Along the northern margin of the Arabia-Eurasia collision zone in the western Greater Caucasus, the Main Caucasus Thrust (MCT) juxtaposes Paleozoic crystalline basement to the north against Mesozoic metasedimentary and volcaniclastic rocks to the south. The MCT is commonly assumed to be the trace of an active plate-boundary scale structure that accommodates Arabia-Eurasia convergence, but field data supporting this interpretation are equivocal. Here we investigate the deformation history of the rocks juxtaposed across the MCT in Georgia using field observations, microstructural analysis, U-Pb and 40 Ar/ 39 Ar geochronology, and 40 Ar/ 39 Ar and (U-Th)/He thermochronology. Zircon U-Pb analyses show that Greater Caucasus crystalline rocks formed in the Early Paleozoic on the margin of Gondwana. Low-pressure/temperature amphibolite-facies metamorphism of these metasedimentary rocks and associated plutonism likely took place during Carboniferous accretion onto the Laurussian margin, as indicated by igneous and metamorphic zircon U-Pb ages of~330-310 Ma. 40 Ar/ 39 Ar ages of~190-135 Ma from muscovite in a greenschist-facies shear zone indicate that the MCT likely developed during Mesozoic inversion and/or rifting of the Caucasus Basin. A Mesozoic 40 Ar/ 39 Ar biotite age with release spectra indicating partial resetting and Cenozoic (<40 Ma) apatite and zircon (U-Th)/He ages imply at least~5-8 km of Greater Caucasus basement exhumation since~10 Ma in response to Arabia-Eurasia collision. Cenozoic reactivation of the MCT may have accommodated a fraction of this exhumation. However, Cenozoic zircon (U-Th)/He ages in both the hanging wall and footwall of the MCT require partitioning a substantial component of this deformation onto structures to the south. Plain Language Summary Collisions between continents cause deformation of the Earth's crustand the uplift of large mountain ranges like the Himalayas. Large faults often form to accommodate this deformation and may help bring rocks once buried at great depths up to the surface of the Earth. The Greater Caucasus Mountains form the northernmost part of a zone of deformation due to the ongoing collision between the Arabian and Eurasian continents. The Main Caucasus Thrust (MCT) is a fault juxtaposing old igneous and metamorphic (crystalline) rocks against younger rocks that has often been assumed to be a major means of accommodating Arabia-Eurasia collision. This study examines the history of rocks along the MCT with a combination of field work, study of microscopic deformation in rocks, and dating of rock formation and cooling. The crystalline rocks were added to the margins of present-day Eurasia about 330-310 million years ago, and the MCT first formed about 190-135 million years ago. The MCT is likely at most one of many structures accommodating present-day Arabia-Eurasia collision.

Although the Greater Caucasus Mountains have played a central role in absorbing late Cenozoic convergence between the Arabian and Eurasian plates, the orogenic architecture and the ways in which it accommodates modern shortening remain debated. Here, we addressed this problem using geologic mapping along two transects across the southern half of the western Greater Caucasus to reveal a suite of regionally coherent stratigraphic packages that are juxtaposed across a series of thrust faults, which we call the North Georgia fault system. From south to north within this system, stratigraphically repeated ~5–10-km-thick thrust sheets show systematically increasing bedding dip angles (&lt;30° in the south to subvertical in the core of the range). Likewise, exhumation depth increases toward the core of the range, based on low-temperature thermochronologic data and metamorphic grade of exposed rocks. In contrast, active shortening in the modern system is accommodated, at least in part, by thrust faults along the southern margin of the orogen. Facilitated by the North Georgia fault system, the western Greater Caucasus Mountains broadly behave as an in-sequence, southward-propagating imbricate thrust fan, with older faults within the range progressively abandoned and new structures forming to accommodate shortening as the thrust propagates southward. We suggest that the single-fault-centric “Main Caucasus thrust” paradigm is no longer appropriate, as it is a system of faults, the North Georgia fault system, that dominates the architecture of the western Greater Caucasus Mountains.

Extension within a continental back‐arc basin initiates within continental rather than oceanic lithosphere, and the geochemical characteristics of magmatic rocks within continental back‐arcs are poorly understood relative to their intraoceanic counterparts. Here, we compile published geochemical data from five exemplar modern continental back‐arc basins—the Okinawa Trough, Bransfield Strait, Tyrrhenian Sea, Patagonia plateau, and Aegean Sea/Western Anatolia—to establish a geochemical framework for continental back‐arc magmatism. This analysis shows that continental back‐arcs yield geochemical signatures more similar to arc magmatism than intraoceanic back‐arcs do. We apply this framework to published data for Triassic‐Jurassic magmatic rocks from the Caucasus arc system, which includes a relict continental back‐arc, the Caucasus Basin, that opened during the Jurassic and for which the causal mechanism of formation remains debated. Our analysis of 40Ar/39Ar and U‐Pb ages indicates Permian‐Triassic arc magmatism from ∼260 to 220 Ma due to subduction beneath the Greater Caucasus and Scythian Platform. Late Triassic (∼220–210 Ma) collision of the Iranian block with Laurasia likely induced trench retreat in the Caucasus region and led to migration of the Caucasus arc and opening of the Caucasus Basin. This activity was followed by Jurassic arc magmatism in the Lesser Caucasus from ∼180 to 140 Ma and back‐arc spreading in the Caucasus Basin from ∼180 to 160 Ma. Trace element and Sr‐Nd isotopic data for magmatic rocks indicate that Caucasus Basin magmatism is comparable to modern continental back‐arcs and that the source to the Lesser Caucasus arc became more enriched at ∼160 Ma, likely from the cessation of back‐arc spreading.