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J. metamorphic Geol., 1999, 17, 573–589

The geometry and timing of orogenic extension: an example from theWestern Italian AlpsS. M. REDDY 1* , J . WHEELER 1 AND R. A. CLIFF2

1Department of Earth Sciences, The University of Liverpool, Liverpool, L69 3BX, UK, 2Department of Earth Sciences, TheUniversity of Leeds, Leeds, LS2 9JT , UK

ABSTRACT Contacts between rocks recording large differences in metamorphic grade are indicative of major tectonicdisplacements. Low-P upon high-P contacts are commonly interpreted as extensional (i.e. material pointson either side of the contact moved apart relative to the palaeo-horizontal ), but dating of deformationand metamorphism is essential in testing such models. In the Western Alps, the Piemonte Ophioliteconsists of eclogites (T#550–600 °C and P#18–20 kbar) structurally beneath greenschist facies rocks(T#400 °C and P#9 kbar). Mapping shows that the latter form a kilometre-wide shear zone (theGressoney Shear Zone, GSZ) dominated by top-SE movement related to crustal extension. Rb–Sr datafrom micas within different GSZ fabrics, which dynamically recrystallized below their blocking tempera-ture, are interpreted as deformation ages. Ages from different samples within the same fabric are reproduc-ible and are consistent with the relative chronology derived from mapping. They show that the GSZ hadan extensional deformation history over a period of c. 9 Myr between c. 45–36 Ma. This overlaps in timewith the eclogite facies metamorphism. The GSZ operated over the entire period during which the footwallevolved from eclogite to greenschist facies and was therefore responsible for eclogite exhumation. Thediscrete contact zone between eclogite and greenschist facies rocks is the last active part of the GSZ andtruncates greenschist facies folds in the footwall. These final movements were therefore not a majorcomponent of eclogite exhumation. Pressure estimates associated with old and young fabrics within theGSZ are comparable, indicating that during extensional deformation there was no significant unroofingof the hangingwall. Since there are no known extensional structures younger than 36 Ma at higher levelsin this part of the Alps, exhumation since the final juxtaposition of the two units (at 36 Ma) seems tohave been dominated by erosion.

Key words: deformation age, eclogite, exhumation, Rb–Sr dating, tectonic.

thermal and baric histories of the footwall andINTRODUCTION

hangingwall, facilitate the recognition of extensionalstructures (Wheeler & Butler, 1994).The processes by which high-P rocks reach the Earth’s

surface are important to an understanding of the Even if genuine, extensional structures may developafter high-P rocks in their footwalls have alreadythermal and barometric evolution of metamorphism

and the preservation potential of peak metamorphic undergone partial exhumation to low-P conditions(Fig. 1). In such cases, the structure associated withassemblages. Erosion is one possible exhumation

process. However, crustal extension has been recog- the metamorphic break observed in the field may havecontributed to only a minor component of thenized as another important unroofing process (Platt,

1986; Dewey, 1988). Diagnosing extensional structures exhumation history of the footwall and the mostsignificant structure may remain unrecognized.within the internal zones of orogens is difficult because

the orientations of layering and shear zones at the The recognition of multiple extensional histories maybe difficult in the field. Late structures may passivelytime of deformation are usually unknown (e.g. Wheeler

& Butler, 1994). Furthermore, post-extensional defor- carry earlier extensional structures in their hangingwallor may completely rework earlier fabrics (Fig. 2). In themation may have significantly modified original geo-

metries by bulk rotation. This may result in extensional former, fabrics preserved in the hangingwall would haveformed during the early extensional deformation historystructures being re-oriented into apparent thrust–sense

displacements (and vice versa). However, criteria based while in the latter only the younger fabrics would bepreserved. Consequently, even when extensional struc-on the structure of major contacts, together with thetures can be established, the relationship betweenextension and exhumation requires quantification of the*Now at: Tectonics Special Research Centre, School of Appliedtime and duration of extensional shearing and theGeology, Curtin University of Technology, PO Box 41987,

Perth, WA 6845, Australia. ([email protected]) timing and rate of exhumation of the footwall. It is

573© Blackwell Science Inc., 0263-4929/99/$14.00Journal of Metamorphic Geology, Volume 17, Number 5, 1999

574 S. M. REDDY ET AL .

Fig. 2. Two models depicting the (a) localized and (b) completereworking of early extensional hangingwall fabrics duringextension. In (a), localized late shearing leads to preservation ofearly extensional fabrics. In (b), all of the early fabrics areoverprinted. Both geometries would be identical in the field but

Fig. 1. Illustration to show that presently observed could be resolved by radiometric dating of the different fabrics.metamorphic breaks (box in bottom figure) need not representthe significant extensional structure related to footwallexhumation (1st extensional phase) but a later, minor structure

structural and geochronological. These are given in(2nd extensional phase).the subsequent two sections, followed by a synthesisand discussion.

therefore essential that the ages of fabrics developed inthe footwall, hangingwall and in the extensional fault

GEOLOGY OF THE WESTERN ALPINEzone be known.

INTERNAL ZONESIn this paper, we assess the role of a regionally

extensive shear zone that is associated with a significant The Pennine Basement Massifs, the Piemonte Unitand Austroalpine Basement are the three major unitsmetamorphic break between eclogite and greenschist

facies rocks. The relationships between deformation of the alpine internal zones. Structurally lowest,the Pennine Basement Massifs (Monte Rosa, Granfabrics and metamorphic assemblages in the hang-

ingwall and footwall of the metamorphic break are Paradiso and Dora Maira) represent European conti-nental basement. The Piemonte Unit comprisesused to infer details of the exhumation process. We

integrate these data with the dating of deformation Jurassic oceanic material that separated the Europeanand African continental plates prior to collision. Thefabrics to constrain the timing and duration of

deformation and relate this to processes taking place Austroalpine Basement (Sesia Zone and Dent BlancheKlippe) forms part of the Adriatic (African) platein other areas of the orogen. Our chosen area for this

study was the Western European Alps because (1) the beneath which the other two units were subductedduring Alpine convergence (see Coward & Dietrich,metamorphic setting is well known, and the large

pressure differences across discrete contacts are well 1989). The Pennine Basement Massifs and theAustroalpine Basement contain evidence of pre-Alpinedocumented; (2) preliminary work (Wheeler & Butler,

