Tracking the transition from contractional to extensional tectonics in the upper plate of a retreating subduction system: A case study in the Aegean forearc, in the Southern Hellenides, Northern Peloponnese, Greece

The Peloponnese in Greece is in the forearc of the Hellenic subduction zone, which was formed during the continuous retreating subduction of the African plate beneath the micro-anatolian plate from Eocene to present day. The Aegean forearc in Peloponnese is composed of different structural domains: Oligo-Miocene fold and thrust belt, high-pressure metamorphic domes of the Quartzite-Phyllite unit (QPU) exhumed on the footwall of low-angle detachments and superimposed Plio-Pleistocene rifts (Corinth, Argos). The transition from the contractional to the extensional tectonic phase, the mechanisms that led to the exhumation of the QPU and the one that controls the style of Miocene to recent extensional tectonics are poorly constrained. Our structural mapping, cross-sections, detailed microstructural analyses are associated with RSCM mapping, P-T-t estimate and Zircons Helium low-temperature thermochronology on the QPU to propose a high-resolution model of the Aegean forearc in the northern Peloponnese.

The results show that the QPU was affected by greenschist to blueschist metamorphism during the contractional phase with maximum P-T constraints between 14-17 kbar and 450-580°+/- 50°. The QPU in northern Peloponnese underwent three main phases of exhumation: (1) a phase of isothermal decompression between 17 kbar and 6 kbar during Oligocene and Early Miocene, (2) a phase of cooling at 9-15 km deep during Middle Miocene, and (3) a regional phase of late-brittle exhumation constrained between 8 Ma and 4 Ma by ZHe ages. The ductile deformation observed within the QPU records both top of NE or SW sense of shear associated with ductile exhumation within the subduction channel and progressive underplating. 3D modelling and cross-section construction around the metamorphic windows suggests that exhumation of the QPU domes was associated with 3D extensional thinning of the orogenic nappes during the Miocene with both arc-parallel and arc-normal extension. The regional low-angle normal faults are cross-cut by a recent network of NW-SE and E-W high-angle normal faults associated with the opening of Plio-Pleistocene rifts surrounding the Peloponnese. The relationships between the exhumation of the HP units, forearc dynamic, syn-orogenic thinning of the nappes as well as recent active rifting are discussed in this work.