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

• Published in: Journal of geophysical research. Solid earth 2022-01, Vol.127 (1), p.n/a
• Main Authors: Priestley, Keith, Sobouti, Farhad, Mokhtarzadeh, Rahil, Irandoust, Mohsen, Ghods, Reza, Motaghi, Khalil, Ho, Tak
• Format: Article
• Language: English
• Subjects: accretionary prism; Belts; Coastal structures; Coastal zone; Continental crust; continetal crust; Crustal structure; Depth; Earthquakes; Eclogite; Immersion plating; Isotopes; Lava; Makran subduction zone; mantle wedge; Moho; Oceanic crust; Oceans; Peridotite; Plates (tectonics); S waves; Sediments; Seismic activity; Seismic analysis; Seismic profiles; Seismic velocities; Serpentinization; Shear wave velocities; Subduction; Subduction (geology); Subduction zones; Surface water waves; Surface waves; Topography; Tsunami hazard; Tsunamis; Upper mantle; Velocity; Volcanic belts; Wave analysis; Wave velocity; Weather hazards; Wedges
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2021JB022942

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 Vs of 3.55 ± 0.05 km s−1, an almost flat Moho 40–45 km deep, and a sub‐Moho mantle Vs 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 Vp/Vs ratio observed for the mantle wedge suggests ∼1%–2% partial melt. Plain Language Summary The Makran Subduction Zone is the primary seismic/tsunami hazard of the northwestern Indian Ocean, but little is known of its on‐shore structure. Although earthquake rates in the Makran are generally low, there have been historic destructive earthquakes accompanied by tsunamis. The poor knowledge of the Makran structure makes it difficult to evaluate the seismic and tsunami potential. From mid‐2016 to late 2020 we operated seismic stations along a ∼400 km‐long profile crossing the Makran Subduction Zone. Our analysis shows a shallow‐dipping oceanic crust beneath thick sediments near the coast. To the north, there is a normal continental crust, but the underlying mantle has a very low velocity. In the transition region, the seismic signature is disrupted in the forearc region where the d