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

Tomographic imaging of the uppermost mantle velocity in the Middle Eas t shows normal Pn velocities (8.0-8.2 km s-1) beneath the Black Sea an d 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 Pl ateau. A region of exceptionally low Pn velocities (<7.6 km s-1) benea th the Lesser Caucasus Mountains and along the Turkish-Iranian border coincides with a region of high Sn attenuation and extensive Neogene v olcanism. All these features suggest that near solidus conditions exis t within the uppermost mantle beneath the Turkish-Iranian Plateau. Suc h conditions may result either from the decompression melting associat ed with upwelling convection cells or by the infiltration of water rel eased from subducted lithosphere into the mantle above. The presence o f a partially melted uppermost mantle weakens the lithosphere beneath the Turkish-Iranian Plateau, thus allowing it to become the locus of d eformation in the Arabian-Eurasian collision zone.