1993) has shown genuine crustal extension associated metamorphism. However, all three major units havelocally developed eclogitic rocks of Alpine age. Thewith greenschist facies shearing above high-P rocks;

(3) complete transects across the Alps are exposed, so units also contain areas that show no evidence ofhaving reached eclogite facies, but record greenschistthat our results can be related to structures and events

elsewhere in these transects. We begin by giving general facies mineral assemblages. In the Piemonte Unit,greenschist facies rocks lie at structurally higher levelsfeatures of Western Alpine geology and then summariz-

ing metamorphic conditions. The main new data are than immediately underlying eclogite facies rocks, and

THE GEOMETRY AND TIMING OF OROGENIC EXTENSION 575

this unit therefore preserves a metamorphic break structurally lower Zermatt-Saas Zone, metamorphosedat eclogite facies conditions, and the overlying green-which may be associated with crustal extension. Here

we investigate this metamorphic break. The nature of schist facies Combin Zone. In the area studied, theZermatt-Saas Zone comprises dominantly metabasic,the breaks in the Sesia Zone is the subject of separate

studies (Reddy et al., 1996; Pickles et al., 1997). metagabbroic and serpentinitic lithologies, while theCombin Zone contains significant amounts of carbon-The area lies to the north of the Val d’Aosta in the

Gressoney and Ayas valleys, which provide excellent ate-bearing lithologies (calcschists) and metabasites.These lithologies are interbanded at the centimetre tooutcrop through the three major units of the internal

zones (Fig. 3). At the lowest structural levels, the tens of metres scale. Serpentinites are also commonwithin the Combin Zone and associated with these areMonte Rosa Unit comprises garnet mica schists,

metagranites and metabasites that are commonly serpentinite breccias that are cemented by a carbonatematrix. Locally metagabbros and quartz schists aredifficult to constrain in terms of metamorphic

conditions. However, metabasites may preserve also present. In the Piemonte Unit of Val Gressoney,the contact between the Zermatt-Saas and Combinomphacite + garnet + glaucophane + zoisite + white

mica, and indicate that Alpine eclogite facies metamor- Zones is a planar feature (Fig. 4a,b). This contact isthe focus of our study. However, before describing it,phism affected this area of Monte Rosa Basement

(Bearth, 1952; Dal Piaz & Lombardo, 1986). At the the metamorphic history of the two units either sideof the contact is discussed.highest structural levels exposed in the working area

(Fig. 4a), the Gneiss Minuti Complex (GMC) of theSesia Zone comprises dominantly fine-grained quartz–

METAMORPHISM IN THE PIEMONTE UNIT OFfeldspar schists, locally with augen orthogneiss, mica VAL GRESSONEYschists and metagabbros. In the study area, there is noevidence of early high-P metamorphism in the GMC. Zermatt-Saas Zone

Between the two basement units, the Piemonte Unitcomprises rocks of ophiolitic affinity that can be The Zermatt-Saas lithologies, particularly the metabas-

ites, commonly retain the high-P mineral assemblagesubdivided into two main lithotectonic units; the

Fig. 3. Geological map of the internal zones of the Western Alps showing location of study area. MR, Monte Rosa; GP, GranParadiso; DM, Dora Maira; DB, Dent Blanche.


Fig. 4. (a) Geological map of the Upper Val Gressoney. All contacts are tectonic but the Combin/Zermatt-Saas contact ishighlighted as a thick line. (b) Geological Cross Sections through the Upper Val Gressoney (see (a) for key and locations).

omphacite and garnet. Metamorphic conditions of grow on the retrograde part of the P–T path, particularlyin zones of deformation (see later structural section).550–600 °C and 18–20 kbar have been suggested for

large areas of the Zermatt-Saas Zone (Barnicoat &Fry, 1986; Ganguin, 1988), while the presence of coesite

Combin Zoneindicates pressures in excess of 26 kbar (Reinecke,1991, 1998). These data suggest either that the Zermatt- The metabasites of the Combin Zone commonly

have the greenschist facies mineral assemblageSaas Zone represents a number of smaller, tectonicallyjuxtaposed units that were originally subducted to actinolite+albite+chlorite+epidote. Relevant reac-

tion lines that constrain the metamorphic conditionsdiffering depths, or it was an original ultra-highpressure unit which was pervasively overprinted by c. are shown in Fig. 5. The Combin metabasite assem-

blages lie:20 kbar assemblages. We cannot distinguish betweenthese alternatives, but note that in both cases, the 1 On the down-pressure side of the line

Tr+Ab+Cln=Gln+Zo+Qtz+W which, for a nom-UHP relics would be older than the surrounding high-P rocks because the UHP rocks are enveloped and inal 400 °C, is <9 kbar.

2 On the up-temperature side of Pmp+Cln+reworked by high-P rocks.The common mineral assemblage within the high-P Qtz=Tr+Zo+W, therefore T>300 °C.

3 On the down-temperature side of Chl+Czo+Zermatt-Saas metabasites is omphacite+garnet+paragonite+phengite+rutile+glaucophane+zoisite+ Qtz=Prp+Tr+W. This places T<500 °C.

However, the univariant reaction used here is fortitanite+hornblende/actinolite±albite. This suite ofminerals does not represent an equilibrium assemblage the Mg end-member system, and the addition of Fe

would stabilize garnet to lower temperatures, as wouldbut the progressive and often complete retrogression ofthe eclogite assemblage during exhumation. Ca and Mn (Evans, 1990). Therefore, we consider

450 °C to be the maximum temperature at which thisGlaucophane may develop both at the peak eclogitefacies in Mg-rich rocks. However, it also appears to assemblage could exist.


pressures at which the Combin and Zermatt-SaasZones were metamorphosed (9 and 18 kbar, respect-ively). This difference cannot be explained by geo-chemical variations in the protolith or by varyingdegrees of greenschist facies overprint (Dal Piaz et al.,1972; Kienast, 1973). Consequently, a metamorphicbreak, with a pressure difference of c. 9 kbar, separatesthe two units.

