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Far-field Tectonic Effects of the Arabia-Eurasia Collision in Anatolia and the Caucasus

Abstract

Mechanical coupling of a collisional orogen and its forelands can induce far-field tectonic stresses and produce significant deformation at distances > 1,500 km from a collision front. Localization of strain far from the collision zone is controlled by spatial and temporal strength variations of the lithosphere, as shown by Peter Ziegler's work. The Bitlis-Zagros orogenic belt of western Asia and the related wide area of deformation within the European foreland to the north is one of the best examples of ongoing continental collision in the world. Present-day reduction of surface area within the collision zone is estimated at 31 × 103 km2/My. Most of the decrease in surface area is being accommodated by coherent westward transport of Anatolia out of the collision zone (approx. 70%) and by shortening along the Bitlis-Zagros and Greater Caucasus orogenic wedges (approx. 15%). The remaining decrease in surface area is distributed across the Anatolian-Iranian plateau and the Lesser Caucasus. A new low-temperature thermochronometric dataset for the Eurasian foreland north of the Bitlis collision zone (i) indicates that present-day crustal dynamics is not representative of the longer-term deformation pattern and (ii) provides an evolutionary template for the sedimentary basins in the area. Two successive stages of Neogene deformation can be inferred. (i) During the Early-Middle Miocene continental deformation was concentrated along the collision zone but tectonic stress was transferred northward across eastern Anatolia, focusing along the eastern Black Sea rheological transition. The Black Sea (quasi)oceanic lithosphere is fundamentally stronger than the polydeformed continental lithosphere to the south and therefore represented a “backstop” resisting deformation and deviating the impinging continental lithosphere. (ii) Since late Middle Miocene time the activation of the North and Eastern Anatolian Fault systems have reduced efficient northward stress transfer. In this new tectonic regime most convergence has been accomodated by the westward motion of Anatolia whereas the eastern Pontides have been mechanically decoupled from the foreland of the collision zone, as shown by the absence of significant seismicity in the area. The following regional-scale topographic uplift of the Anatolian Plateau has not produced enough exhumation to be recorded by low-temperature thermochronometric methods.