Folded continental and oceanic nappes on the ... - Wiley Online Library

TECTONICS, VOL. 24, TC4013, doi:10.1029/2004TC001737, 2005

Folded continental and oceanic nappes on the southern side of Monte Rosa (western Alps, Italy): Anatomy of a double collision suture Jan Pleuger and Nikolaus Froitzheim Geologisches Institut, Rheinische Friedrich-Wilhelms-Universita¨t, Bonn, Germany

Ekkehard Jansen Mineralogisch-Petrologisches Institut, Rheinische Friedrich-Wilhelms-Universita¨t, Bonn, Germany

Received 9 September 2004; revised 6 May 2005; accepted 17 May 2005; published 17 August 2005.

[1] Previous work suggested a double collision suture, including ophiolites from two oceanic basins (Valais and Piemont-Liguria), on the southern side of Monte Rosa (Penninic Alps, northern Italy). This area was studied using field mapping, microstructural analysis, and neutron texture goniometry. After its formation and eclogite-facies metamorphism of continental and oceanic units, the suture was deformed by four successive folding and shearing events under greenschist-facies conditions, all of them taking place between 40 and 28 Ma. After fold retrodeformation, the following tectonostratigraphy results, from base to top: Monte Rosa gneiss (European margin), Balma serpentinite/eclogite unit (Cretaceous crust of Valais ocean), Stolemberg gneiss (Iberia-Briançonnais continent), Zermatt-Saas and Tsate´ ophiolites (Jurassic crust of Piemont-Ligurian ocean), Sesia nappe (continental fragment off the Adria margin). The preservation of this lithological sequence suggests that deep-seated deformation during multiple continent collision produces heterogeneous strain and extreme thinning of nappes but their original stacking order can still be reconstructed using kinematic analysis and overprinting criteria. This is due to the ductile nature of the collisional deformation which retains the continuity of tectonic contacts. Citation: Pleuger, J., N. Froitzheim, and E. Jansen (2005), Folded continental and oceanic nappes on the southern side of Monte Rosa (western Alps, Italy): Anatomy of a double collision suture, Tectonics, 24, TC4013, doi:10.1029/2004TC001737.

1. Introduction [2] Collisional orogens contain ophiolite units derived from former oceanic basins. These ophiolites occur in collision sutures between continent-derived rock units. The ophiolite units in the sutures typically show a develCopyright 2005 by the American Geophysical Union. 0278-7407/05/2004TC001737

opment from sediment-dominated, thick, imbricated, and unmetamorphic in the external part of the orogen, to basement-dominated, thin, folded, and metamorphic in the internal part. In the Alps (Figure 1) where at least two oceanic basins existed in the Mesozoic (Piemont-Ligurian and Valais [Frisch, 1979; Stampfli, 1993]), a double suture zone is expected to occur. [3] Although the geometry of the western Swiss-Italian Penninic nappes (Figures 2 and 3) has been studied for more than hundred years [e.g., Gerlach, 1869; Argand, 1911] and is thoroughly illustrated in maps and cross sections [e.g., Steck et al., 1999], several different paleogeographic restorations of the involved units have been proposed. In particular, the former position of the continental Monte Rosa nappe is a matter of discussion. It is one of the Penninic internal ‘‘massifs’’ (the others are the Gran Paradiso and Dora Maira nappes, Figure 1) and experienced Alpine high-pressure metamorphism of about 12 to 16 kbar [Chopin and Monie´, 1984; Borghi et al., 1996; Engi et al., 2001a]. The question of its origin is inseparably connected to its kinematic history including exhumational stages. [4] It is widely accepted that the Penninic zone comprised, from north to south, the southern European continental margin, the Valaisan or North Penninic basin, the Briançonnais continental fragment, the Piemont-Ligurian or South Penninic basin, and the northern margin of the Adria microcontinent (Figure 4). In addition, a continental fragment (Margna-Sesia fragment [Froitzheim and Manatschal, 1996]) was probably located within the Piemont-Ligurian basin, represented by the Sesia and Dent-Blanche nappes in the western Swiss-Italian Alps. The Piemont-Ligurian ocean opened during the Middle Jurassic [Tru¨mpy, 1975], and the Valais ocean opened later, in the Early Cretaceous, related to sinistral transtension between the Iberian microplate and Europe [Frisch, 1979; Stampfli, 1993]. Recent geochronological work [Liati et al., 2003a, 2005] showed that both Jurassic (circa 160 to 150 Ma) and Cretaceous (circa 93 Ma) oceanic crust existed in the Valais basin. This may be explained by the obliquity between the Piemont-Ligurian and Valais spreading axes (Figure 4b). Assuming that the original geometry of the nappe pile resulted from southeastover-northwest thrusting, paleogeographic restorations depend on the original stacking order of continental and oceanic nappes which, however, was strongly modified by

TC4013

1 of 22

TC4013

PLEUGER ET AL.: ALPINE DOUBLE COLLISION SUTURE

Figure 1. Tectonic sketch map of the Alps, showing the position of the Monte Rosa nappe (MR) and other continental high-pressure nappes of the Penninic zone: Adula (A), Gran Paradiso (GP), Dora-Maira (DM). DB, Dent Blanche nappe; M, Margna nappe; S, Sesia nappe. Rectangle indicates map area of Figure 3. deformations that followed the nappe emplacement. In addition, constraints on major continent displacements derived from plate tectonic models have to be taken into account. The provenance of the Monte Rosa nappe has been located at the northern margin of the Adria microcontinent

TC4013

[e.g., Stampfli et al., 1998], or in the Briançonnais [e.g., Escher et al., 1997; Stampfli et al., 2002], whereas Platt [1986] supposed it was a continental fragment within the South Penninic ocean, and Froitzheim [2001] suggested it was derived from the European margin north of the Valais basin. Figure 5 shows three different interpretations of the nappe geometry. In the first one after Escher et al. [1993, 1997], the Antrona zone represents Piemont-Ligurian ophiolites folded into the underlying Briançonnais basement as a complexly shaped synform (Figure 5a). Around the hinges of this synform, the Monte Rosa nappe is connected with the Bernhard nappe, and the two represent southern and northern portions, respectively, of the Briançonnais. The Antrona zone is connected with the Zermatt-Saas zone through the ‘‘bottle neck’’ of the synform, the Furgg zone. Since the Antrona zone is not Valaisan but PiemontLigurian in this interpretation, the suture of the Valaisan must be in a deeper position, as shown by the dashed line (VO). [5] In the second interpretation after Keller and Schmid [2001], the Antrona zone is the southern part of the Valaisan suture. The connection between Antrona and the SionCourmayeur zone, the undisputed Valaisan, is interrupted by the Simplon fault. The Monte Rosa and Bernhard units are parts of one large Briançonnais nappe. The separation of Monte Rosa and Bernhard nappes by the Furgg zone is only apparent, and ophiolite layers in the Furgg zone are partly Valaisan, folded in from below, and partly PiemontLigurian, folded in from above, leading to the peculiar geometry of two folds with opposite facing directly confronted within the Furgg zone. The layer of Antrona ophiolites underlying the Monte Rosa nappe is not the core of a synform, as in the two other interpretations, but is the

Figure 2. Cross section through the western Swiss-Italian Alps, modified after Escher et al. [1993]. The units are combined according to their paleogeographic origins proposed in this paper. Thin black lines within tectonic units are second-order nappe boundaries and lithological boundaries which are shown in order to highlight the internal structures. Dashed rectangle indicates approximate location of the study area. 2 of 22

TC4013


TC4013

Figure 3. Simplified tectonic map of the Monte Rosa nappe and adjacent units, modified after Steck et al. [1999]. Patterns are as in Figure 2. The large and small rectangles correspond to the mapped areas of Figures 6 and 7, respectively. The dashed line marks the trace of the cross section shown in Figure 2. 2DK, seconda zona dioritico-kinzigitica (second diorite-kinzigite zone).

suture of the Valais ocean between Briançonnais units (Monte Rosa and Bernhard) to the south and above, and European units to the north and below (Figure 5b). [6] According to the third interpretation after Froitzheim [2001], which modifies an earlier reconstruction of Milnes et al. [1981], the Antrona zone is again the southern continuation of the Valais suture, but it is not rooted below the Monte Rosa nappe but above and to the south of that nappe (Figure 5c). The Antrona ophiolites underlying the Monte Rosa nappe form the core of a syncline around the hinge of which the Monte Rosa nappe is connected with the European basement. The Monte Rosa nappe lies structurally below the remnants of the Valaisan ocean that are sought for in the Antrona ophiolite zone and, as its rootward continuation, in the Furgg zone which separates the Monte Rosa nappe from the certainly Briançonnaisderived Bernhard nappe system. The latter wedges out toward south. The ophiolites on the south side of Monte Rosa represent a double suture, of both the Piemont-Ligurian and the Valaisan ocean. [7] We studied this hypothetical double suture zone on the southern side of Monte Rosa in order to gain more insight into the geometry and kinematics of continental collision in the Alps. We found that after retrodeformation of three generations of overprinting folds, the structure of the suture zone is not chaotic but remarkably ordered, with

two separate, although in part very thin ophiolite layers separated by a thin, occasionally interrupted band of continental gneiss. This has important implications for the deformation processes taking place at a deep level during continental collision.