THE STRUCTURAL GEOLOGY OF THE UPPERVAL GRESSONEY

Structure of the greenschist facies rocks

At the highest structural levels, the Gneiss MinutiComplex (GMC) of the Sesia Zone has a subhorizontalfoliation defined by a micaceous schistosity (S1gm)(Fig. 6a). Quartz, actinolite, white mica and feldsparcommonly define a mineral aggregate stretching lin-eation that plunges shallowly to the SE (Fig. 6b). Noevidence of high-P mineral parageneses has beenobserved, and the different assemblages within thedifferent lithologies of the GMC are consistent withgreenschist facies metamorphic conditions. Kinematic

Fig. 5. P–T grid illustrating important reactions constraining indicators show a consistent top-to-SE sense of shearthe metamorphic conditions in the Combin Zone. All mineral at the lowest structural levels (10–100 m) within theabbreviations are from Kretz (1983, 1994). All lines are after GMC. At structurally higher levels, the fabrics in theFrey et al. (1991) except Chl+Czo+Qtz=Prp+Tr+W which

GMC have a different orientation and fewer kinematicis after Evans (1990).indicators.

In the Combin Zone, a greenschist mineral assem-blage (S1co) defines a dominantly SSW dipping foliation(Fig. 6c). The Combin Zone calcschists have mineralCalcschists with calcite+quartz+white mica±

chlorite±zoisite±albite±titanite±tremolite/actinolite stretching lineations defined by calcite, quartz andaggregates of white mica grains. Aligned actinoliteassemblages are intimately interleaved with meta-

basites. The similar style and geometries of fabrics grains define a mineral stretching lineation in themetabasites. The orientation of these lineations issuggest that the two lithologies have equilibrated at

similar conditions. Maximum pressures are constrained NW–SE (Fig. 6d) with the greatest concentration oflineations oriented 100/130. Commonly shear bandsby the reaction Ab+Cln+Qtz=Gln+Pg+W (Fig. 5).

However, there is some disagreement as to the position and mica fish structures provide kinematic indicators.At both the macro- and microscopic scales, mineralof this line in P–T space. Guiraud et al. (1990)

suggested that it lies at c. 11 kbar at 400 °C, while stretching lineations can be traced continuously intoshear bands suggesting that the shear bands are a trueFrey et al. (1991) reported 9 kbar. Neither reaction

provides a tighter constraint than the pressure limit reflection of the overall sense of shear.Abundant kinematic indicators show a top-to-SEfor the metabasites (<9 kbar).

Both metabasites and calcschist contain phengite sense of shear on the eastern side of upper ValGressoney. However, to the west of the valley and inthat can be used to constrain a minimum pressure

estimate based on Si content (Massonne & Schreyer, Val d’Ayas, there is a lens of schist showing top-to-NWkinematics bounded by zones of top-to-SE shearing1987). In many samples this minimum pressure

estimate exceeds the maximum pressure determined by towards the base and top of the Combin Zone (Fig. 4b;sections A & B). The relative timing of this and theother methods and may even place the sample above

the Ab=Jd+Qtz reaction line. The Massonne & SE-directed shear can be inferred from: (i) the discor-dance of the top-to-NW panel to the base of theSchreyer (1987) calibration has been queried by other

workers as yielding pressures that are too high (e.g. Combin Zone (west end of section B in Fig. 4b), and(ii) by truncation of the top-to-NW panel by top-Essene, 1989). In conclusion, the best estimate of P–T

conditions of c. 9 kbar and 300–450 °C from the to-SE fabrics in the east (towards B∞ in section B).These observations suggest that the top-to-NW zoneCombin Zone is derived from the metabasic rocks.

In the Gressoney Valley, there is no evidence for of shear is truncated by top-to-SE shear at both thebase and the top of the Combin Zone. The possibilityCombin Zone rocks having been to eclogite facies.

Therefore, there is a significant difference between the of more than one age of top-to-SE shear is also seen


Fig. 6. Lower hemisphere equal-area stereo-nets from greenschist facies units. (a) Poles to foliation in the Gneiss Minuti Complex;(b) mineral stretching lineations in the Gneiss Minuti Complex; (c) poles to foliation in the Combin Zone; (d) mineral stretchinglineations in the Combin Zone; (e) & (f ) Combin Zone fold data.

in cross-sections across the area. For example, geo- kilometre scale (grid ref. 127786; 45 °51∞N, 07 °52∞E).Parasitic hinges to this major fold are parallel to themetries near B∞ (Fig. 4b; section B) imply truncation

of one panel of top-SE sheared calcschists by another. regional mineral stretching lineation and well devel-oped top-to-SE kinematics are present on both limbsEvidence for macroscopic folding is sparse within

the Combin Zone and a detailed description and of the fold. This fold cannot be traced to lowerstructural levels and it does not fold the Zermatt-Saas/discussion is beyond the scope of this paper. However,

there are two areas where folds are observed. First, in Combin contact. It is therefore spatially restricted tothe uppermost zone of top-to-SE shear that truncatesthe metabasic lithologies exposed in the eastern side

of the Val d’Ayas (grid ref. 046785; 45 °52∞N, 07 °46∞E), the structurally lower top-to-NW fabrics.there are a series of tight folds with SW-dipping axialplanar fabrics (Fig. 6e) and southerly plunging hinges

Structure of the eclogite facies rocks(Fig. 6f ) that are restricted to individual metabasicunits within the overall top-to-NW Combin package. In the eclogite facies rocks structurally underlying the

metamorphic break, a well-developed planar fabricThe folds overprint greenschist fabrics but do notdeform the contacts with the adjacent calcschist (S1zs) is defined by omphacite. Lower pressure glauco-

phanic and greenschist minerals commonly define aunits that preserve top-to-NW kinematic indicators.Therefore, these folds pre-date this episode of shearing. retrograde fabric that lies parallel to the earlier eclogite

facies fabric. The younger fabrics are heterogeneouslyThe second are folds adjacent to the Zermatt-Saas/Combin contact on the eastern side of Val Gressoney developed and represent localized reactivation of the

eclogite facies fabrics. The orientation of S1zs (and(grid ref. 094782; 45 °51∞N, 07 °51∞E). Immediately inthe hangingwall to the contact, isoclinal folds deform subparallel overprinted S1zs) defines a weak girdle

