Deformation of a thrust complex in the Col de la Vanoise, Vanoise Massif, French : Discussion and reply

Discussion

ETIENNE JAILLARD Institut Français d'Etudes Andines, CasiUa 18-1217, Lima 18, Peru HERVE DONDEY 93, rue de Rennes, 75006 Paris, France BRUNO LANDES Institut Dolomieu et UA Centre National de la Recherche Scientifique n° 69, rue Maurice Gignoux, 38031 Grenoble cedex, France

De Roo and Lister (1987) have published a very detailed microstruc- 3. De Roo and Lister ascribed the frontal slice of the Thrust Com- tural study of the Col de la Vanoise area in the Vanoise Massif. The careful plex (UM of Fig. 10b of de Roo and Lister, 1987) to the Vanoise occiden- analysis they made makes most of their results both convincing and relia- tale zone. On one hand, however, this slice comprises an Upper Triassic ble. In this way, they confirm and elaborate the data obtained by several dolomitic series, which is uncommon in the Vanoise occidentale zone; on recent, regional studies on neighboring areas (for example, Raoult, 1980; the other hand, it represents the structural continuation of the Plan des Jaillard, 1984; Detraz, 1984; Jaillard and Detraz, 1984; Broudoux, 1985; Nettes unit, known as a classical example of the Val d'Isère zone (Ellen- Piatt and Lister, 1985a, 1985b; Dondey, 1986,1987; Deville, 1986,1987; berger, 1958). Thus, the frontal slice does not belong to the Vanoise Landes, 1988). Apparent misreading of most of these numerous works, occidentale zone, and the Thrust Complex of the Arpont nappe involves however, has led de Roo and Lister to unconvincing conclusions. units of different paleogeographic origins. 1. Referring to Ellenberger (1958), de Roo and Lister ascribed the 4. The relative autochthonous substratum of the Col de la Vanoise Grande Motte unit to a paleogeographic position between the Briançon- Thrust Complex (Fig. 10b of de Roo and Lister, 1987) consists of Upper nais zone and the Piémontais zone. On the basis of both structural and Permian terms, stratigraphically overlain by Jurassic to Senonian marbles stratigraphie data, however, several authors have proposed that the Grande and breccias (Lombard unit of Deville, 1986, 1987). This unit of internal Motte unit is of intra-Briançonnais origin, having formed a basin between paleogeographic origin extends southward below the Mollard de la Loza the external "Vanoise occidentale" zone and the internal "Val d'Isère- fold and separates this latter from the Paleozoic Arpont schists unit Ambin" zone (sensu Ellenberger, 1958) (see Caron and Gay, 1977; Brou- (Landès, 1988). Thus, the D| thrusting phase involved units of variable doux, 1985; Jaillard and others, 1986; Allenbach and Caron, 1986; paleogeographic origin (see section 3), and the assumption of polyphased Deville, 1986, 1987; Landès, 1988). thrust movements must be discussed. 2. De Roo and Lister (1987, Figs. 2 and 10b) thought that the La In conclusion, in spite of very precise and searching microstructural Réchasse unit belongs to the Grande Motte nappe. The stratigraphie series observations, de Roo and Lister have misinterpreted the paleogeographic of this unit, however, comprises (from base to top) dark-colored brecciated origin of several units. Hence, they have proposed over-simplistic interpre- carbonates (Lias to Middle Jurassic ?), dark-colored quartzite lenses and tations about the primary geometry of the supposed Thrust Complex of the light-colored marbles (Middle to Upper Jurassic), and Senonian to Eocene Arpont nappe. So it seems that the "complete understanding of the Arpont marbles and slates. The lack of any liassic marine terms in this succession nappe" (de Roo and Lister, 1987, p. 389) is far from being "achieved." leads to ascribing the La Réchasse unit either to the Val d'Isère zone (Deville, 1986) or, more probably, to the Vanoise occidentale zone. Con-

sequently, it is impossible to correlate the basal thrust surface of the La MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY 25. 1988 MANUSCRIPT ACCEPTED APRIL 25, 1988 Réchasse unit with the Grande Motte nappe contact. FINAL MANUSCRIPT RECEIVED AUGUST 15, 1988

The article discussed appeared in the Bulletin, v. 98, p. 388-399.

