Deformation of a thrust complex in the Col de la Vanoise, Vanoise Massif, French Alps GcSeK)N S^LOTER* 1 Institute for Earth Sciences, University of Utrecht, 3508TA Utrecht, The Netherlands ABSTRACT to have involved a major component of dolomitic marble, marble, and their brecciation northward translation. and chemical transformation product, known A structural analysis of the area surround- locally as "Cargneule." Before its alteration to ing the Col de la Vanoise in the Vanoise Mas- INTRODUCTION the present form of Cargneule, gypsum served as sif of the French Alps demonstrates the a lubricant on various fault planes at all stages of importance of two major thrusting events in An example is presented of structural analysis the tectonic cycle (Warrak, 1974). Lower and this part of the Alpine orogenic belt. This of a multiply deformed thrust belt by detailed Middle Jurassic sedimentary sequences, if pres- study emphasizes i:he three-dimensional com- investigation of the segment of the belt where ent, involve either thin quartzitic carbonates or a plexity of structure on the kilometre scale successive deformation imprints are best devel- thick pile of (mainly Liassic) banded carbonates. which characterizes the Vanoise Massif. Four oped. Structural relationships derived from such Ellenberger (1958) used this transition to clas- generations of structures can be distinguished a key area, when integrated into regional tecton- sify the Mesozoic stratigraphy in two series, on the basis of systematic overprinting cri- ics, can significantly constrain orogenic evolu- namely, (a) the Vanoise Occidental series and teria on the mesoscale. Tracing of these struc- tion models. (b) the Grande Motte series. The two facies se- tures across the well-developed lithostratig- The study area is part of the Vanoise Massif, ries mark the Jurassic paleogeographic transi- raphy allows correlation with kilometre-scale which is situated in the Briançonnais Zone, a tion from the Briangonnais platform to deep structures, produced during each of these narrow belt defined by Mesozoic platform facies marine environments. Across both facies types, folding events. Thij youngest structures can sedimentation in the Pennine paleogeographic subsequent uniform shelf sedimentation is now be explained by horizontal shortening (D4) domain of the Alpine orogeny (Fig. 1). A zone represented by relatively pure marble (Late Ju- which produced upright folds and kinks. On of steep faults, the "Accident Modane-Cham- rassic) and chloritic marble (Upper Cretaceous- the limb of a majo r D4 dome in the north- pagny" (Raoult, 1980) separates the western Lower/Middle Eocene; Ellenberger and Raoult, western part of the area, originally shallow- side of the Vanoise from Permian-Carboniferous 1979), covered by dark slates, presumably of dipping structures are thrown into steep metasediments of the Zone Houillère. The east- Middle/Upper Eocene age (Ellenberger, 1958). attitudes. D4 may l»e related to either large- ern margin of the massif contains a major tec- Mineral parageneses of the Permian-Eocene scale imbrication associated with the second tonic boundary, where calcareous micaschists cover have been shown to involve medium- to thrust event or to ail event in which consider- (the "Schistes Lustrés") and ophiolites from the high-pressure-low-temperature metamorphism able differential uplift took place. The second Piémont paleogeographic domain are thrust (Ellenberger, 1958; Debelmas, 1974; Goffe, thrusting (D3) took place under greenschist- across the Briançonnais Zone (Debelmas, 1974). 1977, 1982). facies conditions and culminated in move- In the central part of the Vanoise Massif, This paper presents an analysis of t wo sepa- ment eastward along two major gently micaschists and massive metabasites define an rate events of thrusting in the central part of the dipping shear zones. The zone of relatively allochthonous "basement" complex, in which Vanoise fold belt (Fig. 2). It will be demon- low D strain between both horizons allows 3 high-pressure mineral parageneses have been strated that the two stratigraphic fades series of distinction of a pie-D deformation asso- 3 found (Bocquet, 1974a, 1974b; Bocquet and Ellenberger (1958) described above are con- ciated with tight upright folds (D2). This others, 1974; Goffé, 1975, 1977; Piatt and Lis- fined to different, early formed fold nappes and generation of structures overprinted recum- ter, 1978,1985a, 1985b). High-pressure assem- thrust slices, which involve Permian, Mesozoic, bent folds and shear zones which represent blages in the Western Alps are considered to and Tertiary rocks. Formation of this thrust the earliest recognizable deformation (Dj). indicate metamorphism by rapid tectonic