Miocene structural inversion of the Paleogene Adjara-Trialeti back-arc basin (SW Georgia, Caucasus) in the context of the Arabia-Eurasia collision zone

Young back-arc rift basins, because of the not yet dissipated extensional thermal signature, are prone to positive inversion following changes in the local geodynamic regime and/or far-Xeld stress transmission. Structural inversion of such basins tends to exploit and reactivate the former normal faults, particularly if the latter are favourably oriented with respect to the direction of stress transfer. The Adjara-Trialeti fold-and-thrust belt (ATFTB) of SW Georgia perfectly exempliXes this mechanism: it results from the structural inversion of a continental back-arc rift basin developed on the upper plate of the northern Neotethys slab in Paleogene times, behind the long-lived Pontides-Lesser Caucasus magmatic arc. Integration of (i) thermal maturity analyses on the former sedimentary Xll of the Adjara- Trialeti back-arc basin and (ii) low-temperature thermochronological [apatite Xssion-track (AFT) and (U-Th)/He (AHe)] analyses on both the former sedimentary Xll and syn-rift plutons provides constraints on the burial-exhumation history of the ATFTB. Maximum paleotemperatures reached by the oldest (Paleocene) rocks within the former Adjara- Trialeti basin are in the order of about 110-115°C. AFT central ages range between 46 and 15 Ma, while AHe ages cluster mainly between 10 and 3 Ma. The reconstructed maximum paleotemperatures indicate that AFT ages are only partially reset, whereas AHe ages are totally reset. Thermal modelling, integrating AFT and AHe data with independent geological constraints, clearly indicates that the Adjara-Trialeti back-arc basin was inverted starting from the Middle Miocene. This result is corroborated by several independent geological evidence, available for example in the adjacent Rioni, Kartli and Kura foreland basins and in the eastern Black Sea offshore, which all suggest a Middle-Late Miocene phase of deformation linked with the ATFTB building. Adjara-Trialeti structural inversion can be correlated with the widespread Middle-to-Late Miocene phase of shortening and exhumation registered from the eastern Pontides to the Lesser Caucasus, the Talysh and the Alborz ranges. This tectonic phase can in turn be interpreted as a far-Xeld effect of the Arabia-Eurasia collision, developed coevally along the Bitlis-Zagros suture hundreds of kilometres to the south.