SLAB EXTRUSION, CRUSTAL DIAPIRISM AND POST-OROGENIC MELTING: A SPECTRUM OF MECHANISMS FOR THE EXHUMATION OF SUBDUCTED CONTINENTAL CRUST

The presence of high pressure (HP) and ultrahigh pressure (UHP) terranes within most mountain systems and the presence of mantle fragments (orogenic peridotites) within many of these terranes provide evidence that continental crust is subducted into the upper mantle to depths sometimes exceeding 150 km during the continent-continent and arc-continent collisions that occur as ocean basins narrow and close. Continental subduction is usually caused by the pull exerted by the subduction of previously subducted oceanic lithosphere. However, the exhumation of these HP/UHP terranes appears to occur through several mechanisms, all the result of the bouyancy of the silica-rich continental crust relative to the surrounding, denser mantle. Some terranes return as relatively rigid coherent slabs that return towards the surface along the same route they were subducted, bounded below by thrust faults and above by low angle normal faults. These terranes show limited high temperature overprinting and melting. Other terranes return as spaced diapirs (i.e. “massifs”) characterized by an overall domal structure. Their more ductile return towards the surface is consistent with evidence for a high temperature overprint during exhumation accompanied by extensive melting. Still other terranes may not return as coherent solids, but instead melt and return as magmas that rise through the overlying mantle wedge and intrude the overlying continental crust as post-orogenic and even anorogenic granitoids. Exhumation of some terranes may be a hybrid process combining slab return, diapirism and melting. Some factors that determine which of these mechanisms predominates include, but are not restricted to: 1) the chemistry and mineralogy of the subducted slab (particularly the presence or absence of radioactive elements and of hydrous minerals); 2) the initial temperatures of the subducted slab and the enclosing mantle, 3), the length of time the crustal slab lingers in the mantle (i.e. “hang time”); 4) the angle of subduction and 5) the depth of penetration into the mantle. Mountain chains that do not contain HP/UHP terranes, particularly Archean and Proterozoic systems, may nevertheless have evolved through the subduction of continental crust into the mantle where they melted rather than returning as coherent HP/UHP bodies.