TECTONOPHYSICS

ELSEVIER Tectonophysics280 (1997) 261-293

Late Cretaceous,synorogenic, low-angle normal faulting alongthe Schlinig fault (Switzerland,Italy, Austria) and its signiflcancefor the tectonicsof the EasternAlps

N. Froitzheim*,P. Contil, M. vanDaalen

Geologisch-Palcirttologisches Instttut, Universität Basel, BemoLilistr 32, CH-1056 Basel, Swit:erland

Received9 July 1996t accepted10 February 1997

Abstract

The Schlinig fault at the western border of the Ötztal nappe (EasternAlps), previously interpretedas a west-directed thrust, actually representsa Late Cretaceous,top-SE to -ESE normal fault, as indicated by sense-of-shearcriteria found within cataclasitesand greenschist-facies'mylonites.Normal faulting postdated and oflset an earlieq Cretacedus-age, west-directedthrust at the baseof the Ötztal nappe. Shapefabric and crystallographicpreferred orientation in completely recrystallizedquarlz layers in a mylonite from the Schlinig fault record a combination of (1) top-easrsoutheastsimple shear during Late Cretaceous normal faulting, and (2) later north-northeast-directedshortening during the Early Tertiary, also recordedby open folds on the outcrop and map scale.Offset of the basalthrust of the Ötztal nappeacross the Schlinig fault indicatesa normal displacementof l7 km. The fault was initiated with a dip angle of 10'to 15" (low-angie normal fault). Domino-style extension of the competentLate Triassic Hauptdolomit in the footwall was kinematically linked to normal faulting. The Schlinig fault belongs to a system of east- to southeast-dippingnormal faults which accommodatedsevere stretchingof the Alpine orogen during the Late Cretaceous.The slip direction of extensionalfaults often parallels the direction of earlier thrusting (top-W to top-NW), only the slip senseis reversedand the normal faults are slightly steeper than the thrusts.In the western Austroalpine nappes,extension stafied at about 80 Ma and was coeval with subduction of Piemont-Ligurian oceanic lithosphere and continental fragments farther west. The extensional episode led to the formation of AustroalpineGosau basins with fluviatile to deep-marinesediments. West-directed rollback of an east-dipping Piemont-Ligurian subductionzone is proposedto have causedthis stretchingin the upperplate.

Ket'*-ords:tectonics; synorogenic extension; low-angle normal fault; Alps; Austroalpine

1. Introduction nappeabove and Mesozoic sedimentaryrocks of the Engadinedolomites (S-charl nappe) below (Fig. 1). The Schlinig fault is the shallowly dippingcon- It has gained some fame in the Alpine literature ^. tact between pre-Mesozoic basementoI the (Jtztal becauseit is spectacularlyexposed, shows the su- perposition of basementon sedimentaryrocks in a

*Conesponding text-book manner, and has given rise ro different author. Fax: *41 61 267-3613. E-mail: interpretations controversiesamong Alpine ge- froitzheim@ ubaclu.unibas.ch and 'Present address:Drrezione Rilevamento,Via Nuoro 3, 09046 ologists. The kinematics and age of this fault are Srn Vito iCAt. ltaly. not only of local interest but greatly affect the tec-

00.10-1951/97l$17.00O 1997Elsevier Science B.V All riehtsreserved P/1S0040- | 9 5 1( 97)0003 7- 1

t N. FroitzheützeI al./Tectonophysics280 (1997) 267-293

m Valaisbasin, European margin . ] fffi Briangonnais , tl Piemont-Lrguria basrn I ]f] Apulian margin 1oo km

LANDECK Brenner -...l Mesozoic .lauernJ// / Steinach window

öt zta I ,/ ,/ ,/

I/,/,' Brenner 7- SchlinisJfault

S-charl cover I Upper ba sementj Austroalpine schgau Lower Austroalpine ar zone Southern Alps

Penninic units

Helvetic

Ca m p o Tertiary intrusives

{ \< lertlary normal rault <'--4 ,, G iudicari thrust Adamello {-< Cretaceous normal 1. " thrust

O 1O 20 30km

;ig. 1. Tectonic overview of the westernpart of the Eastem Alps. Inset: sketch map of the Alps showing palaeogeographicprovenance of tectonic units. ,4.1H: Meliata-Hallstatt unitsl S : Schlinig fault ZG : Zentralgneiss. tonic interpretation of the westerx part of the Aus- ward thrustingalong the Schlinig fault. Both authors troalpine nappe system. Hammer (1908) and Spitz assumedthat at least the late stagesof thrusting oc- and Dyhrenfurth( 1914) interpretedthe Schlinig fault curred in the Tertiary period, postdating earlier, Late as a W- to WNW-directed thrust of the Ötztal mass Cretaceousdeformation and metamorphism,because over the Engadine dolomites. They were cnticized the Schlinig fault truncatedearly folds of the Enga- by Heim (1922) who postulated that the thrust- dine dolomites and disturbed the zonation of Cre- ing was directed northward. Trümpy (1980) and taceous-agemetamorphism (Thöni, 1980a).Schmid Thöni (1980a). among others, again inferred west- and Haas (1989) studied an east*west-trendingmy- N. Froitzheimet al. /Tectonophlslcs280 (1997) 267 293 ionite zoneat the southernborder ofthe Otztal nappe around SW-NE axes (Domleschgphase, ca.30-25 in the Vinschgau valley, the Vinschgau shear zone Ma; von Blanckenburg,1992), and dissectedby the (Fig. 1), and demonstratedthat it accommodateda Engadine line, an oblique-slip fault which accom- westward transport of the Ötztal nappe relative to modated important block rotations (Figs. I and 2; the underlying Campo nappe,during the Cretaceous. Schmid and Froitzheim, 1993). They assumedthat the Schlinig fault representedthe For this study, the following Austroalpine tectonic continuation of the Vinschgau shear zone towards units are of interest(Figs. I and 2): northwest into shallower levels of the crust. Con- (1) The Ötztal nappe largely consistsof Pre-Per- sequently, they interpreted the Schlinig fault as a mian crystailine basement.Remnants of its Permo- Cretaceous-age,top-west thrust. Mesozoic cover are preserved near the eastern bor- In the following, we will present a kinematic der of the nappe (Brenner Mesozoics). The Permo- study of the Schlinig fault between the Vinschgau Mesozoic rocks of Jaggl, often interpreted as a tec- valley to the south and the Reschenpass to the north, tonic window through the Ötztal nappe(Hess. 1962; and of its southwestwardprolongation, the Gallo Stutz and Walter, 1983),represent, according to our fault (Fig. 2). The result of this study contradictsall results, another remnant of the sedimentary cover of of the aboveinterpretations: the Schlinig fault is not the Ötztal nappe. a thrust, but a Late Cretaceous,top-southeast normal (2) The S-charl nappe. underlying the Ötztal fault postdating earlier nappe imbrication. This has nappe along the Schlinig fault, includes pre-Per- important implications for the tectonic history of the mian basement as well as Permian to Mesozoic Austroalpinenappes. cover rocks. The Permian to Middle Triassic part of the cover was folded and slightly imbricated to- 2. Regional setting getherwith the basement('S-charl-Unterbau'; Spitz and Dyhrenfufih, l9I4), whereas the Upper Trias- The study area belongs to the western Aus- sic Hauptdolomit was detachedfrom the underlying troalpine nappes. These originated from the south- rocks along the evaporite-bearing Raibl Formation, easternmargin of the Mesozoic Tethys ocean.They and deformedindependently ('S-charl-Oberbau' ). were stackedeast-over-west in Late Cretaceoustime (3) The Umbrail-Chavalatsch Schuppenzone and were later, during the Early Tertiary thrust north- (UCS) represents an imbricated stack of crystalline ward as one large thrust mass over the Penninic basementand Mesozoic,mostly Upper Triassic,sed- nappes. The iatter are exposed in the Tauem and imentary rocks (Schmid, 1973). The basementrocks Engadine windows, framed by the overlying Aus- are lithologically identical to ceftain rock types troalpine nappes.The sequenceof deformation in the of the Ötztal nappe. Within the UCS, the base- Austroalpinenappes of Graubündenwas recentlyes- ment/sedimentratio decreaseswestward. Hence, the tablishedas follows (Froitzheim et al., 1994:Fig. 3): whole UCS appearsas a zone of tectonic interfi.nger- Cretaceousdeformation includes west- to northwest- ing of the Ötztal basement, dominating to the east, directed nappe imbrication and associated folding with a stack of sediment thrust sheets to the west. (Trupchunphase, ca. 100-80 Ma), followed by ESE- The largest of these thrust sheetsis the Quattervals directed asymmetric extension of the nappe stack nappe,consisting almost exclusivelyof Upper Trias- (Ducan-Elaphase, ca. 80-67 Ma). Subsequently,in sic carbonates. To the north, the Quattervals nappe the Early Tertiary, the Austroalpine nappe stack was and the UCS overlie the S-charl nappe along the thrust northward over the Penninic nappesand folded southwest- to southeast-dipping Gallo line. In the aroundWNW-ESE axes(Blaisun phase,50-35 Ma, northern part of the study area, the Triassic rocks possibly beginning earlier). This was followed by of Piz Lad near Nauders occupy a similar posi- renewed E-W extension along the Turba mylonite tion as the sedimentary constituentsof the Umbrail- zone located at the base of the Cretaceousnappe Chavalatsch Schuppenzone (Fig. 2). To the north- stack, contemporaneouslywith north-south shorten- east, the Piz Lad sediments rest along a tectonic ing (Turba phase, ca. 35-30 Ma) (Dal Piaz et al., contact on Ötztal basement rocks. To the south, the 1988). Finally, the nappe stack was mildly folded sediments are overlain by the main mass of Ötztal 210 N. hotr:htint et al./Tectottopftr'slcs280 ( lS97) 267-293

