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

Abstract

AbstractThe Neo‐Tethys subduction and subsequent Arabia‐Eurasia continental collision invoked widespread Cenozoic tectono‐magmatism throughout the Iranian Plateau. We herein develop a new method to image the shallow crustal S‐wave velocity (Vs) structure by joint inversion of multifrequency waveforms and horizontal‐to‐vertical ratios around the direct P phase in P‐wave receiver functions. Synthetic tests demonstrate the validity of our method in constraining the absolute Vs values beneath single stations down to ∼12‐km depth. By applying this method to a seismic array of 63 stations with an average spacing of ∼10 km along the main profile, we construct a detailed shallow crustal Vs model across the northwestern Iranian Plateau. The model is characterized by distinct high‐ and low‐velocity anomalies beneath the Iranian hinterland and the Zagros foreland fold‐and‐thrust belt, respectively. In combination with geological observations and laboratory data, the imaged high‐velocity anomalies (with Vs of 3.2–3.9 km/s) may denote arc to intraplate magmatism beneath Central Iran and Alborz to the north, whereas the low‐velocity anomalies (with Vs of ∼1.65 km/s) probably represent the marl/shale layers in Late Cretaceous and Paleocene beneath Zagros. The magmatic rocks at the Iranian hinterland exhibit strong variations in absolute Vs, reflecting different bulk compositions with more mafic inland. The shale/marl layers could have acted as décollements to accommodate crustal deformation. Our observations underline both the key role of lithology‐controlled layering in sedimentary deformation at the Zagros fold‐and‐thrust belt and the change in compositions and forming‐environments of magmatic rocks at the Iranian hinterland.