Geological Society of America Bulletin, v. 101, p. 751-752, May 1989.

751

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J. A. DE ROO Department of Geology, James Cook University, Townsville, 4811 Australia G. S. LISTER Department of Earth Sciences, Monash University, Wellington Road, Clayton, 3168, Australia

We are gratified by the painstaking scrutiny of our paper by Etienne Stratigraphic and structural analysis go hand in hand. One cannot Jaillard, Herve Dondey, and Bruno Landes. We were foolish to write that proceed without the other. If Jaillard, Dondey, and Landès are correct, "complete" understanding had been achieved—we realize well that in an and the topmost structural unit of La Réchasse does not belong to the D] area of such complexity, one proceeds one step at a time. We should point Nappe de la Grande Motte, then a slight modification of the interpretation out, however, that because the work reported in our paper was completed of the 3-D block diagram presented by de Roo and Lister (1987) is before 1983, we felt no compulsion to refer to subsequent studies which necessary. The basal thrust of the D| "Nappe de la Grande Motte" must be confirm and elaborate our original concepts. We acknowledge in particu- moved upward a distance in excess of 100 m. No changes in our conclu- lar the collaboration with John Piatt, over the period of 8 yr during which sions are necessary. these field studies in the Vanoise were carried out. His presence was always In summary, a stimulating one, as Jaillard, Dondey, and Landes must also acknowl- (1) We found the logistic difficulties such that we decided to carry edge, for we understand that they too are somewhat indebted to him by out our geologic field work in the normal manner, and we did not, as way of supervision of their field projects. Jaillard, Dondey, and Landès suggest, use microscopes to look at the rocks We regret that we missed the opportunity to walk with Jaillard, in the field. Dondey, and Landes in the field. We could have corrected some impres- (2) Our structural analysis is not dependent on paleogeographic sions they attempt to convey. "Misreading" of papers had little to do with analysis. On the contrary, we expect that future paleogeographic analyses our conclusions. Our work is based on painstaking observations in the will be dependent on developing a regional understanding of the geometry field, concentrating instead on the observed structures, that is, looking at and kinematics of large-scale structures in the Vanoise. In the meantime, the rocks. opinions of authors such as Jaillard, Dondey, and Landès are interesting, "Microstructure" refers to structures observed in thin section using an demonstrating real interest in a solution of the problems that remain. optical microscope. As is evident from the results published by de Roo and (3) We have made no interpretation of the paleogeographic origin of Lister (1987), we spent considerable time in the field. As a result, we were any unit, and so no misinterpretation is possible. Without data, no inter- able to produce a detailed lithological and structural map, and de Roo was pretation is possible. able to construct a detailed block diagram. Few, if any, of the observations (4) Understanding of the Arpont nappe is a goal to which we de- leading to the synthesis were obtained by loading a microscope in our voted three field seasons. Complete understanding is an ephemeral con- packs and hiking up to the Col de la Vanoise. cept, possibly attainable sometime in the future, certainly involving Jaillard, Dondey, and Landes make only one substantive challenge. detailed structural and stratigraphic analysis and continued interaction On the basis of stratigraphic observations made on a small slice of rock at between individuals of all nationalities interested in the furtherance of our the top of one mountain peak (La Rechasse), these authors argue forcefully science. that we perhaps misidentified the provenance of a small slice of rock,

comprising the top-most structural unit in a pile of intensely deformed COMBINED REFERENCES CITED

