Jan Pleuger Æ Sybille Roller Æ Jens M. Walter Æ Ekkehard Jansen Æ Nikolaus Froitzheim

Transcription

1 Int J Earth Sci (Geol Rundsch) (2007) 96: DOI /s ORIGINAL PAPER Structural evolution of the contact between two Penninic nappes (Zermatt-Saas zone and Combin zone, Western Alps) and implications for the exhumation mechanism and palaeogeography Jan Pleuger Æ Sybille Roller Æ Jens M. Walter Æ Ekkehard Jansen Æ Nikolaus Froitzheim Received: 9 March 2006 / Accepted: 22 May 2006 / Published online: 12 July 2006 Ó Springer-Verlag 2006 Abstract The boundary zone between two Penninic nappes, the eclogite-facies to ultrahigh-pressure Zermatt-Saas zone in the footwall and the blueschistfacies Combin zone in the hanging wall, has been interpreted previously as a major normal fault reflecting synorogenic crustal extension. Quartz textures of mylonites from this fault were measured using neutron diffraction. Together with structural field observations, the data allow a refined reconstruction of the kinematic evolution of the Pennine nappes. The main results are: (1) the contact is not a normal fault but a major thrust towards northwest which was only later overprinted by southeast-directed normal faulting; (2) exhumation of the footwall rocks did not occur during crustal extension but during crustal shortening; (3) the Sesia-Dent Blanche nappe system originated from a continental fragment (Cervinia) in the Alpine Tethys ocean, and the Combin zone ophiolites from the ocean basin southeast of Cervinia; (4) out-of-sequence thrusting played a major role in the tectonic evolution of the Penninic nappes. Keywords Alps Æ Tectonics Æ Exhumation Æ Palaeogeography Æ Quartz texture J. Pleuger (&) Æ S. Roller Æ N. Froitzheim Geologisches Institut, Universität Bonn, Nußallee 8, Bonn, Germany jan.pleuger@uni-bonn.de J. M. Walter Æ E. Jansen Mineralogisch-Petrologisches Institut, Universität Bonn, Außenstelle für Neutronenbeugung im Forschungszentrum Jülich, MIN/ZFR, Jülich, Germany Introduction Tectonic, metamorphic and magmatic processes during continent collision and the resultant orogeny are strongly determined by the precollisional palaeogeography, i.e. the arrangement and shape of continental margins and oceanic basins. Processes that may be influenced or even controlled by palaeogeography are, for example, the formation of orogenic curvature (Zweigel et al. 1998), lateral extrusion (Tapponnier et al. 1982), the formation of ultrahigh-pressure metamorphic rocks (Ernst and Liou 1995), and their exhumation (Froitzheim et al. 2003). Therefore, reconstructing the preorogenic palaeogeography is most important for the understanding of orogenic processes. It may be done using constraints from outside the orogen, i.e. by reconstructing the large-scale plate motions from oceanic magnetic anomalies and palaeomagnetic studies on the continents. This method is not sufficient where independently moving microplates were involved, as is generally the case in the Alpine collision zone. In these cases, the reconstruction relies to a large extent on structural restoration, P-T-path determination, and age dating within the collisional zone. In the present article, we will present results of structural field work and microtectonic analysis on the Penninic zone in the Western Alps (Fig. 1). This work suggests that in addition to the three continents known to have been involved in the Alpine orogeny (Europe, Iberia, Adria), a continental fragment or microcontinent, Cervinia, existed in the Alpine Tethys ocean which collided with Adria at the end of the Cretaceous. This continental fragment should be taken into consideration in future models of the Alpine orogeny. Furthermore, our results have

2 230 Int J Earth Sci (Geol Rundsch) (2007) 96: Fig. 1 Tectonic overview map of the southwestern Penninic and northeastern Graian Alps, modified after Steck et al. (1999) and Bigi et al. (1990). The map area is marked by the rectangle in the inset. E.-L. Etirol-Levaz sliver, G.-R. Glacier Rafray sliver, Mt. E. Mont Emilius klippe Tsaté Dent Blanche E.-L. Tsaté St. Bernard Zermatt- Saas Monte Rosa Balma Simplon fault Antrona Lepontine basement Camughera Periadriatic fault Southern Alps 45 50' 46 00' 46 10' N Mt. E. G.-R. Sesia Gran Paradiso Canavese fault Levone Canavese South Alpine sediments South Alpine basement Combin zone Sesia-type basement slivers Sesia and Dent Blanche nappes Zermatt-Saas zone Briançonnais (Middle Penninic) North Penninic ophiolite units European margin units E N European foreland DM Penninic and Helvetic units MR GP Po plain Austroalpine units South Alpine units MR: Monte Rosa GP: Gran Paradiso DM: Dora Maira Margna-Sesia units 7 20' 7 30' 7 40' 7 50' 8 00' 8 10' 8 20' E 45 30' 45 40' 45 20' important implications for the mechanism by which the high-pressure metamorphic rocks of the study area were exhumed. The Alpine cross section through the Penninic Alps A complete cross section of the Penninic and Southern Alps is exposed between the Rhone valley and the Po plain (Fig. 2). It includes several units mostly derived from continental and oceanic crust, together with their cover sequences, which were deformed and metamorphosed during Late Cretaceous and Tertiary subduction and subsequent exhumation. The area comprises one of the best-studied metamorphic orogens worldwide, providing a huge and valuable database. Most studies were carried out to elucidate the orogenic development of this part of the Alps, focussing on questions concerning the present geometry and kinematics of the units, the exhumation mechanism of high- and ultrahigh-pressure units, and the palaeogeographic arrangement of continental and oceanic domains. With regard to the palaeogeography, large-scale plate tectonic reconstructions (e.g. Stampfli and Borel 2004), structural studies of Alpine cross sections (e.g. Pfiffner et al. 1997) and stratigraphic and geochronological considerations (see Froitzheim et al for a discussion) all consistently provide evidence for the diachronous existence of two oceanic basins, the Valais or North-Penninic ocean and the Piemont or South Penninic ocean, between the European and Adriatic continents and separated by the Briançonnais high, a peninsular spur of Iberia.

