EXHUMATION PATTERN IN THE TALESH MOUNTAINS, NW IRAN "CLIMATE FORCING"

The Talesh Mountains of NW Iran extend between two mountain belts of the Alborz Range in SE and the Lesser Caucasus in NW, forming a narrow gently curved N-trending range west of South Caspian Basin.

The part of the Mountains which stretch out of Iranian border to the Azerbaijan is mainly masked by Tertiary -Eocene- sedimentary and volcanic successions. Within the Iranian border, the Talesh Mountains with about 40-50 km width, display a more complex stratigraphy. It consists of Precambrian to Middle Triassic sedimentary successions deposited along a passive margin of the Palaeotethys Ocean during Early Palaeozoic and, successively, influenced by the opening and closing of the Neotethys Ocean in the Late Palaeozoic-Tertiary. The Eo-Cimmerian event which marks the collision of Iranian Plate with the southern margin of the Eurasia Plate deformed these sequences with large scale displaced nappes. The Shemshak group (Triassic-Jurassic) overlies these nappes with an angular unconformity. The Cretaceous Limestone and volcani-clastic rocks that shows close folds and thrust faults are overlie normally the Late Triassic and Jurassic successions and sealed with Paleocene conglomerate. The Eocene volcanic and volcani-clastic rocks of Karaj Formation succeed the conglomerate and make a main part of Talesh stratigraphic column. These volcanic rocks are reported in literature to be related to active back arc extension of Neo-Thetyan subduction zone.

It is considered that geological evolution of the Talesh Mountain ties closely with the exhumation history of the Alborz Range, and in this study our preliminary assumptions are designed based on the comprehensive studies carried out on the exhumation pattern of the Alborz and its controlling factors.

The general proto-topography of the Alborz and Talesh mountains is assumed to be emerged mainly in Eo-Oligocene ~32Ma when the Eocene extensional tectonic regime converted to the compression, constrained by thermochronomic-stratigraphic data in the Alborz. The stratigraphic evidenced for this event in the Talesh Mountains, in transition of the Upper Eocene to Lower Oligocene strata, has been reported in the literature. It is dominated by fine-grained turbidity current and hemipelagic sediments with slope instability features, which is interpreted to mark the end of rifting and volcanism in the mountains and developing of the Talesh Mountains orogenic wedge as a topographic feature.

The Talesh Mountains with steep slope SW retro-wedge and gentle slope NE pro-wedge provide a solid barrier between the Neogene basins of NW Iran and the South Caspian Basin. There is a strong asymmetry across the range; the NE flank receives, in order of magnitude, more precipitation than the SW flank and erosional unroofing of the mountains must have driven a sediment flux to the adjacent SW and NE depositional basins, which making the area ideal site for studying climate control on erosion and crustal deformation.

This interpretation is compatible with previous study on the climate-exhumation asymmetry in the Alborz Range, located at the SE termination of the Talesh Mountains, displays a higher erosional exhumation rates in the arid-semi arid south flank of the range than in the wet north.

The Talesh Mountains have strong climatic gradients but different lithological gradient between its W and E sides. The Central portion of the mountains is mainly consist of similar erodible rock cover (volcanic and volcani-clastics of the Eocene and Cretaceous) in both retro- and pro-wedge sides. The study will make use of these strong climatic gradients across the Talesh Mountains to constrain effect of the seasonality in exhumation pattern of the orogen. The geomorphic and geodynamic implications of this gradient will be explored, with an emphasis on the possible erosional control on the exhumational and structural evolution of the mountains. Our approach is via combining stratigraphy, structural geology and low temperature Thermochronometric (AFT,AHe) analysis. It's predictable that the western steep flank of the Talesh Mountains has a high exhumation rate with respect to its eastern gentle flank which lasted from late Eocene – Oligocene time in response to low vegetation and high seasonality in retro-wedge steep side of the orogen.