CRITICAL TAPER APPLIED TO THE ALBORZ MOUNTAINS OF NORTHERN IRAN: HAVE THE ALBORZ SHIFTED FROM SUBCRITICAL TO SUPERCRITICAL STATE AND WHAT ROLE HAS CLIMATE PLAYED IN THIS SHIFT?

The Alborz Mountains of northern Iran are a ~100 km wide belt of 2000 m to 5000 m high topography located along the southern margin of the super deep (~25 km of post Jurassic sediment) south Caspian basin. During the last four years we have worked towards understanding Cenozoic spatial and temporal development of the Alborz-South Caspian tectonic architecture, and have proposed models which describe the Alborz as a southward leaning asymmetrical flower structure. An important element of the general model which we have proposed is the existence of a fold and thrust belt wedge that overrides central Iranian basement to the south. Understanding the spatial and temporal development of this wedge is important because it has implications for understanding the role that climate has played in the development of the Alborz south Caspian system over time. New sedimentological and thermochronological data (Ar/Ar and (U-Th)/He) suggests that by ~20 Ma, following a period of Late Eocene to Oligocene intraarc extension which separated the arc volcanic rocks of the Alborz from the the Neotethyan Urumieh Dokhtar volcanic arc, the region along the southern Caspian Margin experienced the onset of contraction and localized uplift. This period of uplift and contraction is correlated with the development of thick (up to 1700 m) sequences of conglomeratic growth strata deposited in large east-west trending intermountain basins. These deposits may represent wedge-top deposits of the early, subcritical (?), south vergent, tectonic wedge that existed throughout the Miocene. Apatites from the Miocene wedge-top basin sediments yield reset (U-Th)/He ages of ~6 Ma to ~2 Ma, indicating that continued contraction during the Latest Miocene and Pliocene deformed and partially exhumed the wedge-top basins. This implies that since the late Miocene the Alborz wedge has shifted to a supercritical state where regional uplift and erosion dominate, and wedge-top deposits of the subcritical phase are removed. The shift from the subcritical to the supercritical state during the Late Miocene coincides with the onset of rapid south Caspian subsidence and the arrival of thick, buoyant Arabian continental lithosphere at the Neotethyan trench. This change in wedge taper probably results in, and may be enhanced by the development of an orographic affect.