Uplift and exhumation of high-pressure metamorphic terrains: the example of the cycladic blueschist belt (Aegean Sea)

The Cycladic blueschists emerged from a depth of more than 50 km during the Tertiary. These rocks are now covered only by a thin fraction of the synmetamorphic overburden (upper plate). Remnants of the upper plate include low-P crystalline rocks and sediments which have been juxtaposed onto the lower-plate blueschists in several stages since the Early Miocene. The low-angle contacts operated as normal faults which removed the overburden from above the Cycladic blueschists. Detailed investigations on Tinos island indicate that the fractured blocks of the upper plate lying above a low-angle normal fault did not rotate. This shows that the faults initiated as rather gently dipping fractures. Similar structures were also reported from other blueschist terrains. Kinematic analysis of such flat-lying faults implies that the synmetamorphic overburden should be mostly located several hundred kilometers away from the present exposure of the blueschists. Considering that the Cycladic blueschist belt exposes successively shallower units in the west, and that rocks of the upper plate have an Anatolian nature, we suggest that these blueschists were dragged from beneath Turkey. We recognize that the blueschist unit has undergone further thrusting in the final stage of the exhumation, as have its counterparts elsewhere in the Hellenides. The emergence of the blueschists in the Tertiary was concomitant with continental collision and thrusting in the Hellenides. The operation of low-angle normal faults with large extensional displacements occurred within this context of collision tectonics. This extension occurred parallel to the strike of the convergent orogen.