PN VELOCITIES BENEATH CONTINENTAL COLLISION ZONES - THE TURKISH-IRANIAN PLATEAU

Tomographic imaging of the uppermost mantle velocity in the Middle East shows normal Pn velocities (8.0-8.2 km s-1) beneath the Black Sea and the southern Caspian Sea. In contrast, low Pn velocities (<7.9 km s-1) are found beneath the Aegean Sea and much of the Turkish-Iranian Plateau. A region of exceptionally low Pn velocities (<7.6 km s-1) beneath the Lesser Caucasus Mountains and along the Turkish-Iranian border coincides with a region of high Sn attenuation and extensive Neogene volcanism. All these features suggest that near solidus conditions exist within the uppermost mantle beneath the Turkish-Iranian Plateau. Such conditions may result either from the decompression melting associated with upwelling convection cells or by the infiltration of water released from subducted lithosphere into the mantle above. The presence of a partially melted uppermost mantle weakens the lithosphere beneath the Turkish-Iranian Plateau, thus allowing it to become the locus of deformation in the Arabian-Eurasian collision zone.