2. Regional Geologic Setting and Lithological Description of the Mapped Units [8] The study area is situated south of the Monte Rosa mountain in the upper parts of the Sesia and Gressoney valleys (Valsesia and Val de Gressoney, Figure 6). The Monte Rosa nappe occupies the lowermost structural position. It is overlain by originally eclogite-facies ophiolites which are largely retrogressed to amphibolite and greenschist facies. According to most authors all these ophiolites belong to the Piemont-Ligurian Zermatt-Saas zone. However, from the western slopes of the Gressoney valley to the eastern parts of the Sesia valley at Colle Mud (Figure 6), a sporadically disappearing band of gneissic rocks divides the ophiolitic rocks into a thin, serpentinite-dominated lower portion and an amphibolite-dominated higher portion (Zermatt-Saas unit). In previous studies and maps, this gneiss unit has either been overlooked completely or interpreted as irregularly intercalated slivers [e.g., Gosso

3 of 22

TC4013


TC4013

nappe which also originates from the Piemont-Ligurian ocean and consists of calcschists and ophiolites (Figure 3). On top, the Penninic nappe stack is delimited by the continental basement rocks of the Sesia zone. [10] On a larger scale, the succession described above appears from north to south since our study area lies on the southern side of the Monte Rosa structural dome where all the aforesaid units dip moderately toward south. Within the upper Sesia valley, however, the rocks are affected by a large south vergent fold (called Cimalegna fold herein; see Figure 7) which locally complicates the otherwise simple geometry and emplaces the Monte Rosa gneisses on top of the ophiolites. The ophiolitic rocks in the lower limb of the Cimalegna fold themselves form two large fold structures (called upper and lower Malfatta fold in the following) whose geometry precludes that they are parasitic structures of the Cimalegna fold. In order to unravel the structure of this area and its deformation history, two smaller valleys at the western side of the Sesia valley, the Valle d’Olen and the Vallone delle Pisse have been mapped in detail (at the scale of 1:10000; Figure 7) up to the ridge dividing the Sesia from the Gressoney valley, and a detailed structural analysis was performed. 2.1. Monte Rosa Nappe

Figure 4. Sketch maps showing the paleogeographic evolution of the Alps. (a) Geometry of the PiemontLigurian ocean including the Margna-Sesia continental fragment in late Jurassic time. (b) Situation after oblique opening of Cretaceous ocean by eastward translation of the Iberia-Briançonnais continent. Because of rerifting of Piemont-Ligurian oceanic crust, the Valais basin north of the Briançonnais peninsula contains both Jurassic and Cretaceous ocean floor. Subduction of the Piemont-Ligurian lithosphere begins in front of the Austroalpine thrust wedge. Light grey indicates Jurassic ocean floor, dark grey indicates Cretaceous ocean floor, and white indicates continental crust.

et al., 1979]. We will show in the following that the gneiss occupies everywhere the same structural level and may therefore represent the most internal remnant of the Briançonnais Bernhard nappe. [9] In the following, we will refer to the gneisses as Stolemberg unit and to the underlying ophiolites as the Balma unit. As a consequence, the term ‘‘Zermatt-Saas zone’’ will be used to denote the South Penninic ophiolite zone above the Stolemberg unit only, although this is not quite true to its definition given by Bearth [1967]. The Zermatt-Saas zone is in turn overlain by the Cimes Blanches nappe. This is a layer of Mesozoic cover sheared off from an unknown continental basement. Along the southern side of the Monte Rosa nappe, it wedges out toward the east between the Zermatt-Saas zone and the overlying Tsate´

[11] The Monte Rosa nappe is built up mainly of preAlpine paragneisses into which large granitoid bodies have intruded. The age of a granitoid from Val d’Ayas was determined as 270 ± 4 Ma and 268 ± 4 Ma (U-Pb sensitive high-resolution ion microprobe (SHRIMP) dating on zircon and monazite by Lange et al. [2000]). Within this rock ensemble, Alpine deformation is heterogeneously distributed. Some Alpine low-strain domains exhibit the primary contacts between paragneisses and the granitoids. Paragneisses with sillimanite- and cordierite-bearing assemblages are remnants of a pre-Alpine temperaturedominated metamorphism. Kyanite, testifying the Alpine pressure-dominated imprint, grew locally at the expense of sillimanite or cordierite [Bearth, 1952; Dal Piaz, 1971]. Alpine high-pressure metamorphism is additionally proven by the local occurrence of eclogite boudins [Dal Piaz and Lombardo, 1986] which, owing to their relatively high competence, may even be preserved in Alpine high-strain areas where the surrounding rocks suffered greenschistfacies retrogression. [12] Within our study area, the Monte Rosa nappe mainly consists of paragneisses and micaschists of variable composition but almost entirely affected by Alpine deformation and late greenschist-facies metamorphism. In the upper limb of the Cimalegna fold (Figures 8 and 9) they are interspersed with numerous lenses of eclogite or retrograde amphibolite. The protoliths of the metabasic rocks are presumably pre-Mesozoic (i.e., neither of North nor of South Penninic origin) because if they were incorporated by imbrication from the ophiolite units, one would expect to find the other lithologies of the Balma and Zermatt-Saas units (serpentinite, calcschist, and metagabbro) as well. Moreover, the Monte Rosa eclogites and amphibolites in the study area were correlated by Dal Piaz [1966] with

4 of 22

TC4013


Figure 5. Schematic cross sections illustrating different paleogeographic restorations of the Penninic nappes. Patterns are as in Figure 2, with the only difference that the Zermatt-Saas and Combin zones are merged (light grey). VO, suture of the Valais (North Penninic) ocean. (a) Model after Escher et al. [1993]. All the ophiolites above the Monte Rosa nappe are of South Penninic origin. The Antrona zone is a complex synform branching off from the Zermatt-Saas zone through the Furgg zone. (b) Model after Keller and Schmid [2001]. The Antrona unit is part of the Valais ocean and lying below the Monte Rosa nappe. Monte Rosa and Bernhard nappes are not separated by the Furgg zone. (c) Interpretation of Froitzheim [2001]. The North Penninic ocean is represented by the Antrona zone but rooted south of the Monte Rosa nappe. The Antrona zone is again interpreted as a complex synform.

5 of 22

TC4013

Figure 6. Geologic map of the upper Sesia and Gressoney valleys, showing all occurrences of Stolemberg and Balma unit rocks at the southern side of the Monte Rosa nappe. The internal arrangement of the Tsate´ nappe in upper Valsesia is modified after Mattirolo et al. [1912, 1927]. Kilometric coordinates in the frame refer to the Italian Gauss-Boaga grid.

TC4013 PLEUGER ET AL.: ALPINE DOUBLE COLLISION SUTURE

6 of 22

TC4013

TC4013


TC4013

Figure 7. Geologic map of the Vallone delle Pisse (to the north) and Valle d’Olen (to the south) on the western side of the upper Sesia valley. The patterns are the same as in Figure 6. The western margin of the mapped area is the ridge between Gressoney valley and Sesia valley with the summits of Stolemberg and Corno del Camoscio. From the latter toward east extends the Cimalegna where amphibolitic and eclogitic lenses within Monte Rosa paragneiss have been mapped to better visualize the structures. The Corno d’Olen is a paragneiss klippe separated from the Cimalegna by a small topographic depression. The dashed lines are the axial traces of the southward closing D3 Cimalegna fold (CF) and its underlying, northward closing counterpart (Molera fold, MoF). The latter bends two older folds (D2), one with eclogite in its core (upper Malfatta fold, UMF) in the southern slopes of the mountain Malfatta and one with serpentinite further east (lower Malfatta fold, LMF), into the lower limb of the Cimalegna fold and thus strongly reorients their fold axes and axial planes. Stars mark sample locations for neutron texture studies. The grey lines within the frame give the traces of the cross sections in Figure 8. Kilometric coordinates in the frame refer to the Italian Gauss-Boaga grid.

Figure 8. North-south cross sections (traces, see Figure 7). The patterns are the same as in Figure 6. (a) Cross section through Corno del Camoscio in the normal-lying upper limb of the Cimalegna fold. The enlarged section shows the complex geology of Corno del Camoscio. (b) Cross section through central parts of the Cimalegna comprising the Cimalegna fold and the upper Malfatta fold with Balma eclogites of maximal thickness in its core. (c) Cross section through eastern parts of the Cimalegna. Below the Cimalegna fold, ophiolites of the Zermatt-Saas and Balma unit are separated by an incoherent band of Stolemberg gneisses. (d) Cross section west of Corno d’Olen. Compared to Figure 8c, the ophiolitic units are less thick, especially the eclogites of the Balma unit wedge out toward east. 7 of 22

TC4013


Figure 8 8 of 22

TC4013

TC4013


Figure 9 9 of 22

TC4013

TC4013


Figure 10. Stolemberg seen from the west. The gneissic rock slice of the Stolemberg unit is separated from the Monte Rosa paragneisses by serpentinite and some amphibolite of the Balma unit. The Zermatt-Saas amphibolites rest on top of the mountain. The structural position is in the normal-lying upper limb of the Cimalegna fold.

those in the Furgg zone at the northern border of the Monte Rosa nappe which have yielded protolith ages of 510 ± 5 Ma [Liati et al., 2001]. Orthogneisses are restricted to the eastern and northern parts of the upper Sesia valley. 2.2. Balma Unit [13] We define the Balma unit (named after the locality ‘‘Alpe Balma’’; Figure 7) in a structural sense as the ophiolitic rocks that overly the Monte Rosa nappe and that are separated from the Zermatt-Saas zone by the structural level in which the gneiss slivers of the Stolemberg unit exclusively occur. Lithologically, all Balma unit rock types are as well displayed by the Zermatt-Saas zone but in different proportions. In the Gressoney valley the Balma unit reaches maximum thicknesses of some dekameters and is built up of serpentinites with intercalated boudins of finegrained eclogite. In the Sesia valley eclogites and serpentinites replace each other laterally. In the area of Stolemberg and Corno del Camoscio (Figure 7), the Balma unit almost exclusively consists of sporadically disappearing serpentinite. This may also apply to the lower limb of the Cimalegna fold but outcrop conditions provide little insight into the

TC4013

structure of this area. The cores of the upper and lower Malfatta folds are occupied by large bodies of eclogite and serpentinite, respectively (Figures 7, 8c, 8d). The latter is again replaced by eclogite in the eastern slopes of the Sesia valley up to Colle Mud (Figure 6). All the eclogites show amphibolite-facies overprint to variable extent and are often completely altered to garnet-(clino)zoisitebearing amphibolite. A sample of the amphibolitized eclogite of the upper Malfatta fold yielded a protolith crystallization age of 93.4 ± 1.7 Ma [Liati et al., 2002] which gives further support to a Valaisan origin of the Balma unit (see discussion below). [14] Garnet-free amphibolites are rare. Metagabbros only occur in small portions in the area of Corno del Camoscio and within the eclogites of the upper Malfatta fold. Only one small layer of calcschist was found 200 m north of Alpe la Balma. Metagabbros and calcschists will be briefly described together with the corresponding Zermatt-Saas rocks. 2.3. Stolemberg Unit [15] The Stolemberg unit (named after the mountain ‘‘Stolemberg’’; Figure 10) is introduced here to collectively denominate the gneiss slivers appearing within the ophiolites. These gneisses are about 50 m thick at Stolemberg; similar thicknesses are reached at Alpe Salza in the Gressoney valley and at Alpe Mud di mezzo in the Sesia valley (Figure 6). As in case of the Balma unit, the Stolemberg rocks are not coherent but always occur in the same structural niveau. They are at least locally absent in the front and the lower limb of the Cimalegna fold. Partial absence and otherwise highly variable thickness can be observed north of Corno d’Olen (Figure 7). In this area the spatial distribution of Stolemberg unit rocks is very complex as a result of polyphase folding. [16] The Stolemberg rocks are exclusively paragneisses without any characteristic lithological differences to those of the Monte Rosa nappe. They vary from relatively massive, quartz-feldspar-rich to strongly microfolded, garnet- and mica-rich gneisses. Although remnants of higher-grade mineral assemblages may locally be preserved, all the gneissic rocks of the Stolemberg unit show a strong greenschistfacies imprint related to the pervasive deformations described below. [17] Only few small, mostly amphibolitic mafic bodies are contained in the Stolemberg unit. Eclogite boudins are found in the vicinity of Alpe Salza.

