Morphology, intersections, and syn/late-diagenetic origin of vein networks in pelites of the Lodeve Permian Basin, Southern France

摘要

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 Lodeve 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 Lodeve 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 Lodeve 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.