Shallow crustal structure in the northwestern Iranian Plateau and its tectonic implications

The crustal structure of the Iranian Plateau bears important information about the details of the tectono-magmatic processes associated with the Neo-Tethys subduction and subsequent Arabia-Eurasia collision. Using a newly developed method of joint inversion of multi-frequency waveforms around and horizontal-to-vertical (H/V) ratios of the direct P arrivals in teleseismic P-wave receiver functions, we construct the shear-wave velocity image of the shallow crust (from surface up to 10-km depth below sea level) along a dense seismic array across the Zagros suture in the northwest Iranian Plateau. The most striking structural feature of the study region is the presence of low- and high-velocity anomalies (LVAs and HVAs) beneath the Zagros fold-and-thrust belt and the Iranian continent, respectively, indicating strong structural differences on the two sides of the suture. Systematic analysis on the velocity estimates and comparison with laboratory measurements and regional geology suggest that the LVAs and HVAs are representatives of Zagros sedimentary rocks and arc to intraplate magmatic rocks, respectively. The LVAs (1.3-2.0 km/s) are characterized by a series of faulted anti-form structures at ~1-7 km depths beneath Zagros. They are likely dominantly composed of shales and mudstones, and could have acted as mechanically weaknesses to accommodate different deformations of surroundings and give rise to the present-day depth-dependent seismicity. The HVAs beneath the central domain and Alborz in the Iranian continent present large ranges in both velocity (3.2-3.9 km/s) and depth (0-10 km), probably suggesting strong lithological variations in these areas. Most of the HVAs above 5-km depth have shear-wave velocities of 3.2 to 3.6 km/s, comparable to those of andesites and basalts dominated in the northwestern Iranian plateau. The deeper HVAs (below 5-km depth), which generally have greater velocities ~3.6-3.9 km/s falling into the velocity range of intrusive rocks such as granodiorites, diorites and diabases, appear to have much larger volumes at depth than that exposed on the surface in the study region. Moreover, the surface projections of the HVAs are spatially coincident with the major faults or tectonic boundaries of the region, suggesting a causal link. Our observations provide evidence for not only the lithology-controlled layering in both sedimentary structure and deformation in the Zagros passive margin but also the much more substantial magma generation and emplacement at depth than faulting-facilitated eruption and exposure on the surface in the Iranian active margin during the subduction and collision processes.