Shear localization and deformation patterns of a regional scale overthrust: an example from the Sestola Vidiciatico Unit (Northern Apennines)

Studying megathrust shear zones is crucial for understanding the mechanics of subduction zone earthquakes, as these zones are recognized as weak interplate faults that localize deformation under low shear stresses. The low effective friction coefficient of megathrust faults, often due to fluid overpressures, facilitates deformation and is evidenced by the occurrence of mineral-filled fracture sets. These fracture networks, which form in response to cyclic stress states and fluid pressures, provide valuable insights into the palaeostress orientations and the characteristics of fault zone fluids over time.

The Sestola Vidiciatico Unit (SVU) in the Northern Apennines is a tectonic unit interpreted as the plate boundary shear zone between the Ligurian prism and the underthrusting Adria microplate during early-to-middle Miocene, active at temperatures up to 170°C. The SVU is 200 - 400 m thick, composed of kilometer-sized tectonically juxtaposed slices of marls, shales, sandstones and mud-rich deposits. The hanging wall, formed by slope sediments along with Ligurian Units incorporated at the toe of the prism, was overthrusted along a basal décollement onto the younger foredeep turbidites of Adria microplate. Here we present the results of a mesoscale structural analysis of a well exposed sector of the basal contact, where the SVU overthrusts foredeep turbidites along a thrust ramp dipping to the south. Early stages of deformation involved soft-sediment deformation, with polygonal normal faults accommodating boudinage and flattening, producing a pervasive flattening foliation and secondary shear surfaces in the hanging wall. As lithification progressed, shear localization occurred, transitioning from distributed shearing to focused slip on a few dominant thrusts lined by thin calcite shear veins, including the basal contact with the turbidites. Along the footwall ramp, irregular and unfavorably oriented shear surfaces were gradually abandoned as slip localized along a sharp, smooth, and planar slip surface, incorporating slices of the hanging wall to the footwall. The deformation within the footwall includes an oblique cleavage in the fine-grained horizons, minor bed parallel shear planes exploiting pelitic horizons, and a conjugated set of NNE-SSW left-lateral and N-S right-lateral subvertical transtensional faults. The latter either crosscut or are crosscut by the basal thrust of the SVU, and are mineralized by at least two carbonate phases, including an early-stage light gray carbonate rich in organic matter. These results highlight a marked contrast in deformation style and stress state between the soft hanging wall and (relatively) strong footwall. Subvertical dilatant shear fractures served as fluid conduits for vertical fluid flow, which is instead very limited in the hanging wall. This study highlights the potential of combining structural and geochemical analyses in megathrust shear zones in providing insights into the interplay between stress state and fluid circulation in both fossil and modern shallow subduction zones.