3 Int J Earth Sci (Geol Rundsch) (2007) 96: SE Cimalegna S. Vanzone Antiform Trift A. Zermatt-Saas Mont Dolin Frilihorn NW Antrona S. Gabbio S. Balm Mischabel Antiform Biel-Balmen A. Mittaghorn S. ahorn A. Berisal S. Glishorn A. Lower Penninic Sion-Courmayeur nappes and external "massifs" Fig. 2 Cross section through the Swiss Italian Western Alps, modified after Escher et al. (1993). Second-order nappe boundaries are drawn within the St Bernard nappe system, the Sesia-Dent Blanche nappe system and the Camughera-Moncucco unit in order to highlight internal structures. Traces of major fold axial planes are marked according to the relative ages of the folds by white (post-d 3 ), light grey (D 3 ), dark grey (D 2 ) and black (D 1 ) triangles. For legend see Figs. 1 and 3 These terranes were subsequently subducted below the Adriatic plate from the Latest Cretaceous onward, beginning with the originally southernmost units and ending in Eocene/Oligocene with the originally northernmost units derived from the European continental margin. Radiometric age data from different rock units interpreted to correspond to peak-pressure mineral assemblages, as well as ages of the youngest sediments deposited on the respective oceanic and continental domains, reflect this progressive consumption by subduction. However, radiometric age data are still incomplete because not all of the presently exposed units have been investigated. The geometry of this orogenic wedge in the Penninic Alps has become of particular interest because some of the units involved show evidence for (ultra)high-pressure metamorphism, and different kinematic concepts and models have been developed or applied in this area. The suggested models differ in many important respects, although they are mostly based on the tectonic scenario described above. Briefly summarised, the following issues are controversial: (1) the palaegeographical origin of some rock units, e.g. the Monte Rosa nappe and the Antrona ophiolite unit, (2) whether or not fragmentary ophiolite slivers in the Canavese zone represent an oceanic suture between the Sesia-Dent Blanche nappe system and the Southern Alps, (3) the number of subduction zones, (4) whether the tectonic units are fold nappes or thrust nappes, (5) the exhumation mechanism of high- and ultrahigh-pressure rocks, especially the question whether or not orogen-perpendicular extensional movements contributed to exhumation, (6) the extent and effect of orogen-parallel normal faulting, and (7) the geometry and style of post-nappe folding. The aim of this contribution is to attempt an updated reconstruction of the kinematic evolution of the Swiss-Italian Western Alps in accordance with recent geochronological (Rubatto et al. 1998; Dal Piaz et al. 2001; Lapen et al. 2003; Liati et al. 2002, 2005) as well as our recent (Pleuger et al. 2005) and new structural observations, and to discuss the consequences of this reconstruction in terms of palaeogeography. Regional geological setting As the lowermost part of the Penninic nappe pile, units derived from the former European margin are mainly exposed in the Central Alps where they build up the Lepontine dome. Southwest of the Simplon fault, the structurally deepest units (also from the southeastern margin of the European continent) are the Camughera-Moncucco unit and the Monte Rosa nappe (Fig. 2) according to reconstructions of Froitzheim (2001) and Pleuger et al. (2005). The Monte Rosa nappe mainly consists of Variscan basement, i.e. paraand orthogneisses and to a minor extent mafic lenses, and small remnants of a sedimentary cover at the northern front of the nappe. Especially in the western, structurally higher, part of the nappe, remnants of a probably Tertiary eclogite-facies imprint (ca kbar/ C; Chopin and Monié 1984; Borghi et al. 1996; Engi et al. 2001) are preserved. The Monte Rosa nappe is generally correlated with the Gran Paradiso and Dora-Maira nappes further to the

4 232 Int J Earth Sci (Geol Rundsch) (2007) 96: southwest. The present geometry of the Monte Rosa nappe is that of a north-closing, refolded, originally recumbent isoclinal fold. Following the lower limb of that fold, its originally south-closing counterpart is the Antrona synform around which the Monte Rosa nappe is connected with the Camughera-Moncucco unit. The Monte Rosa nappe and Camughera-Moncucco unit are separated from each other by the Antrona ophiolite unit in the core of the Antrona synform ( Antrona- Mulde ; Blumenthal 1952). The geometry of these three units is strongly modified by the late, almost upright Vanzone antiform. In the hinge area of this antiform the next higher unit above the Monte Rosa nappe is the Balma unit (Pleuger et al. 2005). Together with ophiolite slivers in the Furgg zone at the northern front of the Monte Rosa nappe and the Antrona ophiolites, the Balma unit forms an incoherent mantle of Valaisan-derived ophiolitic units around the nappes of European origin. Peak-pressure conditions in the Valaisan units were in the eclogite-facies (ca kbar/ C; Colombi and Pfeifer 1986). The next higher tectonic unit is the St Bernard nappe system. It comprises basement and sedimentary cover and is composed of several subnappes (from base to top: Zone Houillière, Pontis nappe, Siviez-Mischabel nappe, Mont Fort nappe; Escher et al. 1993). The St Bernard nappe system is connected laterally with the Briançonnais nappes of the Western Alps and rooted south of the Monte Rosa nappe, where it is represented by only thin slivers of paragneiss (Stolemberg unit; Pleuger et al. 2005). The St Bernard nappe system generally shows no Alpine eclogite-facies but only greenschist- to blueschist-facies imprint (see Bousquet et al and references therein). In the most internal Stolemberg unit (Fig. 3), however, eclogite boudins are present. Ophiolites from the Piemont basin occur in two tectonic units, the structurally lower Zermatt-Saas zone and the higher Combin zone. Ophiolites of the Zermatt-Saas zone are metabasalt, gabbro and serpentinite. Metaradiolarite, marble and calcschists make up the sedimentary cover of the oceanic basement. The Zermatt-Saas zone has been affected by eclogite-facies metamorphism; in one locality coesite was found by Reinecke (1991, 1998). According to Bucher et al. (2005), the P-T conditions of eclogitefacies metamorphism in the entire Zermatt-Saas zone were about kbar and C. The Combin zone comprises two types of units; first, ophiolites and their Late Jurassic to Cretaceous sedimentary cover (Tsaté nappe; Sartori 1987), similar to the Zermatt- Saas zone, except that the metamorphic grade is lower (blueschist facies, ca. 9 kbar/ C; Reddy et al. 1999; locally blueschist eclogite-facies transition, kbar/ C; Bousquet et al. 2004); second, metasedimentary units, including Permo-Mesozoic terrestrial and shallow marine sediments that were deposited on continental crust (Cimes Blanches and Frilihorn nappes; Vannay and Allemann 1990; Escher et al. 1993). The Frilihorn nappe and Cimes Blanches nappe (Fig. 4) occur within and at the base of the Tsaté nappe, respectively. The Cimes Blanches nappe overlies the Zermatt-Saas zone in the south and the St Bernard nappe system in the north. There, the contact of the Cimes Blanches nappe on the Mont Fort nappe, the highest subunit of the St Bernard system, represents a typical case of cover substitution (substitution de couverture), in the sense that the sedimentary cover of the Mont Fort basement was sheared off and replaced by the Cimes Blanches sedimentary rocks (Sartori and Marthaler 1994). On top of the Combin zone lies the Sesia-Dent Blanche nappe system as the uppermost element of the Penninic Alps. The Dent Blanche and Sesia nappes are now separated by erosion but probably formed a single unit. Both Dent Blanche and Sesia comprise three different types of basement. (1) Variscan basement with a Permian granulite-facies overprint (Valpelline series and seconda zona dioritico-kinzigitica in the Dent Blanche and in the Sesia nappe, respectively); (2) Variscan upper-crustal basement (Arolla series and Sesia zone sensu stricto, respectively); (3) Permian gabbros. The Sesia zone sensu stricto is commonly subdivided into the eclogitic micaschist complex ( micaschisti eclogitici ) with eclogite-facies assemblages that were not or incompletely overprinted by a later greenschist-facies retrogression, and the gneiss minuti complex in which this greenschist-facies overprint is pervasive. Maximum P-T conditions in the Sesia zone have been determined as about kbar/ C (e.g. Lardeaux et al. 1982; Pognante 1989), whereas they are considerably lower in the Dent Blanche nappe (10 12 kbar/ C in the Arolla series, Bucher et al. 2004a). Thin slivers of Mesozoic cover rocks within the Sesia-Dent Blanche nappe system are predominantly arranged along or near the contacts between the eclogitic micaschists and gneiss minuti, between the seconda zona dioritico-kinzigitica and the Sesia zone sensu stricto, and along a shear zone within the Dent Blanche nappe which separates the main portion of the latter (Dent Blanche nappe sensu stricto) from an underlying subnappe (Mont Mary klippe ; Fig. 2). The best-preserved remnants of the sedimentary cover occur in the northwestern part of the Dent Blanche nappe at Mont Dolin (Hagen 1948; Ayrton et al. 1982). The most characteristic member is

