EPISODIC EVOLUTION OF A PROTRACTED CONVERGENT MARGIN REVEALED BY DETRITAL ZIRCON GEOCHRONOLOGY IN THE GREATER CAUCASUS

Convergent margins play a fundamental role in the construction and modification of Earth’s lithosphere and are characterized by poorly understood transient processes that occur during the progression from subduction to terminal collision. On the northern margin of the active Arabia-Eurasia collision zone, the Caucasus Mountains provide an opportunity to study the protracted history of a convergent margin that spanned most of the Phanerozoic and culminated in Cenozoic continental collision. However, the main phases of lithosphere (de)formation along this margin remain enigmatic. Here, we use detrital zircon U-Pb geochronology from Paleozoic-Mesozoic (meta)sedimentary rocks in the Greater Caucasus, along with select U-Pb and Hf isotopic data from coeval igneous rocks, to identify and causally link key magmatic and depositional episodes along the Caucasus convergent margin.

Devonian-Carboniferous rocks were deposited prior to accretion of the Greater Caucasus crystalline core onto the Laurussian margin, based on the lack of ~300 Ma grains that are common in both the accreted rocks and younger units derived from them throughout the range. Permian-Triassic rocks document a period of northward subduction and deposition in the forearc of a coeval volcanic arc in the Northern Caucasus and Scythian Platform. Jurassic rocks record the magmatically-active opening of the Caucasus Basin as a back-arc rift during southward migration of the arc front into the Lesser Caucasus. Cretaceous rocks with few Mesozoic zircons indicate a period of magmatic and tectonic quiescence within this basin. Critically, all rocks more than ~20 km north of the Greater Caucasus range front exhibit a provenance affinity with the Paleozoic crystalline core of the Greater Caucasus, indicating that a hypothesized Late Cenozoic suture due to closure of the Caucasus Basin must lie along this structure or another to the south. The transient Paleozoic-Mesozoic magmatic and depositional episodes described here may have guided the unusual location of this Late Cenozoic suture along the southern flank of the Greater Caucasus and thus driven the concentration of shortening within the upper plate of the orogen.