Structural Evolution of a Subducted Continental Sliver: the Northern Dora Maira Massif, Italian Alps

Journal offhe Geological Society, London, Vol. 148, 1991, pp. 1101-1113, 13figs. Printed in Northern Ireland

Structural evolution of a subducted continental sliver: the northern Dora Maira massif, Italian Alps

J. WHEELER Department of Earth Sciences, Liverpool University, Brownlow Street, Liverpool L69 3BX, UK

Abstract: Continental crust of the Dora Maira massif was subducted, in part, to pressures of 30 kbar during early stages of the Alpine orogeny. A new investigation in a transect across the north of the massif has shown that much of the dominant foliation in the upper Dora Maira (Pellice) unit was formed at high pressures of the order of 11 kbar, but at lower pressure than the locally preserved relicts of eclogite-facies metamorphism. This dominant deformation phase produced large recumbent isoclinal folds with intensely flattened limbs in the Pellice unit basement and the overlying Permian- Mesozoic cover (now white augen gneisses and calcschists). The rocks were thus part of the upper crustprior to their subduction, unlessan unrealistically large thickness of Mesozoic rocks (now removed) is hypothesized. The Pellice unit was later emplaced over the Chisone unit in greenschist facies conditions. All shearsense indicators are top-to-west,implying a simple but prolongedhistory in which high-pressure units were emplaced westwards over lower pressure ones whilst being exhumed. The intense horizontal stretching recorded in the basement gneisses may have helped bring them closer to the surface by thinning. Indirect arguments suggest that such tectonic thinning is not, on its own, sufficient to explain the present setting of the eclogite units. One or more sheets of eclogite material mayhave been extruded from the Eoalpine subduction zone in addition. Suchslabs would be bounded below by west-directed, and above by east-directed, shears. As the Dora Maira massif forms the structurally lowest Eoalpine eclogite unit in the Alps, the observed history is not incompatible withthis model. East-or southeast-directed structures in the Alps are commonlyidentified as backthrusts, but an origin for some as the extensional shears predicted by this model should not be excluded.

A remarkable feature of some Phanerozoic orogenies is the addressedwithout a knowledge of the geometry andthe burial of continental material to great depths, in excess of pressure-temperature-time history of particular eclogite 50 km and sometimes to 100 km (Chopin 1984), followed by terrains. its rapid returnto the surface. The most likely burial This paper presents a structural interpretation of part of mechanism is subduction (Ernst & Dal Piaz 1978), implying the Dora Maira massif in the Italian Western Alps, as an anomalousconditions in which buoyantcontinental rocks, element of an ongoing study including metamorphic and ratherthan being incorporatedinto the hangingwall of a geochronological work with the aim of clarifying the uplift subduction zone as they approach it, remain attached to the history of the massif. The massif was chosen primarily downgoing slab.This must imply an unusually strong because of the exceptional burial to 100 km of rocks in its linkage betweensubducting crust and mantle. The southern part (Chopin 1984) during the early, Cretaceous, preservation of eclogite-facies mineral assemblages in such part of the Alpine orogeny (Paquette et al. 1989). Such early rocks, now seen at the surface in the Alps (Compagnoni et eclogite-facies metamorphism, termed ‘Eoalpine’, is of al. 1977, Chopin 1984) andthe Norwegian Caledonides regional significance in the internal Alps and is recorded in (Griffin et al. 1985), is interpreted as a result of rapid uplift both continental and oceanic units (Droop et al. 1990). In precluding both time for retrogression on a decreasing addition, the Dora Maira massif forms part of an orogen for temperaturepath, and time for heating andthermal which much structural and petrological data are available, equilibration with surrounding rocks which would result in andtherefore its regional setting is comparatively well high-pressure granulite facies conditions. In many orogenic constrained. Inthe last part of this paperthe structural terrains erosion is the main process of exhumation, and is evolution of this massifis discussed in relation to overall triggered by several uplift processes including shortening models for the subduction and exhumation of continental and isostatic rebound. However, it is difficult to conceive of rocks. erosion being a fast enough process to uncover such deeply The Dora Maira massif (Vialon 1966, 1967) lies in the buried eclogites before they are re-equilibrated under hotter internal(eastern) parts of the WesternAlpine arc, conditions, and, in addition,the implied volumes of structurally beneath calcschists and ophiolites of the sediment are not apparent(Platt 1986). An alternative Piemonte nappe which are interpreted as oceanic (Fig. 1). process involves unroofing by extension along faults and On its eastern side it is directly onlapped by unmetamor- shear zones. Therefore there are two fundamental questions phosed upper Tertiary sediments of the PO basin. The three concerning these processes: first,what mechanical condi- massifs of Dora Maira, Gran Paradiso (Massonne & Chopin tions prevailed to allow subduction of continental crust, and 1989) and Monte Rosa (Ellis et al. 1989) all contain second, what were the kinematics and mechanics of the eclogite-facies continentalrocks and structurallyunderlie exhumation process? Neither of these issues can be the Piemonte nappe,and are interpreted to havebeen 1101

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BHelminthoid flysch nappe

Internal basementmassif Austroalpinenappes mAustroalpine nappes m 0 km 50 I 0.... Brinconnais Zone Fig. 1. Main tectonic units in the Western Alps, showing the location of the Dora Maira massif.

