BALANCED CROSS SECTIONS RECORD MARINE BASIN CLOSURE AND GROWTH OF A CONTINENTAL COLLISION ZONE: GREATER CAUCASUS MOUNTAINS, REPUBLIC OF GEORGIA

The Greater Caucasus Mountains (GC) define the northern edge of the Arabia-Eurasia collision zone between the Black and Caspian Seas and accommodate ~70% of the modern, orogen-perpendicular component of plate convergence at their longitude. Despite their tectonic significance, the structural geometry and tectonic evolution of this range remain poorly constrained, and estimates of total Cenozoic crustal shortening span an order of magnitude, from <100 km to nearly 1000 km. Here, we address this problem by reporting the first crustal-scale balanced cross sections across the GC. The two orogen-perpendicular sections span the western and central GC in the Republic of Georgia and integrate our own 1:100,000-scale structural mapping and thermochronologic analyses with previously reported geologic, sediment provenance, teleseismic, seismic reflection, and thermochronologic data. Together these data indicate that the orogen comprises a south-vergent imbricate stack of north-dipping thrust sheets that root into a shallowly north-dipping basal thrust beneath the range. The western traverse contains 9 thrust sheets 4-15 km thick with a duplex at depth beneath the central GC. The central traverse is an imbricate stack of 16 thrust sheets that range in thickness from 2 to 15 km. Our balanced cross sections indicate total shortening is at least ~170 km to ~200 km in the western and central GC, respectively. We have minimized slip on structures where stratigraphic cutoffs in the hanging wall have been eroded away and have not accounted for possible non-accretionary underthrusting, making these minimum shortening estimates. Area-balanced cross sections along the same transects yield broadly similar total shortening values, which again are minima. We posit that the basin(s) in which these sediments were deposited must similarly have been at least ~200 km wide prior to initiation of shortening in the GC region. Integrated since an initiation age of ~30-35 Ma, our minimum shortening of 170-200 km implies a minimum average shortening rate of ~6-8 mm/yr, which is greater than the modern GPS-derived shortening rate of ~3-5 mm/yr. Subduction termination and slab detachment provides one mechanism to explain the decrease in shortening rate.