(Ultra-) High-pressure metamorphism and orogenesis: An Alpine perspective

Gondwana Research 18 (2010) 147–166

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(Ultra-) High-pressure metamorphism and orogenesis: An Alpine perspective

Marco Beltrando a,⁎, Roberto Compagnoni a, Bruno Lombardo b

a Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Via Valperga Caluso 35, 10125 Torino, Italyb CNR, Istituto di Geoscienze e Georisorse, Via Valperga Caluso 35, 10125 Torino, Italy

⁎ Corresponding author. Tel.: +39 011 6705111.E-mail address: [email protected] (M. Beltra

1342-937X/$ – see front matter © 2010 International Adoi:10.1016/j.gr.2010.01.009

a b s t r a c t
a r t i c l e i n f o
Article history:Received 24 September 2009Received in revised form 26 January 2010Accepted 29 January 2010Available online 6 February 2010

Keywords:OrogenesisWestern AlpsTectono-metamorphic evolutionUltra-high-pressure metamorphismGeochronologyTethyan ocean–continent transition zones

The Pressure–Temperature–time–deformation evolution of the high- to ultra-high-pressure units of theWestern Alps has been progressively refined in the last 40 years, leading to several paradigm shifts in theunderstanding of orogenesis. This set of information, combined with Cretaceous–Tertiary plate kinematicreconstructions and Mesozoic palaeogeography, indicates that the Western Alps represent the amalgamationof a Cretaceous and an Eocene orogen, which developed at the expense of the Adriatic and European riftedmargins, respectively. In the Cretaceous, NNE-directed drift of the Adriatic plate, parallel to the inheritedJurassic structural trends, led to the development of a highly oblique subduction boundary. In this setting,the Canavese Zone and parts of the Sesia Zone, derived from the hyper-extended Adriatic margin, underwentdeformation and metamorphism at conditions ranging from sub-greenschist to eclogite facies. A LowerEocene switch to NNW-directed motion, perpendicular to the rift-related Jurassic structural trends,culminated in the collision between the proto-Alps and the Briançonnais block at ca. 44 Ma. As a result, theBriançonnais Domain and parts of its hyper-extended margin, preserved in the (U)HP Piemonte Units, wereaccreted to the orogen. Continued convergence was subsequently accommodated by subduction of the moreexternal Valaisan basin and of the thinned European margin. The arrival of thick European continental crustat the subduction zone at ca. 35–30 Ma marked the onset of the final continental collision.To a first order, the orogen grew through the progressive episodic accretion of units located towards north-westerly positions. Accretion of subducting units at the front of the orogen was coeval with kinematicreworking of tectonic contacts in the hangingwall units, locally resulting in renewed deformation/metamorphism.

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1. Introduction

In the last two centuries, several theories on the formation ofmountain belts originated from the European Alps (see Dal Piaz,2001a for a review). According to early fixist theories, the orogenformed in response to vertical movements triggered by differentialcooling/heating of the Earth's crust (e.g., Elie de Beaumont, 1852). Inlater studies, the role of large-scale horizontal movements wasrecognized, and the concept of thrust was introduced (Bertrand,1884). The dualism between fixist and mobilist theories ended infavour of the latter as continental drift was applied to the Alps byArgand (1924), who proposed that convergence between an “Africanpromontory” (Adria) and Europe led to the formation of the AlpineOrogen. As plate tectonics became widely accepted in the early 1970s,this view was confirmed and refined (e.g., Ernst, 1971; Dal Piaz et al.,1972).

Since then, the ever-increasing data set has led to several paradigmshifts in orogenic theories, and many turning points can be identified.

ndo).

ssociation for Gondwana Research.

Ernst (1971) was the first to ascribe the presence of high-pressurerocks in the Western Alps to the descent of a lithospheric plate intothe mantle. Subsequently, the recognition of coesite in the Brossasco–Isasca continental Unit (Chopin, 1984) and in the Lago di Cignanaoceanic Unit (Reinecke, 1991) indicated that both continental andoceanic crust could be buried to ultra-high-pressure (UHP) depthsand then exhumed back to the surface. Up to the mid-1990s, tectonicburial was thought to have occurred at the same time in all HP units atca. 120–100 Ma, before being followed by very slow exhumation andre-equilibration at greenschist facies conditions in the 45–30 Mainterval (e.g., Hunziker et al., 1992). This two-stage view oforogenesis, involving an early high-pressure or UHP event for allunits, followed by slow exhumation through the activity of reversefaults and nappe tectonics, has been superseded by subsequentresearch. It has now been convincingly demonstrated that high-pressuremetamorphism occurred diachronously, from the Cretaceousto the Eocene–Oligocene boundary, in the different units thatconstitute the axial zone of the Western Alpine Belt (e.g., Duchêneet al., 1997; Rubatto et al., 1998). Multi-system geochronology andthermochronology have also shown that in all units metamorphic re-equilibration at high-pressure conditions was followed by rapidexhumation to near-surface conditions, with individual burial–

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148 M. Beltrando et al. / Gondwana Research 18 (2010) 147–166

exhumation cycles lasting as little as 5–10 Ma (e.g., Rubatto andHermann, 2001; Beltrando et al., 2009). These results have led to thenow commonly accepted view of orogenesis as a discrete processrelated to the episodic accretion and exhumation of continental andoceanic units (e.g., Rubatto et al., 1998; Gebauer, 1999; Rosenbaumand Lister, 2005). Rapid exhumation is ascribed to the combined effectof thrusting and erosion, to buoyancy, to the activity of extensionalshear zones, or to a combination of these mechanisms (e.g., Platt,1986; Schmid and Kissling, 2000; Wheeler et al., 2001; Agard et al.,2009). The recent recognition of multiple cycles of Alpine tectonicburial and exhumation recorded in the metamorphic evolution ofindividual rock samples (Beltrando et al., 2007a) points towards

Fig. 1. Tectonic map of the Western Alps (modified from Bigi et al., 1990). A: Acceglio ZoneUnit; C: Combin Zone; ECM: External Crystalline Massifs; F: Furgg Zone; GP: Gran Paradiso MMB: Mont Blanc Massif; MF: Mont Fort Unit; MR: Monte Rosa Massif; MV: Monviso Massif;Serie dei Laghi; SM: Siviez–Mischabel; TPB: Tertiary Piemonte Basin; VA: Vanoise; VO: Voltrilocation of the UHP Lago di Cignana Unit.

further complexities in the evolution of convergent plate margins,which remain to be fully explored.

In this contribution, we review the available literature on thetectono-metamorphic history of the Western Alps, presenting astate-of-the-art overview of the Pressure–Temperature–time–de-formation (PTtd) history of the different units that make up the mosthighly deformed/metamorphosed parts of this limited portion of theAlpine orogenic system. Three well-known regional cross-sectionsare discussed. The historical evolution of the different data sets, withtheir inevitable impact on the orogenic models, is also brieflydescribed. An effort is made to highlight the merits/demerits of theavailable data and the areas in which further research is needed. As is

; AG: Argentera Massif; AM: Ambin Massif; AR: Arolla Series; BI: Brossasco–Isasca UHPassif; IVZ: Ivrea Zone; IZ: Internal Zone; LC: Lago di Cignana UHP Unit; M: Money Unit;P: Pinerolo Unit; PM: Pelvoux Massif; Q: Queyras Schistes Lustrés; R: Rutor Massif; SL:Massif; VS: Valpelline Series; ZH: Zone Houillère; ZS: Zermatt–Saas Zone. Star indicates

149M. Beltrando et al. / Gondwana Research 18 (2010) 147–166

apparent from this contribution, many details of the evolution of theWestern Alps are still debated and several first-order features of thismountain belt remain enigmatic.

2. Geological setting

The Alpine–Himalayan chain is made up of a sequence of orogenicbelts that can be followed for over 18,000 km along strike, from theBetic Cordillera in the Western Mediterranean to New Caledonia and

Fig. 2. Metamorphic map of the Western Alps (modified from Bousquet et al., 2008). Units hrecorded pressure conditions (references in the text). White horizontal dashing indicates unidated units (see explanations in the text). Units with sub-greenschist facies metamorphisPiemonte Units are in white. The slivers of pre-rift sediments located at the interface betweenPT estimates. Traces of cross-sections in Fig. 3 are indicated.

New Zealand in the Southwest Pacific region. The European Alps,where early geological investigations date back to the 18th century,arguably represent the most studied stretch of this extensive orogen(see Dal Piaz, 2001a).

The Alps are a double vergent orogen that developed since theCretaceous as a result of the convergence between Europe and Adria,which is alternatively considered as a promontory of Africa or as anindependent micro-plate (Figs. 1–3; Dewey et al., 1989; Rosenbaumet al., 2002). Convergence resulted in the progressive growth of the

ave been distinguished based on metamorphic grade and age of attainment of highestts for which the age of metamorphism can only be inferred by analogy with comparablem are indicated in white. Outline of continental basement slivers within the eclogiticthe eclogitic and blueschist Piemonte Units have no fillings, due to the lack of published


Alpine Belt in response to the accretion of the rifted margins and ofparts the western Tethys, an oceanic basin that was interposedbetween the two margins (see Sections 6 and 7).

Several domains can be distinguished in the Western Alps on thebasis of lithological associations, type of sedimentary cover, and/orAlpine metamorphism (e.g., Dal Piaz et al., 2003; Schmid et al., 2004;Fig. 1). For the purpose of this paper, the Western Alps are subdividedin three major domains. (1) The first is the Southern Alps, whichconsist of a south/southeast vergent fold-and-thrust belt located onthe Adriatic plate. They display an almost complete section of con-tinental crust, from lower to upper crustal rocks, stratigraphicallycovered by Permian volcanics and Meso/Cenozoic sediments, whichunderwent only minor Alpine metamorphism and deformation.(2) Second is the Axial Belt, comprising both continental units de-rived from the Adriatic and European margins and oceanic unitsoriginated from the Mesozoic Piemonte–Liguria Ocean. The continen-tal basement units derived from the Adriatic margin are referred to as“Austroalpine Units”, while those derived from the European marginand from the Tethys Ocean, locally labelled “Piemonte–Liguria Ocean”,are grouped under the label of “Penninic Units” (Fig. 1). Theseterranes underwent Alpine metamorphism ranging from greenschistfacies to UHP eclogite facies conditions (Fig. 2). The axial zone is

Fig. 3. Simplified cross-sections across the Western Alps showing the metamorphic grametamorphic units (see Fig. 2 for locations of profiles; white dashes for units with uncertainMassif is modified from Escher et al., 1997; profile (b) across the Gran Paradiso Massif is coBucher et al. (2004), Pognante (1989); profile (c) is modified from Lardeaux et al., 2006. StaArolla series; C: Canavese Zone; EMS: Eclogitic Micaschists Complex; ESZ: Entrelor Shear Zoeclogitic and blueschist Piemonte Units); GM: Gneiss Minuti Complex; ICL: Internal CanaveseLM: Lanzo Massif; MF: Mont Fort Unit; PF: Penninic Front; RC: Rocca Canavese Unit; R: RutorPetit–St. Bernard Unit; VS: Valpelline series; ZH: Zone Houllière.

bounded by the Insubric (or Periadriatic) Line and the Penninic Front,dividing it from the less deformed/metamorphosed Southern Alps andExternal Zone, respectively. (3) Third is the External Zone, a nappestack resting upon the European plate. It consists of the HelminthoidFlysch, the Dauphinois–Helvetic Domain, the Molasse Foredeep, thePréalpes and the Jura Mountains, which underwent only anchizone togreenschist facies metamorphism.

