Résumé
Le but de la présente note est la description et l’interprétation d’une structure singulière jalonnant le trajet de la faille du Vuache dans le Jura méridional : le relais de failles transpressif Léaz-Champfromier. La faille du Vuache, qui constitue une structure tectonique majeure du Jura méridional, peut se suivre du sud-est vers le nord-ouest sur environ 100 km depuis le lac d’Annecy jusqu’au front du Jura et se traduit dans la couverture par un décalage sénestre. Notre étude suggère que son trajet dans la couverture, jalonné par des relais de failles complexes entre des parties linéaires bien exprimées, traduit un motif général « en tireté » de l’accident du Vuache au niveau du socle, c’est-à-dire un alignement discontinu de tronçons de fractures verticales (peut-être disposées en échelon sénestre). Dans ce contexte, le relais de failles transpressif Léaz-Champfromier est interprété comme jalonnant l’extrémité nord d’un de ces « tiretés ». Dans la partie médiane de chaque tronçon de fracture de socle, le mouvement sénestre s’effectuerait librement, ce qui se traduit dans la couverture par des portions de failles rectilignes, alors qu’il se bloquerait à leurs extrémités. La déformation se trouve alors préférentiellement absorbée par la couverture, qui doit s’ajuster. Le phénomène se manifeste par l’apparition de failles divergentes verticales ou de rampes peu pentées provoquant des chevauchements, rétrochevauchements, rampes latérales et failles de déchirement qui contrôlent le plissement. L’organisation de la faille du Vuache dans le socle influe certainement sur la distribution de portions sismiquement actives ou passives le long de l’accident. En effet, sur certaines portions de son trajet, le système de la faille du Vuache est responsable de séismes à répétition, dont le dernier en date est celui d’Annecy du 15 juillet 1996, alors que d’autres parties restent apparemment inactives. Les autres failles réputées de la même famille disparaissent sous le bassin molassique genevois sans marquer de leur empreinte la chaîne du Jura. Elles pourraient correspondre à des accidents de socle de moindre extension que celui de la faille du Vuache. Reste à déterminer les structures tectoniques traduisant les « effets d’extrémités » de ces structures dans le bassin genevois.
Abstract
The completion of the 1:50,000-scale Saint-Julien-en-Genevois geological quadrangle achieves a new step in the history of geological exploration in the Jura Haute-Chaîne and the Vuache Mountain initiated more than a century ago by the pioneering work of Schardt (1891). The mapped area lies 25 km west of Geneva, in the Internal Jura bordering the Swiss Molasse plateau. It straddles the path of the Vuache fault system, a left-lateral transcurrent fault zone that is a major tectonic feature of the Southern Jura. Here, we present the results of several years of detailed fieldwork by a team of mapping geologists in a tectonically complex area. Specifically, we discuss the geometric pattern of the Vuache fault system. Because the geological cross-sections cut across this major transcurrent fault, making it difficult to balance them, we have resorted to merely presenting the high-level tectonic structures in detail, as deduced from field mapping, without attempting to balance them. Although a balanced cross-section was presented in the ECORS program for the zone straddling the path of the Vuache fault system (Guellec et al., 1990), this did not apparently take into account the fact that it was crossing a major trans-current fault. The difficulty was further compounded by the poor resolution of the geophysical profile in the Jura mountains, with the result that the Vuache fault system was ignored. We hope our field data will contribute to a realistic image of this part of the internal Southern Jura. The vertical to steeply NE-dipping Vuache fault system crosses the map area from SE to NW (Fig. 2) and exhibits seismically active parts. The stratigraphic record has shown that the Vuache fault system has been active from Permo-Carboniferous times up to the present (Enay, 1966; Krummenacher, 1970; Charollais et al., 1983; Blondel, 1984, 1988; Guyonnet, 1987; Blondel et al., 1988; Gorin et al., 1993; Signer and Gorin, 1995) and its seismic history is well known (Sambeth, 1984; Sambeth and Pavoni, 1988; Amato, 1983, 1985, 1988). It was notably responsible for the Annecy earthquake that occured on July 15th, 1996. The great length of the Vuache fault system suggests that it is located above a major basement fault. For