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Home , Austroalpine nappes, Glarus Alps, Glarus thrust, Helvetic nappes, Infrahelvetic complex, Penninic

001,2-94021 0I I020231 -16 $ 1.50 + 0.2010 Eclogaegeol. Helv.94 (2001)237-252 EirkhäuserVerlag, Basel, 2001

Penniniccover nappesin the Prättigauhalf- (EasternSwitzerland):Structure and tectonicevolution

Manrus Wenl'2 & Nrr

Key words: , , Bündnerschiefer,Falknis ,Prättigau half-window, structural ,Alpine

ZUSAMMENFASSUNG ABSTRACT

Die Sedimentabfolgeder zum Valaisan gehörenden Grava-Decke im Prätti- The sedimentary sequence of the Grava nappe (Valaisan) in the Prättigau gau-Halbfensterder Ostschweizbesteht aus kretazischenSchiefern (Bündner- half-window of easternSwitzerland consistsof Cretaceouscalcschists (,,8ünd- schiefer) und spätkretazisch/alttertiärem . Diese Gesteine wurden durch nerschiefer") and Late CretaceouslEarly flysch. It records the follow- die folgenden alpinen Deformationsvorgänge überprägt: (D1a) Abscherung ing stagesof Alpine deformation: (D1a) d6collement of the cover of der Sedimentbedeckungeiner südwärts abtauchendenLithosphärenplatte und a southward-subductedlithospheric slab and accretion of this cover as a hin- Akkretion der abgeschertenSerien als Duplex an der Basis des Orogenkeils, terland-dippingduplex at the baseof the orogenic wedge,beginning around 50 etrva 50 Ma; (D1b) lokale Verfaltung der D1a-Strukturen; (D2) top-SE- Ma; (D1b) local refolding of D1a structures; (D2) rop-SE shearing and Scherung und "Rückfaltung" in einem extensionalenRegime, zwischen35 und "backfolding" in an exlensionalregime, between 35 and 30 Ma; (D3) renewed 30 Ma; (D3) emeute kompressive Verformung im Zusammenhang mit inter- contractional deformation related to internal shortening of the underthrust ner Verkürzung der unterschobenen europäischenKruste; dadurch Bildung European crust and formation of north-vergent folds (Lunschania antiform, von nordvergenten Falten (Lunschania-Antiform, Valzeina-Synform), kine- Valzeina synform) which are kinematically Linked to a top-north-northeast matisch verbunden mit einer NNE-gerichteten Überschiebung am Kontakt shear zone at the base of the Grava nappe againstthe Infrahelvetic cornplex, zwischen dcr Grava-Decke und dem unterlagemden InfrahelvetischenKom- representing the eastern, along-strike continuation of the overthrust plex, wobei dieseÜberschiebung eine östlicheFortsetzung der Glarner Haupt- (after 30 Ma); (Da) SE-NW-directed additional shortening leading ro open überschiebungdarstellt (nach 30 Ma); (Da) Bildung offener Falten mit süd- folds with southeast-dippingaxial surfaces.The Falknis nappe (BrianEonnais) ostfallenden Achsenebenen durch zusätzliche, SE-NW-gerichtete Verkürzung. in the Gürgaletscharea, at the southern border of the Prättigau half-window, Die Falknis-Decke (BrianEonnais) im Gürgaletsch-Gebiet erfuhr eine ähn- suffereda similar tectonic evolution. It formed its own small duplex consisting liche tektonischePrägung. Sie bildet einen eigenenDuplex auszwei Schuppen of two imbricates (D1a), was locally folded (D1b), and affected by top-SE ex- (D1a), wurde lokal verfaltet (D1b), und schliesslichvon top-SE-gerichteter tensionalshearing and related "backfolding" (D2), kinematically Iinked to the extensionaler Scherung und damit verbundener "Rückfaltung" erfasst (D2), newly identified Gürgaletsch shear zone which forms the northern continua- wobei sich die hier erstmalsbeschriebene Gürgaletsch-Scherzone bildete, wel- tion and termination of the well-known. D2 Turba mvlonite zone. che als Nordfortsetzung und -ende der Turba-Mylonitzone aufgefasstwird.

1. Introduction The study area is located in the Prättigau half-window of The aim of this study, which was carried out as a PhD thesisat Eastern (Figs. 1,2). The study is mainly focussed re Geology-PaleontologyInstitute of Basel University, was to on the Grava nappe, a tectonic unit derived from the Valais investigate the tectonic evolution from oceanicsubduction to paleogeographicdomain (Steinmann 1994),made up by Creta- continent-continent collision in the boundary region between ceousto Tertiary Bündnerschiefer and Flysch , and Eastern and Central Alps in . This was exposedin the interior of the half-window. These sediments, done by analyzing the structures of Bündnerschiefer and being incompetent and anisotropic, are highly susceptible to Flysch sediments. These sediments formed an accretionary folding on all scales.In many outcrops, three to four genera- prism in front of the alpine zone and were affected tions of folding can be clearly distinguished. On the other by polyphase deformation during subduction, collision, and hand, it is difficult and time-consuming to correlate these post-collisional shortenins. structuresover a larger area.Therefore, previous to this study,

1 Geologisch-PaläontologischesInstitut, Universität Basel, Bernoulli-Straße 32, CH-4056 Basel, Switzerland 2 Marti TunnelbauAG, Freiburgstrasse133, CH-3000 Bern 5, Switzerland 3 GeologischesInstitut der UniversitätBonn, Nussallee 8, D-53115Bonn, . E-mail: [email protected]

Structure ofthe Prättigau half-windorv 237 ,{)Z=.--- r\ 3 4t/ - - ^\r Q(t *^teoY )4- "^CtY- (ottn9l =-:

51 Apuliancontinental - marginunits I soutnPenninic -ri7t1 P Prättigauhaltwindow MiddlePenninic E Engadinewindow ISFT1 NorthPenninic Europeancontinental rrn-;Tn L I marginunits 100km l---l tertiaryintrusions

t*a"tc mapof theCentral and with location of study uP9.f au3tu!lpltr.d6dE wareenntnrcu-.ffiusw$aßF X:;1 ffi @ c4nanes za ophbrle-bdq mereg* UpF lttrdphetffil Noö F=:] PerninlcuniBl [[[ neretcunrr

