The geometry and timing of orogenic extension: an example from the Western Italian Alps

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1 J. metamorphic Geol., 1999, 17, The geometry and timing of orogenic extension: an example from the Western Italian Alps S. M. REDDY1*, J. WHEELER1 AND R. A. CLIFF2 1Department of Earth Sciences, The University of Liverpool, Liverpool, L69 3BX, UK, 2Department of Earth Sciences, The University of Leeds, Leeds, LS2 9JT, UK ABSTRACT Contacts between rocks recording large differences in metamorphic grade are indicative of major tectonic displacements. Low-P upon high-p contacts are commonly interpreted as extensional (i.e. material points on either side of the contact moved apart relative to the palaeo-horizontal), but dating of deformation and metamorphism is essential in testing such models. In the Western Alps, the Piemonte Ophiolite consists of eclogites (T # 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 (the Gressoney Shear Zone, GSZ) dominated by top-se movement related to crustal extension. Rb Sr data from micas within different GSZ fabrics, which dynamically recrystallized below their blocking temperature, are interpreted as deformation ages. Ages from different samples within the same fabric are reproducible and are consistent with the relative chronology derived from mapping. They show that the GSZ had an extensional deformation history over a period of c. 9 Myr between c Ma. This overlaps in time with the eclogite facies metamorphism. The GSZ operated over the entire period during which the footwall evolved from eclogite to greenschist facies and was therefore responsible for eclogite exhumation. The discrete contact zone between eclogite and greenschist facies rocks is the last active part of the GSZ and truncates greenschist facies folds in the footwall. These final movements were therefore not a major component of eclogite exhumation. Pressure estimates associated with old and young fabrics within the GSZ are comparable, indicating that during extensional deformation there was no significant unroofing of the hangingwall. Since there are no known extensional structures younger than 36 Ma at higher levels in this part of the Alps, exhumation since the final juxtaposition of the two units (at 36 Ma) seems to have been dominated by erosion. Key words: deformation age, eclogite, exhumation, Rb Sr dating, tectonic. INTRODUCTION thermal and baric histories of the footwall and hangingwall, facilitate the recognition of extensional The processes by which high-p rocks reach the Earth s structures (Wheeler & Butler, 1994). surface are important to an understanding of the Even if genuine, extensional structures may develop thermal and barometric evolution of metamorphism after high-p rocks in their footwalls have already and the preservation potential of peak metamorphic undergone partial exhumation to low-p conditions assemblages. Erosion is one possible exhumation (Fig. 1). In such cases, the structure associated with process. However, crustal extension has been recog- the metamorphic break observed in the field may have nized as another important unroofing process (Platt, contributed to only a minor component of the 1986; Dewey, 1988). Diagnosing extensional structures exhumation history of the footwall and the most within the internal zones of orogens is difficult because significant structure may remain unrecognized. the orientations of layering and shear zones at the The recognition of multiple extensional histories may time of deformation are usually unknown (e.g. Wheeler be difficult in the field. Late structures may passively & Butler, 1994). Furthermore, post-extensional defor- carry earlier extensional structures in their hangingwall mation may have significantly modified original geometries or may completely rework earlier fabrics (Fig. 2). In the by bulk rotation. This may result in extensional former, fabrics preserved in the hangingwall would have structures being re-oriented into apparent thrust sense formed during the early extensional deformation history displacements (and vice versa). However, criteria based while in the latter only the younger fabrics would be on the structure of major contacts, together with the preserved. Consequently, even when extensional struc- tures can be established, the relationship between *Now at: Tectonics Special Research Centre, School of Applied extension and exhumation requires quantification of the Geology, Curtin University of Technology, PO Box 41987, time and duration of extensional shearing and the Perth, WA 6845, Australia. (sreddy@lithos.curtin.edu.au) timing and rate of exhumation of the footwall. It is Blackwell Science Inc., /99/$14.00 Journal of Metamorphic Geology, Volume 17, Number 5,

2 574 S. M. REDDY ET AL. Fig. 2. Two models depicting the (a) localized and (b) complete reworking of early extensional hangingwall fabrics during extension. In (a), localized late shearing leads to preservation of early extensional fabrics. In (b), all of the early fabrics are overprinted. 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 represent the significant extensional structure related to footwall exhumation (1st extensional phase) but a later, minor structure (2nd extensional phase). structural and geochronological. These are given in the subsequent two sections, followed by a synthesis and discussion. therefore essential that the ages of fabrics developed in the footwall, hangingwall and in the extensional fault GEOLOGY OF THE WESTERN ALPINE zone be known. INTERNAL ZONES In this paper, we assess the role of a regionally extensive shear zone that is associated with a significant The Pennine Basement Massifs, the Piemonte Unit metamorphic break between eclogite and greenschist and Austroalpine Basement are the three major units facies rocks. The relationships between deformation of the alpine internal zones. Structurally lowest, fabrics and metamorphic assemblages in the hangingwall the Pennine Basement Massifs (Monte Rosa, Gran and footwall of the metamorphic break are Paradiso and Dora Maira) represent European conti- used to infer details of the exhumation process. We nental basement. The Piemonte Unit comprises integrate these data with the dating of deformation Jurassic oceanic material that separated the European fabrics to constrain the timing and duration of and African continental plates prior to collision. The deformation and relate this to processes taking place Austroalpine Basement (Sesia Zone and Dent Blanche in other areas of the orogen. Our chosen area for this Klippe) forms part of the Adriatic (African) plate study was the Western European Alps because (1) the beneath which the other two units were subducted metamorphic setting is well known, and the large during Alpine convergence (see Coward & Dietrich, pressure differences across discrete contacts are well 1989). The Pennine Basement Massifs and the documented; (2) preliminary work (Wheeler & Butler, Austroalpine Basement contain evidence of pre-alpine 1993) has shown genuine crustal extension associated metamorphism. However, all three major units have with greenschist facies shearing above high-p rocks; locally developed eclogitic rocks of Alpine age. The ( 3) complete transects across the Alps are exposed, so units also contain areas that show no evidence of that our results can be related to structures and events having reached eclogite facies, but record greenschist elsewhere in these transects. We begin by giving general facies mineral assemblages. In the Piemonte Unit, features of Western Alpine geology and then summarizing greenschist facies rocks lie at structurally higher levels metamorphic conditions. The main new data are than immediately underlying eclogite facies rocks, and

