Seismic imaging of the lithospheric structures in the Iranian Makran subduction zone

The Makran subduction zone (MSZ) is located in between the Zagros mountain belt to the west and Himalayan orogen to the east, forming a transition from oceanic subduction to continental collision on both sides along the Tethyan orogenic belt. The Arabian oceanic plate, a narrow remnant of the Neotethys ocean, is subducting northward beneath the Eurasian plate in Makran. Such a unique tectonic setting makes the MSZ an ideal place to investigate the geodynamic processes in response to subduction-collision transition. Since most of the Neotethys has already dived into the deep mantle and the associated geological records are not always well preserved due to the strong collision, the MSZ also provides a special opportunity to explore the evolution history of the Neotethys in a more direct way.

To better understand the deep dynamics of the subduction-collision transition and evolution of the Neotethys, we investigated the lithospheric structure, especially the depth variation of the lithosphere-asthenosphere boundary (LAB), across the Iranian MSZ by S-wave receiver function (SRF) imaging. The teleseismic data used were acquired from 67 broadband stations that were operational from March 2017 to September 2018 in southeastern Iran. This temporary array constitutes the third phase of seismic observations under the “China-Iran Geological and Geophysical Survey in the Iranian Plateau” project.

Our SRF migration images show clear structural variations of both the upper and lower plates in the MSZ. In the upper plate in the southeastern Iranian plateau, we image a thin lithosphere (70-90 km) with monotonic decrease in LAB depth from the plateau interior to the arc region. This arc-ward thinning is probably caused by the focused thermal and chemical erosion at the LAB by arc magmatism. The LAB of the subducting slab is imaged at ~110-90 km depth near the coast but with an unexpected ~20-km deepening along the trench-parallel direction. Assuming a 25-km-thick accretionary wedge (deduced from active-source data), the observed ~85-65-km-thick slab is consistent with the thermal predictions for a mature oceanic lithosphere. However, the trench-parallel LAB step can hardly be explained by the age difference of the Neotethys but may be a result of the Cretaceous plate-mantle plume interaction. The plume-modified slab could be characterized by low density and high viscosity, and thus play an important role in forming low-angle (<10°) subduction beneath the present-day Makran fore-arc region. Our results also suggest that the thin overriding lithosphere is a persistent feature in both the MSZ and the neighboring continental collision/subduction zone, which favors the idea that the vertical-axis rotation and possible convective thinning dominate the evolution of central-east Iranian microblocks during the late Cenozoic. In addition, we detect an east-dipping structure at 70-90 km depth beneath the Zagros-Makran border, perhaps indicating a relatively sharp contact relationship between the oceanic and continental portions of the Arabian plate. These new observations imply a much more complex tectonic evolution than previously envisaged in the MSZ and adjacent subduction-collision transitional area, which deserves future studies to understand the continuous process from Neotethys subduction to continental collision.