b{ü Au*oalphdimenE üededdretüdwuzc'Fly$ uTlTn m Q/ watcwxcaneux v 4 (ate cbeaßl bh, [email protected]|Mth -L.! Ol mftct (m*ed rotdyh theN-prnnhb) |--.1 bü n.@i _-___'- __- @ahltesad E[f aektecEracsustol€dirylrysd cnracous ro T€dtry lrysd *{enea no serious attempts have been made to decipher this valuable r Ars 2fre, Pr.& ElF Ju,stcloc'etahus -e-_g> -l BodneEchtdd BBandtr&e andtr& MtddleP6.ninicsdinsb ^ archive of Alpine deformation. chhBpFildAMstryscn: ^ ßsedhGt $ suurtn mpoe andsftm MpF Except for minor uncertainties,it was possibleto correlate f----l Tedhry tFh NS\l rarnt nape small-scale with large-scaledeformation structures,to corre- L I büoEEc$rer AFadat$h tly*h. Gg coFtd$h *.r zo*, Mg: Mdqrc s&ar !oß. MUi Made96 !nt, NB: Ni.d_B*edn dtar tEE, late these structures throughout the study area, and to assign OA: ftßeslerg srfm, PN: PHls n4f, TM: IuOa nybA . them to four major stagesof deformation(D1 to D4), one of Fig. 2. Tectonic map ofthe Prättigau halfwindow and surrounding area. which is further subdivided into substages(D1a, b). The struc- tural evolution of the Grava nappe reflects two shear-sensere- v, _ ,ls during the Tertiary, from top-north "pro-shear" to top- southeast"retro-shear", and back to "pro-shear". Penninic zone (South Penninic or Piemont-Liguria ' Many additionaldetails of the regionalstructural geology may North Penninic or ) and an intervening continen- be found in Weh (1998).This PhD thesisis availableon request tal , the BrianEonnaisfragment. The Penninic from the Geology-PaleontologyInstitute of BaselUniversity. exposedwithin the half-window include, from top to bottom, the Arosa zone ( and -bearingm6langes from the Piemont-Liguria ocean and the distal Apulian mar- 2. Overview of the gin; Ring et al. 1989), the Sulzfluh nappe (sedimentary rocks The Prättigau half-window is a westward-opening, 25 km x derived from the BrianEonnaisfragment, dominated by Late 25 km embayment of the Austroalpine-Penninic boundary in Jurassicshallow-water ), the Falknis nappe ( Eastern Switzerland (Figs. 1, 2).The half-window is framed by to Early Tertiary sequenceof sedimentary rocks, probably de- the overlying Austroalpine nappeson its northern, eastern, rived from the northern margin of the BrianEonnaisfragment and southern margins. These representthe former continental againstthe Valais basin; Gruner 1981,Weh 1998), and the margin of the Apulian (Adriatic) microcontinent. They were thick Bündnerschiefer-Flysch sequence of the Grava nappe stacked during the Late (Froitzheim et al.1994, (derived from the Valais basin; Steinmann 1994).Sulzfluh and 1996). The Apulian continental margin was, in Cretaceous Falknis nappes do not form continuous sheets but are rather time, divided from by the two oceanicbasins of the made up by lenseswhich reach important thicknessesin some

238 M, weh & N. Froitzheim SLlzlluhnaPPe

10km

LJpperAu.boalpim: Uiddb Pennihic Fdio.nb.y Epp.t: N.Prnninic: Valss 6it

\br-lberg fllll tosa ddmjb. (sedin.^b) N\Nl solzn rh napp" Flysdro( Liedilensiein,Vrd! and |_ Zme ol P[ Terri.Lunsr]ana, '' " uppera.d'o*erValscrmelmge ffi L""r'tat n"ppe (se.lihcnts) 7VZ Oplioi h€-bearingmelanges: Fatrr*snappe Tomülf,appe: t- sitwetE nagpe (s.dim.nt! sdbacsmeßq $ V74 lonü|, Neua-BrusdrgtErn and Gadriolmelange LffiAusb@ldm f]!l Late crehceous loTertiary lysdr Unli of th€ EuioFan contin.nlal marqin ffi Sodi6snlEüdbasem.nt;Ooröcrgnappt $l Sdram nappes -l Jtrösdc lo C{elreous Bin&rersdielet H ürnappe, EJanappe,Mvgna nappe : öolanaPpe Grda nappeand ÄölalsJr lysdr: 9Pmninic [=:l Aduta neppe 1'-iTiy1 CowotTarnbo and ffi r"'t"'ynys.r, l.r''l.U Surenanippes Aar massil end Godrard'messi{' basemenl, l.-f.---:-::l e füil,i?';lli$Jfffl *"0"","*" f]] Btndoersdrierer lnfrahektic md Hctvedcunits ", Al ußti i"- -- Ol overtfrost (onlymaüed in üreN'P$ninic) Middl€ P.nhinic basNnt Eppe i: ------ffi l,.tnr6t"ng" D2 shearzone and axial lrac€ Sl Tamboarü Surenanappes ----- 03 owfnrust afi, axial tace

A:Arblabch t!ysch,MSZ Mst gnaa ihe d lse , N8: Nieh.l.gevefin axal !ec., OA: fuEenbetg 4lilqm, TM: Turbamyldit.

Fig. 3. N-S crosssection through easternSwiss Alps (after Schmid et a|.,1996,modified according to results of this study).

areas and completely pinch out in others. fqwards the west, of the study area, dependent on the dip aDgles of the main the Penninic units of the half-window mostly border against structures in these parts. The exact procedure is described in the underlying , that is, the Aar massif Weh (1998). Minor uncertainties and incorrectnessesof this and auto- and allochthonous sediment units covering it (Pfiff- method arise from the lensoid shape of some tectonic units, ner 1977).This Infrahelvetic complex is overlain to the North e.g., the Falknis and Sulzfluh nappes, and from the strike of t' 'he nappes along the Glarus overthrust (Schmid some major folds deviating from E-W. 1>-;). The border against the Grava nappe only in the northwest corner of the Prättigau half-window (Fig. 2). Infrahelvetic eomplex and Helvetic nappes are derived 3. Structural analysis of the Grava nappe from the European margin of the Valais ocean. 3.1.Stratigraphic and stuctural outline;previous work As follows from the preceding,the structuraily highest and deepest units occur in the East and in the West of the study The Bündnerschiefer and flysch sediments of the Prättigau area, respectively.This situation results from a general east- half-window are part of the Grava nappe (Steinmann 1994, ward dip of the tectonic boundaries and main structural ele- Schmid et al. 1996).This is the lower of two major Bündner- ments in the eastern Central Alps, and makes it possible to schiefer/Flyschtectonic units rooted to the South in the Misox construct cross-sectionsof the nappe stack by projection over zone between the Adula and Tambo basementnappes (Fig. 3). relatively short distances(Figs. 3, 4). The cross-sectionin Fig. 4 The other one is the Tomül nappe which overlies the Grava was produced by east-west-projection of about 50 detailed rappe to the South but does not reach as far north as the Prät- cross-sectionswhich had been recorded mostly along N-S ori- tigau half window, being restricted to the area south of Chur ented stream valleys and mountain ridges. Different anglesof (Fig.2). Within the study area the Grava nappe consistsentire- projection between 0o and 30o were chosen for different parts ly of basin sediments from the Valais basin which was partly

Structure of the Prättigau half-window 239 N S

t' 7,ry27) 4,')t 10km E

-"-' Upper Austroalpine South Penninic North Penninic D1 overthrust -+-- D2axialtrace E Sediments(not differentiated) | ^rosat;ne f----]l Tertiaryf lysch '{-- D2 axialtrace ffi Uppertriassicto cretaceous Middle Penninic f__l Bündnerschjeferof the Grava unit malor --'-- 1.. - I Hauptdolomit(Norian) ll_l srt.ftrhnupp. E Bunon"rschieferoftheTomül unit D2shearzone v Flyschof Liechtenstein,Vaduz 'Ä - D3 axialtrace S- Permianto MiddleTriassic Falknisnappe: ry vzza andlora(lbeß -a a D3 overthrust [--] ea."t"nt nappes n Not differentiatedsediments Units of the Europeancontinental margin Lower Austroalpine u CretaceousandTertiary sedimenls ?--7- Helvelic nappes, Infrahelvetic kompl ex, 1/ / | 74 64ssiv Gotthard E n Triassicand Jurassicsedirnents , "massiv"

GSZ:Gürgaletsch shear zone, MSZ: Martgnas shear zone, NB: Niernet-Beverinaxial trace, OA: Ochsenberg antiform

Fig. 4. N-S cross-sectionof the Prättigau half window.