2.1. Southern Alps

The Southern Alps (Fig. 1), in the hinterland of the Western Alps,are traditionally subdivided into the Ivrea (or Ivrea–Verbano) Zoneand the Strona–Ceneri Zone. These two domains preserve an almostcomplete crustal section, now tilted by 90°, consisting of continentalbasement rocks that underwent the Caledonian and Variscan Orogenybefore abundant mafic magma underplating in the Permian, duringthe Late-Variscan Orogenic collapse (Handy et al., 1999). Mesozoicrifting was accommodated by widespread block faulting in theStrona–Ceneri Zone (Bertotti et al., 1999) and by the activity of low-angle detachment faults along the hyper-extended western margin,now preserved in the Canavese Zone, where exhumed serpentinizedsub-continental mantle is locally found (Fig. 1; Ferrando et al., 2004).

de and age of attainment of highest recorded pressure conditions for the tectono-age of HPmetamorphism has been omitted for clarity). Profile (a) across the Monte Rosampiled and modified after Schmid and Kissling (2000), Le Bayon and Ballèvre (2006),r in profile (a) indicates the approximate position of the Lago di Cignana UHP Unit. AR:ne; FC: Faisceau de Cogne (sliver of pre-rift sediments located at the contact betweenLine; IL: Insubric Line; IVZ: Ivrea Zone; IZ: Internal Zone; LC: Lago di Cignana UHP Unit;Massif; SM: Siviez–Mischabel Unit; T: Tarentaise–Sion–Courmayeur Unit; V: Versoyen–


The Internal Canavese Line, a shear zone that accommodated mul-tiple phases of Alpine (and possibly even pre-Alpine) deformation,separates the Ivrea Zone from the Canavese Zone (Fig. 3a and b; Biinoand Compagnoni, 1989; Schmid et al., 1989; Zingg and Hunziker,1990). Alpine metamorphism in the Canavese Zone and in the partsof the Ivrea Zone located in a proximal position with respect to theInsubric Line reached anchizone to low-greenschist facies conditions,although higher-pressure conditions were probably reached locally,as indicated by the rare occurrence of blue amphiboles (Franchi,1904). Alpine-related deformation is responsible for the 90° tilting ofthe Ivrea Zone, which shows a vertical lithological banding, and forthe presence of rare, large-scale box folds in its northern part (Brodieet al., 1993).

2.2. Axial Belt

2.2.1. Austroalpine

2.2.1.1. Sesia Zone. The Sesia Zone (also known as the Sesia-LanzoZone) is the most easterly unit of the axial belt of the Western Alps. Itis bounded by the Insubric Line (locally also known as the ExternalCanavese Line) to the east, the Piemonte Zone to the west, and theLanzo Massif to the south (Fig. 1). The Sesia Zone is a composite unitthat consists predominantly of Palaeozoic continental basementsimilar to the Ivrea Zone/Strona–Ceneri Zone, characterized by pre-Alpine amphibolite to granulite facies metamorphism. A Mesozoicsedimentary cover, which underwent Alpine metamorphism togetherwith the basement rocks, is found locally (Venturini et al., 1994). AnOligocene post-metamorphic volcano-sedimentary cover crops out inthe proximity of the Insubric Line (Arhendt, 1969). Considerationsbased on metamorphic grade and lithological composition allow thedistinction of three main sub-units (Compagnoni et al., 1977): theEclogitic Micaschists Complex (EMS), the Gneiss Minuti Complex(GM), and the Seconda Zona Diorito-Kinzigitica (2DK; for differentsubdivisions of the Sesia-Lanzo Zone, see Venturini et al., 1994 andBabist et al., 2006).

The EMS consists of a polymetamorphic basement, which includesparagneisses, minor metamafics, and impure marbles, intruded byabundant granitoids andminor gabbros in Carboniferous and Permiantimes (Bussy et al., 1998; Rubatto, 1998). Granitoids are dominant inthe northeastern part of the Sesia Zone, where they are traditionallylabelled “Sesia Gneiss” (Dal Piaz et al., 1972). A probable Mesozoicsedimentary cover, with monometamorphic paragneisses, carbonateschists andmanganiferous quartzites is found in the central part of theEMS (Venturini et al., 1994).

The Gneiss Minuti Complex mainly consists of leucocratic layersinterbeddedwithmoremafic layers rich in albite, whitemica, chlorite,epidote, and actinolite. In several exposures, such as in the lowerAosta Valley, the leucocratic layers are demonstrably orthogneisses,most likely derived from Permian granitoids with the related apliticdyke swarm intruded in a poorly preserved polymetamorphicbasement. In contrast, in several other locations, such as the OrcoValley and the southern Sesia Zone, the Gneiss Minuti Complex doesnot show any trace of pre-Alpine metamorphism and probablydeveloped at the expense of a Mesozoic sedimentary sequencecomprising predominant meta-arkose and rare marble, calcschist,andmetachert (Zambonini, 1922; Gennaro, 1925; Dal Piaz et al., 1971;Minnigh, 1977; Pognante, 1989).

The 2DK consists of slivers of well-preserved pre-Alpine amphib-olite facies garnet–sillimanite–biotite micaschists with local migma-titic leucosome, minor amphibolites, marble, and a small harzburgitebody (Dal Piaz et al., 1971). Alpine re-equilibration under blueschistfacies conditions is restricted to themargins of the slivers or to narrowshear zones (Dal Piaz et al., 1971; Gosso et al., 1979; Passchier et al.,1981; Williams and Compagnoni, 1983). The 2DK crops out discon-tinuously along the contact between the EMS and the GM (Fig. 1).

Another sub-unit of limited extent is found interposed between theEMS and the Canavese Zone along the southeastern end of the SesiaZone, in the proximity of the Insubric Line (Fig. 1). This domain, whichhas been named the “Rocca Canavese Unit” by Pognante (1989),consists of pre-Alpine medium-/high-grade schists similar to thosefound in the 2DK, associated with slices of serpentinized lherzolites.

Alpine metamorphism reached eclogite facies conditions in theEMS, as indicated by the presence of eclogites, eclogitic micaschistscontaining omphacite+garnet, eclogitic gneisses and marbles, andextraordinarily well-preserved jadeite-bearing metagranitoids (Com-pagnoni and Maffeo, 1973). In the Aosta Valley section, the GM rockscontain omphacite but not jadeite (Williams and Compagnoni, 1983),suggesting PT conditions between the albite breakdown and thestability of omphacite, with about 50 mol% of the jadeite component.Similar albite+Fe-omphacite assemblages have been reported fromthe Sesia Gneiss (Lardeaux et al., 1982). The 2DK and the RoccaCanavese Unit recorded PT conditions in the blueschist facies field(e.g., Pognante, 1989).

2.2.1.2. Dent Blanche. The Dent Blanche Unit consists of continentalbasement rocks resting upon the Piemonte Zone, to the north of theAosta Valley (Fig. 1; Ballèvre et al., 1986). It is traditionally subdividedinto (1) the Arolla Series, which consists of metagranitoids andorthogneisses derived from Alpine reworking of Permian intrusivesemplaced into a Palaeozoic metamorphic basement, and (2) theoverlying Valpelline Series, lithologically equivalent to the 2DK. Asedimentary sequence of Mesozoic age is locally found overlying theArolla series in the Roisan Zone (Ballèvre and Kienast, 1987) and inthe Mont Dolin area (Ayrton et al., 1982). Alpine mineral assemblagesrecord blueschist facies conditions (Ballèvre and Kienast, 1987),widely overprinted by greenschist facies parageneses. Due to thelithological similarity between the Arolla+Valpelline series and theGneiss Minuti+2DK pair, the Dent Blanche Unit has often beenconsidered as the western continuation of the Sesia Zone, from whichit has been separated by erosion (Argand, 1911; Schmid and Kissling,2000).

2.2.2. Lanzo MassifThe Lanzo Massif, which crops out to the south of the Sesia Zone

(Fig. 1), consists of sub-continental peridotites that preserve a remark-able record of their polyphase pre-Alpine history (Nicolas et al., 1972),including progressive exhumation and melt infiltration that occurredduring Mesozoic rifting (e.g., Piccardo et al., 2007; Kaczmarek andMüntener, 2008). Serpentinization ismostly limited to the edges of themassif and to two shear zones that divide the massif into Northern,Central, and Southern parts (Nicolas et al., 1972). The presence of aMesozoic sedimentary cover, consisting of meta-ophicarbonate,metabasalt, calcschist, metaquartzite, and monometamorphic gneissresting upon serpentinizedmantle, indicates that parts of theNorthernand Central Lanzo Massifs were exhumed at the bottom of thePiemonte–Liguria Ocean in the Jurassic–Cretaceous (Lagabrielle et al.,1989; Pelletier and Müntener, 2006). The abundance of meta-arkoseand the presence of slivers of continental basement, interpreted asextensional allochthons, are considered to indicate that in theMesozoic the Northern Lanzo Massif was located in a proximalpositionwith respect to a continental margin (Pelletier andMüntener,2006). However, its original pertinence to the Adriatic or Europeanthinned margins is still debated (e.g., Manatschal and Müntener,2009). Alpine re-equilibration at P=2.0–2.5 GPa is generally recordedin the metagabbros along the margins of the massif (Kienast andPognante, 1988; Pelletier and Müntener, 2006).

2.2.3. Penninic Units

2.2.3.1. Piemonte Zone. The Piemonte Zone crops out along the wholelength of the Western Alps and appears in the Alpine literature under


different labels, including Piemonte–Liguria Zone, Ligurian–PiemonteZone, Ligurian–Piemontese Zone, Schistes Lustrés Zone and “Zona deicalcescisti con pietre verdi”. It is bounded by the Sesia Zone and by thePo Plain to the east and by the Briançonnais Units to the west (Fig. 1).The Piemonte Zone samples remnants of the Piemonte–Liguria Oceanand of the sedimentary cover that was deposited along its margins(e.g., Bearth, 1967; Lemoine, 1985; Deville et al., 1992). Lithologicalassociations and Alpine metamorphism allow us to define two mainportions/ensembles, which were first described in the northwesternAlps (Bearth, 1967), where an eclogite facies Zermatt–Saas Zone and ablueschist facies Combin Zone have been distinguished. A similardistinction between eclogite facies Piemonte Units and blueschistfacies Piemonte Units can be made all along the arc of the WesternAlps (Fig. 1; from north to south, see Servizio Geologico d'Italia, 2006;Beltrando et al., 2008; Perotto et al., 1983; Servizio Geologico d'Italia,2002; Agard et al., 2001; Lombardo and Pognante, 1982; Schwartz,2002).

The eclogite facies Piemonte Units consist of abundant serpenti-nized mantle peridotite, locally grading upward to ophicarbonatebreccias, and including bodies of Mg–Al gabbros and Fe–Ti gabbrosconverted to eclogites. Serpentinites are locally overlain by pillowlavas and/or aMesozoic sedimentary cover comprising impure, locallyMn-rich quartzites and calcschists. The blueschist facies PiemonteUnits, instead, are dominated by calcschists, which in the northwest-ern Alps are inter-layered with tabular mafic beds of sedimentary andmagmatic origin (lava flows). To the south of the Susa Valley, largeslices of metabasalts, metagabbro, and ultramafic rocks, which aregenerally interpreted as olistoliths and/or tectonic slices, are oftenfound in the calcschists (Lombardo and Pognante, 1982; Lemoineet al., 1987). The blueschist Piemonte Units are interpreted to sample(1) slivers of Piemonte–Liguria ocean basement with its sedimentarycover, and (2) the sedimentary cover that was deposited along themargins of the oceanic basin, with the local preservation of domainscharacterized by continuous Mesozoic sedimentation over Palaeozoiccontinental basement (“pre-Piemontese” Units: Deville et al., 1992;Tricart and Schwartz, 2006).

