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

The Nestos Shear Zone (NSZ) in the Rhodope Metamorphic Province is a major highstrain 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 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 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 ca. 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 samples 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.