A NEW TECTONIC MODEL DESCRIBING THE LATE TERTIARY AND QUATERNARY STRUCTURAL EVOLUTION OF THE CENTRAL ALBORZ MOUNTAINS, NORTHERN IRAN

The Alborz Mountains, northern Iran, are ~100km wide with average peak heights >3000 m. The range wraps around the S. Caspian forming its southern boundary. Existing tectonic models of the Alborz explain their structural evolution in terms of sinistral transpression with N-vergent thrusting over the SW-Caspian,thin-skinned thrusting, or basement-involved nappe tectonics. These models are inconsistent with our observations from the central Alborz where (1)steep dextral-reverse and dextral-normal faults structurally dominate the high part of the range and (2) dextral transpression was important sometime before 7-8 Ma and has reversed to sinistral transpression more recently. We hypothesize, from regional shallow seismicity, structural geometry, sedimentation patterns, and thermochronometry, that Alborz uplift is occurring above a ~N- or NE-vergent blind thrust wedge of central Iranian continental basement that possibly overrides the southern edge of Caspian mafic basement. The N Alborz and S-most Caspian sediments are tilted N above the wedge tip, with strike-slip faults that cut the hanging wall antiform located above the wedge antiform. Reversal of wrench sense probably occurred in response to onset of eastward extrusion of central Iran. Active N-dipping thrusts along the S range margin splay from the S-vergent wedge roof thrust. In addition the region between the high central Alborz and the southern Alborz has undergone thin-skinned style shortening that involved only the Eocene and younger part of the section. This is the result of the room problem that developed as the growth of large antiformal structures in the southern Alborz progressed. The development of these structures is a response to central Iranian crust being underthrust to the north. The development of topography related to the growth of antiformal structures is reflected in unconformities and unroofing sequences within tilted and folded Neogene coarse clastic sediments located to the north of the structures. These hypotheses will be further tested using U-Th/He thermochronology.