001,2-94021 0I I020231 -16 $ 1.50 + 0.2010 Eclogaegeol. Helv.94 (2001)237-252 EirkhäuserVerlag, Basel, 2001 Penniniccover nappesin the Prättigauhalf-window (EasternSwitzerland):Structure and tectonicevolution Manrus Wenl'2 & Nrr<olaus FRorrzi{ErM1'3 Key words: Alps, Penninic, Bündnerschiefer,Falknis nappe,Prättigau half-window, structural geology,Alpine tectonics 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 Flysch. Diese Gesteine wurden durch nerschiefer") and Late CretaceouslEarly Tertiary flysch. It records the follow- die folgenden alpinen Deformationsvorgänge überprägt: (D1a) Abscherung ing stagesof Alpine deformation: (D1a) d6collement of the sediment 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 Glarus 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 Switzerland (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 sediments, and Eastern and Central Alps in Eastern Switzerland. 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 subduction 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, Germany. 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 Eastern Alps 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 ocean' Many additionaldetails of the regionalstructural geology may North Penninic or Valais ocean) and an intervening continen- be found in Weh (1998).This PhD thesisis availableon request tal terrane, the BrianEonnaisfragment. The Penninic nappes from the Geology-PaleontologyInstitute of BaselUniversity. exposedwithin the half-window include, from top to bottom, the Arosa zone (ophiolites and ophiolite-bearingm6langes from the Piemont-Liguria ocean and the distal Apulian mar- 2. Overview of the regional geology gin; Ring et al. 1989), the Sulzfluh nappe (sedimentary rocks The Prättigau half-window is a westward-opening, 25 km x derived from the BrianEonnais fragment, dominated by Late 25 km embayment of the Austroalpine-Penninic boundary in Jurassicshallow-water limestone), the Falknis nappe (Jurassic Eastern Switzerland (Figs. 1, 2).The half-window is framed by to Early Tertiary sequenceof sedimentary rocks, probably de- the overlying Austroalpine nappes on its northern, eastern, rived from the northern margin of the BrianEonnais fragment 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 Cretaceous(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 Europe 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