The Tertiary structural and thermal evolution of the Central Alps—compressional and extensional structures in an orogenic belt

Tectonophysics 238 (1994) 229-254

The Tertiary structural and thermal evolution of the Central Alps-compressional and extensional structures in an erogenic belt Albrecht Beck, Johannes Hunziker Institut de Minbalogie et PGtrographie,Universite’de Lmsanne, BFSH-2, 1015 Lausanne, Switzerland Received 24 April 1993; accepted 7 January 1994

Abstract The Western Alpine Arc has been created during the Cretaceous and the Tertiary orogenies. The interference patterns of the Tertiary structures suggest their formation during continental collision of the European and the Adriatic Plates, with an accompanying anticlockwise rotation of the Adriatic indenter. Extensional structures are mainly related to ductile deformation by simple shear. These structures developed at a deep tectonic level, in granitic crustal rocks, at depths in excess of 10 km. In the early Palaeogene period of the Tertiary Orogeny, the main Tertiary nappe emplacement resulted from a NW-thrusting of the Austroalpine, Penninic and Helvetic nappes. Heating of the deep zone of the Upper Cretaceous and Tertiary nappe stack by geothermal heat flow is responsible for the Tertiary regional metamorphism, reaching amphibolite-facies conditions in the Lepontine Gneiss Dome (geothermal gradient 2YC/ km). The Tertiary thrusting occurred mainly during prograde metamorphic conditions with creation of a penetrative NW-Se-oriented stretching lineation, X, (finite extension), parallel to the direction of simple shear. Earliest cooling after the culmination of the Tertiary metamorphism, some 38 Ma ago, is recorded by the cooling curves of the Monte Rosa and Mischabel nappes to the west and the Suretta Nappe to the east of the Lepontine Gneiss Dome. The onset of dextral transpression, with a strong extension parallel to the mountain belt, and the oldest S-vergent “backfolding” took place some 35 to 30 Ma ago during retrograde amphibolite-facies conditions and before the intrusion of the Oligocene dikes north of the Periadriatic Line. The main updoming of the Lepontine Gneiss Dome started some 32-30 Ma ago with the intrusion of the Bergell tonalites and granodiorites, concomitant with S-vergent backfolding and backthrusting and dextral strike-slip movements along the Tonale and Canavese Lines (Argand’s Insubric phase). Subsequently, the center of main updoming migrated slowly to the west, reaching the Simplon region some 20 Ma ago. This was contemporaneous with the westward migration of the Adriatic indenter. Between 20 Ma and the present, the Western Aar Massif-Tote culmination was the center of strong uplift. The youngest S-vergent backfolds, the Glishom anticline and the Berisal syncline fold the 12 Ma Rb,/Sr biotite isochron and are cut by the 11 Ma old Rhone-Simplon Line. The discrete Rhone-Simplon Line represents a late retrograde manifestation in the preexisting ductile Simplon Shear Zone. This fault zone is still active today. The Oligocene-Neogene dextral transpression and extension in the Simplon area were concurrent with thrusting to the northwest of the Helvetic nappes, the Prealpes (35-15 Ma) and with the Jura thin-skinned thrust (11-3 Ma). It was also contemporaneous with thrusting to the south of the Bergamasc (> 35-5 Ma) and Milan thrusts (16-5 Ma). 0040-1951/94/$07.00 D

0 1994 Elsevier Science B.V. All rights reserved

A. Steck. J. Hurniker / Tectonophysics 238 (1994) 229-254

230

The Oligocene-Neogene dextral transpression in the Central Alps is of the same age as the thrusting to the west and southwest in the external (French) part and to the east in the internal (Italian) part of the Alpine Arc of the Western Alps.

1. Introduction

data used in our publication is given by Hunziker et al. (1992). This work represents a synthesis of existing as well as recent data. Our tectonic model for the Alps uses the following principal groups of deformational structures of Debelmas et al. (1980): (1) Cretaceous structures; (2) Palaeogene structures; (3) Oligocene-Neogene and Quatemary structures. Following the.proposal by Dal Piaz et al. (1972) and Hunziker et al. (1989), the terms Eoafpine for the Cretaceous structures, Mesoulpine for the Eocene-Early Oligocene structures and Neoalpine for the Late Oligocene and younger tiructures are used (Table 1). We distinguish only one

The purpose of this study is to describe the structural and thermal evolution of the Central Alps in the framework of the Alpine Orogeny (Fig. 1). Different types of extensional structures have been developed on a deep tectonic level of the Alps during the Tertiary continental collision. The tectonic model for the Central Alps is based on temporal relations between deformations1 structures, metamorphic crystallization, intrusion of porphyritic and pegmatitic dikes (Steck, 1990) and radiometric mineral ages (Hunziker et al., 1992). A list of references for the radiometric

Table 1: Clwot~ology

of the Tertiary Alpine sWtures

of the Central Alps Activity Ages

Ma 40

DIV

D III

11 - 3

Jura thrusting

35

25

30

20

(Ma) 15

_____J~_________

11 - 0

Rh@

30-O

To&c

- SiaDplon Line

30 - 0

J..epontine c3cis8 Dome

Lcpotltinc Dome

30 - 0

Backthrurting

clluve& __---___ _ - _ _ _ _ _ _ _ _ ?

30 - 0

B~ckfoldittg s-SEvugmt

-- Vtpzarc --__ __ c*sinl _____

42 - 29

Cdc+&bc intrusions

Lii

5

10

___&&

(doxti)

____ ____ tit& ncim,

0

_ _ _ _ _ _ _ _

-Rb6llc-shnplanLine____________ __-__ __ ? ______? _____

hc _______________________________________ Tme Me ____________________________________

Bergeli- ?

________~lo_______________ br@ Bietir Dikes

35 - 30

Backfolding

D II

35 - 11

Simpbn

DI

40-O

Nwdwtild

Sbr

Gji*

+ B&d

Novate

vcFzzuIc& Mb&r, Waadftubborn Zone

-____----____ - ___ - - ___ __ ___ __ _ _________________ ___

p___?____ peae&s _--_ ?____~.___?*_ I$elwGa

----___----_ ___--____________ _________________ ___?

