Why lithospheric extension separated the Aegean from Turkey
Abstract:The Aegean Sea region in the eastern Mediterranean is one of the classic and best-studied extensional provinces. Inspired by recent 3D geodynamic models of laterally heterogeneous accretion during rollback we discuss the nature of the transition from the Aegean Sea basin (Hellenides) into the Anatolian plateau of west Turkey (Anatolides). The Hellenides and Anatolides experienced similar rates of convergence, but display remarkable differences in lithospheric structure. Whereas the Aegean is characterized by sustained high-pressure metamorphism followed by slab retreat since c. 60 Ma, a south verging greenschist-facies thrust-and-fold belt formed in the Anatolides since c. 45 Ma. Fission-track contour maps show that since c. 24 Ma extension in both regions evolved differently. Gravity data, earthquake locations and seismic velocity anomalies highlight a N–S oriented subvertical boundary in the upper mantle between a fast slab below the Aegean and a slow asthenospheric region below west Turkey, the West Anatolia Transfer Zone (WATZ). Our data support the hypothesis that the WATZ developed as a result of laterally inhomogeneous convergence along the boundary of the Adriatic and Anatolian lithospheres.
3D numerical simulations of laterally inhomogeneous convergence predict a similar evolution, where two distinct domains develop along strike: a region of distributed shortening where the systems gets congested by a microcontinent (Anatolides), and a region of extension associated with rollback of the active subduction zone (Hellenides). Strike-slip deformation concentrates perpendicular to the boundary of the two domains (WATZ). The numerical simulations also predict other salient features of regional geology and geodynamics, including the origin of a lithospheric window below west Turkey, local ocean floor topography, and the formation of the North Anatolian Fault zone. We argue that the seemingly complex tectonic evolution of the Aegean-Anatolian portion of the Tethyan orogen can be explained by a relatively simple model of continuous convergence of laterally heterogeneous lithosphere.