INTRODUCTION to the PREALPES GEOLOGY
A nice map of the Préalpes from the "good old times" (click on the map to have the enlarged version)
27 November 1995; last updated: 2 April 2004
This page has been created by Jon Mosar. Jon.Mosar@unifr.ch
27 November 1995; last updated: 2 April 2004
This page has been created by Jon Mosar. Jon.Mosar@unifr.ch
The following introduction to the Préalpes Médianes structural geology is an excerpt from a paper by MOSAR J., STAMPFLI G.S. & GIROD F.: Western Préalpes Médianes Romandes: timing and structure. A Review. to be published in 1996 in Eclog. geol. helv. 89. Other regions and topics of Préalpes médianes geology are discussed in the Abstractcs from published papers following the general introduction. References cited in the following can be found in a bibliographic reference list containing some 880 titles on the Préalpes geology.
The Préalpes are formed by several allochthonous tectonic klippen resting on the N and NW central European alpine foreland, in front of the Helvetic nappes and the external crystalline basements of the Alps (Schardt 1898; Jeannet 1922; Trümpy 1960; Badoux & Mercanton 1962; Caron 1973; Plancherel 1979; fig.1). Several klippen can be recognized from E of Lucerne (Mythen klippe) to Annecy (France) in the W (Annes klippe). The largest are the Préalpes Romandes NE of lake Geneva in Switzerland and the Chablais Préalpes S of Lake Geneva in Switzerland and France. Within the klippen several tectonic units with different paleogeographical origins can be identified. One can differentiate from top to bottom: [i] the Nappe Supérieure which itself is subdivided into four different units: the Gets Nappe, the Simme Nappe, the Dranses Nappe and the Gurnigel Nappe (Caron 1972, 1976; Trümpy 1980) ; [ii] the Breccia Nappe, resting on the trailing part of the Préalpes Médianes only (Lugeon 1896; Jaccard 1904; Schroeder 1939; Weidmann 1972; Steffen et al. 1993) ; [iii] the Préalpes Médianes nappe and [iv] the Niesen nappe which exists in the Préalpes Romandes only and which today forms the southernmost structural unit (McConnel 1951; Lombard 1971; Matter et al. 1980; Homewood et al. 1984; Ackermann 1986; Caron et al. 1989) . At the base of the Préalpes Médianes and separating them from the Niesen nappe, is the "Zone Submédiane" structural unit (Weidmann et al. 1976) . Elsewhere the sole of the Préalpes is in contact with the underlying units (the Helvetic nappes in the S and the Subalpine Molasse and flysch to the N) by means of the "Ultrahelvetics" (Badoux 1963; Homewood 1977; Jeanbourquin 1991, 1992, Jeanbourquin & Goy-Eggenberger 1991; Jeanbourquin et al. 1992; Lempicka-Münch & Masson 1993) which are characterized by mélanges and cornieules (rauhwacken) associated with anhydrites.
It was Schardt (1884; 1893a; 1898) who first clearly demonstrated that the Préalpes were allochthonous (see also Lugeon 1902) . He also recognized and described the fold and thrust style of the Préalpes Médianes (Favre & Schardt 1887) , and concluded that there was a genetic link between thrusts and folds. Historical accounts of the development of ideas and geologic research in the Préalpes are given in Jeannet (1912-1913) , Bailey (1935) , Masson (1976) and Lemoine (1988) ; more than 800 references on the Préalpes can be found in Mosar & Borel (1995) .
In terms of paleogeography the Préalpes Médianes formed part of the Sub-briançonnais and Briançonnais sedimentation realm (Trümpy 1960; Caron 1972, 1973; Bernoulli et al. 1979; Caron et al. 1980a, 1980b; see general introduction Boillot et al. 1984; Lemoine et al. 1986; Debelmas 1989) . Equivalent stratigraphic units have been found both in the Préalpes Médianes and the Pennine nappes south of the Rhône valley. In these units of the Grand Saint Bernard nappe, the Siviez-Mischabel nappe has been recognized as homeland for the Médianes Rigides and the Pontis nappe for the Médianes Plastiques (Schardt 1907; Ellenberger 1952; Baud & Septfontaine 1980; Sartori 1987, 1990; Escher et al. 1993; Sartori & Marthaler 1994) . The Préalpes Médianes nappe was detached from its basement and underwent thin-skinned tectonics during the Alpine orogeny. The basal décollement occurred in evaporites located at the base of the Middle and Late Triassic. The Zone Submédiane and Niesen nappe paleogeographic homeland is on the northern European margin of the Valais ocean (Stampfli 1993; Stampfli & Marchant in press) . The Breccia nappe has its origin on the north Piemontais margin and represents mainly a synrift deposit of that margin. The Nappe Supérieure comes from the Piemontais oceanic domain (Dranses nappe; see also controversy in Hsü 1989) or the south Piemontais active margin (Gets, Simme and Gurnigel nappes).
The Préalpes Médianes are subdivided into Médianes Plastiques, forming the frontal (NW) part of the nappe and Médianes Rigides, forming the trailing (SE) part of the nappe (Lugeon & Gagnebin 1941) . This subdivision is based on the large scale structural geometry (folds in the Médianes Plastiques and imbricates in the Médianes Rigides), internal deformation (strain) is on the contrary weak and brittle in the Médianes Plastiques and stronger and more "plastic" in the Médianes Rigides. In the Préalpes Romandes, a domain with intermediate structural and sedimentological characteristics exists between the Médianes Rigides and Plastiques: the Gastlosen range.
Comments, questions, suggestions are welcome.