subducted beneath these oceanic rocks during the Eoalpine Fig. 2. The three main units within the Dora Maira massif, event.This term is used to describe ‘early’ Alpineevents distinguished on lithological and metamorphic criteria. Inset shows which involved eclogite-facies metamorphismfrom the position of Fig. 4. mid-Cretaceous onwards, of which the structural setting is strongly modified by later, Tertiary, Alpine shortening. In theMonte Rosa massif, this modification involved major pervasive backfolding and backthrusting (SE directed), and similar phenomena have been recorded at the NW margin of Lithological summary the Gran Paradiso massif (Caby et al. 1978; Butler 1987) and TheDora Maira massif consists of a variety of gneisses, within theDora Maira massif (Vialon 1966). Thus,the often showing gradationsbetween different texturesand so-called ‘massifs’ are not single tectonic units, but stacks of compositions, mostly of granitic or pelitic composition with distinct sheets (Ellis et al. 1989). For consistency the term some basic bodies. In this study, informal unit names were ‘massif will continue to be used here for the main outcrop used (Fig. 3) which correspond roughly to those used in the regions of continental basement, butwithout implication previous studies of Vialon (1966) and Borghi et al. (1984) as that those regions are structurally coherent. In the traverse shown in Table 1. The descriptions given refer tothe through the Dora Maira massif presented here, no evidence particular areas studied hereand may notcorrespond for backthrusting was found, and it is possible that the upper exactly tothe rocks types encounteredelsewhere in the margin of this massif was comparatively less affected by late Dora Maira massif. Alpine events. Nevertheless sheets of calcschist resembling a similar lithology in thePiemonte nappe occur within the Dora Maira gneisses and might suggest modification of the Calcschist original Piemonte nappe contact. The E-W traverse across Grey-brown,soft-weathering calcareous micaschists occur the Dora Maira massif mapped in this study was chosen to within the Piemonte nappe and also as sheets within Dora include one of the larger outcrops of such sheets, north of Maira basement rocks. Quartz,carbonate, phengite and Val Pellice (Figs 2 and 4), to allow investigation of their chlorite are the common minerals, and are of limited value origin. Thistraverse willnow bedescribed in detail, fordetermining grade. Phengite defines astrong shape beginning with adescription of the mappable lithologies fabric. Vialon (1966) does not distinguish the calcschist within andaround the massif. Only the key metamorphic lithologies within the two settings, and it could be inferred aspects are mentioned:a more detailed study of the that they were all oceanic. However, more than one original metamorphism will be published elsewhere. setting is possible for these rocks, as argued below.

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Quaternary Ophiolitic rocks Atthe west end of thetraverse, ophiolitic rocks of the Piemontenappe include serpentinites,rodingites, heavily Mesozoic retrogressed eclogitized gabbros and albitic greenstones (‘prasinites’). Typically the metagabbros show partly BCalcschist alteredomphacite blasts surrounded and slightly wrapped Ophiolite rocks by achlorite fabric. Thisfabric is pervasively crosscut by epidote, albite and random laths of tremolite, indicating a Marble strong greenschist facies overprint postdating deformation. Garnet is notseen except in the rodingites. The chlorite ? Triassic predates theend of the greenschist facies overprint. If it Quartzitic micaschist were itself retrogressive,formed by the breakdown of omphacite and garnet, other sodic and calcic phases should be expected in the same fabric. Their absence suggests that the chlorite fabric relates to a high-pressure event in which .{:e White gneissderived from vdcaniclastic sediments the chlorite and omphacite were stable. sian In the serpentinites, unusual rock types include pods of ?Carboniferous coarse diopside-grossular-epidote rocks and the serpentinites may carry 5 cm crystals of diopsideas a result of Graphiticmicaschist rodingitization (Cametasomatism often associated with ophiolite serpentinites). The diopside is brownish-purple ? Palaeozoic due to clouding with submicroscopic inclusions. 0.. .. Micaschist Chloritoidmicaschist Marble Cream-and buff-coloured boudins and layers of marble Greyaugen gneiss occur sporadically within micaschist and other units, and are Biotitegneiss of uncertain age.

Lumpy gneiss (with distinctivelarge augen) Quartzitic micaschist This lithology occurs within the region of Fig. 4 but, as it was notencountered in the detailedstudy areas, it is not described further. 2°C Main fdiation m, m, Stretchinglineation White gneiss \ Faultsor thrusts Within theDora Maira massif, this lithology forms Fig. 3. Key to all subsequent maps and cross-sections. prominent bands of white, slabby rock with occasional

Fig. 4. Detailed map of the area north of Val Pellice, modified after Vialon (1966). GH shows section line of Fig. 5, and the box shows the Val Subiasco area detailed in Fig. 7.