floored by (Dürr et al. 1993, Florineth & vides the "Flysch" in the senseof Spicher (1980) into what he Froitzheim 1994,Steinmann & Stille 1999).Their metamorphic calls "preflysch" (Turonian to Maastrichtian, that is, Pfävigrat grade as determined from illite crystallinity and vitrinite re- to Eggberg series) and a "flysch" sensu strictu, which is Ter- flectance is anchizonal in the North to epizonal in the South- tiary in age and encompassesonly the Ruchberg series. west (Thum & Nabholz l972,Ferreiro Mählmann et al.1992). During our study we found that the boundaries drawn by Fe-Mg carpholite indicating an earlier with Nänny on his map and profile (Nänny 1948,tables 2,3) are in- high P/T ratio occurs in the Southwestnear Chur (Oberhänsli correct in some areas.This is becauseNänny interpreted the et al. 1995). The age of the sedimentsin the Grava nappe is structure of the half-window in terms of simple, north-facing , -.r'er Cretaceousto Eocene (Nänny 1948,Steinmann 1994). and overall north-vergent folds. In fact, we found that in the The lower part (Lower Cretaceous)is dominated by marl and entire southeasternhalf of the half-window, the main folding sandy limestone, whereashigher up, calcareousand siliciclastic phase(D2) is everywhere south- to east-facing,and more often turbidites and breccia layers, intercalated with shale,become south-vergentthan north-vergent. The simple style of Nänny's increasingly more important. The term "Bündnerschiefer" is profiles has little similarity with the actual situation. Therefore, often used for the lower part of the series, "Flysch" for the we did not adopt his stratigraphic boundaries in our map and upper. There is, however, no agreement about the exact profiles, except for the Cretaceous-Tertiaryboundary which is boundary. The of the Grava nappe in the Prätti- lithologically well-defined and could be adopted from Nänny gau half-window was studied by Nänny (1948). He distin- (1948)with only minor corrections' We will use "Tertiary Fly- guished and partly dated biostratigraphicallyeight series(from sch" in this paper for the Ruchberg series, and "Cretaceous older to younger: Klus, Valzeina, Sassauna,Pfävigrat, Fadura, "for the Cretaceouspart, that is, the "Bündnerschiefer" Gyrenspitz, Eggberg, Ruchberg). Klus to Eggberg series are and "Pre-Flysch" in the senseof Trümpy (1980)' Cretaceousin age, the Ruchberg seriesis Paleoceneto Eocene The Grava nappe is not a nappe in the strict sense(actually, (Nanny 1948). On the tectonic map of Switzerland (Spicher even not in a rather wide sense),but originally represents, as 1980), Klus, Valzeina, and Sassaunaseries are termed "Ilünd- our reconstruction will show, a duplex consisting of "horses" nerschiefer", Pfävigrat, Fadura, Gyrenspitz, Eggberg, and that were imbricated by N-directed D1 thrust faults emplacing Ruchberg series are termed "Flysch". Trümpy (1980) subdi- Cretaceousschists over Tertiary Flysch. At present, the most

240 M. weh & N. Froitzheim ff:\/1i;-A$'"* l'::';''l!t4+i ;;f- ,,x: :--&L4+ 1 ii'

axes Sulzfluh,Arosa, llttr t r U Austroalpine /ltttl I Itr 0-10' 8.1-90"(Plunge angle) Falknisnapp6 N fold dial plane

fold iacing t\ l,,lll r",ra,y -+'" axial trace oJ syntom t- \ vatzetnat cr"ta""ous ...-t"' axial trace of antitorm I synformf E l-_l Helvetic/ transport direction f771'l ( / Infrahelvelicunits (D3 mylonite)

VT Vilanthrust CT Chistensleinthrust JT Jägglischhomthrust

Fig. 5. Orientation of structural elementsin the Prättigau half-window. prominent regional structuresof the Prättigau half-window are Before starting the detailed structural description, we have t' Iarge-scaleD3 folds, the Lunschania antiform and the to explain why we split the first deformation phase, D1, into \ anzeinasynform (Figs. 2, 3, 4). The Lunschania antiform was subphasesD1a and D1b insteadof introducing an additional described by Voll (1976) and Probst (1980) far away from the phase.D2 structurescan be clearly correlated acrossthe study half window, in the area north of the front of the Adula nappe. area and represent a strain field clearly different from initial, Voll (1976) and Steinmann (1994) traced it from there north- top-N thrust sheet imbrication. Therefore we can assumethat eastwardto the vicinity of Chur, and we traced it from Chur to D2 structures are approximately of the same age across the the northeasterncorner of the Prättigauhalf window. This axial study area. In contrast, correlation is more difficult for struc- trace is thus exposedover a length of 75 km. The newly named tures predatingDZ.In places,theDZ folds are predated by two Yalzeina synform accompaniesthe Lunschaniaantiform to the sets of structures. In lower structural levels of the Grava NW and can be traced over the same distance.Using the axial nappe, these structures encompassa cleavage (our D1a) and traces of these D3 folds, three structural zones can be defined folds that deformed this cleavage (our D1b). In higher struc- (Fig. 5): (zone 1) southeastof and above the axial trace of the tural levels and in the Falknis nappe, D2 is locally preceded by Lunschania antiform, (z,one2) between the axial traces of the thrusts(our D1a) and by folds (our Dlb) which do not deform Lunschania antiform and the Yalzeina synform, and (zone 3) a D1a cleavage(because no such cleavageexists there) but are northwest of and below the Valzeina synform. The latter zone occasionallyseen to deform the thrusts (e.g.,Fig, 9). It is not includes the basalshear zone of the Grava nappe. possible,at the present state of knowledge, to temporally cor-

