Delamination in collisional orogens

During continental collision, the weight of the downgoing slab may lead to extensional shearing in the descending plate, permitting separation of the downdip, dense oceanic lithosphere from the adjacent lighter continental lithosphere. Separation may occur by means of a lithosphere-penetrating normal-sense shear zone, by means of pure shear necking of the lithosphere, or by some combination of these mechanisms. During separation, the continental crust attached to the lower plate may undergo an episode of extensional tectonics. The proximity of asthenosphere to crust may result in syntectonic regional metamorphism and plutonism. If the continents continue to converge following delamination, a compressive tectonic regime will be reestablished in the crust. The subsequent cooling of asthenosphere at the base of the crust may lead to the establishment of a continuous flat Moho. The likely imbrication of crustal and mantle lithologies during delamination and during the subsequent compressive tectonic regime, as well as underplating associated with syntectonic plutonism, may account for the seismic complexities observed at Moho depths beneath some mountain belts.