10"30'E

ötztal nappe "

X JAGGL

r ! + + + + + +\ nappe+ + + + +

+ + 10km + + + + + + + + + + + + + +(? + + \<-+L J + i _ + + t. + + + + BORMIO++ +-+ + + t+F<:+ + + + _

30'E -l sedimentary roctJurassic and Lower Cretaceous) Dl (Trupchun lolds Hauptdolomit (Norian), Kössen Fm. (Rhaetisn), Phase) [m and Jurassic breccia of the S-charl nappe D3 (Blaisun Phase) lolds Carnian and oldor s€dimentary cover Mü Münstertal antiform m ot the S-charl nappa Of Ofenpass synform Se Sesvenna antiform Mesozoic sedimenls ot lhe Ortler nsppe E (mostly Upper Triassic and Liassic) _^._-! overthrust Silvretta, Campo, and Sesvenna basement m !-t. normal lault Penninic nappes az) traces of profiles in Fig.4 Fig. 2. Tectonicn.rap of the study area N. Frotr:.heirn at al. /Tectonoph-rsics280 (1997) 267_293 271

Pliocene

Miocene

Ma: intrusion ,r30 of Bergellgranodiorite DoäleTchs Oligocene l'- (v. Blanckenburg1992) \ 32-3f Ma: dykes in easternOrtler nappe I Urba (Dal Piaz et al. 1988)

Eocene

Paleocene i50

80-57 Ma: F2 in Samedan zone (Tietzet aI.1993) Maastrichtian dzn ! \r I Extensional

tren

Santonian Coniacian Turonian

Cenomanian

Aptian

! Radiometric ages biostratigraphic [f ages

Fig 3 Time table fbr cretaceous and rertiary defor'ation of the westernAustroalpine nappes.After Froitzhejm et al. (1994). slightly modified and complemented. basement.along the Schlinig pizLad ) fault. Thus, ap_ (4) The Ortler nappe consistsof permo_Triassic pears as a wed-seof Triassic sediments intercalated to Cretaceoussedimentary rocks and underlies the betweenÖtztal basementrocks .fo aboveand below. Quattervalsnappe and the UCS along the Trupchun_ the northwest,the Piz Lad sedjmentsdirectly over_ Brauiio thrust (Schmid, 1913;Conri er al., 1994). lie cover rocks of the S-charl nappe along a thrust (5) The Campo nappe consists of pre-Mesozoic ornamentedby sliversof basement(Ton-icelli, i956). basementand is divided from the overlvins Ofiler

) I p NW 5E ! PIZ RUSSENNA ROJEN V N OF JAGGL ll I

SCH 3km Piz Lad thrusl

.\tt-n +-+++ +++++ 1 +O 1 2km + a o

Da ta ötztal nappe lwr.rw ESE x- I SCHLINIG V, \ IIITTIIIil mostlv Jurassic & L.Cretaceous LISCHANNA I a I n F+q Hauptdolomit & Jurässic breccias 3km I Raibl f m. -\ rTl F- L.& M. lilassrc ! Verrucano \\ 2 s

x- 1T + 1 ffi mosty serpentinites +++ ++ o F o 2k o t o fii]]a Bündnerschiefer ä

N oo € 3km *'e \ q + I f \ i + N +++ + trl, ++ + + + +\+ 1 o---l- ''n' \ c o

rr--Tl =- FORAZ tavnü - Hauptdolomit r'rl I I \\ SE Raibl dolomite \\ 3km I Raibl evaporites nnn dolomile

Ii mestone

clastics I Km FF+] Dasement

I N. Froit:heim et al. /Tectonop\'sics 280 ( 1997) 267-293 nappeby an Alpine thrust (Zebrü thrust;Conti, i992, 1994). (6) The ntylonite of the VinschgaLrshear zone ts partly derived from pre-Permian basement, partly from Permo-Triassic sedimentary rocks (Schmid and Haas, 1989). This thick layer of mylonite is found between the Ötztal nappe and the UCS above, and the S-charl nappe, Ortler nappe, and Campo nappebelow. 'Engadine The term dolomites' summarizesthe sedimentary portions of the S-charl, UCS, Quatter- vals and Ortler nappes, cropping out in the form of a triangle betweenthe Engadineline. the Öztal, and the Campo nappe(Fig. 2).

3. Results

3.1. Fault geomett)l

The Schlinig fault is exposedover a N*S distance of 20 km between the Reschenpass in the north and Clusio/Schleisin the Vinschgauvailey,.in the south (Figs. 2 and 4). Near Clusio, the fault disappears under the Quaternary fill of the Vinschgau valley. In the North, the fault leaves the sediment-basement contact and becomes an intra-basementfault in the Ötztal nappe. The eastward extension of this fault within the Ötztal nappe has not yet been mapped. shearsense in mylonitesand cataclasites: Between the Reschenpass and Clusio/Schleis,the \ Dl (Trupchunphase) ---rD2 Schlinig fault describesa westwardconvex arc. This (Ducan-Ela phase) is due to a listric shapeof the fault which dips i6' O- @ sample localities (see text) northeastin the south, 8o to 9" east in the middle Fig. 5. Map showing shear direction and shear sensein mylonites part, and 17" southeastin the north (see contour and cataclasites from the Schlinig*Gallo fault system and sur- map in Stutz and Walter, 1983, from which also rounding area. Earlier, Trupchun-phase mylonites are abundant the dip angles were taken). Measured parallel to the in the hanging wall of the normal fault system, whereas in the movement direction (see below) between Piz Rims footwall, they were largely overprinted by top-SE shear. Pl and P2 are intersection points of the Schlinig fault with the basal and Clusio/Schleis (Figs. 5 and 6), the dip angle thrust of the Ötztal nappe in the footwall and in the hangrng is 7.5'. Erosion through the Schlinig fauit has cre- wall. respectively.VCS: Vinschgaushear zone. ated a tectonic window of S-charl-nappesediments surrounded by Ötztal basement rocks in the Rojen valley (Rojen windov,,.Fig. 2'), and severalisolated The Ötztal nappe in the hanging wall consists klippen of Ötztal rocks on top of S-charl-nappesedi- of gneiss and mica schist with amphibolite layers, ments farther west. discordantly cut by the Schlinig fault (see Fig. 1

Fig. 4. Cross-sectionsthrough the Engadinedolomites and the overlying Ötztal nappe.Traces of cross sections:see Fig. 2. Cross-sections (b) and (c) are from Schmid and Haas (1989), with the only modification of the slip sensealong the Schlinig fault, which is top-SE according to this study. SCII : Schlinig fault, El : Engadine line, 51 : basal thrust of the Silvretta nappe. 114 N. Froir:hairn er ul. /Tecrottophr.rlcs 280 ( 1997) 267-293

PIZ RIMS PASS OA SCHLINGIA cLUS!O/ GLORENZA/ (P.27s2) GLURNS I SCHLEIS NW I l SE tl I I xxx O +r_<< xx öttt a I nappe 2km x x 300' 1 310' xx 1 1.67 km S -charl nappe \ .eso" o ..1...... - >300"

Campo 10km Vinschgau shear zone I

Fig. 6. Cross-ser:tionparallel to the slip direction of the Schlinig fault. Trace of section in Fig. 5. Py and P"' are intersectionpoints of the Schlinig tar-rltwith rhe basal thrust of the Ötzrrl nrppe, projectedinto the section.Temperature estimates for the footwall are from a cornbinedillite crystallinity/vitrinitereflectance study of Kürmann (1993). 300"C and 350"C isogradswere drawn under the assumption of a 30"C4