5 Int J Earth Sci (Geol Rundsch) (2007) 96: ' 46 00' 46 10' Dent Blanche GP41 GP38 GP ' 7 50' 8 00' 8 10' Cimes Blanches nappe 7 40' CB3 CB14 DHM4 St. Bernard CK2 Mischabel Antiform CB36 CB32 Saas Stockhorn unit DHM3 CB33 Zermatt- Stockchnubel Stolemberg JA ' B42 CB8 CB7 Mittaghorn Synf. Balma B43 CB5 zo n e Stelli Portjengrat unit Monte Rosa Vanzone Antiform 8 00' MR195 Sesia Simplon fault Balmahorn Antif. Antrona MR199 Gabbio Synf. 2DK Antrona Synf. Ragno Antiform Colmetta Synform Periadriatic 46 10' Monte Leone Moncucco Sesia-Dent Blanche sediments Tsat nappe Cimes Blanches and Frilihorn nappes for other signatures see Figure ' 00' 46 fault 50' 45 Fig. 3 Tectonic map of the Penninic nappe stack between the Sesia-Dent-Blanche nappe system and the Simplon line, modified after Steck et al. (1999). Arrows indicate the transport directions of respective hanging walls during D 1 (black), D 2 (dark grey) and D 3 (light grey). Shear senses inferred from a thick rifting-related breccia of Jurassic age. A similar breccia occurs also in the Cimes Blanches nappe (Vannay and Allemann 1990). Basement rocks with the typical characteristics of the Sesia-Dent Blanche nappe system also occur at a deeper structural level (Nervo and Polino 1976; Ballèvre et al. 1986) as slivers along the contact between the Combin zone and the Zermatt-Saas zone (e.g. the Etirol-Levaz sliver) or within the latter (e.g. the Glacier-Rafray sliver, Fig. 1). Along the Canavese fault, the southwesternmost segment of the Periadriatic fault, thin lenses of ophiolite appear between the Sesia nappe to the northwest and the Canavese zone to the southeast, which represents the distal passive continental margin of the Adriatic continent, characterised by Jurassic riftingrelated breccias. The most complete ophiolite lens is found in the Torrente Levone valley (Elter et al. 1966), quartzite textures are indicated by white dots and labelled with the sample numbers. Traces of major fold axial planes are marked according to the relative ages of the folds by black (D 1 ), dark grey (D 2 ), light grey (D 3 ) and white (post-d 3 ) triangles northwest of Levone (Fig. 1). The ophiolite lens is embedded in low-grade metamorphic calcschists. It comprises serpentinite, strongly altered gabbro, and radiolarite. This rock association is typical for Piemont-Ligurian ophiolites, which led several authors (e.g. Aubouin et al. 1977; Mattauer et al. 1987) to place the suture of the Alpine Tethys between Adria and the Sesia nappe. Further southwest, the Lanzo peridotite occurs west of and in close vicinity to the Canavese fault. In contrast to the low-grade ophiolite sliver at Levone, the Lanzo peridotite experienced Alpine eclogite-facies metamorphism as did the Zermatt-Saas ophiolites. Spalla et al. (1983) documented complex folding of the contact between the Lanzo peridotite and the Sesia nappe, and connected the peridotite with the Zermatt-Saas ophiolites. We follow this interpretation and assume that the Lanzo peridotite is in a deeper structural position than the Sesia-Dent Blanche

6 234 Int J Earth Sci (Geol Rundsch) (2007) 96: Fig. 4 View towards west from Bettaforca Pass (ridge between Val de Gressoney and Val d Ayas) over upper Val d Ayas. The Cimes Blanches nappe (between the black lines) separates greenschist-facies rocks of the Tsaté nappe above from partly eclogite-facies rocks of the Zermatt-Saas zone below. The thin, light-coloured layer above the Cimes Blanches nappe in the slope of Mt Roisetta is the Frilihorn nappe. Peaks in the far background (Valtournenche) are made up by rocks of the Dent Blanche nappe nappe system, in contrast to the Levone ophiolite sliver which is structurally higher and derived from a more internal position than the Sesia-Dent Blanche nappes. Some radiometric age data of importance for reconstructions of the Penninic palaeogeography and the Alpine orogeny After Late Triassic rifting, the Piemont ocean opened from the Middle Jurassic onwards (Trümpy 1975), and protolith ages of ophiolites obtained by U Pb zircon dating mostly on metagabbros cluster around ca Ma (see Liati et al for a compilation). The opening of the Valais domain was preceded by rifting in the Late Jurassic; oceanic spreading occurred from Barremian to Cenomanian (Stampfli et al. 1998). This is confirmed by ca. 93 Ma, i.e. Cenomanian to Turonian, U Pb SHRIMP zircon protolith ages of one eclogite and two amphibolite samples from the Balma unit and the Chiavenna ophiolites (eastern Central Alps), respectively (Liati et al. 2002, 2003). However, a portion of Jurassic oceanic crust was incorporated into the Valais domain due to oblique opening of the latter by overall sinistral movements between Europe and the Briançonnais microcontinent (Stampfli 1993; Liati et al. 2005). Such former Jurassic oceanic crust of the Valais domain, but without radiolarites, is found in the Antrona unit and in the Misox zone (eastern Central Alps; Liati et al. 2005). The oldest data for high-pressure metamorphism are reported from the eclogitic micaschist complex of the Sesia-Dent Blanche nappe system and range between 70 and 60 Ma (U Pb on sphene and Rb Sr on white mica, Ramsbotham et al. 1994; Inger et al. 1996; U Pb SHRIMP on zircon, Rubatto et al. 1999). Eclogite from the Zermatt-Saas zone yielded ages of ca. 44 Ma (U Pb SHRIMP on zircon, Rubatto et al. 1998) and ca Ma (Sm Nd on garnet, Amato et al. 1999; Lu Hf garnet-omphacite-whole rock isochron, Lapen et al. 2003). The Sesia-type gneiss slivers within and at the top of the Zermatt-Saas zone suffered eclogite-facies metamorphism at the same time, together with the Zermatt-Saas ophiolites (Rb Sr on phengitic white mica, Dal Piaz et al. 2001). Concerning the Valaisan ophiolites, eclogite from the Balma unit yielded metamorphic ages of ca. 40 Ma (U Pb SHRIMP on zircon, Liati et al. 2002) and eclogite and amphibolite from the Antrona ophiolites ca Ma (same method, Liati et al. 2005). These different ages reflect a progression of subduction from internal, southeasterly located units (Sesia-Dent Blanche) to external units (Valaisan). The Valais ocean palaeogeographically occupied a more external position than the Piemont ocean and the Sesia-Dent Blanche nappe system in turn occupied a more internal position than the Piemont ocean. The high-pressure metamorphism in the Monte Rosa nappe has not been dated so far. Current models for the evolution of the Penninic nappe pile and the exhumation of high-pressure rocks High-pressure metamorphism was relatively moderate in the Monte Rosa and Gran Paradiso nappes (Borghi et al. 1996), but reached the coesite stability field in the Dora-Maira nappe (Chopin 1984). Together these three units are commonly referred to as the internal crystalline massifs. They are overlain by the Zermatt- Saas zone and correlated ophiolite units in the French- Italian Western Alps, e.g. the Monviso unit. These suffered eclogite-facies metamorphism as well. Several models have been proposed for the exhumation of this complex of high-pressure nappes. They presume either an extrusion-like ascent of high-pressure units within a permanently convergent setting (Wheeler 1991; Michard et al. 1993) or episodes of extensional deformation with exhumation of the high-pressure units accommodated by unroofing along low-angle normal faults (Ballèvre and Merle 1993; Reddy et al. 1999). Either type of tectonic model, exhumation by extrusion or extension, calls for a normal fault at the top of the ascending units. The contact between the Zermatt-Saas and Combin zones coincides with a jump in metamorphic peak