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Table 1. Rock units used in this study elongation lineation defined by elongate quartzand accentuated by mica crenulations with thesame trend. Borghi er 01. Garnet is often obvious, whilst biotite is absent except as a This study Vialon study This (1966) (19W local alteration product of phengite. The phengite forming Ensemble des calcschistes the main shape fabric has a Si content of up to 3.45. A Complesso di temperature estimate of 450-550°C is obtained using the Calcschist Calcschistes calcescisti geothermometer of Green & Hellman (1982) based on Ophiolitic rocks Roches Vertes con pietre Fe-Mg exchange between garnet andphengite. Together verdi with the phengite Si content, this enables a minimum Marble Dolomies Complesso di pressure estimate using the geobarometer of Massonne & marmorisees Depot Schreyer (1987) of 11 kbar.These conditions are broadly Ensemble de Sampeyre transitional between high-pressure greenschist facies and Quartzitic Micaschistes high temperature blueschist facies. In some micaschists from micaschist quartzitiques Conca Cialancia (Fig. 4, and see further discussion below) rare phengite grains occur at a high angle to the main fabric, Ensemble de Droner0 Porphyroides arkosiques Complesso di and this is seen to be a crenulation fabric which has almost White gneiss Jouglard- completely recrystallized an earlier fabric. The early Selleries phengites are strongly zonedin Si with coresas high as Si = 3.58. This implies minimum core pressures of 15 kbar. Graphitic Ensemble graphitique de Complesso di Rims of these grains have compositions comparable to the micaschist Pinerolo Pinerolo recrystallized phengites forming the ‘main’ fabric. Although Ensemble des gneiss these are only minimum pressureestimates for cores and glanduleux rims, it seems likely that the main fabric formed at a much Biotite gneiss Gneiss dioritiques Metatonaliti lower pressure thanan earlyfabric which is only locally Micaschist Micaschistes/Gneiss fins Micascisti - - preserved. Similar patterns havebeen recorded from this Chloritoid area by Scaillet et al. (1990). micaschist Grey augen gneiss Gneiss o6illes homegbnes Complesso del Freidour Lumpy gneiss Gneiss amygdalaire - Chloritoid micaschist Metabasite Amphibolites Metabasiti These rocks are distinct from ordinary micaschists in that the chloritoid tends to form black lenses, a few centimetres across, separated by more micaceous shear bands, giving a feldspar augen. A crude shape fabricis defined by scattered characteristicmottled appearance. Almost invariably the phengite flakes, in a polygonized matrix of quartz,albite shearbands show a consistent asymmetry relative to andminor microcline. Larger millimetre-scale microclines foliation and provide a valuable shear sense indication. are perthitic, cracked and recrystallized, and interpreted as porphyroclasts. The growth of albite may havebeen a delayed recrystallization of strained grains;alternatively it Grey augen gneiss may have been due to retrogression of jadeite since adjacent These rocks tend not to form prominent bandsbut are units preserve evidence of high-pressure metamorphism. otherwise similar theto white gneisses: they are distinguished by context, with the white gneisses frequently Graphitic micaschist occurring adjacent to calcschists, whilst the grey gneisses occur as irregular screens within metabasite bodies. These Dark graphitic micaschists with highly strained con- relationships are discussed fully below. glomeratic horizons are assigned tothe Pinerolo Group, long identified as Carboniferous by analogy tothe carbonaceous rocks of the Brianconnais domain to the west, Lumpy gneiss though no fossils have been found. Little evidence exists for Thisinformal term covers distinctive, variably-deformed high-pressure metamorphism in these rocks. rock types in which large (2-3 cm) white augen,often recrystallized to fine-grained aggregates, occur in a Biotite gneiss sometimes biotite-rich matrix. The occurrence of biotite is Not studied in detail, these rocks occur mainly in the east of distinctive althoughgradations exist between this and the the transect in the lowest tectonic unit (Fig. 4). The biotite other gneiss types. In one place, a completely undeformed forms a shape fabric with chloritewrapping albite lumpy gneiss shows euhedral twinned feldspars set in a porphyroblasts. Small amounts of phengite and garnet are biotitic matrix, suggesting an igneousprotolith for these preserved only within the albite blasts. The main fabric was rocks. In contrast to the ‘biotite gneiss’ type defined above, thus formed at low pressures where biotite was stable, but biotite here does not form a shape fabric but instead appears relicts of an earlier high pressure history may be perserved. aslarge, brown cloudedgrains in relatively unstrained samples. It is interpreted as relict primarybiotite. In contrastlate green biotite is formedas a retrogression Micaschist product of phengite. The phengites have high Si values, Thedominant rock type of the massif, these phengite suggesting a high-pressure history similar tothat of the schists are typically silvery grey and may have a mineral micaschists in which the lumpy gneisses are embedded.

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Metabasite of the transect, moving from west to east and structurally Within theDora Mairametabasites occur asmetre-scale downwards. Each part is given a structural setting and local pods, sheets, and in Val Subiasco as a kilometre-scale folded geographic name. body. Typically they are identified in the field asgarnet amphibolites, sometimes with glaucophane, and the largest Piemonte Nappe : Monte Giulian body containsconcordant foliated sheets of grey augen The ophiolitic sliver exposed at the west end of the traverse gneiss which may represent deformed intrusions. Complex (Fig. 4) represents thenorthern extremity of the large textures are ubiquitousin thin section, butgarnet and Monviso ophiolite complex (Phillipot 1988). Inthe gently glaucophane are prevalent. The largerbodies show west-dipping sliver are seenalbite greenstones above glaucophane shape fabrics so the main deformation was heavily retrogressed eclogitized gabbros, which are in turn probably in blueschist facies. Sometimesglaucophanes are aboveserpentinites. Strong mineral shape fabrics are rimmed by barroisite; more pervasive static greenschist developed generally parallel to lithological contacts. The facies overprints arerare. Pognante & Sandrone (1989) serpentinites contain isolated diopside crystals and boudins record, in addition, eclogites from within the Val Subiasco of rodingite. These, and also the less deformed boudins of body. retrogressedeclogite, show well-developed bending-in fabrics at their margins with themore highly-strained Structure surroundingswhere fabrics are defined by chlorite. These Theentire massif was mapped by Vialon (1966) using always show a top-to-west shear sense. This shearing might lithological divisions similar to thoseabove. This work be synchronous with the greenschist facies retrogression, showed that a principal foliation defined by banding and but, as mentioned above, these assemblages are frequently shape fabrics in all rock types dipped gently west beneath static overgrowths of euhedral crystals, with only chlorite the Piemonte nappe, and was modified by tight and open showing a shape fabric: conceivably this deformation was in E-W-trending folds. A regional stretchinglineation is the eclogite facies. roughly parallel to these fold axes. A new transect has been Beneaththe ophiolite, at Colle Giulian NE ofM. mapped in detail with the aim of identifying high strain Giulian (Fig. 4), about 100 m of calcschist are present with a zones, the shear senses on these, the metamorphic grade of well-developed E-W stretching lineation, which is variable all rocks, andto providecontext a for detailed close tothe base of the ophiolite. The foliation is geochronological andmetamorphic work.Figure 4 shows concordant with the contact with theDora Maira massif anextract, along thetransect, from Vialon's (1966) where it is seen. comprehensive map of theentire massif. Thethree main structuralunits involved arethe Piemonte nappe, the eclogite/blueschist facies tectonic unit occupying most of the Western part of Pellice unit: Tredici Laghi traverse which will be referred to as the Pellice unit, and the This area (Fig. 4) is dominated by micaschists with a structurally lowest, greenschist facies area referred to as the west-dipping foliation affected by later E-W open folds and Chisone unit following the terminology of Borghi et al. usually showing a well-developed west-plunging stretching (1984) (Fig. 2). These units are defined onthe basis of lineation. In Tredici Laghi scattered metabasite boudins can lithology and metamorphism: on the transect studies here, be found, carrying complexly retrogressed garnet- the Chisone unit contains only biotite gneiss and graphitic glaucophane assemblages, and, in contrast, a coherent micaschist, both showing strong greenschist facies fabrics. metabasite sheet. Above this metabasite sheet is a layer of In contrast,the Pellice unitcontains theother basement marble onthe north side of TrediciLaghi corrie, and lithologies, usually carrying high-pressure fabrics with only 20 m-scale boudins of brownish dolomite on the south side. limited staticretrogression under greenschist facies condi- In the centre of this corrie is a lensoid body of lumpy gneiss tions. The hypothesized contact between these two units is around which the main foliation is deflected. Bands of not exposed in the study area. The entityreferred to here as chloritoid-micaschist are common. the Parigi unit is an informal name for the postulated very These rock types display a variety of shear sense high-pressure unit(Chopin 1987) whose bounds are still indicators. The chloritoid-micaschists commonly develop being determined (Henry et al. 1989). A summary cross shear bands, as mentioned above, and these always show a section based on remapping is shown in Fig. 5. Inthe top-to-west shear sense. The lumpy gneiss body contains following description comments are made on various parts discrete west-dipping shear zones in zones of low strain and W W 1

