Structural geometries along the thrust zone of the Umbria-Marche-Sabina mountain front

The Umbria-Marche-Sabina ridge is bounded to the east by a defining a major thrust fault extending from Olevano Romano in the south to the Foglia River Valley in the north. This consists of two main portions arranged in a right-stepping en-échelon pattern. The northern portion («Belforte-Urbino Thrust») extends from the Chienti River Valley in the south to the Cesana Mts. in the north. The southern part (Olevano-Antrodoco-Monti Sibillini, or OAMS Thrust) extends from the Potenza River Valley in the north to Olevano Romano in the south. The former has a general NW-SE trend, changing to WNW-ESE north of the Metauro River Valley. The OAMS Thrust is arc shaped in its northern part, between the Potenza and the Velino River Valleys, and has a N-S strike to the south. The southernmost portion of the thrust consists of two minor segments displaying a right-stepping en-échelon geometry (Ascrea and Canterano Thrusts). The thrust involves the Mesozoic-Cenozoic Umbria-Marche succession and, south of the village of Posta, also involves the Latium-Abruzzi carbonate platform succession and the transitional sequence between the latter and the Umbria-Marche basin. Based on detailed field mapping, integrated also with available subsurface information, 14 geological cross-sections have been constructed. They allowed us to outline the main structural features of the Umbria-Marche-Sabina mountain front, and to infer thrust displacement. The hanging-wall unit mainly consists of an asymmetric, east-verging anticline showing a western gently dipping limb and an eastern vertical to overturned limb. Lens-shaped kilometre-sized tectonic slices also occur along the thrust zone. The thrust surface is locally folded into antiforms and synforms of different size. Locally, erosion associated with structural elevation exposes deep parts of the thrust fault. It displays hanging-wall ramp geometries – with dominant low to moderate cut-off angles – in the stratigraphic units predating the Cerrogna Marls. On the other hand, extensive flats occur in the latter formation. Locally down-section thrust trajectories also occur, as a result of pre-existing footwall deformation. The structural setting of the mountain front is complicated by the occurrence of both pre- and post-thrusting normal faults. The former are commonly deformed during the fold growth and are then offset by thrusts. Locally they also appear to have been reactivated by recent extensional movements. The maximum displacements occur along the Monte Vettore – Accumoli segment of the OAMS Thrust with values of about 10 km. The thrust can be referred to the late Messinian (post-evaporite) time, with locally continuing activity and/or reactivation during the Pliocene. Sub-surface data, only available for the north-western part of the regional structure, suggest an involvement of the basement in structures also affecting the overlying sedimentary cover.