Structure of the Prättigau half-window 241 relate these setsof structuresbetween higher and lower struc- tures.Northeast of and abovethe trace of the Ochsenbergan- tural levels.It may well be, for example.that D1b structuresin tiform, the above-mentionedmedium-scale D2 folds of 1 to 2 higher levels developedcontemporaneously with D1a struc- km amplitude deform minor D1 thrust contactswithin the turesin lower levels,since higher thrust sheets are often folded Grava nappe, along which Cretaceousschists had been trans- rvhenlower tl.rrustsheets are accreted(Boyer & Elliott 1982). ported northward over Tertiary flysch. The Tertiary rocks Therefore, r,vedo not intend to assignan age to the individual form two wedges tapering downward into the Cretaceous pre-D2 structures,but we take the localoccurrence of two sets schists,a southern one at Chistenstein and a northern one at of such structuresinto accountby usingD1a for the older and Jägglischhorn(Figs. 4, 5). The Tertiary rocks in both wedges D lb for the youngerset in anl locarion. rest with stratigraphiccontacts on the Cretaceousschists at their basebut are overthrustby Cretaceousschists at their top. After their formation the D1 wedgeswere deformed by cross- 3.2. Strttctural zone I southeastof the Lunschania antifornt cutting D2 folds as shown on Figure 4. The original geometry This zone is characterizedby post-nappeD2 folds with shal- of thesewedges is interpreted to result from DL south-over lowly dipping axial planes.These represent the dominant fold north imbrication of "horses" forming a duplex. The roof phaseand are generallyassociated with a weli-developedaxial thrust of this duplex is locatedalong the contactbetween the planar cleavage.The facingdirections of the D2 folds are pre- Grava nappe and the overlying Sulzfluh and Falknis nappes, dominanrly south bur vary from E over S to SW (Fig. 5). Their sincethese higher nappesare not incorporatedinto the Ter- vergenceis variableand dependson the position with respect tiary flysch wedges.The floor thrust of the Grava nappe D1 to major D2 folds.Two suchmajor folds are identified in zone duplex is not preservedin the Prättigau half window due to I ar the southern termination of the half-window:The N- the pervasiveD3 deformation in its structurally lowest part clo-singNiemet-Beverin fold (Schmidet al. 1990).representing where a shear zone formed during D3 (see below). The a synform in the study area,and the S-closingOchsenberg fold, Sulzfluh nappe at the northeastern corner of the half window representingan antiform (Figs.2, 3, 4). The axial surfacesof forms another duplex overlying the one of the Grava nappe D2 folds dip gently southeast(in the southernmostpart of (Fig. a, near coordinate 210). structural zone 1) to northeast (in the northern part of zonel). Where the polarity of the strata could be determined from Thus they are warped into a large,open dome (Fig. 4). From gradedbedding and turbidite boltom marks, the facing direc- north to south, the axial trace of the Ochsenbergantiform tion of D2 folds in structuralzone 1 is predominantlysouth to passesfrom the Grava nappe into the structurally higher Falk- east (Fig. 5). This implies that the strata were in an upright nis nappe.In a similarway, the axial traceof the Niemet-Bev- position before D2 backfolding. If major isoclinal D1 folds had erin synform passesfrom north to south up into progressively been presentprevious to D2, this would have led to opposite higher tectonic units. In this sense,both major D2 folds may be D2 facing directions within former upright and inverted D1 describedas backfolds. limbs, which is not observed.However. small-scaleisoclinal D1 In the upper) northern lirnb of the Ochsenbergantiform, folds,termed D1b, do occur in the southernmostpart of struc- the predominant vergenceof D2 folds is towardssouth (Fig. tural zone 1. In this stronglydeformed area it wasnot possible 4). The Cretaceousschists are here in an overall upright posi- to determine the younging direction bf the sediments,and tion (exceptfor the invertedlimbs of severalmedium-scale D2 hence,the isoclinalD1b folding is not monitored by changesin folds with amplitudesof ca. 1 to 2 km). In its lower, southern the facingdirection of D2 folds shown in Figure 5. Open D1b li'-b, which is also the upper limb of the Niemet-Beverinsyn- folds were identified in the northern part of zone 1 (Fig. 5). r -,r, the Cretaceousschists are upside-downand the D2 fold Suchopen folds may be responsiblefor the variability in strike vergenceis towardsnorth (Fig.4). Finally,below the axiai sur- of D2 axesfrom E-W over NE-SW to N-S (Fig. 5). The only face of the Niemet-Beverin synform, in the area southwest of other large-scaleD1 structuresthat can be unambiguousiy the half-windou', the nappe stack is in an upright position identifiedin structuralzone 7 are the thrustsrelated to duplex (Grava nappe below, Tomül nappe above) and the D2 fold formation. These thrusts did not produce inverted limbs, which vergenceis towards south again(see Schmid et al. 1990).Our points to duplex formation during an early, brittle deformation field rvork has shown that the entire volume of Cretaceous slage. schists and Tertiary flysch cropping out in the Prättigau half- In the northeasternpart of structuralzone 1, no D1 cleav- window is structurally above the axial surface of the Niemet- age can be detected, and the first cleavageformed during D2 Beverin synform.in contrastto the profile depictedin Schurid (Fig. 6a). In contrast.in the southwesternmostpart of zone I et al. (1996) where it was assumedto lie almostentirely below near Passugg(Fig. 2), D2 folds deform an earlier cleavage this D2 axialsurface. (D1) and quartz-calciteveins oriented parallel to this cleavage. The Ochsenbergantiform deformsthe D1 thrust contactof Theseveins bear a stretchingIineation formed by quartzfibres the Falknis over the Grava nappe (Fig. 4). The Niemet-Bev- oriented predominantlyNorth-South (Fig. 6b, c. d). Fibres of erin synform deforms the Dl thrust of the Tomül over the the high-PJow-T mineral Mg-Fe-carpholitegrew parallel to Grava nappe and other thrust contactsfurther south (seealso and together with the quartz fibres forming the lineation. Fig. 3). Hence, these D2 folds are clearly post-nappestruc- showing that high-pressureconditions were reached during D1

242 M. Weh& N. Froirzherm Fig.6a Fig.6b

D1 folialion D1 stretchinqlineation

aB do

26 data

Fig.6c Fig.6d

Fig.6e Fig.6f

Fig. 6. Structural elements of the Grava nappe in the Prättigau half-window. a) Recumbent, SE-facingD2 folds in srrucrural zone 1.West side of Jägglischhorn NE of Küblis, Prättigau (see Fig. 2). b) Fibre lineation on Fe-Mg-carpholite-bearingquartz-calcite vein. Rabiusa gorge, ca. 100 m south of P.736,Passugg near Chur (Fig.2).c)D2 and D3 folds deformingD1 veins; samelocalily as (b). d) Orientationof D1 foliarionand stretching(=fibre) lineationin the Rabiusagorge near Passugg,same locality as (b). Lower hemisphereSchmidt projection. e) Fe-Mg-carpholite included in quartz of a D1a vein, crosscutby a later calcite vein. Same locality as (b). f) Upright D2 fold in slructural zone 2. Schraubachvalley NE of Schiers,Prättigau (Fig. 2).

Structure of the Prättigau half-winctow 243 Fig.7a.

Fig.7c Fig.7b

Fig. 7. Overprinting relations in structural zone 2 . Locality: P. 622.Plessur river gorge, 1 km SE of Chur (Fig. 2). a) Overview of the outcrop- b) Polished sample from lower limb of D3 antiform. c) Thin section photograph from upper limb of D3 antiform, showing structures of four deformation phases(D1a: veins and foliation; D1b: first folding; D2: upright. main fold; D3 weak crenulation cleavagedipping to the right).

(see Goff6 & Oberhänsli1992). We assignthe veins at Passugg 3.3. Structuralzone 2 betweenLunschania antiform and to subphaseD1a becausethe veins are similar to other veins at Valzeina synfornt Chur which are in fact D1a becausethey were deforrned by D1b folds (seebelow and Fig. 7). This zone occupies the central part of the half-window. It Throughout structural zone l, the D2 folds and older struc- widenstowards NE due to the fact that the structural thickness tures are overprinted by open D3 folds with steeply to moder- between the Lunschania and Valzeina axial surfacesincreases ately southeast-dippingaxial planes and east- to southeast- in this direction (Fig. 2). The D2 folds which are shallowly ori- plunging axes.The D3 folds are north-vergent,that is, they ented in structurai zole L, change their attitude quite abruptly have long iimbs with shallow D2 cleavageand short limbs in acrossthe axial surface of the Lunschania antiform in order to which the axial planes and cleavageof D2 folds are reoriented become subvertical in structural zorLe2. Structural zone 2 is into a steeply dipping attitude. D3 folds in structural zone I thus characterizedby upright D2 folds. are associatedwith a crenulation cleavageand represent para- An example of an upright D2 fold on the thin section scale, sitic folds on the upper limb of the Lunschania antiform. The from the SW part of zone 2 near Chur, is shown in Fig. 7. The intensity of these folds increasesfrom NE to SW within struc- D2 fold in this example is a similar fold with thickened hinges tural zone 1. At the Passugglocality. D3 folding is quite pro- and an axial plane crenulation cleavage (visible in the upper nounced (Fig.6b, c). part of the thin section in Fig. 7). This D2 fold refolds two