? --__-~~__I___________--___MeoALpINE________________ ____-__-___-

~____________

(D I - D IV arc. four types of ducfik

I

___oli~____

I

_________l@Jce#&__________

shear zones maponaibk for rock extension. Fig. 3.)

IpliQI

__--____ _

A. S&d, .f. Hmiker

/ Thmcpkysks

238 (1!W4j 229-254

231

232

A. Steck, J. Hunxiker / Teciorwp&sics 238 Il994) 22%.254

MOLASSE a

HELVETICS

Fig. 2. Distribution of the Cretaceous high-pressure metamorphism, after Pfeifer et al. (1991).

Tertiary metamo~hism in the Lepuntine Gneiss Dome. The Mesoalpine metamorphism passes gradually into the Neoalpinrs! metamorphism. In the Alpine steep belt north of the Insubric Line, the Mesoalpine and NeoaIping structures may be dist~guished by the Oligocene ma~matic dikes of the Periadriatic Line. 1.1. me crf?tacf?ousstmctclres Pfeifer et al. 09911, H~rne~h~d~ and Frank (1991) and others showed that the mineral parageaesis and their relicts of the Cretacequs high-pressure metamorphism are widespread in the Penninic and part of the Austroafpine zone of the Central Alps (Fig. 2). The Cretaceous Oregeny is characterized by subduction, the closure of the Piemont Ocean, continental coBion and creation of high-pressure m~tam~hism. Mineral cooling curves (Oberhlnsli et al., 1985; Hunziker et al., 1989; Hurford and Hunziker, 19893, observations on time successions of Cretaceous and

Tertiary mineral par~n~is @earth, 195-2; C&mbi, 19891 and preservation of hi&pressure minerals suggest that in the Sesia zone and -Penninic nappes to the west of the Ossoia Valiey the high-pressure rocks were upiifted dad cooied to greenschist-facies temperature co~~~~s before ree~uilibratiun of the isotherms by t&rest&I heat flow. This rapid uplift occurred in the Paleocene, before 60 Ma ago, Different kinematic mocIels explaining this uplift aqd cooiing by a Late Cretaceous-Early Tertiary detachment fault-~are proposed by Platt et al. 11985) and Merle and &&&re C1992). Note that the deformation of the Alp&t: chain during the Cretaceous continental collision is very important and it is often difficult to d&tinguish the older Cretaceous from the. younger Tertiary deformations, as shown by-Ring and Merle 0992& They observe that in the- northern front of the Monte Rosa Nappe a Cretqzeous stretching lineation, formed at temper&ures in excess of SOWC, has been transposed bY_the later Tertiary X, stretching lineation under retrograde

A. Steck, J. Hunziker / Tectonophysics 238 (1994) 229-254

greenschist-facies conditions. Further discussion of the Cretaceous Orogeny lies outside the scope of this paper. The interested reader is referred to Hunziker et al. (1989, 1992).

233

1.2. The Tertiary structures Table 1 shows the chronology of the Tertiary Alpine structures of the Central Alps. In the

D II

Fig. 3. Map of Tertiary schistosities and stretching lineations of the Central Alps, after Steck (1990, Plate 1). Four types of ductile shear zones, Dt-D,,, are distinguished. The shear sense is indicated by arrows. Schistosities and related stretching lineations X,-X, are turned to horizontal. X, has been formed during nappe thrusting and extension in a NW direction. X,, corresponds to the SW-oriented extension direction in the ductile Simplon Shear Zone. The discrete Rhhne-Simplon Line repre ents a late manifestation in this shear zone. Xm indicates the direction of backthrusting along the Canavese Line. The dextral hear zones % Dtv, with their sub-horizontal stretching lineation, have been formed after backthrusting and backfolding and concurrently with the movements on the RhGne-Simplon Line.

234

A. Steck, J. Hunziker / Tectonophysics 238 (I 994) 229-254

Simplon transect, we distinguish four types of ductile shear zones and related stretching lineations (Fig. 3; Steck, 1990). These structures are responsible for extension in the Alps. A first SE-oriented stretching lineation, Xi, is related to the Palaeogene NW-vergent recumbent foldnappes (D, in Table 1 and Fig. 3). This lineation was created during underthrusting of the European Plate below the Adriatic margin by ductile shear of the upper part of the European crust and of the lower part of the Adriatic Plate Urgand, 1916; Heim, 1919-1922; Voll in Nabholz and Vail, 1963; Steck, 1980, 1984, 1987; Malavieille et al., 1984, Lacassin, 1987; Merle et al., 1989; Escher et al., 1993) below the more stable erogenic lid (Laubscher and Bernoulli, 1982: “trameau Ccraseur rigide” of Termier, 1903) formed by the overthrust margin of the Adriatic Plate. The second, SW-oriented stretching lineation, Xn7 is related to a NE-SW extension of the ductile Simplon Shear Zone. This Oligocene and Neogene structure overprints the Palaeogene nappe stacking (Steck, 1980, 1984, 1987, 1990; Mancktelow, 19851990, Mantel and Merle, 1987; Merle et al., 1989). A third stretching lineation, XIII, is related to a SE-oriented backthrusting on the Canavese Line (Schmid et al., 1989). The fourth, E-W- and NE-SW-oriented stretching lineation, XIV, was formed during continuation of the movements of dextral transpression, after the Oligocene backfolding (Steck, 1984, 1990).

