THE SEISMIC STRUCTURE OF THE EURASIAN-ARABIAN COLLISIONAL BELT

Continent-continent collision plays a critical role in the development and growth of the continents on earth, yet it is one of the least understood processes in plate tectonic theory. Recent seismic experiments in the Anatolian and Iranian plateaus, the archetypal example of young continent-continent collision, have begun to yield a clearer picture of the lithospheric structure along this plate boundary. The collision between the Arabian and Eurasian plates along the Zagros thrust zone is generally thought to have begun after the final closure of the Neo-Tethys in the Miocene. Recent investigations within Eurasian/Arabian plate margin have demonstrated that the majority of Iranian and Anatolian continental plateaus are underlain by an uppermost mantle low velocity zone (LVZ) that is also characterized by low seismic attenuation factors (Q). Preliminary models suggest that this LVZ is not connected with a deeper anomaly in the upper mantle beneath the Anatolian and Iranian plateaus. The location of the LVZ is consistent with the location of Quaternary volcanics and is similar to that observed beneath the central and northern Tibetan plateau; it may be the remnant effect of subduction of the Neo-Tethys oceanic lithosphere. Crustal structure along the Eurasian-Arabian plate boundary is generally characterized by an increase in crustal thickness. This can be inferred from the blockage of Lg waves that cross the plate boundary. Furthermore, recent receiver function results within the Iranian and Anatolian plateau offer some evidence of increase in crustal thickness; however, the values are smaller than expected, given the average elevation of these continental plateaus. The negative Bouguer gravity anamolies in the region are consistent with the location of tomographically mapped uppermost mantle LVZ. We suggest that much of these continental plateaus are supported dynamically by the less dense uppermost mantle beneath the plateaus. Also, accurate measurements of earthquake hypocenters within the plateau suggest that there is little or no subcrustal seismicity and that the majority of the earthquakes occur within the upper 20 km of the crust; this is consistent with their being no continental subduction or underthrusting beneath these plateaus.