-central Alborz the shortening rate may not be constant over time.