3D seismic imaging of P–wave velocity structure for upper–most mantle of the Zagros collision zone using full waveform inversion

Convergence between Arabian and Eurasian plates since the onset of subducting of Neo-Tethys ocean beneath Eurasia in Jurassic (Berberian et al. 1981) until closure in the late Cretaceous (Agard et al. 2005), then continental-continental collision in the late Eocene to Oligocene (Agard et al. 2011) 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 is important from the Zagros collision zone point of view. For this, we need a new tomographic model of the lithospheric mantle beneath Zagros, coming from avoiding artifacts in traditional methods, which is the motivation of this study. We report on the first fully three-dimensional (3D) waveform model for P–wave velocity structure beneath the Zagros collision zone by analyzing phase measurements.We used regional events which occurred inside the border of the country of Iran. They were recorded between 2012 to early 2016 by three-component sensors with 120s, 240s, and 360s cut-off periods. All used stations belonged to International Institute for Earthquake Engineering and Seismology (IIEES) and the Iranian Seismological Center (IRSC) in Iran. We also incorporated public seismograms from IRIS located in Turkey to satisfy the criterion. The explosion in computational power of the past decades has opened the door for use of the entire seismogram. Therefor, in this study, waveforms of 37 earthquakes were proccessed and we followed 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 (Fichtner et al. 2018) and applied the adjoint method and the Broyden–Fletcher–Goldfarb–Shanno (L-BFGS) optimization algorithm to reconstruct the upper–most mantle P-wave velocity structure. The Zagros collision zone consists of the margin of the Arabian platform ― the Zagros Fold and Thrust Belt ― and the margin of the Eurasian Plate ― the Iranian microplates. Unlike the required number data in tomography, using the full waveform inversion method was able to calculate a fully three-dimensional waveform model by a limited number of earthquakes, only for structures with long wavelengths. A strong velocity contrast is observed in the upper part of lithospheric mantle and its lower part throughout the Zagros collision zone, and we interpret it as the interaction of the mantle-lithosphere structure of the Iranian plateau with the Arabian platform during early stages of continent-continent collision after the end of subduction of the Netothetis oceanic plate, which indicates the difference in the extent of the higher-velocity structure; So the behavior of the convergence of the Arabian plate towards the Eurasian plate in the northern Zagros is different from the central Zagros. A high-velocity anomaly is resolved beneath the Lut block, and the anomalies in the P–wave velocity beneath the Sanandaj–Sirjan zone are interpreted as a suture zone between two tectonic structures of the southwestern margin of the Eurasian plate and the northeastern margin of the Arabian platform.