Deformation of a thrust complex in the Col de la Vanoise, Vanoise Massif, French

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 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.

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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

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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). V/H = 1; line of sight 40>030. Saw teeth indicate Grande Motte and Vanoise Occidentale Dj thrusts. Note the block indexation A-J; in which shades indicate removed parts of blocks. Inset block diagram: approx- imate location of D3 shear zones. Note the klippe formed by coinciding Dt and D3 shear zones in block D (upper level of saw teeth).

la vallette

A

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Figure 2. (Continued).

Loza (2538m)

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amphitheatres are s pectacular geomorphological Eastward Thrusting (1985a, 1985b) in the southern Vanoise, where expressions of the steep downwarp, where in- D3 is associated with eastward thrusting. De- itially subhorizontal D3 mylonites are in subver- Outside areas affected by kinks and chevron belmas and Kerckhove (1973), Debelmas tical positions. Without systematic analysis, folds, a penetrative, predominantly subhorizon- (1974), and Piatt and Lister (1985a, 1985b) these horizons could be mistaken as the result of tal cleavage (S3) is observed. In some cases S3 is have termed the regional eastward movement late-stage steep faults. represented only by fold axial planes, without "backthrusting," making a comparison with the The folds and kink structures form a distinct cleavage development. The subhorizontal folia- backthrusting and backfolding in the Swiss Alps deformation generation, consistently overprint- tion is associated with eastward-vergent recum- (Milnes, 1974,1978; Milnes and others, 1981). ing a subhorizontal foliation produced during bent folds and appears to be the same deforma- There are two horizons of very high D3 strain D3. It is possible that D4 structures merely re- tion D3 as recognized by Piatt and Lister which cut across the area (Fig. 2, inset). flect ongoing shortening associated with the D3 thrusting event, for example, as by-products of large-scale thrust imbrication. Alternatively, D4 may be associated with large-scale differential vertical uplift in the last stages of the Alpine orogeny.

o

MARKER HORIZON OF UNSPECIFIED STRAT/GRAPHIC LEVEL

TRIASSIC QUARTZITE

•RV^J GLAUCOPHANE MICASCHIST

BACKTHRUSTING

Figure 3. Orthographic block diagram on the same scale and covering the same area as Figure 2. The planes shown represent enveloping surfaces of foliations. Bedding attitudes are represented by form surfaces parallel to unspecified stratigraphic horizons and by the outline of the Triassic quartzite and glaucophane micaschist, shown as massive bodies. Thrust planes have been omitted.

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 Figure 4. Structural map of the Col de la Vanoise. Location and lithology symbols correspond with Figure 2, blocks E and F.

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In between, there: is a cascade of large-scale The dome in S2 produced at La Rechasse by the vergent D3 "backfold," as outlined in Figure 2, recumbent folds in the 1600-m-thick pile of vertically heterogeneous strain profile and by block I. The shear zone associated with this banded quartzitic carbonate rocks of La Grande varying degrees of rotation of the S2/S3 intersec- structure is tentatively correlated with the upper Casse (Figs. 2 and 3). Westward of the summit tion lineation is represented in Figure 3. high strain system with accompanying nodule (3,855 m), and toward the valley at the eastern The trace of D3 fold axes in outcrops outside attenuation west of La Grande Casse. The upper base of the mountain, fold wavelength decreases the major shear horizons also marks the effect of eastward thrusting horizon is best exposed at the as fold asymmetry and strain intensity increase, a strain gradient in S3 normal to the extension summit of La Rechasse, where it envelops the culminating in zones of intense attenuation in direction. This is manifested by various stages of Di thrust horizon of the allochthonous Liassic the upper and the lower D3 shear horizons. The fold axis realignment (Fig. 6, lower left) and fold banded rarbonates of the Grande Motte series resulting strain gradient on the kilometre scale is attenuation. Further evidence of heterogeneous (Fig. 2, block D). Where the Dj thrust surface is also exposed southwest of La Grande Casse, strain distribution outside the two shear zones is folded downward to the northeast, under La where ellipsoidal nodules of limestone are in- provided by the Permian rocks below Le Mol- Grande Casse, the eastward thrusting horizon creasingly stretched as one climbs toward the lard de la Loza (2,538 m), where D3 is highly can no longer follow the older surface, and the upper shear horizon. Elongated nodules are ori- variable in intensity. Here S ] has been crenu- two shear zones diverge. The D3 movement ho-