:q Hammer. 1908). The regional amphibolite-grade south (SW in the westernpart and SE in the eastern -,retamorphism of these rocks is Hercynian in age part). It truncatesearlier thrust faults and recumbent (Thöni, 1980b).The Alpine defom.rationof the Ötz- folds ofthe Trupchun phase(see figs. 15 and 16 of tal rocks is restricted to the lowermost fer.l'metres Schmid, 1973)and is itself,in its easternpart, gen- where a cataclasticor. in the southernmosiexposed tly folded around later, open, WNW-ESE-strikrng parl of the Schlinig lault. mylonitic ovärprintof anti- and synformswhich are ascribedto the Blaisün the Hercynian structuresis observed.The footwall phase (Fig. 2: Mür'rster-talantiform, Ofenpass syn- of the Schlinig fault rnainly consistsof Mesozoic form; Schmid, 1973 Conti. i994). We assumethat sedimentsof the S-charl nappe or, in the northern- the Gallo and Schlinig faults are connectedaround most part, the Piz Lad unit. An older, pre-Schlinig- the hinge of the east-dipping Sesvennaantiform, fault deformationof the S-charl nappeis represented also fonned during the Blaisun phase(Fig. 2). The by SW-NE-striking, northwest-fäcingfblds of the connectionis buried underthe Vinsch-qauvalley Iill. Trupchun phase which deform the sediments and In addition to the Schlinig and Gallo faults, the basement-sedimentcontact in the footq'all but fault rocks were also sampledalong the Arlui fauit are truncatedby the Schlinig fault (Stutz and Wal- (Figs. 2 and 4a). This is the moderatelyto steeply ter, 1983; Fig. 2). In the area northwestof Clu- southeast-dippingcontact between Permo-Mesozoic sio/Schleis(Figs. 2 and 4b), the thicknessof the rocks of Jaggl, to the southeastand above the fault, sedimentsdecreases torvards the southeastso that and Ötztal basement.to the northwestand below. ttre Ötztal nappe ahnost directly rests on the base- rr-ientof the S-charl nappe.In this area,a myionitic, 3.2. Kinematic anal)tsisof nrylonitesand _ jchlinig-fault-related overprint of the footwall base- cataclasites mentrocks is not restrictedto the vicinity of thefault itself but is observedover a structural thicknessof Although the Schlinig 1äultis well visible from a severalhundred metresinto the footwall. Kinematic distancethrough the contrastbetween gneissin the analysisof fault rocks (see below) revealedthat the hanging wall and sedimentar-yrocks in the footwali, Gallo fault (Böse,1896; Hess. 1953;Schmid, 1973; good outcropsof the fault contactitself are rare. The Conti, 1994; Fig. 2) representsthe southrvestward best outcropsalong the fault are locations 1 to 3 lsee prolongation of the Schlinig fault. The Gallo fault Fig. 5) describedbelow. separatesthe S-charl nappe to the north from the Locotion 1 is an approximately 100 m thick sec- overiying Umbrail*Chavalatsch-Schuppenzoneand tion of mylonites exposedon the northern side of Quattervalsnappe, to the south. It generally drps the stream west of Clusio/Schleis (Swiss coordi- N. Froit:heint et al. / Tectotloplrlsics280 ( 1997) 267-293 J75 nates835.200/176.150). These my'lonites are derived main foliation dipping shallowly SE, and abundant from basementgneisses of the S-char'lnappe. sericite shearbands, also dipping SE but at a higher angle.A piiyllite (representingPermian volcaniclasticrocks), slickensidelineation is formed on the foliation and Lower Triassic sandstone.and Middle Triassic car- on the shear bands. This lineation is oriented SE- bonatcrocks. M1 lonitized Ötztal gneissoccurs in l{W. perpendicularto the intersectionof foliation and the uppermostpart of tire section.Triassic sediments shearbands. Hence, the lineation.foliation. and shear and older rocks are imbricated, producing strati- bands are interpretedto be all related to the same graphic repetitions. The mylonites exhibit a well deformation process that formed the cataclasites. developedfoliation dipping shallowlynorth to east. From the asymmetry of the shear band-foliation and a stretchinglineation in quartz-richlayers which relationship,a top-to-the-SEsense of shearis clearly is oriented SII to ESE. Additionaily, a weak post- recognizedin the field. mylonitic crenulation,subparallel to slightlyoblique In thin section, three types of pianar elements u,ith respectto the stretchinglineation, is observedin are observed:(1) a foliation ('s' in Fig. 7f) deflned mica-rich layers. In the field, the shearsense of the by mica grains, elongatedfragments of chloritized mylonitesis difficult to assess;only indistinctshear and sericitized gneiss protolith, and thin layers of band/foliation relationshipsand occasionalporphy- ultracataclastic,very fine-grainedmaterial; (2) shear roclast systens reveal a top+o-the-SE shear sense surfaceswith ultracataclasticmaterial, oriented par- (Fig. 7a). allel to the Schlinig fault ('c' in Fig. 7f), (3) shear Thin sectionsperpendicular to the foliation and bands, inclined with respectto the Schlinig fault. parallel to the lineation show abundantshear sense A slightly shearedand recrystallizedpseudotachy- criteria(Fig. 7b-et. includingshear band/foliation lyte vein was observedin one sample. Criteria on relationships(Fig. 7b). S-C fabrics(Fig. 7c), asym- the microscopicscale (S-C structure,foliation/shear metric porph,vroclastsvstems (Fig. 7d), asymmetric band relations, asymmetric clasts) clearly confirm mica fish (Lister and Snoke, 198,1),and grain-shape the top-SE shearsense. preferredorientations in thin quartz layers (Fig. 7e). Quafiz is mainly observedas porphyroclastsem- The porphyroclastsare feldspar-.quaftz-. and tour- beddedin a fine-grainedfoliated matrix of feldspar. maline-grains.Generally the observedporphyroclast white mica, and quartz. The quarlz porphyroclasts systemsare of the o-type (Passchierand Simpson, exhibit older (pre-Alpine)microstructures formed by 1986).The shearsense indicated by thecriteria listed grain boundarymigration recrystaliization,a mecha- aboveis consistentlytop-to-the-southeast (Fig. 5). nism which is active at eievatedtemperatures (Guil- Quartz is obsen,edin thin layers between mica- 1op6and Poirier, 1979;Drury and Urai, 1990),higher rich la-vers and as forner porphyroclasts in a than the temperatures reached during the Alpine sericite-feldspar-quartz-richmatrix. Quartz is com- orogeny in this area.This former, higher-tenperature pletely dynamically recrystallized.The grain size in deformationphase is responsiblefor the large bulges the recrystallizedlayers is approrimately 100 p.m. and serratedgrain boundaries observed inside the The shapeof the former porphyloclastscan still be porphyroclasts.These older microstructuresare over- discemed,but they now consistentirely of recrystal- printed by the colder deformation during movement lized grains. The shape fabric of the recrystallized along the Schlinig fault. Quartz porphyroclastsand grainsin thethin quafizlayers indicates top-SE shear layers show strong undulatory to patchy extinction, sense(Fig. 7e). deformationbands, and subbasaldeformation lamel- Locatiort2 is about 10 km fartherto the norlhwest lae. The latter usually indicate high flow stresses (Swisscoordinates 828.125/186.00). The footwall in (Ar,6 Lallemant and Cafier, 1971: Drury, 1993).The this area consistsof mylonitized, Middle Triassic initiation of dynamic recrystallization is observed limestone with an E- to SE-dipping foliation and a along narow bandswith newly recrystallizedgrains pronouncedESE- to SE-striking stretchinglineation. ßize 20 pm, see Fig. 7g), but completely recrys- Foliated,basement-derived cataclasite occurs in a 10 tallized quartz layers as observed at location 1 are to 20 m thick layer at the base of the overlying not present.This indicateslower temperatureduring Ötztal gneiss. Typically. these cataclasiteshave a deformationat location2 as comparedto location 1. 3

jl c

c- \o

N h

:a \ N

Asymmetryinclicatesdextral(top-SE)shearsense.Diameterofcoinis2cm.Loc.lncarClusio/Schleis.(b)Mylonite

Clusio/Schleis.

I N. Froitzheimet al./Tectonoplr1'sics280 (1997) 267-293 211

.=.= 9 S.= :

- o 4 -Y @i )r a., o -. !

AIa 3lq.: =.= r --) ^- -; I v- a .2.2 -ö ( :='=*=^'

l- N;^:.: ?:,:*i'-! !s:v5>! ;G =-_u ;'t,, \ I tr >- .J : - o i-.(:!- ,'JY e o:Jn i I

! ! > -- '= ü a -= J bOE ;.i Y I N ! ;4, :so"-j-- ='Z.Y ? t g.; *tEilrlEi_:i

E r " I- '.- >.=a =

! Ä-= N = I ". : o i - :

va i J >o o i:. Caail, - ö o Ec

- ü:= ä i 9,o'äE *i säFi3. ! 9- ? 0 äzäHcr ,?- ä8. -- =l,. o Y - ! j - " I S ., öo öb.