Slivers of continental basement rocks and pre-rift sediments arelocally found along the contact between the eclogite- and blueschistfacies Piemonte Units (Hermann, 1937; Perotto et al., 1983; Ballèvreet al., 1986; Dal Piaz, 1999). The association of serpentinized mantle,slivers of continental basement, and pre-rift sediments has recentlybeen ascribed to a Mesozoic Ocean–Continent Transition Zone, partlyreworked during the Alpine Orogeny (Dal Piaz, 1999; Beltrando et al.,2010). Therefore, the eclogitic Piemonte Units, at least in thenorthwestern Alps, probably sample the part of the Piemonte–LiguriaOcean that was originally attached to the hyper-extended Briançon-nais margin.

2.2.3.2. Briançonnais Units. The Briançonnais Units form a remarkablycontinuous exposure of continental basement and Mesozoic/Tertiarycover rocks bordering the Piemonte Zone to the east and the Valaisanand the Helvetic Domains to the west (Fig. 1). Several sub-units havebeen defined within this domain on the basis of lithologicalcomposition/association and/or grade of Alpine metamorphic over-print (Figs. 2 and 3). The reader is referred to Escher et al. (1997),Malusà et al. (2005a), Michard et al. (2004), and references therein fora detailed description of the lithological, metamorphic, and structuralfeatures of the different sub-units along the Monte Rosa, GranParadiso, and Dora–Maira transects, respectively. Given the mainfocus of this paper, the different sub-units will be grouped anddiscussed according to their Alpine metamorphic overprint. A generalincrease in metamorphic grade from low-greenschist to epidoteblueschist facies conditions is observed from the western to theeastern units (Bousquet et al., 2008).

The greenschist facies Briançonnais belt comprises several sub-units, which in the northwestern Alps are the Siviez–Mischabel

Nappe, the Pontis Nappe, and the Zone Houillère. For details on theseveral greenschist facies Briançonnais sub-units of the Dora Mairatransect, see Table 1 in Michard et al. (2004). The Zone Houillèrerepresents the most external Briançonnais unit along the GranParadiso and Monte Rosa transects. It consists of a Carboniferous–Permian sequence of continental deposits locally overlain by Triassicquartzite and dolomitic marble (Burri and Jemelin, 1983). The moreinternal Siviez–Mischabel and Pontis Nappes consist of a polymeta-morphic basement overlain by a sedimentary cover of Carboniferous–Permian clastic sediments, Triassic quartzite and dolomitic marble. Inthe Siviez–Mischabel Nappe, a sedimentary sequence continuous upto the Eocene is also found (Thélin et al., 1990).

The blueschist facies Briançonnais belt crops out between themoreexternal greenschist facies Briançonnais Units and the more internalPiemonte Zone. Higher-T (epidote-bearing) and lower-T (lawsonite-bearing) blueschist units have been distinguished (Figs. 2 and 3). Theformer units comprise, from north to south, the Mont Fort Nappe, theInternal Zone (Bucher and Bousquet, 2007; seeMalusà et al., 2005a foran alternative subdivision of the Internal Zone), the Vanoise andAmbinBasementUnits (Malusà et al., 2002; Ganne et al., 2003) and theAcceglio Unit (Schwartz et al., 2000; Michard et al., 2004). The MoneyUnit and the Pinerolo Unit, which crop out underneath the GranParadiso and Dora–Maira Nappes, respectively, are also ascribed to theBriançonnais Units sensu stricto (Figs. 1, 2, and 3; Argand, 1911;Michard, 1967; Compagnoni et al., 1974). They consist of Permianmetagranites (Le Bayon and Ballèvre, 2004) intrusive into a mono-metamorphic sequence consisting of conglomerates and graphite-bearing micaschists (Compagnoni et al., 1974), which underwentAlpine blueschist to eclogite facies metamorphism (Borghi et al.,1996).

2.2.3.3. Internal Crystalline Massifs. The culminations of continentalbasement, which are surrounded by the Piemonte Zone, are generallygrouped under the label of Internal Crystalline Massifs (ICM; Fig. 1).They comprise the Monte Rosa, Gran Paradiso, and Dora–MairaMassifs and the small Arcesa–Brusson Window. These massifs consistof a Variscan amphibolite facies basement, intruded by abundantPermian porphyritic granitoids (e.g., Bertrand et al., 2005). The latterwere deformed into augengneisses during the Alpine Orogeny. Thin,discontinuous slivers of dolomite and marble, which are interpretedas remnants of the Mesozoic sedimentary cover, are found both in theGran Paradiso (Elter, 1972) and Dora–Maira Massifs (Vialon, 1966;Michard, 1967). In contrast, the Mesozoic cover sequences croppingout in the proximity of the Monte Rosa Massif, such as the Furgg Zoneand the Gornergrat Unit, are of more controversial provenance (Fig. 1;cfr. Dal Piaz, 2001b; and Keller and Schmid, 2001). While the GranParadiso and Monte Rosa massifs are relatively homogenous (Dal Piazand Lombardo, 1986), the Dora–Maira can be divided into a northernpart, which is similar to the other ICM, and a southern part, which ischaracterized by several thin tectonic slivers with different Alpinemetamorphic overprint. The Brossasco–Isasca Unit, which underwentUHP metamorphism at PN4.0 GPa (Hermann, 2003), is sandwichedbetween the underlying Pinerolo Unit and the overlying slivers of thesouthern Dora–Maira Massif (Figs. 1, 2, and 3c).

The Internal Crystalline Massifs are generally considered torepresent a transitional domain between the Briançonnais Domainand the Piemonte–Liguria Ocean (e.g., Elter, 1972; Dal Piaz, 2001b),although other origins have also been suggested (Platt, 1986;Froitzheim, 2001).

2.2.3.4. Valaisan Domain. The Valaisan Domain is bounded by theBriançonnais Domain to the east and by the Penninic Front to the west(Fig. 1), and it represents themost external unit of theWestern Alps thatexperiencedAlpinehigh-pressuremetamorphism(Figs. 2 and3;Bocquet,1974; Bousquet et al., 2002). The Valaisan Domain consists of theVersoyen–Petit St. Bernard Unit and the Sion–Courmayeur–Tarentaise


Unit (Masson et al., 2008). The Versoyen–Petit St. Bernard Unit, which isexposed in the proximity of the Petit St. Bernard Pass, at the French–Italian border (Elter and Elter, 1965), consists of Palaeozoic magmaticrocks (Schärer et al., 2000; Beltrando et al., 2007b; Masson et al., 2008)intrusive into a basement composed of graphite-bearing meta-siltstonesand meta-greywackes. These basement rocks are overlain by Liassiccalcschists,with the local interposition of Triassic quartzite, dolomite, andlimestone (Antoine, 1971). The Sion–Courmayeur–Tarentaise Nappeconsists of a thick detrital series of undetermined post-Jurassic age,transgressive on a much thinner Late Palaeozoic to Jurassic basal series(Barbier, 1948; Masson et al., 2008).

Alpine metamorphism resulted in the re-equilibration of theVersoyen–Petit St. Bernard Unit at P=1.5–1.6 GPa and T=425–500 °C (Cannic et al., 1995; Bousquet et al., 2002), while low-Tblueschist facies conditions were reached in the Sion–Courmayeur–Tarentaise Nappe.

2.3. External zone

The units located in a more external position with respect to thePenninic Front underwent Alpine metamorphism ranging fromanchizone to low-greenschist facies conditions (Figs. 1 and 2). Theirlithological composition and Alpine evolution are treated here onlybriefly. The reader is referred to Escher et al. (1997) and referencestherein for further details. The Helminthoid Flysch, which overlies theHelvetic–Dauphinois Domain in the proximity of the Penninic Front,consists of Late Cretaceous–Palaeocene Flysch Units, which under-went sub-greenschist faciesmetamorphism. The Helvetic–DauphinoisDomain consists of continental basement, which is exposed in theExternal Crystalline Massifs (i.e., from north to south: Mont Blanc/Aiguilles Rouges, Belledonne, Pelvoux, and Argentera; Fig. 1), and of aLate Permian-to-Lower Oligocene sedimentary cover sequence.Alpine metamorphism at sub-greenschist facies conditions (P=0.3–0.5 GPa and T=250–300 °C) is recoded in the External CrystallineMassifs. The Jura Belt is a typical fold-and-thrust belt that formed inthemost external part of the Alpine chain (Escher et al., 1997) from ca.11 Ma (Laubscher, 1987). The Molasse exposes a peripheral foredeepthat developed subsequently to the collision between the AlpineOrogen and the European plate, from late Oligocene to MiddleMiocene, and that is now partly imbricated in the Alpine Orogen(Sissingh, 2001). The Préalpes, which rest upon the external part ofthe Helvetic Domain, consist of several cover nappes that aregenerally considered as the detached sedimentary cover of differentPenninic basement units. Metamorphic grade increases from west toeast, reaching anchizone conditions in the most internal nappes(Escher et al., 1997).

Fig. 4. Selected pressure–temperature estimates for the Sesia Zone (a), Zermatt–Saas Zone (from the early to the most recent studies. References: (1) Bocquet, 1974 (2) Pognante, 198Barnicoat, 2002; (6) Bucher et al., 2005; (7) Borghi et al., 1996; (8) Le Bayon et al., 2006; (

3. Pressure–temperature paths

Petrographic and petrologic studies led to the early recognition ofthe high-pressure nature of the Alpinemetamorphism experienced bythe internal units of the Western Alps (Ernst, 1971; Dal Piaz et al.,1972). Progressively updated regional compilations of the metamor-phic data set can be found in Frey et al. (1974), Desmons et al. (1999),Frey et al. (1999), Oberhänsli et al. (2004), and Bousquet et al. (2008).Although the general pattern of metamorphic grade has not changedsignificantly since the first syntheses, the absolute peak-pressurevalues and the shapes of the Pressure–Temperature (PT) paths havebeen progressively refined. Fig. 4 shows how the estimates of thehighest pressures recorded in the different tectono-metamorphicunits have increased in time. Such an increase can be ascribed torefinements of the thermo-barometers used for the estimates and tothe ever-growing number of studies, leading to new discoveries, suchas the coesite findings in the Brossasco–Isasca Unit of the Dora–MairaMassif (Chopin, 1984) and in the Lago di Cignana Unit of the PiemonteZone (Reinecke, 1991). Significantly, in the early studies theindividual points in the PT space, determined on the basis ofthermo-barometric estimates on specific mineral parageneses, werenot linked in a continuous PT path. The habit of connecting thesediscrete data points with smooth, direct lines, thereby implicitlyimplying that the rocks followed the depicted continuous changes inPT conditions and not any of the infinite possible tie-lines between thefew estimated data points, is relatively recent. While in specificcircumstances thismethodmay lead to plausible results, in principle itgives a gross over-representation of the resolution that is achievablein the determination of the PT evolution of a metamorphic rock.Recent studies have shown that this approach may lead to the over-simplification of the PT paths, which in detail may be much morecomplex than commonly thought (Beltrando et al., 2007a; Harriset al., 2007; Umhoefer et al., 2007).