ented with their long axes parallel to the min- lated by S2 and/or S3 (Fig. 6). rizon then rises over the summit of La Grande eral lineation L3 on S3, thus defining the Mineral lineations and rotated fold axes, Casse. east-southeast-trending extension direction combined with asymmetrical shear band cleav- The lower shear zone is to be found in the (Fig. 5). Nodule elongation is accompanied by age (Piatt and Vissers, 1980), define an east- western part of the area, along a northeast- progressive rotation of foliations toward an southeast movement direction around the Col southwest-trending belt of highly deformed orientation subparallel to S3. The strain gra- de la Vanoise (Fig. 5). East of La Rechasse, rocks, including Permian greenschist, quartz- dients represent D3 strain localization which cascades of lobate mesoscopic antiforms with phengite schist, lenses of Triassic quartzite, and culminated on the regional scale with the crea- attenuated lower limbs show closure to the east. marble. This zone can be seen in the block dia- tion of two separate movement horizons asso- The lobe-shaped folds independently reflect the gram (Fig. 2, blocks A, B, C). Along the belt, a ciated with intense penetrative stretching. style and movement direction of regional "back- pre-existing thrust, marked by highly attenuated The inhomogeneous strain profile across both folding" toward the east. The direction of stratigraphic sequences, may have been partially of the D3 high strain zones is typical of large movement is subparallel to the direction of D3 reactivated by D3. Subsequent deformation has ductile shear zones (compare Grocott and Wat- backthrusting in the southern Vanoise Massif. warped the D3 shear zone locally into a steeply terson, 1980; Ramsay, 1980) and is marked by On the northwestern side of the Col de la dipping attitude (Fig. 3). The eastward thrusting

mylonites which encompass various stratigraph- Vanoise, upright D2 folds flank a steeply to horizon is found also below the central quartzite ic units, but which are most spectacular in gently dipping tectonic contact. The complete mass shown in blocks C, D, and E of Figure 2, carbonate rocks. In the mylonite zones, symmet- stratigraphic sequence from Permian greenschist arching into the Zone des Cirques, as a partially rical and asymmetrical shear band cleavages and to Eocene dark slate is incorporated in the con- reactivated Dj surface. Toward the north, this foliation boudinage (Piatt and Vissers, 1980) tact zone; Cargneule is present everywhere. lower hoiizon remains below the D[ thrust slices suggest that the zones of coaxial stretching are North of the Refuge du Col de la Vanoise exposed at Le Mollard de la Loza and the al- separated by zones of intense non-coaxial shear. (Fig. 4), the shear zone is gently inclined; older lochthonous pile of Liassic banded carbonates The shear bands represent a high strain stage of rocks to the northwest are generally found (Fig. 2, inset).

mylonite development in shear zones, after crea- above younger units to the southeast. Locally On the large scale, the two D3 movement tion of a strong foliation anisotropy (White and inclined and attenuated D2 folds, together with zones may represent part of an anastomosing others, 1980; Piatt and Vissers, 1980). Other the inverted contact, form part of a giant east- system of ductile shear zones separating D3 diagnostic features of very high ductile strain in nappes. The macroscopic strain heterogeneity the mylonite zones are penetrative mineral involved allows recognition of pre-Dj structures (stretching) lineations, rotation of intrafolial fold N in the domain of low D3 strain separating the axes toward the extension direction, and compo- two shear horizons. A horizontal shortening sitional lamination with dolomite boudinage in event with large upright refolds and associated marbles. steeply inclined cleavages has been recognized in Planar fabric elements in suitable attitudes this domain. with respect to the D3 shortening axis were folded to form eastward-vergent folds, whereas Horizontal Shortening Leading to Tight planes in the simple shear field have been Upright Folding Prior to Eastward Thrusting stretched and rotated. There are examples of

both types of behavior, but in particular, we Outside zones of high D3 strain, S2 can be note how pre-existiog foliations have been in- identified as a spaced cleavage, a penetrative corporated into the mylonitic layering by being slaty cleavage or a differentiated layering, de- rotated and stretched toward parallelism with pending on lithology. This cleavage is axial S3. This progressive deformation of pre-existing plane to tight upright folds, such as depicted on structures is well displayed across the lower the western front of block F in Figure 2. Tracing