.;-X>rl = ^. *- il,

.=:d=- €:x!{ -:i 4 q,_-. == 9i:.:

s i E g "o- =IEüYä9.=--- -; ! ?: N

Y!i;:=.^ E+T: E. sd;^ =93,ö= .9hP-s 9 6 --a-=-- h- 4",!^9 -C.-g- ^- E6-* -"E ä- ;, , E ? Y oc- o .=9_ts;* E - ^9 r. 9.= v : ? = 7 = Eo. r ^. e € o .= : : : .: t!7 i.: s:(

)

- N. Ftotr:hcim et aL./Tecronopltysics 280 ( 1997) 267-293

Location -l is still farlher north, southwestof Piz mica fish in these fault rocks consistently yield Lad (Swisscoordinates 830.388/191.787). Here, the a southeast-side-downsense of displacement.Tri- footwall is formed by Llpper Triassic Hauptdolornit assic dolomite belonging to the Permo-Mesozoic of the Piz Lad unit. Cataciasticdeformation of Ötztal successionsof Jaggl, directly overlying these foot- basementrocks is restrictedto a thin zoneof 2 to 3 m wail-derivedfault rocks, is cut by numeroustop-east (Schmid and Haas, 1989).Differently fiom location normal fauits, also indicating normal displacement 2, foliation. iineation, and shear bands are not well along the Arlui fault. developedand do not yield a consistentshear sense. The study of the fault rocks yielded the following Again the pre-Alpine deformation microstructures main results: (1) the shearsense along the Schiinig are observed in the quartz porphyroclastsand are fault and the easternpart of the Gallo fault is con- clearly overprintedby the later colder deformation sistently top-SE to top-ESE, with the exception of phase. This is indicated by incipient bulge nucle- the northernmostlocality 3 along the Schlinig fault ation observodon the old, largebulges. Fufihermore, where shearsense could not be determined;and (2) the quaftz grains have subbasaldeformation lamel- the microstructuresof quarlz in the fault rocks in- lae, some Böhm lamellae.and subgrainboundaries dicate a systematicnorthwestward decrease of the parallel to the prisn planes, correspondingto the marimum defornation temperature.Täken together, deformation bandsdescribed by Carter et al. (1964) the top-southeastshear sense,the overall eastward (Fig. 7h). There is no evidencefor recrystallization dip of the fault, and the increaseof syn-deforma- of quafiz, which indicatesstill lower temperaturesas tion temperaturein the transpoft direction clearly - comparedto location2. documentthe nomal fault characterof the Schlinig fault. Obviously, this normal fäult movement must 3.2.1.Gallo line havepostdated nappe imbrication, because basement Algng the easternponion of the Gallo line. a rocks overlie Mesozoic sedimentsalong the fault. la1'er of ml,lonite is found in the uppermost palt The Gallo fault prolongatesthe Schlinig fault to of the footwall. The protoliths are basementgneiss the southwest.The top-southeastshear sense of the and Permo-Triassicvolcaniclastic rocks. The folia- Schlinig fault is incompatible with the consistent tion of the mylonites dips shallowly southeast,and top-west shearsense documented for the Vinschgau the stretchinglineation is orientedparallel to the dip shearzone by Schmid and Haas (1989). Therefore, direction. SE to ESE (Fig. ,5). Quartz microsrruc- the latter cannot representthe prolongation of the tures are very similar to those frorn the southern Schlinig fault torvardsdeeper crustal levels but rep- part of the Schlinig fault (Ioc. 1), indicative of resentsan older Alpine shearzone, truncatedby the lower-greenschist-faciesconditions during deforma- Schlinig fault (seeFig. 6). tion. The shearsense in the mylonitesis consistently The Arlui fault is also a top-southeastnormal top-southeast.Along the western, NW-SE-striking fault and is intelpreted as a hangingwail splay of porlion of the Ga11oline (outsidethe areaof Fig. 5), the Schlinig fault (Fig. 4a). Consequently,the Jaggl both sides of the fault are formed by Triassic sedi- Permo-Mesozoic rocks do not, according to our ments,mainly dolomite.The deformationof the wa1l results, represent a window of the S-charl-nappe rocksis cataclasticin thisarea. Slickenside lineations cover underlying the Ötztal nappe. as was often ,- along the fault are generallyhorizontal and NW-SE assunred(Hess, 1962; Thöni, 1913,1980a;Stutz and oriented,but indicationsof the slip sensecould not Walter, 1983;Schmid and Haas, 1989),but a piece be found. of Otztal-nappecover (Staub, 1937) thrown down towardsthe southeastalons the Arlui normal fault. 3.2.2.Arluifault Foiiated cataclasitesand lower-greenschist-facies 3.3. Shapefabric and ulstallograpltic prefened mylonites derived from Ötztal basementrocks were orientationof quartT also found along the Arlui fault (Figs. 2 and 4a. and Fig. -5). Foliation/shearband relations, S-C Shapefabric and crystallographicprefened oden- structures,asymmetric porphyroclast systems, and tation were analyzedquantitatively in a sarnple from N. Froit:heint et al. /Tectottoplrl,sics280 ( 1997) 267-293 location1 along the Schlinigfault (E409).Both the slight deviationrvhich is anticlockwisefor the grain shapefabric and the crystallographictexture turned long axesand clockwisefor the grain boundaries. out to be strongLyinffuenced by iater,post-mylonitic, As argued above, the non-coaxial,top-ESE my- NNE-SSw-directed shortening(Blaisun phase),not lonitic shearing (Ducan-Ela phase) is likely to be visible on the hand specimen.ln thin sectionsper- responsiblefor the oblique orientation of the grain pendicularto foliation and stretchinglineation, how- long axes with respect to the foliation in section ever.mica-rich layers exhibit a crenulatior.rresulting E4094. During the follorving coaxial, N-S-directed from this shortening(see Fig. 8a). shortening(Blaisun phase),a microscopic crenuia- tiou developedwith axessubparallel to the stretching 3.3.1.Shapc preJ'errctl orietttatiott tSPO) lineation produced b1, the Ducan-Ela phase. It is The SPO was analyzedin three sections(Fig. 8); suggestedthat this shorteningevent accountsfor the (1) parallel to lineation and normal to foliation (X-Z microstructurein section E409D but did not oblit- section), (2) norrnal to lineation and foliation (Y-Z erate the obiiquity of the grains with respectto the section), and (3) parallel to lineation and foliation foliation in sectionE4094. (X-f section). In the geographic referenceframe, the foliation dips 12'ESE (107'), and the stretching 3.3.2. Crystallographicpreferred orientation (CPO) lineation (X direction; plunges ESE 1103")at an Quartz CPO was measuredin sectionE4094 (X- angle of 12'. SURFOR and PAROR methods of Z section)with the X-ray texturegoniometer and the shapeanalysis (Panozzo, 1983, 1984)were applied. U-stage.With respectto the foliation and macroscop- The three-dimensionalshape fabric is unusualfor a ically visible stretchinglineation, the measurements n.ryloniteand can be explained by two successive yielded an unusuaitype of texture.From the field ob- defomrationevents. servatious,tl.re shear sense criteria in the X-Z piane, The axial ratio of the elongatedgrains in the X- , and the measuredSPO. this sectionwas supposedto Z section(E409A) is 1.95 calculatedby PAROR. In contain the principal kinematic direction.The a-axrs this sectionthe grain long axesdistnbution (PAROR) maxima, however,are perpendicularto the stretching hasits maximum orientedobiique with respectto the lineation (Fig. 9a). By rotating the pole figure 90" foliation planeby 20' (Fig. 8c). The maxin-rumof the around an axis normal to the foliation, it becomes grain boundary distribution (SURFOR) is inclined clear that sectionE409D (perpendicularto lineation) 15' u'ith respectto the foliationplane (Fig. 8c).The containsthe kinematic referenceframe for that pan obliquity of the maxima with respectto the foliation of the deformation which led to the presentquartz is consistentwith top-ESE shear. CPO. Fig. 9b shows the rotated pole figures. The The axial ratio of the quartz grains in rhe Y-Z c-axis pole figure is almost symmetric and resem- section (E409D) is 1.57. The SPO suggestsflatten- bles a type II cross-girdle(Lister, 1977; Schmid and ing parallel to the f axis and extensionparallel to Casey. 1986) or two-girdle pattern (Sander, 1970). the Z axis. The maximum of the long axes of the We assumethat this texture was fonned by coax- grains is inclined 80" to the foliation. The maximum ial NNE-SSW shortenirrgin the Biaisun phase.The of the preferredorientation of the grair.rboundaries deviation from a normal type II cross-girdle(com- is inclined 90" to the foliation. This carrnotbe ex- pare Schmid and Casey, 1986) may be explained plained by the mylonitic shearingbecause from this by a preexisting texture that formed during the top- we would expect the grains to be elongated par- ESE shearing and was not completely obliterated allel to the foliation. Instead,these microstructures dunng the N-S shortenin-q.The above suggeststhat are consistent with the later compressionalevent the quartz CPO reflects two successivedefonnation (Biaisunphase). phases,which is quite unusual.According to Lister Section E409F, the X-X section parallel to the and Williams (1919),the skeletaloutline of a pattem foliation, has an axial ratio of 1.16. The maxima of preferredorientation will only rememberthe last of the prefened orientationsof the grain long axes strainincrement. In our case,the incompleteobliter- and of the grain boundariesare subparallelto the ation of the older texture is understandablebecause macroscopicallyvisible stretching iineation. with a the overprintingdeformation was rather weak. 280 N. Froir:lrcint eI al. /Tecronophl'sics280 ( 1997) 267-293

8409Fparallel to lineationand ioliation

IOOUm

whitemba

Z ,*+#\ l" h quarEvetn ioliation =F. \)2.

oeroendicuarlo lineation& Joliation

)

PAROR

)

- N. Froit:hein et al. /Tectonophl,sirt tUO ( 1997) 267-293 281

referencelrames c-axes a-axes

&

b

Fig. 9. (a) Quartz c-axis and a-axis pole figures as measured in section E409A. the X-Z section of the mylonitic deformation. W is to the left. The c-axis girdle does not passthrough the centerofthe stereonet,and the a-axis maxima are at high anglesto this section.(b) Pole figures after a 90" rotation around Z, that is, viewed rn the Y-Z section. N is now to the left. a-axis maxima are at the periphery now and c-axesshow a type-Il cross girdle, interpretedas indicating postmylonitic coaxial shorteningin the I direction, that is, perpendicularto the mylonitic sheardirection ()1. Numbers below the pole figuresgive the intensity of the maxima (e.g. 4.05 times uniform).

