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Beaumont-Escher-1997.pdf

Pergamon Journal of Sfrucrural Geology., Vol. 19, No. I. pp. 955 to 974. 1997

C 1997 Elsevier Science Ltd

PII: S0191-8141(97)00022-9 All rights reserved. Printed in Great Britain

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Formation, burial and exhumation of basement nappes at crustal scale: a geometric model based on the Western Swiss-Italian Alps

ARTHUR ESCHER

Institut de GCologie, BFSH-2, UniversitC de Lausanne, CH-1015 Lausanne, Switzerland

and

CHRISTOPHER BEAUMONT

Oceanography Department, Dalhousie University, Nova Scotia, Halifax, Canada B3H 45 1

(Received 23 May 1996; accepted in revisedform 3 1 January 1997)

Abstract-Information from geological and reflection seismic data from the Western Swiss-Italian Alps, and from numerical models, is used to build a geometrical model that can explain some of the major tectonic-metamorphic features of Alpine-type basement nappes. The model gives a geometrical and mechanical explanation for the initiation, burial, and subsequent uplift and partial exhumation of basement nappes at a crustal scale. Three main tectonic stages during convergence are distinguished and each correlated with the formation of specific nappe structures. The first two stages are single vergent (NW) and correspond to the subduction of continental margin crust, and the formation and uplift ofhigh-pressure rocks. Simple-shear flow and superimposed wedge-shaped pure shear flow is proposed for the creation and intrusion of high-pressure nappes of the Monte Rosa type. The third stage is characterized by a doubly-vergent style with both pro- and retro-movements. The former created NW- vergent nappes, as seen in the external Alpine massifs and the latter caused backfolding and thrusting, typical of the internal Alps. This third stage corresponds to the Neoalpine movements in the Western Swiss-Italian Alps, and is accompanied by a generalized uplift, mountain building and molasse sedimentation. 0 1997 Elsevier Science Ltd.

INTRODUCTION

The scope of this paper is to give a geometrical and mechanical explanation for the initiation and subsequent uplift and stacking of basement nappes in an Alpine-type orogen. The proposed model is based mostly on informa- tion from geological and reflection seismic data from the Western Swiss-Italian Alps and, for the last (doubly- vergent) stage, on plane-strain finite-element models (Beaumont et al., 1996). It tries to explain the main tectonic-metamorphic features of basement nappes and includes a relatively new concept concerning the rapid uplift and decompression of units that underwent high- pressure metamorphism. The model only deals with some of the essential characteristics of basement nappes and their evolution, and not with the highly complex details of the Swiss-Italian Alps. Moreover, it is based on two- dimensional reconstructions along vertical transverse sections which, although they contain most of the early Tertiary Alpine strain movements, do not include important longitudinal components. For these reasons the following pages are only meant to show how some old and new concepts can be applied to the erogenic deformation of basement rocks at a crustal scale. They certainly should be confirmed by more detailed work, in the field and laboratory, as well as by finite-element and analogue modeling techniques. In order to understand better the proposed model, an outline of the structure and evolution of the Western Swiss-Italian Alps is given below.

STRUCTURE OF THE WESTERN SWISS- ITALIAN ALPS

Since the early investigations (Gerlach, 1869; Schardt, 1907; Argand, 1911, 1916; Lugeon, 1914; Heim, 1921; Hermann, 1937), the geological knowledge of the Western Swiss-Italian Alps has been steadily refined by a large number of geologists. The results of the recent deep seismic survey of the Ecors-Crop and NFP-20 programs (Frei et al., 1990; Tardy et al., 1990; Heitzmann et al., 1991; Pfiffner, 1992; Marchant, 1993; Escher et al., 1997; Steck et al., 1997) have allowed us for the first time to control to some extent the geometric extrapolations and reconstructions of the deep Alpine structures. The result, although not very different to Argand’s brilliant reconstruction, gives a better impres- sion of the geometry of each Alpine unit at a crustal scale. It also shows the probable relationship between the Alpine nappe stack and the European and Adriatic lithospheres. In a simplified way, these data are represented in the geological cross-section of Fig. 1 which trends from the Mont Tendre (Jura) to the Val Sesia (Sesia-Ivrea). The site of this profile was chosen because it crosses one of the best-known parts of the Alpine chain; because many of the geological interpreta- tions of the seismic profiles WI-W5 can be directly projected onto it (Escher et al., 1987, 1993; Steck et al., 1989; Marchant, 1993) and because the NW-SE orienta- tion of the section coincides with the major stretching and flow direction during the paroxysm of Tertiary

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Basement nappes in the Swiss-Italian Alps 957

deformation (0, phase according to the classification by Steck, 1984, 1990).

As recognized by Trtimpy (1973,1980,1988, 1992), Stampfli and Marthaler (1990) Stampfli (1993) and Marchant and Stampfli, 1997, the Western Swiss-Italian Alps result from the collision between at least five main lithospheric units, from the northwest or external to the southeast or internal part (Fig. 1): (1) the European continental lithosphere; (2) the Valais oceanic litho- sphere; (3) the Brianconnais continental lithosphere; (4) the Piemont oceanic lithosphere; and (5) the Austroal- pine and South Alpine lithospheres.