244 M. Weh & N. Fröitzheim older cleavages.The youngerone of these,related to D1b, rep- earlier folds correlatedwith D2 of struclural zones1 and 2. To- resents the axial planar cleavageof isoclinal folds with a small- ward south and towards west within zone 3, the intensity of D3 er amplitude than D2. These D1b folds deform a first, penetra- deformation increasesand no older folds or cleavagesare pre- tive cieavageas weli as quartz veins oriented parallel to this served. D3 folds are locally overprinted by open, north-ver- cleavage.Cleavage and quartz veins thus represent the oldest gent, weak D4 folds with moderately to steeply southeast-dip- deformation stage,D1a. Note that the D1a and D1b cleavages ping axial surfacesand without a new cleavage.Associated can be clearly distinguished only in the hinge zone of the D2 with the small-scaleD4 structures are large, open anti- and fold, trut not on the limbs. The D1a q\artz veins are of the synformsnorth of Landquart. same type as the ones that contain fibrous Fe-Mg carpholite at In a downward direction, towards the baseof the Bündner- the Passugglocality (see above). Therefore, we attribute the schiefercomplex (which is not exposedbut covered by the qa- formation of this high-P-low-Tmineral to D1a. ternary fill of the valley), D3 folds become more and The D1a, D1b, and D2 structures are all overprinted by D3 more isoclinal and finally disappeardue to the complete trans- folds and associatedcrenulation cleavage53 (Fig.7a, c). This position of the layering into parallelism with the D3 foliation. 'dips crenulation cleavage shallowly to moderately east to The D3 axial plane cleavagethus becomesa mylonitic foliation southeast.The D3 folds are parasitic folds of the Lunschania which bears a NNE-SSW oriented stretching lineation ("trans- antiform. The structural evolution of the Chur outcrop (Fig. 7) port direction" in Fig. 5). Viewed perpendicularto the lin- can thus be summarizedas follows: D1a - penetrative cleavage eation, asymmetricstructures (o clasts,shear bands,S-C struc- and quartz veins; D1b - isoclinal folding; D2 - open to tight tures) indicate a top-NNE shear-sense(Fig. 8a). The mylonites folding and crenulation cleavage;D3 - open folding and crenu- are overprinted by minor, open D4 folds with steeply SE-dip- lation cleavage. ping axial planes(Fig. 8b). Tn the northern part of structural zone2, the samestructur- This mylonite zone formed during north-directed thrusting ar r-rases can be identified, but the associateddeformation is of the Bündnerschiefer over the Infrahelvetic complex ex- generally weaker and less penetrative than in the South. The posed on the other, western side of the Rhine valley. This tec- main folds are upright, open to tight D2 folds (Fig. 6f). Most of tonic contact,which representsthe Penninic basal thrust in the these are upward facing, a minority is downward facing.These study area,is therefore a D3 structure. It is probable, however, opposite facing directions result from earlier folding: Upward that the floor thrust of the D1 duplex was already located in D2 folding is observedon upright Dlb limbs, downward facing the samezone but was completely overprinted by later D3 my- on inverted DLb limbs. The upward-facingD2 folds are the lonitization. equivalent of the southward-facingD2 folds in structural zone At Vilan in the northernmost part of zone L, a wedge of 1, verticalized by folding around the hinge of the Lunschania Tertiary flysch, including the Ruchberg type locality, occurs antiform. Tight D1b folds are observed in some outcrops, and between Cretaceousschists below and above (Figs. 2,3, 4). these deform a penetrative cleavage,quartz-caicite veins, and This flysch restswith a depositional contact on the Cretaceous mullion structures,all belonging to D1a. The steeply dipping schistsbelow and is overthrust by the ones above, along the axial-planecleavage of the D2 folds is crenulatedby weak D3 Vilan thrust. We interpret this thrust as a D1 structure, like folds with subhorizontal to shallowly southeast-dipping_axial similar thrusts at Jägglischhornand at Chistenstein. surfaces.These are parasitic folds of the Lunschania antiform. They are in turn locally overprinted by a younger crenulation 3.5. Structure of the Grava n(tppe: Summary (D4) with moderately southeast-dipping axial surfaces.D4 is very weak and rarely observed in structural zone 2, absent in Large-scaleD1 structures are, from S to N, the Chistenstein, s :tural zone L, but becomes more ubiquitous in structural Jägglischhornand Vilan thrusts that subdivide the Bündner- zone 3, i.e. belorv the axial surface of the Valz-einasynform. schiefer/Flyschcomplex of the Prättigau half-window into four "horses". The thrusts are brittle and the orientations of cut-off lines defined by the intersection of the thrust surfaceswith the 3.4. Structurql zone 3 nortlu'est of the Valzeina synfornt Cretaceous-Tertiaryboundary constrain thrusting to be north- This zone is situated between the quaternary fill of the Rhine directed. The roof thrust of the Bündnerschiefer/Flyschduplex valley to the West and the axial trace of the Valzeina synform was below the Falknis nappe, and the floor thrust was at the to the East. Like zone 2, it widens towards north (Fig. 5). base of the Bündnerschiefercomplex but has later been com- Across the Valzeina synform, an abrupt change occurs from pletely overprinted by D3 mylonitisation. Small-scaleDla steeply oriented layering (folded by upright D2 folds) in zone structures include a penetrative foliation, mullion structures, 2, to a dominantly shallow orientation of the layering in zone 3. and quartz-calciteveins in which Fe-Mg carpholite has grown The dominant folds in zone 3 are tight D3 folds with NE- in the southernmost part of the half-window. D1b structures to E-plunging axes, shallowly to moderately SE-dipping axial are isoclinal folds deforming the veins and the foliation. Such surfacesand axial-planecleavage, well exposedin the Klus small-scalestructures are found in structural zones2 and 3 and gorge eastof Landquart (Fig. 5). In the northern part of zone in the southwesternmostpart of zone 1. Open D1b folds occur 3, NE of Landquart, thesefolds overprint south- to east-facing in the northern part of zone 1 (Fig. 5). Here, however, they do

Structure of the Prättigau half-window 245 Fig.Ba

Fig.Bb

Gürgaletschshear zone D2 folds Gürgale'tschshear zone:

" stretchinglineation , mylonilicfoliation

D2 folds:

. D2 lold axis x D2 axialplanes and foliation

Fi.r.Bc Fig.Bd

Fill8. Sttu.tutesfromthebasalshearzoneoftheGravanappeandfromtheFalknisnappeatGürgaletsch.a)Myloniticcalcschistinthebasal shearzone(D3)of the Grava nappe. Asymmetric, sheared fragments of quartz-calciteveins (arrow) indicate top-NNE shear sense.Road from Trimmis to Says,near Trimmis (see Fig. 2). b) D4 fold overprinting basal shearzone. Same locality as (a). c) o clast (shearedfragment of earlier vein?) in mylonitic calcschistof the Upper Cretaceous Couches Rouges formation in the Gürgaletsch shear zone. indicating top-SE normal shearingduring D2. Falknis nappe at Gürgaletsch; outcrop is nearP.2447 south of Gürgaletsch (Fig, 2), west of Parpaner Schwarzhorn.d) Orientation of D2 structural elementsin the Gürgaletsch area. Lower hemisphereSchmidt pro- Jection.D2 fold axial planesare parallel to mylonitic foliation of Gürgaletschshear zone. D2 fotd axis distribution shows progressiverotation towards the shear di- rection of the Gürgaletschshear zone (asrepresented by stretchinglineations).

not deform a D1a foliation becauseno suchcleavage is present. around recumbent "backfolds", including the Ochsenberg an' We tentativelyassume that D1a small-scalestructures formed tiform and,southwest of the half window, the Niemet-Beverin together with the iarge-scaleD1 thrusts,during duplex stack- synform. In the inverted limbs of these folds, the originally ing, and that D1b folding overprintedthe horsesof the dupiex south-dipping thrusts were rotated into a northward-dipping after their formation.Duplex stackingmust havebeen diachro- orientation. The entire Grava nappe of the Prättigau half-win- nous,starting in the south and endingin the north (Boyer & El- dow is structurally above the axial surface of the Niemet-Bev- liou 1980),and D1b folding may havebeen diachronous, roo. erin synform.As will be shown beiow, D2 structuresresulted During D2 the D1 thrusts were shortened by folding from top-southeastextensional shearing.