Early studies (Frey et al., 1974) suggested that Alpine metamor-phism in the axial zone produced two main groups of mineralassociations, with an early high-pressure group followed by a secondgroup at low to intermediate pressures and variable temperatures,generally at greenschist facies conditions. A third low P and Tparagenetic group was restricted to the external parts of the belt, inthe Helvetic Domain. The metamorphic grade was considered toincrease from the external to the internal parts of the belt (Ernst,1971; Frey et al., 1974). Metamorphic zonations were chiefly definedon the basis of the presence/absence of sodic amphiboles, jadeite, and,occasionally, kyanite. Petrographic evidence, particularly from theEclogitic Micaschists Complex, in the Sesia Zone, indicated that thereare several generations of pyroxene and Na-amphibole, implying that

b), and Gran Paradiso Massif (c). Note the progressive increase in estimated pressures9; (3) Konrad-Schmolke et al., 2006; (4) Ernst and Dal Piaz, 1978; (5) Cartwright and9) Gabudianu Radulescu et al., 2009.


individual rock units underwent multiple stages of metamorphic re-equilibration during the Alpine Orogeny (Bocquet, 1974; Frey et al.,1974). The crystallization of high-pressure minerals was thenfollowed by retrogression under greenschist facies conditions, withgrowth of chlorite, green biotite, epidote, blue-green amphibole,actinolite, stilpnomelane, paragonite, and chloritoid (Frey et al.,1974). It was soon recognized that the extent of such retrogressionwas heterogeneously distributed, being pervasive mainly along thecontact between the Sesia Zone and the Piemonte Zone, in the GneissMinuti Complex and Combin Zone, and in the proximity of thecontacts between the Internal Crystalline Massifs and the overlyingmeta-ophiolites of the Piemonte Zone. The growth of oligoclase rimsaround albite porphyroblasts, commonly observed in a late stage ofthe greenschist facies evolution of several units, including the InternalCrystalline Massifs and the Piemonte Zone, was commonly inter-preted as a consequence of a late increase in temperature (Borghi etal., 1996; Desmons et al., 1999), generally related to the OligoceneBarrovian metamorphism observed in the Central Alps (e.g., Frey andFerreiro-Mählmann, 1999).

Over 50 years of petrographic/petrologic studies have resulted in aremarkable data set of PT evolutions of the main units of the WesternAlps (see Figs. 2–5). The concept of the general eastward increase inpeak pressure has now been overcome by the recognition of asignificantly more complex pattern of metamorphism (Figs. 2 and 3;Bousquet et al., 2008). In particular, significant “inverted” pressuregaps are found:

(1) between the high-pressure Valaisan Domain (Bocquet, 1974;Bousquet et al., 2002) and the low-greenschist Zone Houillère(P gap=1.0–1.4 GPa; Figs. 2 and 3b);

(2) between the eclogite facies Piemonte Units and the blueschistfacies Piemonte Units, particularly in the Valtournenche (Pgap=1.4–1.7 GPa; Figs. 2 and 3a). The extent of this P gap isbased on the observation that the eclogitic Piemonte Unit, tothe north of the Aosta Valley, experienced pressures in excessof 2.7 GPa (Bucher et al., 2005);

(3) between the UHP Brossasco–Isasca Unit and the rest of thesouthern Dora–Maira tectonic slices (Figs. 2 and 3c: PgapN2.0 GPa; Matsumoto and Hirajima, 2000; Castelli et al.,2007).

The recent discovery of multiple burial–exhumation cycleswithin individual tectono-metamorphic units introduces furthercomplexities into the evolution of the Western Alps and of orogenicbelts at large (Fig. 5). Beltrando et al. (2007a) showed that in theEocene the eclogitic Piemonte Unit, to the north of the Gran ParadisoMassif, underwent a first episode of eclogite facies metamorphism atPN1.5 GPa, followed by rapid exhumation to near-surface condi-tions, before renewed burial to epidote–amphibolite conditions, atca. 0.8 GPa. Analogous multiple burial–exhumation cycles areprobably much more common than hitherto recognized both in thedifferent parts of the eclogitic Piemonte Units (Beltrando et al., 2008)and in other tectono-metamorphic units of the Western Alps. Suchfindings are in accordance with recent discoveries from otherorogenic belts (Harris et al., 2007; Umhoefer et al., 2007) and withnumerical (e.g., Gerya and Stöckhert, 2005) and conceptual modelsof orogenesis (Lister et al., 2001; Collins, 2002; Wells and Hoisch,2008).

4. Geochronology

Geochronological data have traditionally held a prominentposition in providing constraints for tectonic models of orogeny.Similarly to the petrological studies, progressive refinement ofanalytical techniques and the availability of larger data sets resultedin significant changes in the understanding of the timing of meta-morphic and deformational events in the orogen. Comprehensive

reviews of the Alpine geochronology can be found in Hunziker et al.(1992), Gebauer (1999), and Rosenbaum and Lister (2005).

Early K–Ar studies on metamorphic mineral assemblages (seeFrey et al., 1974 for a review) were interpreted to indicate that theWestern Alps had experienced three main metamorphic phases, asfollows. (1) First there was the “Eo-Alpine” metamorphic event,duringwhich the Sesia Zone and themeta-ophiolites of the PiemonteZone underwent eclogite/blueschist facies metamorphism at ca.100–60 Ma. Ages of 100–80 Ma were estimated from glaucophaneand crossite from the ophiolitic units (Hunziker, 1974; Bocquet,1974), while phengites from the Sesia Zone yielded ages of 90–60 Ma(Hunziker, 1974). (2) Second was the Mid-Tertiary “Meso-Alpine”metamorphic event, during which the Central Alps underwentamphibolite facies metamorphism, while the Western Alps, apartfrom the Monte Rosa region, underwent greenschist facies meta-morphism. This metamorphic “event” was placed in the 50–30 Mainterval, with a significant age cluster at ca. 38 Ma (Dal Piaz et al.,1972). (3) Third was the low-grade “Neo-Alpine” event, duringwhich sediments that had been deposited from the Oligoceneonwards in the foreland underwent greenschist facies metamor-phism. The wealth of subsequent geochronological studies was laterclassified within slightly refined age groups (Hunziker et al., 1989,1992), with a longer Eo-Alpine “event” at ca. 140–60 Ma, followed bya relatively quiescent period, prior to the Meso-Alpine “event” at 45–30, which was then followed by the Neo-Alpine “event” at 30–0 Ma.The Eo-Alpine event was itself subdivided into an early, eclogitefacies phase, between 140 and 85 Ma, and a later, glaucophane-bearing phase, between 85 and 60 Ma. Eo-Alpine “ages” were foundboth in the Eclogitic Micaschists Complex of the Sesia Zone and in thePenninic Domain (Piemonte Zone, Internal Crystalline Massifs,Briançonnais and Versoyen).

These results were at odds with a large number of geologicalobservations, which excluded the possibility for early Cretaceousorogenesis in the Western Alps, including (1) plate kinematic studies(e.g., Dewey et al., 1973; Livermore and Smith, 1985; Dewey et al.,1989) showing that the onset of the Europe–Africa convergence hadto be bracketed between anomaly M0 (118 Ma) and anomaly 34(84 Ma), and (2) the presence of a Tertiary sedimentary cover onthe Penninic Units (Desmons, 1986). Furthermore, the presence ofremarkably well-preserved eclogites was considered to be incompat-ible with the long time lag estimated for their slow exhumation to thesurface (Dal Piaz et al., 1972).

Later studies showed that some of the anomalously old agesobtained in several geochronological studies could have beenaffected by the presence of “extraneous argon”, unrelated to the Kpresent in the dated micas (e.g., Arnaud and Kelley, 1995; DiVincenzo et al., 2006), or to isotopic disequilibrium between mineralphases used to define isochrones for the Rb–Sr method (e.g., Hurfordet al., 1989).

The widespread use of high-resolution in-situ dating techniquessince the mid-1990s has allowed to clarify many paradoxes arisingfrom the geochronological data illustrated above. Several studies,chiefly conducted with high-resolution U–Pb techniques on zircon,allanite, or titanite, or with dilution methods on garnet or rutile and,to a lower extent, with 40Ar/39Ar on white micas, showed a few well-defined age clusters for the high-pressure metamorphism of thedifferent units of the Western Alps:

(1) Ages of ca.70–65 Ma have been obtained from the EclogiticMicaschists Complex of the Sesia Zone [U–Pb on zircon(Rubatto et al., 1999), Lu–Hf on garnet (Duchêne et al., 1997),40Ar/39Ar on white mica (Inger et al., 1996)].

(2) Ages of ca. 55 Ma and ca. 47 Ma have been obtained in the LanzoMassif from high-pressure zircons and allanites, respectively.These estimates have been interpreted to indicate that themassif resided at high-pressure depths for that time span


(Rubatto et al., 2008). K–Ar geochronology onwhitemicas fromthe Mesozoic cover of the Dent Blanche Nappe yielded similarresults (Ayrton et al., 1982), with a large age spread from ca.55 Ma to ca. 34 Ma, probably related to partial overprint andisotopic resetting at greenschist facies conditions.

(3) Ages of 47–42 Ma have been estimated from several ophiolitesof the Piemonte Zone, including the Zermatt–Saas Zone [U–Pbon zircon (Rubatto et al., 1998), Sm–Nd on garnet (Lapen et al.,2003), 40Ar/39Ar on white mica (Gouzu et al., 2006)], the BalmaUnit, overlying the Monte Rosa Massif [Lu–Hf on garnet(Herwartz et al., 2008)], the eclogitic Piemonte Unit of theUrtier Valley [(40Ar/39Ar on white mica (Beltrando et al.,2009)], the Monviso Massif [Sm–Nd on garnet (Duchêne et al.,1997); U–Pb on zircon (Rubatto and Hermann, 2003)], and theVoltri Massif [40Ar/39Ar on white mica (Federico et al., 2007)].The slivers of continental basement associated with the eclo-gitic Piemonte Units in the northwestern Alps also underwenthigh-pressure metamorphism at ca. 47–44 Ma [40Ar/39Ar onwhite mica (Dal Piaz et al., 2001; Beltrando et al., 2009); U–Pbon zircon (Beltrando et al., 2010)]. Middle Eocene ages havebeen obtained also from the continental basement of the FurggZone [U–Pb on rutile (Lapen et al., 2007)], which the authorsextend to the Monte Rosa massif, and from the calcschists ofthe blueschist facies Piemonte Unit near Cogne [40Ar/39Ar onwhite mica (Beltrando et al., 2009)].

(4) Another well-defined cluster of high-pressure mineral growthage is restricted to the Gran Paradiso Massif and the Brossasco–Isasca Unit, in the Dora–Maira Massif. Recent U–Pb high-resolution geochronology on allanites and monazites revealedthat the Gran Paradiso Massif experienced high-pressuremetamorphism at 37–33 Ma (Gabudianu Radulescu et al.,2009). These results are at odds with the age of 43.0±0.5 Maprovided by Meffan-Main et al. (2004) with the Rb–Sr methodon the apatite–white mica mineral pair. The Brossasco–IsascaUnit has been the subject of numerous geochronologicalstudies, which have convincingly shown that the UHP event

Fig. 5. Pressure–time evolution of the tectono-metamorphic units of the axial zone ofthe Western Alps. ZFT: Zircon Fission Track ages from the Gran Paradiso and Dora–Maira Massifs. References: (1): Inger et al., 1996; (2) Zingg et al., 1976; (3): Cartwrightand Barnicoat, 2002; (4): Ganne, 2003; (5): Reddy et al., 2003; (6): Beltrando et al.,2009; (7): Gebauer et al., 1997; (8): Malusà et al., 2005b; (9): Konrad-Schmolke et al.,2006; (10): Pelletier and Müntener, 2006; (11): Hermann, 2003; (12): Groppo et al.,2009; (13): Gabudianu Radulescu et al., 2009; (14): Ganne, 2003; (15): Beltrando et al.,2007a. Pt path of EMS is compiled after Konrad-Schmolke et al., 2006. Pt path ofSouthern Gneiss Minuti, in proximity of the Lanzo Massif, is inferred from the path ofthe Lanzo Massif (explanation in the text).

must have occurred at ca. 38–35 Ma [U–Pb on zircon (Tiltonet al., 1991; Gebauer et al., 1997), Sm–Nd on garnet (Tiltonet al., 1991; Duchêne et al., 1997), U–Pb on titanite (Rubattoand Hermann, 2001), Rb–Sr on phengite-whole rock (DiVincenzo et al., 2006)]. It is important to note that recentgeochronological studies from the several other tectonic slicesof the Dora–Maira Massif are lacking. Therefore, caution shouldbe exerted when extending the geochronological results of theBrossasco–Isasca Unit to the whole Dora–Maira Massif. Nota-bly, 40Ar/39Ar ages of ca. 40 Ma have been reported fromphengites of the northern Dora–Maira Massif (Scaillet et al.,1990).

