Three-dimensional Seismic Velocity Model for Upper-Most Mantel of Zagros Collision Zone Using Full Wave Form Inversion

We report on the full three-dimensional (3D) waveform model for seismic velocity structurebeneath the Zagros collision zone by analyzing phase measurements. We use entire seismograms from59 earthquakes and follow a multiscale approach for periods between 20 and 80 seconds. We startedwith the first generation of the Collaborative Seismic Earth Model and applied the adjoint method andthe Broyden–Fletcher–Goldfarb–Shanno (L-BFGS) optimization algorithm to reconstruct theuppermost mantel seismic velocity structure. The Zagros collision zone consists of the margin of theArabian platform - the Zagros Fold and Trust Belt - and the margin of the Eurasian Plate - the Iranianmicroplates. The retrieved model reveals strong shear wave velocity contrast around 180 km depthalong the Zagros mountain belt at the surface is a piece of evidence that the deformation of thetransition zone along the Zagros is stopped. We interpreted that interaction between two continentallithospheres is terminated at this depth. We observed a relatively low-velocity sub-crustal lithospherewith relatively high velocities structures beneath the Central Iran and Lut bock at 80-150 km depthsand a strong one throughout the Arabian lithosphere at 70-200 km depths. It explains continentalcollision caused earlier thickening during the convergence of the Arabian platform toward thenortheast. This observation indicates the lithosphere of Iranian microplates has a relatively warmstructure and non-uniform distribution of a sharp velocity contrast between this structure and thestrong low-velocity structure underlying it, showing the boundary between the lithosphere andasthenosphere (LAB). Our results indicate this boundary is located at 119 km depth on average. Incomparison, we observe a thickened and cold lithosphere for the margin of the Arabian lithosphere.