Atypical crustal structure of the Makran subduction zone and seismotectonic implications

Abstract

The Makran subduction zone is atypical due to the thick sediment input and shallow subduction angles, and thus is highly susceptible to megathrust earthquakes. However, the thick sedimentary layers and their underlying crustal structure of the Makran slab are still poorly understood. Here we present a high-resolution crustal velocity model of the Makran subducting slab offshore Pakistan obtained from an active-source wide-angle ocean bottom seismic experiment, revealing fine structure in density and porosity. The results reveal that the incoming sedimentary layer is up to 8.5 km thick with an abrupt gradient change in porosity at 4–5 km depth, indicating a critical depth of compaction and consolidation. The igneous crust is 6–12 km thick with an average subduction dip angle of ∼2°. On either flank of the Little Murray Ridge, divergent crustal structures suggest the plausible existence of distinct tectonic provenances or divergent tectonic evolutionary trajectories for the oceanic crust on each side. The Little Murray Ridge may represent a remnant of the paleo-oceanic boundary, possibly associated with the phenomenon of low-density underplating. The subducting consolidated sediments and uppermost crust with high-water content could significantly influence earthquake rupture mechanisms at both the décollement and igneous basement interfaces. These findings suggest that the décollement and igneous basement interface both have the potential to trigger large earthquakes in the Makran subduction zone.