The general consistency of mineral ages obtained with multi-system geochronology has often resulted in the rapid dismissal ofpublished mineral ages inconsistent with the above clusters. Morespecifically, published pre-70 Ma “anomalous” 40Ar/39Ar ages are nowgenerally discarded as being related to “excess argon”, even when noassessment of the presence of such component can be performed,generally due to the lack of the necessary microchemical informationin the old publications (e.g., Ruffet et al., 1995, 1997). In somecircumstances (Inger et al., 1996), “anomalous” age spectra from theSesia Zone in the 90–70 Ma range are surprisingly dismissed despitepassing the “inverse isochron test” (McDougall and Harrison, 1999;Kuiper, 2002).

Phengitic micas yielding 40Ar/39Ar ages of ca. 82–92 Ma arecommonly found in the sediments deposited in the Tertiary PiemonteBasin (TPB in Fig. 1). This age cluster has been interpreted to indicatethat deformation in the Western Alps may have already started in theMiddle Cretaceous (Carrapa and Wijbrans, 2003). Although such aconclusion is consistent with plate kinematic reconstructions (seebelow), the ages reported by Carrapa and Wijbrans (2003) fromdetrital micas could in principle be affected by the presence of excessargon, similarly to some continental basement rocks of the WesternAlps (e.g., Arnaud and Kelley, 1995; Di Vincenzo et al., 2006).

The interpretation of the 40Ar/39Ar data from the Western Alps isfurther complicated by the possible preservation of pre-Alpine argonreservoirs within white micas that escaped re-equilibration duringthe orogenic deformation and metamorphism. Several studies haveshown that pre-Alpine micas preserved in microlithons can occasion-ally survive Alpine metamorphism at least up to blueschist (Monié,1990; Markley et al., 1998) and possibly even to eclogite faciesconditions (Darbellay et al., 2009). Such micas can be found both inthe continental basement and in the Mesozoic/Tertiary sediments(Takeshita and Itaya, 1993).

Therefore, after more than 40 years of extensive geochronologicalstudies carried outwith all available techniques, it can be concluded thatthe ages of high-pressure minerals from theWestern Alps are clusteredinto several relatively well-defined age groups: (1) ca.70 Ma; (2) ca.55 Ma; (3) 48–42 Ma; (4) 38–33 Ma (Fig. 5).

The calcschists of the blueschist facies Piemonte Units of theCottian Alps, in the southwestern Alps, display a somewhat morecomplex age pattern with respect to the rest of theWestern Alps, withhigh-pressure micas seemingly forming during multiple episodes ofdeformation/metamorphism in the 62–40 Ma period (Takeshita et al.,1994; Agard et al., 2002). Such an age spreadmay reflect themixing ofdifferent argon reservoirs, possibly related to the original detritalmicas (Takeshita et al., 1994) or continuous deformation/recrystalli-zation during that time span (Agard et al., 2002), suggesting that theblueschist facies Piemonte Units of the Cottian Alps may sample afossil accretionary prism (Agard et al., 2001, 2009).

5. Age, metamorphic grade and kinematics of Alpine shear zones

While early studies of the axial zone of the Western Alps werechiefly concerned with determining the highest pressure and


temperature reached by the different tectono-metamorphic units, inthe last 15 years the focus has progressively moved towards theexhumation history of the (U)HP units. The contacts between thedifferent units have attracted increasing attention, with the aim ofdetermining the mechanisms leading to tectonic burial and exhuma-tion. Although early studies considered that all tectonic contacts hadaccommodated exclusively shortening deformation, in the last twodecades an extensional origin has been proposed for several shearzones found in the Western Alps.

The first extensional shear zones detected in the Western Alpshave been interpreted as related to lateral extrusion of crustalmaterial in a direction parallel to the strike of the orogen, in anoverall shortening regime (e.g., Simplon Line; Mancktelow, 1985,1992). Later studies have suggested that a large number of thetectonic contacts found in theWestern Alps may have accommodatedextensional deformation, at least for part of their history (e.g., Ballèvreet al., 1990; Blake and Jayko, 1990; Philippot, 1990; Ballèvre andMerle, 1993; Wheeler and Butler, 1993; Avigad, 1996; Cannic et al.,1995; Inger and Ramsbotham, 1997; Reddy et al., 1999; Cannic et al.,1999; Rolland et al., 2000; Agard et al., 2001; Bousquet et al., 2002;Cartwright and Barnicoat, 2002; Reddy et al., 2003; Avigad et al.,2003; Forster et al., 2004; Ganne et al., 2003, 2005; Beltrando et al.,2008). Extensional deformation immediately following each of theepisodes of high-pressure metamorphism that affected the axial zoneof the Western Alps has been indicated as an important factor forrapid exhumation and preservation of high-pressure mineral assem-blages (Avigad, 1996; Rosenbaum and Lister, 2005; Avigad et al.,2003).

However, field studies often provide contradictory evidence forthis hypothesis (cf. Avigad, 1996; Babist et al., 2006). Indeed, therecognition of the original nature of a tectonic contact is oftenhampered by re-orientation and/or kinematic reworking, which arecommon in polydeformed terranes (Wheeler and Butler, 1994).Significantly, eclogite facies fabrics are found only locally, and mostshear zones in the internal Western Alps preserve exclusively fabricsthat formed at greenschist (Cannic et al., 1999) or, occasionally,blueschist facies conditions (Reddy et al., 1999, 2003; Avigad, 1996;Babist et al., 2006). The general lack of an HP structural record is at thebasis of the significantly different models that have been proposed forthe early stages of exhumation of Alpine (U)HP rocks.

The existing geochronological data on the formation of metamor-phic mineral fabrics in the different parts of the Western Alps isreviewed in the following sections. The Pressure–Temperature–time(PTt) range of activity of the shear zones in the Monte Rosa transect,for which the largest data set is available, has been indicated in Fig. 6.

Fig. 6. Simplified cross section across theWestern Alps (see Fig. 3a) with the metamorphic gdifferent tectonic units (in black) are from Escher et al., 1997. Numbers indicate references: (1996; (5) Reddy et al., 1999; (6) Reddy et al., 2003; (7) Cartwright and Barnicoat, 2002; (8)which they have been projected. Contacts between different tectono-metamorphic units, asFront; V: Valaisan Domain; SM: Siviez–Mischabel Unit; C: Combin Zone (blueschist PiemonICL: Internal Canavese Lines; Piemonte MZ: Piemonte Movement Zone.

5.1. Cretaceous to Early Eocene deformation

The oldest fabrics recognized in the Western Alps are associatedwith the Sesia Zone and the neighbouring Canavese Zone. The firstevidence of shear zone activity in the Western Alps is found in theCanavese Zone, in the area of Montalto, near Ivrea, where K–Argeochronology of white micas from sheared Mesozoic sedimentsyielded ages in the 75–60 Ma range (Zingg et al., 1976; Figs. 5 and 6).These results were interpreted to date the onset of sinistraltranspression along the Adriatic margin (Biino and Compagnoni,1989). Later kinematic reworking of this area, located in the proximityof the Insubric Line, resulted in local, multi-stage fabric re-equilibra-tion and isotopic resetting in the Tertiary (Zingg and Hunziker, 1990,1990; Schmid et al., 1989; Babist et al., 2006).

The Sesia Zone provides evidence of the best-preserved and mostcontinuous tectonic contact that formed at high-pressure conditionsof the entire Western Alps. A blueschist facies mylonitic belt, whichhas been recently dated at ca. 65–60 Ma with the Rb–Sr method(Babist et al., 2006), characterizes well-defined sections of theEclogitic Micaschists Complex, particularly at the contact with the2DK (Fig. 6). Early exhumation of the EMS and juxtaposition to the2DK have been ascribed to shearing along this mylonitic belt, whichhas been named “Chiusella Shear Zone” (Ridley, 1989; Avigad, 1996;Inger and Ramsbotham, 1997; Babist et al., 2006). This mylonitic belthas been alternatively interpreted as an extensional (Avigad, 1996) ortranspressional shear zone (Ridley, 1989; Babist et al., 2006) thataccommodated the exhumation of the EMS. Such contrastingobservations provide a vivid illustration of the limits that affect thegeometric observations in highly deformed terranes, where theoriginal fabrics are severely deformed and re-oriented duringsubsequent deformation. In the specific case, several folding eventsfollowed the formation of the blueschist mylonitic belt (Williams andCompagnoni, 1983; Inger and Ramsbotham, 1997; Babist et al., 2006)suggesting caution when inferring paleo-kinematics from present-day geometries.

Eclogite facies fabrics/mineral assemblages are preserved at thecontact between the Gneiss Minuti Complex and the Lanzo Massif(Fig. 1). In the Balangero mine, near Lanzo, a continuous layer ofomphacitite is found at the contact between the serpentinites of theNorthern Lanzo Massif and the GM (Compagnoni and Sandrone,1986), which here probably represent a detrital sequence of Mesozoicage (Pognante, 1989). Omphacitite represents a common metaso-matic reaction zone between juxtaposed ultramafic and acidic rocks athigh-pressure conditions. This observation indicates that the juxta-position between the two units pre-dates the high-pressure

rade of shear zones for which radiometric age estimates are available. Contacts between1) Zingg et al., 1976; (2) Zingg and Hunziker, 1990; (3) Babist et al., 2006; (4) Inger et al.,Freeman et al., 1998. Results in (8) are from localities slightly off the cross section, ontodefined in the text and in Figs. 1, 2, and 3, are shown in black. BBF: Basal Briançonnaiste Unit); Z: Zermatt–Saas Zone (eclogitic Piemonte Unit); GM: Gneiss Minuti Complex;


metamorphism (Compagnoni and Sandrone, 1986). An early juxta-position is also suggested by structural studies, showing that thecontact between the two units is affected by all deformation eventsthat can be observed in the field (Spalla et al., 1983). Some of theimplications of such observations, which have generally been over-looked in Alpine syntheses, will be discussed in the last section of thispaper.

Another distinct generation of shear zones is observed in thenorthwestern parts of the Sesia Zone, which host the Ometto ShearZone, a mylonitic belt that generally dips to southeast and ischaracterized by top-to-the-east kinematics at greenschist faciesconditions. This shear zone has been interpreted to have accommo-dated normal motion (Babist et al., 2006). Overprinting relationshipswith the older Chiusella SZ andwith the younger PiemonteMovementZone (see below) indicate that its activity has to be bracketed betweenca. 60 Ma and ca. 42 Ma.

