Subduction tectonics and exhumation of high-pressure metamorphic rocks in the Mediterranean orogens

Abstract

Using the peri-Mediterranean blueschist belts as a case study we discuss the mechanisms of syn-orogenic exhumation of high-pressure (HP) and ultra-high-pressure (UHP) rocks. The Mediterranean examples, within an overall convergent zone, show variations in the rates of convergence, rates of slab retreat, available space, and various stages of maturation of accretionary complexes. After a compilation of the deformation history and the kinematic boundary conditions and their evolution through time, we discuss P-T-t paths. Most of the structures found in the field today relate to the exhumation stage and often to quite superficial events related to syn-orogenic detachments. However a significant vertical motion occurred from the depth of eclogites (or UHP eclogites) to the depth of the blueschist or greenschist facies along cold P-T paths. Two types of mountain belts are described: those where a single thermal gradient was recorded throughout its’ history, like the Franco-Italian Alps, which suggest a steady-state evolution; and those where the thermal gradient has changed through time (and space), such as the Aegean region, which suggest a non-steady-state evolution. Slab retreat within a subduction complex does not lead to the exhumation of UHP rocks because the open subduction channel allows for fast circulation and detachment of sediments from the subducting basement. Subducted sediments lubricate the subduction channel and the basement is thus not involved in the return flow. Early exhumation is fast and the thermal regime in the subduction channel is partly controlled by the velocity of convergence and the velocity of slab retreat as well as by the nature of the subducted material. Final exhumation occurs within the accretionary complex at a much slower rate below extensional detachments. The removal of the overburden is achieved primarily by extension in the upper part of the accretionary complex. Extensional faults and shear zones are rooted in the brittle-ductile transition of the accretionary complex. Deeper “extensional” shear zones result from shearing along the base of the upper plate. We discuss a model with several levels of circulation of subducted material and compare it with available thermo-mechanical models.