The continent-derived units are preserved as crustal basement and cover nappes, whereas only ophiolite slices and associated sediments remain of the oceanic litho- spheres, forming remnants of accretionary prisms. By definition, the Alpine cover rocks were deformed by the Alpine orogeny only. Most basement rocks were also affected by earlier events. Cover rocks are mostly represented by sediments or metasediments of Late Permian-Pliocene age (Fig. 1).

Units derivedfrom the European continental lithosphere

The European upper continental crust and cover is well represented in the external (NW) part of the belt, now forming the major component of the Swiss Alpine chain. Most structures show an initial N- to W-

vergence, whereas later backfolding (S-vergence) is only observed southeast of the Aiguilles Rouges massif. The large majority of basement nappes, from the external Mont Blanc to the internal Monte Leone nappe, display fold-nappe structures. with a normal limb, a frontal hinge part and an overturned lower limb (Steck, 1984, 1987). It is likely that the most external Aiguilles Rouges and Infra Rouges basement units are also fold nappes, as inferred from seismic data (Steck et al., 1997) and from outcrop features (Badoux, 1962). Alpine deformation of the basement gneisses is intense in the overturned limbs, and decreases towards the core and upper limb of each fold nappe. As a general rule, the amount of strain increases considerably from the external Aiguilles Rouges, where it is partitioned into separate shear zones, to the more internal units, where it resulted in a well-developed, penetrative and principal schistosity. This early deformation took place at greenschist-facies conditions in the external nappes, and at amphibolite facies in the more internal ones. Most internal nappes were formed during at least two successive early phases, resulting in spectacular superposed structures. Later backfolding is mostly characterized by retrograde greenschist-facies metamorphism; it always refolds ear- lier NW-vergent nappes.

Most of the cover of the European upper crust was detached during the formation of the basement nappes. It was displaced to the northwest, the distance of transport increasing toward the southeast. This cover is now found

as external thrust sheets (Jura) or as a stack of more internal thrust and fold nappes, forming the Helvetic

cover nappes (Fig. 1). Most cover thrust nappes used weak layers such as Triassic evaporites as detachment horizons. Basement and cover nappes were generated simultaneously, often by different mechanisms, ductile for the basement fold nappes and brittle for the cover thrust sheets (Escher et al., 1993; Epard and Escher, 1996).

The European lower crust is well defined on most reflection seismic profiles and can be constructed down to ca 45 km depth (Steck et al., 1997). It shows a remarkable continuity and appears to be almost undeformed by the Alpine orogen.

Units derivedfrom the Valais oceanic lithosphere

Remnants of the Valais oceanic lithosphere and associated sediments are found in a discontinuous zone at the boundary between nappes derived from the European crust and those of Brianconnais origin (Fig. 1). The ductile oceanic sediments of the Valais accre- tionary prism must have formed a weak structural link between the European and Brianconnais nappe piles.

Units derivedfrom the Briaryonnais continental lithosphere

The basement nappes derived from the Brianconnais upper continental crust form a central zone between the Valais and the Piemont ophiolitic nappes (Fig. 1). Most of the Brianconnais sedimentary cover was separated from its basement during early Alpine deformations, and translated to the northwest. It forms the bulk part of the Prealpine nappes. Like the European units, the Brian- connais-derived basement nappes mostly display fold features with intensely sheared inverted limbs and less- deformed cores and upper limbs (Lacassin, 1987; Escher, 1988; Sartori, 1990). A dominant penetrative foliation, resulting from the earliest phases of deformation, is present almost everywhere. It clearly indicates that here, too, crustal nappes were formed at an early stage by heterogeneous ductile shear. Prograde greenschist-facies metamorphic conditions prevailed during the early deformation phases in the external Brianconnais nappes (Pontis, Siviez-Mischabel). In the most internal units (Mont Fort and Monte Rosa) remnants of early mineral paragenesis indicate that they were formed under high- pressure-intermediate-temperature conditions with values of ca 15 kbars and 500°C for parts of the Monte Rosa nappe (Bearth, 1952; Hunziker, 1970; Frey et al., 1976; Colombi, 1989). Subsequently they must have been elevated, decompressed and cooled to greenschist-facies conditions before any significant heating above ca 500°C could take place by the terrestrial heat flow. Late SE- vergent backfolding, associated with retrograde greens- chist-facies metamorphism, affected all the Brianconnais basement nappes. The Brianconnais lower crust has not been identified anywhere.

958 A. ESCHER and C. BEAUMONT

Units derived,from the Piemont oceanic lithosphere

Remains of the Piemont oceanic domain form an

important and continuous zone separating the Briancon- nais units from the Austroalpine and Adriatic ones (Fig. 1). It is composed of two main units.