5.2. Middle Eocene deformation (48–38 Ma)

While pre-48 Ma shear zones are limited to the Sesia Zone and tothe Canavese Zone, geochronological evidence indicates that signif-icant deformation affected the Penninic Units and, to a lesser extent,the western margin of the Sesia Zone, in the Middle and UpperEocene, in the 48–38 Ma interval. High-pressure mineral fabricsformed at 48–44 Ma in the eclogitic Piemonte Units (40Ar–39Ar andwhitemica and Rb–Sr onwhitemica and coexisting calcite, epidote, orfeldspar; Reddy et al., 1999, 2003; Dal Piaz et al., 2001; Beltrando et al.,2009), in the Briançonnais basement (40Ar–39Ar and Rb–Sr on whitemica; Monié, 1990; Markley et al., 1998) and cover units (Rb–Sr onthe white mica/calcite pair; Freeman et al., 1998). Due to the per-vasive subsequent overprint, such fabrics are often difficult to inter-pret in terms of kinematics and deformation regime. While somefabrics have been related to axial plane foliations to folds (Markleyet al., 1998), others have been ascribed to top-to-the-northwest shear,possibly related to burial of the units to HP conditions and/or earlystacking at depth (Reddy et al., 2003). This interpretation is supportedby the overlap between the ages of these HP mineral fabrics and theestimated ages for HP metamorphism in the Piemonte Units (seeabove).

A widespread deformation episode, characterized by top-to-the-east kinematics andmetamorphic conditions evolving from blueschistto greenschist facies at ca. 45–38 Ma is then observed to affect all unitsof the Penninic Domain. Shear zones ascribed to this deformationevent are common in the Piemonte Units in the Monte Rosa transect(Wheeler and Butler, 1993; Reddy et al., 1999; Cartwright andBarnicoat, 2002; Reddy et al., 2003; Forster et al., 2004; Fig. 6), in theGran Paradiso transect (Inger and Ramsbotham, 1997; Beltrando et al.,2008, 2009) and in the Dora–Maira transect (Agard et al., 2002).Furthermore, published structural/metamorphic studies from otherparts of the Piemonte Units suggest that such shear zones arewidespread (e.g., Perotto et al., 1983; Caby, 1996; Schwartz, 2002; seeBeltrando et al., 2008 for a review). Top-to-the-southeast shear in thebottom part of the Gneiss Minuti Complex, in the Sesia Zone, has alsobeen ascribed to this episode (Wheeler and Butler, 1993; Reddy et al.,1999; Fig. 6). Similarly, coeval top-to-the-east shear evolving fromblueschist to greenschist facies conditions has been reported alsofrom the Ambin Massif, in the Briançonnais Domain (Ganne, 2003).

The Monte Rosa transect provides important insights into thisdeformation phase because the presence of the UHP Lago di CignanaUnit, which was exhumed mainly during that time interval, hasattracted considerable attention in the geological community.Detailed structural and geochronological studies have shown thatthe Combin Zone, the upper part of the Zermatt–Saas Zone, and thelower part of the Gneiss Minuti Complex and Arolla Gneisses haveaccommodated significant deformation in the 42–36 Ma interval. Thishighly strained domain is here referred to as the Piemonte Movement

Zone (Fig. 6). High-pressure shear fabrics are only locally preserved inthe Lago di Cignana area (Van der Klauw et al., 1997; Forster et al.,2004), while the Combin Zone, in its hangingwall, has accommodatedseveral phases of deformation under different metamorphic condi-tions. Early top-to-the-southeast shear at blueschist facies conditionsat 42–41 Ma was followed by a switch to top-to-the-northwestkinematics at 39–37 Ma (Reddy et al., 1999), before renewed switchto top-to-the-southeast kinematics, generally localized to the Com-bin–Zermatt interface. The significance of these different deformationevents is highly debated, and the crustal excision currently found atthe Zermatt–Combin interface has been alternatively ascribed toextensional deformation (Ballèvre and Merle, 1993; Wheeler et al.,2001; Reddy et al., 2003; Forster et al., 2004), to the activity of an“extraction fault” (Froitzheim et al., 2006) or to the effect of thrusting(Pleuger et al., 2007). The variable kinematics observed along thismovement zone have been related either to strain partitioning along acomplex extensional shear zone (Reddy et al., 1999) or to switchesfrom shortening to extensional deformation (Ballèvre and Merle,1993; Beltrando et al., 2008). Such switches have been advocatedpartly to account for the complex tectono-metamorphic stratigraphyfound in the proximity of the Zermatt–Combin interface, particularlyregarding the presence of slivers of continental basement and of theUHP Lago di Cignana Unit. In this respect, recent studies have shownthat P estimates from the meta-ophiolites of the Zermatt–Saas Zoneand the Lago di Cignana Unit differ by less than 0.3 GPa. Indeed, asshown by Groppo et al. (2009), the presence/absence of coesite is theonly discriminating factor between UHP and HP eclogites, thecomposition and zoning pattern of the major minerals being identical.

Despite the details and controversies listed above, several lines ofevidence indicate that the Piemonte Movement Zone accommodatedsignificant crustal excision through extensional deformation duringan early phase of its activity (Ballèvre and Merle, 1993; Reddy et al.,2003): (1) the Combin and Zermatt–Saas Zones underwent peakpressures at the same time, at ca. 48–44 Ma, were juxtaposed by ca.38 Ma, and shearing was recorded in the movement zone throughoutthe entire time interval between 44 and 38 Ma; (2) the PiemonteMovement Zone intersects the Earth's surface when traced out in adirection opposite to that of the hangingwall transport (Wheeler andButler, 1994; Wheeler et al., 2001).

5.3. Late Eocene–Early Oligocene deformation (36–30 Ma)

Direct geochronological evidence of shear zone activity in the axialzone of the Western Alps in the 36–30 Ma interval is relatively scarceand is invariably associated with mineral assemblages indicative ofgreenschist facies conditions. Shear zones of Uppermost Eocene–Lower Oligocene age have been reported from themargins of the GranParadiso Massif and from the external parts of the axial zone of theWestern Alps, while the activity of several other contacts can bebracketed to this time interval with independent geochronologicaldata.

The controversial Entrelor Shear Zone (Fig. 3b), which marks thewest-dipping contact between the Briançonnais and the PiemonteUnits to the west of the Gran Paradiso Massif, provides the mostinternal evidence of ductile shearing that has been dated in theWestern Alps. This shear zone, for which a Rb–Sr age of 34±1 Ma hasbeen estimated (Freeman et al., 1997), has been alternatively inter-preted as a backthrust (Butler and Freeman, 1996; Freeman et al.,1997), as an extensional shear zone (Brouwer et al., 2002), as an olderthrust contact between the Briançonnais and the Piemonte Units thathas been subsequently folded (Bucher et al., 2003), or as anextensional shear zone reactivated during shortening (Malusà et al.,2005a, 2005b). The backthrust hypothesis was originally formulatedto explain the origin of the west-dipping contact between theBriançonnais and the Piemonte Units, which can be followed fromthe Entrelor area to the Dora–Maira transect (Fig. 3). Zircon fission


track ages show that the Briançonnais basement of the immediatehangingwall of the Entrelor SZ was at shallower crustal levels withrespect to the Gran ParadisoMassif, located in the footwall, during theactivity of the shear zone (Malusà et al., 2009). This observation maysuggest that the footwall of the Entrelor SZ was exhumedwith respectto its hangingwall, thus implying that their contact accommodatedcrustal excision. However, there is evidence of crustal thickening ofsimilar age immediately to the east of the Entrelor SZ (Vearncombe,1985; Beltrando et al., 2008, 2009). Therefore, further thermo-chronological studies will be needed to solve the Entrelor SZconundrum. It is indeed possible that the extensional deformationthat can be inferred from the fission track data post-dates the crustalthickening reported by Vearncombe (1985) and Beltrando et al.(2008).

The margins of the Gran Paradiso Massif and of the Brossasco–Isasca Unit preserve evidence of shear zone activity in the 36–30 Marange. A single Rb–Sr age of 33.2±0.4 Ma has been estimated fromthe greenschist facies shear fabrics found at the contact between thenorthwestern part of Gran Paradiso Massif and the overlying eclogiticPiemonte Unit (Freeman et al., 1997). The activity of other greenschistfacies shear zones, which are found associated with the Gran ParadisoMassif and the Brossasco–Isasca Unit, has been bracketed between theages of 36–34 Ma estimated for the (U)HP metamorphism (Gabu-dianu Radulescu et al., 2009; Gebauer et al., 1997) and the zirconfission track ages of 32–29 Ma (Malusà et al., 2005a, 2005b; Gebaueret al., 1997). This is the case for the greenschist facies mylonite foundat the contact between the Gran Paradiso Massif and the underlyingMoney Unit (Fig. 3b) and for the west-dipping shear zones that roofthe Brossasco–Isasca Unit and the overlying units of continentalbasement in the southern part of the Dora–Maira Massif (Fig. 3c;Avigad et al., 2003). However, until direct dating of such myloniticrocks is performed, the inferred age for their activity hinges on thecorrectness of the data used to bracket it.

The contact between the Briançonnais and the Versoyen–Petit St.Bernard Units has been dated in the Petit St. Bernard area, where Rb–Sr geochronology on the white mica–calcite pair, separated fromgreenschist facies mylonites, yielded ages in the 32–27 Ma range(Fig. 6; Freeman et al., 1998). This contact has been alternativelyconsidered either as an extensional shear zone, which accommodatedpart of the exhumation of the underlying unit from high-pressureconditions, underneath the greenschist facies Zone Houillère (Cannicet al., 1995) or as a thrust along which the Briançonnais was carriedover the more external units, after that the early phases ofexhumation had already taken place (Freeman et al., 1998). Indeed,the conspicuous lack of high-pressure fabrics (Fügenschuh et al.,1999) has so far hindered any unambiguous inference on the earlystages of exhumation of the Versoyen–Petit St. Bernard Unit.

Althoughmost of the Upper Eocene–Lower Oligocene deformationwas limited to the axial zone of the Western Alps, a recent study ofshear zones in the Pelvoux Massif, in the Helvetic Domain (Fig. 1), hasshown that ductile deformation and tectonic burial to greenschistfacies conditions were also affecting the external parts of the WesternAlps at ca. 34–30 Ma (Simon-Labric et al., 2009).

6. Evolution of the Western Alps: From palaeogeographyto orogenesis

The petrological, structural, and geochronological data reviewedabove have been the basis for several models attempting to explainthe tectono-metamorphic evolution of the Adria–Europe convergentplate margin from Cretaceous to Oligocene (for reviews, see Dal Piazand Gosso, 1984; Hunziker et al., 1989). Most models agree inconsidering the Western Alps as consisting of slices of sediments andcrust scraped off the African (Adriatic) and European margins and thePiemonte–Liguria oceanic lithosphere (Coward and Dietrich, 1989).“These scrapings, the Alpine thrust sheets or nappes (Heim, 1921), are

each at most a few kilometers thick, yet stacked on top of each otherthey have produced pronounced crustal thickening” (Coward andDietrich, 1989).

Despite this common starting point, several significantly differentmodels have been proposed to account for the present-day relativeposition of the different units and for their PTtd evolution. A few endmembers, which have been progressively modified in order to fit theever-increasing data set, can be identified (Fig. 7). As is immediatelyapparent from Fig. 7, the orogen dynamics predicted by the differentmodels are highly dependent on the starting geometry of the platemargin prior to orogenesis.

Indeed, the Mesozoic palaeogeography of the area that laterbecame part of the Western Alps has been the source of much debatein the Alpine geological community (for reviews, see Hunziker et al.,1989; Froitzheim, 2001; Rosenbaum and Lister, 2005). Significantlydifferent pre-orogenic settings have been proposed, depending on thecriteria adopted to constrain the original palaeogeographic position ofthe units now stacked in the axial zone of the belt.