7 Int J Earth Sci (Geol Rundsch) (2007) 96: pressure from kbar in the footwall (Bucher et al. 2005) to kbar in the hanging wall (Bousquet et al. 2004). Hence, a pressure gap of ca. 12 kbar appears to exist between these units, equivalent to a missing crustal thickness of ca. 43 km. This contact is thus clearly tectonic and has been described under the names of Combin fault (Ballèvre and Merle 1993) and Gressoney shear zone (Reddy et al. 1999). Even without quantifying the gap, the difference between the greenschist calcschist association of the Combin zone and the eclogite-dominated Zermatt-Saas zone is remarkable and at the origin of hypotheses postulating a major southeast-vergent extensional detachment fault that accommodated the exhumation of underlying eclogite-facies rocks. Ballèvre and Merle (1993), at a time when highpressure metamorphism in the Penninic nappes was regarded as Cretaceous in age, proposed that the Combin fault was a Late Cretaceous top-to-thesoutheast low-angle normal fault which was reactivated as a top-to-the-northwest thrust in the Tertiary. Reddy et al. (1999) studied the relations between Combin zone and Zermatt-Saas zone and provided a detailed kinematic model to account for an Eocene age of highpressure metamorphism in the footwall. Reddy et al. (1999, 2003) argued for Tertiary extensional faulting with a large total displacement, affecting the whole Combin zone and the base of the Sesia zone in earlier stages (from ca Ma onward), afterwards (ca Ma) temporarily either replaced by pervasive northwest-vergent shearing or at least coupled with conjugate domains of top-to-the-northwest shear sense, and finally localised in the lowermost part of the Combin zone (ca Ma). Only the early stages of southeast-vergent shear are held responsible for the exhumation of eclogite-facies rocks in the footwall, whereas northwest-vergent and late southeast-vergent shear led to a late and comparatively weak overprint. Reddy et al. (1999) introduced the term Gressoney shear zone for this extensional, dominantly top-to-thesoutheast shear zone comprising the entire Combin zone and the base of the Sesia zone. Wheeler et al. (2001) proposed that the top-to-the-southeast displacement along the Gressoney shear zone amounted to 100 km, which would make this shear zone by far the most important synorogenic normal fault in the Alps. In contrast to this view, Ring (1995) found that the Combin/Zermatt-Saas contact represented mainly a northwestward thrust of Eocene age, overprinted by later top-to-the-southeast normal faulting of much lesser importance than assumed by Reddy et al. (1999) and Wheeler et al. (2001). Structural history of the Penninic nappes between the Simplon fault and the Aosta valley Structural studies of the Penninic nappes between the Simplon fault and Aosta valley (e.g. Sartori 1987; Steck 1990; Pleuger et al. 2005) have shown that the oldest structures that are abundant enough to be correlated throughout the whole region were formed by northwest-vergent shearing (D 1 ), then overprinted by structures related to west- to southwest-vergent shearing (D 2 ) and finally overprinted by structures related to east- to southeast-vergent shearing (D 3 ). During D 1 and D 2, shearing led to folding with fold axes parallel to the actual stretching direction. D 3 fold axes are also parallel to the D 3 stretching direction in southern, i.e. deeper, parts of the nappe stack (e.g. C- imalegna fold; Fig. 2) but oblique to perpendicular to the stretching direction in the north (e.g. Mischabel fold; Fig. 2). Thus age relations between the deformation phases can macroscopically best be recognised from fold overprinting geometries or folded stretching lineations. The mylonitic foliation runs (sub)parallel to nappe contacts and is in large portions of the nappe stack a composite foliation of D 1, D 2 and D 3. However, the imprint of each deformation phase shows marked spatial variability normal to the foliation and lithological layering as well as more gradual variations along strike. Due to the variably strong overprint of D 2 and D 3 on D 1 and of D 3 on D 2, evidence for all three deformation phases can be found above and below the contact between the Combin zone and the Zermatt- Saas zone. Our study is focussed on this contact and therefore largely deals with the Cimes Blanche nappe (Fig. 4), which is exposed over large distances along the base of the Combin zone. The Cimes Blanches nappe is composed of pervasively mylonitised Mesozoic sediments. These are mainly carbonatic but also include a Triassic layer of quartzite which provided most of our samples for microstructural and textural studies. In southwestern Valtournenche, where the Cimes Blanches nappe occurs only sporadically, the Etirol-Levaz slice occupies a similar structural position. In the Val de Gressoney and east of it, the Cimes Blanches nappe is almost completely missing; therefore, no textures from the base of the Gressoney shear zone could be obtained for this area. Microstructural studies on thin sections mainly served to gain information about the metamorphic conditions underwhichdeformationtookplaceandtoascertain the shear senses related to macroscopic stretching lineations. Along the base of the Combin zone, stretching lineations mostly trend southeast northwest (Fig. 3). In