Figure 9. Tectonic interpretation of the north-south cross sections as shown in Figure 8. The patterns are the same as in Figure 2. (a) Tight south vergent folds which are partly D2, partly D3. They are overprinted by D4 folds; for example, the Corno del Camoscio is positioned in the steep southern limb of a large D4 anticline. The Balma and Stolemberg units disappear only sporadically. The boundary between the Tsate´ nappe and the Zermatt-Saas zone is poorly exposed but cuts the layering of the Zermatt-Saas rocks. (b) Frontal parts of the Cimalegna fold (CF). Rocks of the Balma and Stolemberg units are restricted to the cores of some tight to isoclinal D2 and/or D3 folds. (c) Malfatta folds (UMF, LMF) clearly overprinted by D3 folding (Cimalegna fold). In the upper limb of the Cimalegna fold, the Tsate´ nappe was not involved into D2 and D3 folding but emplaced upon the Monte Rosa gneisses during D4 southeast directed detachment. (d) D2 fold axial planes that dip slightly steeper toward south than parasitic structures of the large D3 north closing fold. 10 of 22

TC4013


2.4. Zermatt-Saas Zone [18] The Zermatt-Saas zone is rooted in the southern steep belt immediately southeast of (i.e., structurally above) the Monte Rosa nappe. Disregarding the local occurrences of the Balma and Stolemberg units, the Monte Rosa nappe is surrounded along its southern and western margins by the Zermatt-Saas zone which lies in the northwest below the Mischabel antiformal backfold and above the Portjengrat unit of the Briançonnais-derived Bernhard nappe system (Figure 3). [19] Ultrabasic and basic members of an ophiolite suite are by far the predominant rocks of the Zermatt-Saas zone. The serpentinites contain locally olivine or titanclinohumite but are in most cases almost monomineralic. Generally, eclogites are quite abundant; P-T estimates carried out on these rocks gave 18 – 20 kbar and 550 – 600°C [e.g., Barnicoat and Fry, 1986] and, in the single case of a coesite-bearing metachert at Lago di Cignana, west of our study area, 26– 28 kbar and 600°C [Reinecke, 1991]. In our study area, eclogites are mostly retrogressed and therefore subordinate to amphibolites. [20] Within the metagabbro, aggregates of green hornblende have replaced the primary augite and are deformed to elliptic or sometimes schlierenlike shapes. They are surrounded by a whitish matrix which mainly consists of clinozoisite, albite, chlorite, and in many cases fuchsite. Amphibolites are partly retrogressed eclogites with similar mineral assemblages as the metagabbro, but a different structure, i.e., greater homogeneity in mineral distribution and smaller grain size in case of the amphibolite, and partly result of intense deformation of metagabbros which led to homogenization and a reduction of grain size. The gradual transformation of metagabbro into amphibolite can be followed in the southern slope of Corno d’Olen. Because of these transitions between amphibolites and metagabbros the classification of these rocks during map work may sometimes be arbitrary. Both rock types, metagabbro and amphibolites, commonly reveal greenschist-facies conditions during the latest deformational stages which are conspicuously displayed by abundant porphyroblastic growth of albite in many amphibolites. The greenschistfacies imprint makes it in most cases impossible to decide whether the amphibolite had been eclogite or metagabbro before. [21] Calcschists, usually rich in quartz, clinozoisite/ epidote and chlorite or biotite, are infrequent but an almost reliable distinguishing feature between the Zermatt-Saas zone and the Balma unit because the latter is practically devoid of them. Two more peculiar units occur in the area of Corno del Camoscio. First, a greenschist breccia with mostly centimeter- to decimetersized components of amphibolite and metagabbro similar to those mentioned above and, additionally, granitoids and actinolite aggregates incorporated in a quantitatively dominating prasinitic matrix. Second, a me´lange of amphibolite, calcschist, the above described greenschist breccia, and minor amounts of metagabbro and ophicarbonate. Included in this me´lange is paragneiss consisting mainly of quartz, chlorite, plagioclase, and epidote that we

TC4013

interpret to be metagraywacke. This me´lange is possibly a southern equivalent of the ‘‘Rifelbergzone’’ of Bearth [1967]. [22] A synopsis and a discussion of radiometric protolith ages obtained for the ophiolites of the Pennine Alps are given by Liati et al. [2003a]. For the Piemont-Ligurian basin they range between 148 Ma and 166 Ma; for the Zermatt-Saas zone in particular U-Pb SHRIMP ages of 164 ± 2.7 Ma and 163 ± 1.8 Ma have been found by Rubatto et al. [1998]. 2.5. Combin Zone and Sesia Zone [23] The Combin zone comprises several subnappes, two of which occur in our study area. One is the Cimes Blanches nappe that consists of Mesozoic metasediments (mainly quartzite, calcite and dolomite marble, and metaarkose). It is generally overlain by the Tsate´ nappe. The Tsate´ nappe comprises almost exclusively greenschist-facies rocks. Only few indications of elevated pressures have been reported, suggesting lower blueschist-facies metamorphism not in excess of 9 kbar [e.g., Balle`vre and Merle, 1993; Reddy et al., 1999]. However, recent findings of the relic paragenesis garnet, Mg-rich chloritoid, and phengite indicate higher pressures in the range of the blueschist-eclogite transitional facies [Bousquet et al., 2004]. The pervasive greenschistfacies reequilibration and the fact that the Tsate´ nappe continues to the north along the upper limb of the Mischabel backfold above the Bernhard nappe and that toward the east it cannot be traced far into the southern steep belt (Figure 2), are remarkable differences to the Zermatt-Saas zone. [24] Within the study area, the Tsate´ nappe is mainly represented by an alternation of meter- to dekameter-thick greenschist and calcschist layers. The greenschists are prasinites rich in albite, epidote, chlorite, and locally bluegreen hornblende and biotite. Calcschists are often rather impure marbles but in some cases contain less calcite than quartz and white mica. Although the calcite content is variable, it is on average higher than in calcschists of the Zermatt-Saas zone. Embedded in this greenschist-calcschist alternation is a large serpentinite body extending southwestward from Corno Rosso down to the Gressoney valley. [25] Gneisses and metapelites of the Sesia zone and equivalent Dent Blanche nappe cover the Pennine nappe stack. They contain remnants of Latest Cretaceous to Early Paleocene eclogite-facies metamorphism [Rubatto et al., 1999] and, being coupled with the Tsate´ nappe during exhumation [Dal Piaz et al., 2001], experienced mainly greenschist-facies Tertiary metamorphism as well. In the study area, the Sesia zone appears as a klippe at the summit of Punta Straling and in close vicinity to Monte Rosa nappe rocks between Alagna and Colle Mud (Figure 6) where the Zermatt-Saas unit thins out. Our tentative paleogeographic reconstruction (Figure 4) is based on the assumption that the Tsate´ nappe was formed as an accretionary wedge during subduction of the ocean basin southeast of the Margna-Sesia continental fragment, thrust toward northwest over the latter, and buried under the Dent Blanche and Sesia nappes by a later out-of-sequence thrust. The out-of-sequence character of the basal thrust of the Dent-Blanche nappe is

11 of 22

TC4013


Figure 11. Photographs of x-z thin sections under crossed polarizers. (a) Detail of MR139 from the Stolemberg unit in the eastern slope of Corno del Camoscio (5080470/ 1412510). North (6°) is to the right. The asymmetric garnet porphyroclast indicates dextral top-to-the-north sense of shear (D1). (b) Detail of MR13 from the Monte Rosa nappe northeast of Alpe Mud di mezzo (5080730/1419020). Southeast (129°) is to the left. Asymmetric porphyroclasts of garnet (G) and an aggregate of albite and microcline (F) indicate D3 sinistral top-to-the-southeast sense of shear as well as a shear band (c0) and the shape-preferred orientation of quartz (arranging the grain long axes from upper left to lower right) in the top part of the photograph. obvious from Figure 2 where the basal thrust is unconformable to folded internal thrust planes within the Dent Blanche nappe.

3. Structures [26] The first part of the Alpine deformational history, including subduction and early stages of exhumation, is

TC4013

poorly recorded by structures. Though structures developed under high-pressure conditions such as a foliation and lineations defined by elongate amphibole grains are frequently preserved within eclogites, they can hardly be interpreted in kinematic terms since they are usually strongly reoriented and restricted to the local occurrences of eclogites. [27] Four generations of correlated structures are distinctive throughout the study area. They overprint the eclogitefacies structural relics under greenschist-facies conditions and are thus retrogressive. We are aware that therefore our deformation phases D1, D2, . . . as described below should rather be denoted as ‘‘D1+x, D2+x, ..’’, but for the sake of simplicity we desist from this notation. From thin section observations no characteristic differences in metamorphic grade during these deformation phases can be concluded. Within the boundary zone between Monte Rosa nappe and Zermatt-Saas zone (including the Balma and Stolemberg units), the main mylonitic foliation (S) is composite in that it accommodated all shear movements from D1 to D3. The mylonitic foliation is, at least at outcrop scale, parallel to the lithologic layering. Only in fold hinges can different generations of foliation be distinguished. Folds of D1, D2, and D3 in some cases developed their own schistosities within mica-rich rock types of low competence. Stretching lineations are generally defined by elongate mineral grains or grain aggregates. Apart from the results of neutron texture goniometry described below, shear senses have been determined by asymmetric porphyroclasts, shear bands and in thin section additionally by oblique foliations. [ 28 ] The first observed deformational stage D 1 is characterized by top-to-the-north to -northwest shearing (Figure 11a). D1 structures can be recognized with certainty in the northwest of the study area north of Stolemberg where later imprints are quite weak. The stretching lineation L1 trends (sub)parallel to the axes of tight to isoclinal folds. Compared to folds of later generations, D1 folds are relatively small-scale structures with wavelengths in the dimension of some decimeters. As already advised above, the rocks are in large areas strongly affected by later folding. Especially in the area of the Cimalegna and Malfatta folds (corresponding to the central part of Figure 7), D1 folds are therefore mostly reoriented and can rarely be unambiguously addressed. The same is true for to the D1 stretching lineation which is, moreover, largely erased by later shearing. [29] D2 led to originally southeast vergent folding with northeast-southwest trending fold axes, especially the two north closing, isoclinal Malfatta folds with rocks of the Balma unit in their cores (Figures 9c and 9d), but also abundant parasitic structures. The geometry of these parasitic folds generally depends on the rock type, for example they are isoclinal within the Zermatt-Saas amphibolites of Corno del Camoscio and tight in the proximately underlying paragneisses of the Stolemberg unit. The stretching lineation L2 is (sub)parallel to D2 fold axes (Figure 12). It is less frequent than L1 and L3. In normal lying positions the shear sense is top-to-the-southwest but it may be reoriented into roughly southeast direction where D3 folds led to an

12 of 22

TC4013


TC4013

planes compared to D3 folds. D4 shear sense indicators and a stretching lineation L4 cannot be separated from the respective D3 features. However, continuing southeast vergent shear movements after D3 must be inferred mediately from the fact that the last increments of movement between the Tsate´ and Zermatt-Saas nappes were southeast vergent but postdate the D3 Cimalegna folding in which the Tsate´ nappe is not involved (Figure 9c). Therefore D4 seems to have evolved from D3 and these deformation phases are not separated by a fundamental kinematic change.

