Upper mantle S-velocity structure and Moho depth variations across Zagros belt, Arabian–Eurasian plate boundary

Abstract

The collision of the Arabian and Eurasian plates in the early Miocene, after the subduction of Neo-Tethys ocean beneath Eurasia, formed the Zagros belt, a seismically active continental-continental plate boundary in southwest Iran. The Zagros suture zone is an important seismotectonic boundary indicating an abrupt cutoff between the intense seismicity of the Zagros and the almost aseismic Central Iran plateau. Compared with other more evolved plate boundaries, little is known about the deep lithospheric structure of the Zagros belt, such as the fate of subducted Neo-Tethys plate and ambiguity in the presence of intermediate and deep earthquakes under the Zagros suture zone. In this study, we use the partitioned waveform inversion (PWI) method to image the upper mantle S-velocity structure and Moho depth variations across Zagros collisional zone. The resulting Moho depth along the profile shows the average Moho depth value of order of 40–45 km for most parts of the profile with abrupt crustal thickening in the middle of the profile up to about 65 km. As expected, the derived models show that the relatively old and cold Arabian plate has higher velocity at depth than the younger lithosphere farther north in Central Iran. A sharp and steep subcrustal boundary is found roughly coincident with the surficial expression of the Main Zagros Thrust (MZT), separating two different mantle domains. A high-velocity anomaly, possibly representing a fragment of subducted lithosphere, has been imaged beneath Central Iran at a depth between 350 and 600 km, which is quite similar to the case of Indo-Asian collision in Qinghai-Tibetan plateau. These observations as well as the sudden changes of shallow velocities along the cross-section with a sharp boundary under the Zagros suture zone support the idea that subducted oceanic lithosphere has broken-off under the region of maximum Moho depth. A relatively low velocity region beneath the Arabian plate is also imaged in our high resolution tomography model which is interpreted as an indication of lithospheric delamination within the Arabian lithosphere.