boundary of the subhorizontal calc-mylonites of of D2 fold axes shows that these folds are highly La Rechasse (3,212 m; Fig. 2, block D), which noncylindrical; in many cases, their present mark the locus of the upper D3 thrusting ho- Figure 5. Plot of the D3 stretching lin- shape may be in large part due to effects related rizon. The progressive reorientation of an in- eations in the Col de la Vanoise region; 55 to the geometry of SQ prior to the D2 deforma- itially steeply dipping S2 can be followed from readings contoured at 1%, 8%, 20%, and 30% tion, as well as to effects related to heterogene- the northwest toward the southeast in Figure 4. (lower hemisphere of equal-area projection). ous deformation during D2. Another feature

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 Figure 6. Structural map of the area around Le Mollard de la Loza, the location of which coincides with the white marble band in the center. Compare with Figure 2, block D. For section AB see Figure 7.

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which characterizes D2 is the widespread devel- oriented normal to both cleavages. This small- a transition from gentle dips to steep attitudes

opment of cuspate folds, reflecting marked scale structure represents a similar continuous before D3 (Fig. 7). competency contrasts between pure and impure switch in dominant foliation as one climbs down The rotation and refolding of pre-existing marble that are not observed in later the 300-m-high western wall of the valley below structures during D2, forming upright folds with deformations. Le Mollard de la Loza (Fig. 6), until one en- steep cleavages in several localities, can be inter- On the large scale, the effect of D2 has been to counters the lower D3 shear horizon (Fig. 2, preted as reflecting a regional horizontal shorten- refold earlier formed recumbent folds into steep inset). ing. As suggested by the distribution of D2 attitudes, as shown, on the southern side of block D2 rotation of older foliations into steeply structure« preferably along the front of a large, E in Figure 2. Reorientation and refolding are dipping attitudes took place progressively. This pre-existing recumbent fold system, D2 may accompanied by :i steep axial plane cleavage can be demonstrated by tracing pre-D2 struc- have involved regional horizontal shortening ac- (S2), which is refolded by D3 about subhorizon- tures from the eastern base of La Rechasse, commodated at the front of the Dt structure by tal axial planes (Fig. 3). Le Mollard de la Loza where D2 left little imprint, to Le Mollard de la upright folding and high-angle faulting. Piatt and provides excellent access to a three-dimensional Loza where the imprint of D2 was particularly Lister (1985a) argued that this event was the representation of i:his type of structure, so this strong. North of Le Mollard de la Loza, there is continuation of their Dj thrusting event. outcrop was examined in detail (Figs. 6,7). Dif- an allochthonous slab consisting of sediments of ferent cleavages are locally parallel, and one fo- the Vanoise Occidentale series, molded into a Nappe Emplacement during the Earliest liation varies rapidly in its dominance over giant spoon shape by D2 (depicted in Fig. 3, Recognizable Deformation Event another (bottom left of Fig. 7). Therefore folia- right of center). The spoon-shaped slab has over- tions represented in the diagram have been ridden sediments of the Grande Motte series Excluding sedimentary structures, the earliest traced continuously on the outcrop with consid- along a steeply dipping tectonic contact. Pro- deformation that we have recognized is repre- erable care to prevent errors in establishing tim- gressive deformation during D2 involved con- sented by tight to isoclinal folds accompanied by ing of deformation events. The overprinting siderable rotation of this allochthon along the a slaty cleavage or differentiated layering (Si). relationship shown in the bottom left of Figure 7 contact, accompanied by upright folding and In Le Mollard de la Loza, these tight to isoclinal is enlarged for clarity; the true distance over subvertical D2 cleavage development (Fig. 8). folds have been rotated to vertical attitudes dur- which Si ceases to be dominant and is over- Rotation of this structure and of the refolded ing D2 and subsequently refolded in D3 (Fig. 7). whelmed by S3 is less than a few centimetres. recumbent folds of Le Mollard de la Loza con- When the early folds are reconstructed by re- Note that S[ can be seen to be actually over- centrated deformation along attenuated fold moving later deformation effects, or if they are printed by D2, whereas S3 is not affected. S2 is limbs and pre-existing shear zones, which show traced into an area not involving strong D2 or

Figure 7. Section AB of the eastern side of Le Mollard de la Loza, showing the different foliations traced (for location AB, see Fig. 6). Diagram is not to scale; AB = 350 m. Closed So-loops reflect boudinage and/or noncylindrical folds. Saw teeth indicate D] shear zones.

