EXHUMATION EPISODES IN THE ALBORZ MOUNTAINS WITHIN IRANIAN-TURKISH PLATEAU

The Alborz Mountains in the north Iranian-Turkish Plateau form a first-order structural system accommodating convergence between Arabia and Eurasia during the Tertiary. In this study we demonstrate the temporal pattern of exhumation in the Alborz coincides with the main phases of deformation within Iranian-Turkish Plateau.

Located between the ultra deep basin of the South Caspian Sea and the micro-continental plateau of central Iran, the Alborz Mountains take up ~ 40% of the crustal shortening due to convergence of the Central Iranian Plateau and Eurasia. This has resulted in the construction of a ~4 km high mountain belt composed of Precambrian-Paleozoic and Mesozoic basement rocks and inverted Cenozoic basin fills. The timing of this construction is particularly important because it may have been coincided with the closure of the Neo-Tethyan seaway linking the Indian Ocean and the Mediterranean Sea, with broad consequences for global ocean circulation. The mountain belt has also interrupted the air flow off the Russian plains and the Caspian Sea, producing a strong climatic gradient with a wet, densely vegetated north flank and a dry, barren south flank. We have used a combination of low temperature thermochronometric and stratigraphic observations to constrain the onset of mountain building in the Alborz and the timing and location of its stepwise exhumation. Apatite fission tracks record a phase of magmatic activity associated with crustal extension during the Middle-Late Eocene, ca. 39 Ma, followed by rapid exhumational cooling during the Eocene-Oligocene transition at ca. 32 Ma. This exhumation phase has caused the deposition of coarse clastic sediments of Lower Red Formation in fringing basins. Accompanied by a switch from submarine to subaerial volcanic activity in southern Alborz, it is interpreted to reflect the onset of convergence and mountain building in north Iran.

This first episode of exhumation recorded in the Alborz coincides with initiation of collision in the Arabia-Eurasia continental collision zone has been constrained by the other studies.

It has been followed by a period with relatively little tectonic activity resulted in deposing of carbonate Qom Formation. This phase of tectonic quiescence lasts for 15-20 My.

The second phase of rapid exhumational cooling has been started at ca. 16 Ma caused the deposition of coarse clastic sediments of Upper Red Formation in the Middle Miocene. It has been interpreted in the literature as the second stage of the collision resulted from the arrival of unstretched Arabian continental lithosphere in the Neo-Tethyan subduction zone.

The Iranian-Turkish Plateau has undergone a main phase of exhumation at NE portion of the Eurasia-Arabia collision zone between 18 and 13 Ma. The second episode of the Alborz exhumation falls within this interval.

The third exhumation pulse, from ca. 5 Ma has been recorded by apatite (U-Th)/He. This episode coincides with a major reorganization of the collision zone occurred at ca. 5 Ma.