Abstract 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.

Abstract We propose a theoretical model, supported by a field study, to describe the patterns of fault/fracture meshes formed within dilational stepovers developed along faults accommodating regional scale wrench-dominated transtension. The geometry and kinematics of the faulting in the dilational stepovers is related to the angle of divergence (α), and differs from the patterns traditionally predicted in dilation zones associated with boundary faults accommodating strike-slip displacements (where α = 0°). For low values of oblique divergence (α<30°) and low strain, the fault–fracture mesh comprises interlinked tensile fractures and shear-extensional planes, consistent with wrench-dominated transtension. At higher values of strain, a switch occurs from wrench- to extension-dominated transtension, leading to the reactivation and/or disruption of the early-formed structures. These structural processes lead to the development of a geometrically complex and kinematically heterogeneous fault pattern, which may affect and/or perturb the development of a through-going fault linking and facilitating the slip transfer between the two overlapping fault segments. As a result, dilational stepover zones will tend to form long-lived sites of localized extension and subsidence in regional transtensional tectonic settings. Cyclic increases/decreases of structural permeability will be related to slip on the major boundary faults that control the distribution of fluid-flow paths and, consequently, the long- and short-term structural evolution of these sites. Our model also predicts complex and more realistic subsurface fluid migration pathways relevant to our current understanding of hydrothermal ore deposits and hydrocarbon migration and storage.

Abstract Fault segments belonging to a fault population can link and interact, eventually forming a single larger fault, and thus affecting the estimation of the maximum expected earthquake. We present throw distribution data along the Quaternary normal faults of the Colfiorito fault system (central Italy), which consists of four main fault segments and where a seismic sequence occurred in 1997–1998. Throw values along the two central overlapping, en-échelon segments (8.5–9.5 km long) were measured on a good stratigraphic marker, by constructing a set of closely spaced geological cross-sections, perpendicular to the fault strike. As these faults are commonly retained active and border Quaternary basins, we compare morphological and geological throws in order to verify the faults neotectonic activity. Geological and morphological throw distributions show good correlation, testifying that recent faulting affected the topographic surface and suggesting that the observed offset completely accumulated during the Quaternary. The throw distribution along the fault segments is asymmetric and reaches maximum values (500–550 m) within the zone of fault overlap, suggesting mechanical interaction between the studied faults. Maximum length-throw correlation suggests that the studied faults grew according to a linear scaling relationship.

Abstract During the past 18 Ma extensional tectonism has migrated from the Tyrrhenian sea eastward into the Northern Apennines of Italy. The extension is due in part to lowangle east-dipping normal faults, that are now exhumed in the Tyrrhenian islands and Tuscany, while additional extension is still occurring in the Apennine chain (Umbria region, c. 200 km eastward). This tectonic framework is an example where active extensional processes affecting the Umbria region can be studied in exhumed faults that are no longer active. Here a comparison between the Zuccale Fault (ZF), cropping out in the Isle of Elba, and the Altotiberina Fault (ATF), revealed by geophysical data, seismology and seismic profiles crossing the Umbria region, provide insights into the processes affecting low-angle normal fault development and evolution. Recorded microseismicity suggests that the ATF is presently active under a vertical σ 1 . Structural analysis of the ZF depict a comparable scenario with fluid involvement during the activity. The comparison of these two structures suggests movements with fluid involvement along gently dipping planes under a vertical σ 1 , implying that these faults are mechanically weak.