The Rhodope Metamorphic Complex as a case for extreme vertical extrusion

We explore the fate of subducted continental margins using thermomechanical modeling with constant-force boundary conditions. Under certain conditions, large volumes of upper crust can get subducted to mantle depth, detach from the slab, and rise through the upper plate’s lithosphere even against considerable compressive tectonic stress. At the surface, such an event is expressed as a phase of intense horizontal extension and magmatism internal of the oceanic suture zone. The process can create normal-fault-bounded core complexes hundreds of kilometers wide, in which metamorphic continental crust derived from the subducting plate is exposed. Horizontal tectonic stress and the thickness of the downgoing upper crust control the width of complexes, their topographic elevation, and the subduction depth of exposed rocks. We propose that the Rhodope Metamorphic Complex on the Balkan Peninsula represents a prime example for such vertical extrusion internal of a suture zone. Lower tectonic units in this domain exhibit Eocene high-pressure metamorphism and nappe stacking followed by massive magmatism and large-offset normal faulting. Despite more than 100 km of extension in Cenozoic times, the area still has thick crust. Our modeling results support schemes that attribute the lower units of the Rhodope Metamorphic Complex to the subducted Adriatic plate.