DEFORMED RIVER TERRACES SHOW ACTIVE SHORTENING IN THE WESTERN GREATER CAUCASUS, REPUBLIC OF GEORGIA

The Greater Caucasus Mountains (GC) delineate the northern margin of the Arabia-Eurasia collision. Geodetic data¹ suggest that most orogen-perpendicular shortening within this sector of the collision is accommodated in the GC, and the region has a history of large earthquakes, such as the Mw 7.0 1991 Racha earthquake. However, the location, kinematics, and slip rates of major structures accommodating this deformation remain enigmatic. At the eastern end of the GC, in Azerbaijan, recent work² indicates that active shortening is now located in the foreland Kura fold-thrust belt, along the southern margin of the GC. At the western end of the GC, in Georgia, the Rioni fold-thrust belt (RFTB) is structurally equivalent to the Kura fold-thrust belt, and structures within the RFTB are active^3,4. However, the kinematics and shortening rate within the RFTB have not been determined.

To address this problem, we use deformed river terraces along the antecedent Enguri River to quantify Quaternary deformation across the western RFTB. By integrating differential GPS survey data of folded terrace surfaces with surficial and bedrock geologic mapping, we build a kinematic model describing both terrace deformation and the finite deformation recorded by underlying folded late Neogene strata within the Dzumi fold. Luminescence dates from 3 samples in a loess cap on a folded terrace give a maximum age for terrace abandonment of 94.1 ± 8.34 ky. Monte Carlo simulations accounting for uncertainty in depth to detachment, fault dip, and fold width indicate 139 ± 15 m of displacement since formation of the folded terrace, implying an average shortening rate of ~1.5 mm/yr over the past 100 ky. Our work suggests that structures in the western GC are capable of producing surface-deforming ruptures in the foreland basin, in addition to blind ruptures within the fold-thrust belt and in the main range (e.g., the Racha earthquake). An understanding of the structural kinematics and rupture potential of the RFTB has significant societal implications, as much of the region’s population and infrastructure, including the capital cities of both Georgia and Azerbaijan, lie within this zone.

¹ Reilinger et al., 2006, JGR v111

² Forte et al., 2013, Tectonics v32, p688

³ Tibaldi et al., 2017, J Struc Geol v96 p35

⁴ Tsereteli et al., 2016, Tectonophysics v691 p328