Difference in petrophysical properties between foliated and dilatant fault rocks in deeply buried clastics: The case of the Gr  es d’Annot Formation, SW French Alps Thibault Cavailhes, 1 Pierre Labaume, 1 Jean-Pierre Sizun, 2 Roger Soliva, 1 Claude Gout, 3 Jean-Luc Potdevin, 4 Martine Buatier, 2 Aur elien Gay, 1 Alain Chauvet, 1 Delphine Charpentier 2 and Anna Trav e 5 1 G eosciences Montpellier (UMR 5243), Universit e Montpellier 2 – CNRS, Montpellier, France; 2 Chrono-Environnement (UMR6249), Universit e de Franche-Comt e – CNRS, Besanc ßon France; 3 Total Exploration et Production, Pau, France; 4 G eosyst emes (UMR 8217), Universit e Lille 1 – CNRS, Villeneuve d’Ascq, France; 5 Facultat de Geologia, Universitat de Barcelona, Barcelona, Spain ABSTRACT This study describes normal fault zones formed in foreland arkosic turbidites (the Gr es d’Annot Formation, SW French Alps) under deep diagenesis conditions (~200 °C) and high- lights the occurrence of two markedly different fault-rock types: (1) the foliated fault rocks of the Mouti ere-Restefond area; and (2) the dilatant fault rocks of the Estrop area. The deformation of (1) is dominated by intra- and transgranular fracturing, pressure solution of quartz and feldspar grains and syn-kinematic phyllosilicate precipitation resulting from feldspar alteration. The combination of these mechanisms results in a strongly anisotropic strain with intense shortening normal to the foliation (pressure solution) and extension par- allel to the foliation (quartz- and calcite-sealed extension veins). This deformation implies local mass transfer that may be achieved without (or with limited) volume change. The deformation of (2) is expressed as dilatant quartz-sealed veins and breccia textures in which the main mechanisms are trans- granular fracturing and quartz precipitation. Type (2) implies fault volume increase, isotropy of deformation and mass transfer at distances larger than in type (1). This study dis- cusses the origins of (1) and (2) and shows that the perme- ability of (1) is anisotropic, with higher values than the host rocks parallel to the Y main deformation axis (i.e. perpendicu- lar to the slip vector), whereas the permeability of (2) is isotropic and equivalent to that of the host rocks. Terra Nova, 26, 298–306, 2014 Introduction Predicting the hydraulic behaviour of faults is one of the most uncertain processes in the understanding of upper-crust fluid flow (Corrigan, 1993; Fisher and Knipe, 2001). Fault- related fluid redistribution is signifi- cantly influenced by hydraulic gradient, in-situ stress conditions, fluid characteristics and fault-rock properties (e.g. Faulkner and Rutter, 1998; Manzocchi et al., 1999; Sibson, 2000; Yielding, 2002; Eichhubl et al., 2009; Faulkner et al., 2010; Gud- mundsson et al., 2010). The latter are a key parameter that is mainly related to the nature and mechanical proper- ties of the protolith, the fault offset, the rock and fluid pressures and tem- peratures at the time of faulting and how the resulting fluid–rock inter- actions enhanced mineral dissolution or/and precipitation (e.g. Solum et al., 2005, 2010; Ajdukiewicz and Lander, 2010; Onasch et al., 2010; Pollington et al., 2011). Understand- ing the relationships between the different kinds of fault rocks and their petrophysical properties is par- ticularly important for the 5- to 8-km burial interval, which corresponds both to part of the seismogenic zone (Maggi et al., 2000) and to deeply buried reservoirs, an increasingly important target for hydrocarbon and mining exploration (Fisher et al., 2003; Laubach and Ward, 2006). Here, we study two types of fault rocks related to normal fault zones formed in the Gr es d’Annot Forma- tion, an arkosic turbidite succession of the SW-Alpine foreland basin, under deep diagenesis conditions (around 200°C) (Cavailhes, 2012; Lecl ere et al., 2012; Cavailhes et al., 2013a,b). Two study areas were selected, such that the following parameters were similar at the time of faulting: the protolith lithology, the fault offset, the mechanical prop- erties linked with burial diagenesis and the P–T conditions. This similar- ity in properties and conditions allows us to discuss the role of fluids in the development of two different types of fault-zone structures where mechanical and chemical processes that control the deformation mecha- nisms are intimately linked. We also assess the anisotropy in permeability of the fault rock in accordance with the structural deformation orienta- tion and compare it with the host- rock values. This study highlights the relationship between the strain ellip- soid and the permeability ellipsoid in a fault zone. Geological setting The Gr es d’Annot Fm. was depos- ited during the Priabonian-Rupelian in the SW-Alpine foreland basin and buried under the Embrunais-Ubaye nappes soon after deposition (Ker- ckhove, 1969; Joseph and Lomas, 2004) (Fig. 1A). The Miocene uplift associated with basement thrusting resulted in the exhumation of the Gr es d’Annot (Labaume et al., 2008) and the formation of several genera- tions of normal faults (Labaume et al., 1989). The studied normal faults are located in two distinct Correspondence: Thibault Cavailhes, DNO International ASA, Bryggegata 9, Aker Brygge, 0250 Oslo, Norway. E-mail: thibault.cavailhes@dno.no 298 © 2014 John Wiley & Sons Ltd doi: 10.1111/ter.12100