Fault-zone anatomy and fracture distribution in poorly lithified sediments of the Tarquinia basin (central Italy): Implications for fault zone evolution

Syn-sedimentary, sub-seismic scale extensional faults in poorly lithified Pliocene sandstones are well exposed in an abandoned quarry SW of the Tarquinia village (Central Italy). Geometry and spatial distribution of main normal fault segments, and related deformational features (including secondary synthetic and antithetic faults, fault rocks, joints, deformation bands) were carefully mapped in detail both along and across major fault strikes. Fault segments are 20-30 m long and strike mainly NW-SE. They are partially overlapped and connected by either relay ramps or transfer faults. Kinematic analyses on fault slickensides indicate nearly pure dip-slip sense of movement. Estimated displacements (throws) never exceed 15m. Fault damage zones are 5-10m wide and consist of subsidiary synthetic and antithetic normal faults, extensional fractures and deformation bands. Their relative abundance and distribution is strongly controlled by the position within the fault zones. Subsidiary faults and deformation bands accommodate most of the total deformation in approaching the fault tips. On the other hand, indurated master slip surfaces accommodate most displacement in the central sectors of the fault zones. Near vertical, 5-10 cm spaced joints are frequent in their footwall damage zones. Cumulative statistical analysis of joint data indicates that they are systematically orthogonal to the fault strike. Joint paralleling the fault strike are strongly subordinated. Detailed AMS analyses were also carried out in the proper rock types along transects perpendicular to fault strikes. Results show that K1 orientations are consistent with the stretching directions inferred from structural data. Magnetic fabrics appear reworked in approaching fault surfaces.Based on these integrated information, we propose a structural evolutionary model for normal fault growth in poorly lithified sediments, which involves the development of fault strike-perpendicular joints during the lateral migration of fault tips. Numerical-analytical modelling (FRAPtre) validates the proposed evolutionary pathway and shows that such a fracture evolution and distribution may have a considerable impact on the fluid flow modelling for the successful exploitation of fractured reservoirs.