Fig. 5. Cross-section GH across the Dora Maira massif as indicated on Fig. 4. AB' delimit the more detailed cross-sectionof Fig. 9. Large arrows highlight regional shear sense as deduced from several criteria.

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also develops asymmetric quartz preferred orientation fabrics. Some marble units show asymmetric open folds with a fainta axial-planar fabric, verging west. The common micaschists may sometimes show shearbands, and sometimesdiscrete shear zones (for example nearthe summit of Pta Cialancia at the east side of Tredici Laghi) dipping west less steeply thanthe foliation which are equivalent to the overturned limbs of metre-scale tight folds of the main foliation, and into which the latter is transposed. All these different features indicatea top-to-west shear sense. In Conca Cialancia (NE of TrediciLaghi, Fig. 4) is exposed a slightly lower structural level than that of Tredici a b Laghi. Here, large open folds arefound in afoliation defined by banding andshape fabrics, in which an axial N planar fabric dips gently west. These are shown schematically below PtaCornour on Fig. 5. Field criteria donot allow a correlation between these structures and those seen further west (andabove). The folds could be late, local features in which the ‘main’ fabric is overprinted by ‘late’ folds and fabrics. Alternatively, the axial-planar fabric could be the temporal equivalent of the main fabric seen further west. As mentioned earlier, the axial-planar fabric micas in these folds have similar composition to those of the main fabric further west. Early relict micas belonging to the first fabric are more phengitic in their cores, and strongly zoned outto less phengitic rims of composition comparableto C d those new micas defining the axial planarfoliation. This Fig. 6. Structural data from the west of the massif plotted on suggests two points: first, the folded fabric is early and the equal-area stereonets. (a) Stretching lineations from the Tredici axial-planar fabric is thesame ageas the ‘main’ fabric Laghi-Conca Cialancia area (see Fig. 4). (a) Poles to main foliation further west. Secondly, the early fabric may have formed at in the Tredici Laghi-Conca Cialancia area. (c) Stretching lineations substantially higher pressures (15 kbar)than the second, from the Val Subiasco area (Fig. 7). (a) Poles to main foliation axial-planar fabric(referring tothe geobarometer of from the Val Subiasco area. Massonne & Schreyer 1987). Occasional zones of lineations of ‘anomalous’ trends are found and account for some of thescatter on Fig. 6(a). calcschists, a stream section of gently dipping rock units was NW-trending lineationsoccur locally: in a thin quartz- logged indetail (Fig. 8). It will beseen that, with the mylonite band east of Colle Giulian, a top-to-NW shear exception of the topmost sheet of calcschists which lie sense is deducedfrom asymmetric quartz fabrics. N-S directly below micaschists, everycontact of calcschist is trends occur in the north of Tredici Laghi, and SW trends in against white gneiss. The thinnest white gneiss sheet is only Conca Cialancia. The significance of these swings is not 20cm thick. Figure 9 shows the sheet of calcschist, rimmed clear. by white gneiss, beneath the large lens of lumpy gneiss in the north of the valley. The consistent presence of white gneiss both above and below calcschist layers suggest that Central part of Pellice unit: ValSubiasco these are isoclinal folds, though closures are rarely exposed. Structurally beneaththe TrediciLaghi area occurs a At the top of the uppermost calcschist layer, white gneisses complex zone involving calcschist, white gneisses and a large are often absent, suggesting that movement on adiscrete metabasitebody (marked as box on Fig. 4). In this area, zone has excised parts of the white gneiss layer. No fault although all the rocks are intensely deformedand rocks have been found at these contacts. Almost everywhere metamorphosed,structural arguments can be used to else white gneisses are preserved, even though the layer identify the likely pre-Alpine setting. ranges in thickness from 20 cm through to about 200 m. In Fig. 10 asynoptic cross section through the area is Structure. A map of this area is shown in Fig. 7. The shown, in which data have been projected horizontally north inclusions of calcschist sheets within the basement could be from various parts of the valley ontothe section. As due either to several thrusts stacking Dora Maira material important N-S lateralvariations in structure are evident onthe calcschist, or to isoclinal folding. The second from themap, this section shouldbe regarded only asa hypothesis is preferred for the following reasons. It will be summary of the available information. In particular, the notedfrom the map that white gneisses frequently follow structurecannot be traced northwards into upper Val the contact between calcschist sheets and otherrock types of Angrogna (Fig. 4), even though the principal thinning of the massif. In places where this is not illustrated on the map, the packet as a whole is eastwards. The packet of folded it is sometimes due to a genuine absence of white gneisses, calcschists and other rocks thickens westwards along the but more often due to the impossibility of representing very transect and may be traced south to join the western margin thin sheets of white gneiss on the map. To emphasize the of the Dora Maira massif in the upper PO valley (Vialon trueabundance of white gneiss sheetsadjacent to 1966). Thus the westernpart of the massif may forma

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.top

Fig. 8.