From the analysisof the SPO and the CPO and shearing(Schlinig norrnal fault, Ducan-Ela phase), the comparison of the two, the following conclu- and the secondwas coaxial, NNE-SSW shortening sions can be drawn. (1) Both SPO and CPO can (Blaisun phase).(2) Although the N-S shorteningis best be explained by two successivedeformation macroscopicallyinvisible in this particularsample, it events,the first of which was noncoaxial, top-ESE signiflcantlymodified the SPO and CPO.

Fig. 8. Shapefabrics in a dynamically recrystallizedquartz vein from a Schiinig-fault mylonite. (a) Sketch of the sample, iliustrating the relationship between the macroscopicstretching lineation developedduring Schlinig fault movement and microscopic crenulation developedduring later N S shortening.(b) Quartz grain boundariesin the X-)'section (E409F) and resulting PAROR and SURFOR diagrams. (c) Shape fabric in X-Z section (E409A). (d) Shape fabric in Y Z section (E409D). Foliation is always E-W in the rose diagrams. PAROR rose diagrams represent the orientation distribution of the long axes of the grains, SURFOR diagrams represent the srain boundarv onentationdistribution.

) t.82 N. Ft'oit:lteinteI al. /Tecttnopft,r'slcs280 (1997) 267-293

3.4. Antourttof displacententacross th.e Schlini.g later rotatedinto its presentshallow orientation?An fault estimate of the initiation angle can be made us- ing the variation in peak metamorphictemperatures Since extension along the Schlinig fault post- acrossthe footwall, provided that (1) thesetempera- dates nappe i.mbrication,the offset of thrust faults turesreflect the thermal stateimmediately preceding by the normal fault may be used for determining extensionalfaulting, (2) the preextensionalgeother- the amount of displacement.The basal thrust of the mal gradient is known, and (3) the isothermswere Ötztal nappe is exposedin the hanging wall and in horizontalprior to normal faulting (Wemicke, 1995). the footq'all of the Schlinig fault. In the hanging For the footwall of the Schlinig fault, palaeotem- wall it is representedby the Vinschgau shearzone. perature data exist from petrologic and geochrono- along the northern side of the Vinschgauvalley. The logicalwork of Thöni (1980b,1981), and from com- top of this north-dipping shearzone is truncatedby bined illite crystallinity/vitrinite reflectancestudies the Schlinig fault in the vicinity of Glorenza/Glurns, of Kürmann (1993) and Henrichsi19931. Ötztal under the Quaternary valley fill (P2 in Fig. 5). In gneissabove the Schlinig fault yielded undisturbed the footrvall, the thrust at the base of the Piz Lad Variscan K-Ar biotite ages, whereasbiotite in the unit (Piz Lad thrust, Fig. 2) is interpreted to rep- footwall basement was parlly rejuvenated during resent the basal thrust of the Ötztal nappe. West Cretaceousmetamorphism (Thöni, 1980a,b).Hence, of Piz Lad, this thrust places Triassic rocks of the the Schlinig fauit emplaced colder on hotter rocks Piz Lad unit on Jurassicrocks of the S-charl nappe and is discordant with respect to the tempera- Fig. 4a). Slivers of basementrocks occur along the ture zonation. Therefore the temperature peak of - thrust.Farther nofiheast, the thrust is betweenÖtztal Cretaceousmetamorphism in the footwall (S-charl basementrocks above and S-charl basementbelow, nappe) must have predated normal faulting aiong with a thin layer of rnylonitizedN4esozoic sediments the Schlinig fault. On ihe other hand, the footwall in between.This Piz Lad thrust is truncateäby the did not cool down significantly before the no.-ui Schlinigfault at P1 fig. 5). Fig. 6 is a cross-section faulting event, becausemicrostructures in Schlinig parallel to the movement direction of the Schlinig mylonites and cataciasites(see above) indicate syn- normal fault, from Glurrrsto the Piz Rims klippe. P1, faulting temperatures that were still near the peatr< and P2,are the intersectionpoints of the Ötztai basal metamorphic temperatures.Therefore, the tempera- thrust with the Schlinig fault in the footwall and ture data from the footwall can be taken as repre- in the hangingwall, respectively,projected into the senting the thermal state immediately before normal cross-section.The projectionof P1,is not well con- faulting. For the southem S-charl nappe, Kürmann strainedbecause the Piz Lad thrust is not planar.The (1993) estimateda geotherm of 35 :t 5'C/km from intersectionpoint in the cross-sectionmust, however, illite crystallinity/vitrinite reflectance determinations be locatednorlhwest of the Piz Rims klippe because at different structural depths. Henrichs (1993), us- at Piz Rims, the base of the Ötztal rocks is still the ing the same method, found geothermsof 40"C/km cataclastictop-southeast norrnal fauit, and there are for the western S-charl nappe and 43'Clkm for the no remnantsof an older top-west thrust. Hence, the southeastemS-charl nappe. For Cretaceousmeta- offset of the Ötztal basal thrust acrossthe Schlinig morphism in the southwesternÖtztal nappe.Schmid "ault measuredparallel to the movement direction and Haas (1989) inferred conditions of ca. 440'C amountsto about 17 km (Fig. 6). and 4 kbar for the chloritoid-in boundary. Assum- ing a density of 2.7 g/cmr. this correspondsto a 3.5. initiation angle geothermof 29.l"Ckrn From theseestimates, a gra- dient between30"C/km and 45"CÄanticline (Fig. 10). This directly zons, the evaporite-bearingRaibl Fornation of Car- results from the keystone-likeshape of the dominos nian age below, and calcareousand shaly Jurassic u'hich are wider at the top than at the base. The to Cretaceousfbrmations above.In the southeastern anticline may be interpretedas a fault-bendanticline S-charlnappe, the Hauptdolomit is presentas a layer related to a thrust undemeath. The most important 'domino' of northwest-side-downtilted blocks sep- result of this retrodeformation is that the normal arated by southeast-dippingnormal faults (Fig. 4b. faults did not form simultaneouslywith the folds c). This layerof tiltedHauptdolomit blocks ('S-charl nor before the folds, but that they postdatefolding. Oberbau') is decoupledfrom the lower part of the The amount of extensionof the Hauptdolomit layer Triassicsediments and the basement('S-charl Llnter- can be directly read from Fig. t0 (distance A'--B' bau') along the Raibl formation. As demonstratedby minus distanceA-B). It is 7.85 km, corresponding Schmid and Haas (i989), the importanceof the nor- to a stretch | * e of 2.67 (.A'-B' divided by A- mal faults dissecting the Hauptdolomit diminishes B). This is in the range of what Schmid and Haas northwestward.In the northwest oarl of the S-charl (1989) calculated (.1+ e : 3.45) with a different 28.1 ,Y. Frcit:hcim ?t ol./Tectonophl'sics280 (1997)267 293

SE 4.7 km

Hau ptdolomil

B^ basemenl Hermtan I /-./ Ca rnian .-' -----= \o +l*\l\

SCH __s \..--_

--l- ) - \f

+ ++ TTT + -f + \+'- 16.1km to' g-i---iz k-

Fig. 10. Retrodeformationof domino-st1'leextension in the fbotwall of the Schlinig fault. Lower sectionis taken from Schmid and Haas (1989) and is identical to Fig. ,lc. Llpper sectionshou's pre-normal^faulting siruation, restored by fitting the fault blocks together.SCH: Schlinig fault. A, B, C etc. are referencepoints. For further expJenation.see texr.

method.using the angularrelationships of faults and fault, and is compatible with top-SE shear of the beddingplanes. Minor normal täulting also affected Hauptdolomit layer (comparefig. 6 in Schmid and 'S-charl 'Unterbau' the Unterbau'. Extension of the Haas, 1989), with the domino-normal faults acting ar.nountsto 3.15 km (C'-D' minus C-D). Hence, in a similar way as syntheticRiedel shearsor shear the Hauptdolomit was extended by 7.85 km, and bandsin a mylonite. 'Unterbau' the only by 3.15 km. The difference is ,1.7 km. This extra extension of the Hauptdolomit 4. Discussion must have been transfered to deepercrustal levels along the Schlinig normal fault (if we do not as- 4.I . Reconstructionof pre-normalfauLt geomettj sume rootless extension of the Hauptdolomit, e.g., by gravitationalgliding, for which we do not seeany An attemptat restoringthe geometryof the nappe evidence).Therefore. extension of the Hauptdolomit stack before extensionaifauiting is made in Fig. I1. was kinematicaily linked to normal faulting along The cross-sectionused is the one of Fig. 6. Fig. 1la the Schlinig fault. is the restoredsection. We assumedan initiation an- To conclude.we assumethat the NW-SE ex- gle of 15'for the Schlinig fault, in accordancewith tension of the Hauptdolomit layer postdatesnappe the field relations (see above). The basal thrust of imbrication and formation of NW-facing folds, and the Ötztal nappeis used as a referenceievel for the is kinematically linked with the Schlinig normal reconstruction.The Arlui fault is projectedinto the fault. The rotation of the Hauptdolomit blocks is an- section from the north. In the reconstructedsection, tithetic (NW-side-down)with respectto the Schlinig it joins the Schlinig fault at the point where the N. Froitzheintet aL./Tectortopb,sics 280 (1997) 267-293 285

