Magnetic fabric of Pleistocene continental clays from the hanging-wall of an active low-angle normal fault (Altotiberina Fault, Italy)

Abstract

Anisotropy of magnetic susceptibility (AMS) represents a valuable proxy able to detect subtle strain effects in very weakly deformed sediments. During the last decades a large number of AMS studies have documented that in compressive tectonic settings the maximum susceptibility axes (i.e. the magnetic lineations) are parallel to fold axes (and thrust faults) and local bedding strikes, while in extensional regimes they are perpendicular to the normal faults and, thus, parallel to the strata dip directions. One of the most striking active tectonic structures of the northern Apennines is represented by the Altotiberina Fault (ATF), a NE-dipping low-angle normal fault bounding the High Tiber Valley. The ATF represents a primary detachment of the Plio-Quaternary extensional tectonics affecting the Apennine belt. The long-lasting activity of the ATF produced 5 km of total displacement and up to 1200-m-thick basin infill of syn-tectonic, sandy-clayey continental succession. Thus, the AMS analysis of the sediments lying above the ATF represents a unique opportunity to document the strain field affecting the hanging-wall of low-angle normal faults. We collected 129 oriented cores at 12 different localities within the High Tiber Valley, and measured the AMS with a spinner Multi-Function Kappabridge. Most of the sites show a magnetic fabric typical of sediments at the earliest stages of deformation, characterized by oblate AMS ellipsoids and a well defined magnetic lineation, while prolate AMS ellipsoids at two sites are suggestive of pervasive tectonic effects. The magnetic lineation is well-developed at all sites and 20 has a prevailing N-S direction. At five sites the bedding is tilted and the magnetic lineation is subparallel to local bed-strikes, implying that these sites underwent a maximum horizontal shortening along an E-W direction. At two sites the magnetic lineation is sub-perpendicular to the trend of the ATF, and may be related to extensional strain. Our results reveal the existence of both compressional and extensional structures at the hangingwall of the ATF, and suggest that the early Pleistocene sequence of the High Tiber Valley is arranged in gently, local folds (hardly visible in the field) ~N-S trending. We interpret these compressivestructures as the result of local superficial stress induced by irregularities of the fault plane at depth. Accordingly, the strain field we documented from the High Tiber Valley can not be used to infer the regional tectonic regime acting during the ATF activity. We conclude that the long-lasting debate on the extensional vs. compressional Plio-Quaternary tectonics of the Apennines orogenic belt should be revised evaluating the importance of compressional structures resulting by local effects