8 236 Int J Earth Sci (Geol Rundsch) (2007) 96: thin section, shear sense indicators like shear bands, feldspar porphyroclasts and mica fish indicate both northwest- and southeast-vergent senses of shear. Only locally could southwest-plunging stretching lineations be observed with associated top-to-the-southwest shear senses. This phase of southwest-vergent shearing (D 2 ) overprints northwest-vergent shearing (D 1 ) and is in turn overprinted by southeast-vergent shearing (D 3 ). Where the D 2 imprint is missing, age relations between D 1 and D 3 are difficult if not impossible to recognise because D 1 and D 3 were almost coaxial. Moreover, along the Combin/Zermatt-Saas contact south of the area affected by the Mischabel fold, folding was not intense and folds overprinting earlier structures can rarely be found. Due to the fact that overprinting relations within the Cimes Blanches nappe itself are largely missing, most of the mylonitic structures and textures investigated in the present study could not be ascribed to D 1, D 2 or D 3 by overprinting relations but only according to the related shear senses (Fig. 5). This seems justified as the average transport directions of D 1, D 2 and D 3 are separated by large angles and for each deformation phase the variation in transport directions within a certain area turned out to be comparably small where the relative age of structures could be recognised. In impure quartzitic samples from the Cimes Blanches nappe (Fig. 6b, c), the quartz grains always show evidence for deformation by subgrain rotation and in many cases also minor influence of grain boundary migration recrystallisation. These deformation mechanisms may concurrently act at temperatures of about 500 C and subgrain rotation is interpreted to become dominant with decreasing temperatures (Stipp et al. 2002). Therefore, greenschist-facies metamorphic conditions not in excess of 500 C can be assumed for the observed deformations in quartzitic rocks of the Cimes Blanches nappe although the mineral content (apart from quartz, some feldspar and white mica) is too meagre to confirm this by direct petrological observations. In accordance with the implied metamorphic conditions, microstructures related to the above-described shearing events D 1 to D 3 were formed in metabasic rocks of the Tsaté nappe and the Zermatt- Saas zone when albite, chlorite and/or actinolite were stable, i.e. in the greenschist facies. Neutron texture goniometry on quartzitic mylonites Quartz textures have been analysed from samples of 1 8 cm 3 with the neutron texture diffractometer SV7-b at Forschungszentrum Jülich (Jansen et al. 2000) in order to obtain information about the kinematics and mechanisms of deformation. In Figs. 7, 8, and 9, the orientation distributions of the c axes, normals to first-order prism planes {1 0 0} (hereafter {m}), and normals to the second-order prisms {1 1 0} (hereafter {a}) are shown in upper-hemisphere stereographic projections. The pole figures of {a} and {m} are measured ones; the c axis pole figures were calculated from the respective orientation distribution functions because the reflections of the basal planes {c} are too weak to be accurately measured. The trace of the foliation is always horizontal, the stretching lineation at the intersections between the trace of the foliation and the edge of the diagram. The pole figures are plotted in such a way that the more southerly direction is to the left. For some samples, the textures indicate a stretching direction different from that displayed by the macroscopic mineral lineation. In these cases, the textures were rotated around the normal to the foliation so that, in the c axis pole figures, axes lying in the foliation plane meet the centre of the diagram (samples DHM3, CB5, CB7), i.e. in the case of a girdle distribution the girdle runs through the centre, or that the absolute {a} maximum falls onto the margin of the diagram (samples CB8, CB36, MR199, CB3, B43). Such rotations Fig. 5 Lower-hemisphere Schmidt projection plots of D 1, D 2 and D 3 stretching lineations (triangles) and poles to mylonitic foliation (dots). Corresponding shear senses are shown in Fig. 3 N N N D 1 D 2 D 3 S: 19 values L: 19 values S: 12 values L: 12 values S: 18 values L: 18 values

9 Int J Earth Sci (Geol Rundsch) (2007) 96: Fig. 6 Photographs of XZ-thin sections of quartzitic mylonites under crossed polarisers. Sample locations (see also Fig. 3) are given in coordinates of the Italian Gauss-Boaga grid. a Detail of MR195 from Valle d Antrona ( / ). Quartz vein (lower half) in a kyanite-bearing paragneiss (upper half; ky kyanite). Quartz grains are comparatively large and have isometric and polygonal shapes suggesting higher temperatures during postkinematic annealing than in the other samples shown in this figure and probably also during deformation (D 1 ). The texture of the quartz vein (Fig. 7b) indicates dextral shear sense (top-to-the-west-northwest, D 1 ). b Detail of impure mylonitic quartzite GP41 from the Cimes Blanches nappe in Valtournenche ( / ). Dextral shear sense (top-to-the-northwest, D 1 ) is indicated by C -type shear bands and shape-preferred orientation of quartz (see also the respective texture, Fig. 7d). c Detail of impure mylonitic quartzite GP38 from the Cimes Blanches nappe in Valtournenche ( / ). Sinistral shear sense (top-to-the-southeast, D 3 ) is indicated by albite sigma clasts and shape-preferred orientation of quartz and consistent with the shear sense indicated by the respective texture (Fig. 9c). d Detail of impure quartz mylonite B42 from the base of the Sesia nappe on the ridge between Val de Gressoney and Valsesia ( / ). Quartz grains indicate opposite shear senses in domains above (dextral) and below (sinistral) the white line by different shape-preferred orientations. The respective texture (Fig. 9f) shows that altogether domains with sinistral shear (top-to-the-southeast, D 3 ) dominate over conjugate domains with dextral shear (see also MacCready 1996) are based on two assumptions. First, Æaæ is the only important slip direction in slip systems that act under greenschist-facies conditions (e.g. Bouchez 1978; Schmid and Casey 1986) and one of the {a} maxima should align with the direction of shear, i.e. normal to Y and near to X. Second, in the case of initially randomly distributed crystallographic axes, homogeneous deformation should, irrespective of the flow type, produce textures with twofold internal symmetry, either rotational with respect to Y (e.g. samples MR199, B43) or with the shear plane as a mirror plane (e.g. CB5). Despite many differences between the textures, in all cases in which the {c} pole figure meets these criteria without having to be rotated, the {a} pole figure shows a more or less pronounced single maximum close to X. This suggests that indeed all the quartz textures were formed by intracrystalline slip in the Æaæ direction. Another indication that rotations of textures are permissible is that in some samples two or even three mineral lineations occur, and in these cases the rotation brings one of these lineations into the X position of the diagrams. The occurrence of more than one mineral lineation is made possible by growth of grains parallel to the direction of instantaneous stretching during shearing. Those textures which formed during D 1 north-vergent shearing (Fig. 7) show girdle distributions of {c}. Four of them (CB5, GP41, JA17, CB14) were collected from the base of the Combin zone, the other two (DHM3, MR195) from the Monte Rosa nappe for comparison. Whereas DHM3 (Fig. 7a) and CB14 (Fig. 7f) are almost perfect single and type I crossed girdles (Lister 1977), respectively, the other {c} patterns are intermediate between these two types. The {c} pole figure of MR195 (Fig. 7b) yields a single girdle distribution similar to that of DHM3 kinked in the positions where, in the case of a crossed girdle, the two other branches would be expected to join the main

