Description:

_Journal of Structural Geology 32 (2010) 1392–1402_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com locate jsg Microtectonics of low-P low-T carbonate fault rocks Andrea Billi* Dipartimento di Scienze Geologiche, Università ‘‘Roma Tre’’, Largo S. L. Murialdo 1, 00146 Rome, Italy Article info Article history: Received 21 November 2007 Received in revised form 6 May 2009 Accepted 8 May 2009 Available online 19 May 2009 Keywords: Carbonate Fault Fault rock Microtectonics Abstract With the aim of deducing some general microtectonic processes responsible for the development of carbonate fault cores, rock samples were collected in ten of such structures, which are different in size, attitude, kinematics, displacement and tectonic environment. Samples were thin-sectioned and analysed under an optical microscope. Microscopic evidence (i.e., at the scale of tens-to-hundreds of microns) shows that grain size reduction occurred mostly by cataclasis and occasionally by pressure solution. Cataclasis involved three main processes here named intragranular extension fracturing, chipping and shear fracturing. Intragranular extension fracturing is more common in the early stages of cataclasis and produces a coarse breccia consisting of angular grains. In a few cases, pre-existing weaknesses and flaws control the fracture pattern associated with intragranular extension fracturing. Chipping is more common in the advanced stages of cataclasis and produces a gouge consisting of a few survivor rounded grains within a fine matrix. Shear fracturing seems less frequent than the other two processes and usually occurs in the advanced stages of cataclasis. By considering the microscopic and mesoscopic evidence, and the dissimilar frequency of dissolution structures in the analysed fault cores and damage zones, it is inferred that the studied fault zones probably acted as conduit–barrier permeability systems. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Faults in low-pressure low-temperature carbonate rocks are known both as earthquake foci (Amato et al., 1998; Di Bucci and Mazzoli, 2003; Del Gaudio et al., 2007) and as complex permeability structures within hydrocarbon, water and geothermal reservoirs (Eberli et al., 2004; Mancini et al., 2004; Mazzullo, 2004; Celico et al., 2006; Rossetti et al., 2007a,b). Their study at all scales is therefore relevant for structural geologists dealing with seismic faulting or working in the hydrocarbon, water and geothermal industries. Until about 1990, carbonate fault rocks were hardly studied (e.g., Turner et al., 1954; Rutter, 1974; Mimran, 1976, 1977; Friedman and Higgs, 1981) compared to fault-related crystalline and silicoclastic rocks (e.g., Engelder, 1974; Sibson, 1977; Sammis et al., 1986; Sammis and Biegel, 1989; Blenkinsop, 1991). In the last fifteen years, the study of carbonate fault rocks has significantly advanced and become systematic mostly because of its importance in the hydrocarbon industry (Burkhard, 1993; De Bresser and Spiers, 1993; Hadizadeh, 1994; Newman and Mitra, 1994; Babaie et al., 1995; Kennedy and Logan, 1997; Salvini et al., 1999; Graham et al., 2003; Kim et al., 2003; Storti et al., 2003; Llana-Funez and Rutter, 2005). Studies of fault core permeability (Ghisetti et al., 2001; Agosta and Kirschner, 2003; Micarelli et al., 2006; Agosta et al., 2007), grain shape evolution with fault slip (Storti et al., 2007) and some earthquake indicators obtained in laboratory-simulated faults (Han et al., 2007a,b; see Billi and Di Toro, 2008 for a review) have recently improved our understanding of the mechanical and hydraulic behaviour of carbonate fault rocks. In particular, faulting simulations performed at seismic slip rates (about 1 m/s) in Carrara marble revealed very promising results and showed that the seismic (i.e., frictional) process and related indicators have to be investigated at the microscale (Han et al., 2007a,b). Unfortunately, the microtectonics of low-pressure low-temperature fault-related carbonate rocks is still poorly emphasized, and published microscopic images of these rocks are relatively rare (Wenk, 1985; Pieri et al., 2001a,b; Barnhoorn et al., 2004, 2005; Billi, 2005, 2007; Tondi et al., 2006; Tondi, 2007; Billi et al., 2008; Ferrill and Morris, 2008; Mort and Woodcock, 2008). This lack of knowledge prevents advances in the understanding of the processes responsible for the formation of carbonate fault cores and, therefore, in the understanding of the frictional and hydraulic behaviours of these structures. The main goal of this paper is to contribute in knowing and understanding the microscopic processes that are responsible for the development of carbonate fault cores. To reach this goal, A. Billi Journal of Structural Geology 32 (2010) 1392–1402 1393 microscopic images from low-pressure (