Ayoub Kaviani

Seismic imaging of the lithospheric structure of the Zagros mountain belt (Iran)

Abstract We present a synthesis and a comparison of the results of two temporary passive seismic experiments installed for a few months across the Central and Northern Zagros. The receiver function analysis of teleseismic earthquake records gives a high-resolution image of the Moho beneath the seismic transects. On both cross-sections, the crust has an average thickness of 42±2 km beneath the Zagros fold-and-thrust belt and the Central domain. The crust is thicker beneath the hanging wall of the Main Zagros Reverse Fault (MZRF), with a greater maximum Moho depth in the Central (69±2 km) than in the Northern Zagros (56±2 km). The thickening affects a narrower region (170 km) beneath the Sanandaj–Sirjan zone of the Central Zagros and a wider region (320 km) in the Northern Zagros. We propose that this thickening is related to overthrusting of the crust of the Arabian margin by the crust of Central Iran along the MZRF, which is considered as a major thrust fault cross-cutting the whole crust. The fault is imaged as a low-velocity layer in the receiver function data of the Northern Zagros profile. Moreover, the crustal-scale thrust model reconciles the imaged seismic Moho with the Bouguer anomaly data measured on the Central Zagros transect. At upper mantle depth, P-wave tomography confirms the previously observed strong contrast between the faster velocities of the Arabian margin and the lower velocities of the Iranian micro-blocks. Our higher-resolution tomography combined with surface-wave analysis locates the suture in the shallow mantle of the Sanandaj–Sirjan zone beneath the Central Zagros. The Arabian upper mantle has shield-like shear-wave velocities, whereas the lower velocities of the Iranian upper mantle are probably due to higher temperature. However, these velocities are not low enough and the low-velocity layer not thick enough to conclude that delamination of the lithospheric mantle lid has occurred beneath Iran. The lack of a high-velocity anomaly in the mantle beneath Central Iran suggests that the Neotethyan oceanic lithosphere is probably detached from the Arabian margin.

Generalization of the H‐κ stacking method to anisotropic media

We investigate the effect of anisotropy on estimates of crustal thickness H and average bulk VP/VS-ratio κ. Here we extend the stacking approach of Zhu and Kanamori (2000) to include all 20 P-to-S converted waves and their crustal reverberations that are generated in the anisotropic case—instead of only five phases in the isotropic case. The ray-based algorithm of Frederiksen and Bostock (2000) is used to calculate the amplitude and arrival time of each phase. Synthetic tests are performed to investigate the feasibility and robustness of the stacking approach. For simplicity, we assume hexagonal symmetry and a horizontal symmetry axis, but more general anisotropy may be considered. The tests reveal that the estimates of H and κ can be significantly affected by the presence of crustal anisotropy. We verify the feasibility of the stacking approach for real data by applying the method to examples from three different tectonic regions. The results show that the anisotropic stacking scheme presented here can provide a much better constraint on the estimation of H and κ than is achieved using isotropic stacking. Anisotropic stacking can also help resolve the ambiguity in determination of H and κ that arises when several maxima from stacking amplitudes of receiver functions occur in the case of complex crustal structure.