Fig. 7. Lithological and structural map of the Val Subiasco area as indicated on Fig. 4. Planar and linear features marked are the dominant foliation and stretching lineation. Fig. 8. Detailed log of completely exposed stream section EF as shown on Fig. 7. ‘Top’ marks the top of the uppermost calcschist sheet seen in the Val Subiasco area. White gneisses are shown schematically as the most protruding layers; calcschists as the least protruding. Key as in Fig. 3.

kilometre-scale downwards verging fold nose involving the folding may have led to the local thickening of the white contactbetween calcschists and basementrocks, as gneiss unit. The E-W thickening and thinning of various indicated on Fig. 5. Thisvergence is in accord with the rock units within the overall isoclinal fold structure is not consistent top-to-west shear bands seen in the chloritoid always regular: in particular some calcschist sheets thicken micaschists both here and in Tredici Laghi. eastwards before closing. The most spectacular example of Almost all the isoclinal fold closures in Fig. 10 are this irregularity is the large lens of lumpy gneiss near the top inferred, andare notexposed. This is afunction of the of thestructure. These geometries suggest boudinage of degree of exposure in Val Subiasco. The dominant foliation morecompetent sheets within the isocline, with the less in all rock types is parallel to lithological contacts and only competent calcschists accommodatingthis boudinage, and in certain places are folds seen in the main foliation. They the white gneisses showing an intermediate behaviour. This are common within the structurally lower, eastern part of could be due either to continued simple shear within the the white gneiss outcrop in Val Subiasco wherecomplex, fold after it had become isoclinal, or to an additional variably-trending open and tight folds in the main foliation pure-shear flattening component in the overall deformation. occasionally developa new axial planar foliation.This Beneath the lower limb of this major fold occurs a

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7). This fold, and its continuation east to the SW slopes of M. Vandalino (Fig. 4), affects the mainfoliation and the included bands of grey augen gneiss (Fig. 11). The strong disharmonic nature of this fold is indicated in the N-S moss section (Fig. 12) in which the synformal core is interpreted to die out upwards: this disappearance over a shortdistance is demonstrated clearly in Val Rospard (Fig. 11). It is likely that the main foliation seen in Val Subiasco is synchronous with the isoclinal folding and, given the presence of garnet-glaucophanemetabasite sheets (e.g. at localgrid reference 524676,Fig. 7) transposed by this foliation and containingglaucophane fabrics, is of comparablemetamorphic grade to the ‘main’ foliation in Tredici Laghi.

Znferred pre-Alpine scheme. The white gneisses are not part of the ‘jumble’ of other Dora Maira rock types, but occupy a distinct position separating calcschists from other gneiss types. Theyare thus inferred to be a highly deformed sedimentaryunit. Vialon (1966) andothers interpret the protolith of this unit, by analogy with rocks elsewhere in the internal zonesof the Alps, as Permian acid volcaniclastic rocks, perhaps partlyepiclastic. Pognante & Sandrone (1989) indicatean intrusive igneous origin for similar gneisses further north. The structural setting of the white gneisses militates against an intrusive origin here. However they are similar in appearance to the grey augen gneisses which appear in contact with a variety of other basement gneisses in thearea. It is possible thatthese were minor intrusions, broadly comagmatic with the eruptive rocks, and are the analogues of the rocks mentioned by Pognante & Sandrone (1989). A key question concerns the original setting of the calcschists. There are two possibilities. (1) The calcschists were part of the Piemonte ophiolitic sequence.They are lithologically indistinguishablefrom thosefound in the main outcrop of the Piemonte nappe, Fig. 9. Photograph of 2 m band of calnchist between basement and, as traced south from thestudy area, become associated gneisses, below RocciaPeyroun (at 513 675 on Fig. Note the 7). with metabasites(Vialon 1966). In thissetting the prominent bands of white gneissbordering both sides of the calcschist layer. calcschists would have been deposited on oceanic crust. (2) The calcschists were part of sequence deposited on continentalbasement of theDora Maira massif. Further kilometre-scale metabasite body, reported by Pognante & north in Val Susa,Pognante (1980) describesvarious Sandrone (1989) tocontain relict eclogiteassemblages. In tectonic units containing calcschists between the Dora Maira contrast to the gently warped lower contact of white gneiss and Ambin massifs. Some of these are assigned to oceanic with micaschist, this body shows a large, tight E-W-trending units, others to a Dora Maira cover sequence. Possibly the fold, well displayed on the west face of Gran Guglia (Fig. Val Subiasco calcschists are analogous to the latter. Only

W W

m.10. Synoptic cross-section acrossVal Subiasco alongABB’ (see Fig. 7 and Fig. 5).

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Fie. 11. (a) Photograph looking NE from Pertuselinto the Rospard valley andup to Monte Vandalino (Fig. 4). (b) Sketch of same view showing strongly disharmonic folds in metabasite unit (unshaded)with bands of grey gneiss (hatched), which die out upwards and downwards into foliated lumpy gneisses (stippled) of relatively uniformorientation below La Gardiola.