246 M. weh & N. Froitz-heim During D3, a top-N mylonite zone overprintedthe baseof Swiss coordina!es: the Grava nappe.The nort-vergentLunschania and Valzeina 186 folds formed contemporaneouslyin the hangingu'allof the my- lonite zone. In the steep limb between the Lunschaniaand Valzeina axial surfaces,the originally recumbent D2 folds were verticalized by D3. When shearingin the basal shear zone had ceased,open, N-vergent D4 folds accommodated some further shortenins.

4. Structure of the Falknis nappe in the Gürgaletscharea

4.1. Field relations FaJknisnapPe: c.",u.uou. OA Ochsenberg äntiform (D2) The Falknis nappe is a cover nappe without ,com- I US Urden stf,cline (D I b) PA Alpsrein antacline(D I b) prising a sedimentarysequence from Jurassicto Eocenewith I'r*i,lr**.,. facing direction rare slivers of rocks at the base (Allernann 1957, ,> Gruner 1981).Polymict brecciasand conglonerates(Faiknis Fig. 9. Simplified crosssection of the Falknis nappe in the Gürgaletsch area, breccia) are interlayered with Upper Kimmeridgian to showing dominant recumbentD2 folds kinematically Iinked with the Gür- galetschshear zone and overprinting earlier D1 thrusts and folds. Lower Tithonian carbonates in the Falknis nappe. These were shed from a partly subaeric high to the Southeast (C-rrner 1981). Most authors assumethat this source area w-- part of the Briangonnaisswell, and, therefore, that the Falknis nappe originates from the northern margin of this presentlydip north. The Alpstein and Urden syncline swell (Stampfli 1993, Steinmann 1994; for a different view, also deform the basal thrust of the Gürgaletsch Schuppe over see Gruner 1981).Two major outcrop areas of the Falknis the Grava nappe. Hence these two folds may be assigned to nappe exist: the Falknis area at the N margin of the half- D1b, and the thrust to D1a. window, and the Gürgaletscharea to the south. The meta- Further towards south in the profile (Fig. 9), close to the morphic grade is anchizonal in both areas (Ferreiro baseof the overlyingArosa zone, a tectonic m6lange devel- Mählmann et aI. 1992). Only the Gürgaletsch area will ops. The incompetent Couches Rouges marls form the matrix be discussedin the present paper (see Weh 1998for details while disrupted hingesof D2 ,consisting of older for- concerningthe Falknis area). mations,form the components.The D2 axial-planefoliation The profile through the Gürgaletscharea (Fig. 9) showsre- developsinto a mylonitic foliation in the Couchesrouges, dip- cumbent D2 folds that are characteristicfor this area. They ping shallowlySE, parallel to the contact with the overlying have shallowlyeast- to southeast-dippingaxial surfaces,E-W Arosa zone.This foliation bearsa SE-plungingstretching lin- to SE-NW axes(Fig.8d), and face towardssouth (Fig.9). La-te eation. Viewed perpendicular to the lineation, asymmetric Jurassic Falknis breccia is exposed in the North of the profile structures(o clasts,shear bands) consistently indicate top-SE (northern part of Gürgaletschschuppe). Towards south, we directed transport (Fig. 8c). This mylonitic structure stops encounter progressivelyyounger formations up to the Upper abruptly at the base of the Arosa zone which is marked by a ClretaceousCouches Rouges Formation and the Paleocene briltle fault. C 'rorotalien-Schichten(Lichtsteiner 1992).Along the con- The recumbentD2 folds observedwithin the Gürgaletsch tact marked "D1 overthrust" in Fig. 9, theseCouches Rouges Schuppealso deform the thrust contact between Falknis nappe border againstJurassic rocks which form the baseof a second and Bündnerschieferat the northern end of the profile (Fig. 9). sequence (Malakoff schuppe), again younging towards south These folds can be traced acrossthe thrust contact into the and deformed by south-facing, recumbent D2 folds. The D1 southeast-facingD2 folds of the Ilündnerschiefer in the lower overthrust betu'eenCouches Rouges and Jurassicrocks is fold- limb of the Ochsenbergantiform (Fig.9). The Gürgaletschand ed by the same D2 folds. During D1, the southernsequence Malakoff schuppenare underlain by Tertiary flysch and Creta- (Malakoff schuppe) was thrust towards north over the north- ceousBündnerschiefer along a gentlyN-dipping contact that is ern one (Gürgaletschschuppe). The or,erthrustwas subse. entirely covered by Quaternary deposits. This contact proba- quently, during D2, rotated clockwise(looking east),short- bly representsthe D1 floor thrust of the Gürgaletsch-Malakoff ened, and folded. These relations were in principle already rec- duplex. It may, however,have been strongly overprinted by ognizedby Gruner (1981). D2 extensionalshearing, because the northern continuationof A syncline-anticlinepair occurs in the structurally higher the Martegnasshear zone, another D2 top-SE shearzone com- part of the GürgaletschSchuppe (Alpstein anticline,Urden parable to the Turba mylonite zone (Figs. 2.3, 4), probably syncline).These folds are attributed to D1 becausethey are runs very close to or directly at the base of the Gürgaletsch- overprinted bv recumbent D2 folds and their axial surfaces Malakoff duplex.