burial, complex was followed by development of two The Dj event culminated in formation of fold possibly in relation to plate collision (for exam- major shallow-dipping shear zones in which nappes and thrust slices and can be inferred ple, see Ernst, 1973; Frey and others, 1974). mylonites formed. On these horizons, move- The high-pressure metamorphics are overlain ment eastward took place in a tectonic event by Permian greenschist and quartz-phengite known in the western Alps as "backthrusting" "Present addresses: (de Roo) Department of Geol- schist (Ellenberger, 1958; Goffé, 1975). The or "retrocharriage" (Debelmas and Kerckhove, ogy, James Cook University, Townsville, 4811 1973; Debelmas, 1974; Raoult, 1980; Piatt and Australia; (Lister) Department of Earth Sciences, Mesozoic metasedimentary sequence has pure Monash University, Wellington Road, Clayton, 3168 quartzites at its base, covered by a Triassic evap- Lister, 1985a, 1985b). This analysis of part of Australia. orite sequence represented now by gypsum, the pile of "cover" thrusts in the Vanoise sup- Geological Society of America Bulletin, v. 98, p. 388-, 9, 10 figs., April 1987. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/98/4/388/3419480/i0016-7606-98-4-388.pdf by guest on 02 October 2021 DEFORMATION OF THRUST COMPLEX, FRENCH ALPS 389 Figure 1. Geological setting of the Vanoise Massif (after Piatt and Lister, 1985a). Arpont Schist: pre-Permian metamorphic complex. Zone Houillère: low-grade Carboniferous and Permian clastic metasediments, and foliated metagranite. Open circles: pre-Triassic metaclastic rocks of probable Permian age. Bricks: Triassic to Eocene cover. V-pattern: Schistes Lustrés. Inset: Western Alps. Horizontal lines: External Zone cover. Crosses: external crystalline massifs. Diagonal lines: Austro-Alpine and Southern Alpine domains. ports basic structural relations Piatt and Lister confirmed. In the Col de la Vanoise, the major The arrangement of asymmetrical kinks and (1985a, 1985b) inferred regionally on the basis deformations involve complex interference pat- folds indicates differential uplift of the central of their study of the Arpont Nappe, composed terns that require reconstruction of successive axis of the Col de la Vanoise region with respect mainly of the pre-Permian Vanoise "basement" events backward in time, as follows. to the eastern and western parts of the area. The schists. At the Col de la Vanoise, full advantage uplift is reflected by the shape and disposition of has been taken of the well-understood lithostra- Horizontal Shortening Leading to the glaucophane micaschist unit shown in Fig- tigraphy and excellent exposure to achieve a Downwarping in the Northwestern Part of ure 3. The domal antiform continues southward complete understanding of the structure of the the Vanoise Massif through the Arpont region described by Piatt thrust complex. and Lister (1985a, 1985b). Westward of the Re- The youngest set of structures in the central fuge du Col de la Vanoise (2,516 m), situated in STRUCTURAL ANALYSIS part of the Vanoise consists of upright chevron the central part of the studied region, there is a folds and kinks with steeply dipping axial zone dominated by D4 kinking on the meso- Systematic and detailed structural analysis of planes. These are found across the entire area in scopic scale (Fig. 4; lower left). This zone forms the Col de la Vanoise allowed construction of various attitudes, but north-south axial plane the northern termination of a megakink on the block diagrams (Figs. 2 and 3) representing the strikes dominate. The structures are most northwestern side of the dome. The "Zone des three-dimensional, large-scale geometry of the strongly developed in the northwestern part of Cirques" is an array of huge natural amphithea- area. The sequence of deformation events rec- the area. Chevron-style folds with rounded tres along the megakink. They have been carved ognized by Piatt and Lister (1985a, 1985b) in hinges rather than abrupt flexures are in some out by selective erosion of the subvertical pelitic the southern Vanoise has been independently cases accompanied by an axial-plane cleavage. schists on blocks A, B, and C of Figure 2. The Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/98/4/388/3419480/i0016-7606-98-4-388.pdf by guest on 02 October 2021 THE GEOLOGY OF THE COL DE LA VA NOISE REGION DARK SLATE CARGNEULE Cg CHLORITIC MARBLE QUARTZITE C •it. 1 1 MARBLE till QUARTZ-PHENGITE SCHIST M Pi I . 1 qUARTZITIC CARBONATE GREENSCHIST D im BANDED LyJ QUARTZITIC CARBONATE METABASITE DOLOMITE l MARBLE GLAUCOPHANE MICASCHIST Tdc Figure 2. Orthographic block dia- gram showing interpretation of de- tailed mapping of the area surrounding the Col de la Vanoise (see Fig. 1 for location).