Architecture and deformation mechanism of a basin-bounding normal fault in Mesozoic platform carbonates, central Italy

We studied the mechanisms and intensity of deformation across a large, active, basin-bounding normal fault zone in Mesozoic platform carbonates. Based on the modes, orientation, crosscutting and abutting relationships of various structural elements preserved within the fault zone and in the surrounding host rocks, we propose a conceptual model of normal fault growth under an extensional tectonic regime that follows an earlier contractional regime. Normal faults initiated by shearing of the pre-existing elements, predominantly pressure solution seams inherited from the contractional regime, formation and subsequent shearing of new seams and joints/veins, and localization of pods of fragmented carbonates within the individual mechanical layers. With ongoing deformation and emersion from depth, two sets of conjugate normal faults developed within the fault zone through the fragmented pods. The end result is normal fault zone with a maximum of 600 m of throw that includes deformed basinal sediments in the hanging wall, and up to 1-m thick fault core and 100-m thick damage zone in the footwall. The fault core is made up of matrix-supported and cement-supported fault rocks and major slip surfaces. The damage zone consists of small faults and fragmented carbonates; the intensity of deformation generally increases towards the fault core.