2. The Mesoalpine stmctums

of the Centrai A&p8

2.1. The NW-oriented thrusting and regional metamorphism According to Trtimpy (1980), the lower bracket for the onset of the M&oalpine or Palaeogene Orogeny is given by the youngest sedimentary formations present in each thmst belt. These flysch sediments are of up to M&l&e Eocene age in the Perminic units to the south (Ellenberger, 1953; Sartori, 1987) and of Late Eocene to Early Oligocene age in the Helvetic units farther to the

north. These erogenic sediments may be deposited on nappe structures during their formation. Therefore the time of onset of the Palaeogene continental collision may be older and is not exactly known. The Mesoalpine continental collision starts with a SE-oriented underthrusting of the European below the Adriatic Plate, with production of NW-vergent nappes. The main Mesoalpine nappe thrusting was synmetamorphic and the deformation at temperatures above 300°C was ductile in the granitic crustal rocks (Voll, 1976). During the ductile shear of the upper part of the European crust and under prograde metamorphic conditions a strong stretching lineation, X,, parallel to the NW-oriented nappe transport was developed (deformational structures D, in Fig. 3; Heim, 1919-1922; Voll in Nabholz and Voll, 1963; Steck, 1980, 1990; Malavieille et al., 1984; Lacassin, 1987; Merle et al., 1989). Heating by geothermal heat flow of the subducted European upper crust is responsible for the Tertiary regional metamorphism (Fig. 4; Niggli and Niggli, 1965; Wenk and Keller, 1969; Trommsdorff, 1972; Colombi, 1989). This model and the geothermal models of England (19781, Oxburgh and England (1980) and Thompson and England (1984), indicate that the nappe stack responsible for the regional metamorphism by terrestrial heat flow cannot be only of a Palaeogene age. It must be of Palaeogene and Late Cretaceous age if there was no other heat source, such as an elevated magma chamber. That means, that in the Lepontine area the Cretaceous nappe structures remained at higher temperatures and at a certain depth between the Cretaceous and Tertiary thrusting. In the Simplon region, the geothermal gradient is approximately 25”/ km (Frank, 1983; Hammerschlag, 1985; Colombi, 1989). Streckeisen et al. (1974) explained the steep and overturned isograd surfaces of the Simplon area by a thermal dome structure, whereas Milnes (1975) suggested that the steep and overturned isograds of the Tertiary metamorphism of the Simplon area may also be explained by a synmetamorphic ductile thrusting and folding of these isograds. This study favours the latter. The

SOUTHERN

ALPS

-

50 km

RR- BERNINA

Fig. 4. Isograds of the Tertiary metamorphism in the Central Alps. Plagioclase-hornblende isograds after Wenk and Keller (1969) and Colombi (1989), diopside-quartz-calcite isograd after Trommsdorff (1972) and biotite isograd in granitic rocks after Steck and Burri (1971); Ma = Maggia syncline, and Ve = Verzasca anticline after Steck (1990); A-A’, B-B’, C-C’ and D-D’ = location of the geological sections of Figs. 10, 11 and 12.

METAMORPHIC

236

A. Steck, J. Hunziker / Tectonophysics 238 (1994) 229-254

A. Steck, J. Hunziker / Tectonophysics 238 (1994) 229-254

cooling of white micas started on the eastern and western border of the Lepontine metamorphic zone some 38 Ma ago (Fig. 5). These oldest Tertiary white mica ages have been interpreted by Hunziker (1969) as the time of the culmination of the Tertiary metamorphism. In the Lepontine Gneiss Dome, we distinguish only one Tertiary metamorphism. The Mesoalpine metamorphism passes gradually into the Neoalpine metamorphism.

237

2.2. The Lower Oligocene structures 2.2.1. The dextral Simplon Shear Zone In the Central Alps, movements of dextral transpression began after the culmination of the Tertiary metamorphism under amphibolite-facies conditions and before the Oligocene magmatic activity. The most important structure is a zone of ductile shear, with a SW-oriented stretching lineation, X II, that crosses the Alps in the Simplon

OLIGOCENE AND NEOGENE TECTONICS OF THE ALPS

\ 100 km

L

+---Q-

“backfolds”

T %T T -T 7 h.....C

thrust fault thrust fault (sub-surface) strike-slip fault normal fault low angle normal fault SimDlon shear zone

Fig. 6. Oligocene, Neogene and Quatemary structural pattern of the Central Alps. This structural pattern represents the product of the collision of Adria and Europe and the anticlockwise rotation of the Adriatic indenter relative to Europe. The ductile Simplon Shear Zone and the later Rhane-Simplon Line are extensional structures, which are related to dextral transpres&m between Europe and the Adriatic indenter. Ad = Adamello gabbros, tonalites and granodiorites (42-29 Ma); C = Cremosina Line; Be = Bergell tonalites and granodiorites (32-30 Ma); Bi = Biella monzodiorite (30 Ma).

138

A. Steck, .I. Hunziker / Tectonophysics 238 (1994) 229-254

region CD,, in Figs. 1, 3 and 6). This ductile Simplon Shear Zone developed during a long period. Deformation started some 35-30 Ma ago under ductile conditions and remained active in the late discrete RhBne-Simplon Line until the present. This ductile shear zone is 8 km wide and crosses the zone of highest grade (kyanite-sillimanite) of the Tertiary metamorphism between Locamo and Domodossola, north of the Insubric Line (Figs. 1,3 and 7). Further to the west, the zone turns around the younger Vanzone backfold. In the Simplon area it corresponds to a lo-15 km thick low-angle normal shear zone and

changes gradually into a dextral shear zone in the Rhbne Valley and the Rawil Pass region. The z-shaped geometry of the ductile Simplon Shear Zone has been created by the younger phase of backfolding, evidenced by the Vanzone anticline to the south and Berisal syncline and Glishorn anticline to the north (Figs. 8, 9 and 10). This zone was originally dipping with a low-angle to the south (Steck, 1984). East of Locarno, this oldest dextral shear zone and the roots of the Monte Rosa and Sesia zones are cut by the younger discrete Canavese and Tonale Lines created under greenschist-facies conditions (Fig. 1:

Fig. 7. The Central Acl#r,with the Lepontiae Gaeiss Dome and related deformationalstruc1uresof Oligocene-Qwemary &se. Deep structures after the NW20 seismic survey (Marchaat et al., 1993). AA = Au~tr~aiphe~ riappas; AM - & F+&df; e An 1741omMenda isograd Mar We& An-AiliiqorioN~witi ltad grmadbb (32-B CMa&i,1989);AR-~ blah C = Cmnosina Line; G&X= L. = C!mmese L&c; D B-- Dart Bbche Nappe; E = E@&iaa Line, biarksvi& w&if? dots = Zerrk-Sass ophiolkic Nappe; W&t#aMy ruled -Tsat.i optioiitic nappe; black-Antmna ophioiitic nappe; heavy dots = ophiotiticnappesof eastern Switzerland; light dots = Ultr&clWic and VaIaisunits.

