Study reveals double slab tearing at a subduction–collision transition

An international research team has used seismic imaging to identify two slab tears beneath the Zagros–Makran transition zone located in southeastern Iran near the Strait of Hormuz. The findings suggest that the tears formed through distinct geodynamic mechanisms and may have influenced regional tectonic deformation and magmatic activity.

The study was led by Profs. CHEN Ling and LAN Haiqiang at the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS), in collaboration with international researchers, and was published in Nature Geoscience on November 4.

The transition from oceanic subduction to continental collision is a fundamental process in plate tectonics, yet it is rarely observed directly. The Tethyan domain—a broad geological region extending from the Mediterranean into Southeast Asia—is one of the few places where these two processes occur simultaneously. Therefore, the Zagros–Makran transition zone, which falls within this area, is a valuable natural setting for studying this transition.

The researchers deployed a temporary broadband seismic array across the transition zone, comprising one main profile and five supplementary profiles. Using nearly 18 months of teleseismic waveform data collected between 2017 and 2018, they obtained approximately 57,000 P- and S-wave travel-time measurements across multiple frequency bands.

They then applied finite-frequency travel-time tomography to reconstruct the 3D seismic velocity structure of the upper mantle beneath the region.

The resulting images reveal striking differences in slab geometry and continuity between the eastern and western parts of the region. Beneath Makran in the east, the oceanic slab forms a roughly 300-km-long, low-angle segment beneath the forearc, then steepens beneath the volcanic arc and extends continuously to a depth of at least 450 km.

Beneath Zagros in the west, by contrast, the continental lithosphere reaches depths of only about 300 km. Below it lies a remnant of ancient oceanic slab, separated from the overlying continental slab by roughly 100 km. These contrasting structures place key constraints on slab tearing during the transition from oceanic subduction to continental collision.

For the tear beneath Zagros, the researchers propose that the originally connected oceanic and continental lithosphere entered the subduction zone in succession. The denser oceanic portion continued to sink into the mantle, while the more buoyant continental portion resisted further descent.

This differential motion initiated a nearly horizontal tear along the subducted ocean–continent transition, which then propagated laterally and eventually produced a slab window separating the shallower continental slab from the deeper oceanic slab.

A second tear lies near the junction between the Zagros–Makran transition boundary and the Makran trench. A low-velocity anomaly separates the adjacent oceanic and continental slabs there, suggesting a vertical slab tear.

Unlike conventional tearing models, which are dominated by trench retreat, the researchers propose that this tear was triggered by the continued northward advance of the Zagros collision front, which progressively lengthened the transition boundary. On the basis of regional tectonic evolution and plate-motion constraints, they infer that the vertical tear began about 8 million years ago and propagated roughly 200 km southward.

The study suggests that the two tears formed at different locations and with different orientations, jointly facilitating the progressive decoupling of the continental lithosphere to the west from the oceanic lithosphere to the east. Notably, seismic imaging reveals a locally continuous high-velocity anomaly between the two tears, indicating that a connecting segment of lithosphere may still remain. Such a connection could help maintain low-angle subduction beneath Makran while limiting slab rollback.

According to the researchers, the slab windows created by tearing may also have provided pathways for hot asthenospheric mantle to rise, potentially influencing regional magmatic activity since the late Miocene.

The study presents an evolutionary model in which nearly horizontal and vertical slab tears develop together in the absence of significant trench retreat. It offers new insights into the progressive separation of oceanic and continental lithosphere during the early stages of the subduction–collision transition.

The resulting slab segmentation may also reorganize the forces acting on the subducting lithosphere and the surrounding mantle flow, providing key seismic constraints on the contrasting tectonic evolution of orogenic belts.

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