4. Neutron Texture Studies of Quartz Figure 12. Lower hemisphere Schmidt projection plots of Mylonites structural elements in the study area. The data have been selected out of much larger numbers of measurements in order to only plot data that can be unambiguously addressed to D2 and D3. For D2, lineations and fold axes are almost parallel and partly reoriented into west-northwest direction by wrapping around D3 folds. Fold axial planes are variable due to later deformations. For D3, the fold axial planes were originally flat lying but are in some cases reoriented by D4. inversion of the facing direction, namely, in the lower limb of the Cimalegna fold. Accordingly, the stretching lineations dip shallowly in southwestern to northwestern directions (Figure 12). In the latter case the shear sense is top-to-the-southeast and L2 cannot be distinguished from L3. This is one reason why L2 appears less frequently than L3, the others are that L2 is in some levels originally missing, particularly in deeper parts of the Monte Rosa nappe, and that in other parts it is extinguished by D3 shearing. [30] Southeast vergent shear movements during D3 (Figure 11b) were accompanied by the formation of the south closing Cimalegna fold whose fold axis runs westnorthwest/east-southeast and its north closing counterpart below (called Molera fold in the following). As with the D2 Malfatta folds, parasitic structures of the Cimalegna and Molera folds are abundant tight to isoclinal folds of variable dimensions, reaching from folds with wavelengths of some dekameters down to crenulations with wavelengths of some millimeters. D2 and D3 parasitic folds in some cases cannot be addressed to one of these phases, especially in the lower limb of the Cimalegna where the D2 fold axes are deflected into southeast-northwest trending directions. In the respective area the only reliable criterion to distinguish between D2 and D3 is that the first-order parasitic folds of D2 and D3 have opposite vergences, i.e., southwest vergent for D2 and northeast vergent for D3. D3 produced the most widespread stretching lineation L3, again (sub)parallel to the fold axes (Figure 12). L3 is penetrative within the Monte Rosa nappe rocks in the core of the Cimalegna fold and in the overlying units. It failed to develop only in very mica-rich rock types. [31] D3 fold axial planes have variable orientations (Figure 12) as they are influenced by the formation of almost coaxial D4 folds. D4 folds have greater, open to close, interlimb angles and more steeply dipping fold axial

[32] Textures of eight almost monomineralic quartz mylonites have been analyzed using the texture diffractometer SV7-b operated at the Forschungszentrum Ju¨lich [Jansen et al., 2000]. Since the reflections of the base pinacoid {c} planes are very weak, the orientation distribution of the c axes was calculated from the orientation distribution function (ODF) obtained from reflections of the first-order prism {m}, the second-order prism {a}, {111} and the intrinsically overlapped positive and negative rhombs {r + z}. All diagrams are upper hemisphere Wulff projections. [33] Northwest vergent D1 shearing is testified for the area of Indren glacier around the ‘‘Rifugio Citta` di Mantova’’ (approximately 2 km northwest of Stolemberg) by samples MR8 and MR11. The orientation of MR8 in geographic coordinates is S 72/31 and L 350/4. Apart from an inferior maximum in the upper left quadrant the c axes distribution forms an incoherent girdle inclined to the left. The absolute maximum of the c axis pole figure (Figure 13a) lies between the center and the periphery in the lower half of the hemisphere, another maximum is on the girdle in the middle of the upper right quadrant. According to the absolute c axis maximum, the {a} pole figure yields three maxima on a great circle around the {c} maximum. The strongest {a} maximum is on the periphery of the diagram. Both the asymmetries of the {a} and the {c} pole figures indicate dextral sense of shear which is geographically north vergent. Sample MR11 (Figure 13b) shows a c axes girdle that does not meet the center of the stereonet but suggests a strain x direction trending about 35° more to the north than the measured orientation of L 325/14. Such a lineation, however, is neither visible in macroscopic nor in microscopic view. Two explanations for the triclinic symmetry seem reasonable: the texture may have been modified during later deformation phases or the instantaneous strain orientation changed to a certain extent during D1 with the shape fabric reacting more sensitively to the change than the texture or vice versa. A case of a texture behaving more inertly than the shape fabric is documented in MR140. The sample was taken from the immediate vicinity of a huge eclogitic lens of the Monte Rosa nappe which crops out over a distance of more than 1 km on Cimalegna and is up to about 30 m thick. The sample location is situated about 100 m above the Cimalegna fold axial plane. The D1 stretching lineation is defined by amphibole needles and

13 of 22

TC4013


Figure 13. Upper hemisphere Wulff projections of quartz textures obtained from neutron diffraction. The {a} and {m} quartz pole figures were observed; the c axis pole figures were calculated from the orientation distribution function derived from the 100, 101/011, 110, and 111 reflections. The sample orientation in geographic coordinates is given below the c axis pole figure for every sample. The azimuths of the x direction (stretching lineation) are given above the diagrams. The trace of the foliation is normal to z in the diagrams. Those of the sample locations that are in the map area of Figure 7 are shown in that map. All samples were taken from the Monte Rosa nappe, except for MR126 (Stolemberg unit). (a) Sample MR8 from the ‘‘Rifugio Citta` di Mantova’’ (5083260/1410800). The microstructure of this sample is shown in Figure 14a. (b) Sample MR11 from southeast of the ‘‘Rifugio Citta` di Mantova’’ (5083095/1410995). (c) Sample MR140 from Cimalegna (5080710/1413780). (d) Sample MR204 from approximately 2 km west of Alpe Salza (Figure 6; 5081700/1406020). (e) Sample MR55 from east of Malfatta, close to the hinge of the large north closing D3 fold (5082090/1414720). (f) Sample MR126 from the hinge zone of the lower Malfatta fold southeast of Alpe la Balma (5081270/1414830). MR126 is the only sample from the Stolemberg unit. (g) Sample MR48 from Cimalegna (5080470/1413250). The microstructure of this sample is shown in Figure 14b. (h) Sample MR146 from Cimalegna (5080650/ 1413440). 14 of 22

TC4013

TC4013


parallel to the axes of decimeter-scale tight D1 folds. {c}, {a}, and {m} tend to form girdle distributions only disconnected close to the center of the diagrams (Figure 13c). The {c} maximum and the {a} maximum are at the periphery of the projection rotated in a clockwise sense from the vertical and the horizontal direction, respectively, and give a top-tothe-northwest shear sense. If under greenschist-facies conditions of D1 basal hai slip (with some additional rhomb hai slip) was active [e.g., Stipp et al., 2002] a single c axis maximum at the periphery should indeed be developed [Schmid and Casey, 1986]. This texture is unaffected by D3 although an additional stretching lineation L3, again defined by elongate amphibole grains, is present. [34] MR 204 stems from the upper Val Gressoney, near the western bounding ridge of this valley (about 2 km west of Alpe Salza superiore, Figure 6). The texture corresponds to an intense D2 stretching lineation which is widely distributed in this area where D2 left a particularly strong imprint. {c} is arranged on a band steeply inclined to the right which is pronounced close to center (Figure 13d). Accordingly, {a} and {m} are preferentially spread over the edges of their diagrams; {a} with an absolute maximum that as well as the inclination of the {c} band evidences top-tothe-southwest shearing. Similarly, top-to-the-southsouthwest shearing is revealed in the texture of MR55 (Figure 13e). The stretching lineation L 205/19 is defined by the elongation of quartz grains which is visible only microscopically in x-y thin section. The c axis pole figure yields a single maximum at the periphery of the diagram and somewhat rotated from the z direction in a counterclockwise sense. Around this maximum, the {a} and {m} maxima are arranged on great circles as single maxima as well. The similarly oriented MR126 (L 186/10) is a sample from the Stolemberg unit in the core of the lower Malfatta fold. Prominent maxima of the c axis pole figure in the upper left and lower right quadrants (Figure 13f) between the center and the edge are interpreted to have formed by rhomb hai slip. The asymmetry of the {c} pole figure as well as the position of the absolute {a} maximum, which is shallowly inclined to the left with respect to x, confirm sinistral top-to-the-south shearing ascribed to D2. [35] Sample MR48 (Figure 13g) yields a single girdle distribution of {c}, indicating top-to-the-south-southeast shear sense, that contains a remarkably strong (16.54 multiples of random distribution) absolute maximum again between center and periphery of the stereonet. In accordance with the elongate shape of the c axis maximum the {a} and {m} distributions form elongate maxima, too, that combine to wavy girdles. A second texture formed during D3 top-to-the-southeast shearing is that of MR146. The texture resembles that of MR48 but the c axis girdle shows a tendency to branch into a cross girdle in the upper half of the diagram, and the {a} and {m} maxima on a great circle around the absolute c axis maximum are more clearly pronounced (Figure 13h). The pole figures’ obliquity again displays top-to-the-southeast (sinistral) shearing along L 121/38. Both in case of MR48 and MR146 it seems unlikely that the respective absolute {c} maxima should have developed by rhomb hai slip because in that case from