SE

S'charl lcoverll lti nappe {0"=.-.lT r ä : before extension

b: present

Campo

Fig. 11. R""onrt-"tion of the Schlinig fault. (a) Pre-normal-faulting geometry of the Cretaceousnappe stack. Stippled lines are trajectories of future normal faults. (b) Present-day situation after normal faulting along the Schlinig and Arlui faults. A : Arlui fault; S : Schlinig faüt; VSZ - Vinschgau shear zone. latter passesthrough the Otztal basal thrust. Two 4.2. Age of nornmlfaulting large thrust sheetsexist in the reconstructedsection, the lower S-charl-Campothrust sheet(including the The Schlinig-Gallo fault systemoverprints older Ortler nappesouth of the section)and the higher Ötz- Alpine structures, mainly thrust faults and open to tal thrust sheet (including the Umbrail-Chavalatsch isoclinal folds. These older structuresare attributed zone and the Quatten'als nappe). These were dis- to the west- to northwestward imbrication of the sected by the Schlinig-Gallo fault. The S-charl- Austroalpine nappe stack during the Late Creta- Campo thrust sheet was divided into a part in the ceous Trupchun phase. Trupchun-phasemylonites hanging wall of the Schlinrg-Gallo fault (Campo with top-westto top-northwestshear sense are abun- nappe, Ortler nappe) and a paft in the footwall dant in the Umbrail-ChavalatschSchuppenzone and (S-charlnappe). The reconstructionshows some fea- in the Ortler nappe south of the Gallo line (Conti, tures that we regard as typical for the Late Cre- 1994: Fig.5). In the footwall of the Gallo-Schlinig taceous collapse in the western Austroalpine. The fault system, they are largely overprinted by top- normal faults have a similar orientation as the ear- ESE shear due to normal fauiting (Fig. 5). On the lier thrusts,only that they are often slightly steeper other hand, the Schlinig and Gallo normal faults and the slip senseis reversed.They partly reactivate are themselvesslightly folded around ESE-striking earlier thrusts,as was observedalong the baseof the axes, leading to anti- and synforms on the regional Silvretta nappe (Froitzheim et al., 1994), but partly scale (Fig. 2), and to the quartz microfabrics de- follow new trajectoriesat small anglesto the thrusts. scribedabove on the microscopic scale.These folds The extensionis strongly asymmetric,to such a de- are assignedto the Early Tertiary Blaisun phase. gree that antithetic,norlhwest-dipping normal faults The normal faulting along the Schlinig and Galio are hardly ever observed. faults can therefore be attributed to the Ducan-Ela

I 286 N. Froit:lteint er al. /Tectonophl'sics280 (1997) 267-293

extensional phase as deflned in the area west of assumedage of the Ducan-Ela extensionalphase, the Engadine line. We will briefly summarize the which suggeststhat cooling was related to exten- age constraintsfor the Ducan-Elaphase in order to sionalexhumation. discussthe lalge-scaletectonjc scenario. The youngest sedimentsin the Engadine dolo- 4.3. In searchof a tectonicmodel for Late mites are Cenomanianto possibly middle Turonian Cretaceousextension foraminiferal marls in the core of a Trupchun-phase synciine at the western end of the Ortler nappe A Late Cretaceousextensional overprint has been (Caron et al., 1982). Trupchun-phasetluusting and demonstratedin many areas of the Austroalpine folding in this area can therefore only have be- realm. Prior to the present study, such extensional gun after about 93.5 Ma (age of the Cenomanian- faulting was already reported by Krohe (1987), Turonian boundary after Gradsteinet a1.,1994). On Ratschbacheret al. (1989), Froitzheim (1992). Wer- the other hand, the youngest fossils found in Pen- ling (1992),Froitzheim et al. (1994),Neubauer et al. ninic units of the Engadinewindow, underlying the (1995),Fügenschuh (1995), and severalothers. The Austroalpinenappes, are Paleocene to Lower Eocene extensionalfaults and shearzones identified by these Formanifera found in a flysch unit in the westem authorsgenerally overprint earlierthrust faults, have part of the window (Ziegler,1956; Rudoiph, 1982; a transport direction towards east (NE to SE), and Eiermann, 1988). Hence the thrusting of the Aus- have important displacements,often in the range of -oalpine nappesen bloc ot'er the Penninic units of tens of kilometres.In many areas,but not generaiiy rhe window, occuned soon after 50 Ma. Overprint- (seeKrohe, 1987),the transportdirection is opposite ing relations (seeabove and Froitzheim et a1.,1994) to the directionof earlierthrustins. show that this thrusting postdatesthe Ducan ex- tension. Theiefore, from biostratigraphicevidence, 1.3.1. Late Cretaceoüsextension and the Gosau Trupchun-phasethrusting and subsequentnormal basins faulting along the Schlinig*Gallo fault system oc- According to Krohe (1987), Ratschbacheret al. cured between93.5 and 50 Ma. (1989)and Neubaueret al. (1995),Late Cretaceous Radiometric data further constrain the timrng. normal faulting is related to the formation of basins Dating of extension-relatedpseudotachylyte in the in which sedimentsof the Gosau Group were de- Silvretta nappe (79-73 Ma using Rb-Sr, Thöni, posited. The Gosau Group is of late Turonian to 1988) and of mica grown along Ducan-Eia-phase Early Eocene age (90 to 50 Ma) in the Northern cleavagein the Lower Austroalpine (80-67 Ma us- CalcareousAlps and of Santonianto Maastrichtian ing K-Ar, Tietz et al., 1993)suggest that the Ducan- age (86 to 65 Ma) in the Central Austroalpinebase- EIa extensionaiphase in the western Austroalpine ment east of the Tauern window (Wagreich, 1995; realm lastedfrom about 80 Ma to 6l M4 with some Neubaueret al., 1995).The Lower Gosau Subgroup uncertaintydue to the scarcityof data. Alpine meta- (Wagreichand Faupl, 1994) restsunconformably on morphism in the Engadinedolomites, Campo nappe, folded and imbricated older rocks and comprises flu- and Ötztal nappewas radiometricallydated by Thöni viatile (at the base) to shallow marine (at the top) .nd coworkers(Thöni, 1980b, 1981, 1986, i988; sedimentsdeposited in small, fault-boundedbasins -fhöni and Miller, 1987, reviewed in Schmrd and (Wagreich,1995). The Upper GosauComplex com- Haas, 1989). Virtually all the ages from the study prisesdeep-sea fan facies,deposited partly below the area are between 100 and 70 Ma. These include CCD, and marly slope facies,and was not restricted both formation ages and cooling ages. K,/Ar ages to single basinsbut coveredthe entire Northern Cai- from fine-grainedmuscovite/phengite in the south- careousAlps (Wagreich, 1995). The boundary be- easternS-charl nappe clustering around 80 * 6 Ma tween the Lower and lJpper Complex is reiated to a are interpretedas cooling ages,whereas ages from major subsidencepulse that occured diachronously, the northeasternS-charl nappe around 90 :l 5 Ma betweenlate Turonian/Santonian(90 to 83.5 Ma) and are interpreted as formation ages (Thöni, 1980b). Maastrichtian(71 to 65 Ma) (Wagreich, 1995). The The cooling ages approximately coincide with the correlationbetween Late Cretaceousnormal faulting

