Architecture and deformation mechanism of a basin-bounding normal fault in Mesozoic platform carbonates, central Italy Fabrizio Agosta * , Atilla Aydin Rock Fracture Project, Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA Received 6 December 2005; received in revised form 30 March 2006; accepted 5 April 2006 Available online 12 July 2006 Abstract We studied the mechanisms and intensity of deformation across a large, active, basin-bounding normal fault zone in Mesozoic platform car- bonates. 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 car- bonates within the individual mechanical layers. With ongoing deformation and emersion from depth, two sets of conjugate normal faults de- veloped 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. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Platform carbonates; Failure modes; Normal fault growth; Fault architecture; central Apennines 1. Introduction Fault zones have two compartments: core and damage zones (Chester et al., 1993; Antonellini and Aydin, 1994; Caine et al., 1996). The fault core includes fault rocks that de- velop around the major slip surfaces due to comminution, dis- solution/precipitation, mineral reactions, and other mechanical and chemical processes that destroy the fabric of the host rock (Sibson, 1977; Chester and Logan, 1986). The damage zone flanks the fault core and consists of a wider zone of fractures and smaller faults that do not completely obliterate the host rock fabric (Cowie and Scholz, 1992). Both core and damage zones are surrounded by the host rock, which has a background value of deformation intensity. Characterization of both deformation mechanisms and structural elements that charac- terize the fault zones is crucial to understanding the processes of fault growth (Martel et al., 1988; Davatzes and Aydin, 2003; Myers and Aydin, 2004) and fault architecture and permeabil- ity (Caine et al., 1996; Aydin, 2000). Faulting in carbonate rocks involves the sequential forma- tion and shearing of joints, veins, and/or pressure solution seams (Alvarez et al., 1978; Marshak et al., 1982; Peacock and Sanderson, 1995; Kelly et al., 1998; Salvini et al., 1999). Three main deformation mechanisms along faults in carbonates have been documented in some detail. Strike-slip faulting in the dolomitic rocks of the Sella Group, in northern Italy, was initiated by echelon joints that were later sheared producing cross joints, which broke up the joint-bounded bridges and eventually formed throughgoing slip surfaces and fault breccia (Mollema and Antonellini, 1999). In another case, normal faults within the platform carbonates at the front of the Maiella thrust sheet in central Italy developed entirely * Corresponding author. Tel.: þ1 650 723 4788. E-mail address: bizio@pangea.stanford.edu (F. Agosta). 0191-8141/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2006.04.006 Journal of Structural Geology 28 (2006) 1445e1467 www.elsevier.com/locate/jsg