(Fig. 7a). The extent of the greenschist facies overprintgreenschist fabrics and have axial planes lying parallelto the main contact and hinges lying subparallel to increases towards the Combin/Zermatt-Saas contact

and is associated with a re-orientation and transpo-mineral stretching lineations. These folds are inter-preted to be related to contact-related shear. sition of early eclogite fabrics (S1zs) into high-strain

greenschist fabrics (S2zs) lying parallel to the contactStructurally higher in the GSZ, the contact betweenthe Combin Zone and the GMC is folded at the (Fig. 4b; sections A & B).


Fig. 7. Lower hemisphere equal-area stereo-nets from the eclogitic Zermatt-Saas andMonte Rosa Units. (a) Poles to foliation in theZermatt-Saas Zone; (b) mineral stretchinglineations in the Zermatt-Saas Zone; (c) polesto foliation in the Monte Rosa Unit; (d)mineral stretching lineations in the MonteRosa Unit; (e) & (f ) F2ZS fold data fromZermatt-Saas; (g) & (h) F2MR fold data fromthe Monte Rosa Unit.

Omphacite and glaucophane mineral lineations are in the Zermatt-Saas Unit (Fig. 7c). The absence ofdiagnostic mineral assemblages within the Monte Rosalocally preserved and are broadly subparallel to

mineral lineations developed in the overlying Combin fabrics makes the conditions of foliation developmentdifficult to constrain. However, at the Monte Rosa/Zone (Fig. 7b). In detail, lineation orientations

change from E–W to SE–NW as the contact with the Zermatt-Saas contact, where Zermatt-Saas metabasitesare strongly retrogressed, greenschist fabrics within theCombin Zone is approached. Omphacite lineations

and foliations formed at eclogite grade are not usually Monte Rosa and Zermatt-Saas Zones are subparallel.This suggests that juxtaposition of the two unitsassociated with good kinematic indicators.

Glaucophane and actinolite lineations are dominantly probably post-dates eclogite facies metamorphism.Mineral stretching lineations are again generallyassociated with top-to-SE kinematics although

occasionally top-to-NW kinematics are observed NW–SE oriented (Fig. 7d). Kinematic indicatorsassociated with Monte Rosa show both top-to-SE and(Fig. 4a). Shear bands developed during the greenschist

facies overprint are best developed in the zone of S2zs top-to-NW shearing.Both the Monte Rosa Basement and Zermatt-Saasand these record a consistent top-to-SE kinematic

framework. Foliation data from the Monte Rosa Unit Zone contain isoclinal folds that have axial planeslying subparallel to the regional orientation of thein Val Gressoney (S1mr) define a girdle similar to that


foliation. In the Monte Rosa Unit, isoclinal folds tapers to the east, and, although cutting top-to-SEfabrics, is cut by younger SE-directed deformation at(F1mr) deform lithological layering (defined by pre-

Alpine aplites within the garnet–white mica schists) the shear zone margins.4 The base of the GSZ has a planar contact thatand a well-developed foliation. In the Zermatt-Saas

Unit, S1zs, defined by eclogite facies minerals, is also truncates E–W greenschist facies folds developed inthe Zermatt-Saas and Monte Rosa rocks in itsisoclinally folded (F1zs). This folding is associated with

the retrograde growth of glaucophane and titanite and footwall.post-dates the peak metamorphic assemblage. Isoclinalfolds also affect the Monte Rosa/Zermatt-Saas contactto the east in Val Sesia (F1mr/zs). These folds deform

GEOCHRONOLOGYgreenschist facies fabrics that lie parallel to the MonteRosa/Zermatt-Saas contact. Kilometre-scale folds Field mapping demonstrates the relative age of some

structural features in the GSZ. However, it is difficult(F2mr/zs) also deform the contact. These two foldingepisodes therefore post-date the juxtaposition of the to tie these to footwall evolution because of the

demonstrably late movement between the Zermatt-Monte Rosa and Zermatt-Saas Units (Fig. 4b;section C). Axial planes of parasitic folds to the major Saas and Combin Zones. To clarify the history of the

GSZ with respect to possible contemporaneity ofF2mr/zs structures dip gently to the north (Fig. 7e,g).Fold hinges are subparallel to the pole of the great footwall decompression, absolute dating techniques

are required. The homogenization of Sr isotopes duringcircle defined by the S1zs and S1mr foliations (compareFig. 7e,g with a,c). In the field, these folds again clearly deformation (e.g. during syntectonic recrystallization)

may allow the age of the deformation event to bedeform greenschist facies fabrics in the Zermatt-SaasZone. Variations in shear sense in the Monte Rosa constrained if the temperature of deformation were

significantly lower than the closure temperature for Srand Zermatt-Saas Zone can be related to the inversionof shear bands by these folds, which have fold hinges diffusion in the mineral being dated (c. 500 °C for

white mica) (Cliff, 1985). This simple picture may besubparallel to the mineral stretching lineation (cf.Figure 7b,d,f ). These folds rarely show axial planar complicated by incomplete recrystallization and the

mixing of different mica populations which will resultfabrics in the field. However, in thin section, greenschistfacies minerals define an axial planar orientation which in inheritance of radiogenic Sr. However, the Piemonte

rocks analysed here have no pre-Alpine metamorphicindicates that F2mr/zs folding occurred at greenschistfacies conditions. Importantly, these folds do not affect history and therefore problems of inheritance should

be minimal.the Combin Zone (Fig. 4b; section 3).In summary, field mapping shows: Here we present Rb–Sr data from the different areas