Structure of the Prätligau half-window 247 4.2. Structural interpretution of the Gürgaletscharea to mesoscale structures formed during D1 indicate a two-stage evolution. D1a comprises mullion structures, The newly discoveredzone of intense,mylonitic top-SE shear- a penetrative cleavageand, parallei to this cleavage,quartz ing along the top of the Falknis nappe at Gürgaletschis termed veins bearing the mineral Fe-Mg carpholite which formed "Gürgaletsch shearzone". We assumethat it is syngeneticwith at 7.0 t 1.0 kbar and less than 300'C (Weh 1998). D1b D2 folding for the following reasons:(1) the D2 axial plane fo- comprises isoclinal folds deforming the cleavage and the liation developsinto the mylonitic foliation of the shear zone; veins.We were not able to reconstructthe geometry of D1b (2) the shortening and clockwise rotation (looking E) of the folds in the deeper part of the Grava nappe due to strong D1 overthrust between the Gürgaletschschuppe and Malakoff D2lD3 overprint. In the Falknis nappe at Gürgaletsch, a schuppe requires a large, top-S directed component of shear- north-directed D1a thrust emplaced the Malakoff schuppe ing during D2. consistentwith the shear senseof the Gür- over the Gürgaletsch schuppe. A D1b synform-antiform galetschshear zone. pair deformed the Gürgaletschschuppe and its basal thrust The following structural histor,vis envisagedfor the Gür- over the Grava nappe. The pre-D2 reconstruction of these galetscharea: In a regime of south-over-northimbrication dur- folds suggestthat they were north-vergent but had rather ing D1, two schuppenof Falknis-type sediments,Gürgaletsch steepaxial surfaces(Fig. 10). Schuppe and Malakoff Schuppe,were accretedat the base of During D1 the Penninic sediment nappeswere sheared off the Arosa zone to form a small, hinterland-dippingduplex (Fig. from their southward subductedbasement and accreted to the 10). As a result of this duplex geometry,the basalthrust of the base of the orogenic wedge. The latter consistedof the Aus- Falknis nappe dipped south in the northernmost part and be- troalpine nappes and, at their base, the Arosa mdlange zone. ' re subhorizontal farther south. The Gürgaletsch Schuppe These higher tectonic units had been stacked already during and its basal thrust were subsequentlydeformed by two north- the Late Cretaceous,whereas accretion of the Penninic nappes vergent folds, Alpstein antiform and Urden'syncline (D1b). discussedhere only began in the Early Tertiary, as indicated This must have happened after the Grava nappe had become by the age of the youngestsediments in these units. These rep- accretedbecause the basal thrust of the Gürgaletschschuppe-is resent Paleoceneto possibly Eocene in the Falknis nappe at the same time the roof thrust of the Grava nappe duplex. (Allemann 1957)and the "lower part of the Lower Eocene" in During D2, Ihe overall shear sensechanged to top-SE, that is, the Grava nappe (Ruchberg series;Nänny 1948).There is no the Arosa zone and overlying moved evidencefor pre-Tertiary deformation of the Grava nappe. On down towards SE relative to the Falknis and Grava nappes,and the contrary, the oldest structures of the higher Grava nappe, a broad shear zone developed below the Arosa zone. Now the the D1 thrusts, buried the Ruchberg seriesand must hence be south-dipping sedimentary layering of the two schuppen,the Tertiary in age.Our field data thus suggestthat accretion of the thrust of the Malakoff over the GürgaletschSchuppe, and the Grava nappe did not begin before the Eocene (see also Schmid basal thrust of the Gürgaletsch Schuppein the North, all had a et aL.1996). The possibility still exists,however, that deforma- component of dip in the direction of shearingthat was greater tion of the lower parts of the Grava nappe, that is, the southern than the dip angle of the shear zone itself, and therefore were parts of structural zones2 and 3, beganalready earlier. No Ter- in an orientation for shortening and folding. tiary sedimentswere identified here, and hence, a late Creta- The Gürgaletschshear zone can be followed towards south ceous age of D1a deformation and concomitant carpholite to Tiefencastel and the Oberhalbstein area,rvhere it continues formation in this part of the Grava nappe cannot be excluded. under the name of Turba mylonite zone (Nievergelt et al. In the Falknis nappe, which is of more southerly origin t6). Northeastof the Gürgaletscharea, on the other hand, than the Grava nappe, the youngest sediments,of Paleocene tIe mylonite zone disappears. There, the deformation is no to possibly Early Eocene age, rest unconformably on older longer localized in a shear zone but distributed over a larger formations, indicating pre-Paleocenetectonic movements. volume of rock. More specifically, this volume is the zone of These movements began already in the Upper Campanian to important D2 folding in the upper Grava nappe (Ochsenberg Lower Maastrichtian (Allemann 1957).No thrusts or folds re- ) antiform and minor backfolds further north, seeFig. a). lated to this pre-Tertiary deformation can be identified. In the absenceof such structures,we assum.ethat was ei- 5. Summary, discussion,and correlation with adjacent areas ther caused by uplift related to block tilting, e.g., in a strike- slip regime, or by arching related to a flexural forebulge of 5.1. Early Tertiary accretion of sediment nappes (DI) the Piemont-Ligurian subduction zone located farther to the In the Grava nappe, D1 structures include three north- South. directed thrusts emplacing Cretaceous on Tertiary flysch (Chistenstein, Jägglischhorn, and Vilan thrusts). 5.2.Bockfolding and top-SE extensionalshearing (D2) These thrusts are part of a duplex whose roof thrust was located at the base of the Falknis nappe and whose floor The D2 folds are backfolds in the sensethat their axial sur- thrust is not preserved due to intense D3 overprint. In the faces climb into progressively higher structural units towards deeper parts of the nappe (structural zones2 and 3), small- south, that is, into more southerly-derived nappes. This is ex-

248 M. Weh& N. Froitzheim l_, phasein the Tambo and Suretta nappes)."Extensional set- ting" in this context means that the Penninic nappes of Graubünden were thinned and extended in a SE-NW direc- tion at that time. We do not imply, however, that N-S conver- genceacross the Alps ceased,which is in fact rather improba- ble (Schmid et al. 1996), nor do we exclude synchronous shortening at deeper structural levels. D2 backfolding, the Niemet-Beverin phase, occurred be- tween about 35 and 30 Ma, that is, in the Oligocene.The mini- mum age is constrained by the 30 Ma Bergell granodiorite in- Fig. 10. Reconstructed pre-D2 geometry of the Falknis nappe in the trusion that truncates the Turba mylonite zone (Nievergelt et Gürgaletsch area, al. 1996).The upper age bracket,35 Ma, comesfrom the ob- servationthat the Niemet-Beverin phasepostdates a consider- able part of the exhumation of the Adula eclogiteswhich had suffered peak pressures around 40 Ma (see discussion in emplified by the Niemet-Beverin fold whose axial surface Schmidet al. 1996). climbs from north to south through the following nappes: It is important to note that the D2 extensional shearing is Grava nappe (Valais) - Tomül nappe (Valais) - Schamsnappes not responsiblefor the bulk of the exhumation of the Adula (Briangonnais)- Surettanappe (Brianqonnais)- Avers Bünd- high pressure rocks but that a large part of this exhumation nerschiefer (Liguria) (Fig. 3). In the lower limb of the Niemet- and the juxtaposition of the high-pressurerocks against the Be 'rin synform, the nappe stack remained in an upright posi- overlying lower-pressurerocks of the Tambo nappe occurred ti<,-whereas it became overturned in the upper limb (Schmid already before D2 extensional shearing (Schmid et al. 1996, et al. 1990,Schreurs 1993). Our study has shown that the entire Partzsch1997). The samehoids for the Fe-Mg-carpholite-bear- volume of the Grava nappe in the Prättigauhalf-window lies ing rocks of the Grava nappe in our study area: high-P/low-T above the axial surface of the Niemet-Beverin synform, and conditions were present during D1a, and during D1b the that a whole seriesof comparableD2 folds, only of smaller carpholite was already replaced by chlorite (Weh 1998). In the scale,developed above and to the north of the Niemet-Beverin Bündnerschiefer of the Engadine window, representing the fold. The Ochsenbergantiform follows above the Niemet-Bev- eastward continuation of the Grava nappe of the Prättigau erin synform; it brings the inverted nappe stack of the upper half-window, Bousquet et al. (1999) identified a top-NW de- limb of the Niemet-Beverin fold back into an upright position. tachment fault, formed during the local D1. This fault accom- The even higher synforms again locally invert the nappe stack modated the exhumation of carpholite-bearinghigh-pressure for short distances,as is shown by the locally overturned, inter- rocks located in its footwall. We did not observe such a de- nal D1 thrustsof the Grava nappe (Fig.4). tachment in the Prättigau area.It may have been present, how- The D2 folds are kinematically linked to top-SE shear ever, in fhe southwestern part of the half-window near Pas- zones, as was sho-wnabove for the case of the Gürgaletsch- sugg, i.e., above_the carpholite-bearing rocks. If it existed, it Turba shear zone. Another newly identified shear zone was obliterated by the intense D2 and D3 deformation in this (Martegnas shear zone), also with a top-SE transport direc- area. lion, occurs at a slightly deeper tectonic level (Figs. 2,3,4). The Turba shearzone coincides with a metamorphic disconti- 5.3 "Out of sequence"N-directed thrusting qnd the wedging- n ', that is, it has lower-grade rocks in the hanging wall out of the Helvetic nappes(D3) resTrngon higher-grade rocks in the footwall, a certain tem- perature range being omitted in between (Ferreiro N-S shortening within the Penninic sediment nappes of the Mählmann 1995,Nievergelt et al. 1996).It is for this reason study area resumed during D3. Structures belonging to this that we interpret the Gürgaletsch-Turba shearzone as a post- phasedominate the lower part of the Grava nappe (structural nappe extensional fault, and the entire D2 stage as an exten- zone 3). In contrast,D2 is the dominant folding phasein the sional event during the Tertiary orogenic evolution. The upper part of the Grava nappe (zone 1) and the Falknis nappe change from D1 accretion to D2 extension coincided with a at Gürgaletsch. Large, north-vergent D3 folds like the Lun- reversalof the shearsense from top-N to top-SE (to E in the schaniaand Valzeina structuresare kinematically linked to the caseof the Turba zone further south,Nievergelt et al. 1996). basal,syn-D3, mylonitic shear zone of the Grava nappe (sec- In this new kinematic frame, the sedimentarylayering and tion 3.4). D4 folds accommodated some further shortening many of the nappe boundaries were in an orientation for after theD3 mylonite zone becameblocked. shortening. This led to widespread D2 backfolding. In our The structural relations in the basal shear zone are very view, these folds did not develop in a contractional but rather similar to the ones observed in the basal part of the Glarus in an extensionalsetting (as was demonstratedby Marquer Verrucano (Helvetic nappes) above the Glarus overthrust, ex- 1991 and Marquer et al. 1996 for the same deformational posedwest of the Rhine valley (Fig. 2). In both areas,a pene-

