Boudinage of a stretching slablet implicated in earthquakes beneath the Hindu Kush

As the fragments of Gondwana (Africa, Arabia, India and Australia) moved northward, arc-shaped belts with intervening basins formed in the Alpine-Himalayan mountain chain during and after collision. This was accompanied by subduction (or sinking) of the ancient Tethyan oceanic plate (or slab) into the underlying mantle. The arc-like shapes could in part be the end result of processes related to drips forming in the less-viscous mantle layer at the base of the Earth's rigid outer shell and then falling into the deeper mantle. Alternatively, the arcs could have formed because slabs constituted of intervening small ocean basins were independently subducted during convergence, and have now disappeared. The subducting slabs tend to stretch, tear and eventually break off, leaving behind thin, vertical strips of colder material that can easily be mistaken for mantle drips. Previous work indicates the presence of such remnant material beneath the Hindu Kush region, close to the collision zone between the Indian and Eurasian continental plates. Here, we analyse a cluster of intermediate-depth earthquakes beneath this region and suggest the existence of an elongate boudin, a lens-shaped feature bounded by ductile faults or shear zones. Our data do not support mantle drip and instead offer a snapshot into the process of break-off, as a thin strip of vertically stretching slab tears free before descending deeper into the underlying mantle.