of greenschist facies fabrics to establish the absolute1 A major 1–2 km wide zone dominated by top-SEshear at greenschist facies in the Combin Zone that ages of the different fabrics within the GSZ and assess

the duration of deformation across the shear zone.extends structurally upwards into the GMC. We referto this as the Gressoney Shear Zone (GSZ). The shear Our approach is to analyse several samples from the

same structural level to test for reproducibility of datazone dies out upwards (probably gradually) in thelower 100 m of the GMC, and has a lower boundary from a particular structure. The age data are also

checked for consistency with structural overprintingagainst the eclogite facies Zermatt-Saas Zone.2 Within the Zermatt-Saas Zone, glaucophane and/or relationships obtained from field mapping. We also

present data from above and below the GSZ, whichgreenschist facies fabrics overprint eclogite facies fabricsformed at conditions close to the metamorphic peak. provide a context in which to interpret the deformation

ages. Within the footwall (Monte Rosa and Zermatt-These later fabrics are heterogeneously developed andlie subparallel to the early fabrics. These fabrics and Saas Zone), metamorphic temperature estimates are

generally close to the closure temperature for Srthe underlying Monte Rosa Unit are deformed at thekilometre scale by greenschist facies folds. Localized, diffusion in white mica (Dal Piaz & Ernst, 1978;

Barnicoat & Fry, 1986; Dal Piaz & Lombardo, 1986).pervasively developed, dynamically recrystallized,greenschist facies fabrics are developed toward the top White micas from these units may yield closure ages.

In the GMC, rocks with syntectonic greenschist faciesof the Zermatt-Saas Zone and overprint these folds.These fabrics and associated mineral lineations are assemblages are likely to have crystallized below the

closure temperature for white mica and hence givesubparallel to those in the immediately overlyingCombin Zone and show well-developed top-to-SE deformation ages.shear indicators.3 Within the GSZ, regions with different kinematic

Analytical techniqueshistories have been identified and these illustrate acomplex deformation history within the shear zone. Rock samples were crushed and mineral separations

carried out using standard magnetic and heavy liquidOverprinting relationships allow the relative age offabrics within different parts of the shear zone to be techniques. Where the white mica fraction contained

both paragonite and phengite it was possible to enrichconstrained. A discrete panel of NW-directed shear


the phengite proportion magnetically. However, some trometer in the School of Earth Sciences, Universityof Leeds. Analytical errors in the Rb/Sr ratio stemparagonite remains even in the most magnetic fraction

(compare the Rb and Sr concentrations of the two almost entirely from variable mass fractionation duringRb runs. Replicate analyses of unspiked Rb indicatewhite mica fractions from S3-75b in Table 1). Calcite,

epidote or feldspar separates were analysed to provide that 95% confidence limits on the Rb/Sr ratios are±0.8%. Errors in the 87Sr/86Sr ratios were betweencontrol on initial 87Sr/86Sr ratios. Minerals were first

spiked with a mixed 87Rb–84Sr spike, decomposed, 0.004 and 0.014%.and Rb and Sr separated using standard ion exchangeprocedures. The Rb/Sr ratio of the spike was checkedby analysis of SRM 607 K-feldspar (Cliff et al., 1985).

ResultsIsotopic compositions of Rb were measured on a VGMicromass 30 while Sr was measured using dynamic Results are given in Table 1. Data are summarized in

terms of the GSZ and hangingwall/footwall ages.dual collection on the VG Isomass 54E mass spec-


Fig. 8. Distribution of mica ages in the study area. All the data are from this paper (Table 1) except for sheared GMC sample56140 reported in Inger et al. (1996). Star shows location of UHP unit at Lago Cignara.

Gressoney Shear Zone data

Samples were strongly foliated calcschists to impuremarbles with aligned mica flakes in a matrix of quartzand carbonate. Calcite grains form an interlockingmosaic of elongate grains with aspect ratios up to 251.Micas in all rock types show limited unduloseextinction with localized stronger kinking. Two foliatedGMC (orthogneiss) samples from the GSZ were alsodated. We interpret the micas within these greenschistfacies rocks to have grown or recrystallized syn-deformationally within the evolving foliation. Thelocation of samples and the distribution of ages areshown in Fig. 8 while their relationship to the basickinematic framework of the GSZ is illustrated inFig. 4(b). All ages from the GSZ fall between 45 and36 Ma. An important result is that the youngest ages(37.5–36.5 Ma) come from the base of the GSZ andare from rocks associated with SE-directed shear.Samples from the zone of top-NW shear yield agesfrom 39.2 to 37.2 Ma. The oldest ages from the CombinZone of the GSZ (42.3–41.2 Ma) are recorded fromrocks with top-SE fabrics that are truncated by thebasal top-SE shear and the structurally higher top-NWzone of shear (Fig. 4b). Older ages are also found inthe highest structural levels of the GSZ within theGMC (39.9–44.6 Ma).

These data illustrate that there are consistent andreproducible groupings of ages from adjacent sampleswithin the GSZ but that different parts of the shearzone record different ages (Fig. 9). Furthermore, the Fig. 9. Summary of age data from the GSZ in relation to the

kinematic and structural setting.different Rb–Sr ages are consistent with relative


chronologies of deformation inferred from cross-cutting isotopic heterogeneity for this age discrepancy. First,not all areas of the Zermatt-Saas Unit may haverelationships of different fabrics (Fig. 4b). The ages

from the GSZ are not consistent with the data reached peak metamorphic conditions at the sametime. This is suggested by recent data from therepresenting closure through a particular temperature

but are consistent with the Rb–Sr ages dating the structurally and metamorphically equivalent Monvisoarea which yields interpreted peak metamorphic agestiming of deformation.of 62 and 49 Ma (Duchene et al., 1997; Cliff et al.,1998). However, as argued above, the UHP Unit at

Hangingwall and footwall dataCignana cannot have crystallized after the high-Peclogites which surround it. An alternative explanationSample 0-70 from the GMC in Val Sesia is in the

hangingwall to the GSZ and contains a fabric that for the different ages is that zircon growth in theUHP Unit may not have coincided with peak eclogitebecomes progressively overprinted towards the GSZ.

