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

Keith Priestley,Khalil Motaghi,Tak Ho,Reza Ghods,Rahil Mokhtarzadeh,Mohsen A Irandoust,Farhad Sobouti

doi:10.1029/2021jb022942

Abstract

AbstractThe 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.

Highlights

The Makran Subduction Zone (Figure 1), which extends ∼1,000 km from the Strait of Hormuz in the west to Sonmiani Bay near Karachi in the east, results from the ongoing northward subduction of the Arabian Sea Plate beneath southeastern Iran and southwestern Pakistan
We have derived a shear wave velocity model that extends to a ∼100 km depth beneath a ∼400 km-long profile of seismographs oriented approximately parallel to the convergence vector and stretching from the Makran coast to a point ∼400 km north on the southern Iranian Plateau (Figure 1)
Modeling of receiver function and surface wave data from coastal sites shows the presence of a ∼22–28 km-thick low wavespeed sedimentary cover

Summary

Introduction

The Makran Subduction Zone (Figure 1), which extends ∼1,000 km from the Strait of Hormuz in the west to Sonmiani Bay near Karachi in the east, results from the ongoing northward subduction of the Arabian Sea Plate beneath southeastern Iran and southwestern Pakistan. It is characterized by a shallow, gently-dipping subduction interface (e.g., Kopp et al, 2000; Smith et al, 2012), a high sedimentation rate (e.g., Ellouz-Zimmermann, Deville, et al, 2007; McCall, 1997), an extremely broad accretionary prism which is largely sub-aerial and no bathymetric trench. Improved knowledge of the Makran Subduction Zone structure is a prerequisite in evaluating its tsunami potential

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Discussion

Conclusion