a number of authors, the imprint of the Vuache Fault can be followed along a general N 150° trend from Lake Annecy between the Bauges and the Bornes subalpine massifs in the southeast, to the overthrusted Jura front in the Bresse basin (Wildi et al., 1991) approximately 100 km to the northwest, thus linking the Alpine and Jura thrust fronts (Signer and Gorin, 1995) (Fig. 1). It would belong to the so-called “radial faults” (Pavoni, 1975), a family of left-lateral transcurrent faults cutting radially through the arcuate Jura mountain chain. In the Geneva area, this family comprises the La Caille, Cruseilles, Coin (Fig. 2) and Arve faults. Our study deals with the newly mapped area extending from the latitude of the Vuache Mountain in the south to that of Reculet in the north (Figs. 2, 4 and 5). Description of the tectonic structures related to the Vuache fault system (figs. 2, 5 and 6) In our work area, the Vuache Fault can be divided into three parts that are, from south to north: - south of the Rocher de Léaz, in the Montagne du Vuache, where the trajectory of the Vuache Fault is supposedly simple and rectilinear; - north of the Rocher de Léaz, where the fault splits and opens to the northwest into a “horse-tail” pattern that extends across most of the study area. It includes a main eastern branch and a secondary curved western branch that together define a triangular area, the “Léaz-Champfromier left-lateral transcurrent transfer zone” which is limited to the north by the Monnetier backthrust; - north of the Monnetier backthrust where the system continues as the left-lateral en echelon Haute-Crête-Les Bouchoux fault zone. The above pattern defines three contrasted tectonic units: - the Haute-Chaîne-Chalam unit to the east, which contains the Reculet and Crêt de Chalam thrusts; - the Retord-Giron unit to the west, which is a tabular kink-folded area; - the Léaz-Champfromier unit in between, which displays a complex tectonic pattern. The stratigraphic column (Fig. 3), which has been refined in the course of the mapping, comprises regularly alternating soft (ductile) and hard (competent) horizons, responsible for the particular Southern Jura tectonic style. The Haute-Chaîne-Chalam unit comprises two N 30°-trending ramp anticlines thrust towards the west-northwest: the Reculet and Crêt de Chalam thrusts (Fig. 5). The cross-section (Fig. 7-A) shows the horizontal shortening to be respectively 4 km for the Reculet thrust and 2 km for the Crêt de Chalam thrust. Both thrusts are bounded on their western side by the Vuache fault zone: the main branch of the Vuache fault system is a lateral ramp to the Crêt de Chalam thrust and a tear fault to the Reculet thrust. This implies that the Vuache fault system and the Haute-Chaîne-Chalam unit have a common deformation history. The Retord-Giron unit consists of several plateaus separated by conjugate left- and right-lateral transcurrent faults (Figs. 4, 5 and 6), respectively trending N 110° and N 80°. The tectonic backbone consists of a succession of N 0°- to N 15°-trending anticlines and synclines bounded by kink-style rectilinear and steeply dipping limbs, locally faulted and/or overturned to the west (Aeschlimann, 1996). Two types of fold occur, both exhibiting the same kink-fold style: major folds rooted on inverted, former extensional, probably Oligocene faults of kilometric strike length (Glangeaud, 1944, 1947; Bienfait, 1981; Bergerat, 1985) and shallow kink-bands of hectometric strike length. The Retord-Giron unit evolved independently from the Vuache fault system. The Vuache fault system The Vuache Mountain monocline dips 40° to 70° NE (Schardt, 1891; Scolari, 1955; Blondel, 1984 1988; Blondel et al., 1988). The Vuache fault zone is marked by huge cliffs in the western slope of the mountain. A preliminary study of the replicas of the July 15th, 1996 earthquake in the Annecy area (25 km south of our study area) have shown the Vuache Fault to be dipping 80° NE along two parallel planes (F. Thévenot, 1996, oral comm.) The Léaz-Champfromier transcurrent fault-transfer zone is framed by the main and secondary branches of the Vuache fault system and by the Monne-tier backthrust (see above, and Fig. 5). It contains a complex assemblage of tectonic structures with a N 90° to N 140° direction of shortening, which indicates an internal compressional deformation consistent with a left-lateral displacement along the borders of the fault-transfer zone (Loubat, 1963; Arikan, 1964; Tripet, 1966; Wernli and Jaquet, 1972; Mage, 1983; Blondel, 