Textural observations show that the biotite in this facies conditions.In conclusion, there are some problems with inter-rock appears to have recrystallized along with white

mica during deformation. We interpret the 46 Ma preting the Zermatt-Saas age data. It is possible thatthe Zermatt-Saas Zone represents a number of smallerwhite mica data as a deformation age and 30 Ma from

biotite as cooling. These age data and others reported units that experienced different P–T histories beforebeing juxtaposed. Our best interpretation of availableby Inger et al. (1996) appear to show increasingly

older deformation ages with increasing structural level data is that at least some of the footwall was ateclogite facies conditions at 44 Ma.in the GMC.

Three ages are reported from the footwall of theGSZ. One from the Monte Rosa Unit gave an age of

DISCUSSION226 Ma. We interpret this age to be the result ofincomplete isotopic re-equilibration during Alpine

Evolution of the Gressoney Shear Zonemetamorphism. This is consistent with previouslypublished data (Frey et al., 1976). Two petrographically The contact between the Combin and Zermatt-Saas

Zones is a significant metamorphic break separatingdistinct samples from the Zermatt-Saas Zone haveexperienced only Alpine metamorphism but yield eclogite facies rocks in the footwall from greenschist

facies rocks in the hangingwall. Previous studiesdifferent ages. A metabasite sample from Val d’Ayas(S3-75b) which gave a white mica age of 40.5±0.6 Ma around this contact have suggested several alternative

interpretations to explain the nature of the metamor-contains randomly oriented micas and epidoteintergrown with blue-green amphibole. The micas and phic break. Milnes et al., (1981) and Ellis et al. (1989)

suggested that the contact was related to SE-directedepidote are interpreted to be a relatively low-Poverprint and the age represents crystallization of backthrusting (i.e. crustal shortening) which has been

subsequently rotated. It has also been suggested thatretrograde mica. A calcschist from Valtournanche(S4-60) consists of strongly foliated phengite enclosing the contact may be extensional and associated with

exhumation of Zermatt-Saas eclogites in the footwall.epidote and titanite in a quartz-rich matrix with garnetporphyroblasts. This is interpreted as an eclogite facies Within this framework, Butler (1986) and Ballevre &

Merle (1993) suggested SE-directed extension, whileassemblage. A white mica–titanite regression gave anage of 46.4±0.6 Ma and an initial 87Sr/86Sr ratio of Platt (1986) suggested NW-directed movement. None

of these authors presented detailed kinematic or timing0.7121, whereas a white mica–carbonate regressionproduced an age of 47.5±0.5 Ma and an initial information from the Combin Zone against which to

test the various models.87Sr/86Sr ratio of 0.7118. Petrography shows that thecalcite grains are altered to an opaque phase around Our work, together with that of Wheeler & Butler

(1993), shows that the Combin Zone is dominated bytheir margins and along cleavage planes: this alterationmay well have affected the 87Sr/86Sr ratio. We consider top-SE shear for at least 10 km along strike.

Throughout the study area, the GSZ dips beneath thethe 46.4±0.6 Ma age more reliable. The foliatedphengite is clearly part of the eclogite facies fabric and structurally higher units of the Sesia Zone. No

comparable shear zone re-emerges further south-easttherefore crystallized above 500 °C. We interpret the46.4 Ma age as a cooling age. and this regional geometry is therefore consistent with

the GSZ being a zone accommodating crustal exten-Our Zermatt-Saas data must be interpreted togetherwith other existing age data from the Zermatt-Saas. sion. This contrasts with backthust-related shear that

should cut up structural section to the south-east.In particular, Rubatto et al. (1998) obtained a zirconcrystallization age of 44±0.7 Ma that has been Additional criteria also support the extensional nature

of the GSZ. Fundamentally, if the GSZ were a rotatedinterpreted as a peak metamorphic age for the UHPUnit at Lago Cignana. This must be reconciled with backthrust, the implication is that the Combin Zone

and the Sesia Zone were situated to the north-west of,the phengite data from the calcschist (S4-60) with anapparently older cooling age. There are several and at deeper structural levels than, the Zermatt-Saas

Zone. If the GSZ were extensional, the Combin Zonepossible explanations other than the potential for


Fig. 10. Schematic evolution of Gressoney Shear Zone in its Alpine context. 45 Ma, onset of extension localized on GSZ; 39 Ma,continuing SE-directed extension, with some top-NW component possibly related to pure shear in hangingwall; 36 Ma, final stagesof extension truncating folds in footwall (not shown), and creating present-day geometry of metamorphic break; 30 Ma, majorbackthrusting on shear zones and brittle structures, and local rotation of units in hangingwall, moving Alpine internal zones uprelative to Ivrea Zone. Uplift triggers major erosion. Note that backthrusting was probably active over a prolonged period. PresentDay, geometry (after Escher et al., 1988) is dominated by backthrust and backfold structures, although earlier extension wasresponsible for equally major vertical displacements.

and Sesia Zone would also be to the north-west of the their present geometry therefore post-dated F2MR/ZS(Fig. 4b). Since these folds deform greenschist faciesZermatt-Saas Zone prior to GSZ movement but these

zones would have been at higher structural levels, that fabrics, the eclogitic footwall must have already beenpartly exhumed when the GSZ and the underlyingis in their generally accepted structural position above

other internal zone units. We conclude that the eclogites were juxtaposed in their present position.This illustrates a fundamental point about the naturemetamorphic break therefore corresponds to the

contact between eclogite facies rocks and a greenschist of metamorphic breaks; that is, the contact itself maybe a late structure that is not responsible for initial orfacies zone of extensional shear.

