EVOLUTION OF TEXTURE AND COMPOSITION ALONG THE SUBDUCTION MEGATHRUST UP-DIP OF THE SEISMOGENIC ZONE

Along the subduction megathrust, the top of the seismogenic zone occurs at 100 to 150 °C and the region up-dip hosts diverse modes of fault slip. The transition is thought to occur due to evolving material properties that promote seismicity but are poorly understood. The Sestola-Vidiciatico Tectonic Unit (SVTU) has been interpreted to represent an ancient analog of an erosive plate boundary shear zone (PBSZ) at conditions near the up-dip limit of the seismogenic zone, with peak deformation temperatures up to ~150 °C corresponding to ~5 km vertical depth. The SVTU accommodated the early Miocene convergence between the subducting Adriatic plate and overriding accretionary prisms of the European plate. Field observation suggest deformation within the SVTU was dominantly accommodated within a meter scale along the basal contact. Samples from 5 outcrops along the basal decollement were analyzed to provide new temperature estimates and characterize the composition and microstructure with increasing depth. Structures within the SVTU evolve from being distributed across the 500 m thick SVTU to being localized along the decollement and cut the basal decollement and footwall at greater depths. These outcrops have inferred burial depths between ~3 and ~5 km based on vitrinite reflectance temperature estimates at proximal sites within the hanging and footwall. We present measurements of pyrolysis on samples from the decollement to more accurately constrain paleotemperatures during subduction. Using these new temperature constraints, combined with preliminary compositional and microstructural data, we outline an evolution of diagenesis, deformation mechanisms, and strain partitioning at conditions between ~90 and ~150 °C.