1984; Copson, 1984; Guyonnet, 1987, 1988; Blondel et al., 1988; Nussbaumer, 1995; Graezer, 1995; Meyer, 1995; Vilpert, 1996). The left-lateral Haute-Crête-Les Bouchoux en echelon faults develop to the north of the mapped area. Still farther to the north, the trace of the Vuache Fault can be followed along the left-lateral rectilinear Vulvoz - Molinges Fault (Fig. 2). Between the Léaz-Champfromier fault-transfer zone and the Haute-Crête-Les Bouchoux en echelon faults, the fault system is intercalated by the Combe d’Evuaz molasse syncline (Fig. 5). Offsets along the Vuache fault system are measured by two types of displacement marker: previously existing tectonic structures (cartographic offsets) and stratigraphic markers along geological cross-sections (tectonic shortening). Although the two types of structural marker do not have the same significance, we combine them for a total estimate of offsets. They give minimum values, because there is as yet no independent means for estimating the total offset and shortening induced by the play of the Vuache fault system during its entire lifetime. It is interesting to note that the mapped offset along the (vertical or steeply E-dipping) main branch of the Vuache fault system in the vicinity of the Crêt de Chalam thrust is of the same value as that of the horizontal shortening measured along this thrust (2 km) (Figs. 5, 6 and 7A). This confirms the cartographic pattern indicating that the main branch acts as a lateral ramp for this thrust (see above). In our work area, the total minimum horizontal offset along the main branch is estimated at 6 km; that along the secondary branch at 1 km; that induced by the en echelon Haute-Crête-Les Bouchoux fault zone at 2 km; and the horizontal shortening in the Léaz-Champfromier fault-transfer zone at around 5 km (where the shortening induced by the Monnetier backthrust alone amounts to around 3.5 km). The vertical offsets are highly variable. They can reverse along the trend of a fault, which is typical of strike-slip faults. Significance of the Vuache fault system geometry The geometric pattern of the Vuache fault system suggests that the left-lateral displacement along the rectilinear trans-current Vuache Fault was blocked northward of the Vuache Mountain. The tectonic regime changes abruptly from a sliding regime in the south, to a compressive one in the north (Léaz-Champfromier fault-transfer zone). The fault and ramp deformation in the Léaz-Champfromier fault-transfer zone is interrupted north of a line defined by the Crêt de Chalam thrust and the Monnetier backthrust (Fig. 5). It resumes farther to the north in the La Haute-Crête-Les Bouchoux en echelon fault zone and in the rectilinear left-lateral Molinges Fault. The question is: what induces the blocking of the left-lateral sliding along the Vuache fault zone? To answer this question, we invoke a pattern in the basement where the Vuache fault zone would consist of separate segments aligned (or arranged in a left-lateral en echelon pattern) along the general trend of the Vuache Fault. Sliding would be easy along the middle part of the segments, but would be blocked at their ends. Here the strain would have to climb to a higher level and invade the cover rocks, producing a pattern of divergent, vertical and gently dipping faults (thrusts, backthrusts, lateral ramps or tear faults) giving rise to folds. If this suggestion is correct, then the occurrence of fault-transfer zones between rectilinear fault portions in the cover would indicate “extremity effects” of rectilinear fault segments in the basement. This mechanism could be relevant to the distribution of seismic and aseismic segments along the Vuache fault zone, where earthquake foci would be distributed in the vicitnity of areas where the basement is involved. Also of note is the contrast between the Vuache fault system and the related left-lateral la Caille, Cruseilles, le Coin and Arve fault zones (Fig. 2). Although, in the south, they all appear as transcurrent fault zones across the Salève mountain chain, only the Vuache Fault reappears on the northwestern side of the Molasse basin and continues clearly in the Jura mountains. We can surmise that the rectilinear trace of the other faults in the cover occurs above vertical basement fractures that are less developed than the Vuache fault system. It remains to identify, in (or under) the Molasse basin and masked by the Quaternary cover, the tectonic patterns reflecting the “extremity effects” of these faults.
Dernière mise à jour le 02.07.2015