Mesostructural analysis of S-C fabrics in a shallow shear zone of the Umbria-Marche Apennines (Central Italy)

Several examples of foliated fault zones characterized by penetrative S-C fabric are exposed in the Umbria-Marche region, often associated with major thrusts. These tectonites, dominated by pressure-solution processes, are developed at shallow depths (,3 km) in sedimentary rocks. In this paper, we perform a detailed structural analysis of the Scheggia Thrust Zone (STZ), a significant example of S-C tectonite spectacularly exposed in the Umbria-Marche Apennines. We reconstruct both the architecture and internal structure of the shear zone, focusing on the factors controlling the genesis and development of the tectonic pattern such as: (a) anisotropy; (b) lithological contrast; and (c) distance from the fault core. The STZ shows an intensely cleaved fault core (S-C fabric) and a thick and asymmetric damage zone developed in the footwall rocks. By analysing the spacing of the S-surfaces, we observe that the magnitude of deformation decreases with distance from the fault core. However, far from the fault core the magnitude of deformation is strongly controlled by the lithology of the protolith: almost undeformed calcareous beds alternate with intensely sheared intervals, localized along weak, marly horizons, also characterized by flattened S-planes. Based on these data we have divided the STZ into three subzones where the deformation is mainly driven by: (1) distance from the fault core; and (2) the lithology of the protolith. Marly horizons into these subzones, with respect to the calcareous horizon, show that: (i) S-planes are more closely spaced; and (ii) the angle between C- and S-planes is smaller. Our study confirms that the rhythmic alternation of calcareous and marly rocks can favour the development of foliated fault zones, also producing an increase in pressure-solution cleavage at shallow crustal conditions.