IMPLICATIONS OF PRELIMINARY (U-TH)/HE COOLING AGES AND U/PB CRYSTALLIZATION AGES FROM THE CENTRAL ALBORZ MOUNTAINS, IRAN

We sampled vertical transects in the central Alborz for (U-Th)/He apatite thermochronometry and U/Pb zircon geochronology. (U-Th)/He apatite ages record cooling through ~45°-70°C. Based on ~20% of the samples, the (U-Th)/He ages within any transect are elevation correlated. Ages are older on the north and south flanks of the range than in the central parts. The Pan-African-age pluton (U/Pb ages of 552±6 and 551±6) near Lahijan by the Caspian coast records a (U-Th)/He age of ~13.5 Ma. On the south side of the range near Tehran, Eocene Karaj Formation above an active(?) thrust with Plio-Pleistocene footwall strata records a (U-Th)/He age of ~11.3 Ma. The significance of these ages is unclear because other samples from these transects remain undated. In contrast, (U-Th)/He ages from the interior central Alborz range from ~2.8 to 6.8 Ma. The base of a Cretaceous pluton near Nusha (U/Pb ages of 98±1 and 100±1.7 Ma) yields the 2.8 Ma age, whereas a sample ~2 km higher in that transect yields a 6.7 Ma age. (U-Th)/He ages of samples collected at similar elevations on both sides of a dextral fault that offsets the pluton ~12 km are similar and latest Miocene-Pliocene ages from a transect across a major thrust zone north of Dizan show no age breaks at the thrusts. On the basis of this and other ongoing work (Guest et al., this volume; Axen et al., 2001, Geology) uplift of the central Alborz was well underway by ~6-8 Ma and possibly began as long ago as ~11-14 Ma. Quaternary sediments in thrust footwalls along the southern margin of the Alborz suggest that thrusting may have migrated from north to south and that uplift of the interior central Alborz is mainly due to thrusts along the southern margin of the range.