SEISMIC IMAGES OF A YOUNG CONTINENT-CONTINENT COLLISION

The Eastern Turkey Seismic Experiment (ETSE) was designed to image the crustal and upper mantle velocity structure beneath the northernmost Arabian plate and the Anatolian Plateau, constraining geodynamic models for young continent-continent collision. A temporary 29 station broadband PASSCAL array was deployed from October 1999 until August 2001. The array consisted of 2 V-shaped transects, with sufficiently close station spacing to enable high-resolution 2-D imaging, and 10 interior stations, to improve control for long wavelength 3-D velocity structure and earthquake location within the plateau. Hypocentral locations from the first 9 months of deployment reveal a possible continuation of the East Anatolian Fault Zone (EAFZ) into the easternmost portion of the Anatolian Plateau. We found no evidence for sub-crustal earthquakes in eastern Anatolia which is consistent with earthquake depth distribution in the Iranian and Tibetan plateaus. A receiver function profile along the western transact of the ETSE array indicates that there is no significant crustal thickening across the EAFZ and within the Anatolian subplate. This indicates that the northward vergence of the Arabian plate is being largely accommodated by the westward extrusion of the Anatolian subplate. Our results from regional wave attenuation tomography and Pn travel time tomography imply a fundamental change in the properties of the uppermost mantle in the vicinity of the Bitlis suture and EAFZ. We observe very low Pn velocities and very high Sn attenuation beneath the easternmost Anatolian Plateau. These observations are best interpreted to indicate that the mantle lid beneath the eastern Anatolian plateau is anomalously hot and that the lithospheric mantle is very thin. Shear-wave splitting analysis indicates that there is no large change in upper mantle azimuthal anisotropy across the Bitlis suture and EAFZ. Comparisons with Pn anisotropy indicate that most of the shear-wave polarization anisotropy may be located in the asthenospheric mantle. Using upper mantle seismic anisotropy as a proxy for mantle flow, this would indicate that there is, to a large degree, decoupling of the lithospheric from asthenospheric deformation.