3 THE GEOMETRY AND TIMING OF OROGENIC EXTENSION 575 this unit therefore preserves a metamorphic break structurally lower Zermatt-Saas Zone, metamorphosed which may be associated with crustal extension. Here at eclogite facies conditions, and the overlying green- we investigate this metamorphic break. The nature of schist facies Combin Zone. In the area studied, the the breaks in the Sesia Zone is the subject of separate Zermatt-Saas Zone comprises dominantly metabasic, studies (Reddy et al., 1996; Pickles et al., 1997). metagabbroic and serpentinitic lithologies, while the The area lies to the north of the Val d Aosta in the Combin Zone contains significant amounts of carbon- Gressoney and Ayas valleys, which provide excellent ate-bearing lithologies (calcschists) and metabasites. outcrop through the three major units of the internal These lithologies are interbanded at the centimetre to zones (Fig. 3). At the lowest structural levels, the tens of metres scale. Serpentinites are also common Monte Rosa Unit comprises garnet mica schists, within the Combin Zone and associated with these are metagranites and metabasites that are commonly serpentinite breccias that are cemented by a carbonate difficult to constrain in terms of metamorphic matrix. Locally metagabbros and quartz schists are conditions. However, metabasites may preserve also present. In the Piemonte Unit of Val Gressoney, omphacite + garnet + glaucophane + zoisite + white the contact between the Zermatt-Saas and Combin mica, and indicate that Alpine eclogite facies metamor- Zones is a planar feature (Fig. 4a,b). This contact is phism affected this area of Monte Rosa Basement the focus of our study. However, before describing it, (Bearth, 1952; Dal Piaz & Lombardo, 1986). At the the metamorphic history of the two units either side highest structural levels exposed in the working area of the contact is discussed. ( Fig. 4a), the Gneiss Minuti Complex (GMC) of the Sesia Zone comprises dominantly fine-grained quartz METAMORPHISM IN THE PIEMONTE UNIT OF feldspar schists, locally with augen orthogneiss, mica VAL GRESSONEY schists and metagabbros. In the study area, there is no evidence of early high-p metamorphism in the GMC. Zermatt-Saas Zone Between the two basement units, the Piemonte Unit comprises rocks of ophiolitic affinity that can be subdivided into two main lithotectonic units; the The Zermatt-Saas lithologies, particularly the metabasites, commonly retain the high-p mineral assemblage Fig. 3. Geological map of the internal zones of the Western Alps showing location of study area. MR, Monte Rosa; GP, Gran Paradiso; DM, Dora Maira; DB, Dent Blanche.

4 576 S. M. REDDY ET AL.

5 THE GEOMETRY AND TIMING OF OROGENIC EXTENSION 577 Fig. 4. (a) Geological map of the Upper Val Gressoney. All contacts are tectonic but the Combin/Zermatt-Saas contact is highlighted 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, particularly C and kbar have been suggested for in zones of deformation (see later structural section). large areas of the Zermatt-Saas Zone (Barnicoat & Fry, 1986; Ganguin, 1988), while the presence of coesite indicates pressures in excess of 26 kbar (Reinecke, Combin Zone 1991, 1998). These data suggest either that the Zermatt- The metabasites of the Combin Zone commonly Saas Zone represents a number of smaller, tectonically have the greenschist facies mineral assemblage juxtaposed units that were originally subducted to actinolite+albite+chlorite+epidote. Relevant reaction differing depths, or it was an original ultra-high lines that constrain the metamorphic conditions pressure unit which was pervasively overprinted by c. are shown in Fig. 5. The Combin metabasite assemblages 20 kbar assemblages. We cannot distinguish between lie: these alternatives, but note that in both cases, the 1 On the down-pressure side of the line UHP relics would be older than the surrounding high- Tr+Ab+Cln=Gln+Zo+Qtz+W which, for a nom- P rocks because the UHP rocks are enveloped and inal 400 C, is <9 kbar. reworked by high-p rocks. 2 On the up-temperature side of Pmp+Cln+ The common mineral assemblage within the high-p Qtz=Tr+Zo+W, therefore T >300 C. Zermatt-Saas metabasites is omphacite+garnet+ 3 On the down-temperature side of Chl+Czo+ paragonite+phengite+rutile+glaucophane+zoisite+ Qtz=Prp+Tr+W. This places T <500 C. titanite+hornblende/actinolite±albite. This suite of However, the univariant reaction used here is for minerals does not represent an equilibrium assemblage the Mg end-member system, and the addition of Fe but the progressive and often complete retrogression of would stabilize garnet to lower temperatures, as would the eclogite assemblage during exhumation. Ca and Mn (Evans, 1990). Therefore, we consider Glaucophane may develop both at the peak eclogite 450 C to be the maximum temperature at which this facies in Mg-rich rocks. However, it also appears to assemblage could exist.

