A CRYPTIC CONTINENT-CONTINENT SUTURE IN THE CAUCASUS? CLOSURE OF RELICT OCEAN BASINS MAY ACCOMMODATE CONTINENTAL CONVERGENCE WITH MINIMAL CRUSTAL SHORTENING

Current budgets of crustal shortening in several active orogens are hundreds to thousands of kilometers smaller than known amounts of post-collisional plate convergence determined from plate reconstructions. For example, McQuarrie and van Hinsbergen, (2013) document a 220-420 km deficit in crustal shortening since 35 Ma within the eastern Arabia-Eurasia collision. It has proven challenging to identify the structural systems responsible for absorbing such deficits, leading to a number of proposed explanations, such as younger collisional ages than indicated by geologic observations or subduction of thinned continental or oceanic crust on the leading margin of incoming continents. A potential additional mechanism for balancing such deficits may be the closure of modestly sized (<500 km wide) relict ocean basins trapped within the interior of the overriding plate in the early stages of continental collision. Here we suggest on the basis of regional geology and detrital zircon geochronology that the Greater Caucasus Mountains, at the northern margin of the Arabia-Eurasia collision zone, record the closure of such a relict ocean basin. In particular, we show that detrital zircon populations from Mesozoic flysch on opposite sides of the Greater Caucasus define two distinct provenance domains that are juxtaposed along a north-dipping system of range-parallel thrusts in the interior of the range. These two domains most likely represent the former margins of a relict back-arc basin that was originally contiguous with the eastern Black Sea and South Caspian Basin, and that was subsequently consumed by underthrusting along its northern margin starting in the Eocene. This eventually led to ~5 Ma collision of the Lesser Caucasus with the southern margin of Scythia to form the Greater Caucasus Mountains and a hidden suture within them. This hypothesis is further supported by a new synthesis of regional seismicity indicating the presence of a ~150 km deep, north dipping slab beneath the central and eastern Greater Caucasus. Relict basins, such as the one consumed within the Greater Caucasus, likely form along the margin of overriding plates during protracted subduction and terrane accretion prior to continental collision at the end of a Wilson cycle, and thus may represent a common occurrence in the early stages of orogeny.