A. Steck, J. Hunziker/ Tectonophysics238 (1994) 229-254

A. Colombi and H.-R.+ Pfeifer, pers. ~~un., 1993). It is important to note that during dextral shear in the Simplon Shear Zone, late thrusting of the Aar Massif front and the Helvetic nappes occurred simultaneously (Steck, 1984; Lacassin, 1989). Therefore with a constant NW-SE orientation of the greatest normal stress, the little normal stress is approximately horizontal in the dextral shear zone and vertical in the zone of thrusting.

2.2.2. Early S-vergenr folding In the Lepontine region, “‘backfolding” starts under amphibolite-facies conditions and continues during retrograde metamorphic conditions with the creation of the Verzasca anticline, Maggia syncline, the Saimone anticline and the Wandfluhhom antiformal syncline. These S-vergent folds deform the Penninic nappe stack (Figs. 7 and 12). The trace of the labradorite (An 50)hornblende isograd envelops the Maggia syncline (Fig. 4; Wenk and Keller, 1969). This geometric relationship may be explained by two different processes, which probably have acted simultaneously. Wenk and Wenk (1984) discussed the model of an elevated (anisotropic) heat flow parallel to the zone of subvertical schistosity of the Maggia structure. An other ~~ibili~ is a s~metamorphic folding during high-grade metamorphism concurrent with erosion and cooling. In the second case the Verzasca anticline is more rapidly cooled down than the underlying Maggia syncline. Early cooling below 500°C of white micas, about 26 Ma ago, in a zone that corresponds to the crest line on the upper limb of the Verzasca anticline may be an argument for the second model (Fig. 5). We distinguish two cases, either the Verzasca and Maggia folds formed during the peak of amphi~lite-facies grade metamo~hism or in the second case during cooling after the peak of metamorphism and near amphibolitegreenschist facies transition. Note that the Bosco thrust, suggested by Merle et al. (19891, cutting the hinge of the Wandfluhhom recumbent fold does not exist (Hunziker, 1966; Plate III in JHger et al., 1967; Hall, 1972; Steck et aI., 1989).

239

3. Oligocene magmatic activity Oligocene magmatic intrusions are widespread along the Periadriatic Line. The most important intrusions are the Biella, the Bergeil and the AdamelIo bodies, as well as widespread trachyandesitic dikes (Table 1, Figs. 6 and 7). The geochemistry of the Oligocene magmas indicates a mantle origin and a strong crustal contamination (Beccaluva et al., 1983; Reusser, 1987). Most of them intruded in a very short time interval between 31 and 29 Ma (Dal Piaz et al., 19881, suggesting an extensional event at this time. The Adamello and Bergell intrusions are situated in the zone of finite extension between conjugate strike-slip faults. The ages from the composite Adamello intrusions vary between 42 to the southwest and 29 Ma to the northeast, with the younger intrusions in the northeast comer, next to the intersection point of the dextral Tonale and sinistral Giudicarie Lines. KlStzli (1988) found a K-Ar biotite age of 29 Ma for a porphyritic (rhyo~itic) dike from the Loana Valley. Monazites and xenotimes of two muscovitepegmatite dikes from the Centovalli yield an age of 26 Ma (Urs Schiirer, pers. commun., 1993). The extensional event of the Periadriatic Line is probably temporatly related to the Oligocene extension in the European crust, d~umented by a phase of opening of the Rhine, Saline and Limagnes graben systems (Laubscher, 1983; Bergerat, 1987; Le Pichon et al., 1988). The Oligocene dikes provide excellent temporal discrimination between the Mesoalpine and Neoalpine structures (Table 1). In the Alpine root zone between Locamo and Domodossola, these dikes cut the older horizontal amphibolitefacies stretching lineation of the ductile Simplon Shear Zone. AI1 the dikes are overprinted by dextral shear under greenschist-facies conditions, related to the movements of the young and dextral Tonale, Canavese and Centovalli strike-slip fault system. The Tonale strike-slip fault (Heitzmann, 1987) is a younger discrete fault, formed under greenschist-facies conditions, which cuts the older amphi~lite-facies grade dextral shear zone at Locamo (Fig. 1; Steck, 198’7, 1990).

240

A. Beck* 1. Hunziker f Tectonaphysics 238 (1994) 229-Z%

SOUTHERN

ALPS

llO-60Ma 40-1OMa

A *

LEGENDE

Fig. 9. Rb-Sr biotite ages and isochrons. These indicate cooling ages below a temperature of about 300°C of the Lepontine Gneiss Dome during the Oligocene and Neogene uplift, erosion and tectonic unroofing.

Rb-Sr Biotite Ages

242

A. Sreck, J. Hunziker / Tectorwphysics 238 (19941229-254

4. The Neoalpine structum

On the local scale of the Central Alps, dextral transpression in the Simplon region, NW-vergent folding and thrusting in the Aar Massif, the Helvetic nappes and the Preafps and S-vergent folding and thrust~ug along the fnsubric Line, the Bergamasc and Milan belts were concurrent (Figs.

6 and 7). During uplift, erosion and cooling of the Lepontine Gneiss Dome, the ductile SimpIon Shear Zone was migrating to the deeper and hi~b-temp~rat~e part of the Lepontine region. At the same time the diffuse ductile deformation of the high-grade metamorphic rocks is gradually replaced by discrete (still ductile) shear zones formed under retrograde metamorphic conditions. The younger retrograde structures are often characterized by S and C fabrics (Berthe et al., 1979).

Fig. 10. Cooling curves for the VaUe d’0ssoia-A~ Massif section (location on map Fig. 4). The Ivrea zone had ~w$xI tu below 3OO“Csome 19Q Ma ago. The q@ng curve W .Cr~~ppo(Monte Rosa Nappe) shows the rapid uplift and er&on since 30 Ma related to backlbkding and backthrt@ing (Argabd’s Iksubt’ic phase). Cblin$ rrCthe Brig region started some 20 Ma ago, f&owed by the Gondo area. Note the strong variation of the ~wling rate in the Brig and Gondo cooling cmves.

