Lateral variation of crust and upper mantle structures in NW Iran derived from surface wave analysis

Abstract

To obtain the shear velocity structure across North-West of Iran and surrounding areas to a depth of 160 km, we performed a namely Hedgehog nonlinear inversion on Rayleigh wave group velocity dispersion curves in the period range from 7 to 60 s. The distributed dispersion curves are the results of our surface wave dispersion tomography using the data of 280 local and regional seismic events, recorded by the medium-and broad-band seismic stations in the region. We outline different crust and upper mantle structures for the study area based on calculated group and shear velocities. Our results reveal relatively low velocities at the shorter periods (7-10 s) in the presence of sedimentary basins (e.g., South Caspian Basin) and for eastern Anatolia and relatively high velocities along the Sanandaj-Sirjan Metamorphic zone, Alborz, Talesh, and the Lesser Caucasus Mountains. By depth inversion of group velocities, we observed 14-km-thick sediments in South Caspian Basin and Kura Depression. Based on our maps at 20 s, we outline different crustal models for the region and highlight the differences between South Caspian Basin and NW Iran, on one side, and the similarities between the South Caspian Basin and Kura Depression that extend beneath Talesh, Alborz, and Lesser Caucasus, on the other. Comparing the shear velocity of lower crust in South Caspian Basin and Kura Depression with that of NW Iran proves different origination of lower crust in the basin, probably oceanic source, because of its significant higher shear velocity rather than NW Iran. In Talesh, we observe indications of an under-thrusting of the lower crust of SCB beneath NW Iran while the middle crust is locked. The analysis of group velocities at longer periods (≥ 35 s) and obtained shear velocity models allows us to outline different lithospheric structures and crustal depth in the region. The high group velocities in Talesh, South Caspian Sea, and Lesser Caucasus on one side and Zagros Folding and Thrust Belt on the other, beside the result of shear velocity models, suggest the presence of a stable and thick mantle lid that seems to be thin or absent in the eastern Anatolia and much of NW Iran. The shallowest Moho

High‐Resolution Lithospheric Structure of the Zagros Collision Zone and Iranian Plateau

“…The deeper levels of the crust under the Talesh have a uniformly high wave-speed (V s ∼4 km s −1 , Figures 4c and 4d). The high-velocity anomaly in the lower crust (at about 30-40 km depth in Figure 6 profile A) extending from the SCB to just under the eastern flank of the Talesh Range was already reported by Bavali et al (2016) and Mortezanejad et al (2018). This high V s structure might delineate the basement slab of the SCB.…”

“…(2016) and Mortezanejad et al. (2018). This high V s structure might delineate the basement slab of the SCB.…”

“…Taghizadeh-Farahmand et al ( 2010) also reported the same eastward deepening of the Moho in NW Iran, but with a steeper slope (38.5-53 km). Based on V s changes with depth, Mortezanejad et al (2018) calculated a deeper Moho in the Talesh (53 km) and a shallower one in NW Iran and SCB (∼46 km).…”

Seismic anisotropy and mantle deformation in NW Iran inferred from splitting measurements of SK(K)S and direct S phases

“…Two body wave tomographic models using joint regional and teleseismic phases (Bavali et al 2016;Motaghi et al 2018) in NW Iran have shown a sharp contrast between high-velocities in the upper mantle under the SCB and low-velocities in the neighbouring NW Iran and western Alborz regions. Mortezanejad et al (2019) converted the Rayleigh wave group velocity dispersion data for shear wave velocity in NW Iran and derived a map of lithospheric thickness variations in our study area (Fig. 1).…”

“…The SK(K)S fast directions obtained from previous studies are also shown. Contour lines show variations of lithospheric thickness adopted from Mortezanejad et al (2019). The black and grey arrows are the plate motion vectors from Kreemer et al (2014) and GPS vectors (relative to stable Eurasia) from Khorrami et al (2019), respectively.…”