Relamination Styles in Collisional Orogens

Abstract

During continental collision, a part of the lower-plate material can be subducted, emplaced at the base of the upper plate, and eventually incorporated into its crust. This mechanism of continental-crust transformation is called relamination, and it has been invoked to explain occurrences of high-pressure felsic rocks in different structural positions of several orogenic systems. In the present study we reproduced relamination during continental collision in a thermomechanical numerical model. We performed a parametric study and distinguished three main types of evolution regarding the fate of the subducted continental crust: (i) return along the plate interface in a subduction channel or wedge, (ii) flow at the bottom of the upper-plate lithosphere and subsequent translithospheric exhumation near the arc or in the back-arc region (“sublithospheric relamination”), and (iii) nearly horizontal flow directly into the upper-plate crust (“intracrustal relamination”). Sublithospheric relamination is preferred for relatively quick convergence of thin continental plates. An important factor for the development of sublithospheric relamination is melting of the subducted material, which weakens the lithosphere and opens a path for the exhumation of the relaminant. In contrast, a thick and strong overriding plate typically leads to exhumation near the plate interface. If the overriding plate is too thin or weak, intracrustal relamination occurs. We show that each of these evolution types has its counterpart in nature: (i) the Alps and the Caledonides, (ii) the Himalayan-Tibetan system and the European Variscides, and (iii) pre-Cambrian ultrahot orogens.