243

A. Beck, J. Hunziker / Tectonophysics 238 (1994) 229-254

The alpine nappe stack is overprinted by the backfolds related to Argand’s Insubric phase such as the Vanzone and Mischabel backfolds to the south and the Glishom anticline and Berisal syncline to the north. The different backfolds do not have the same age and are formed over a long period (Table 1). This backfolding is contemporaneous with or followed by dextral shear. Movements on the Canavese Line, the southern limb of the Monte Rosa Nappe and the LorancoZermatt Line correspond to oblique dextral underthrusting in the overturned limbs of the Boccioletto, the Vanzone and the Mischabel anticlines (Figs. 3, 6, 7 and 10). In Valtournanche south of TEMPERATUREw

2 MONTE ROSA

NEOALPINE

Zermatt and in the Aosta Valley the dextral underthrusting passes gradually into W-oriented underthrusting and to the well-known badkthrusting of the Western Alpine Arc (Caby, 1968; Debelmas and Kerckhove, 1980; Tricart, 1984; Platt et al., 1989). 4.2. The Rh&ze-Simplon Line The discrete RhGne-Simplon Line develops during this late period of dextral transpression. This structure represents a dextral strike-slip fault north of Domodossola and passes gradually into a low-angle normal fault in the Simplon area (Figs.

a. 4GONDO

38RlGOANTER

CALC-ALKALIC MESOALPINE

9 W. BERGELL

\\

II

8 N. SURETTA

COOLING HISTORY OF THE LEPONTINE ALPS Fig. 11. Cooling curves for Monte Rosa-Bergell sections (locations on map Fig. 4). Rapid cooling between 38 and 2E Ma of the Monte Rosa and Siviez-Mischabel units may be related to updoming during the early dextral transpression. Cooling of the Lepontine Gneiss Dome related to updoming and erosion started some 30 Ma ago in the Bergell area and migrated tolthe Simplon region, where rapid cooling started some 20 Ma ago.

244

A. Sfeck, J. Hmziker / Tectonophysics 238 (19941 229-254

h

l

A. Steck, J. Hunziker / Tectonophysics 238 (1994) 229-254

1, 3, 6, 7 and 8). Near the Simplon Pass, this fault cuts the Berisal syncline and the Glishorn anticline (Bearth, 1956; Hunziker and Bearth, 1969; Steck, 1980; Merle et al., 1986; Mancktelow, 1990) and is represented farther to the west in the Rhane Valley by a set of dextral strike-slip faults (Steck, 1984, 1987, 1990; Mancktelow, 1990; Sartori, 1993). One branch of the strike-slip faults continues into the Chamonix syncline, between the Aiguilles Rouges and the Mont Blanc Massifs. Gourlay (1984) showed that the fault in the Chamonix syncline acts in this late phase as a zone of oblique SW-oriented overthrusting. 4.3. The Lepontine Gneiss Dome During the oblique collision of Europe and Adria, a pull-apart structure was created, as evidenced by the Valpelline depression to the southwest and the Aar-Tote culmination to the east of the Simplon Pass (Figs. 1, 6, 7 and 8; Steck, 1990). The Aar-Tote culmination represents the western and youngest part of the Lepontine Gneiss Dome. The geometry of this gneiss dome, composed by the Ticino sub-dome to the east and the Tote sub-dome to the west, is well described by Wenk (1955, 1970) and Spicher (1972; Fig. 1). The main uplift of the Lepontine Gneiss Dome starts with the intrusion of gabbros, tonalites and granodiorites of a mantle source in the Bergell area, some 32-30 Ma ago (Villa and Von Blanckenburg, 1991). The development of the Lepontine Gneiss Dome is well documented by the Rb-Sr biotite cooling ages, corresponding to a closure temperature of about 300°C (Fig. 9) and the cooling curves represented on Figs. 10 and 11. Rapid cooling of the gneiss dome started some 30 Ma ago along the Tonale and Canavese Lines. The 32-30 Ma old Bergell tonalite intruded at a depth of about 17 km. It cooled to 300°C some 28 Ma ago by a rapid uplift of about 7 km in 2 Ma (uplift rate N 3.5 mm/a> concomitant with erosion (Reusser, 1987). Subsequently, the zone of rapid cooling has gradually migrated to the west, reaching the Simplon region some 20 Ma ago (Hunziker et al., 19921, probably parallel with the E-W migration of the Adriatic promontory and the development of the dextral Tonale and

245

Canavese Lines (Schmid et al., 1989; Steck, 1987, 1990). The youngest biotite cooling ages of 10 Ma are reported in the western Aar Massif (Aar-Tote culmination) (Hunziker and Bearth, 1969; Hunziker et al., 1992). In the Simplon region, the SE-vergent Glishorn anticline and Berisal syncline fold the 12 Ma biotite isochron. These folds are cut by the younger discrete Rhone-Simplon Line (Figs. 9 and 10). The chlorite-biotite isograd in gneissmylonites of the Rhone-Simplon Line can not be younger than the 11 Ma biotite cooling age at the same place (Aubry, 1984; Steck, 1984). Therefore the folding of the Glishorn-Berisal structure and the younger RhGne-Simplon Line are not very different in age. Schmid et al. (1989), Baudin and Marquer (1993) and Froitzheim (1992) observed E-dipping low-angle extensional faults on the eastern border of the Lepontine Gneiss Dome (Figs. 7 and 12). These faults are interpreted as extensional structures related to a late period of updoming of the Lepontine Gneiss Dome. 4.4. Age, rate and mechanisms Lepontine Gneiss Dome

of cooling of the

The cooling curves represented in Figs. 10 and 11 are based on the closure-temperature concept, assuming the following closure temperatures: N 690°C for U-Pb monazite 500°C for Rb-Sr white mica, 350°C for K-Ar white mica, 300°C for Rb-Sr and K-Ar biotite, 225°C for zircon fission tracks and 120°C for apatite fission tracks (Hurford, 1986; Hunziker et al., 1992). In the Central Alps the cooling is irregular. Periods of rapid cooling alternate with periods of slow cooling. The cooling of the Lepontine Gneiss Dome is well illustrated in Fig. 12. Note the converging time gradients of mineral cooling ages on both the eastern and western sides of the Ticino and Tote culminations and the RhBne-Simplon Line. Note also the strange behaviour of the U-Pb monazite ages, in Fig. 12 always younger than Rb-Sr white mica ages, suggesting a closure or crystallisation temperature around 450°C (Kiippel et al., 1980) and not around 690°C a$ normally assume (Hunziker et al., 1992). This anomalous behaviour could be explained by a strong water-