Structure of the Prättiqau half-windorv 249 Fig.11. Structural evolution of the Prättigau half-window. Upper panel: D1 duplex geometry. Middle panel: Over- print by top-SE shearingduring the Niemet-Beverin phase (D2). Lower panel: Present-dayprofile, showing main D3 structures - the , the Valzeina synform and the Lunschania antiform. Note that the older Penninic basal thrust (the floor thrust of the D1 duplex) id folded around the Lunschania and Valzeina folds to the South. Stippled: Tertiary flysch of the Grava nappe; vertical ruling: Falknis nappe. Other units may be identified by comparison s'ith Fig. 3. "P" indicates Passuggcarpholite locality.

trative foliation ( phase along the Glarus thrust, D3 in not necessarythat the Helvetic nappes wedge out completely our study area), axial-planar with respect to N-vergent folds, becausethin remnants of Helvetic Verrucano may still be hid- curves downward into parallelism with the thrust contact.This den below the quaternary Rhine valley fill. foliation is associatedwith a stretching lineation trending N-S We assumethat this wedging-out (or at least extreme thin- t'. the Verrucano above the Glarus thrust (Siddans1979), and ning) of the Helvetic nappes results from the "out-of-se- , E-SSW in the Cretaceous schists at the bottom of the quence((nature of the D3 thrust, interfering with older struc- Grava nappe (Fig.5). The penetrative foliation is in both areas tures. In contrast, Trümpy (1,992)proposed that the omission overprinted by small-scaleopen folds and a crenulation cleav- of Helvetic nappes in the Rhine Valley between Chur and age dipping steeply SSE (Ruchi phasealong the Glarus thrust, Landquart is the result of a late Alpine N-S oriented normal D4 in our case).It is for these reasonsthat we equate the D3 fault, the "Churer Störung". We did not find any structural ev- basal shear zone between Chur and Landquart with the Glarus idence for such a fault and, in addition, zircon fission track thrust at the base of the Helvetic nappes, that is to say, the data of Fügenschuhand Weh (data in Weh 1998)argue against amount of northward thrusting that occurred along the Glarus its existence. thrust west of the study area (Helvetic nappes over the Infra- Equating the D3 basal shear zone of the Grava nappe with helvetic complex), is accommodatedby the basalshear zone in the Glarus thrust implies that this shear zone is rooted, togeth- our study area. The Helvetic nappes laterally wedge out to- er with the Glarus thrust, between the Aar and Gotthard mas- wards the East, i.e., the Chur Rhine valley (Fig. 2). The apex of sifs. It follows that the "Penninic basal thrust" (Schmid et al. the Helvetic nappesin the northern part of the Rhine valley is 1996) south of the Gotthard massif, that is, the thrust which at Fläscher Berg (Fig. 2). Another, similar apex exists to the emplacesPenninic Bündnerschieferonto the cover of the Got- Southwest, in the vailey SW of Chur (Fig. 2). thard massif(a cover that is itself allochthonous,Etter 1987),is Therefore, within the study area, the Grava nappe was trans- not identical with the D3 basal shearzone of our study area. In ported directly over the Infrahelvetic complex. However, it is fact, the Penninic basal thrust south of the Gotthard massif

250 M. Weh & N. Froitzherm must be older than our D3 deformation becauseit is folded Tab. 1. Correlation of deformation phases between the study area and adjacent around the Valzeina synform and the LunSchania antiform areas.Phases in the Schamsand Suretta nappesafter Schreurs (1993) and Schmid et al. (1996),phases in the Helvetic nappesafter Miines & pfiffner (Fig. 11). Our basal shearzone, in contrast,is cogeneticwith (1977\. lhesefolds. The age of the Glarus thrust and the Calanda phase in its footwall is constrained by the youngest sediments affected, i.e.Lower Oligocene (ca. 35 Ma) flysch of the Aar massifcover. Deposition of these sediments,however, also predates the older Pizol phase, that is, the emplacementof "exotic sheets" of South Heivetic origin on the Aar massif cover, which were later buried "out of sequence"by the Glarus thrust. This Pizol phase may be tirne-equivalent to our D2. This wouid then imply that thrust-sheetemplacement took placein the Helvetic duplex of the Grava nappe already had an antiformal geome- realm while the Penninic nappes were extended, favouring a try, as is suggestedby the occurrence of Falknis nappe units "gravity spreading" model as envisagedby Milnes & Pfiffner north and south of the half-window (see Fig. 11, upper panel). (1977,i980) for the same time interval. A tentative correlation (2) Formation of the Lunschania antiform and Valzeina syn- and estimated ages of the deformation phases are given in form (D3) contributed significantly to the shape of the half- Table 1. window. The Lunschania antiform is responsiblefor the sharp bend of the Austroalpine basal thrust at the NE corner of the 6. Conclusions half-window. (3) Additional arching of the half-window oc- curred after D3, as is evidenced by fission-track work of Fü- r he Grava and Falknis nappes formed hinterland-dipping genschuhand Weh (in Weh 1998) and by the antiformal shape (south-dipping) duplexes when they were detached in the of the Glarus thrust and of the D3 fold axial traces (Fig. 11, Early Tertiary from the southward-subducting lithosphere lower panel) - although this antiformal shapemay in part be a and accreted to the base of the orogenic wedge. Burial of the primary feature of the Glarus thrust (Schmid 1975). Grava nappe led to a pressure-dominatedmetamorphism with formation of Fe-Mg carpholite in the southermost part of the Prättigau half-window. The accretion stage (D1) is lo- cally subdivided in two subphases:thrusts, cleavage,and Acknowledgments veins formed during D1a and were tightly to isoclinally fold- Support by SwissNational Sciencefoundation grants Nr.21-39182.93 and 20- ed during D1b. 45174.95is gratefully acknowledged.Careful and constructivereviews by Ste- The kinematic picture changed abruptly around 35 Ma fan Schrnid and Mark Steinmann improved the manuscriot. Jearmette Schaub when the Penninic nappeswere affected by top-SE extensional is thanked for drafting Fig. 5. shearing. A series of backfolds, of which the Niemet-Beverin synform is the most important but_by far not the only one, formed synchronously with a system of top-SE extensional REFERENCES shear zones (Turba-Gürgaletsch,Martegnas). 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