Chronology of Neogene and Quaternary uplift and magmatism in the Caucasus:constraints from K-Ar dating of volcanism in Armenia

The Greater Caucasus is one of Earth's highest actively-uplifting mountain ranges; the adjoining Caspian Sea basin contains a substantial proportion o f its hydrocarbon reserves. Like other parts of the former Soviet Union, th e Neogene and Quaternary chronology of these important regions has not prev iously been well-defined. It has thus been impossible to obtain reliable es timates for rates of processes such as uplift of the Caucasus and sedimenta tion in the Caspian Sea. Previous studies have established the relative tim ings of events in the region, using correlation schemes between volcanism, glaciations, and the stratigraphy of the Caspian basin. However, a range of absolute chronologies has previously been proposed for these sediments and igneous rocks, based mainly on different interpretations of their magnetos tratigraphic records. By K-Ar dating, we determine the ages of volcanism at three localities in Armenia as 1.1, 0.8 and 0.8 Ma. Using these data and o ther evidence, we propose a revision to the chronology of this region, in w hich a distinctive brief interval of normal magnetic polarity in the local sedimentary and volcanic magnetostratigraphic records is matched to the Cob b Mountain event in the global record rather than the Olduvai event or an e arlier subchron as had previously been thought. We thus interpret a similar to 1.5 Ma timing for the start of volcanism in the Lesser Caucasus, and al so suggest a similar to 1.2 Ma timing for the Late Akchagyl transgression o f the Caspian Sea, a key event in the regional stratigraphy when this water body reached its greatest extent. We tentatively correlate this transgress ion with the melting event following glaciation during stage 36 of the oxyg en isotope timescale, which was thus the first time during the Pleistocene when eastern Europe was covered by a lowland ice sheet. Time-averaged since similar to 1 Ma, the flanks of the eastern Greater Caucasus mountains are shown to have uplifted at similar to 0.6 mm a(-1) and the Lesser Caucasus a t similar to 0.3 mm a(-1). We show that the rate and spatial scale of this uplift are too great to be the result of plate convergence, and suggest ins tead that this uplift is caused by crustal thickening due to inward lower-c rustal flow to beneath these mountain ranges. At the start of magmatism in both the Greater and Lesser Caucasus, the estimated crustal thickness was s imilar to 45 km. We thus suggest that this magmatism has been caused by hea ting of the mantle lithosphere due to earlier crustal thickening, the tempe rature rise required to initiate magmatism being the same in both cases. (C ) 1999 Elsevier Science B.V. All rights reserved.