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rock interaction during Miocene times. The cooling related to updoming started after the peak of the Tertiary metamorphism some 38 Ma and the cooling related to normal faulting on the RhGne-Simplon Line some 20 Ma ago. The cooling was very rapid after 30 Ma on the eastern border of the Bergell intrusion and along the Insubric Line and in the Monte Rosa Nappe, followed by rapid cooling of the Verzasca anticline after 25 Ma and very rapid cooling of the Ticino culmination after 22 Ma. Rapid cooling reaches the Gondo region in the foot&all of the

Rhone-Simplon low-angle normal fault some 20 Ma ago, in an early stage accompanied by an accelerated cooling in the hanging wall of the RhBne-Simplon Line. The rapid cooling in the footwall of the RhBne-Simplon Line continues until about 10 Ma, with a second period of rapid cooling between 12 and 10 Ma. This second phase is related to the formation of the discrete Rhone-Simplon Line. This period is followed by slow cooling between 10 and 3 Ma and very rapid cooling after 3 Ma, concomitant with ranid cooling of the Tote culmination.

Fig. 13. Apatite fission track isochrons, corrected for an elevation of 1000 m above sea level, after Soom (1990). Note that the 11 Ma old discrete Rhbe-Simpkm Line remains active during the last 5 Ma.

A. Steck, J. Hunziker/ Tectonophysics238 (1994) 229-254

The late cooling after 5 Ma is documented for the block north and east of the Rh8ne-Simplon Line by the apatite fission track ages. The map of Fig. 13 shows the data of apatite fission track isochrons, corrected for a height of 1000 m above sea level, after Soom (1990). This map illustrates the cooling below 120°C during the last 15 Ma. The strong age gradient perpendicular to the Rh8ne-Simplon Line shows that this structure has been active during the last 5 Ma. Between Visp and Sierre, Soom determined a relative vertical uplift rate of the northern block on the Simplon normal fault varying between 0.5 and 0.7 mm/a during the last 5 Ma.

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The results of 80 years of high-precision levelling measurements by the Federal Service of Topography from Bern, show that the Region of Visp has the highest relative uplift rates of 1.5 mm/a with respect to an arbitrarily chosen zero point near Aarburg (Fig. 14; Gubler et al., 1981). Between Fiesch in the upper RhGne Valley and Brig, the highest uplift rates are coincident with the trace of the axial surface of the still active Glishorn anticline. From Brig to Sion, highest uplift rates follow the Rhane-Simplon Line, a region of abundant seismic activity. The fault plane solutions of earthquakes identified by Pavoni (1980) for this region, suggest E-W-ori-

&CENT I

: I

I

CRUSTAL MOVEMENTS after

FEDERAL OFFICE OF TOPOGRAPHY, 1981 I

Fig. 14. Curves of iso-uplift rates for the Central Alps for the last 80 Ma relative to an arbitrarily chosen 0 point near Aarburg, after Gubler et al. (1981).

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A. Reck. f. Nunziker / Tectonophysics 238 (I 994) 229-254

ented dextral strike-slip movements. The relative uplift of the Glishorn anticline and the related southwestern end of the Aar Massif is due to the 20” southwest dip of the strike-slip movement direction. These observations show that the dextral RhBne-Simplon Line and the Glishorn backfold are still active today. Geo-thermometric and -barometric studies by Frank (19831, Hammerschlag (1985) and Colombi (1989) indicate an approximate geothermal gradient of 25”C/km for the high-temperature Verampio gneisses in the Tote culmination. The gradient was probably in excess of this value during uplift and erosion, after nappe emplacement. These results indicate that 24 km of overburden have been eroded since the peak of metamorphism at 38 Ma. This yields an average uplift and erosion rate of 0.63 mm/a. The suggested cooling rates (Fig. 12) and a geothermal gradient of 25”C/ km indicate uplift and erosion rates between 0.2 and 2.2 mm/a in the Tote culmination. These values are similar to those observed today in the Central Alps (Fig. 14; Gubler et al., 1981). There are two mechanisms responsible for the cooling of the Lepontine Gneiss Dome after peak temperature conditions: (1) updoming, backfolding and erosion of the ductile high-grade metamorphic rocks related to dextral transpression in front of the Adriatic indenter; (2) after 20 Ma, tectonic unroofing along the SW-dipping Simplon low-angle normal fault and along the E-dipping low-angle normal faults on the eastern border of the Lepontine Gneiss Dome. The formation of the Lepontine Gneiss Dome during longitudinal extension and NW-SE compression was certainly facilitated by the presence of highly ductile hot crustal rocks in the lower part of the alpine nappe stack, with temperatures above 600°C as suggested by Wenk (1970) and demonstrated by laboratory models by Van Den Driessche et al. (1993). The normal faulting after 20 Ma was facilitated by the inclined limbs of the dome structure and truncation of the gneiss dome by the erosional surface. This mechanism of tectonic unroofing has been suggested in the thermal models by Hunziker (1969) and Mancktelow (1985) and for the Brenner Line in the Eastern Alps by Selverstone (1988).

5. Estimation of the relative dextral strike-slip displacement of the Adriatic and European Plates during the Mesoalpine and Neoalpine periods of the Tertiary Orogeny The displacement to the southwest of the Dent Blanche unit along the ductile Simplon Shear Zone relative to the Verampio gneiss is greater then 40 km and probably of the order of 80 km. 40 km is the displacement distance obtained from a simple shear model (Ramsay, 1967) for an extension value 1 + e = 4 and a shear zone thickness of 10 km. The extension value has been measured by deformation markers in the more competent conglomerates and granitic gneisses and, if the deformation in the Mesozoic calcschists is much stronger, yields a distance of displacement of the order of 80 km (Steck, 1980). Heitzmann (1987) measured a distance of 60 km for the deformation of the Bergell intrusives by dextral strike-slip movements on the Tonale Line (Figs. 1, 6 and 7), which was active after 30 Ma. In the case of the 11 Ma old discrete Simplon low-angle normal fault, Hunziker (1969) and Mancktelow 0985) calculate displacements of approximately 12 km parallel to the 30” SW-dipping extension direction.