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D4 deformation, they define approximately of the main allochthon have caused multiple orientation of sheath folds indicate that D[ may north-vergent recumbent anticlines with attenu- repetitions of part of the stratigraphy. Bands of have involved a substantial component of ated lower limbs (see Fig. 2; and compare chloritic marble are markers of this repetition northward thrusting in the Col de la Vanoise blocks D and E). As corresponding synclines are (see Fig. 2, blocks D, E, and F, and also Figs. 4 region of this part of the Grande Motte thrust strongly attenuated (Fig. 7) or missing altogether and 6). and underlying units. This need not be incom- (Fig. 2), these early formed structures can only These allochthons are overlain by the Grande patible with the northwest direction of thrusting be interpreted as north-vergent imbricate thrusts, Motte Nappe (Ellenberger, 1958), which is lith- in Di inferred by Piatt and Lister (1985a) in the with individual imbricates having a fold nappe ologically defined by the Grande Motte facies southern Vanoise, as transport directions of dif- geometry (Fig. 8). A similar geometry may be series. The base of this thrust is marked by ferent parts of a thrust system may diverge dur- exhibited on the regional scale, but this is ex- Cargneule, and a mylonite horizon which affects ing semicontinuous progressive deformation ceedingly difficult to demonstrate because of the a range of rocks encompassing the stratigraphic (Merle, 1982; Merle and Brun, 1984). complicating effects of subsequent deformations. sequence of the upper limb of the Vanoise Oc- A sketch of the suggested early Dj geometry In the region of the Col de la Vanoise the cidental Nappe. Thrusting of the Grande Motte of the cover units (Fig. 10a) shows the thrust imbricate thrust slices are built up of basically Nappe appears to have involved kilometre-scale slices of the Vanoise Occidental series, with similar stratigraphic series, namely components boudinage of dolomite strata. A tectonic doub- imbrication along shear zones and north-vergent of the Vanoise Occidental facies series. There- ling of this dolomite horizon north of the Col de fold closures. Lithological units of the Grande fore the imbricate stack can be defined as the la Vanoise has been interpreted as a recumbent Motte Nappe have not been involved in this Vanoise Occidental Nappe, subdivided in the fold cut by a Cargneule-filled fault horizon (Fig. imbrication, as demonstrated at Le Mollard de area in a main allochthon below La Rechasse 2; block H). Sheath folds (Cobbold and Quin- la Loza (Fig. 7), which might be expected if both (Fig. 2), the thrust slices at Le Mollard de la quis, 1980) in banded quartzitic carbonates nappes were in place at the time of imbrication. Loza, and the spoon-shaped allochthonous slab within this recumbent structure (Fig. 9) illustrate In addition, the parasitic recumbent folding in to the north of these imbricates (Figs. 3, 6, 8). the high degree of D| attenuation. The sheath the upper limb of the Vanoise Occidental al- Imbrication took place along gypsum (now folds are not caused by the D3 thrusting event as lochthon locally involves narrow shear zones transformed into Cargneule), and also along observed elsewhere, because they have a stretch- encompassing various attenuated stratigraphic phyllonite horizons of Tertiary dark slates and ing axis at high angles to the local extension units, among which no Liassic banded carbon- Permian quartz-phengite schists. Refolded and lineation L3. ates (diagnostic of the Grande Motte Nappe) attenuated parasitic Dj folds on the upper limb Reconstruction of Dj fold closures and the have been observed, despite excellent exposure. Hence we suggest the Grande Motte Nappe was emplaced later than the imbrication de- scribed above, moving on horizons of Cargneule and mylonitized carbonates (Fig. 10a). The space between the overriding thrust and the D2 antiforms in the underlying units was occupied by accumulations of highly deformed dark slate and chloritic marble. Large masses of gypsum, now represented by Cargneule, have also been concentrated in these locations by the overriding thrust. Erosion has found its way into the Carg- neule furrow thus created, to carve out the pres- ent Col de la Vanoise (Fig. 2). The thrust was emplaced before the forma- tion of the D2 synclines in the Grande Motte series on the northern flank of the Col de la Vanoise valley. The location of these synclines with steeply inclined axial plane cleavage S2 in- dicates a D2 downwarp of the thrust plane of the Grande Motte Nappe from the summit of La Rechasse into the valley (Fig. 10b).

