­Crustal Structure of the Western Anatolian Extensional Province: Evidence for a ductile lower crust through the joint inversion of Receiver Functions and Dispersion Data

Monday, 15 December 2014
Jonathan R Delph1, Metin Kahraman2, George Zandt1, Susan L Beck1, Arda A. Ozacar3 and Niyazi Turkelli2, (1)University of Arizona, Tucson, AZ, United States, (2)Kandilli Observatory, Istanbul, Turkey, (3)Middle East Technical University, Ankara, Turkey
Western Anatolia is one of the most seismically active regions in the world, undergoing broad N-S extension associated with the rollback of the Aegean slab in the south. This region is characterized by high heat-flow, recent volcanism, and core-complex formation. We use >3500 receiver functions from a dense array of 47 stations located in western Turkey and dispersion data from a recent regional-scale, ambient noise tomography study to invert for shear-wave velocity as a function of depth. Using this technique, we obtain an unprecedented three-dimensional shear-wave velocity model that characterizes the crust of western Turkey.

We find a sharp Moho transition beneath much of western Anatolia, with crustal thickness varying from ~25 km near the Aegean Sea to ~35 km beneath the Fethiye Lobe correlating with the topography in western Turkey. The lower crust exhibits a relatively continuous ~10-15 km thick low shear-wave velocity layer (LVL) beneath most of western Anatolia, reaching velocities below 3.0 km/s in some places. The top of this LVL marks the lower boundary of crustal seismicity. These characteristics suggest that the lower crust in western Anatolia is behaving as a ductile solid, as seismic velocities in this range at lower crustal depths are indicative of the presence of fluids (possibly partial melts when considering the high heat-flow in the region). We propose that the lower crust in this region may be undergoing local crustal flow due to the N-S dominated stress regime, which led to the formation of the sharp, low-relief Moho observed in western Turkey.

Across the Fethiye-Burdur Fault Zone, we see the slowest S-wave velocities in the lower crust, reaching ~2.8 km/s. Unlike elsewhere in western Turkey, this region has relatively low surficial heat-flow and no recent volcanism. Therefore, the anomaly in the Fethiye Lobe is likely due to a different mechanism than western Turkey. We also observe a very slow uppermost mantle beneath this region, with shear-wave speeds < 4.2 km/s. We attribute this anomaly to subduction related processes, such as the introduction of aqueous fluids from the underthrusting of thick, hydrated sediments or alternatively, to possible serpentinization of the upper-mantle. Mantle wavespeeds transition to more typical velocities across the Fethiye-Burdur Fault Zone.