A paleoseismic investigation of a frontal foreland thrust in the Greater Caucasus

The Caucasus defines the northern margin of the Arabia-Eurasia collision zone between the Black and Caspian Seas within the Alpine-Himalayan collision. Most orogen-perpendicular convergence within this sector of the Arabia-Eurasia collision is absorbed within the Greater Caucasus, as indicated by seismicity, GPS velocity gradients and Neotectonic geology. Despite significant historical seismicity in the region, Important earthquakes include Lechkhumi-Svaneti earthquake of 1350, Ms 7.0, I0=9; the Alaverdi earthquake of 1742, Ms 6.8, Io=9. active faults with the potential for seismogenic rupture remain poorly characterized. Earthquake focal mechanisms in the Greater Caucasus generally have thrust mechanisms, with recent earthquakes on the main thrust of the Caucasus region being the 1991 Racha Mw=7.0 and the 1992 Barisakho earthquake Mw=6.4. GPS data indicate 2-10 mm/y of convergence across the Greater Caucasus. To investigate potential coseismic rupture along the southern margin of the Greater Caucasus we are conducting a paleoseismic study in the Shida Kartli foreland fold-thrust belt, 40 km NW of Tbilisi. In 2015 we excavated a 24 m long and 5 m deep trench across a potential fault scarp on the southern limb of a south-vergent fold near the Tbilisi-Gori highway between the villages of Okami and Igoeti. This fold deforms deposits mapped as Meotian-Pontian in age. The following soil layers were excavated: the deepest horizon is Unit 1, which is buried topsoil (?), Unit 2 yellow sandy-clayey layer and Unit 3 dark-brown topsoil. In addition, we identified fault in middle of this trench, this is thrust fault that dips 3450to the north and cuts all layers, except the first. The Unit 1 horizon shows about 3-4 meters of separation along the fault. Unit 1 in the hanging wall are deformed by an overturned anticline. We interpret this thrust fault to have ruptured the surface cutting Unit 1 during one ore more significant earthquakes. We have constructed photo mosaics using Structure from Motion and are now logging the trench in detail. Snail shells provide potentially dateable material to help constrain the timing of deformation exposed in the trench. The logging, retrodeformation, and dating are all ongoing.