Giant Porphyry Copper Deposits Caused by a Slab Jamming in the Mantle Transition Zone

Abstract

Two giant porphyry copper deposits in the Southern Central Andes formed during the Miocene–Pliocene transition when a bend in the subducting Juan de Fernández hotspot chain jammed in the mantle transition zone, causing mega-scale slab-kinking. This geometry implies mechanical resistance that caused East–West compression and eventually a thrust-mode failure in a now flattened slab segment at depth. The geometric incompatibility of the kinking zone, compared to the subducting slab adjacent, caused transverse, down-dip slab tears parallel to (but not coincident with) the Juan de Fernández Ridge. One torn flat slab segment acted as a trapdoor, which opened, gaped, and then detached. The largest copper deposits formed along the prolongation of the trapdoor tear, where it intersected tears at the southern edge of the flat slab kink segment. Fluid rising from the sinking slablet beneath escaped for only a limited period before the slab gape was over-ridden by ongoing subduction.