Within the eclogite facies rocks, SE-directed shear significant exhumation of the footwall (Fig. 1). Withinthe GSZ, the basal shear is clearly the most recentbands, developed from eclogite through to greenschist

facies conditions, suggest that the Zermatt-Saas Zone area of localized deformation. Therefore, the possibilityremains that the penetrative fabrics within the exten-was cooling and moving to lower pressures during

top-to-SE shearing and consequently that these struc- sional shear zone, but above the basal shear, mightstill be responsible for initial exhumation of thetures are contemporaneous with the exhumation

process (Fig. 10). The simplest model is that the non- footwall (Fig. 2). This cannot be tested based on thepresent geometry of these units. Importantly, the Rb/Srcoaxial, top-to-SE deformation in the Zermatt-Saas

Unit represents non-pervasive deformation associated data show that the majority of calcschist fabrics aresignificantly older (by up to 9 Myr) than those withinwith exhumation and cooling, formed in the same

extensional regime as the GSZ. However, there the basal shear zone of the GSZ. This supports amodel of shear at the base of the GSZ carrying earlierare some complications to this model. These are

addressed below. fabrics that did not undergo recrystallization in thefinal stages of GSZ deformation.

Truncation of greenschist fabrics in the footwallTiming of shear and exhumation of the Gressoney Shear Zone

In the footwall to the GSZ, lithological layering, themain foliation and large folds (F2MR/ZS) which affect the The Si contents of phengite from various structural

levels within the GSZ show no clear correlation withMonte Rosa/Zermatt-Saas contact are truncated bythe planar Zermatt-Saas/Combin contact. The juxtapo- age (Table 1). Absolute pressure estimates using the

phengite geobarometer of Massonne & Schreyer (1987)sition of the greenschist and eclogite facies units into


seem artificially high, perhaps by as much as 3 kbar. zone related to SW–NE extension, but it passesbeneath the study area and cannot have contributedHowever, Si content will be some guide to relative

pressures. Inspection of the chemistry and age data to unroofing. Unless there were other such structureswithin the Austroalpine nappes (for which there isindicates that calcschists in the GSZ were at pressures

at least as great as that for sample 0–70 in the no evidence), later exhumation must have beenaccomplished by erosion.hangingwall, but as much as 10 Ma later. There are

two important conclusions. First, portions of the shearzone were deeper relative to the hangingwall when

Significance of NW-directed shearthey were deforming than their current positionsuggests. This is to be expected in an extensional shear There are two possible explanations for the develop-

ment of NW-directed shear in the GSZ. One is anzone. Second, the pressure in the GSZ in the last stageof movement was not significantly less than the episode of crustal shortening; the other is a component

of ‘pure shear’ extension in and above the GSZ whichpressure in the immediate hangingwall before move-ment commenced. Therefore, there was little unroofing partitioned into SE- and NW- directed shear. In the

first model, the NW-directed shear would transferof the hangingwall (by erosion or hypothetical higherlevel structures) during the 9 Myr period of shear. downwards into deeper shortening structures and

would relate to a major change in deformation style,which then switched back to late extension. In theTiming of shear and footwall exhumationsecond model, the strain in the GSZ would involvesome overall pure shear stretch and this would alsoParts of the Zermatt-Saas Unit were at eclogite facies

at 44 Ma. The crucial relationship between this age include the hangingwall. The GMC generally containsgently dipping fabrics that, immediately above theand those of the GSZ is that they overlap: the GSZ

registers crustal extension occurring throughout the Combin Zone, are non-coaxial and SE-directed.Structurally higher, kinematic indicators are lackingperiod that the footwall was unroofed from eclogite to

greenschist facies. Shear indicators within the eclogite and this could be due to pure shear. The GMC directlyabove the study area has been removed, so evidence isfacies rocks also record the same kinematic framework

as the GSZ throughout exhumation to greenschist circumstantial. We prefer the second hypothesis as itbetter explains the near-simultaneous top-NW andfacies conditions. The GSZ records greenschist facies

(c. 9 kbar) throughout this time period with no evidence top-SE shear.for major pressure changes. This indicates that theextensional GSZ was the major agent of unroofing the

Implications for the exhumation of Alpine eclogites andZermatt-Saas Unit from >18 kbar at 44 Ma to orogen evolutionc. 9 kbar by 36 Ma. Taking a notional density of2.8 g/cm3 (there being no direct evidence for the nature

The significance of the timing of extension at 36–45 Maof the material that was originally above the eclogites),this corresponds to removal of at least 32 km of Initial estimates of the timing of eclogite facies

metamorphism suggested an Early Cretaceous agematerial in not more than 8 Ma. The average rate oftectonic exhumation over this period was therefore at (Oberhansli et al., 1985) and these were supported by

Cretaceous argon ages (Hunziker, 1974; Stockhertleast 4 mm/a.Remarkably, the oldest ages of extensional move- et al., 1986). However, recent data shows that the

Western Alpine eclogites are significantly younger andment in the GSZ actually appear to pre-date thecrystallization of some of the eclogites found in its record a decrease in age passing to structurally lower

levels through the sequence of Austroalpine, Piemontefootwall. This might appear to be a serious contradic-tion, since the eclogites must have been buried by and Pennine Units. In the Sesia Zone, U–Pb dating

suggests that eclogite facies metamorphism took placelithospheric shortening. However, the time of ‘peak’metamorphism does not necessarily relate to that of at c. 65 Ma (Ramsbotham et al., 1994). Recent Lu–Hf

dating yields similar estimates (Duchene et al., 1997).peak pressure, and peak temperature is a more likelycandidate. It is therefore possible that the eclogite In the Zermatt-Saas Zone, Sm–Nd dating of garnet

suggests a Tertiary age for metamorphism (Bowtellassemblages crystallized during the early stages ofexhumation. et al., 1994), which is consistent with recent U–Pb

zircon ages of 44 Ma (Rubatto et al., 1998). In theThe GSZ was still at considerable depth (9 kbar,with only continental material above it) when the Pennine Basement Massifs, eclogite facies metamor-

phism in Dora Maira appears to have peaked betweenyoungest part of the shear zone was active. There isno evidence in our study area relating to later 32 and 38 Ma (Tilton et al., 1989; Duchene et al.,

