A slab window caused by subducted seafloor fabrics beneath North Sumatra

Slab break-off, or detachment, is a key process in the tectonic evolution of the lithosphere and underlying mantle. Even though slab break-off is commonly advocated to explain a wide range of geological observations, the mechanisms that initiate slab tearing with eventual break-off are still uncertain. Slab tearing beneath North Sumatra has been suggested from evidence based mainly on passive-seismic studies. However, the detailed slab morphology and main factors responsible for slab tearing are unknown due to a lack of convincing constraints on the properties of the subducting slab. We obtained a new 3-D high-resolution tomographic model that reveals the fine crust-upper mantle structure of North Sumatra and a well-defined, relatively low P-wave velocity channel that cuts across the subducting slab. The high-resolution images finally identified the existence of a slab window below North Sumatra, consistent with an enriched potassium content around the Toba volcano. According to the linear earthquake cluster, the location of seafloor fabric structures (such as fracture zones and fossil ridges in the Wharton basin) and the corresponding lower velocity window in our robust tomographic images, we infer that the shear stress resulting from the interaction between oblique subduction system and seafloor fabrics led a mechanism weakness in the corresponding fossil-spreading ridge, which eventually formed a slab window. The window allows the subslab materials to support the arc volcanism.