Morphology, intersections, and syn/late-diagenetic origin of vein networks in pelites of the Lode `ve Permian Basin, Southern France Ghislain de Joussineau * , Loı ¨c Bazalgette, Jean-Pierre Petit, Michel Lopez Laboratoire de Dynamique de la Lithosphe `re (UMR 5573), c.c 060, Universite ´ Montpellier II, Place Euge `ne Bataillon, 34095 Montpellier cedex 5, France Received 17 June 2003; received in revised form 22 January 2004; accepted 30 June 2004 Abstract This paper presents the results of a field study aiming to describe and to interpret new types of relationships between vein sets. Three vein sets of the Lode `ve Permian Basin (Languedoc, Southern France) were studied. They consist of a family of N100E–N120E, widely opened sinuous veins with a composite infilling (sparite, calcite fibres, barite), hereafter called Sparitic Sinuous Veins, and of two orthogonal families of slim veins presenting a fibrous infilling of calcite, and oriented N10E–N20E and N90E–N100E, respectively. These two latter families are hereafter called N20 Fibrous Slim Veins and N90 Fibrous Slim Veins. The intersections between the three vein sets are shown to be original and more complex than those classically observed in the case of joint sets (abutting, crosscutting, .). The analysis of these intersections permitted a relative chronology of formation of the different vein sets to be established: the N20 Fibrous Slim Veins post-date the N90 Fibrous Slim Veins, which themselves post-date the Sparitic Sinuous Veins, the two former vein sets being shown to reopen contemporaneously. Furthermore, the vein intersections were not as simple as expected. Indeed, the successive propagating cracks of one set could in certain cases crosscut the pre-existing veins of an earlier set, or could be stopped at contact with earlier veins in other cases. This implies a physical change in the interface between the pre-existing veins and the host rock (pelites) during the formation of the latest cracks. Taking into account the well-known tectonic history of the Lode `ve basin, and the field observations (vein features, intersections, .), the origin of each vein set is discussed. We put forward that the Sparitic Sinuous Veins formed during the burial history of the basin: their morphological characteristics suggest that they appeared during the phase of active compaction of the basin, in response to the Permo- Triassic NS extension, and that their formation was assisted by fluid pressure. Second, we propose that the two orthogonal sets of Fibrous Slim Veins formed during the same extensional phase, their formation being favoured by the tectonic relaxation occurring at the beginning of the uplift of the basin at the end of the Permian. We attribute their particular characteristics (straight morphology of joints but systematic mineral infilling) to their origin in a medium already compacted but still incompletely lithified, and containing a large amount of fluids. We suggest that the fracturing history of the Lode `ve basin occurred in the time interval between the Late Permian (Thuringian) and the Middle Triassic (Anisian). Finally, the observed evolution in the propagation path between the Sparitic Sinuous Veins and the N90 Fibrous Slim Veins is discussed, and bears out the idea that the rock material was not completely lithified when the vein sets formed. q 2004 Elsevier Ltd. All rights reserved. Keywords: Vein intersections; Pelites; Lode `ve basin; Compaction; Burial and uplift; Thuringian; Anisian 1. Introduction In spite of a hundred years’ studies on joints (Pollard and Aydin, 1988) and veins (Durney and Ramsay, 1973; Hancock and Atiya, 1975; Ramsay, 1980; Cox, 1987; Urai et al., 1991) many fundamental aspects of mode I fracture morphology and genesis still have to be understood. A basic one is that of the morphological and mechanical transitions between joints (long and straight planar discontinuities showing opening displacements with no appreciable shear displacement) and veins (discontinuities often presenting en-e ´chelon or sinuous geometries and significant opening but no shear displacement, filled by a cement derived from hydrothermal processes). These two fracture types are 0191-8141/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2004.06.016 Journal of Structural Geology 27 (2005) 67–87 www.elsevier.com/locate/jsg * Corresponding author. Present address: Geological and Environmental Sciences Department, Stanford University, Stanford, CA934305, USA E-mail address: dejoussi@stanford.edu (G. de Joussineau).