Understanding continental subduction: A work in progress
A study by Froitzheim et al. (2016, p. 223 in this issue of Geology) adds new constraints to our rapidly evolving ideas and models regarding the process of continental subduction. Classic plate tectonics concepts suggested that continents do not subduct. Instead, when two continents collide at a convergent boundary following the consumption of an ocean by subduction, they accommodate the shortening within the lithosphere, which is thickened up to twice the normal values. The subducted oceanic slab that brought the continents together stalls and eventually breaks off and sinks into the mantle due to its negative buoyancy. In contrast to that view, modern petrologic, tectonic, and geophysical observations have completely changed this picture still prevalent in many textbooks: continental lithosphere does, in fact, subduct to great depths at major long-lived collisional boundaries, and the two colliding plates can be separated by a section of convective upper mantle (mantle wedge) similar to the case of oceanic subduction.