6 578 S. M. REDDY ET AL. pressures at which the Combin and Zermatt-Saas Zones were metamorphosed (9 and 18 kbar, respectively). This difference cannot be explained by geochemical variations in the protolith or by varying degrees of greenschist facies overprint (Dal Piaz et al., 1972; Kienast, 1973). Consequently, a metamorphic break, with a pressure difference of c. 9 kbar, separates the two units. THE STRUCTURAL GEOLOGY OF THE UPPER VAL GRESSONEY Structure of the greenschist facies rocks At the highest structural levels, the Gneiss Minuti Complex (GMC) of the Sesia Zone has a subhorizontal foliation defined by a micaceous schistosity (S1 ) gm (Fig. 6a). Quartz, actinolite, white mica and feldspar commonly define a mineral aggregate stretching lineation that plunges shallowly to the SE (Fig. 6b). No evidence of high-p mineral parageneses has been observed, and the different assemblages within the different lithologies of the GMC are consistent with greenschist facies metamorphic conditions. Kinematic Fig. 5. P T grid illustrating important reactions constraining indicators show a consistent top-to-se sense of shear the metamorphic conditions in the Combin Zone. All mineral at the lowest structural levels ( m) within the abbreviations are from Kretz (1983, 1994). All lines are after GMC. At structurally higher levels, the fabrics in the Frey et al. (1991) except Chl+Czo+Qtz=Prp+Tr+W which is after Evans (1990). GMC have a different orientation and fewer kinematic indicators. In the Combin Zone, a greenschist mineral assemblage (S1 ) defines a dominantly SSW dipping foliation co Calcschists with calcite+quartz+white mica± (Fig. 6c). The Combin Zone calcschists have mineral chlorite±zoisite±albite±titanite±tremolite/actinolite stretching lineations defined by calcite, quartz and assemblages are intimately interleaved with metabasites. aggregates of white mica grains. Aligned actinolite The similar style and geometries of fabrics grains define a mineral stretching lineation in the suggest that the two lithologies have equilibrated at metabasites. The orientation of these lineations is similar conditions. Maximum pressures are constrained NW SE (Fig. 6d) with the greatest concentration of by the reaction Ab+Cln+Qtz=Gln+Pg+W (Fig. 5). lineations oriented 100/130. Commonly shear bands However, there is some disagreement as to the position and mica fish structures provide kinematic indicators. of this line in P T space. Guiraud et al. (1990) At both the macro- and microscopic scales, mineral suggested that it lies at c. 11 kbar at 400 C, while stretching lineations can be traced continuously into Frey et al. (1991) reported 9 kbar. Neither reaction shear bands suggesting that the shear bands are a true provides a tighter constraint than the pressure limit reflection of the overall sense of shear. for the metabasites (<9 kbar). Abundant kinematic indicators show a top-to-se Both metabasites and calcschist contain phengite sense of shear on the eastern side of upper Val that can be used to constrain a minimum pressure Gressoney. However, to the west of the valley and in estimate based on Si content (Massonne & Schreyer, Val d Ayas, there is a lens of schist showing top-to-nw 1987). In many samples this minimum pressure kinematics bounded by zones of top-to-se shearing estimate exceeds the maximum pressure determined by towards the base and top of the Combin Zone (Fig. 4b; other methods and may even place the sample above sections A & B). The relative timing of this and the the Ab=Jd+Qtz reaction line. The Massonne & SE-directed shear can be inferred from: (i) the discordance Schreyer (1987) calibration has been queried by other of the top-to-nw panel to the base of the workers as yielding pressures that are too high (e.g. Combin Zone (west end of section B in Fig. 4b), and Essene, 1989). In conclusion, the best estimate of P T (ii) by truncation of the top-to-nw panel by topconditions of c. 9 kbar and C from the to-se fabrics in the east (towards B in section B). Combin Zone is derived from the metabasic rocks. These observations suggest that the top-to-nw zone In the Gressoney Valley, there is no evidence for of shear is truncated by top-to-se shear at both the Combin Zone rocks having been to eclogite facies. base and the top of the Combin Zone. The possibility Therefore, there is a significant difference between the of more than one age of top-to-se shear is also seen

7 THE GEOMETRY AND TIMING OF OROGENIC EXTENSION 579 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 stretching lineations in the Combin Zone; (e) & (f ) Combin Zone fold data. in cross-sections across the area. For example, geo- kilometre scale (grid ref ; N, E). metries near B (Fig. 4b; section B) imply truncation Parasitic hinges to this major fold are parallel to the of one panel of top-se sheared calcschists by another. regional mineral stretching lineation and well developed Evidence for macroscopic folding is sparse within top-to-se kinematics are present on both limbs the Combin Zone and a detailed description and of the fold. This fold cannot be traced to lower discussion is beyond the scope of this paper. However, structural levels and it does not fold the Zermatt-Saas/ there are two areas where folds are observed. First, in Combin contact. It is therefore spatially restricted to the metabasic lithologies exposed in the eastern side the uppermost zone of top-to-se shear that truncates of the Val d Ayas (grid ref ; N, E), the structurally lower top-to-nw fabrics. there are a series of tight folds with SW-dipping axial planar fabrics (Fig. 6e) and southerly plunging hinges ( Fig. 6f ) that are restricted to individual metabasic Structure of the eclogite facies rocks units within the overall top-to-nw Combin package. In the eclogite facies rocks structurally underlying the The folds overprint greenschist fabrics but do not metamorphic break, a well-developed planar fabric deform the contacts with the adjacent calcschist (S1 ) is defined by omphacite. Lower pressure glaucophanic and greenschist minerals commonly define a zs units 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 The second are folds adjacent to the Zermatt-Saas/ facies fabric. The younger fabrics are heterogeneously Combin contact on the eastern side of Val Gressoney developed and represent localized reactivation of the (grid ref ; N, E). Immediately in eclogite facies fabrics. The orientation of S1 (and zs the hangingwall to the contact, isoclinal folds deform subparallel overprinted S1 ) defines a weak girdle zs greenschist fabrics and have axial planes lying parallel (Fig. 7a). The extent of the greenschist facies overprint to the main contact and hinges lying subparallel to increases towards the Combin/Zermatt-Saas contact mineral stretching lineations. These folds are interpreted and is associated with a re-orientation and transpo- to be related to contact-related shear. sition of early eclogite fabrics (S1 ) into high-strain zs Structurally higher in the GSZ, the contact between greenschist fabrics (S2 ) lying parallel to the contact zs the Combin Zone and the GMC is folded at the (Fig. 4b; sections A & B).

