Full seismic waveform tomography for upper-most mantle structure in the Zagros seismotectonic provinces using adjoint and spectral-element methods.

Convergence between Arabian and Eurasian plates since the onset of subducting of Neo- Tethys ocean beneath Eurasia in Jurassic until closure in the late Cretaceous, then continental-continental collision in the late Eocene to Oligocene has continued to the present. Because of the convergence, the Zagros folded zone has formed. So it is a necessary geophysical constraint for assessing the geodynamic evolution of Zagros orogeny. Indeed, it has importance from the Zagros collision zone point of view. For this, we need a new tomographic model of the lithosphere beneath Zagros, coming from avoiding artifacts in traditional methods, which is the motivation of this study. 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 37 earthquakes and follow a multiscale approach (Bunks et al., 1995) 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).