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<:+ + + + _