The exposed base of a collapsing wedge: The Nestos Shear Zone (Rhodope Metamorphic Province, Greece)

Abstract

The Nestos Shear Zone (NSZ) in the Rhodope Metamorphic Province is a major high-strain zone between two metamorphic terranes. Microdiamond-bearing ultrahigh-pressure (UHP) rocks occur in the NSZ which was therefore interpreted as a suture zone where subduction and exhumation of these rocks and terrain accretion occurred during the Mesozoic. Our petrological study of samples from the lower part of the NSZ, together with monazite dating of a microdiamond-bearing garnet schist, structural observations, already published results from the upper part, and other published timing constraints, results in a fundamentally different picture: the NSZ is the base of an Eocene age southwestward thrust wedge which included not only the structurally higher parts of the Rhodope Metamorphic Province but also the entire Internal Hellenides. The UHP rocks, for the peak pressure of which we derive an age of ∼200 Ma by monazite dating, are unrelated to the tectonic processes in the NSZ and probably represent slivers of a higher tectonic unit captured by thrusting along the NSZ. Pressure decrease in the footwall and regional extension and basin formation in the hanging wall during the activity of the NSZ show that the overlying thrust wedge was collapsing in late Eocene times. We hypothesize that the transition from subduction to continental collision may start with a pronounced accretion event when the first detachment horizon forms in the middle crust of the downgoing plate. Such an event could trigger slab retreat, an extensional collapse of the internal wedge, and subsequent magmatism. Copyright 2011 by the American Geophysical Union.