TC4013

quartz symmetry considerations one would expect another comparably strong c axis maximum ‘‘mirrored’’ by the trace of the slip plane. The latter is parallel to the {a} maximum at the periphery, which corresponds to the slip direction of the shear zone, and to y in the center of the projection [e.g., Bouchez, 1978; Burg and Laurent, 1978]. A speculative explanation for these single absolute c axis maxima is that they formed before D3, probably during D2, and are inherited elements in the D3 textures. [36] To summarize, the three macroscopically manifested deformation phases D1 to D3 are confirmed by the texture observations. D1-related textures are best preserved in comparably deep levels of the Monte Rosa nappe in northwestern parts of the study area, whereas D3-related textures are prevalent in rocks affected by the Cimalegna fold. The sample coined by D2 from the upper Gressoney valley (MR204) indicates that rhomb hai, basal hai, and especially prism hai slip acted concurrently. In the upper Sesia valley, textures developed by D2 shearing seem restricted to the hinge zones of the D2 Malfatta folds, perhaps because in such positions the orientation of the composite foliation is not favorable to accommodate southeast vergent D3 shearing that overprints D2 elsewhere. Our observed D2 textures from this area are characterized by individual c axis maxima formed by rhomb hai slip in one (MR126) and basal hai slip in the other case (MR55). The D3 c axes reveal girdle distributions formed by combined rhomb hai, basal hai, and prism hai slip [Schmid and Casey, 1986; Law et al., 1990] and D1 textures are intermediate. The observed textures correspond to predominant grain boundary migration recrystallization during D1 and D2 with increasing influence of subgrain rotation and bulging recrystallization toward D3 (Figure 14). Despite the differences, all observed textures and fabrics are well in line with greenschist-facies conditions during deformation. In some cases (MR140, MR48, and possibly MR11) the grain shape fabric adjusted faster to changing strain orientations than the texture.

5. Summary of the Structural Evolution [37] Looking at the overall nappe geometry, D1 folding with northwest-southeast trending fold axes can be neglected since D1 folds are mostly smaller than map scale. On the other hand, both field observations and neutron textures show that D1 shearing was probably penetrative and accommodated significant amounts of relative movements [see also Steck, 1990], although D1 postdates the eclogite-facies high-pressure metamorphism and occurred under greenschist-facies conditions. This implies that large parts of the kinematic history including substantial exhumation happened prior to the formation of the D1 structures. [38] Top-to-the-southwest shearing (D2) is not ubiquitous but also reported from the contact between the Bernhard nappe and the Combin zone in southern Turtmanntal [Sartori, 1987] and from northeastern parts of the Monte Rosa nappe and the neighboring Antrona and Bernhard nappes in the Simplon area [Steck, 1984, 1990], and D2 is the dominant deformation phase farther west in Val d’Ayas.

15 of 22

TC4013


TC4013

the axes of D2 and D3 folds are oblique to each other, the upper Malfatta fold is only refolded into the inverted limb of the Cimalegna fold in the Sesia valley but not in Gressoney valley. In the rock face east of the peak Malfatta, the axial plane of the lower Malfatta fold is almost vertical, corresponding to a position in the hinge zone of the Molera fold (Figure 16). Further to the east, the base of the Tsate´ nappe cuts progressively down into the Cimalegna fold: A portion of paragneiss south of Alpe Grande Halte is interpreted as Monte Rosa nappe rocks in the core of the Cimalegna fold or one of its parasitic structures in the inverted limb. In an overturned sequence the gneisses rest upon Zermatt-Saas amphibolites but the normal limb of the D3 structure is cut off by the contact to the Tsate´ nappe. At the eastern margin of our study area near Colle Mud the Cimalegna fold is absent; it was in our interpretation entirely cut away along the base of the Tsate´ nappe during D4 shearing. [39] With respect to the Stolemberg unit, the complex overprinting of D2 and D3 is reflected in the highly irregular shape of its outcrop area (Figure 7) and the occurrence of this unit in different positions. Shearing is held responsible for the omission of the Stolemberg and Balma units in the hinge zone and lower limb of the Cimalegna fold and the otherwise variable thickness of these units which, in the case of the Balma unit, is greatest in the two large D2 antiforms in the Malfatta area. In the hinge zones of these folds, especially the lower one, the Stolemberg gneiss is intensely squeezed out into a band ending east of Alpe Balma. This gneiss is again tectonically thickened by

Figure 14. Photographs of x-z thin sections of quartz mylonites under crossed polarizers. (a) Detail of quartzitic mylonite MR8 from the area of ‘‘Rifugio Citta` di Vigevano’’ (Monte Rosa nappe, 2 km northwest of Stolemberg, 5083260/1410800). The irregular grain shapes were formed by grain boundary migration recrystallization during D1 (top-to-the-north shearing, compare Figure 13a). (b) Detail of quartzitic mylonite MR48 from the Cimalegna (5080470/1413250). The sample comes from the Monte Rosa nappe. Large grains are separated from smaller ones by low- to high-angle boundaries generated by subgrain rotation recrystallization during D3. Southeast vergent shearing is indicated by shape-preferred orientation of quartz. It was followed by more intense top-to-the-southeast shearing (D3) that led to the strong textures of MR48 and MR146 (Figures 13g and 13h). In the upper Sesia valley, D2 and D3 produced large folds with parasitic structures at all scales and fold axes (sub)parallel to the respective stretching lineation. The results of D2 and D3 fold overprinting are visualized schematically in Figure 15. As the D2 fold axial planes dip more steeply toward south than those of D3, and

Figure 15. Schematic drawing of the fold geometry in the upper Sesia valley. The D2 Malfatta folds are refolded by the D3 Cimalegna fold. Thus the D2 shear senses are locally inverted. The folded surface represents the upper boundary of the Monte Rosa nappe.

16 of 22

TC4013


TC4013

ogy; the Balma unit is almost exclusively composed of retrogressed eclogite and serpentinite whereas the ZermattSaas unit additionally contains huge amounts of amphibolite and metagabbro locally accompanied by calcschists and a characteristic greenschist breccia. Second, in the east of the study area between Alpe Mud di mezzo and Colle Mud, paragneisses of the Stolemberg unit are opposed to augengneisses of the Monte Rosa nappe from which they are separated by a thin layer of eclogite. If this was only a local duplication of the lithologic sequence due to minor faulting, one would expect the Monte Rosa and Stolemberg gneisses to be of the same type. Third, the strongest argument for different origins of the Zermatt-Saas and Balma ophiolites is that at Stockchnubel (Figure 3), northwest of Monte Rosa, a layer of serpentinite with eclogite boudins, lithologically identical with the Balma unit, occurs between the Monte Rosa nappe below and various gneisses above which belong to the Stockhorn unit of the Bernhard nappe system. These are strong indications in favor of a correlation of the Stolemberg gneiss with the Stockhorn gneiss. We interpret the Stolemberg unit to be the most internal remnant of the rootless Bernhard nappe system and therefore the Balma unit to be of Valaisan origin. [41] The Monte Rosa nappe as the lowermost unit of the nappe pile represents the former European margin which is separated from the Briançonnais by the Valais suture which, after unfolding the large-scale postnappe folds within the nappe pile, has to be looked for in the Furgg zone [Milnes et al., 1981; Froitzheim, 2001]. As the nature and tectonic significance of the Furgg zone are controversial, we will in the following shortly summarize the geological data about this zone and discuss its origin as far as it is relevant for the present study. Figure 16. D2 antiform with serpentinite of the Balma unit in its core (lower Malfatta fold) east of the Malfatta mountain seen from southeast. In this area the D2 fold axis trends roughly north-south due to its reorientation by D3 folds. The D2 fold axial plane is bent by parasitic folds of the D3 Molera fold.

almost isoclinal folding in the hinge zone of a parasitic fold of the Molera antiform north of Corno d’Olen. [40] Within this complex deformational pattern, the Balma and Stolemberg units consistently occupy their level between the Monte Rosa nappe and the Zermatt-Saas zone. By stepwise retrodeformation of D4/D3 and D2 deformation (Figure 17) we arrive at a post-D1 situation where the Monte Rosa nappe is overlain first by the Balma unit, then the Stolemberg gneiss, and finally the Zermatt-Saas ophiolites (Figure 17c). Since the direction of thrusting during D1 was north to northwest, this means that before D1 the units were arranged in the following order from southeast to northwest: Zermatt-Saas – Stolemberg – Balma – Monte Rosa. Although it might be imaginable that our Balma and Stolemberg units are a local repetition of the Monte Rosa and Zermatt-Saas units induced by imbrication prior to D2, there are three indications that this is not the case: First, the Balma and Zermatt-Saas units differ significantly in lithol-

6. Nappe Structure Versus Me´lange Structure and the Controversy About the Furgg Zone [42] The controversy about the Furgg zone [Jaboyedoff et al., 1996; Froitzheim, 2001; Keller and Schmid, 2001; Dal Piaz, 2001; Kramer et al., 2003] is centered on the question whether it is a me´lange or not, and on the nature of amphibolite boudins in the Furgg zone. The Furgg zone sensu stricto is located between the Monte Rosa basement and the Portjengrat basement unit, along the northern front of the Monte Rosa nappe [Bearth, 1952] (Figure 3). However, also rock units at the northwestern border of the Monte Rosa nappe, between the Monte Rosa basement and the Stockhorn basement unit (‘‘Schuppenzone des Stockknubel’’ [Bearth, 1953]), and rocks on the southern side of Monte Rosa [Dal Piaz, 1966] were parallelized with the Furgg zone. [43] The Furgg zone senso strictu in the corridor between the Monte Rosa nappe and the Portjengrat unit is characterized by abundant amphibolitized eclogite boudins in a matrix of schistose to gneissic metasediments. Layers of ophiolite (amphibolite, serpentinite, calcschist) extend from the Antrona ophiolite unit toward southwest into the area under question. They are locally accompanied by slivers of Mesozoic metasediments (marble, quartzite). Ophiolites and

17 of 22

TC4013


TC4013

Figure 17. Stepwise retrodeformation of folding and shearing events on the southern side of Monte Rosa. The according part is marked by a dashed rectangle in Figure 18. Patterns are as in Figure 2. Cones indicate directions and senses of shearing which were oblique to the plane of cross section during D1 to D3. (a) Present-day situation. (b) Situation after D2, showing southwest vergent D2 folds. (c) Situation after D1, showing the nappe stack comprising Monte Rosa, Balma, Stolemberg, Zermatt-Saas, and Tsate´ nappes, from base to top. Note different scale in Figure 17c.