(1) The Tsate nappe is probably the remnant of an accretionary prism formed during the Early-Middle

Cretaceous subduction and closure of the Piemont oceanic domain (Marthaler and Stampfli, 1989; Stampfli and Marthaler, 1990). It is mostly composed of calcschists containing lenses of ophiolitic rocks. The metamorphic history of the Tsate nappe is characterized by an early, middle-high-pressure event resulting in the formation of greenschist to blueschist metamorphic assemblages (Dal Piaz, 1976; Caby, 1981; Ayrton et al., 1982; Pfeiffer et al., 1989, 1991). It was followed by a pervasive greenschist-facies episode.

(2) The Zermatt-Saas and Antrona zones are large slices of Piemont oceanic lithosphere, associated with some oceanic sediments (Bearth, 1967). They underwent a very high-pressure metamorphism, probably during the Eoalpine subduction, with P-T conditions of ca

18 kbars/550”C (Hunziker, 1974; Meyer, 1983;

Barnicoat and Fry, 1986; Barnicoat et al., 1991). A later Tertiary greenschist (Zermatt-Saas Fee) and amphibolite facies (Antrona) overprint is well documented (Laduron, 1976; Colombi and Pfeiffer, 1986; Ganguin, 1988; Steck, 1989). In Fig. 1, the Lanzo oceanic unit is interpreted as the internal and southwest continuation of the Zermatt- Saas Fee ophiolites as proposed by Blake et al. (1980) and

Lagabrielle et al. (1989).

Units derived from the Austroalpine and South Alpine continental lithospheres

South of the Piemont ophiolite suture zone, the western equivalents of the Austroalpine nappes are present in the Dent Blanche klippe and in the Sesia zone. Both are made of the same two superposed Austroalpine basement thrust nappes characterized by the absence of inverted limbs and the presence of important basal mylonites (Fig. 1).

(1) The lower nappe is composed of the Arolla series, the Gneiss Minuti and the Eclogitic Micaschist Complex (lower Sesia). It is made of (Adriatic?) upper crust and contains relic zones of high-pressure paragenesis of Cretaceous age (Venturini et al., 1991; Venturini, 1995).

(2) The upper nappe comprises the Valpelline zone and the II-DK (second dioritic kinzingitic) zone, and consists of lower crust gneisses displaying well-preserved pre- Alpine granulite-facies assemblages (Argand, 1934; Dal Piaz et al., 1971; Pognante et al., 1988). These rocks are quite similar to those of the Ivrea (Adriatic) lower crust (Rivalenti et al., 1984; Zingg et al., 1990; Rutter et al., 1993). High-pressure metamorphism is only present in the II-DK zone where it reaches high-grade blueschist facies.

The Vanzone and Boggioletto backfolds affect the Austroalpine nappes, as well as the underlying Piemont, Brianconnais and European crusts.

The Canavese zone, represented by a tectonically thinned zone of crustal basement and cover rocks, forms an independent unit between the exhumed Aus- troalpine system and the Southern Adriatic Alps. Rare serpentinized peridotite and metabasalt lenses suggest that a Canavese oceanic lithosphere has existed. The

Canavese zone is limited to the northwest by an important thrust surface: the Canavese Line.

The Ivrea zone represents a unique cross-section through the Adriatic lower continental crust (Fig. 1). It is here that the Southern Alpine Moho comes closest to the present erosional surface. The lower crust is com- posed of granulite-facies pre-Alpine gneisses containing slices of mafic and ultramafic rocks (Schmid, 1967; Bertolani, 1968; Steck and T&he, 1976; Zingg et al., 1990). Alpine greenschist-facies metamorphism is only observed along some isolated shear zones and along the Canavese Line. The present position and the subvertical to overturned dips of the Canavese and Ivrea rocks are the result of Tertiary ductile backfolding associated with shear zones, because ‘brittle’ backthrusting alone could not have caused the observed orientations and dips of the gneisses.

Flysch and molasse deposits

Flysch-type sediments are, by definition, marine sediments deposited in tectonic active regions. Their age varies from Middle Cretaceous in the southeast to Late Eocene in the northwest. These flysch basins thus migrated from the southeast to the northwest, together with the advancing front of deformation (Fig. 2). During the Early Oligocene the flysch sedimentation changed into molasse type (mostly continental) in both northwest and southeast frontal basins. This coincided with the first backward movements in the southeast.

OUTLINE OF THE EVOLUTION OF THE WESTERN SWISS-ITALIAN ALPS

Following proposals of Dal Piaz et al. (1972) Triimpy (1973, 1980), Debelmas et al. (1980) Hunziker and Martinotti (1984) Hunziker et al. (1989, 1992), Steck and Hunziker (1994), Escher et al. (1997), the erogenic history of the Western Swiss-Italian Alps can be divided into three main periods of tectono-metamorphic activity (Fig. 2).

(1) The Eoalpine erogenic events, Cretaceous-Early Paleocene in age with a peak of metamorphic pressure reached at about 110 Ma and a temperature peak at 85 Ma. These events are characterized by the formation of high-pressure mineral assemblages.

(2) The Mesoalpine erogenic events, which we propose

Basement nappes in the Swiss-Italian Alps 959

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