the lithologic association (presence or absence of large Eastern part of Pellice unit: Monte Vandalino-Val amounts of basic/ultrabasicmaterial) serves to distinguish Angrogna these affinities. This area (Figs 4 and 5) consists of micaschists and one sheet If (1) is correct,then there must be a major tectonic of grey augen gneiss with gently W-dipping foliation and a contactsomewhere between the calcschists andthe W-plunging stretching lineation, and occasional metabasite basement gneisses in Val Subiasco. This would be above, sheets. within, or at the base of the white gneisses. This raises the issue of whether those gneisses were themselves part of the Piemonte nappe, in which case their lithology is Chkone unit:Bric Poi anoamalous, or whether they were cover to the Dora Maira basement, in which case any additional cover was removed Thispoorly exposed area was not studied in detail and andsubstituted by the oceanic calcschists. Boththese containsscattered outcrops of graphitic micaschist, biotite possibilities are problematic. Inaddition, metabasites are gneiss and quartzite. The contact between micaschists above absent from within the calcschists Valin Subiasco. and the Pinerolo group rocks below hasnot been found, Therefore (2) is preferred, in which the white gneisses and but micaschists near the contact as mapped by Vialon (1966) calcschists formed part of a probably Permian and Mesozoic show top-to-west shear bands, and quartzite microstruc- sequence deposited on the Dora Maira basement, analogous tures confirm this. However the mineralogy of the graphitic tothat recorded by Pognante (1980) furthernorth. The micaschists and the biotite gneisses suggest the main fabric occasional marble boudins associated with calcschist could formed in greenschist facies, whilst the mineralogy of also be part of this cover. A consequence of this is that there micaschists at Tredici Laghi indicateshigher temperature must be major contacts perhaps within calcschist units and much higher pressures. (outside this study area), separating lithologically similar calcschists of differing origins. Structural synthesis In this section the structural observations made on the new traverse are synthesized. The three main units, as well as having different structural positions and metamorphic grade, have different lithologies. The Piemonte nappe preserves ophiolites and oceanicsediments, whilst the Pellice unit containsmetapelites and metamorphosed rnafic and felsic igneous rocks originally directly overlain by Permian, possibly volcaniclastic, sediments(now the white gneiss), and calcschists. In contrast the Chisone unit consists almost exclusively of grey augen gneisses overlain by graphitic micaschist and conglomerates of the Carboniferous Pinerologroup. Vialon (1966) interpreted thiscontrast as due to the local development of Carboniferous basins within an area of already metamorphosed, possibly Variscan, Fig. 12. North-south cross-section down Val Subiasco (CD on Fig. basement. Permian andlater Mesozoic sedimentsover- 7) showing strongly disharmonic folds in metabasite unit. steppedthese basins to rest directly on basement. In this

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model, the westwards steepening of the dominant Dora nappe contact into an isoclinal fold in which the Permian Maira foliation, andthe emplacement of micaschists cover was drastically thinned on isoclinal fold limbs but structurallyabove the Pinerolo group, were dueto local rarely lost itscontinuity. Local boudinage of competent east-directed backthrusting with relatively minor shortening. lithologies, and possible thickening in boudin necks, suggest Borghi et al. (1984) preferred, following Argand (1911) and strong flattening. This folding was contemporaneous with Kerckhove (1979), amodel in which thePiemonte nappe the development of the main fabric in the Pellice unit, and and Pellice unit were emplaced westwards over the Chisone occurred in the low-pressure eclogite or high-temperature unit and perhaps originated a considerable distance to the blueschist facies around 11 kbar. Locally it is seen that the east. They also emphasize the lithological similarity of the main fabric is axial-planar to folds in higher pressure Chisone unit to the Brianconnais zone. The shear criteria foliations formed at least 15 kbar, and is itself affected by presentedhere show aconsistent west-directed sense of later open E-W folds. movement through the Dora Maira massif in the rocks of In the western part of the Pellice unit deformation various grades, including both the high pressure rocks in the probably ceased during the blueschist facies metamorphism: steep western margin and the lower pressure rocks in the some rocks show post-tectonicglaucophane growth, and Bric Poi area. The early quartz fabric work of Laurent & local greenschist facies overprints do notseem related to Etchecopar (1976) was alsoperformed in Pinerolo group additional deformation.This contrasts with the Chisone rocks from the Chisone unit in Val Germanasca and this, unit in which enormous strains occurred during greenschist too, indicated west-directed shear within this unit.Thus facies west-directed shear. Tight and isoclinal folds with there is no evidence for ductile movement with backthrust E-W axes affected theentire unit and are particularly sense, nor have any brittle features beenfound with this evident in the ‘basement-cover’ contact of Pinerolo group sense of movementin a reconnaisance from Val Pellice above grey gneisses. Intense prolate strains developed with northto the Rocciavre area.In summary, the second E-W maximum stretching, most spectacularly in the scheme is preferred in which the distinct lithology, structural Pinerolo group conglomerates (Borghi et al. 1984, author’s position and metamorphic grade of the Pellice unit are a own observations). The Pellice unit was emplacedas a result of its pre-alpine position some way to the east of the relatively undeformedunit during or after this intense Chisone unit. deformation. The actualcontact between the Pellice and It will be assumed herethat the highest grade of Chisone units is not exposed on or near this traverse: further metamorphism preserved in each unit relates to its greatest north, in the Rocciavre region, the Pellice unit thins so that depth of burial, rather than to reworking so pervasive as to Piemonte nappe ophiolites occur not far above the Chisone remove all traces of earlier higher-pressure events. This is unit. unlikely to be rigorously true:as Oxburgh & Turcotte (1978) and Thompson & England (1984) demonstrate, in metamorphismduring overthrusting the thermal peak will Implications for exhumation of the Dora Maira occur during exhumation, rather than at peak pressure, and massif this is when the ‘peak’ assemblage is likely to develop. In The evolution deduced from this Val Pellice transect is the absence of any more detailedpressure-temperature- entirely one of west-directed transport. The first Eoalpine time histories, the available approximate P-T estimates will event recorded was emplacement of the eclogitic Piemonte be used as a guide to their maxima. In particular, there is no nappe onto the Pellice continental unit, yet, as the pressure evidence that any of these tectonicunits reached the conditions of 9-13 kbarrecorded in metabasites of the pressures of near 30 kbar recorded in the Dora Maira massif Pellice unit (Pognante & Sandrone 1989) are not quiteas further south. deep as those of parts of the Piemonte nappe such as the The earliestevents were eclogite-facies metamorphism Rocciavre ophiolite (12-15 kbar, Pognante 1985), exhuma- in the Piemonte nappe and Pellice unit. Rocks of this grade tion of the Piemonte nappe had alreadybegun. Further are now preserved locally, and the main fabric in the Pellice pervasive deformation followed in the blueschist facies, unit close tothe Piemonte nappe is slightly lower grade, affecting much of the Pellice unit and severely modifying the perhaps blueschist facies. It is thus notclear whether the original contact with the Piemonte nappe.Later, the Piemonte nappe was emplaced onto the Pellice unit during composite Pellice-Piemonte unit was emplaced westwards, or after eclogite conditionsprevailed. The initial contact with little furtherdeformation, over the Chisone unit, between Piemonte nappe calcschist and Pellice unit rocks is inducing intensestrain at greenschist facies in thelatter. difficult to place dueto the ambiguous affinities of the Once again, substantial exhumation of the allochthon must calcschists at the western margin of the Dora Maira massif have preceded this emplacement. these lie just above Dora Maira basement, in which case Thereare several ‘end-member’ models for eclogite they could be part of a cover sequence, but are lithologically exhumation which must now be assessed in relation to this identical to those found above the M. Giulian ophiolite, in study (Fig. 13). One of them, simple rebound and erosion, which casethey may belong to the Piemonte nappe. The is quickly dismissed, but is given here for completeness. For white gneisses and the Val Subiasco calcschists and marbles a model of eclogite exhumation to be viable, it must be (1) are argued to be part of a Dora Maira cover sequence. The compatible with the observed geometries of eclogite terrains implication is that, unless the cover was very thick (for in orogens; (2) compatible with the inferred kinetics of which there is no evidence), the Pellice unit formed a high movement and P-T evolution,and (3) incorporatea crustal level prior to its Eoalpine burial. This negates the driving force and mechanism. The following end-member possibility that the Dora Maira rocks were part of the lower models refer to Fig. 13. continentalcrust, and suffered less severe Eoalpine burial (a) Isostatic reboundanderosion. Metamorphic prior to their exhumation: most of the burial was tectonic. complexes exhumed in this way would show abroadly Continued westwards shearing deformedthe original concentricset of isograds, with no discontinuities in