In the past 15 years, palaeogeographic reconstructions of theAdriatic and European rifted margins have benefited from theadvances in the understanding of the geometry of present-daymagma-poor rifted margins (Froitzheim and Manatschal, 1996;Manatschal and Bernoulli, 1999). As a result, most authors attributethe Sesia Zone, and possibly the Dent Blanche Nappe, to the hyper-extended Adriatic margin, which was formed during the Jurassicopening of the Tethys (Froitzheim and Manatschal, 1996; Dal Piaz,1999; Babist et al., 2006). It is still unclear whether those continentalbasement units represented the western prolongation of the thinnedAdriatic plate, tapering out to the west (e.g., Lemoine, 1985; Pognanteet al., 1987), the remnants of micro-continents (Platt, 1986), orklippen of thinned continental basement isolated from the Adriaticplate sensu stricto by exhumed mantle (Dal Piaz, 1999).

The Penninic Units, instead, are interpreted to sample the thinnedEuropean margin and the remnants of the Piemonte–Liguria Ocean(e.g., Lemoine et al., 1986). The latter was probably characterized bydifferent branches and V-shaped basins (Trümpy, 1980), similar to thepresent-day margin of Iberia (Péron-Pinvidic et al., 2007). TheBriançonnais Domain is considered to sample a block that was sepa-rated from the European plate by the North Penninic Basin (also calledValaisan Basin). This domain of thinned continental crust was, at leastlocally, floored by exhumed serpentinized mantle (Figs. 8 and 9a;Masson, 2002; Manatschal and Müntener, 2009).

Most reconstructions assume that the stacking order reflects theoriginal relative position of the different terranes and that Alpinedeformation did not significantly alter it (e.g., Dal Piaz et al., 1972;Coward and Dietrich, 1989; Rubatto et al., 1998). Therefore, units thatare located inamore external position (i.e., towards thewest) and lowerin the present-day architecture of the Western Alps are considered toderive from domains that were located progressively closer to theEuropean plate (e.g., Frisch, 1979; Fig. 7b, c, d, and e). This view issupported by several lines of evidence, including (1) type of pre-orogenic cover (e.g., Mohn et al., submitted to publication), (2) type andage of syn-orogenic cover (e.g., Frisch, 1979; Ford et al., 2006), (3) theprogressive younging of the ages of high-pressure metamorphism fromthe more internal to the more external units, which is interpreted toindicate a younging in the age of accretion (e.g., Rubatto et al., 1998).

In this palaeogeographic context, characterized by an oceanicbasin with several branches, Alpine convergence would have beenaccommodated either by a single subduction zone or by twosubduction zones. In the first scenario, the episodic accretion ofcontinental units to the orogenic belt resulted in the progressivemigration of the subduction zone hinge towards the northwest(Fig. 7e; Frisch, 1979; Platt, 1986; Rosenbaum and Lister, 2005). In thesecond scenario, instead, two subduction zones were active at thesame time along the Piemonte and the Valaisan sutures (Fig. 7d and e;Laubscher, 1974; Avigad et al., 1993; Herwartz et al., 2008).

Fig. 7. Selected models of theWestern Tethys palaeogeographic setting and of the tectonic evolution of theWestern Alps. Colours for the different units are the same as in Fig. 1. HD:Helvetic–Dauphinois; ICM: Internal Crystalline Massifs.


Other models consider orogenesis as a more chaotic processinvolving significant tectonicmixing of the different units, making anydirect correlation between present-day architecture and Mesozoicpalaeogeography ambiguous. This alternative view is based on thefrequent ambiguities in the basement-cover relationship in severalhighly deformed/metamorphosed units and on the controversiesregarding the age of HP metamorphism in the Internal CrystallineMassifs (see discussion below). In this view, the basal traction exertedby the downgoing lithosphere on the hangingwall leads to the so-called “tectonic erosion” of units from the Adriatic plate and to theiraccretion to the mountain belt (Dal Piaz et al., 1972). This idea,originally proposed to account for the origin of the Sesia Zone (Fig. 7b;Dal Piaz et al., 1972), was later developed further (Hunziker andMartinotti, 1984; Polino et al., 1990; Stöckhert and Gerya, 2005) topropose that all of the Penninic Units of continental affinity wereoriginally located on the Adriatic margin. Tectonic erosion of suchmargin during the Cretaceous–Tertiary subduction of the Piemonte–Liguria oceanic lithosphere led to the burial to HP conditions (Fig. 7a).Such models were designed mainly to account for the very limitedthickness of the continental basement units found in the WesternAlps, which never exceed a few kilometers. Indeed, if such thin unitshad to represent slivers of basement scraped off the downgoingmicro-plates, unrealistic amounts of continental crust and lithosphereshould have been subducted underneath the Alps (Polino et al., 1990).This mass balance problem has later been successfully solved with therecognition that some Alpine terranes may sample thin slivers ofcontinental basement located in proximity of the hyper-extendedcontinental margins (Froitzheim and Manatschal, 1996).

7. Evolution of the Western Alps: An updatedtectonic reconstruction

The plate kinematic framework provides important constraints forthe interpretation of the geodynamic significance of the PTtd data

from the axial zone of the Western Alps (Fig. 8). Plate kinematicreconstructions based on Atlantic magnetic anomalies allow a poorresolution as to the onset of the N-ward motion of the African plate,constraining it to the 120–83 Ma interval, during the CretaceousNormal Superchron (Dewey et al., 1989; Rosenbaum et al., 2002). Thewidespread, synchronous inversion of sedimentary basins and theevidence of basement thrusting on the European plate from Provenceto Scandinavia and in North Africa starting at ca. 90–86 Ma aregenerally considered to provide a better estimate of the onset of NNEmotion of the African plate (Ziegler et al., 1995; Kley and Voigt, 2008).High-pressure metamorphism in the Eastern Alps at 90–88 Ma (Thöniet al., 2008) and in the Malenco area at 91–83 Ma (Villa et al., 2000)and the formation of a fold-and-thrust belt in the Graubunden area, inthe westernmost Eastern Alps, at ca. 93–90 Ma (Froitzheim et al.,1994) provide further evidence for a major change in plate kinematicsat that time.

This NNE-ward drift ceased between 60 and 52 Ma (Rosenbaumand Lister, 2005; Fig. 8), with the onset of a relatively constant NNW-ward direction of relative motion, which has been maintained untiltoday (DeMets et al., 1994).

As is apparent from this brief outline of Cretaceous to Tertiary platekinematics (Fig. 8), the evolution of the Western Alps cannot betreated as a 2-dimensional process, where the current Alpinegeometry is “unfolded” into a Cretaceous palaeogeography with allunits aligned along a NW–SE profile. Significant Cretaceous N-wardmotion of the Adriatic plate, which has been estimated at ca. 200–300 km (Rosenbaum et al., 2002), suggests that the formation of theWestern Alps is characterized by two well-defined phases: (1) an earlyphase, during which the Cretaceous Alps formed at the expense of thehyper-extended Adriatic margin, along a plate boundary sub-parallel torelative plate motions; and (2) a second phase, which started in thePalaeogene, during which the relative plate motion was generallyperpendicular to the main structural trends inherited from the Jurassicrifting (Fig. 9; e.g., Lemoine, 1985). Therefore, the present-dayWestern


Alps represent the amalgamation of these twodifferent orogenic phasesthat, as will be discussed below, show specific characteristics.

7.1. The Cretaceous Alps: Oblique subduction

As summarised above, several lines of evidence bracket the onsetof the northward motion of the African plate at ca. 93–86 Ma.Evidence of deformation/metamorphism in the Western Alps at thattime is scarce. Ages of 90–70 Ma have been found with 40Ar/39Argeochronology in the Eclogitic Micaschists Complex (Inger et al.,1996; Ruffet et al., 1995). K–Ar dating of metamorphic white micasfrom the anchizone-sub-greenschist facies rocks of the Canavese Zoneyielded ages as old as 75 Ma (Zingg et al., 1976), suggesting thatAlpine metamorphism affected that area in the Campanian. Platekinematic and palaeogeographic reconstructions imply that thewestern margin of the Adriatic plate was oriented at a low anglewith respect to the direction of relative plate motion, resulting in apredominant strike–slip deformation (Coward and Dietrich, 1989;Figs. 8 and 9a, b). Sinistral strike–slip deformation is recorded alongthe Internal Canavese Line and predominant strike–slip deformationhas been proposed for the burial and exhumation of the EclogiticMicaschists Complex (Ridley, 1989; Babist et al., 2006). Remarkablyconstant N–S stretching lineations, marked by high-pressure amphi-boles and pyroxenes, are also found throughout the EclogiticMicaschists Complex (Vuichard, 1986). Therefore, the CretaceousAlps developed at least starting from ca. 75 Ma at the expense of thethinned Adriatic margin, in a geodynamic setting characterized byhighly oblique subduction of the Piemonte–Liguria slab (Fig. 9b). Thedifferent tectono-metamorphic units that are now grouped in theSesia Zone either could have been part of a single extensionalallochthon (Froitzheim and Manatschal, 1996) or may have repre-sented multiple independent slivers of continental basement genet-ically related to the hyper-extended Adriatic crust (Dal Piaz et al.,2001; Babist et al., 2006).

The mechanisms that led to the burial, accretion, and exhumationof the individual tectono-metamorphic units are difficult to constrain,due both to the significant overprint experienced by the Cretaceous

Fig. 8. Plate reconstruction of the Western Tethys and surrounding areas in theSantonian (84 Ma) [modified fromManatschal and Müntener (2009)] and motion pathof Africa relative to Europe, in the 110–20 Ma interval, shown by one point (45°N/8°E)that moves together with Africa (from Rosenbaum and Lister, 2005). Numbers indicatetime in millions of years. In the Late Cretaceous, Africa moves towards the NE withrespect to Europe, while after 60 Ma convergence switches to the NNW direction.AA=Adria thinned margin and allochthons.

structures during subsequent deformation and to the lack of adequatethermo-chronological studies. Available data indicates that the EMS,after reaching high-pressure conditions at ca. 70–65 Ma (Inger et al.,1996; Duchêne et al., 1997; Rubatto et al., 1999), was rapidly ex-humed to blueschist facies conditions and juxtaposed to the 2DK byca. 65–60 Ma (Babist et al., 2006). Transpression (Ridley, 1989; Babistet al., 2006) and transtension (Avigad, 1996) have been proposed asfactors responsible for such exhumation.

7.2. Lower Eocene Alps: Accretion of the Lanzo Massif/Southern Gneiss/Minuti Complex/Dent Blanche Nappe

SSE–NNW-directed convergence, perpendicular to the Jurassicstructural trends, followed the Cretaceous strike–slip motion at ca.60–52 Ma (Fig. 8). The onset of high-pressure metamorphism in theLanzo Massif (Rubatto et al., 2008) and in the Dent Blanche Nappe(Ayrton et al., 1982), constrained at ca. 55 Ma, may be related totectonic burial and accretion of these units induced by the abruptchange in plate kinematics. Parts of the Gneiss Minuti Complex mayhave undergone a similar evolution, as suggested by the presence of alayer of omphacitite at the contact between the northern edge of theLanzo Massif and the GM in the southern part of the Sesia Zone(Compagnoni and Sandrone, 1986). Therefore, while the GneissMinuti Complex is commonly abscribed to the Sesia Zone (Fig. 1),with the implicit assumption that it underwent HP metamorphism inthe Cretaceous, it is indeed possible that it underwent eclogite faciesmetamorphism significantly later than the EMS–2DK pair (Fig. 2). Thispossibility needs to be tested by direct dating of the eclogite faciesmetamorphism of the Gneiss Minuti Complex, which is still lacking.

The Ometto Shear Zone, which accommodated extensionaldeformation under greenschist facies conditions in the northernpart of the Eclogitic Micaschists Complex between 60 Ma and 42 Ma(Babist et al., 2006), suggests that ductile deformation was probablyaffecting the hangingwall units while the LanzoMassif, Gneiss Minuti,and Dent Blanche were being accreted.