Mesozoic metasediments are not part of the Furgg zone as defined by Bearth [1952, Figure 1] [see also Keller and Schmid, 2001]. Froitzheim [2001, p. 606] assumed that the Furgg zone is a me´lange and that the amphibolitized eclogite boundins may either represent Jurassic-Cretaceous oceanic basalts, pre-Mesozoic basic rocks, or PermianMesozoic dikes. Liati et al. [2001] dated an amphibolitized eclogite from the southern border of the Furgg zone. U-Pb SHRIMP analysis of magmatic zircon yielded a Cambrian age of 510 ± 5 Ma. Magmatic zircon from a leucocratic gneiss in the Furgg zone yielded a Permian age of 272 ± 4 Ma [Liati et al., 2001]. Finally, a mafic boudin in the ‘‘Schuppenzone des Stockknubel’’, which is lithologically identical to the Furgg zone senso strictu, yielded a Permian age of 269 ± 3 Ma for the magmatic protolith, also using U-Pb SHRIMP on zircon [Liati et al., 2003b]. So far, the radiometric ages of the Furgg zone senso strictu and the ‘‘Schuppenzone des Stockknubel’’ yielded Cambrian and Permian ages. Therefore the Furgg zone most probably represents Hercynian basement (as assumed by Bearth [1953]) with Permian intrusions which was affected by extremely strong shearing deformation during the Alpine orogeny, leading to the complete boudinage of mafic layers. This basement in our opinion belongs partly to the Monte Rosa nappe, partly to the Portjengrat unit. It does not represent a me´lange (as assumed by Froitzheim

[2001]) in the sense that completely heterogeneous rocks were mixed together. The probably Mesozoic metasediments are remnants of the sedimentary cover of this basement. The ophiolite layers were juxtaposed with the Furgg zone rocks by imbrication during subduction and were later affected by strong shearing deformation together with them. We still assume that an oceanic suture is located between the Monte Rosa nappe and the Portjengrat unit, represented by the main layer of ophiolite extending from the Antrona unit toward southwest, pinching out, but found again at Stockchnubel. This is in our view the suture of the Valais ocean, connected to the east with the Antrona unit and to the south with the Balma unit. [44] On the south side of Monte Rosa, the rock units that are similar to the Furgg zone and which were assigned to the Furgg zone sensu lato by Dal Piaz [1966] are predominantly paragneisses with abundant amphibolite/eclogite layers and boudins (Figure 7). In the study area, it is quite clear that these rocks are part of the Hercynian basement and belong to the Monte Rosa nappe (e.g., at Cimalegna, our Figure 7 and Dal Piaz [2001]). The basement slivers of the Stolemberg unit also locally comprise eclogite and amphibolite boudins. This supports the assumption that the main lithology of the Furgg zone, schist and gneiss with abundant mafic boudins, is the product of strong shearing deformation of quite normal Hercynian para-

18 of 22

TC4013


gneiss/amphibolite basement. The extent of chaotic mixing and me´lange formation between Hercynian and Mesozoic rocks has previously been overestimated [e.g., Froitzheim, 2001]. Although the boundaries of the Monte Rosa nappe were strongly sheared, the original tectonic units were in most cases preserved as coherent bodies. The area is characterized by nappe structure rather than me´lange structure. True me´lange only occurs locally within the ZermattSaas unit (Figures 6 and 7).

7. Age Data and Their Implications for the Kinematic Evolution [45] Structural observations and data provide a good insight into the complex kinematic history of the Monte Rosa nappe and overlying units under greenschist-facies conditions whereas structures related to high-pressure metamorphism are far too rare to permit well-constrained reconstructions of that part of the history. Indirect information about the prograde kinematic evolution results mainly from age dating and general considerations concerning the nappe pile geometry. Since the prograde evolution of the Penninic nappes during the Tertiary proceeded by southeastward subduction [e.g., Stampfli et al., 1998], nappe accretion took place along top-to-the-north(west) thrusts. Consequently, the ages of high-pressure metamorphism become successively younger from internal to external units [e.g., Gebauer, 1999]. [46] The youngest radiometric age data interpreted to reveal the pressure peak for the South Penninic ZermattSaas nappe were gathered by Rubatto et al. [1998] and Dal Piaz et al. [2001] and are 44.1 ± 0.7 Ma (U-Pb SHRIMP on metamorphic zircon) and 42 to 45 Ma (Rb-Sr on phengitic micas), respectively. [47] For the Balma and Antrona units U-Pb SHRIMP dating yielded 40.4 ± 0.7 Ma [Liati et al., 2002] and 38.5 ± 0.7 Ma [Liati et al., 2005], respectively, on amphibolitized eclogites. These similar ages confirm that both units share the same metamorphic history and that they are derived from a more external origin than the Zermatt-Saas zone, i.e., Valaisan. An age of 93.4 ± 1.7 Ma was determined for oscillatory-zoned, magmatic zircon domains of an eclogite from the Balma unit [Liati et al., 2002] that was collected 400 m south-southeast of Alpe Balma (Figure 7). This age is interpreted to date the crystallization of the gabbroic protolith. This protolith age is essentially the same as the 93.0 ± 2.0 Ma reported by the same authors for the North Penninic Chiavenna ophiolite (eastern central Alps). These late Cretaceous ages represent the stripe of Cretaceous oceanic crust that was formed in the Valais basin due to sinistral movement of Iberia relative to Europe (Figure 4b). It is most unlikely that the Late Cretaceous gabbro belongs to the Piemont-Ligurian oceanic crust, because there was no spreading in this ocean from 121 Ma (Barremian-Aptian) onward [Stampfli and Borel, 2004]. This is a strong argument for the Valaisan nature of the Balma unit, in addition to and independent of our structural arguments. A Late Jurassic protolith age of the Antrona ophiolites has been reported by Liati et al. [2005]. The Antrona ophiolites in

TC4013

our view represent Jurassic Piemont-Ligurian oceanic crust captured in the Valais basin by the eastward movement of Iberia-Briançonnais (Figure 4b). [48] No radiometric age data are available for the pressure climax of the Combin zone, the Stolemberg unit or the equivalent Bernhard nappe system, and the Monte Rosa nappe. Some constraints are given by the data compiled above: 44 Ma, i.e., the age of eclogite-facies metamorphism in the Zermatt-Saas zone, and the 41 to 38 Ma for the Valaisan domain are age brackets for the maximum burial of the Briançonnais (Stolemberg and Bernhard units). Since the Monte Rosa nappe is underlying the North Penninic Balma unit it must be of European origin and eclogite-facies metamorphism is therefore expected to be slightly younger than in the Balma and Antrona units. [49] The age of circa 40 Ma for the eclogite-facies metamorphism in the Balma unit can be regarded as a maximum age bracket for D1 in our study area. This is because D1 postdates the eclogite-facies metamorphism and occurred under greenschist-facies conditions. On the other hand, 28.85 ± 0.55 Ma resulted from 40Ar/39Ar dating of hydrothermal muscovite in a gold vein from Valle Mud whose growth postdated the ductile deformation and thus the formation of the Vanzone fold [Pettke et al., 1999] which effectuated the steepening of the so-called root zone north of the Insubric line and, looking at kinematic indicators and fold geometries, has to be correlated to D4. Therefore the entire sequence of deformation phases described here took place after 40 and before 29 Ma, that is, in the Late Eocene and Early Oligocene. A zircon fission track age of 24 Ma [Hurford et al., 1989] from the Monte Rosa nappe in our study area dates further cooling through the annealing temperature of zircon (240 ± 60°C after Yamada et al. [1995]). [50] Greenschist-facies metamorphism has been dated applying the Rb-Sr method on mica by Reddy et al. [1999]. Main results are that northwest vergent shearing took place from 39.2 to 37.2 Ma in the Tsate´ nappe and was postdated by D3 and/or D4 southeast vergent shearing closely above the contact of the Tsate´ and Zermatt-Saas nappes between 37.5 and 36.5 Ma. The top-southeast shearing is probably equivalent to our D3. If these data are correctly interpreted, our D1 to D3 would have taken place in a very short period of time, between 40 and 36 Ma.

8. Discussion and Conclusions [51] The sequence of deformation phases defined on the south side of Monte Rosa is closely related to tectonic processes along the Insubric line and the exhumation of the Lepontine metamorphic dome between the Monte Rosa and Adula nappes (Figure 1). Upright D4 folds belong to a system of antiforms, one of which is the Vanzone fold, that parallel the Insubric line (Periadriatic line) to the north (Figure 3). In the southern limb of the Vanzone and equivalent folds, the tectonic units are rotated from their original, shallowly south dipping orientation into a more steeply south dipping to vertical and eventually overturned position. The study area is in the broad hinge region of the

19 of 22

TC4013


TC4013

Figure 18. Schematic cross section illustrating a paleogeographic restoration of the Penninic nappes according to results of this study. Dashed rectangle indicates part of the section retrodeformed in Figure 17. VO, suture of the Valais (North Penninic) ocean. Ophiolites at the southern border of the Monte Rosa nappe are interpreted to represent a double suture of the Valais (Balma unit, B) and Piemont-Liguria oceans, with remnants of the Briançonnais (Stolemberg unit, S) in between. Farther north, the Valais suture is represented by an ophiolite layer in the Furgg zone, the Antrona zone, and, offset along the Simplon line, the Sion-Courmayeur zone. The Monte Rosa nappe is the southernmost exposed part of the European continental margin.