metasediments. We are quite content to consider the possibility. Let us Allenbach, B., and Caron, J. M., 1986, Relations lithostratigraphiques et tectoniques entre les séries mésozoïques de la do so. boedure SW du massi! d'Ambin (Alpes occidentales): Eciogae Geologicae Helvetiae, v. 79/1, p. 75-116. Broudoux, B., 1985, Géologie des unités de Vanoise septentrionale et méridionale de Pralognan à (Alpes de Savoir) If Jaillard, Dondey, and Landes are correct, then we must adjust the [thesis of 3rd cycle]: Lille, France, University of Lille, 229 p. Caron, J. M., and Gay, M., 1977, La couverture mésozoïque du massif d'Ambin, transition entre le domaine briançonnais position of the basal thrust above which lie the complexly deformed et le domaine piémontais: Eciogae Geologicae Helvetiae, v. 70/3, p. 643-665. metasediments of the Nappe de la Grande Motte. Little of significance has de Roo, J. A., and Lister, G. S., 1987, Deformation of a thrust complex in the Col de la Vanoise, Vanoise Massif, : Geological Society of America Bulletin, v. 98, p. 388-399. changed except the elevation of the basal thrust. The structural analysis is Detraz, G., 19 84, Etude géologique du bord interne de la zone houillère briançonnaise entre la vallée de l'Arc et le massif de Péclet-Polset (Alpes de ) [thesis of 3rd cycle]: Grenoble, Franœ, University of Grenoble, 176 p. not tied to this discussion on where the sediments of the nappes may have Deville, E., 1986, Données nouvelles sur le cadre stratigraphique et structural de l'unité de la Grande Motte (Massif de la originally been deposited; nor is it tied to accurate chronostratigraphic Vanoise, Alpes de Savoie). Conséquences paléogéographiques: Géologie Alpine, Grenoble, v. 62, p. 51-61. 1987, Etude géologique en Vanoise orientale (Alpes occidentales françaises, Savoie) [thesis]: Chambéry, France, identification of the rocks in every minor thrust slice. 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Jaillard, E., 1984, Etude géologique des unités briançonnaises de Vanoise occidentale au Sud-Ouest de Pralognan hope that our detailed structural work in the field, examining as much of (Savoie). Evolutions sédimentaire et structurale [thesis of 3rd cycle]: Grenoble, France, University of Grenoble, the exposure as our three field seasons permitted, will help Jaillard, Don- 213 p. Jaillard, E., and Detraz, G., 1984, Tectoniques superposées en Vanoise occidentale (Savoie): Société géologicque de dey, and Landes in continuing their arduous, and sometimes thankless, Frano:, eds.. Réunion annuelle des Sciences de la Terre, 10°, Bordeaux, p. 302. Jaillard, E., Dondey, H., and Debelmas, J-, 1986, Reconstitutions paléogéographiques de la zone briançonnaise de task. Vanoise (Alpes occidentales): Nouveaux arguments pour une origine intra-briançonnaise de l'unité de la Grande It is worthwhile that they continue, for as we tried to acknowledge, Motte: Comptes Rendus à l'Académie des Sciences, Paris (II), t. 302, p. 1091-1094. Landès, B-, 1988, Les unités briançonnaises de la bordure interne du massif de Vanoise méridionale (Savoie) [thesis]: without the painstaking stratigraphic analysis which has taken place in the Grenoble, France, University of Grenoble, 200 p. Plait, J. P., and Lister, G. S., 1985a, Structural history of high-pressure metamorphic rocks in the Southern Vanoise Massif, Vanoise (Ellenberger, Debelmas, Raoult), it would not have been possible French Alps, and their relations to alpine tectonic events: Journal of Structural Geology, v. 7, p. 19-35. for us to concentrate on the structural geology, as we have done. 1985b, Structural evolution of a nappe complex. Southern Vanoise Massif, French Pennine Alps: Journal of Structural Geology, v. 7, p. 145-160. It is difficult, even impossible, to make a paleostratigraphic map Raoult, J. F., 1980, Interpretation nouvelle de la Vanoise (zone briançonnaise, Alpes françaises): Revue de Géologie Dynamique et de Géographique Physique, v. 22, p. 303-312. without understanding the geometric and kinematic significance of the large-scale structures which exist in an area. We admire those who have the courage to attempt paleogeographic reconstruction without that MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 7, 1988 information. MANUSCRIPT ACCEPTED APRIL 25, 1988

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