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Cretaceousand Tertiary shortening. In particular, much early syn-orogenic sediment is now partly buried within the Alpineorogen or may even have beensubducted. Platt (1986) argues nevertheless that the amounts and timing of synorogenic sedimentation are incompatible with the unroofing of the internalzones. The overburdenabove eclogite regions would have consisted of blueschist and Initial eclogite facies rocks. Detrital glaucophane and lawsonite are found locally in flysch as early as Cretaceous (Hsu 1989), although not in large volumes. It is unlikely that the Dora Maira coesite-bearing eclogites, in particular, were overlain by100 km of thickenedcontinental rocks, because it is difficult tosee how such a welt would have formed. Instead the Dora Maira eclogites could have been overlain by amantle wedge (Butler 1986; Chopin 1987). Conse- quently, if these rocks were unroofed by erosion, enormous ...... volumes of detritus derived from garnet peridotite should a b have been deposited in the Alpine flysch basins. There is no evidence for this. (c) Gravityspreading with extension and foreland- directedstructures. Platt (1986) hasargued that gravitational instability in the Alpine orogenic wedge could have given rise to extension, with consequent unroofing of high-grade terrains.Extensional deformation during exhumation has beendemonstrated in specific areas (e.g. Selverstone 1988). The considerable thinning recorded on the limbs of now-subhorizontal isoclinal folds in the Dora Maira study area might besymptomatic of larger scale extension. However, once again the postulated existence of a mantle wedge above the internal zones is critical to this model. If the main transportdirection, whether along thrusts or extensional structures, was towards the foreland, thenremnants of the mantle wedge would have been e f emplaced onto more external zones, and either been eroded Fig. 13. Six different ‘end-member’geometries which might enable or preserved as major ultramafic nappes. Such remnants are unroofing of deeply-buried continental units within a subduction not seen. zone. The top diagram shows the initial subduction geometry. (a) (d) Asa possible alternative mechanism of tectonic Erosion only; (b) thrusting and erosion; (c) gravitational spreading; unroofing, Butler (1986) invoked regional plate divergence (a) extension along the subduction zone; (e) return of a ‘pip’ to the giving rise to hinterland-directed extensional structures. In surface; (Q return of a continental sheet to the surface. this fashion, the mantle wedge may simply slide back down thesubduction zone, bringing high-grade material in the footwall to shallow levels. Whilst this model avoids some of recordedpressure andtemperature patterns. Whilst some the problems with other processes, there is little evidence complexes, such as the Dalradian of the Scottish for a phase of actual plate divergence at some time between Caledonides, show such patterns, this simple scenario the Eoalpine and laterAlpine convergent histories. cannotaccount forthe juxtaposition of different (e)For continental rocks overlain by mantlematerial, metamorphic grades seen in the Alps. decoupling above and below could lead to the simultaneous (b) Continued thrusting and erosion. It is manifest that operation of normal-sensea and reverse-sense shear, the Alps evolved over a considerable time with a general allowing buoyant uprise of a pip of rock (Platt 1987). Such a foreland-propagating locus of deformation, in which pip would cease to ascend once it reached the base of the high-grade internalunits were frequently thrust over less dense continental wedge. lower-grade, more external, units. This model accounts for These models are not mutally exclusive. In particular, it the ‘reverse-sense’ metamorphic breaks in which high-grade is suggested herethat the process of tectonic unroofing rocks are found above lower-graderocks. Normal-sense could involve both foreland and hinterland-directed metamorphic breaks, such as that between the greenschist extensional structures, or foreland-directedthrusting syn- facies Combin zoneand the structurally underlying chronous with hinterland-directed extension (Fig. 13f). This Zermatt-Saas eclogite facies zone (Platt 1986) could, in is similar to a situation implicit in the discussion of Dal Piaz principal, be accounted for by out-of-sequence thrusting or et al. (1972). This latter geometry allows the subducted backthrusting within the orogenic wedge (Coward & continentalslab to rise up relative tothe mantle wedge Dietrich 1989). In this model, erosion is still the sole agent whilst net convergence continues. The mechanical basis for of exhumation, so that sediments equal in volume and gravitational spreading of an orogenic wedge is not adequate composition to the eroded material would be produced. In to explain how this geometry might develop. However, as the case of the Alps, it is not trivial to determine thevolume discussed by a number of workers (e.g. England & Holland and destination of sediments produced at each stage in the 1979; Platt 1987) subducted continental material, even when

