EXHUMATION BY LOW-ANGLE NORMAL FAULTING: EXAMPLES FROM THE AEGEAN SEA, GREECE

The Aegean Sea is a world-class example of continental extension and low-angle normal faulting related to a retreating subduction zone. Low-angle normal faulting and exhumation occur in two general settings: (1) considerable exhumation but little extension in the forearc when the thermal gradient is low and (2) little exhumation but larger extension in an intra/back-arc position when the thermal gradient is elevated. In the fore-arc case, the low-angle normal faults appear to reactivate former subduction thrusts. The low-angle normal faults form the upper and the subduction thrusts the lower boundary of extrusion wedges, i.e. normal faulting is intimately associated with thrust faulting during lithospheric convergence and did not cause much extension. Gaps in P-T histories and age constraints suggest considerable displacement and large slip rates on the faults bounding the extrusions wedges. Because displacement on the faults may exceed 100 km and slip rates may exceed 10-20 km/Myr, the faults contribute significantly to rapid exhumation of high-pressure rocks. During progressive subduction-zone retreat rocks gradually shift from a fore-arc into an intra/back-arc position. In the Aegean, this shift is associated with pronounced arc-related magmatism during lithospheric divergence. Weakening by magmatic activity and pronounced stretching of the lithosphere caused low-angle normal faulting and generating two different kinds of metamorphic core complexes in the intra/back-arc setting. Numerical simulations suggest that the viscosity of the lower crust plays a fundamental role in the mode of core-complex formation. "Hot” core complexes are characterized by large-scale normal faults with relatively large slip and high cooling rates, and the development of an overall symmetric geometry expressed by a relatively late secondary antithetic normal fault caused by symmetric, plume-like extrusion of the hot lower crust. "Cold” core complexes are strongly asymmetric and have considerably smaller displacements and cooling rates. Commonly the low-angle normal faults root at the brittle/ductile transition. Low-angle normal faulting in the intra/back-arc settings did not contribute significantly to the exhumation of high-pressure rocks in the Aegean but caused considerable horizontal extension.