Differential exhumation across the eastern Greater Caucasus from low-temperature thermochronology: Implications for plate boundary reorganization and foreland basin deformation

The Greater Caucasus, stretching from the Black Sea to the Caspian Sea, are the highest mountain range in Europe, and form the northern boundary of the Arabia-Eurasia collision zone. The role that the Greater Caucasus plays, and has played, in accommodating strain within this orogen, however, remains elusive. Estimates of the onset of rapid exhumation and deformation of range span a large fraction of the Cenozoic, from Eocene or Oligocene at the western end of the range in Georgia, to late Miocene or early Pliocene in the central Greater Caucasus of Russia, and Pliocene at the eastern end of the range in Azerbaijan. Such controversies have also served to obscure the role of the Greater Caucasus in responding to a major, and widely recognized, plate boundary reorganization at ~5 Ma, characterized by the genesis of many active faults, increased supply of detritus from the Greater Caucasus to its foreland basins, and the onset of deformation of foreland basin sediments. Here we present low-temperature thermochronology data, including apatite (U-Th)/He and fission-track data, from a north-south transect at ~48°E across the eastern Greater Caucasus in Azerbaijan that demonstrates differential exhumation rates and timing across the range. In the northern portion of the range, which is underlain by sedimentary strata of the Scythian Platform, rounded and substantially abraded detrital apatites were recovered from Cretaceous and Jurassic sandstones. Thermal modeling of these data reveal onset of cooling of the northern Greater Caucasus at ~20 Ma at 2-4^oC/My and a significant increase in cooling rate at ~5 Ma to rates >10^oC/My. Maximum exhumation of northern Caucasus strata is ~4 km. Strata of the northern Greater Caucasus are separated from those of the southern Greater Caucasus by the Zangi thrust, south of which strata of the Vandam Zone are comprised of early Cretaceous volcaniclastic sediments of andesitic composition. Detrital apatites from these strata are pristine and euhedral, and are derived from clearly proximal sources. Fission-track ages of these grains are indistinguishable from depositional ages, and presumably represent eruption ages of the source volcanic rocks, while (U-Th)/He ages are Pliocene. Thermal modeling of these data require rapid burial of these strata, and a long period of isothermal holding at temperatures of ~90^oC, prior to rapid exhumation at 4 Ma. Similarities of volcaniclastic strata of the Vandam zone to those described from scientific boreholes within the Kura basin and exposed in the Lesser Caucasus, suggest that a Pliocene increase in exhumation rate of the Greater Caucasus, and propagation of this deformation southward into the Kura basin, may have resulted from collision of the Lesser Caucasus arc with the Scythian platform, concluding consumption of the intervening Greater Caucasus basin. If so, this event may mark the final removal of oceanic or transitional crust from the Arabia-Eurasia orogen, potentially driving plate boundary reorganization as a result of through going continental-continental collision.