7.3. Middle Eocene Alps: Collision with the Briançonnais Block

Ongoing NNW-directedmovement of the Adriatic plate resulted inthe progressive reduction of the Piemonte–Liguria oceanic basin(Fig. 9c). Remnants of such a basin, which originally was severalhundreds of kilometers wide (Dewey et al., 1973; Lemoine et al.,1986; Stampfli et al., 1998; Lombardo et al., 2002), are now foundexposed in the Piemonte Units. As noted by several authors, theophiolitic basement and cover that are presently cropping out in theWestern Alps represent only a small fraction of the original volume ofrocks (Agard et al., 2009; Manatschal and Müntener, 2009). This massdeficit is not surprising in ophiolitic units because negatively buoyantoceanic lithosphere has a well-known tendency to escape accretionand sink into the asthenosphere (e.g., Garfunkel et al., 1986). Tomo-graphic images of the Western Alps area show the presence ofabundant lithospheric material, which may represent the remnantsof the Piemonte–Liguria slab, underneath the Adriatic plate (Lippitschet al., 2003). Petrographic investigations of the eclogitic rocks of Lagodi Cignana support this view, revealing a rapid decrease in thegeothermal gradient in the 50–44 Ma interval (Li et al., 2004; Groppoet al., 2009). This variation has been interpreted to indicate a markedincrease in the vertical velocity of the subducting Piemonte–Liguriaslab during subduction (Groppo et al., 2009).

The accretion of a relatively small section of ophioliticmaterial wasprobably made possible by the arrival of more buoyant Briançonnaiscontinental crust at the subduction zone (Frisch, 1979; Nur and Ben-Avraham, 1982; Brun and Faccenna, 2008). Two lines of evidenceindicate that the Piemonte meta-ophiolites were originally located ina proximal position with respect to the thinned Briançonnais margin:(1) the presence of slices of pre-rift sediments of Briançonnais and

Fig. 9. Schematic evolution of theWestern Tethys andWestern Alps from Jurassic rifting to Alpine orogenesis. (a) Shows a simplified Jurassic palaeogeography at the onset of mantleexhumation in the Valais and Piemonte–Liguria Basins (modified after Mohn et al., submitted to publication). Note that the Valais is interpreted to die out laterally, whereas thePiemonte–Liguria Basin is evolving into an embryonic oceanic domain. TheWestern Alps represent the amalgamation of a Cretaceous orogen (b), which developed at the expense ofthe Adriatic Ocean–Continent Transition Zone along a highly oblique subduction boundary and of an Eocene orogen (c), which formed as a result of the collision of the proto-Alpswith the Briançonnais terrane and the European plate sensu stricto. PLO=Piemonte–Liguria Ocean; PA=Proto-Alps.


Pre-Piemontese affinity resting upon, or in close proximity with, theophiolitic basement (Dal Piaz, 1999); (2) the presence of slivers ofcontinental basement in the same structural position (Hermann,1937; Ballèvre et al., 1986; Dal Piaz, 1999; Beltrando et al., 2010). Suchfeatures are considered to be characteristic of Ocean–ContinentTransition (OCT) Zones, which are preserved with little Alpinedeformation in the Central Alps and have been drilled off along theAtlantic margin (Manatschal, 2004).

While the OCT underwent (U)HP metamorphism, the thinnedsoutheasterly margin of the Briançonnais block re-equilibrated atvariable metamorphic conditions ranging from eclogite facies in theMonte Rosa, to blueschist facies in the Ambin Massif, to progressivelylower grades towards the foreland. Despite the general agreement onthe crucial role of the arrival of buoyant crust at the subduction zonefor the accretion of the thinned Briançonnais margin and of theBriançonnais OCT (Angiboust et al., 2009; Groppo et al., 2009), themechanisms that led to the rapid exhumation of the different units tonear-surface conditions are still debated. Even though the role of

extensional deformation in the exhumation of the different units fromblueschist facies conditions has been increasingly recognized anddocumented (Ballèvre and Merle, 1993; Wheeler and Butler, 1993;Reddy et al., 1999, 2003; Wheeler et al., 2001; Ganne, 2003; Forsteret al., 2004; see Froitzheim et al., 2006 for contrasting views), little isknown with regard to the early phases of exhumation of the eclogiticPiemonte Units. The lack of eclogite facies fabrics that could beunambiguously related to the general kinematic framework leavesroom for speculation and for largely different views as to the mecha-nisms driving the early phases of exhumation.

The widespread presence of blueschist to greenschist faciesextensional shear zones throughout the units that had already beenaccreted to the Western Alps points towards a generalized episode ofextensional deformation affecting the orogenic belt at ca. 42–38 Ma.According to Rosenbaum and Lister (2005), this widespread extensionmay have been related to the rapid retreat of the hyper-extendedEuropean margin that, after the accretion of the Briançonnais Block,was being subducted underneath the Western Alps (Frisch, 1979;


Stampfli et al., 1998). Although the presence of mafic crust betweenthe Briançonnais block and the European plate sensu stricto is highlycontroversial, several studies have shown that the Valaisan Basin was,at least locally, floored by exhumed serpentinized mantle (Fig. 9a;Masson, 2002). Therefore, the “Valaisan lithosphere”, which repre-sented the thinned-out European margin, would have been stillcharacterized by the negative buoyancy typical of oceanic lithosphere.

7.4. Upper Eocene–Lower Oligocene: The onset of collision

The arrival of the European continent sensu stricto at the sub-duction zone marked the onset of continental collision in theWesternAlps. This event is recorded by the formation of greenschist faciesshear zones in the External Crystalline Massifs (Simon-Labric et al.,2009) and by the sedimentary evolution of the foreland basins (e.g.,Sissingh, 2001). Renewed tectonic burial of Piemonte rocks, whichhad already been exhumed to shallow depths, together withwidespread shortening deformation in the Alpine orogen after ca.36 Ma (Beltrando et al., 2009) suggest a link between the onset ofcontinental collision and the deformation regime of the orogenic belt.

The onset of collision was followed by the rapid exhumation of theGran Paradiso Massif and Brossasco–Isasca Unit from (U)HP depths.This rapid exhumation of rock units has been ascribed to a short-livedepisode of lithospheric thinning that affected the whole Alpine Belt.This tectonic event, which is well-preserved in the structural, meta-morphic, and magmatic record of the Central Alps, has beenalternatively ascribed either to the break-off of the Piemonte slabunderneath the orogenic roots (von Blanckenburg and Davies, 1995)or to the roll-back of the north-dipping African slab (Beltrando et al.,submitted for publication). However, the details of the LowerOligocene evolution of the Western Alps go beyond the purpose ofthis paper, and the reader is referred to the above references for adetailed discussion.

Renewed, post-30 Ma shortening, responsible for the propagationof the Western Alps front to the European foreland, is largelyresponsible for the lithospheric-scale structures imaged in seismicsections and for the low-grade deformation recorded within the axialzone of the belt, especially along the Insubric Line (e.g., Schmid et al.,1989; Fig. 3a, b).

8. Open problems

Several first-order features of the Western Alps, which haveimportant implications for the orogen dynamics, are still enigmatic:

1. The origin and nature of the Gneiss Minuti Complex. This unit istraditionally considered to be part of the Sesia Zone, implying that itunderwent HPmetamorphism in the Cretaceous. However, specificparts of the GneissMinuti Complex sample aMesozoic sedimentarysequence that, at least in the area of Lanzo, was deposited overexhumed serpentinizedmantle of the Piemonte–Liguria Ocean. Theonly published ages for the high-pressure metamorphism of theLanzoMassif currently indicate an Eocene age (Rubatto et al., 2008),implying that at least parts of the GMwere accreted to the evolvingWestern Alps significantly later than the Eclogitic MicaschistsComplex and the 2DK. Further studies on the nature of the GMand on the age of its high-pressure metamorphism will indicatewhether or not this is a common feature.

2. The palaeogeographic position of the Lanzo Massif. The Lanzo Massifhas been alternatively placed (a) between the Adriatic plate andthe Sesia Terrane (Rosenbaum and Lister, 2005), (b) along theAdriatic rifted margin, or (c) along the European rifted margin(Manatschal and Müntener, 2009). The age of the HP metamor-phism of the Lanzo Massif (Rubatto et al., 2008) and the currentposition of the LanzoMassif/Southern Gneiss Minuti pair in the belt

seem to suggest that the Lanzo Massif was originally located to thenorthwest of the Eclogitic Micaschists Complex.

3. The palaeogeographic position of the Brossasco–Isasca, Gran Paradiso,and Monte Rosa Units and their age of accretion to the Alpine Belt.These units have been alternatively considered to derive eitherfrom the easternmost part of the hyper-extended BriançonnaisDomain (e.g., Elter, 1972) or from the European margin sensustricto (Froitzheim, 2001). Indeed, the possibility that the InternalCrystalline Massifs did not originate from a unique palaeogeo-graphic position should also be considered.

4. The significance and role of extensional deformation within the axialzone of the belt. It is unclear whether extension represents a localexception to bulk shortening or whether it can be a transientdeformation mode that dominates at the scale of the orogen. In thefirst case, extension is related to strain partitioning within anevolving lithosphere-scale accretionary prismorwithin a subductionchannel (e.g., Platt, 1986; Wheeler et al., 2001; Agard et al., 2009),while in the second case extension is linked to short-lived episodes ofcrustal or lithospheric thinning along the convergent Africa–Europeplate boundary (Rosenbaum and Lister, 2005; Beltrando et al., 2008).

The above issues have important implications for understandingthe dynamics of orogenic belts and, more specifically, for the long-standing debate between the “para-autochthonous” view of orogen-esis, according to which the stacking order reflects the originalpalaeogeographic distribution of the different units (e.g., Dal Piaz et al.,1972; Coward andDietrich, 1989; Rubatto et al., 1998; RosenbaumandLister, 2005) and the “chaotic” views of orogenesis, which put anemphasis on the complex dynamics of orogenic wedges, where theoriginal relationships between palaeogeographic units are significant-ly altered by the formation of orogen-scale tectonic mélanges (e.g.,Polino et al., 1990; Gerya et al., 2002; Bousquet, 2008).

9. Conclusions

The evolution of the Western Alps is best understood by con-sidering that they developed at the expense of two passive con-tinental margins that formed during the opening of the Tethys. Riftingresulted in a complex pre-orogenic palaeogeography, characterizedby ocean–continent transition zones on both margins. Integration ofthis palaeogeographic setting with first-order plate kinematicreconstructions indicates that the Cretaceous Alps developed alonga highly oblique subduction boundary at the expense of the AdriaticOCT. A switch to NW-directed motion of the Adriatic plate led to thecollision of the proto-Alps, first with the Briançonnais block and thenwith the European plate sensu stricto The onset of final continentalcollision led to renewed shortening deformation and, at least locally,to a second episode of tectonic burial of the hangingwall units. Theaccretion of rock units during each collisional episode, at ca. 44 and35 Ma, was followed by rapid exhumation at ca. 42–38 Ma and 33–32 Ma. While the last phases of exhumation were demonstrablyaccommodated by extensional deformation, several uncertaintiessurround the earlier parts of the burial–exhumation history. Morespecifically, the mechanisms leading to the accretion of the UHP unitsto the orogen and to the first stages of their rapid exhumation are stillunclear.

Acknowledgments

M.Malusà, G. Manatschal, H. Masson, J. Hermann, C. Groppo, and C.Cigolini are gratefully thanked for discussions.

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(Ultra-) High-pressure metamorphism and orogenesis: An Alpine perspective