The M. Tancia regional thrust: geometric, kinematic and mechanical characterization within the Structural Geology course at La Sapienza

Within the Structural Geology course of the BSC degree at La Sapienza University we have studied the geometry, kinematics and mechanics of a regional thrust fault exposed near Rieti (Lazio) and deformed during the Apenninic compressional phase. The fault is a km-scale displacement structure trending N-S and gently (10-20°) westward dipping. The thrust superposes the Maiolica (Early Cretaceous) over the Scaglia Variegata (Eocene) Formations and it is represented by a 6-8 m thick foliated (SCC’ tectonites) shear zone. Small displacement normal faults showing cataclastic fault rocks, cut the foliated fabric. We have used SCC’ tectonites to characterize the kinematic evolution of the fault zone. We have detected two sets of SCC’ fabric: one set shows a top to the east sense of shear that is consistent with the compressional phase, the second set overprints the first one and is characterized by a top the west kinematics likely due to the quaternary extensional phase of the area. The fault zone is characterized by two main types of fault rocks: principal slipping zones are affected by cataclastic processes, producing grain size reduction and localization; fluid assisted dissolution and precipitation processes promote the development of a distributed foliation within the entire fault zone. During the compressional phase the fault accumulated kilometers of displacement as a well-oriented structure within an Andersonian compressional regime. In the following Quaternary extension, some portions of the shear zone were reactivated via optimally oriented small normal faults, other portions of the fault zone were reactivated as misoriented structures (gently dipping planes) within the Andersonian extensional regime. This misoriented reactivation was likely promoted by clay concentration within the fault planes following pressure-solution processes.