8 580 S. M. REDDY ET AL. Fig. 7. Lower hemisphere equal-area stereonets from the eclogitic Zermatt-Saas and Monte Rosa Units. (a) Poles to foliation in the Zermatt-Saas Zone; (b) mineral stretching lineations in the Zermatt-Saas Zone; (c) poles to foliation in the Monte Rosa Unit; (d) mineral stretching lineations in the Monte Rosa Unit; (e) & (f ) F2 ZS fold data from Zermatt-Saas; (g) & (h) F2 MR fold data from the Monte Rosa Unit. Omphacite and glaucophane mineral lineations are in the Zermatt-Saas Unit (Fig. 7c). The absence of locally preserved and are broadly subparallel to diagnostic mineral assemblages within the Monte Rosa mineral lineations developed in the overlying Combin fabrics makes the conditions of foliation development Zone (Fig. 7b). In detail, lineation orientations difficult to constrain. However, at the Monte Rosa/ change from E W to SE NW as the contact with the Zermatt-Saas contact, where Zermatt-Saas metabasites Combin Zone is approached. Omphacite lineations are strongly retrogressed, greenschist fabrics within the and foliations formed at eclogite grade are not usually Monte Rosa and Zermatt-Saas Zones are subparallel. associated with good kinematic indicators. This suggests that juxtaposition of the two units Glaucophane and actinolite lineations are dominantly probably post-dates eclogite facies metamorphism. associated with top-to-se kinematics although Mineral stretching lineations are again generally occasionally top-to-nw kinematics are observed NW SE oriented (Fig. 7d). Kinematic indicators (Fig. 4a). Shear bands developed during the greenschist associated with Monte Rosa show both top-to-se and facies overprint are best developed in the zone of S2 top-to-nw shearing. zs and these record a consistent top-to-se kinematic Both the Monte Rosa Basement and Zermatt-Saas framework. Foliation data from the Monte Rosa Unit Zone contain isoclinal folds that have axial planes in Val Gressoney (S1 ) define a girdle similar to that mr lying subparallel to the regional orientation of the

9 THE GEOMETRY AND TIMING OF OROGENIC EXTENSION 581 foliation. In the Monte Rosa Unit, isoclinal folds tapers to the east, and, although cutting top-to-se (F1 ) deform lithological layering (defined by pre- fabrics, is cut by younger SE-directed deformation at mr Alpine aplites within the garnet white mica schists) the shear zone margins. and a well-developed foliation. In the Zermatt-Saas 4 The base of the GSZ has a planar contact that Unit, S1, defined by eclogite facies minerals, is also truncates E W greenschist facies folds developed in zs isoclinally folded (F1 ). This folding is associated with the Zermatt-Saas and Monte Rosa rocks in its zs the retrograde growth of glaucophane and titanite and footwall. post-dates the peak metamorphic assemblage. Isoclinal folds also affect the Monte Rosa/Zermatt-Saas contact to the east in Val Sesia (F1 ). These folds deform mr/zs greenschist facies fabrics that lie parallel to the Monte GEOCHRONOLOGY Rosa/Zermatt-Saas contact. Kilometre-scale folds Field mapping demonstrates the relative age of some (F2 ) also deform the contact. These two folding structural features in the GSZ. However, it is difficult mr/zs episodes therefore post-date the juxtaposition of the to tie these to footwall evolution because of the Monte Rosa and Zermatt-Saas Units (Fig. 4b; demonstrably late movement between the Zermattsection C). Axial planes of parasitic folds to the major Saas and Combin Zones. To clarify the history of the F2 structures dip gently to the north (Fig. 7e,g). GSZ with respect to possible contemporaneity of mr/zs Fold hinges are subparallel to the pole of the great footwall decompression, absolute dating techniques circle defined by the S1 and S1 foliations (compare are required. The homogenization of Sr isotopes during zs mr Fig. 7e,g with a,c). In the field, these folds again clearly deformation (e.g. during syntectonic recrystallization) deform greenschist facies fabrics in the Zermatt-Saas may allow the age of the deformation event to be Zone. Variations in shear sense in the Monte Rosa constrained if the temperature of deformation were and Zermatt-Saas Zone can be related to the inversion significantly lower than the closure temperature for Sr of shear bands by these folds, which have fold hinges diffusion in the mineral being dated (c. 500 C for subparallel to the mineral stretching lineation (cf. white mica) (Cliff, 1985). This simple picture may be Figure 7b,d,f ). These folds rarely show axial planar complicated by incomplete recrystallization and the fabrics in the field. However, in thin section, greenschist mixing of different mica populations which will result facies minerals define an axial planar orientation which in inheritance of radiogenic Sr. However, the Piemonte indicates that F2 folding occurred at greenschist rocks analysed here have no pre-alpine metamorphic mr/zs facies conditions. Importantly, these folds do not affect history and therefore problems of inheritance should the Combin Zone (Fig. 4b; section 3). be minimal. In summary, field mapping shows: Here we present Rb Sr data from the different areas 1 A major 1 2 km wide zone dominated by top-se of greenschist facies fabrics to establish the absolute shear at greenschist facies in the Combin Zone that ages of the different fabrics within the GSZ and assess extends structurally upwards into the GMC. We refer the duration of deformation across the shear zone. to this as the Gressoney Shear Zone (GSZ). The shear Our approach is to analyse several samples from the zone dies out upwards ( probably gradually) in the same structural level to test for reproducibility of data lower 100 m of the GMC, and has a lower boundary from a particular structure. The age data are also against the eclogite facies Zermatt-Saas Zone. checked for consistency with structural overprinting 2 Within the Zermatt-Saas Zone, glaucophane and/or relationships obtained from field mapping. We also greenschist facies fabrics overprint eclogite facies fabrics present data from above and below the GSZ, which formed at conditions close to the metamorphic peak. provide a context in which to interpret the deformation These later fabrics are heterogeneously developed and ages. Within the footwall (Monte Rosa and Zermattlie subparallel to the early fabrics. These fabrics and Saas Zone), metamorphic temperature estimates are the underlying Monte Rosa Unit are deformed at the generally close to the closure temperature for Sr kilometre scale by greenschist facies folds. Localized, diffusion in white mica (Dal Piaz & Ernst, 1978; pervasively developed, dynamically recrystallized, Barnicoat & Fry, 1986; Dal Piaz & Lombardo, 1986). greenschist facies fabrics are developed toward the top White micas from these units may yield closure ages. of the Zermatt-Saas Zone and overprint these folds. In the GMC, rocks with syntectonic greenschist facies These fabrics and associated mineral lineations are assemblages are likely to have crystallized below the subparallel to those in the immediately overlying closure temperature for white mica and hence give Combin Zone and show well-developed top-to-se deformation ages. shear indicators. 3 Within the GSZ, regions with different kinematic histories have been identified and these illustrate a Analytical techniques complex deformation history within the shear zone. Rock samples were crushed and mineral separations Overprinting relationships allow the relative age of carried out using standard magnetic and heavy liquid fabrics within different parts of the shear zone to be techniques. Where the white mica fraction contained constrained. A discrete panel of NW-directed shear both paragonite and phengite it was possible to enrich