I N. Froitzheimet al. /Tectonoplrlslcs280 (1997) 267-293 281 and Gosau basir.rformation is u,ell establishedfor stretched in an east-west direction (80-67 li4a). the Gosaubasins located in the CentralAustroalpine On the other hand, the Late Cretaceousextension basementeast of the Tauem window (Neubaueret postdatesolder Cretaceouseclogite metamorphism aI., 19951,and is also probablefor the Northern Cal- within the Austroalpinenappes. These older eclog- careousAlps becauseOrtner (1994) and Eisbacher ites formed during collision after the closure of and Brandner (1996) dernonstratedthat Gosau sedi- the Meliata-Hallstatt ocean,remnants of which are ments of the westem Northern CalcareousAlps seal found in the easternNCA (Fig. 1, inset). The ex- SW-NE-trending folds (: Trupchunphase) and are humation of these eclogites was at least partly themselvesdeformed bv WNW-ESE-trendins folds accommodatedby the Late Cretaceousextension (: Blaisr-rnphase). (Neubauer,1994; Froitzheim et al., 1996).The con- temporaneityof extensionin the Austroalpinerealm 4.3.2.Relation betw-een ertensiort and strike-slip aird subduction along the southeastemmargin of faulting the Piemont-Ligurian oceanputs an important con- Neubauer et al. (1995) demonstratedthat in the straint on the tectonic model: Late Cretaceousex- Central Austroalpine area east of the Tauern win- tension occurred in the upper plate of an active dow (Fig. 1), Late Cretaceousextensional shearing subductionzone. and Gosau basin formation were coeval with sinis- A model for Late Cretaceousextension of the tral strike-slipalong E-W- to NE-SW-striking shear Austroalpineregion is constrainedby the following zones.On the other hand, Ortner (199:l) and Eis- observations:(1) regional subsidenceto below-CCD bacher and Brandner (1996) demonstratedthat for- depth concomitantwith extension;(2) connectionof nation of Gosau basins in the Northern Calcareous east-west-directedextension with dextral strike-slip Alps (Fig. l) was reiated to dextrcilslip along faults in the north and sinistral strike-slip in the south; that presentlytrend SE-NW but trendedE-W dur- (i) contemporaneityof extensionwith subduction ing the Late Cretaceous(fig. 4 of Eisbacher and along the Austroalpine-Penninicboundary. Regional Brandner. 1996). Therefore, the extension was ap- extensionof previously thickened crust in orogenic parently associatedwith sinistral strike-slip in the belts is caused by a change in the stress regime more southeriy located Central Austroalpine, and within this crust from deviatoric compression to with dextral strike-slipto the north. in the NCA. deviatorictension. Several models have been put for- ward to explainthis (Fig. 12). In modelsof Fig. 12a- 1.3.3. Contentporaneityof exrettsictnand subductiott d (left side),extension is driven by locally generated In the westernAustroalpine, the extensionalphase forces associatedwith topography.In the orogenic is coeval with subduction of Piemont-Ligurian wedge model (Platt, 1986; Fig. 12a) extensionoc- oceanic lithosphere and continental slivers below curs when the surface slope of the wedge exceeds the Austroalpine continentai crust. As discussed a dynamically stablecon{iguration, for example,due in Froitzheimet al. (1996), the Sesiazone in the to underplatingof material at the baseof the wedge. Western Alps (Fig. 1, inset) probably representsa A decreaseor cessationof plate convergencemay continental slir,'erin the Piemont-Ligurian oceanic also lead to a decreaseof horizontal normal stress basin.near the Apulian (Austroalpine-SouthAlpine) in the orogen and hence to a change from devi- continentalmargin. The widespreadeclogite facies atoric compressionto tension (Fig. 12b; see Mol- metamorphism of the Sesia zone (Compagnoni et nar and Lyon-Caen, 1988). Convective removal of aI., 1911) developed when this continental sliver lithosphere(Fig. 12c;Fleitout and Froidevaux,1982; was subductedunder the Apulian margin. Recent Platt and England, 1994) and slab breakoff (Fig. IZd; geochrorrologicalstudies (Ramsbotham et aL, 1994; Wortel and Spakman, 1992; von Blanckenburgand Rubatto et ai., 1995) indicate that peak pressures Davies, 1995, 1996) both lead to a rapid increasein were reached at or shortly before 65 Ma. Hence surface elevationwhich may result in extensionof the Sesiarocks must have been on their way down the orogen. Models where the extension is mainly the subduction zone at the same time when the dnven by forces createdoutside the extending area Austroalpine crust in the upper plate was severely are sketched on the nght side of Fig. 12 (e*g):

I N. Froit:.lrcitnet aL./Tectottoplr,isic.s 280 (1997) 267-293

gravitational adjustmenl nf nrnnanin rrradno pull-apart extension

^^^^^+i^^ ^{ ^l^+^ uY5>dUUt r ur PrdrE convergence t^+^.^l

srr f1 zT ....j] extrusion 'n 4 fl *'ra convective removal of Iithosphere t))n

@ c subduction rollback slab breakoff

,.,H

d g

Fig. 12. Different modeis explaining regional extension in orogens. (a) Gravitational adjustmentof an unstableorogenic wedge (after Plan. 1986). (b) Cessationof plate convergence.(c) Convectiveremoval of lithospheric root (after Fleitout and Froidevaux, 1982). (d) Sl:rbbreakoff (after von Blanckenburgand Davies, 1996,sJightly modified). (e) Pull-apan extension.(f) Lateral extrusionfrom a collision zone (after Retschbacheret al., 19S9, 1991. Note that this model r.vasdesigned for Miocene, not Cretaceous.extension in the Eastern ,+lps).(S) Subductionroilback 1;rfterHetzel et al.. 1995). Stippled representsmantle lithosphere,black is oceaniccrust. pull-apart extension in a strike-siip regime, lateral to keep the surface slope in the orogenic wedge extrusionfrom a collision zone (Ratschbacheret al., constantin order to maintain the dynamicaliy stable 1991),and subductionrollback (Royden and Burch- configuration (Platt, i986). This processcan there- nel,1989). fore hardly expiain the subsidenceof the surface Gravitationaladjustment of an unstableorogenic of the orogenic wedge to below the CCD, as ob- wedge (Fig. 12a; Plan, 1986), was proposedby served during extensionin the Austroalpine reaim. Ratschbacheret al. (1989) and Froitzheim et al. Secondly,underplating of the Zentralgneissterrane (1994) to explain Late Cretaceousextension in the postdatedthe Late Cretaceousextension of the Aus- Austroalpinerealm. N4orespecificaliy, Ratschbacher troalpine nappes(Kurz et al., 1996). A cessationor et al. (1989)assumed that the extensionwas caused significant decreaseof plate convergence(Fig. 12b) 'Zentralgneiss by underplating of the buoyant ter- may be ruled out as weil becausecontemporaneous rane' exposedin the Tauem window of the Eastern eclogitization of Sesia rocks (see above) indicates Alps (Fig. 1, inset).The model appearsunlikely for ongoing convergence.Convective removal of litho- two reasons.Firstly, gravitational adjustmenttends sphere (Fig. 12c) or slab breakoff (Fig. 12d) may N. Froit:heint et al. / Tectonophysics280 ( 1997) 267-293 have occurred in the CretaceousAlps (von Blanck- ocean,bounded by strike-slip zones on both sides. enburg and Davies, 1996), but we doubt that these The driving force of the rollback is gravity acting mechanismshave caused the extension.In both mod- on a densesubducting slab (Royden,i993). We pro- els, extensionis mainly the result of a rapid increase pose that this model applies to the Late Cretaceous in surface elevation, leading to a high mountain extension of the Austroalpine area (Fig. 13). The chain or plateau (Platt and England, 1994; von marginal,southeastern part of the Piemont-Ligunan Blanckenburg and Davies, 1996). The absenceof oceaniclithosphere which was subductedunder the volumetrically significantmolasse-type sediments of Austroalpine-South Alpine margin during the Late Cretaceousage within the Austroalpine and i.n the Cretaceous(probably beginning at about 100 to 90 neighbouring areas makes the existenceof such a Ma, seediscussion in Froitzheimet a1.,1996), was high range unlikely. Dunng the Cretaceousperiod, in fact dense.It was old (formed since about 165 the Austroalpine area representeda sea with tem- Ma) and devoid of a well-deveiopedcrustal sec- porarily emergenthighs rather than a high mouniain tion, because it had been formed by extensional range (see palaeogeographicmaps in Oberhauser, denudationof subcontinentalmantle (Lemoine et al., 199s). 1987; Froitzheim and Manatschal,1996). The con- Neubauer et al. (1995) proposed that Late Cre- temporaneityof the extensionwith subductionalong taceous extension in the easternparl of the Aus- the plate boundary (see above) strongly favours this troalpine nappes resulted from a releasing bend model. Furthermore,we can explain the association in an east-west-trendingsinistral wrench coridor of extensionalstructures with dextral strike-slip in (Fig. 12c). This cannot serve as a general expla- the north (NorthernCalcareous Alps) and with sinis- nation for Late Cretaceousextension, since exten- tral strike-slip in the south (Central Austroalpine). sion is connectedwith dextral strike-siipalong east- These two strike-slip systems accommodatedthe west faults in other parts of the Austroalpineregron westward retreat of a small segmentof the subbuc- (see above).Lateral extrusion from a collision area tion boundary (Fig. l3c). Typical for a subduction (Fig. 12f.1is unlikely as well becauseLate Creta- rollback scenario is also the low topographic ele- ceousextension in the Austroalpineregion occurred vation (Royden, 1993) of the Austroalpine area in before the continental coilision between Europe the Late Cretaceous,below sea level for the most and the Austroalpine region, when the Piemont- part. Liguria ocean was still open (e.g., Neubauer,1994; The assumedwestward retreat of the subduction Froitzheimet al.. 1996,;. boundaryin the Late Cretaceouswould also explain why Late Cretaceouseast-west extensionis ubiq- 1.3.1. Late Cretaceoussubdttctiort rollback in tlrc uitous in the Austroaipine nappesbut absentin the P ientont-Li guria oceant SouthernAlps: the SouthernAlps were locatedsouth Roydenand Burchfiel(1989) and Royden(19935 of the sinistrai strike-slip fault delimiting the west- proposedthat systematicvariations in the style of ward retreatingsegment of the subductionboundary. thrust belts in the Alpine system resulted from dif- This assumption implies that the dextrally trans- ferent ratios betweenthe rate of subductionand the pressive,Tertiary-age Insubric line (Fig. 1), forming rate of plate convergence.Contemporaneous subduc- the limit between Austroaipine and SouthernAlps, tion and back-arc extensionoccur when the rate of had a sinistral forerunner in the Late Cretaceous subduction of one piate is greater than the rate of (palaeo-Insubricline in Fig. l3c). In fact. there is overall plate convergence.This resultsin oceanward some structuralevidence that Late Cretaceoussinis- retreatof the subductionboundary (subductionroll- tral strike-slipzones exist adjacentto the Periadriatic back, Fig. 12g). Royden (1993) assumedthat this fault in the EasternAlps (Unzog, 1989). processcan affect limited segmentsof a subduction After the stage sketchedin Fig. 13b and c, the boundary while other segmentsof the same bound- main direction of convergencebetween the Aus- ary remain stationary(with respectto a point in the troalpine area and Europe changed from E-W to interior of the upper plate) which leadsto the forma- N-S. In this new kinematic framework, the closure tion of strongly arcuatesegments retreati.ng into the of the Piemont-Lieurian ocean and continentalcol-