SYNTHESIS

Two early thrusts have been identified in the Permian-Eocene cover sequence in the central part of the Vanoise Massif (Figs. 1, 2), namely, (a) the Vanoise Occidentale thrust and (b) the Grande Motte thrust. South of the Cargneule furrow marking the Col de la Vanoise (Fig. 2), Figure 8. The northern front of the Vanoise Occidentale Nappe (a) seen from the west, the thrust front of the former unit can be defined showing the spoon-shaped slab (UM; LM = the thrust slices at Le Mollard de la Loza, VO - as an imbricated stack of thrust slices, each con- the main allochthon). The stippling (b) outlines the slab UM in (c); dashes illustrate the lobate sisting of a north-vergent recumbent fold with shape of the nappe front; (c) this shows the slab as it has been folded and rotated by D2, and attenuated lower limb. Stratigraphic repetitions refolded by D3. UM at present covers an area of400 x 400 m, indicating the scale of the figure. formed by attenuation of parasitic folds in 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 Figure 9. Dt sheath fold ¡in banded quartzitic carbonates. The ledge casting its shadow on the outcrop contains a lineation

defined by parasitic fold axes orthogonal to the flat wall. Dt folds in adjacent outcrop have variably rotated fold axes.

Figure 10. Thrust complex development around Le Mollard de la Loza in southwest-northeast cross station, corresponding to the southern wall of block E in Figure 2. Cargneule, chloritic marble, and dark slate have been omitted. These units occupy the space between the overriding thrust and the marble antiforms in Figure 10b. (a) This shows northward-advancing thrust slices of the Vanoise Occidental Nappe. These imbricates are overridden by the Grande Motte Nappe. Figure 10a is not to scale; reconstructed displacements are relative and not quantified.

Figure 10b shows the result of D2 shortening of the structures represented in 10a. Note the present position of the frontal slab (UM) of the Vanoise Occidental thrust above its former cover of the Grande Motte Nappe.

