Analysis of fracture network geometries and orientations within a fold-and-thrust structure in the Northern Apennines, Italy

Abstract

This research focuses on fracture networks in sedimentary rocks within the Umbria-Marche fold-and-thrust belt in the Northern Apennines, Italy. The aim of this research is twofold, namely to correlate the geometry of fracture networks with tectonic position and lithology and to correlate the orientation of fracture networks with the origination of a fold-and-thrust structure. The fold-and-thrust belt within the area strikes about N160° and developed in the Miocene within a compressional regime as the result of the collision between the European Corsica-Sardinia Margin and the Adriatic plate, accompanied by back-arc extension due to rollback. In order to analyze geometries of fracture networks software named DigiFract is used to digitize outcrops in the field. Fracture orientation, density, spacing, height and termination are analysed for different lithologies within the Umbria-Marche succession. Orientations of fracture sets are correlated to different structural stages during the development of the fold-and-trust structure. The first stage in which fractures develop is layer parallel shortening, during which bedding-normal pressure-solution surfaces develop, striking parallel to the hinge line. Subsequently, longitudinal joints striking parallel to the hinge line develop during fold initiation. This is followed by amplification and tightening of the fold, causing development of transversal joints, striking perpendicular to the hinge line. In theory, fold limbs are preferred sites for deformation within an active-hinge fault-related anticline, rather than the corresponding anticlinal crests (Salvini and Storti, 2001; Salvini and Storti, 2004). Our data is not substantial to proof this theory, but is in accordance to it. Chert, primarily present in the Maiolica Fm and the Diasprini Fm, and marl, present in the Bisciaro Fm, act as non-fractured or very low density-boundary between fractured beddings. Siliclastic turbidites are less fractured than carbonates at similar tectonic positions.