DETERMINING SHORTENING RATES BY INTEGRATED STRUCTURAL MODELING OF DEFORMED FLUVIAL TERRACES AND UNDERLYING FOLDED STRATA: AN EXAMPLE FROM THE WESTERN GREATER CAUCASUS

Seismic hazard assessment relies in part upon estimates of slip rates on active structures. In active shortening systems, faults are often blind, with hanging wall erosion and footwall deposition making it difficult to identify piercing lines. In such systems, folds deforming Quaternary deposits and geomorphic surfaces may provide the best opportunity for assessing shortening rates. The geometry of surface deformation is dictated by the geometry of structures accommodating shortening at depth. Thus, calculating a geologic shortening rate requires both characterization of the surface deformation and a geometric model that describes corresponding slip on structures at depth. Prior work in active thrust systems has produced multiple such geometric models, each with specific constraints on both subsurface structural geometry and surface deformation patterns.

Here we report the first late Quaternary (Q) geologic shortening rate across an active fold/thrust belt in the Greater Caucasus orogen determined by dating and reconstructing folded landforms. In the Rioni foreland basin along the southwestern margin of the Greater Caucasus, deformed fluvial terraces of the Enguri River record Q shortening in the Rioni Fold/Thrust Belt. We investigate fluvial terraces that have been folded by the south-vergent Tsaishi anticline along the southernmost exposed frontal thrust. Terrace surfaces are well preserved and together with underlying strata record clear deformation. In contrast, subsurface data are limited and provide minimal constraints on structural geometry at depth. Using high-resolution topographic data from differential GPS surveys, we compare multiple geometric models and their efficacy at describing both terrace deformation and finite shortening across the anticline recorded by folding of Mesozoic to Neogene strata. We calculate that the deformed terraces record at least ~80 - 130 m of slip since terrace abandonment. Optically stimulated luminescence dating of quartz in a loess cap on this terrace indicate it was abandoned before 94.1 ± 12.8 ka, implying a geologic shortening rate of 0.85 - 1.38 mm/yr since ~100 ka. This work confirms that structures are Q-active, but falls short of regional geodetic rates of 3-5 mm/yr, with significant implications for seismic hazard in the region.