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upper limb of the highest recumbent fold in the cover thrusts on their substratum before D2 sen, Ramon Loosveld, John Piatt, Wiebe stack (Fig. 2, block E), further support a model folding. Pronker, and Reinoud Vissers for many stimu- of nappe formation involving ductile folding. The two thrusts were redeformed during D3 lating days in the field. The manuscript has been The imbricated fold nappe front was overridden with progressive reorientation of linear structur- improved by comments from Andrew Duncan by the Grande Motte thrust, essentially a thick al elements toward the D3 extension direction, and Cees Swager, and by our reviewers Bob sheet of Liassic banded carbonates, moving which is subparallel to the backthrusting direc- Knipe, Geoff Milnes, and Andrew Siddans. north on calc-mylonites and a décollement ho- tion documented in the southern Vanoise by REFERENCES CITED rizon presently occupied by Cargneule. The Piatt and Lister (1985a, 1985b). Planar elements in the extension field were stretched and rotated Bocquet, J., 1974a, La métamorphisme alpin dans les Alpes occidentales: Grande Motte thrust has been correlated by Géologie Alpine Grenoble, v. 50, p. 27-38. Piatt and Lister (1985b) with the Parrachée to parallelism with S3; planes in the D3 shorten- 1974b, Le socle briançonnais de Vanoise (): Arguments en faveur de son âge anté-alpin et de son polymétamorphisme: Comptes rendus de Nappe in the southern Vanoise. At the Dent ing field were folded to form progressively re- l'Academie des Sciences Paris, ser. D, v. 278, p. 2601-2604. cumbent folds. A lobate shape of these east-ver- Bocquet, J., Delaloye, M., Hunziker, J. G, and Krummenacher, P., 1974, Parrachée (Fig. 1), Liassic banded carbonates of K/Ar and Rb/Sr dating of blue amphiboles, micas and associated this thrust were emplaced directly on top of the gent "backfolds" can be argued on the basis of minerals from the Western Alps: Contributions to Mineralogy and Pe- trology, v. 47, p. 7-26. Vanoise "basement" defined by the Arpont D3 fold axial traces (Fig. 6) and mesoscopic fold Cobbold, P. R., and Quinquis, H., 1980, Development of sheath folds in shear geometry. Strain intensified into two horizons, regimes: Journal of Structural Geology, v. 2, p. 119-126. Nappe. This took place before the second re- Debelmas, J, 1974, Géologie de la France 2: Paris, Doin, p. 387-438. gional thrusting event (D3), as both units have along which horizontal eastward movement Debelmas, J., and Kerckhove, G, 1973, Large gravity nappes in the French- Italian and French-Swiss Alps, in de Jong, K. A., and Schölten, R., eds.. been turned up on end in a D3 "backfold" (Piatt took place as part of a regional thrust event Gravity tectonics: New York, John Wiley & Sons, p. 189-200. tentatively dated as of Oligocene age (Tricart, Ellenberger, F., 1958, Étude géologique du pays de Vanoise: Bureau des Re- and Lister, 1985a). cherches Géologiques et Minières, Mémoire de la Carte Géologique de 1984). Differential uplift has complicated the la France, 561 p. The Di allochthons are overlain by a major Ellenberger, F., and Raoult, J. F., 1979, Les enseignements géologiques des early thrust, which transported calcareous mica- deformation pattern by kink-like warping of rochers de la Loze à Pralognan (Massif de la Vanoise, Savoie): Travaux parts of the backthrusting system into steeply Scientifiques du Parc National de la Vanoise, v. 10, p. 37-69. schists (Schistes Lustrés) from a more internal Emst, W. G., 1973, Interpretative synthesis of metamorphism in the Alps: dipping attitudes. Associated domal culmina- Geological Society of America Bulletin, v. 84, p. 2053-2078. position in the Alpine chain (the Piémont Zone) Frey, M., Hunziker, J. C„ Frank, W„ Bocquet, J., Dal Piaz, G. V., Jäger, E„ across the Vanoise Massif (Ellenberger, 1958; tions may form part of north-south-trending an- and Niggli, E., 1974, Alpine metamorphism of the Alps: A review: ticlinal ridges formed in the backthrusts across Schweizerische Mineralogische und Petrographische Mitteilungen, Debelmas, 1974; Tricart, 1984). The Liassic v. 54, p. 248-278. banded carbonates incorporated in the Grande the region (Tricart, 1984). Goffé, B., 1975, Étude structurale et pétrographique du venant occidental du massif paleozoîque de Chasseforét (vanoise méridionale) [Thèse 3me Motte thrust according to their model originated Cycle]: Paris, l'Université de Paris Sud, Centre d'Orsay. 