Vanzone antiform (Figure 2). Therefore the geometric effects of D4 are only minor in this area. D3 deformation occurred in a wide shear zone with a top-to-southeast sense of shear. This shear zone has the geometry of a normal fault. From the study area toward northeast, the D3 shear zone approximately follows the southern border of the Monte Rosa nappe and becomes steeper and eventually overturned, due to the steepening effect of D4. The rotated normal fault component led to a relative uplift of the northern block. Farther toward northeast, the D3 shear merges into the steeply north dipping mylonite belt along the Insubric fault (Figure 1). If the D3 shear zone still has the north-side-up component, which we think is likely, it follows that the metamorphic contrast across the Insubric line in the central Alps, between high-grade amphibolite facies in the Lepontine metamorphic dome to the north and unmetamorphic rocks to the south [Bousquet et al., 2004], is not entirely due to south directed backthrusting [e.g., Schmid et al., 1987], but may be partly the result of a top-to-southeast normal shear zone which was later rotated into the north dipping position. Very similar relations were observed in the southern Adula nappe (Figure 1) by Nagel et al. [2002]; they also found a wide, top-to-southeast extensional shear zone (their D2) which toward south becomes steepened and overturned by a later antiform (their D3) located north of the Insubric line. [52] Top-to-southwest shearing during D2 occurs not only in the Monte Rosa nappe but also in a wide shear zone, the Simplon ductile shear zone [Steck, 1990], comprising parts of the Bernhard nappe and the underlying units. The geometry of the shear zone is that of a ductile normal fault dipping toward southwest. This deformation reflects the earliest step of extensional unroofing of the Lepontine metamorphic dome. Top-to-northwest shearing during D1 is directed toward the Alpine foreland and may therefore

reflect thrusting during continent collision. It occurred in the study area under greenschist-facies conditions, but may have started under higher pressures. D1 deformation clearly postdates, however, the pressure peak which is recorded only in eclogite boudins. Similar relations are observed in the Adula nappe at the eastern border of the Lepontine dome where top-to-north to -northwest shearing (Zapport phase) occurred during and after the exhumation from eclogite-facies to greenschist-facies conditions [Nagel et al., 2002; Pleuger et al., 2003]. [53] The occurrence of the Balma and Stolemberg units on the southern side of Monte Rosa supports an interpretation of the nappe geometry where the Valais and Briançonnais units are rooted south of the Monte Rosa nappe (Figure 18). As a result, we regard the Monte Rosa nappe to be part, in terms of paleogeography, of the European continental margin, originally located northwest of the Valais ocean. It represents the most distal, oceanward part of that margin, southeast of the Lower Penninic Monte Leone and Moncucco nappes (Figures 2 and 3). Taking into account the radiometric age constraints for the deformation phases (see above), this requires that subduction of the Valais and distal European margin started before complete closure of the sedimentary basins in the Valais-Briançonnais transition zone at circa 37 to 34 Ma [Bagnoud et al., 1998; Stampfli et al., 1998]. The ophiolite-bearing zone south of Monte Rosa represents a double suture, formed by the closure of the Piemont-Ligurian and Valais basins. Out-ofsequence thrusting did occur during and after continental collision, but because of the ductile character of that thrusting, the original stacking order of tectonic units can be reconstructed by means of careful structural restoration. One such major out-of-sequence thrust is located at the base of the Monte Rosa nappe, where the nappe was emplaced over the more internally derived and structurally higher

20 of 22

TC4013


Valais ophiolites of the Antrona zone. As we do not know the kinematics of deformation preceding our D1, our paleogeographic reconstruction is not the only possible one. It is, however, the simplest solution based on the existing data. Other solutions are possible if complicated pre-D1 deformation events are inferred without the support of field data. [54] Because of the regional axial plunge toward southwest, deeper portions of the collisional suture are exposed from the Monte Rosa nappe toward northeast, in the steep belt that follows the Insubric line to the north. In this part of the suture zone, deformation is even more complex than in our study area, to such a degree that original geometries can hardly be reconstructed. This area was described as a tectonic accretion channel by Engi et al. [2001b], that is, a narrow channel where successive subduction, accretion, decoupling, and obduction events led to a mixture of rock units with different paleogeographic provenance and different metamorphic histories. However, the tectonic accretion channel does not include the Monte Rosa nappe and units above it, as was also clearly stated by Engi et al. [2004]. [55] It is remarkable that the units in the double suture south of Monte Rosa are extremely thinned and affected by several phases of folding, but not mixed in a chaotic manner. Compared to the paleogeographic map for the Cretaceous (Figure 4b), our reconstruction implies that some hundred kilometers of oceanic and continental crust, including the South Penninic, Briançonnais, and North Penninic domains, were consumed between the Monte Rosa nappe and the southern Alpine units during subduction. As result, in a postcollisional stage (post-D1, Figure 17c), the remnant units show evidence for extreme shearing and large-scale boudinage; for example, in the case of the Stolemberg unit, the entire continental crust of the Brian-

TC4013

çonnais is only represented by a thin and interrupted layer of gneiss. However, the geometry reveals a high degree of order neither disturbed by local transport oblique to the main shear direction nor by chaotic material flow. This suggests that the strain in a collisional suture zone was, at least in this case, relatively homogeneous during nappe stacking and subsequent stages of exhumation. Another result is that in an advanced stage of thinning in the suture zone, oceanic units are reduced to serpentinite with mafic boudins (Balma unit), and continental units are reduced to gneiss without cover remnants (Stolemberg unit). Any sedimentary rocks are sheared off during the descent of the terranes into the subduction zone, and are piled up in more external parts of the suture zone. [56] We deduce from this study that suture zones are not generally chaotic but may be ordered, and that therefore their geometry and kinematics can be clarified using the methods of structural geology, in combination with metamorphic petrology and geochronology. Thus structural geology is an important tool for understanding plate tectonic processes in collisional orogens. We stress this point because in the past some authors [e.g., Polino et al., 1990] have claimed that the internal parts of collisional orogens are so chaotic that they cannot be palinspastically restored using structural geology, forcing the geologist to rely entirely on petrology, geochronology, and modeling techniques.

[57] Acknowledgments. The authors gratefully acknowledge constructive reviews by G. M. Stampfli, M. Engi, and Associate Editor A. Pfiffner that helped to improve earlier versions of this paper. J. Albus, B. Schneider, and L. Wirths are thanked for preparing geologic maps of Val de Gressoney and placing them at our disposal. The study was supported by DFG (grants FR700/6-1 and -2).

References Argand, E. (1911), Les nappes de recouvrement des Alpes Pennines et leurs prolongements structuraux, Mater. Carte Geol. Suisse, 31, 1 – 26. Bagnoud, A., R. Wernli, and M. Sartori (1998), Dećouverte de foraminife`res planctoniques paleóge`nes dans la zone de Sion-Courmayeur a` Sion (Valais, Suisse), Eclogae Geol. Helv., 91, 421 – 429. Balle`vre, M., and O. Merle (1993), The Combin Fault: Compressional reactivation of a Late CretaceousEarly Tertiary detachment fault in the western Alps, Schweiz. Mineral. Petrogr. Mitt, 73, 205 – 227. Barnicoat, A. C., and N. Fry (1986), High-pressure metamorphism of the Zermatt-Saas ophiolite zone, Switzerland, J. Geol. Soc. London, 143, 607 – 618. Bearth, P. (1952), Geologie und Petrographie des Monte Rosa, Beitr. Geol. Karte Schweiz, 96, 94 pp. Bearth, P. (1953), Geologischer Atlas der Schweiz 1:25000, Blatt Zermatt 29, Erlaüterungen, Schweiz. Geol. Kommission, Bern, Switzerland. Bearth, P. (1967), Die Ophiolithe der Zone von ZermattSaas Fee, Beitr. Geol. Karte Schweiz, 132, 130 pp. Borghi, A., R. Compagnoni, and R. Sandrone (1996), Composite P-T paths in the internal Penninic Massifs of the western Alps: Petrological constraints to their thermo-mechanical evolution, Eclogae Geol. Helv., 89, 345 – 367. Bouchez, J.-L. (1978), Preferred orientations of quartz a-axes in some tectonites: Kinematic inferences, Tectonophysics, 49, T25 – T30. Bousquet, R., M. Engi, G. Gosso, R. Oberha¨nsli, A. Berger, M. I. Spalla, M. Zucali, and B. Goffe´

(2004), Explanatory notes to the map: Metamorphic structure of the Alps Transition from the western ¨ sterr. Miner. Ges., 149, to the central Alps, Mitt. O 145 – 156. Burg, J. P., and P. Laurent (1978), Strain analysis of a shear zone in a granodiorite, Tectonophysics, 47, 15 – 42. Chopin, C., and P. Monie´ (1984), A unique magnesiochloritoide-bearing, high-pressure assemblage from the Monte Rosa, western Alps: Petrologic and 40 Ar-39Ar radiometric study, Contrib. Mineral. Petrol., 87, 388 – 398. Dal Piaz, G. V. (1966), Gneiss ghiandoni, marmi ed anfiboliti antiche del ricoprimento Monte Rosa nell’alta Valle d’Ayas, Boll. Soc. Geol. Ital., 85, 103 – 132. Dal Piaz, G. V. (1971), Nuovi ritrovamenti di cianite alpina nel cristallino antico del Monte Rosa, Rend. Soc. Ital. Mineral. Petrol., 27, 437 – 477. Dal Piaz, G. V. (2001), Geology of the Monte Rosa massif: Historical review and personal comments, Schweiz. Mineral. Petrogr. Mitt, 81, 275 – 303. Dal Piaz, G. V., and B. Lombardo (1986), Early Alpine eclogite metamorphism in the Penninic Monte Rosa-Gran Paradiso basement nappes of the northwestern Alps, in Blueschists and Eclogites, edited by B. W. Evans and E. H. Brown, Geol. Soc. Am. Mem., 164, 249 – 265. Dal Piaz, G. V., G. Cortiana, A. Del Moro, S. Martin, G. Pennacchioni, and P. Tartarotti (2001), Tertiary age and paleostructural inferences of the eclogitic imprint in the Austroalpine outliers and Zermatt-

21 of 22

Saas ophiolite, western Alps, Int. J. Earth Sci., 90, 668 – 684. Engi, M., N. C. Scherrer, and T. Burri (2001a), Metamorphic evolution of pelitic rocks of the Monte Rosa nappe: Constraints from petrology and single grain monazite age data, Schweiz. Mineral. Petrogr. Mitt, 81, 305 – 328. Engi, M., A. Berger, and G. T. Roselle (2001b), Role of the tectonic accretion channel in collisional orogeny, Geology, 29, 1143 – 1146. Engi, M., R. Bousquet, and A. Berger (2004), Explanatory notes to the map: Metamorphic structure of ¨ sterr. Miner. Ges., the Alps central Alps, Mitt. O 149, 157 – 173. Escher, A., H. Masson, and A. Steck (1993), Nappe geometry in the western Swiss Alps, J. Struct. Geol., 15, 501 – 509. Escher, A., J. C. Hunziker, M. Marthaler, H. Masson, M. Sartori, and A. Steck (1997), Geologic framework and structural evolution of the western Swiss-Italian Alps, in Deep Structure of the Swiss Alps, edited by O. A. Pfiffner et al., pp. 205 – 221, Birkhaüser, Basel, Switzerland. Frisch, W. (1979), Tectonic progradation and plate tectonic evolution of the Alps, Tectonophysics, 60, 121 – 139. Froitzheim, N. (2001), Origin of the Monte Rosa nappe in the Pennine Alps: A new working hypothesis, Geol. Soc. Am. Bull., 113, 604 – 614. Froitzheim, N., and G. Manatschal (1996), Kinematics of Jurassic rifting, mantle exhumation, and passive-