6. The Neoalpine structures at the scale of the Western Alpine Arc During the Tertiary convergence and dextral transpression of the European and Adriatic Plates, several types of structures develop simultaneously by strain partitioning, creating the complicated interference pattern of the Western Alps. After the Oligocene-Neogene and Quaternary periods of continental collision, the Alps represent a patchwork of domains (fields) characterized by a combination of the following different types of deformation (Figs. 3, 6 and 7). (a) Dextral shear: Simplon Shear Zone and Rh&e-Simplon Line, Tonale-northern Canvese, Centovalli and Cremosina Lines, southern Belledonne and Pelvoux regions. (b) E-W extension: Brenner Line.

A. Beck, .l. Hunziker / Tectonophysics 238 (1994) 229-254

(c) NE-SW extension: ductile Simplon Shear Zone and Rhone-Simplon Line. (d) Sinistrul shear: Giudicarie and Engadine Line, Austroalpine nappes, Prealps, Jura. (e) NW-uergent thrusts: Helvetic nappes, Prealpes and Jura. (f) W-uergent rhmsts: Dauphin6 zone of Western Alps. (g) SW-uergent tmsrs: Parpaillon nappe and Digne thrusts. (h) S-verged thrusts: Bergamasc and Milan belts. (i) SE-uergent thrust: Canavese Line. (i> E-uergent th rusts: Penninic zone of Western Alps. The contemporaneous deformations of the different structural domains (with exception of the extensional structures) produce a thickening, uplift and erosion of the crustal rocks of the Alpine Arc as well as subsidence and sedimentation in the Molasse foreland basins of Switzerland and the PO Valley (Matter et al., 1980; Triimpy, 1980; Menard, 1988). The erogenic thickening is partly compensated by extension in ductile shear zones: extension in the ductile thrust zones and NE-SW extension in the Simplon region and on the Brenner Line on the western border of the Tauern window. The structural pattern of the Central and Western Alps may best be explained by a NW-SE compression between the colliding European and Adriatic Plates, concomitant with an anticlockwise rotation of the Adriatic Plate relative to Europe (Gidon, 1974; Tapponnier, 1977; Steck, 1984, 1990; Menard, 1988; Vialon et al., 1989). Our new interpretation of Lowrie’s (1986) palaeomagnetic study of the Adriatic Plate, suggests that the anticlockwise rotation of this plate is of about 17-25”. The Southern Alps are not rotated. They have been accreted in the Late Cretaceous on the Central Alps and detached on the Bergamasc and Milan thrusts from the Adriatic Plate during the Tertiary compression.

7. Summary and conclusions The complex structural pattern of the Western Alpine Arc represents the result of the Creta-

249

ceous and Tertiary orogenies (Figs. 1, 6 and 7). Structural, metamorphic and chronological investigations suggest that the Tertiary structures have been created during the collision of the European and the Adriatic Plates, with anticlockwise rotation of the Adriatic indenter. The preexisting geometry of the European and Adriatic plate margins represents important boundary conditions. The main Tertiary nappe emplacement by thrusting to the northwest of the Austroalpine, Penninic and Helvetic nappes occurred during the Mesoalpine period of the Tertiary Orogeny (Table 1). Thrusting occurred mainly during prograde metamorphic conditions with creation of a penetrative NW-SE-oriented stretching lineation, Xi, parallel to the direction of extension by simple shear. Heating of the deep zone of Tertiary nappe stacking by geothermal heat flow is responsible for the regional metamorphism. Early cooling some 38 Ma ago, related to updoming and erosion, is recorded by the cooling curves of the Monte Rosa and Mischabel nappes to the west and the Suretta Nappe to the east of the Lepontine Gneiss Dome. In the Lepontine Alps, the onset of dextral transpression and the early S-vergent “backfolding” took place some 35-30 Ma ago during retrograde amphibolitefacies conditions and before the intrusion of the Oligocene dikes of the Periadriatic Line. The main updoming of the Lepontine Gneiss Dome started some 32-30 Ma ago with the intrusion of the Bergell tonalites and granodiorites, concomitant with S-vergent backfolding and backthrusting and dextral strike-slip movements along the Tonale and Canavese Lines (Insubric Line, Argand’s Insubric phase). The center of the updoming migrated slowly to the west, arriving at the Simplon region some 20 Ma ago, concurrent with the migration to the west of the Adriatic indenter. Between 20 Ma and the present, the Western Aar Massif-Tote culmination experienced uplift related to normal fault extension along the ductile Simplon Shear Zone and the younger Rhone-Simplon fault. The Brig region is characterized by a recent uplift rate of 1.2-1.5 mm/a (Gubler et al., 1981). A steep geothermal gradient of up to 59”C/km is measured in the Simplon railway tunnel (Hayoz and Zuber, 19904.

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A. Steck, .I. Hunziker / Tectonophvsics 238 (1994) 229-254