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eclogitized, will be less densethan surrounding mantle. extensional relative to the Earth’s surface,and could still Therefore a buoyancy force acts on the subducted crust. The accomplish pressurea decrease itsin footwall. The forcescontributing tothe net motion of a ‘pip’ of geometry of these early structures would then be disrupted continental crust in asubduction zone were discussed by by later genuine backthrusting such as that emplacing the Platt (1987), and similar forces would act in the model of Brianconnais zone eastwards over the Piemonte nappe. In Fig. 13f. For the continental crust to be subducted, it must summary, the original attitude and tectonic significance of be strongly attachedto the downgoing slab, and this features described asbackthrusts in the internalAlps are attachment must be weakened to allow return to shallower worthy of further investigation. levels. Local buoyancy effects would cease to operate once the slab reached the base of continental crust. However, if Conclusions thebounding shear zoneshave the geometryas shown in Fig. 13f, so that displacement is transferred intothe The northern Dora Maira massif, as examined in a mapped thickened continental crust, then continued motion of slabs traverse north of Val Pellice, shows rocks derived from the denser than non-eclogitic continentalmaterial could be upper part of the pre-Alpine continental crust which have favoured by the amount of subducted continental material been buried to eclogite facies. The eclogite facies Piemonte still remaining within the subduction zone. Thisbuoyant nappe overlies the eclogite-bearing Pellice nappe which material would provide a ‘push from below’, forcing the slab underwent intense top-to-west deformation under blueschist up into the orogenic welt. The stage at which the continental facies conditions(i.e. during exhumation), tightly folding crust might detach would becontrolled by the evolving the basement-cover contact. Sedimentary cover rocks were rheology of the system, speculation on which is beyond the enormously stretchedat this time onthe isoclinal limbs, scope of this paper. with still-continuous formationsthinned down to 20 cm. A final point will be made, in the Alpine context, Morecompetent lithologies wereboudinaged. These two concerning postulatedeast-directed extensional structures. units were in turn carried westwards as a relatively coherent Several of the models in Fig. 13, including (f), predict that nappe over the Chisone unit which was intensely deformed bodies of high-pressure rocksin the Alps returned to the under greenschist facies conditions. surface would boundedbe below by west-directed It is argued that this west-directed stacking of high grade structuresstacking high-pressure rocks on lower-pressure units upon lower-grade ones, and associated erosion, cannot ones.This is thepattern seen in the Val Pellice section have been the only processes leading to unroofing of these across DoraMaira, but it is not diagnosticin itself. The rocks and particularly the very high-pressure eclogites present attitude of the Dora Maira massif would imply that further south. The most plausible way of removing the west-directed structures wereextensional, but this is overlying units was by east-directedextensional shearing likely to be a result of late Alpine warping of the Piemonte contemporaneous with continuedsubduction and west- nappe and underlyingunits, as in the case of the domal directed stacking. Noting thatanimportant belt of Gran Paradiso massif. The arguments outlined above have east-directedstructures exists justabove theDora Maira predicted the occurrence of east-directedextensional massif, which coincides approximately with the upwards shearing above the units of highest-pressure rocks. No such decrease in metamorphic grade, it is speculated that these structures with presentlyextensional sense have been structures, ratherthan being latebackthrusts, may also recorded in theDora Maira massif oradjacent units. incorporateearly extensional shears. The highest-grade Nevertheless the present attitude of structureshas been rocks in theDora Maira massif would thenbe in units strongly influenced by later Alpine events andit is the shear emplaced west overlower-grade units and with other sense which should be considered. lower-grade units emplaced eastwards on top. This is exactly Running near the edge of the Dora Maira massif is a the geometry which would result fromnappes being major zone of east-directed structures. These are sometimes expelled from an active subduction zone. entirely within the Piemonte nappe (for instance, east of the Pellice traversedescribed here),but sometimesencroach I am indebted to P. Vialon for useful discussions on the Dora Maira intothe continentalbasement as recorded further south massif at an early stage of this project, and to C. Chopin for helpful (Phillipot 1988). Thesestructures havebeen assigned in comments. D. Dietrich and J. Ramsay are thankedfor the many parts of the Alps to backthrusting (retrochurriuge) invitation to visit ETH Zurich and for their stimulating comments thickening the orogenic wedge at a late stage in its history. on this work. N. Fry and K. Brodie provided useful suggestions. However their present attitude and metamorphic grade are Special thanks go to J. Lynch for drawing the figures. NERC not proof of a late backthrust origin. It is speculated here funded this work through PostdoctoralResearch Fellowship that some east-directed structures are not late backthrusts CTS/F/88/GS/l. but, at the time of their formation, were extensional relative to the surface, and perhaps operating at some depth within References the subduction zone. Continuing operation as high-pressure ARGAND,E. 1911. Les nappes de remuvrement des Alpes Occidentales et rock were returned to the surface would eventually lead to leurs prolongements structuraux. Materiaux Carte Geologiquede la low-grade rocks above high-grade units separated by Suicse, 31, 1-26. BORGHI,A., CADOPPI,P,, PORRO,A., SACCHI,R. & SANDRONE,R. 1984. east-directed structures, which in its simplest form is what is Osservazionigeologiche nella Val Germanasca e nella mediaVal seen in the southern part of the massif. The fact that some Chisone(Alpi Cozie). Bollettino del Museo Regionale di Scienze backthrustsduplicate original or tectonic-interleaved rock Naturale-Torino, 2, 503-530. units, implying shortening,does not militate against this BUTLER, R. W. H. 1986. Thrust tectonics, deep structure and crustal possibility. As Fig. 13 shows, rock units are inclined within subductionin the Alps andHimalayas. Journal of the Geological Society, London, 143,857-873. andnear the top of the subductionzone. Thus a feature -1987. Thrust sequences. Journal of the Geological Society, London, 144, cutting up section, and duplicating rock units, could still be 619-634.

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Received 25 May 1990; revised typescript accepted 28 May 1991.

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