10 582 S. M. REDDY ET AL. the phengite proportion magnetically. However, some trometer in the School of Earth Sciences, University paragonite remains even in the most magnetic fraction of Leeds. Analytical errors in the Rb/Sr ratio stem (compare the Rb and Sr concentrations of the two almost entirely from variable mass fractionation during white mica fractions from S3-75b in Table 1). Calcite, Rb runs. Replicate analyses of unspiked Rb indicate epidote or feldspar separates were analysed to provide that 95% confidence limits on the Rb/Sr ratios are control on initial 87Sr/86Sr ratios. Minerals were first ±0.8%. Errors in the 87Sr/86Sr ratios were between spiked with a mixed 87Rb 84Sr spike, decomposed, and 0.014%. and Rb and Sr separated using standard ion exchange procedures. The Rb/Sr ratio of the spike was checked by analysis of SRM 607 K-feldspar (Cliff et al., 1985). Isotopic compositions of Rb were measured on a VG Results Micromass 30 while Sr was measured using dynamic Results are given in Table 1. Data are summarized in dual collection on the VG Isomass 54E mass spec- terms of the GSZ and hangingwall/footwall ages.

11 THE GEOMETRY AND TIMING OF OROGENIC EXTENSION 583 Fig. 8. Distribution of mica ages in the study area. All the data are from this paper (Table 1) except for sheared GMC sample 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 impure marbles with aligned mica flakes in a matrix of quartz and carbonate. Calcite grains form an interlocking mosaic of elongate grains with aspect ratios up to 251. Micas in all rock types show limited undulose extinction with localized stronger kinking. Two foliated GMC (orthogneiss) samples from the GSZ were also dated. We interpret the micas within these greenschist facies rocks to have grown or recrystallized syndeformationally within the evolving foliation. The location of samples and the distribution of ages are shown in Fig. 8 while their relationship to the basic kinematic framework of the GSZ is illustrated in Fig. 4(b). All ages from the GSZ fall between 45 and 36 Ma. An important result is that the youngest ages ( Ma) come from the base of the GSZ and are from rocks associated with SE-directed shear. Samples from the zone of top-nw shear yield ages from 39.2 to 37.2 Ma. The oldest ages from the Combin Zone of the GSZ ( Ma) are recorded from rocks with top-se fabrics that are truncated by the basal top-se shear and the structurally higher top-nw zone of shear (Fig. 4b). Older ages are also found in the highest structural levels of the GSZ within the GMC ( Ma). These data illustrate that there are consistent and reproducible groupings of ages from adjacent samples within the GSZ but that different parts of the shear zone record different ages (Fig. 9). Furthermore, the different Rb Sr ages are consistent with relative Fig. 9. Summary of age data from the GSZ in relation to the kinematic and structural setting.