1977, Succession de subfaciès métamorphiques en Vanoise méridionale in a paleogeographic position between the Bri- CONCLUSIONS (Savoie): Contributions to Mineralogy and Petrology, v. 62, p. 23-41. ançonnais platform to the west and the Piémont 1982, Définition du faciès à Fe-Mg carpholite chloritoide, un marqueur de métamorphisme de HP-BT dans les métasediments alumineux. Vol- ocean basin to the east. Piatt and Lister (1985a, The tectonic history of two Pennine thrusts in umes I and II [Thèse d'État]: Paris-VI, l'Université Pierre et Marie Curie, 233 p. 1985b) suggested that the nappes formed during the central part of the Vanoise Massif can be Grocott, J., and Watterson, J., 1980, Strain profile of a boundary within a large meso-Alpine collision-related underthrusting but ductile shear zone: Journal of Structural Geology, v. 2, p. 111-117. explained in terms of a regional syn- or post- Merle, O., 1982, Cinématique et déformation de la Nappe du Parpaillon that the presently exposed Dj structures were Eocene nappe transport northward (D[), paral- (Flysch à Helminthoides de l'Embrunais-Ubaye, Alpes Occidentales) [Thèse 3me Cycle]: Rennes, France, l'Université de Rennes. established in a tectonic uplift-related event of lel to the present mountain chain of the Western Merle, O., and Brun, J. P., 1984, The curved translation path of the Parpaillon horizontal extension. This is proposed on Nappe (French Alps): Journal of Structural Geology, v. 6, p. 711 -719. Alps, followed by a phase of folding about ap- Milnes, A. G., 1974, Structure of the Pennine Zone (Central Alps): A new (micro) structural grounds and on the basis of proximately east-west-trending axes. The earlier working hypothesis: Geological Society of America Bulletin, v. 85, p. 1727-1732. juxtaposition of metamorphic assemblages re- thrusts are overprinted by an anastomosing set 1978, Structural zones and continental collision, Central Alps: Tecto- flecting different P/T conditions across thrust nophysics, v. 47, p. 369-392. of major shear zones that have profoundly modi- Milnes, A. G., Greller, M., and Müller, R., 1981, Sequence and style of major contacts; for example, glaucophane and jadeite fied the structure of the Vanoise region. Asso- post-nappe structures, Simplon-Pennine Alps: Journal of Structural Geology, v. 3, p. 411-420. are found only in the Arpont Nappe. Further- ciated eastward thrusting and shortening (D3 Piatt, J. P., and Lister, G. S., 1978, Déformation, métamorphisme et méca- more, the Liassic banded carbonates of the nismes d'écoulement dans le Massif de la Vanoise, Alpes penniques and D4) of the nappe pile took place in the françaises: Comptes rendus hebdomaires des Séances de l'Academie, Grande Motte thrust were carried to a position Oligocene or later. ser. D, v. 287, p. 895-898. 1985a, Structural history of high-pressure metamorphic rocks in the directly above the Arpont high-P/T metamor- The control of polyphase deformation (in- southern Vanoise Massif, French Alps, and their relation to Alpine phics, which was ascribed to extensional de- tectonic events: Journal of Structural Geology, v. 7, p. 19-35. volving noncylindrical interference structures 1985b, Structural evolution of a nappe complex, southern Vanoise tachment onto a stratigraphically inverted foot- Massif, French Pennine Alps: Journal of Structural Geology, v. 7, at various scales) on the finite geometry of a p. 145-160. wall thrust defined by the Arpont Nappe. complex of lobate allochthons can best be Piatt, J. P., and Vissels, R.L.M., 1980, Extensional structures in anisotropic rocks: Journal of Structural Geology, v. 2, p. 397-410. A downwarp of the Grande Motte thrust reconstructed by successively removing major Ramsay, J. G., 1980, Shear zone geometry: A review: Journal of Structural Geology, v. 2, p. 83-99. along the Col de la Vanoise (Fig. 2) indicates deformational imprints in reverse order, em- Raoult, J. F., 1980, Interprétation nouvelle de la Vanoise (zone briançonnaise, north-south shortening (D2) of the two cover phasizing the three-dimensional aspects by Alpes Françaises): Revue de Géologie Dynamique et de Géographie Physique, v. 22, p. 303-312. thrusts and their substratum after the Vanoise presentation in block diagrams. Tricart, P., 1984, From passive margin to continental collision: A tectonic scenario for the Western Alps: American Journal of Science, v. 284, Occidentale Nappe had been overridden by the p. 97-120. Grande Motte thrust. The event is marked by ACKNOWLEDGMENTS White, S. H., Burrows, S. E., Carreras, J., Shaw, N. D„ and Humphreys, F. J., 1980, On mylonites in ductile shear zones: Journal of Structural Geol- steep faults, upright folds, and S2 cleavages tran- ogy, v. 2, p. 175-187. Warrak, M., 1974, The petrography and origin of dedolomitized, veined or secting the early recumbent folds. These D2 Field work by de Roo for this paper was part brecciated carbonate rocks, the "cornieules" in the Fréjus region, French structures are particularly well developed along of a doctoral thesis at the University of Utrecht. Alps: Journal of the Geological Society of London, v. 130, p. 229-247. the Vanoise Occidentale thrust front (Fig. 7). The authors feel indebted to their colleagues Lu- MANUSCRIPT RECEIVED BV THE SOCIETY JUNE 19,1986 REVISED MANUSCRIPT RECEIVED DECEMBER 8,1986 Fold relationships suggest emplacement of the cien de Bakker, Jurriaan Gerretsen, Roel Jans- MANUSCRIPT ACCEPTED DECEMBER 8, 1986

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