10 238 Int J Earth Sci (Geol Rundsch) (2007) 96: a SE (145 ) NW (325 ) d SE (135 ) NW (315 ) S 244/25 L' 325/4 S 314/15 L 315/14 b ESE (111 ) WNW (291 ) e SE (126 ) NW (306 ) S 247/63 L 291/28 S 216/25 L 126/5 c SSW (195 ) NNE (15 ) f ESE (120 ) WNW (300 ) S 4/50 L' 15/49 Fig. 7 Upper-hemisphere stereographic projections of D 1 quartzite textures obtained from neutron diffraction. {c} pole figures were calculated from the orientation distribution function derived from observed {1 0 0}, {1 0 1/0 1 1}, {1 1 0} and {1 1 1} reflections. {a} and {m} pole figures are from direct experimental observation. Textures rotated around Z are indicated by L instead of L in the geographical sample orientation, which is given below the {a} pole figure (see explanation in the text). The azimuths of the stretching lineation are given above the diagrams. Sample locations (see also Fig. 3) are given in coordinates of the Italian Gauss-Boaga grid (signed I ) or Swiss coordinates (signed CH ) depending on the sample S 300/35 L 300/35 location. a Sample DHM3 from the top of the Monte Rosa nappe (I / ). b Sample MR195 from the lowest part of the Monte Rosa nappe above the Antrona synform (I / ). The microstructure of this sample is shown in Fig. 6a. c Sample CB5 from the Cimes Blanches nappe between the Portjengrat unit (Fig. 3) and the rest of the St Bernard nappe system (CH /108675). d Sample GP41 from the Cimes Blanches nappe (I / ). The microstructure of this sample is shown in Fig. 6b. e Sample JA17 from the base of the Tsaté nappe (I 1440/ ). f Sample CB14 from the Cimes Blanches nappe adjacent to the Stockhorn unit (CH / 93500; Fig. 3) girdle. The {c} pole figures of CB5 (Fig. 7c), GP41 (Fig. 7d) and JA17 (Fig. 7e) show an increasing tendency to form a type I crossed girdle. In terms of flow type, this tendency can be interpreted to result from a decrease of the rotational shear component (Schmid and Casey 1986). In contrast to the other samples, the thin section of JA17 shows that strain is strongly partitioned into almost equally large alternating domains of northwest- and southeast-vergent shear. This can be seen by preferential addition or subtraction colours when inserting a lambda plate and by different grain shape-preferred orientations. The bulk strain of JA17 apparently has only a weak rotational component. Another, less pronounced trend in the samples formed by D 1 is that in the textures with {c} cross girdles (GP41, CB14), Æaæ tends to spread over small circles at high angles around the shear plane normal. We interpret this to be due to a larger flattening component to the strain compared to those textures with single girdles of {c}. Looking at the sample localities, these findings indicate that D 1 shearing had a stronger rotational component in the Monte Rosa nappe and perhaps a more important flattening component in the Combin zone. The textures formed during D 2 (Fig. 8) are very variable and distinctly different from the D 1 textures. Four relevant samples come from the base of the Combin zone (CB8, CB36, CB32, CB33) and one is from the Monte Rosa nappe (DHM4). The texture of the latter (Fig. 8a) shows an incomplete single girdle of {c} with an absolute maximum close to Y. Perpendicular to this {c} maximum, the {a} and {m} maxima are located at the periphery of the diagrams. This suggests increasing importance of prism Æaæ slip. Generally, prism Æaæ slip becomes dominant over basal Æaæ and rhomb Æaæ slip at higher temperatures (Wilson 1975), roughly those of

11 Int J Earth Sci (Geol Rundsch) (2007) 96: Fig. 8 D 2 quartzite textures obtained from neutron diffraction. a Sample DHM4 from the top of the Monte Rosa nappe (I / ). b Sample CB8 from the Cimes Blanches nappe between the Portjengrat unit and rest of the St Bernard nappe system (CH / ). c Sample CB32 from the Cimes Blanches nappe (CH /96275). d Sample CB33 from the Cimes Blanches nappe (CH / 96825). e Sample CB36 from the Frilihorn nappe (CH /96950) a SSW (195 ) NNE (15 ) S 238/9 L195/6 b SW (232 ) NE (52 ) S 279/8 L' 232/20 d W (260 ) E (80 ) S 264/42 L 260/40 e SW (227 ) NE (47 ) S 301/42 L' 227/14 c SSW (192 ) NNE (12 ) S 240/15 L 192/10 upper greenschist- to amphibolite-facies conditions (e.g. Schmid and Casey 1986; Stipp et al. 2002). Comparison of the DHM4 texture with those developed in the Monte Rosa nappe during D 1 (DHM3, MR195) therefore suggests that temperatures may have increased between D 1 and D 2. Indeed, Borghi et al. (1996) report petrological evidence for a temperature increase after considerable decompression of the Monte Rosa nappe. CB8 is another sample with top-to-the-southwest sense of shear indicated by the obliquity of the {c}, {m} and {a} pole figures (Fig. 8b). In contrast to DHM4, basal Æaæ slip is in this case inferred to have been the dominant slip system during development of the texture from the positions of the maxima. Three other D 2 textures show orthorhombic symmetries, from which no rotational component of strain can be concluded. The c axis pattern of CB32 (Fig. 8c) is very similar to that of CB5 and has the geometry of half a type I cross girdle, i.e. its left half is well developed whereas the right is almost completely missing. As in the case of CB14, there is a tendency for {a} to form small circle distributions around Z, which may indicate flattening strain. Although all three pole figures shown here ({c}, {m}, and {a}) are symmetric with respect to the foliation and suggest coaxial strain, the XZ thin section surprisingly exhibits asymmetric plagioclase porphyroclasts, mica fish, and shear bands proving a rotational top-to-thesouthwest strain component. Well-defined flattening strain is again documented by the {c}, {m} and {a} pole figures of CB33 (Fig. 8d), which have an almost perfect rotational symmetry around Z. These findings are obviously contradicted by the texture of CB36 (Fig. 8e), which is characterised by a fairly distinct {a} maximum parallel to X and a rudimental great circle distribution of {c} around X, suggesting constrictional strain (e.g. Schmid and Casey 1986). All in all, the wide variation in the D 2 textures presented here is consistent with largerscale observations that D 2 strain was strongly heterogeneous, as can be seen from the fact that D 2 structures are unevenly distributed and that D 2 was the main episode of large-scale folding at least below the Combin/ Zermatt-Saas contact. Among the {c} pole figures related to D 3 (Fig. 9), there are again some examples of type I cross to single girdles (CK2, GP38, GP40) and patterns whose shapes are determined by stronger single c axis maxima (CB3, B43, B42, MR199, CB7). Sample CK2 (Fig. 9a) from the Cimes Blanches nappe yields a texture with a {c} pattern somewhat intermediate between type I and type II crossed girdles. According to established interpretations (e.g. Schmid and Casey 1986), this and the tendency for {a} to be arrayed around the stretching lineation suggests constrictional strain. GP40 and GP38 (Fig. 9b, c), which were sampled close to each other from the Cimes Blanches nappe, show similar textures that are characterised by kinked c axis single girdles. In the case of GP38, the two {a} maxima display shapes elongated along small circles around Z and

