Architecture and evolution of an extensionally-inverted thrust (Monte Tancia thrust, central Apennines, Italy): geological, structural, geochemical, and K-Ar geochronological constraints

Deformation in the upper crust is heterogeneous and mostly localized along brittle faults. Faults and fault rocks may be weak compared to surrounding host rocks and are likely to accommodate repeated slip episodes due to structural reactivation during commonly fluid-assisted faulting events. Thrust fault reactivation by subsequent normal faulting has been commonly documented in orogenic wedges, where extensional tectonics often follows contraction. Fault inversion may lead to variable overprinting of the early tectonic fabrics, which prevents a straightforward interpretation of the complete fault kinematics and deformation history. For this reason, and due to the seismogenic potential of upper crustal faults, much effort has been invested into a better understanding of the architecture of faults, of their deformation mechanisms as well as kinematic evolution through time. In this study, we adopt a multidisciplinary approach to reconstruct the tectonic evolution of the exhumed Monte Tancia Thrust (MTT), central Italy, consisting of a W-dipping thrust fault later inverted into a normal fault. To this goal, we combined: 1) 1:5000 scale geological mapping of an area of about 15 km2 and detailed structural analysis both in the hanging wall and in the footwall of the MTT; 2) microstructural analysis of the thrust wall and fault rocks; 3) cathodoluminescence microscopy (CL); 4) X-ray diffraction (XRD) analysis of the clay content of the MTT fault rocks and surrounding undeformed blocks to evaluate their thermal evolution and 5) K-Ar dating of authigenic synkinematic illite. Further geochemical analyses such as stable and clumped isotopes of calcite mineralizations are in progress. The MTT is a 200 m thick shear zone characterized by a pervasive S-CC’ fabric associated with contractional top-to-ENE kinematics (S1 foliation). Within the first few metres below the thrust surface, a zone dominated by the partial superimposition of S-CC’ fabric showing an extensional top-to-W-SW movement (S2 foliation) on top of the earlier compressional fabric is prominent. Cathodoluminescence suggests that the compressional and extensional deformation stages were facilitated by fluid ingress in the deformation zone, but that the fluid did not undergo significant changes in chemistry. To evaluate the thermal evolution of the MTT and constrain the timing of the tectonic inversion, XRD and K-Ar datings are respectively now in progress. Based on our observation, we speculate that the S1 foliations mainly developed by repeated events of fluid overpressure leading to multiple crack-and-seal events characterized by several veining episodes, followed by pressure-solution creep. S2 foliation characterized by fibrous calcite developed under fluid pressure fluctuations simultaneous with the fault slip in a normal sense.