12 584 S. M. REDDY ET AL. chronologies of deformation inferred from cross-cutting isotopic heterogeneity for this age discrepancy. First, relationships of different fabrics (Fig. 4b). The ages not all areas of the Zermatt-Saas Unit may have from the GSZ are not consistent with the data reached peak metamorphic conditions at the same representing closure through a particular temperature time. This is suggested by recent data from the but are consistent with the Rb Sr ages dating the structurally and metamorphically equivalent Monviso timing of deformation. area which yields interpreted peak metamorphic ages of 62 and 49 Ma (Duchêne et al., 1997; Cliff et al., Hangingwall and footwall data 1998). However, as argued above, the UHP Unit at Cignana cannot have crystallized after the high-p Sample 0-70 from the GMC in Val Sesia is in the eclogites which surround it. An alternative explanation hangingwall to the GSZ and contains a fabric that for the different ages is that zircon growth in the becomes progressively overprinted towards the GSZ. UHP Unit may not have coincided with peak eclogite Textural observations show that the biotite in this facies conditions. rock appears to have recrystallized along with white In conclusion, there are some problems with inter- mica during deformation. We interpret the 46 Ma preting the Zermatt-Saas age data. It is possible that white mica data as a deformation age and 30 Ma from the Zermatt-Saas Zone represents a number of smaller biotite as cooling. These age data and others reported units that experienced different P T histories before by Inger et al. (1996) appear to show increasingly being juxtaposed. Our best interpretation of available older deformation ages with increasing structural level data is that at least some of the footwall was at in the GMC. eclogite facies conditions at 44 Ma. Three ages are reported from the footwall of the GSZ. One from the Monte Rosa Unit gave an age of 226 Ma. We interpret this age to be the result of DISCUSSION incomplete isotopic re-equilibration during Alpine metamorphism. This is consistent with previously Evolution of the Gressoney Shear Zone published data (Frey et al., 1976). Two petrographically The contact between the Combin and Zermatt-Saas distinct samples from the Zermatt-Saas Zone have Zones is a significant metamorphic break separating experienced only Alpine metamorphism but yield eclogite facies rocks in the footwall from greenschist different ages. A metabasite sample from Val d Ayas facies rocks in the hangingwall. Previous studies (S3-75b) which gave a white mica age of 40.5±0.6 Ma around this contact have suggested several alternative contains randomly oriented micas and epidote interpretations to explain the nature of the metamor- intergrown with blue-green amphibole. The micas and phic break. Milnes et al., (1981) and Ellis et al. (1989) epidote are interpreted to be a relatively low-p suggested that the contact was related to SE-directed overprint and the age represents crystallization of backthrusting (i.e. crustal shortening) which has been retrograde mica. A calcschist from Valtournanche subsequently rotated. It has also been suggested that (S4-60) consists of strongly foliated phengite enclosing the contact may be extensional and associated with epidote and titanite in a quartz-rich matrix with garnet exhumation of Zermatt-Saas eclogites in the footwall. porphyroblasts. This is interpreted as an eclogite facies Within this framework, Butler (1986) and Ballèvre & assemblage. A white mica titanite regression gave an Merle (1993) suggested SE-directed extension, while age of 46.4±0.6 Ma and an initial 87Sr/86Sr ratio of Platt ( 1986) suggested NW-directed movement. None , whereas a white mica carbonate regression of these authors presented detailed kinematic or timing produced an age of 47.5±0.5 Ma and an initial information from the Combin Zone against which to 87Sr/86Sr ratio of Petrography shows that the test the various models. calcite grains are altered to an opaque phase around Our work, together with that of Wheeler & Butler their margins and along cleavage planes: this alteration (1993), shows that the Combin Zone is dominated by may well have affected the 87Sr/86Sr ratio. We consider top-se shear for at least 10 km along strike. the 46.4±0.6 Ma age more reliable. The foliated Throughout the study area, the GSZ dips beneath the phengite is clearly part of the eclogite facies fabric and structurally higher units of the Sesia Zone. No therefore crystallized above 500 C. We interpret the comparable shear zone re-emerges further south-east 46.4 Ma age as a cooling age. and this regional geometry is therefore consistent with Our Zermatt-Saas data must be interpreted together the GSZ being a zone accommodating crustal extenwith other existing age data from the Zermatt-Saas. sion. This contrasts with backthust-related shear that In particular, Rubatto et al. (1998) obtained a zircon should cut up structural section to the south-east. crystallization age of 44±0.7 Ma that has been Additional criteria also support the extensional nature interpreted as a peak metamorphic age for the UHP of the GSZ. Fundamentally, if the GSZ were a rotated Unit at Lago Cignana. This must be reconciled with backthrust, the implication is that the Combin Zone the phengite data from the calcschist (S4-60) with an and the Sesia Zone were situated to the north-west of, apparently older cooling age. There are several and at deeper structural levels than, the Zermatt-Saas possible explanations other than the potential for Zone. If the GSZ were extensional, the Combin Zone

