PRELIMINARY SEDIMENTARY-TECTONIC HISTORY AND AR/AR AGES FROM THE “NEOGENE” REDBEDS, TALEGHAN VALLEY, ALBORZ MOUNTAINS, IRAN

The synformal, intramontane Taleghan basin contains redbeds, evaporites, and carbonates that record the tectonic history of the central Alborz Mountains, about 100 km west of Tehran. Its N side is largely bounded by moderately N-dipping reverse faults that probably formed during transpression, apparently initially dextral and later switching to the sinistral regime recorded by historical seismicity (Axen et al., 2001, Geology). In the eastern end, basalt flows are interbedded with fine-grained clastic rocks and lacustrine, spring-fed carbonates near the base of the section. Two groundmass Ar/Ar plateau ages from these flows are 32.7±0.3 and 32.9±0.2 Ma, indicating that the base of the section is actually Oligocene, the age of the Qom Formation, deposited in a shallow inland sea in central Iran. Up section, deposits become increasingly coarse-grained and record unroofing of the volcanogenic Eocene Karaj Formation and underlying Paleozoic carbonates. Upper parts appear to be synorogenic and display apparent growth-related structures and sediment fans. The stratigraphic position and age of earliest syntectonic sedimentation are presently unknown. several kilometers west of Dizan, reverse faults place Karaj Formation on the redbeds and one is intruded by a series of subvertical, ~N-S striking dikes that display minor sinistral separation. Eight hornblende, biotite, and groundmass Ar/Ar ages from these dikes range from 6.7±0.1 to 8.7±0.2 Ma, indicating that major motion occurred before then. Overlying the redbeds in Taleghan and Alamut Valleys are undeformed but deeply incised flows with hornblende, groundmass, and biotite ages of 0.24±0.03 to 2.86±0.8 Ma, indicating that deformation of the basins had ceased before then and suggesting that uplift migrated south onto the still-active thrusts that bound the Alborz.