Mouvement post-messinien sur la faille de Nîmes; implications pour la sismotectonique de la Provence

Abstract

The seismicity of southern France probably results from the convergence between Africa and Europe which proceeds at a rate of approximately 0,8 cm/year at the Provence longitude [Nuvell-DeMets et al., 1990]. The potentially active faults delimit a large panel in the Mesozoic cover. It includes E-W compressive structures (Mont-Ventoux, Montagne de Lure to the north, Luberon, Costes et Trevaresse to the south) and NE left-lateral strike slip (Durance to the east and Nimes, to the west, and possibly the Cevennes Fault) [e.g., Grellet et al., 1993; Sebrier et al., 1997; Lacassin et al., 1998]. The Nimes Fault, which is considered as one of the main faults of southeastern France [Combes, 1984; Grellet et al., 1993; Ghafiri, 1995] is associated with only few and small seismic events, but paleoseismic evidence for larger earthquakes, with magnitudes possibly as large as 6.5, were found on a subsidiary fault near Courthezon [Combes et al., 1993]. Here, we try to quantify fault activity over a longer period of time than that accessible from the usual geomorphic approach, by assessing possible displacement of Messinian markers on the Nimes fault. In the early Miocene a regional erosion surface of Burdigalian age (around-20 Ma) was formed. This surface is still preserved and has not been much deformed west of the Nimes fault. To the east, this surface is only gently folded due to E-W anticlines [Champion, 1999; Champion et al., 2000]. This contrast suggests that the Nimes fault has been active and has accommodated N-S shortening after the abandonment of the Burdigalian erosion surface. The Nimes and Pujaut faults can be followed in the topography between Nimes and Sauveterre where they are generally bounded by outcrops of Mesozoic limestone (fig. 2A-3). To the NE, the Nimes fault can be roughly traced across the Quaternary Rhone alluvium, between Sauveterre and the Mont Ventoux. It is marked by disruption of the continuity of the terrasses of Chateauneuf-du-Pape. The signal is only tenuous and cannot be used to infer precisely the fault location and segmentation but suggests that the faults have been active during the Quaternary. During the Messinian, starting at about -5.95 Ma, the Mediterranean sea level fell by about 1500 m [Clauzon, 1975; Krijgsman et al., 1999; Gautier et al., 1994; Cande et Kent, 1992-1995; Clauzon et al., 1995]. The major tributaries were forced to cut down and formed deep and narrow valleys. The Mediterranean sea rose up to an elevation of +80 m NGF at -5.32 Ma, flooding the canyons, and remained stable until about -3.8 Ma [Vail and Mitchum, 1979; Benson et al., 1991; Cita, 1975; Haq et al., 1987; Hilgen et Langeries, 1993]. After -5.32 Ma the canyons were filled with Pliocene sediments. The canyon formed by the Rhone incision during the Messinian crisis is well documented [Clauzon, 1982; Clauzon et al., 1995; Clauzon et al., 1999; Rubino et al., 2000]. We found evidence for a tributary canyon on an old seismic line ELF M2S8. The canyon strikes E-W between the "Barre de Roquemaure" and "Barre de Caderache" and should cross the Nimes Fault. In order to constrain more tightly its geometry near the Nimes Fault, we have implemented three seismic lines. If we trace the position of the southern border of the canyon using the different profiles and the surface geology, we find that the horizontal offset at the fault cannot be much larger than about 500 meters. In order to image a possible smaller offset we have determined the geometry of the canyon from a microseismic zoning technique [Nakamura, 1989; Duval et al., 1997; Ibs-von Seht M. and Wholenberg, 1999; Sabourault, 1999]. Measurements were conducted at 37 points which were used in complement. The depth to the canyon bottom was determined using the velocities derived from the seismic profiles and was cross-checked from the comparison with geological log at points F1 and F2. The geometry of the southern edge of the Messinian canyon, shows a left-lateral offset of 440 m + or -50 m, which might be taken to reflect post-Messinian fault motion. Assuming that the observed 440 m offset of the Messinian canyon at the Nimes fault is due to fault motion, and that the fault slip rate has not varied significantly since the Messinian crisis, we derive a left-lateral slip rate of 0.06 to 0.09 mm/year. Given that the observed deflection of the flank of the canyon might in part be of non tectonic origin, our study basically places an upper limit of 0.09 mm/yr on the slip rate on the Nimes faults. Such a low slip rate is comparable with estimates obtained on the Durance Fault [Baroux, 2000] and on the E-W folds and thrust faults east of the Nimes faults such as Ventoux-Lure and Alpilles-Costes-Trevaresse [Champion, 1999; Champion et al., 2000]. Although the details of the kinematics scheme of active deformation of Provence remain a matter of discussion, these various faults are probably linked and must have similar slip rates.