A Slab Window Beneath North Sumatra Revealed by P-Wave Mantle Tomography

Passive seismic studies have suggested that slab tearing occurs beneath North Sumatra, albeit the slab morphology is somewhat poorly constrained. The lack of resolution on the slab means that the presence of a tear has not been validated, and that the main factors responsible for slab tearing are hard to discern. We use a large seismic data set to determine a new 3-D high-resolution tomographic model of the crust and upper mantle beneath North Sumatra, which reveals a well-defined low-velocity channel that cuts across the subducting slab through a cavity. This feature cannot be reconciled with a tear in the slab, as previously suggested. Our relocated earthquakes exhibit a protruding cluster of seismicity along the subducted Investigator Fracture Zone, indicating that the high reliefs of the Investigator Fracture Zone possibly enhance the coupling between the subducted slab and the overriding plate. Shear stress resulting from the differential coupling in the slab may have created an area of mechanical weakness in correspondence of the fossil-spreading ridges, which eventually led to the formation of a slab window. This window allows the exchange of mantle material from the subslab region up to the Toba volcano. We propose that the previously damaged parts (e.g., fracture zones and spreading centers) of the oceanic plate could help promote the growth of a slab window.