Three-dimensional waveform model of seismic speed for uppermost lithosphere of the Arabian-Eurasian collision zone using full waveform inversion

We report on the first full three-dimensional (3D) waveform model for seismic velocity structure beneath the Zagros collision zone by analyzing phase measurements. We use entire seismograms from 59 earthquakes and follow a multiscale approach for periods between 20 and 80 seconds. We started with the first generation of the Collaborative Seismic Earth Model and applied the adjoint method and the Broyden-Fletcher-Goldfarb-Shanno (L-BFGS) optimization algorithm to reconstruct the uppermost mantel seismic velocity structure. The Zagros collision zone consists of the margin of the Arabian platform - the Zagros Fold and Trust Belt - and the margin of the Eurasian Plate - the Iranian microplates. The retrieved model reveals strong shear wave velocity contrast around 180 km depth along the Zagros mountain belt at the surface is a piece of evidence that the deformation of the transition zone along the Zagros is stopped. We interpreted that interaction between two continental lithospheres is terminated at this depth. We observed a relatively low-velocity sub-crustal lithosphere with relatively high velocities structures beneath the Central Iran and Lut bock at 80-150 km depths and a strong one throughout the Arabian lithosphere at 70-200 km depths. It explains continental collision caused earlier thickening during the convergence of the Arabian platform toward the northeast. This observation indicates the lithosphere of Iranian microplates has a relatively warm structure and non-uniform distribution of a sharp velocity contrast between this structure and the strong low-velocity structure underlying it, showing the boundary between the lithosphere and asthenosphere (LAB). Our results indicate this boundary is located at 119 km depth on average. In comparison, we observe a thickened and cold lithosphere for the margin of the Arabian lithosphere.