13 THE GEOMETRY AND TIMING OF OROGENIC EXTENSION 585 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 stages of extension truncating folds in footwall (not shown), and creating present-day geometry of metamorphic break; 30 Ma, major backthrusting on shear zones and brittle structures, and local rotation of units in hangingwall, moving Alpine internal zones up relative to Ivrea Zone. Uplift triggers major erosion. Note that backthrusting was probably active over a prolonged period. Present Day, geometry (after Escher et al., 1988) is dominated by backthrust and backfold structures, although earlier extension was responsible for equally major vertical displacements. and Sesia Zone would also be to the north-west of the their present geometry therefore post-dated F2 MR/ZS Zermatt-Saas Zone prior to GSZ movement but these (Fig. 4b). Since these folds deform greenschist facies zones would have been at higher structural levels, that fabrics, the eclogitic footwall must have already been is in their generally accepted structural position above partly exhumed when the GSZ and the underlying other internal zone units. We conclude that the eclogites were juxtaposed in their present position. metamorphic break therefore corresponds to the This illustrates a fundamental point about the nature contact between eclogite facies rocks and a greenschist of metamorphic breaks; that is, the contact itself may facies zone of extensional shear. be a late structure that is not responsible for initial or Within the eclogite facies rocks, SE-directed shear significant exhumation of the footwall (Fig. 1). Within bands, developed from eclogite through to greenschist the GSZ, the basal shear is clearly the most recent facies conditions, suggest that the Zermatt-Saas Zone area of localized deformation. Therefore, the possibility was cooling and moving to lower pressures during remains that the penetrative fabrics within the exten- top-to-se shearing and consequently that these struc- sional shear zone, but above the basal shear, might tures are contemporaneous with the exhumation still be responsible for initial exhumation of the process (Fig. 10). The simplest model is that the non- footwall (Fig. 2). This cannot be tested based on the coaxial, top-to-se deformation in the Zermatt-Saas present geometry of these units. Importantly, the Rb/Sr Unit represents non-pervasive deformation associated data show that the majority of calcschist fabrics are with exhumation and cooling, formed in the same significantly older (by up to 9 Myr) than those within extensional regime as the GSZ. However, there the basal shear zone of the GSZ. This supports a are some complications to this model. These are model of shear at the base of the GSZ carrying earlier addressed below. fabrics that did not undergo recrystallization in the final stages of GSZ deformation. Truncation of greenschist fabrics in the footwall In the footwall to the GSZ, lithological layering, the Timing of shear and exhumation of the Gressoney Shear Zone main foliation and large folds (F2 ) which affect the MR/ZS The Si contents of phengite from various structural Monte Rosa/Zermatt-Saas contact are truncated by the planar Zermatt-Saas/Combin contact. The juxtaposition of the greenschist and eclogite facies units into levels within the GSZ show no clear correlation with age ( Table 1). Absolute pressure estimates using the phengite geobarometer of Massonne & Schreyer (1987)

14 586 S. M. REDDY ET AL. seem artificially high, perhaps by as much as 3 kbar. zone related to SW NE extension, but it passes However, Si content will be some guide to relative beneath the study area and cannot have contributed pressures. Inspection of the chemistry and age data to unroofing. Unless there were other such structures indicates that calcschists in the GSZ were at pressures within the Austroalpine nappes (for which there is at least as great as that for sample 0 70 in the no evidence), later exhumation must have been hangingwall, but as much as 10 Ma later. There are accomplished by erosion. two important conclusions. First, portions of the shear zone were deeper relative to the hangingwall when they were deforming than their current position Significance of NW-directed shear suggests. This is to be expected in an extensional shear There are two possible explanations for the development zone. Second, the pressure in the GSZ in the last stage of NW-directed shear in the GSZ. One is an of movement was not significantly less than the episode of crustal shortening; the other is a component pressure in the immediate hangingwall before movement of pure shear extension in and above the GSZ which commenced. Therefore, there was little unroofing partitioned into SE- and NW- directed shear. In the of the hangingwall ( by erosion or hypothetical higher first model, the NW-directed shear would transfer level structures) during the 9 Myr period of shear. downwards into deeper shortening structures and would relate to a major change in deformation style, Timing of shear and footwall exhumation which then switched back to late extension. In the second model, the strain in the GSZ would involve Parts of the Zermatt-Saas Unit were at eclogite facies some overall pure shear stretch and this would also at 44 Ma. The crucial relationship between this age include the hangingwall. The GMC generally contains and those of the GSZ is that they overlap: the GSZ gently dipping fabrics that, immediately above the registers crustal extension occurring throughout the Combin Zone, are non-coaxial and SE-directed. period that the footwall was unroofed from eclogite to Structurally higher, kinematic indicators are lacking greenschist facies. Shear indicators within the eclogite and this could be due to pure shear. The GMC directly facies rocks also record the same kinematic framework above the study area has been removed, so evidence is as the GSZ throughout exhumation to greenschist circumstantial. We prefer the second hypothesis as it facies conditions. The GSZ records greenschist facies better explains the near-simultaneous top-nw and (c. 9 kbar) throughout this time period with no evidence top-se shear. for major pressure changes. This indicates that the extensional GSZ was the major agent of unroofing the Implications for the exhumation of Alpine eclogites and Zermatt-Saas Unit from >18 kbar at 44 Ma to orogen evolution c. 9 kbar by 36 Ma. Taking a notional density of 2.8 g/cm3 (there being no direct evidence for the nature of the material that was originally above the eclogites), The significance of the timing of extension at Ma this corresponds to removal of at least 32 km of Initial estimates of the timing of eclogite facies material in not more than 8 Ma. The average rate of metamorphism suggested an Early Cretaceous age tectonic exhumation over this period was therefore at (Oberhänsli et al., 1985) and these were supported by least 4 mm/a. Cretaceous argon ages (Hunziker, 1974; Stöckhert Remarkably, the oldest ages of extensional move- et al., 1986). However, recent data shows that the ment in the GSZ actually appear to pre-date the Western Alpine eclogites are significantly younger and crystallization of some of the eclogites found in its record a decrease in age passing to structurally lower footwall. This might appear to be a serious contradiction, levels through the sequence of Austroalpine, Piemonte since the eclogites must have been buried by and Pennine Units. In the Sesia Zone, U Pb dating lithospheric shortening. However, the time of peak suggests that eclogite facies metamorphism took place metamorphism does not necessarily relate to that of at c. 65 Ma (Ramsbotham et al., 1994). Recent Lu Hf peak pressure, and peak temperature is a more likely dating yields similar estimates ( Duchêne et al., 1997). candidate. It is therefore possible that the eclogite In the Zermatt-Saas Zone, Sm Nd dating of garnet assemblages crystallized during the early stages of suggests a Tertiary age for metamorphism (Bowtell exhumation. et al., 1994), which is consistent with recent U Pb The GSZ was still at considerable depth (9 kbar, zircon ages of 44 Ma (Rubatto et al., 1998). In the with only continental material above it) when the Pennine Basement Massifs, eclogite facies metamorphism youngest part of the shear zone was active. There is in Dora Maira appears to have peaked between no evidence in our study area relating to later 32 and 38 Ma (Tilton et al., 1989; Duchêne et al., exhumation of these rocks. There are many younger 1997). Unfortunately, there is little recent data con- structures in the Alps, commonly forethrusts and straining the age of eclogite facies metamorphism in backthrusts, but it is difficult to reconcile these with the Monte Rosa massif. In support of Tertiary eclogite tectonic exhumation of the GSZ from 36 Ma. The ages, recent 40Ar/39Ar dating (Arnaud & Kelley, 1995; Simplon Line (Mancktelow, 1985) is a younger shear Ruffet et al., 1995; Reddy et al., 1996; Scaillet, 1996;

