Comparison of plate convergence with the timing and magnitude of upper crustal shortening in collisional orogens indicates both shortening deficits (200–1700 km) and significant (10–40%) plate deceleration during collision, the cause(s) for which remains debated. The Greater Caucasus Mountains, which result from postcollisional Cenozoic closure of a relict Mesozoic back‐arc basin on the northern margin of the Arabia‐Eurasia collision zone, help reconcile these debates. Here we use U‐Pb detrital zircon provenance data and the regional geology of the Caucasus to investigate the width of the now‐consumed Mesozoic back‐arc basin and its closure history. The provenance data record distinct southern and northern provenance domains that persisted until at least the Miocene. Maximum basin width was likely ~350–400 km. We propose that closure of the back‐arc basin initiated at ~35 Ma, coincident with initial (soft) Arabia‐Eurasia collision along the Bitlis‐Zagros suture, eventually leading to ~5 Ma (hard) collision between the Lesser Caucasus arc and the Scythian platform to form the Greater Caucasus Mountains. Final basin closure triggered deceleration of plate convergence and tectonic reorganization throughout the collision. Postcollisional subduction of such small (102–103 km wide) relict ocean basins can account for both shortening deficits and delays in plate deceleration by accommodating convergence via subduction/underthrusting, although such shortening is easily missed if it occurs along structures hidden within flysch/slate belts. Relict basin closure is likely typical in continental collisions in which the colliding margins are either irregularly shaped or rimmed by extensive back‐arc basins and fringing arcs, such as those in the modern South Pacific.

[1] Constraining the timing of onset and rates of deformation within the Greater Caucasus mountains is key to understanding their role in accommodating deformation across the Arabia-Eurasia orogen. We present new low-temperature thermochronometric constraints on the Cenozoic thermal evolution of the central Greater Caucasus that elucidate a three-phase cooling history. Between 50 and 30 Ma, cooling within the range was negligible. In Oligocene time, cooling rates throughout the range increased to ∼4°C/Myr. These rates remained constant until the early Pliocene time, when they increased again, reaching ∼25°C/Myr along the axial part of the range. Rates and timing of Oligocene exhumation are consistent with previous results from the western Greater Caucasus and are proposed to result from onset of subduction of the Greater Caucasus back-arc basin. Rapid exhumation of the Greater Caucasus, beginning in Pliocene time, contrasts with previously reported thermal histories for other portions of the range. Pliocene exhumation of the central Greater Caucasus appears to be tectonically driven and coincides with widespread evidence for a major reorganization of the Arabia-Eurasia plate boundary. We hypothesize that this exhumation, and regionally observed plate reorganization, results from the collision of the Lesser Caucasus with Eurasia, completing the subduction of oceanic lithosphere across this segment of the Arabia-Eurasia plate boundary.

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[1] New detrital low-temperature thermochronometry provides estimates of long-term erosion rates and the timing of initiation of river incision from across the interior of the Tibetan Plateau. We use the erosion history of this region to evaluate proposed models of orogenic development as well as regional climatic events. Erosion histories of the externally drained portion of the east-central Tibetan Plateau are recorded in modern river sands from major rivers across a transect that spans >750 km and covers a region with no published thermochronometric ages. Individual grains from eight catchments were analyzed for apatite (U-Th)/He and fission track thermochronometry. A wide distribution in ages that, in most cases, spans the entire Cenozoic and Late Mesozoic eras requires a long period of slow or no erosion with a relative increase in erosion rate toward the present. We apply a recently developed methodology for inversion of detrital thermochronometric data for three specified erosion scenarios: constant erosion rate, two-stage erosion history, and three-stage erosion history. Modeling results suggest that rates increase by at least an order of magnitude between 11 and 4 Ma following a period of slow erosion across the studied catchments. Synchroneity in accelerated erosion across the whole of the Tibetan Plateau rather than a spatial or temporal progression challenges the widely held notion that the plateau evolved as a steep, northward-propagating topographic front, or that south to north precipitation gradients exert a primary control on erosion rates. Instead, we suggest that accelerated river incision late in the orogen's history relates to regional-scale uplift that occurred in concert with eastern expansion of the plateau.