I 290 N. Ftrtt.h.eim er al./Tectonoplrysics280 (1997) 267 293

Margna-Sesia wNw f ragment Piemont'Liguria Austroalpine Briangonnais ocean a

Cretaceous collisional ._=\ thrust belt subducted Meliata- Hallstatt ocean Late Cretaceous extens ion - + fto Mt b

W-ward retreat ot subductionboundary extension and dextral + shearing in Northern T Calcareous Alps

Brianconnais

paleo-lnsubric line

'' ''''' extension and sinistral I Piemont-Liquria' shearing in Central Austroalpine area ocea n , :: l, : .../ Southern Alps c

=500 km

Fig. 13. Hypothetical large-scalescenario for the Late Cretaceousevolution of the Austroalpine nappes. (a) Situation at 100 Ma (Trupchun phase). Crustal stacking within the Austroalpine realm results from footwall propagation of thrusting after continental collision along the Meliata-Hallstatt suture. Subduction of Piemont-Ligurian oceanic lithosphere is just about to begin. (b) Situation at 70 Ma (Ducan-Ela phase). Premont-Ligurian lithosphere is being subductedat a rate higher than the overall plate convergence, :ading to oceanward retreat of the subduction boundary and extension of the Austroalpine upper plate by ESE-dipping normal faults, - contemporaneouswith subductionof the Sesiazone to eclogite-faciesdepth. (c) Map view of the situation at 70 Ma. Westwardretreat of the subductionboundary is restnctedto the Austroalpinesegment of this boundary and is accommodatedby a sinistral shearzone to the south ('palaeo-Insubricline') and a dextral shearzone to the north.

lision (: Blaisun phase)took place in the Paleocene 5. Conclusions to Eocene.Final1y, Late Oligoceneto Miocene NW- SE-convergenceled to a reversalof the shear sense The Schlinig fault is a LaIe Cretaceousnormal along the Insubric line which then accommodated fault overprinting a stack of thrust sheetsimbricated dextral strike-slip and south-directedbackthrusting slightly earlier,also during the Late Cretaceous.The (Schmidet al.. 1987). siip direction of the nornal fault is SE to ESE,

I ^1.Froit:heinr et al./Te(tottoplr1,sic.r180 (1997) 267-293 z9l

and the displacementin this direction amountedto References about 17 km. The present dip angle of the fault. measuredparallel to the slip direction, is between Av6 Lallenant, H.J. and Carter,N.L.. 1971. Pressuredependence of quartz deformation lamellae orientations.Arr. J. Sci, 270: 5o and 10'. The initial dip an-cieof the fault was 218-235. less than 20o. Hence, the Schlinig fault represents Böse. E., 1896. Zur Kenntnis der Schichtenfolgeim Engadin. Z. a low-angle nonnal fault. Although apparentll, at Dtsch.Geol. Ges., 48: 557-631. variance with Andersonianfault mechanicsand the Caron, M., Dössegger,R., Steiger,R. and Trümpy, R., 1982.Das seismic record (Jackson, 1987), existenceof such Alter der jüngsten Sedimenteder Ortler-Decke (Oberostalpin) in der Val Tmpchun (SchweizerischerNationalpark, low-angle normal faults is indicatedb}, an increasing Graubün- den). EclogaeGeol. Helr,.,15: 159-169. body of field evidence(e.g. John and Foster,1993; Carter,N.L., Christie, J.M. and Griggs, D.T., 1964.Experimental Wernicke, 1995)to which the presentstudy adds. deformation and recrystallizationof quartz.J. Geol., 12: 681- Shape fabric and crystallographicpref-erred ori- 733. entation in dynamically recrystallizedquartz layels Corrpagnoni,R., Dal Piaz. G.V., Hunziker,J.C., Gosso,G., Lorr- of a Schlinig mylonite record two subsequentde- bardo, B. and Williams, P.F., 197'7.The Sesia-Lanzo zone, a slice of continentalcrust with Alpine high pressure-low tem- formation events,first the top-ESE shearrelated to peratureassemblages in the western Italian AIps. Rend. Soc. Late Cretaceousnormal faulting, and second,short- Ital. Mineral. Petrol.,33(l): 281-334. ening in the NNE-SSW direction related to Early Conti, P, 1992. Tettonicadelle falde Austroalpinenelle Dolomiti Tertiary collisional deformation.The samesequence dell'Engadina: un'ipotesi di lavoro. Atti Ticinensi Sci. Terra, of eventsis reflectedby large-scale,open folding of 35:61-64. Conti, P., 1991. La Falda dell'Ortles: struttura intema e suo the normal fault aroundESE-trending axes. Domino- ruolo nell'evoluzione tettonica delle Dolomiti dell'Engadina style extensionof the competentHauptdolomit layer (Prov. Sondrio. Bolzano e Cantonedei Grigioni). Ph.D. thesis, in the footwall of the Schlinig fault was kinemati- UniversitätBasel. Basel, I 63 pp. cally linked with nomra[ slip along the fault itself. Conti, P., Manatschal,G. and Pfister,M| 1994. Synrift sedimen- Extrusion models which assumedomino extension tation, Jurassicand Alpine tectonicsin the centralOrtler nappe (EastemAlps, Italy). EclogaeGeol. Helv., 87: 63-90. to be caused by thrusting do not apply in this Dal Piaz,G.V., Del Moro, A., Martin. S. and Venturelli,G.. 1988. case. Post-collisional magmatism in the Ortler-Cevedale Massif Late Cretaceousextensional deformation in the (Northem Italy). Jahrb. Geol. Bundesanst.Wien, 131: 533 Austroalpine realm accommodated the formation 551. of the Gosau basins, u,as associatedwith sinistral Drury. M.R.. 1993.Deformation lamellae in metalsand minerals. In: J.N. Boland and J.D. Fitz Gerald (Editors). Defects and strike-slip in the soutl.r(Central Austroalpine) and Processesin the Solid State: GeoscienceApplications. The with dextral strike-slip in the north (Northern Cal- Mclaren Volume, Elsevier,Amsterdam. pp. 195-212. careousAlps), and was coevalwith subductionalong Drury, M.R. and Urai. J.L., 1990.Deformation-related recrystal- the Austroalpine-Penninic boundary.Westward re- liz-ationprocesses. Tectonophysics, 112: 235-253. treat (rollback) of a pafl of this subductionboundary Eiermann, D.R., 1988. Zur Stellung des Martegnas-Zuges.Eclo- gae is proposed to have caused the extension.Ile+-re- Geol. Heh'.. 81: 259-212. Eisbacher,G.H. and Brandner,R., 1996. Superposedfold-thrust structures and high-angle faults, Northwestern Calcareous @ Alps, Austria. Eclogae Geol. Helv., 89: 553-571. Fleitout, L. and Froidevaux,C., 1982. Tectonicsand topography Acknowledgements for a lithospherecontaining densitv heterogeneities.Tectonics, 1:21-56. Froitzheim, N., 1988. Synsedimentaryand synorogenicnormal We thank S. Schmid and H. Stünitz for stimulat- faults within a thrust sheet of the Eastern Alps (Ortler zone. ing discussions,F. Neubauerand F. v. Blanckenburg Graubünden.Switzerland). Eclogae Geol. Helv..81: 593-610. for very constructive reviews, and M. Thöni and Froitzheim, N., 1992. Fomation of recumbent folds during syn- B. Fügenschuhfor some interesting remarks. Sup- orogenic crustal extension(Austroalpine nappes, Switzerland). ported by Swiss National ScienceFoundation grants Geology, 20:923-926. Froitzheim, N. and Manatschal,G., 1996. Kinematics of Jurassic No. 21-25252.88 and 20-29869.90 (N. 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