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

Abstract

Summary We present the results of a shear wave splitting analysis performed on the teleseismic SK(K)S and direct S wave recordings of 68 temporary broadband stations to investigate the mantle deformation on the northern side of the Arabia-Eurasia collision zone in NW Iran. We used the Reference Station Technique to overcome potential contamination from the source-side anisotropy on the direct S-wave signals. This method enabled us to expand our splitting measurement database beyond the usual SK(K)S phases. The average splitting delay time over the entire region was found to be 1.14 ± 0.42 s for the SK(K)S wave and 1.36 ± 0.26 s for the direct S wave. In most parts of the study area, the fast polarization directions for both shear phases are consistent and show a uniform NE-SW direction with an average of 36° and 37° for SK(K)S and S wave-derived results, respectively. This direction is in close agreement with the direction of the absolute plate motion vector in NW Iran (N39° E). The fast directions are associated with neither the surface geological trends, nor the geodetic strain fields. We propose that the observed anisotropy is mainly controlled by the LPO fabric developed due to the shearing of the asthenospheric layer in response to the motion of the lithosphere relative to the deeper mantle. Only in a narrow region near the tectonic boundaries of central Iran with NW Iran and the Alborz, NW-SE oriented SK(K)S fast directions tend to align with the major geological structures. Fast directions obtained from direct S-wave indicate significantly smoother variations in the same regions and mostly continue to be aligned in the NE-SW direction. We attribute these differences to the change in the structure of the lithosphere in the tectonic boundary zone. The western margins of central Iran possess a strong deformational fabric as evidenced by the major active strike-slip zones there. Considering that the depth extent of this fabric expands over a relatively narrow zone in the mantle, it can locally influence the SK(K)S phases. The direct S-waves, on the other hand, have a larger footprint and therefore average over a larger region, and relative to the SK(K)S phases, are influenced more strongly by the asthenospheric fabric due to their larger angles of incidence, which results in a larger zone of influence for station average anisotropy parameters.

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

“…In the western Alborz, the orientation of anisotropy changes from NW‐SE at shallow depths to NE‐SW near the crust‐mantle boundary. In the mantle lithosphere and the sub‐lithosphere, the rocks have a consistent NE‐SW oriented anisotropy throughout, as the agreement between the SKS directions (Arvin et al., 2021) and the Rayleigh wave directions suggest. Likewise, the SCB also has a variable fabric of deformation inside its crust since a significant variation of anisotropy with depth is observed.…”

“…The SKS fast directions calculated over the Plateau show variable patterns (Arvin et al., 2021; Gao et al., 2022; Kaviani et al., 2009, 2021; Sadeghi‐Bagherabadi et al., 2018). Beneath NW Iran and western Alborz where the lithosphere is not very thick, the SKS fast axes broadly trend NE‐SW, subparallel with APM vectors (Arvin et al., 2021; Kaviani et al., 2021; Sadeghi‐Bagherabadi et al., 2018). Over vast regions in central, eastern, and northeastern Iran, the SKS fast directions show complex and variable patterns, in many places unrelated to APM vectors (Gao et al., 2022; Kaviani et al., 2021), and in the Zagros they change from range‐parallel to range‐perpendicular over short distances (Kaviani et al., 2020; Priestley et al., 2012; Sadeghi‐Bagherabadi et al., 2018).…”

“…The SKS fast directions calculated over the Plateau show variable patterns (Arvin et al., 2021; Gao et al., 2022; Kaviani et al., 2009, 2021; Sadeghi‐Bagherabadi et al., 2018). Beneath NW Iran and western Alborz where the lithosphere is not very thick, the SKS fast axes broadly trend NE‐SW, subparallel with APM vectors (Arvin et al., 2021; Kaviani et al., 2021; Sadeghi‐Bagherabadi et al., 2018).…”

Shallow Crustal Response to Arabia‐Eurasia Convergence in Northwestern Iran: Constraints From Multifrequency P‐Wave Receiver Functions

“…The Iranian Plateau has witnessed significant Cenozoic tectono‐magmatism as a consequence of the long‐standing Arabian‐Eurasian convergence (Agard et al., 2011; Hatzfeld & Molnar, 2010), which is thus an ideal area to study the shallow crustal response to subduction and collision. Numerous investigations have been conducted in the Iranian Plateau, providing key insights into the deformation mechanisms either near the surface or at the crustal or lithospheric scales (e.g., Arvin et al., 2021; Casciello et al., 2009; Chu, Allen, et al., 2021; Chu, Wan, et al., 2021; Dashti et al., 2020; Gao et al., 2022; Guest et al., 2006; Kaviani et al., 2020; Paul et al., 2006, 2010; Pilia et al., 2020; Rahmani et al., 2019; Wu et al., 2021). However, studies on the shallow crustal structures of the plateau are still lacking, and the existing models provide only limited information on the shallow crustal deformation because they are either dedicated to the specific interface structures or suffering from a low resolution (e.g., Dashti et al., 2020; Maheri‐Peyrov et al., 2020; Teknik & Ghods, 2017).…”