Upper mantle melt caused by a subducted slab in the Indian-Eurasian continental subduction zone

A low-velocity layer atop the mantle transition zone has been extensively observed worldwide. In subduction zones, this layer is widely explained as partial melting triggered by slab subduction on a regional or global scale. However, direct observational evidence is still absent, and the response of the layer to slab subduction is not well known. Here, we image the seismic velocity around the mantle transition zone by matching synthetic and observed triplicated seismic P and sP waveforms in the Indian-Eurasian continental subduction zone. Our observations reveal a laterally varied low-velocity layer atop the mantle transition zone beneath the Hindu Kush, where a subducted slab extends to the mantle transition zone. The geometric morphology of the low-velocity layer documents that it is a partially molten layer induced by the subducted slab on a regional scale. Interestingly, our observations also support that the layer has a low viscosity. The decreased viscosity may facilitate slab motion in the deep mantle, contributing to slab stretching, tearing and break-off and its resultant rare recurring large intermediate-depth earthquakes in an intracontinental setting. A low-viscosity layer atop the mantle transition zone may facilitate slab sinking and recurring intermediate-depth seismicity, according to an analysis of seismic observations and velocity models from the Indian-Eurasian collision zone.