The youngest S-vergent backfolds, the Glishorn anticline and the Berisal syncline fold the 12 Ma Rb/Sr biotite isochron. The formation of the Glishorn anticline and the related backfolding of the internal Aar Massif is about of the same age as the Jura thrust (Table 11. The Glishorn and Berisal folds are cut by the 11 Ma old RhBneSimplon Line. The discrete RhBne-Simplon Line represents a late retrograde manifestation in the preexisting ductile Simplon Shear Zone. This fault zone is still active today. Two mechanisms are responsible for the cooling of the Lepontine Gneiss Dome. The first is the updoming, backfolding and erosion of the high-grade metamorphic rocks to the north of the Insubric Line, in a zone of longitudinal extension situated in front of the Adriatic indenter. The southwest and E-dipping structures on the western and eastern side of the Lepontine Gneiss Dome are cut during the first process by the erosion surface. They act in a following stage as low-angle extensional normal faults. The resulting second mechanism is the tectonic unroofing by extensional movements on the ductile SW-dipping Simplon Shear Zone. Later movement occurs along the Rhane-Simplon Line and along E-dipping low-angle normal faults on the eastern border of the Lepontine Gneiss Dome, as observed by Schmid et al. (1989), Froitzheim (1992) and Baudin and Marquer (1993). The displacement to the southwest of the Dent Blanche Nappe along the ductile Simplon Shear Zone relative to the Verampio gneiss is greater than 40 km and may be on the order of 100 km. The displacement to the southwest of the hanging wall along the Rhone-Simplon Line is of the order of 12 km. The dextral strike-slip displacement along the Tonale Line is greater than 60 km (Heitzmann, 1987). The Oligocene-Neogene dextral transpression in the Simplon area is contemporaneous with thrusting to the northwest of the Helvetic nappes, the Prealpes (35-15 Ma) and the Jura thinskinned thrust (11-5-O Ma). It is also contemporaneous with thrusting to the south of the Bergamast (> 35-5 Ma) and Milan thrusts (16-5 Ma). The Oligocene-Neogene dextral transpression in the Central Alps is of the same age as the thrust-

ing to the west and southwest in the external (French) part and to the east in the internal (Italian) part of the Alpine Arc of the Western Alps. The Insubric Line (also called the Periadriatic Line) has a complex Cretaceous and Tertiary history. This fault zone consists of the Canavese Line to the west and the Tonale Line to the east of Locarno. Metamorphic muscovites from the Canavese Jurassic limestones, which are incorporated in the Canavese mylonites, have Cretaceous and Tertiary K-Ar ages (Zingg and Hunziker, 1990). Cretaceous ages of 76-60 Ma are found south of Ivrea. The Tertiary ages decrease from 43 Ma near Biella to 20 Ma near Locamo. The ages around 30 and 20 Ma between Rimella in the Sesia Valley and Locarno are related to backthrusting with a down-dip lineation and dextral shear with a subhorizontal lineation. The Canavese Line and the Canavese sediments are folded by the Fela fold to the west of Finer0 (Steck and T&he, 1976; Steck et al., 1989) and by the Boccioletto backfold near Biella (Fig. 7). East of Locarno, the amphiboiite-facies mylonites of the 35-30 Ma old dextral Simplon Shear Zone disappear into the 30-20 Ma old ductile mylonites related to uplift and backthrusting of the Central Alps. These structures are cut by the straight and 80-90” north-dipping Tonale inverse and strikeslip fault (Heitzmann, 1987). This late brittle fault was active after cooling below 300” and after 21 Ma (Fig. 9; Hunziker et al., 1992, fig. 12). The Tonale Line continues to the west in the Centovalli Line (Fig. 1). We conclude with a statement concerning the topic of this special volume: “Late erogenic extension in mountain belts”. Continental collision and thrusting are responsible for crustal thickening in the Alps. The seismic survey of the Swiss (NFP-20) research programs shows that the European crust has been thrust below the Adriatic plate boundary and that the central part of the orogen has been thickened by thrusting and folding (Fig. 7; Marchant et al., 1993). The Moho discontinuity dips uniformly to the southeast, with a depth of 35 km below the Prealps and the front of the Helvetic nappes to the north of the Aar Massif. It has a depth of

A. Steck, _I.Hunziker / Tectonophysics 238 (1994) 229-254

approximately 60 km below the Biella (Bi) and the Bergell (Be) intrusions (Fig. 7). Crustal thickening during the Alpine Orogeny does not allow the simple conclusion that all the deformations have contributed to crustal thickening. In the Central Alps we distinguish different types of extensional structures that have been formed during the same period of continental collision and that are responsible for crustal thinning during ductile deformation. (1) The gneiss folds of the external massifs Aiguilles Rouges, Mont Blanc, Aar and Gotthard and the fold nappes of the Penninic zone were created during ductile deformation. This deformation produces extension parallel to the direction of NW-oriented thrusting and thinning perpendicular to the fold-nappe boundaries and the main schistosity (deformation D, with extension direction X, in Fig. 3). (2) The Alpine nappe stack is cut by ductile zones of dextral strike-slip movements parallel (or sub-parallel) to the colliding plate boundaries. These shear zones are characterized by a W- to SW-oriented extension direction and a shortening perpendicular to the main schistosity (deformations D,, and D,, in Fig. 3). In the two cases, (1) and (21, extension in the movement direction and shortening perpendicular to the schistosity plane correspond to the great axes X and the little axes 2 of the finite deformation ellipsoid formed by the mechanism of simple shear. This mechanism of extension reduces the amount of crustal thickening in the Alpine thrust belt. (3) More difficult to understand is the formation of a pull-apart structure in a zone of dextral transpression, with the Valpelline depression to the southwest and the Aar-Tote culmination and the Lepontine Gneiss Dome to the northeast of the Simplon Line (Figs. 1 and 7). This deformation started some 30 Ma ago and is still active today. The E-W translation of the Adriatic indenter relative to Europe is simultaneously responsible for: (a) the squeeze and updoming of the ductile amphibolite-facies grade rocks of the Lepontine Gneiss Dome and the Gotthard and Aar Massifs; (b) for pushing the Aar Massif and the Alpine foreland to the northwest; and (c) for

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the southwest lateral motion of the block containing the Valpelline depression, limited by the Rhone-Simplon Line to the north and the Canavese Line to the east. These movements are accompanied by SW-oriented extension and thinning in the ductile Simplon Shear Zone and along the discrete Rhone-Simplon Line after 11 Ma (Fig. 1, R-S in Fig. 7, Fig. 9). During these movements the more rigid upper mantle of the Adriatic indenter slides along the uniformly SEdipping surface of the European upper mantle (Fig. 7; Marchant et al., 1993). In the same period of deformation, the external front of the Western Alps was thrust to the west and the internal front to the east. Therefore, all these deformations, including extensional deformation, are related to movements of compression and transpression between the colliding Adriatic and European Plates. The example from the Central Alps suggests that extensional movements related to simple shear must be the major deformational mechanism in the deep and ductile levels of a compressional mountain belt.

Acknowledgements

John Platt, Dan Marshall, Michel SCranne and Pierre Tricart are gratefully acknowledged for critical reviews of the manuscript.

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