1997). Unfortunately, there is little recent data con-exhumation of these rocks. There are many youngerstructures in the Alps, commonly forethrusts and straining the age of eclogite facies metamorphism in

the Monte Rosa massif. In support of Tertiary eclogitebackthrusts, but it is difficult to reconcile these withtectonic exhumation of the GSZ from 36 Ma. The ages, recent 40Ar/39Ar dating (Arnaud & Kelley, 1995;

Ruffet et al., 1995; Reddy et al., 1996; Scaillet, 1996;Simplon Line (Mancktelow, 1985) is a younger shear


Pickles et al., 1997) clearly demonstrates that early plate divergence (Butler, 1986). A fourth, more recentmodel has illustrated how normal-sense shear zonesargon studies from the different eclogite units were

probably affected by excess argon. may develop due to crustal velocity gradients aboverock packages where thrusting is taking place at theThe difference in metamorphic ages between the

different major tectonic units places very important base of the unit (Beaumont et al., 1996). Such a modelmay be applicable to the Alps (Escher & Beaumont,constraints on Alpine tectonic evolution. If we assume

that eclogite facies metamorphism resulted from sub- 1997). However, we reiterate that such models mustbe tested using geochronological data to prove theduction of crustal material (e.g. Ernst, 1971), then

available age data suggests prolonged and continuous synchroneity of structures that are hypothesized to bedynamically linked. Escher & Beaumont (1997) relatedSE-directed subduction. The post-metamorphic cooling

and exhumation histories of the different eclogites eclogite facies metamorphism to Cretaceous events,which now seems unlikely (see discussion above), andmust have been taking place while the subduction

process was still operating. Fundamentally, the Sesia do not present other timing data. The fundamentalpoint is that the extensional geometry from oneZone eclogites had undergone a significant part of

their exhumation history before the Piemonte eclogites structural level is not diagnostic of the mechanism,and additional information is required to infer thehad reached their metamorphic peak. Similarly, the

data in this paper illustrate that the Zermatt-Saas driving forces for extensional deformation.In the Alps, there are many thrusts in the externalZone had undergone significant exhumation by the

time the Pennine Basement (Dora Maira) eclogite zones that carry the internal zones in their hang-ingwalls. To have some constraint on the geodynamicfacies metamorphism took place. The data therefore

suggest a westerly migrating accretion of material to mechanism responsible for extension, it is crucial toknow whether these thrusts were active simultaneouslythe hangingwall of the subduction zone.

The 36–45 Ma ages reported for extensional defor- with extension. Kinematic reconstructions of platemovement at the time of SE-directed extension showmation at the top of the Zermatt-Saas Zone correspond

to the time immediately after, or overlapping, attain- that the relative motion of the African plate withrespect to Europe was towards the north-east (e.g.ment of the eclogite facies metamorphic peak. Tectonic

extension was therefore responsible for the initial Dewey et al., 1989). This seems to exclude net platedivergence as a driving force, and restricts possibleexhumation of the Zermatt-Saas eclogites over 9 Ma

following their formation. If all of the Western Alpine models to those in which buoyancy forces play arole.eclogites record a similar extensional unroofing story,

the geometry of the different units may also require awesterly migration of the extensional structures respon- CONCLUSIONSsible for eclogite exhumation. Currently, data tosupport this are limited. We have shown that in the Alps, mafic eclogites that

crystallized at 44 Ma were unroofed by hinterland-directed extensional shear that began to operate even

Exhumation dynamicsbefore the presently exposed eclogites had crystallized.Extension began at 45 Ma and became more localizedWe have demonstrated here the kinematics of eclogite

unroofing in this part of the Alps (summarized in before ceasing at c. 36 Ma. As the footwall wasunroofed, sporadic overprinting, shearing and large-Fig. 10), and have deliberately avoided tying the

geometry and kinematics of extension to a particular scale folding occurred on the retrograde path. Thebase of the GSZ truncates these folds because it is latedriving force. This is because the most that can be

deduced from structural, metamorphic and geochrono- and discordant relative to earlier fabrics formed in thesame kinematic regime. Consequently, the presentlylogical study is the nature and timing of relative

movement of rock bodies. Many studies do not make observed metamorphic break was only responsible fora small component of the exhumation history of theclear that a given extensional geometry can be caused

by more than one driving force. For instance, Wheeler footwall eclogites. The total amount of unroofingaccommodated by extension brought the eclogites(1991) illustrates several scenarios for unroofing. Three

of these involve hinterland-directed extension similar from c. 60 to c. 30 km. The lack of systematic pressuredecrease in the GSZ over the period in which it wasto that demonstrated in this contribution. In the first

two models, a ‘pip’ or ‘wedge’ of material rises via active shows that syn-extensional erosion was insig-nificant. In contrast, subsequent removal of thethrusting at its base and extension at its top. In both

of these, the driving force was thought to be the remaining 30 km of overburden since 36 Ma appearsto have been dominated by erosion. We infer that theintrinsic average buoyancy of the pip or wedge relative

to its surroundings (Wheeler, 1991). In the case extensional shear zone reached at least 60 km depth,and is probably linked to the relict subduction zone.described here, such a driving force might be possible

if the mafic eclogites of the Zermatt-Saas Zone were Internal buoyancy forces rather than overall platedivergence appear to have driven extension, althoughattached to continental material of the Monte Rosa

Unit. In the third model, extension relates to overall it remains for this to be conclusively demonstrated.


Ernst, W. G., 1971. Metamorphic zonations on presumablyACKNOWLEDGEMENTS subducted lithospheric plates from Japan, California and the

Alps. Contributions to Mineralogy and Petrology, 34, 45–59.This work was funded by the Natural Environment Escher, A. & Beaumont, C., 1997. Formation, burial andResearch Council (NERC grant GR3/8606). Thanks exhumation of basement nappes at crustal scale: a geometric

model based on the Western Swiss–Italian Alps. Journal ofto B. Jamieson and S. Schmid for detailed reviews ofStructural Geology, 19, 955–974.the manuscript and K. Lancaster for help with drafting

Escher, A., Masson, H. & Steck, A., 1988. Coupes geologiquesof figures. des Alpes occidentales suisses. Memoires de Geologie

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