New Constraints for the On‐Shore Makran Subduction Zone Crustal Structure

Abstract

Abstract: The Makran Subduction Zone is the primary seismic/tsunami hazard of the northwestern Indian Ocean, but little is known of its on‐shore seismic structure. We derived a shear wave velocity model extending to > $ > $ 100 km depth beneath a ∼400 km‐long seismic profile oriented parallel to the convergence vector of the Arabian Sea Plate. Receiver function/surface wave analysis shows that the average structure in the coastal region comprises a ∼22–28 km‐thick low wavespeed sedimentary cover and a 6–8 km‐thick gradient zone overlying > $ > $ 100 km‐thick high wavespeed upper mantle. The ocean‐basement interface dips gently northward, remaining a positive impedance contrast to ∼50 km depth at ∼250 km north of the coast where it disappears as the basaltic/gabbroic oceanic crust has probably transformed to eclogite. Further north, a weak arrival at ∼5 s in the receiver functions appears, grading northward into the Moho arrival of the continental Iranian Plateau. This disruption in the seismic signature of the Moho occurs in the forearc region where the dip of the subducting oceanic plate steepens. The southern Iranian Plateau's continental crust has an average V s of 3.55 ± 0.05 km s−1, an almost flat Moho 40–45 km deep, and a sub‐Moho mantle V s of 3.75 ± 0.05 km s−1 in the 50–80 km depth range. Weak Moho conversions probably result from ∼20% serpentinization of peridotite in the mantle wedge. Receiver functions indicate a flat continental Moho – no crustal root beneath the high topography region of the volcanic belt, which therefore must be compensated by low upper mantle densities. The high V p /V s ratio observed for the mantle wedge suggests ∼1%–2% partial melt.