12 240 Int J Earth Sci (Geol Rundsch) (2007) 96: Fig. 9 D 3 quartzite textures obtained from neutron diffraction. a Sample CK2 from the Cimes Blanches nappe (I / ). b Sample GP40 from the Cimes Blanches nappe (I / ). c Sample GP38 from the Cimes Blanches nappe (I / ). The microstructure of this sample is shown in Fig. 6c. d Sample CB3 from the Cimes Blanches nappe (CH /96200). e Sample CB7 from the Cimes Blanches nappe between the Portjengrat unit and rest of the St Bernard nappe system (CH /106300). f Sample B42 from the Sesia zone (I / ). The microstructure of this sample is shown in Fig. 6d. g Sample B43 from a greenschist layer in the uppermost Tsaté nappe (I / ). Note that the {0 1 1/1 0 1} instead of the {1 0 0} pole figure is shown. h Sample MR199 from the lowest part of the Monte Rosa nappe above the Antrona syncline (I / ) a SE (130 ) NW (310 ) S 300/19 L 310/18 b SE (137 ) NW (317 ) S 341/35 L 317/33 c SE (142 ) NW (322 ) S 350/19 L 322/16 d SSE (159 ) NNW (339 ) e SSE (165 ) NNW (345 ) S 276/26 L' 345/10 f SSE (168 ) NNW (348 ) S 118/30 L 168/21 g SSE (156 ) NNW (336 ) S 186/5 L' 156/4 h WSW (250 ) ENE (70 ) S 290/12 L' 339/8 S 57/17 L' 70/17 therefore suggest flattening strain. Single {c} maxima are shown by two more samples from the Cimes Blanches nappe, CB7 and CB3 (Fig. 9d, e). Possible interpretations of these two textures have to take into account that these had to be rotated to bring the respective strongest {a} maxima onto the periphery of the pole figures, i.e. to construct the possible X strain directions. However, in neither hand specimen could a corresponding stretching lineation be observed. Since the {c} patterns have triclinic symmetries even after the rotation, some doubt is cast over whether the constructed X directions are meaningful and if so, whether they represent only certain strain increments (specifically the latest strain increments according to MacCready 1996; Lebit et al. 2002) or the finite strain. The fact that the {c} maximum of CB7 and one of the {c} maxima of CB3 lie in a position between the centre and the margin of their diagrams suggests that rhomb Æaæ slip was operating (e.g. Schmid and Casey 1986; Mancktelow 1987; Law et al. 1990). Both in CB7 and CB3 there are indications that the latest strain was D 3. In the c axis pole figure of CB7, a tail-shaped appendix of the single maximum suggests southeast-vergent shear. The same is true for the stronger of two {c} maxima on the margin of the diagram for CB3. Assuming that the {c} maxima between the centre and the periphery were formed before D 3 by basal Æaæ slip, as is likely for deformation under greenschist-facies conditions, their position would be consistent with a formation during D 2 southwest-vergent shearing. Two samples (B42 and B43) stem from the base of the Sesia zone. B42 (Fig. 9f) shows two {c} maxima close to Z. The stronger and the weaker one are rotated by about 20 towards X in a counterclockwise and clockwise sense, respectively. {m} and {a} are almost equally disposed each on two girdles perpendicular to the {c} maxima. This texture corresponds well with thin section observations (Fig. 6d). Ribbon-shaped domains of quartz display opposite but internally consistent senses of shear, as clearly documented by grain shape-preferred orientations defining oblique foliations and by addition and subtraction colours when applying a

13 Int J Earth Sci (Geol Rundsch) (2007) 96: lambda plate. By total volume, ribbons with southeastvergent shear sense, corresponding to the stronger {c} maximum, dominate over those with northwest-vergent shear sense. Although within each domain deformation was probably close to plane strain and simple shear, the bulk strain of the conjugate set of ribbons was therefore flattening with a weaker overall rotational component than simple shear (Pauli et al. 1996). The texture of B43 (Fig. 9g) is dominated by strong single maxima of {c} and{a} at the periphery of the respective pole figures, indicative of basal Æaæ slip. Appendices of the {c} maximum extend towards the interior of the pole figure and document some additional influence of rhomb Æaæ slip. In the pole figure of {0 1 1}/{1 0 1}, a strong single maximum lies on the margin of the diagram whereas the other {r} reflections occupy a great circle around the absolute {r} maximum. Opposite to the absolute {c} maximum, the position of the absolute {r} maximum is about 20 away from Z in a counterclockwise direction. This suggests that slip on {r} acted in the opposite sense to that of the basal Æaæ slip and thus represents a conjugate slip system to the latter, in the same way as discussed for the weaker {c} maximum and corresponding {a} girdle in the case of B42. Consequently, for B43 the same interpretation with regard to strain applies as for B42. A D 3 texture from the Monte Rosa nappe (MR199, Fig. 9h) has a similar {c} pattern to that of B42 but {a} and {m} are not that properly arranged on girdles. From thin section there is no evidence for strain partitioning into conjugate domains but the microfabric is fairly homogeneous. This texture suggests a similar bulk strain as for B42, but it may not be representative for the Monte Rosa nappe, as D 3 textures characterised by c axis single girdles have also been found there (Pleuger et al. 2005). Tectonic interpretation of the quartzite textures Although flow patterns and strain geometries can be gathered from textures of quartzitic mylonites only in a qualitative way, the textures described here provide valuable information about the kinematics of the Gressoney shear zone at least for D 1 and D 3. D 1 textures from the Cimes Blanches nappe are more or less complete cross girdles with exclusively almost orthorhombic symmetries and therefore indicate only a small rotational component of shear (Schmid and Casey 1986). These results are consistent for samples taken from widely separate localities and are in remarkable contrast to D 1 samples from the Monte Rosa nappe, which show single girdles indicating larger components of rotation and thus larger amounts of transport to the northwest. The fact that D 2 textures are ambiguous is best accounted for by the strong heterogeneity of D 2 strain, related to intense largescale folding. Variable strain geometries must also be concluded from D 3 textures in samples from the Cimes Blanches nappe. However, in spite of these differences, all of them indicate strongly rotational shear and are therefore well in line with the conclusion of Reddy et al. (1999, 2003) that the base of the Combin zone was affected by particularly strong southeast-vergent shearing late in its kinematic history. These authors also recognised that this shearing postdated the main period of exhumation of the Zermatt-Saas zone, because it also postdated northwest-vergent shearing that took place under greenschist-facies conditions both in the Zermatt-Saas and Combin zones. Reddy et al. (1999, 2003) therefore argued that a first phase of southeast-vergent shearing, exhuming the Monte Rosa and Zermatt-Saas units from eclogite-facies to greenschist-facies conditions, should have affected the whole Combin zone prior to northwest-vergent shearing. Structural evidence for this shearing should have been erased later in lower parts of the Combin zone. Structures related to southeast-vergent shearing in higher levels, i.e. in the upper parts of the Tsaté nappe and the base of the Sesia nappe, are attributed to this first stage of exhuming deformation. For several reasons, however, this concept of exhumation of highpressure rocks by pre-d 1 southeast-vergent shearing in the Gressoney shear zone must be called into doubt. The textures of samples from the base of the Sesia zone (B42 and B43, Fig. 9f, g) indicate bulk flattening strain and only a small rotational component. These results are in agreement with strain analyses carried out by Ring (1995) along the boundary between the Dent Blanche nappe and the Combin zone, also indicating flattening strain and only a small rotational component both for northwest- and southeast-vergent shearing. Considering these findings, the actual thickness of the Gressoney shear zone on the order of 1 3 km is at odds with top-to-the-southeast displacement on the order of 100 km as postulated by Wheeler et al. (2001). Furthermore, to the north the Gressoney shear zone must continue on either the upper or lower limb of the Mischabel fold. In the first case, a significant metamorphic gap should be expected between the Briançonnais units and units within the Gressoney shear zone, which is not present. In the second case, the possible displacement would be restricted to the distance between the Mischabel fold hinge and that of the synform below, i.e. about 10 km. In this area, southeast-vergent shearing in the Combin and Zermatt-Saas zones