TC4013


margin formation in the Austroalpine and Penninic nappes (eastern Switzerland), Geol. Soc. Am. Bull., 108, 1120 – 1133. Gebauer, D. (1999), Alpine geochronology of the central and western Alps: New constraints for a complex geodynamic evolution, Schweiz. Mineral. Petrogr. Mitt, 791, 191 – 208. Gerlach, H. (1869), Die Penninischen Alpen: Beitra¨ge zur Geologie der Schweiz, Denkschr. Schweiz. Naturforsch. Ges., 23, 132 pp. Gosso, G., G. V. Dal Piaz, V. Piovano, and R. Polino (1979), High pressure emplacement of early-alpine nappes, postnappe deformations and structural levels, Mem. Sci. Geol. Ital., 32, 15 pp. Hurford, A. J., M. Flisch, and E. Ja¨ger (1989), Unravelling the thermo-tectonic evolution of the Alps: A contribution from fission track analysis and mica dating, in Alpine Tectonics, edited by M. P. Coward, D. Dietrich, and R. G. Park, Geol. Soc. Spec. Publ., 45, 369 – 398. Jaboyedoff, M., P. Be´gle´, and S. Lobrinus (1996), Stratigraphie et e´volution structurale de la zone de Furgg, au front de la nappe du Mont-Rose, Bull. Soc. Vaudoise Sci. Nat., 84, 191 – 210. Jansen, E., W. Scha¨fer, and A. Kirfel (2000), The Ju¨lich neutron diffractometer and data processing in rock texture investigations, J. Struct. Geol., 22, 1559 – 1564. Keller, L. M., and S. M. Schmid (2001), On the kinematics of shearing near the top of the Monte Rosa nappe and the nature of the Furgg zone in Val Loranco (Antrona valley, N. Italy): Tectonometamorphic and paleogeographical consequences, Schweiz. Mineral. Petrogr. Mitt, 81, 347 – 367. Kramer, J., R. Abart, O. Mu¨ntener, S. M. Schmid, and W.-B. Stern (2003), Geochemistry of metabasalts from ophiolitic and adjacent distal continental margin units: Evidence from the Monte Rosa region (Swiss and Italian Alps), Schweiz. Mineral. Petrogr. Mitt, 83, 217 – 240. Lange, S., L. Nasdala, U. Poller, L. Baumgartner, and W. Todt (2000), Crystallization age and metamorphism of the Monte Rosa granite, western Alps, paper presented at 17th Swiss Tectonic Studies Group Meeting, Geol. Insti., ETH, Zu¨rich, 31 March to 1 April. Law, R. D., S. M. Schmid, and J. Wheeler (1990), Simple shear deformation and quartz crystallographic fabrics: A possible natural example from the Torridon area of NW Scotland, J. Struct. Geol., 12, 29 – 45. Liati, A., D. Gebauer, N. Froitzheim, and C. M. Fanning (2001), U-Pb SHRIMP geochronology of an amphibolitized eclogite and an orthogneiss from the Furgg zone (western Alps) and implications for its geodynamic evolution, Schweiz. Mineral. Petrogr. Mitt, 81, 379 – 393. Liati, A., D. Gebauer, and N. Froitzheim (2002), Late Cretaceous basic oceanic magmatism in the Valais ocean, western and central Alps: Geochronological evidence and paleogeographic implications, paper presented at Annual Meeting of the Swiss Academy of Natural Sciences, Davos, Switzerland. Liati, A., D. Gebauer, and C. M. Fanning (2003a), The youngest basic oceanic magmatism in the Alps (Late Cretaceous; Chiavenna unit, central Alps):

Geochronological constraints and geodynamic significance, Contrib. Mineral. Petrol., 146, 144 – 158. Liati, A., D. Gebauer, and N. Froitzheim (2003b), Permian basic magmatism, Upper Eocene and lower Oligocene metamorphism in the Furgg zone (western Alps), paper presented at EGS-AGU-EUG Joint Assembly, Nice, France, 6 – 11 April. Liati, A., N. Froitzheim, and C. M. Fanning (2005), Jurassic ophiolites within the Valais domain of the western and central Alps: Geochronological evidence for re-rifting of oceanic crust, Contrib. Mineral. Petrol., 149(4), 446 – 461. Mattirolo, E., V. Novarese, S. Franchi, and A. Stella (1912), Carta geologica d’Italia, scale 1:100000, folio 29 (Monte Rosa), Serv. Geol. d’Ital., Rome. Mattirolo, E., V. Novarese, S. Franchi, and A. Stella (1927), Carta geologica d’Italia, 1:100,000, Foglio 30 (Varallo), Servizio Geologico d’Italia, Rome. Milnes, A. G., M. Greller, and R. Mu¨ ller (1981), Sequence and style of major post-nappe structures, Simplon-Pennine Alps, J. Struct. Geol., 3, 411 – 420. Nagel, T., C. de Capitani, M. Frey, N. Froitzheim, H. Stu¨nitz, and S. M. Schmid (2002), Structural and metamorphic evolution during rapid exhumation in the Lepontine dome (southern Simano and Adula nappes, central Alps, Switzerland), Eclogae Geol. Helv., 95, 301 – 321. Pettke, T., L. W. Diamond, and I. M. Villa (1999), Mesothermal gold veins and metamorphic devolatilization in the northwestern Alps: The temporal link, Geology, 27, 641 – 644. Platt, J. P. (1986), Dynamics of orogenic wedges and the uplift of high-pressure metamorphic rocks, Geol. Soc. Am. Bull., 97, 1037 – 1053. Pleuger, J., R. Hundenborn, K. Kremer, S. Babinka, W. Kurz, E. Jansen, and N. Froitzheim (2003), Structural evolution of Adula nappe, Misox zone, and Tambo nappe the in San Bernardino area: Constraints for the exhumation of the Adula eclo¨ sterr. Geol. Ges., 94, 99 – 122. gites, Mitt. O Polino, R., G. V. Dal Piaz, and G. Gosso (1990), Tectonic erosion at the Adria margin and accretionary processes for the Cretaceous orogeny in the Alps, in Deep structure of the Alps, edited by F. Roure, P. Heitzmann, and R. Polino, Mem. Soc. Geol. Fr., 1, 345 – 367. Reddy, S. M., J. Wheeler, and R. A. Cliff (1999), The geometry and timing of orogenic extension: An example from the western Italian Alps, J. Metamorph. Geol., 17, 573 – 589. Reinecke, T. (1991), Very-high-pressure metamorphism and uplift of coesite-bearing metasediments from the Zermatt-Saas zone, western Alps, Eur. J. Mineral., 3, 7 – 17. Rubatto, D., D. Gebauer, and M. Fanning (1998), Jurassic formation and Eocene subduction of the Zermatt-Saas Fee ophiolites: Implications for the geodynamic evolution of the central and western Alps, Contrib. Mineral. Petrol., 132, 269 – 287. Rubatto, D., D. Gebauer, and R. Compagnoni (1999), Dating of eclogite-facies zircons: The age of Alpine metamorphism in the Sesia-Lanzo zone (western Alps), Earth Planet. Sci. Lett., 167, 141 – 158. Sartori, M. (1987), Structure de la zone du Combin entre les Diablons et Zermatt (Valais), Eclogae Geol. Helv., 80, 789 – 814.

22 of 22

TC4013

Schmid, S. M., and M. Casey (1986), Complete fabric analysis of some commonly observed quartz c-axis patterns, in Mineral and Rock Deformation: Laboratory Studies—The Patterson Volume, Geophys. Monogr. Ser., vol. 36, edited by B. E. Hobbs and H. C. Heard, pp. 263 – 286, AGU, Washington, D. C. Schmid, S. M., A. Zingg, and M. Handy (1987), The kinematics of movements along the Insubric Line and the emplacement of the Ivrea Zone, Tectonophysics, 135, 47 – 66. Stampfli, G. M. (1993), Le Briançonnais, terrain exotique dans les Alpes?, Eclogae Geol. Helv., 86, 1 – 45. Stampfli, G. M., and G. D. Borel (2004), The TRANSMED transects in space and time: Constraints on the paleotectonic evolution of the Mediterranean domain, in The TRANSMED Atlas, edited by W. Cavazza et al., pp. 53 – 80, Springer, New York. Stampfli, G. M., J. Mosar, D. Marquer, R. Marchant, T. Baudin, and G. Borel (1998), Subduction and obduction processes in the Swiss Alps, Tectonophysics, 296, 159 – 204. Stampfli, G. M., G. Borel, R. Marchant, and J. Mosar (2002), Western Alps geological constraints on western Tethyan reconstructions, J. Virtual Explor., 8, 77 – 106. Steck, A. (1984), Structures de de´formation tertiaires dans les Alpes centrales (transversale Aar-SimplonOssola), Eclogae Geol. Helv., 77, 55 – 100. Steck, A. (1990), Une carte des zones de cisaillement ductile des Alpes centrales, Eclogae Geol. Helv., 83, 603 – 627. Steck, A., B. Bigioggero, G. V. Dal Piaz, A. Escher, G. Martinotti, and H. Masson (1999), Carte tectonique des Alpes de Suisse occidentale et des re´gions avoisinantes, Carte Speć. 123, 4 maps, scale 1:100,000, Serv. Hydrol. Geól. Nat., Bern, Switzerland. Stipp, M., H. Stu¨ nitz, R. Heilbronner, and S. M. Schmid (2002), The eastern Tonale fault zone: A ‘‘natural’’ laboratory for crystal plastic deformation of quartz over a temperature range from 250 to 700°C, J. Struct. Geol., 24, 1861 – 1884. Tru¨mpy, R. (1975), Penninic-Austroalpine boundary in the Swiss Alps: A presumed former continental margin and its problems, Am. J. Sci., 275, 209 – 238. Yamada, R., T. Tagami, S. Nishimura, and H. Ito (1995), Annealing kinetics of fission tracks in zircon: An experimental study, Chem. Geol., 122, 249 – 258.

N. Froitzheim and J. Pleuger, Geologisches Institut der Universita¨t Bonn, Nussallee 8, D-53115 Bonn, Germany. ([email protected]; jan.pleuger@ uni-bonn.de) E. Jansen, Arbeitsgruppe fu¨r Neutronenbeugung, Mineralogisch-Petrologisches Institut der Universita¨t Bonn, Aussenstelle im Forschungszentrum Ju¨lich, MIN/ZFR, D-52425 Ju¨lich, Germany. ([email protected])