15 THE GEOMETRY AND TIMING OF OROGENIC EXTENSION 587 Pickles et al., 1997) clearly demonstrates that early argon studies from the different eclogite units were probably affected by excess argon. The difference in metamorphic ages between the different major tectonic units places very important constraints on Alpine tectonic evolution. If we assume that eclogite facies metamorphism resulted from subduction of crustal material (e.g. Ernst, 1971), then available age data suggests prolonged and continuous SE-directed subduction. The post-metamorphic cooling and exhumation histories of the different eclogites must have been taking place while the subduction process was still operating. Fundamentally, the Sesia Zone eclogites had undergone a significant part of their exhumation history before the Piemonte eclogites had reached their metamorphic peak. Similarly, the data in this paper illustrate that the Zermatt-Saas Zone had undergone significant exhumation by the time the Pennine Basement (Dora Maira) eclogite facies metamorphism took place. The data therefore suggest a westerly migrating accretion of material to the hangingwall of the subduction zone. The Ma ages reported for extensional deformation at the top of the Zermatt-Saas Zone correspond to the time immediately after, or overlapping, attainment of the eclogite facies metamorphic peak. Tectonic extension was therefore responsible for the initial exhumation of the Zermatt-Saas eclogites over 9 Ma following their formation. If all of the Western Alpine eclogites record a similar extensional unroofing story, the geometry of the different units may also require a westerly migration of the extensional structures responsible for eclogite exhumation. Currently, data to support this are limited. Exhumation dynamics We have demonstrated here the kinematics of eclogite unroofing in this part of the Alps (summarized in Fig. 10), and have deliberately avoided tying the geometry and kinematics of extension to a particular driving force. This is because the most that can be deduced from structural, metamorphic and geochronological study is the nature and timing of relative movement of rock bodies. Many studies do not make clear that a given extensional geometry can be caused by more than one driving force. For instance, Wheeler ( 1991) illustrates several scenarios for unroofing. Three of these involve hinterland-directed extension similar to that demonstrated in this contribution. In the first two models, a pip or wedge of material rises via thrusting at its base and extension at its top. In both of these, the driving force was thought to be the intrinsic average buoyancy of the pip or wedge relative to its surroundings (Wheeler, 1991). In the case described here, such a driving force might be possible if the mafic eclogites of the Zermatt-Saas Zone were attached to continental material of the Monte Rosa Unit. In the third model, extension relates to overall plate divergence (Butler, 1986). A fourth, more recent model has illustrated how normal-sense shear zones may develop due to crustal velocity gradients above rock packages where thrusting is taking place at the base of the unit (Beaumont et al., 1996). Such a model may be applicable to the Alps (Escher & Beaumont, 1997). However, we reiterate that such models must be tested using geochronological data to prove the synchroneity of structures that are hypothesized to be dynamically linked. Escher & Beaumont (1997) related eclogite facies metamorphism to Cretaceous events, which now seems unlikely (see discussion above), and do not present other timing data. The fundamental point is that the extensional geometry from one structural level is not diagnostic of the mechanism, and additional information is required to infer the driving forces for extensional deformation. In the Alps, there are many thrusts in the external zones that carry the internal zones in their hangingwalls. To have some constraint on the geodynamic mechanism responsible for extension, it is crucial to know whether these thrusts were active simultaneously with extension. Kinematic reconstructions of plate movement at the time of SE-directed extension show that the relative motion of the African plate with respect to Europe was towards the north-east (e.g. Dewey et al., 1989). This seems to exclude net plate divergence as a driving force, and restricts possible models to those in which buoyancy forces play a role. CONCLUSIONS We have shown that in the Alps, mafic eclogites that crystallized at 44 Ma were unroofed by hinterlanddirected extensional shear that began to operate even before the presently exposed eclogites had crystallized. Extension began at 45 Ma and became more localized before ceasing at c. 36 Ma. As the footwall was unroofed, sporadic overprinting, shearing and largescale folding occurred on the retrograde path. The base of the GSZ truncates these folds because it is late and discordant relative to earlier fabrics formed in the same kinematic regime. Consequently, the presently observed metamorphic break was only responsible for a small component of the exhumation history of the footwall eclogites. The total amount of unroofing accommodated by extension brought the eclogites from c. 60toc. 30 km. The lack of systematic pressure decrease in the GSZ over the period in which it was active shows that syn-extensional erosion was insignificant. In contrast, subsequent removal of the remaining 30 km of overburden since 36 Ma appears to have been dominated by erosion. We infer that the extensional shear zone reached at least 60 km depth, and is probably linked to the relict subduction zone. Internal buoyancy forces rather than overall plate divergence appear to have driven extension, although it remains for this to be conclusively demonstrated.