14 242 Int J Earth Sci (Geol Rundsch) (2007) 96: between 42 and 37 Ma already occurred under greenschist-facies conditions (Cartwright and Barnicoat 2002), leaving little freedom to speculate that part of the structurally preserved southeast-vergent shearing event exhumed the Zermatt-Saas zone and the Monte Rosa nappe. And finally, all the evidence from the Combin zone and lower part of the Sesia-Dent Blanche nappe system indicates that northwest-vergent shearing predates southeast-vergent shearing (see also Mazurek 1986; Sartori 1987; Steck 1989, 1990; Ring 1995). This is confirmed by our study. Southeastvergent shearing below the Sesia-Dent Blanche nappe is a late deformation event under greenschist-facies conditions (D 3 ); it was preceded by northwest- (D 1 ) and west- to southwest-vergent shearing (D 2 ). Incremental restoration of earlier nappe stack geometries Some general remarks on the construction of the cross sections Figure 10 shows cross sections giving stepwise retrodeformations of the D 1 to D 3 structures described above as well as post-d 3 deformations. The maximum relative displacements between the units caused by shearing could not be depicted since shearing of all deformation phases had transport directions oblique to the north south orientation of the cross sections. As a result, and because no information exists about exactly how large these displacements were, strict area balancing is neither possible nor reasonable. An important constraint for constructing the cross sections was that all deformations treated here took place under greenschist-facies conditions (at most, transitional to amphibolite-facies in lower parts of the nappe stack) so that the units must not be moved to depths corresponding to eclogite-facies by retrodeformation. As a result, the retrodeformations are quite conservative with respect to the effects of D 1 to D 3 shearing. Restoration of D 2, D 3, and later deformation Figure 10d largely shows the present-day geometry of the nappe stack, except that the effects of top-to-thesouthwest directed ductile and brittle movements along the Simplon fault zone are eliminated, which took place from ca. 19 to 3 Ma (Mancktelow 1992; Grasemann and Mancktelow 1993). Ductile movements were almost entirely below the brittle Simplon line and accommodated the largest part of a total vertical exhumation of about 15 km for units of the Lepontine dome. More than half of the displacement is supposed to have been concentrated in the uppermost part of the shear zone (Grasemann and Mancktelow 1993). While the hanging wall was hardly affected by these deformations, they cut out the units of Valaisan origin between the outcrop areas of the Antrona unit and the Sion-Courmayeur zone (Fig. 2). In consequence, these units appear connected in our reconstruction. Additionally, the Berisal synform and Glishorn antiform have been unfolded in this first step since they formed contemporaneously with ductile Simplon faulting (e.g. Steck 1990; Mancktelow 1992). The second step (Fig. 10c) effectuates unfolding of the Vanzone antiform (Fig. 2). Minimum absolute ages for the formation of the Vanzone antiform are given by 40 Ar/ 39 Ar dating of muscovite from hydrothermal goldbearing veins that cut across this structure, ranging between 24.5 and 32.3 Ma (Pettke et al. 1999). The restored cross section takes into account that formation of the Vanzone antiform was mainly due to steepening and thinning of the units now located in the fold s southern limb, i.e. the Southern Steep Belt of Milnes (1974). In contrast, the northern limb was rotated without much change of the internal geometry. This mega-scale folding was probably kinematically linked with shearing along the Periadriatic fault. With this second step of retrodeformation, we arrive at the post- D 3 situation. D 3 was the last deformation phase in which shearing affected almost the whole nappe stack, although with spatially variable intensity. As described above, shearing was particularly strong at the base of the Combin zone but can also be traced in all other units, at least above the Antrona synform. The most conspicuous D 3 structures are the large backfolds affecting the internal parts of the Briançonnais-derived units, namely the Mischabel antiform, Mittaghorn synform, and Balmahorn antiform (Fig. 2). Müller (1983) has shown that the Mischabel antiform in its inverted limb accommodates backward shear which further north becomes progressively localised in a discrete shear zone subparallel to the nappe boundaries. This observation led to the concept that D 3 backfolds were generally formed by relative movements of comparatively slightly deformed competent blocks along shear zones situated in the inverted limbs of the folds. This kinematic model is indeed supported by the fact that the axes of those folds mentioned above are at high angles to the mean top-to-the-southeast shear direction. The retrodeformation (Fig. 10b) has been done accordingly. Passing from the Briançonnais units into the Combin zone, the broad hinge of the Mischabel antiform splits into a series of small parasitic folds (Sartori

15 Int J Earth Sci (Geol Rundsch) (2007) 96: S -10km N -20km d -30km -40km 0-10km post-d 3-20km -30km c -10km -40km -20km post-d 2-30km 0-40km b -10km -20km post-d 1-30km -40km a Fig. 10 Stepwise retrodeformation of folding and shearing events in the western Penninic Alps. Cones illustrate overall shear senses during D 1, D 2 and D 3, whereas the cross sections reconstruct the respective situations after D 1, D 2, D 3 and post-d 3 faulting and folding. Traces of major fold axial planes are marked according to the relative ages of the folds by black (D 1 ), dark grey (D 2 ), light grey (D 3 ) and white (post-d 3 ) triangles. For legend, see Fig. 11. For names of the folds, see Fig. 2. a Post-D 1 situation: the present-day stacking order in upper parts of the nappe stack is already accomplished while south-over-north thrusting is still going on within the European basement. We assume that the later Monte Rosa nappe has begun to separate from the rest of the European basement by out-of-sequence thrusting onto the later Antrona ophiolites during D 1 (see Fig. 11b). b Post-D 2 situation: the Penninic nappe stack was affected by intense folding during D 2, generating (amongst others) the isoclinal Antrona, Biel-Balmen and Trift folds. Additionally, we assume a north-facing counterpart of the Biel- Balmen fold roughly in the position of the later D 3 Mittaghorn synform. D 2 folding was accompanied by heterogeneous west- to southwest-vergent shearing which was particularly strong along D 2 fold axial planes. c Post-D 3 situation: in southern parts of the cross section, D 3 shearing was particularly strong in the lowest parts of the Combin zone (i.e. within the structural level of the Cimes Blanches nappe). Within the St Bernard nappe system farther north, D 3 folding was accommodated by southeastvergent shearing in the lower limbs of the Balmahorn and Mischabel antiforms. d Situation after the formation of the Vanzone antiform with the Penninic nappes updomed north of the Periadriatic fault. Shearing along the Periadriatic fault led to strong thinning of the nappes in the southern limb of the Vanzone antiform