Festschriftzum 60. Geburtstag von Helfried Mostler Geol. Paläont. Mitt. Innsbruck, ISSN 0378-6870, Bd. 20, S. 389--406 ROLE OF lllGH-ANGLE FAULTS DURING HETEROAXIAL CONTRACTION, INNTAL THRUST SHEET,NO RTHERNCALCAR EOUS ALPS, WESTERN AUSTRIA Gerhard Eisbacher & Rainer Brandner With 7 figures Abstract: During Late Cretaceous/two-stage contraction of sedimentary strata within the Austroalpine accretionary wedge initial fold-thrust detachment and subsequent heteroaxial shortening were controlled by low-strength Stratigraphie heterogen­ eities and by the propagation of transverse high-angle faults. For the Inntal thrust sheet of the Northern Calcareous Alps (NCA) about 20 of NW -directed thrust movement was accompanied by internal shortening and by distributed dextral displacement along NW -striking transfer faults by about 15 to 20 Thrust sheet segmentation along high­ angle transfer faults led to significant relid between stratal panels of variable vergence and accounts for local depositi­ km on in and patchy preservation of Upper Cretaceous syndeformational clastic basins. One of the authors (R.B.) inter­ km. prets orogen-parallel striking normal faults with Upper Cretaceous scarp breccias as an indication of today's NW-SE extension of the early alpine nappe edifice. Superimposed NNE-SSW-oriented heteroaxial contraction in latest Cretaceous-Paleogene time by about 10 reac­ tivated initial transfer faults as high-angle reverse faults, with a new set of NE-striking high-angle sinistral faults pro­ pagating from the footwall into the frontal Inntal hangingwall. This increased the plunge of pre-existing fo lds and pro­ km duced a new set of plunging folds within fault-bounded panels. High-angle faults thus accommodated polyphase shor­ tening of the NCA-wedge and superimposed basins that formed along transverse zones with major structural relief. Zusammenfassung: Während der oberkretazischen, zweiphasigen Einengung des sedimentären Schichtenstapels innerhalb des ostalpinen Akkretionskeiles werden die initialen Abscherhorizonte der Faltenüberschiebungen und die nachfolgenden heteroaxia­ len Krustenverkürzungen durch stratigraphisch vorgegebene Horizonte geringerer Scherfestigkeit und durch propagie­ rende steilstehende Querstörungen kontrolliert. Die -gerichtete Überschiebung der Inntaldecke im Ausmaß von mindestens 20 km wurde sowohl von interner Verkürzung als auch von etwa 15-20 weiten dextralen Seitenver­ schiebungen entlang NW �streichenden Transferstörungen begleitet. Die Segmentierung des Deckenkörpers durch die NW steilstehenden Transferstörungen führte zu einem signifikanten Relief der mit variabler Vergenz gelagerten Schichtsta­ km pel, das die synorogene klastische Sedimentation innerhalb der oberkretazischen Gosaubecken nach sich zog. Einer der Autoren (R.B.) sieht zudem in orogenparallel verlaufenden Abschiebungsstrukturen, die von Scarp-Breccien be­ gleitet werden, einen Hinweis auf eine distensive Gosaubeckenbildung. Die ursprünglichen Transferstörungen werden in der obersten Kreide und im Paläogen durch eine NNE-SSW -orien­ tierte Krustenverkürzung von ca. 10 als steile Aufschiebungen reaktiviert. Ein neues Set von sinistralen, NE-strei­ chenden Blattverschiebungen setzt sich von der Liegendscholle in den frontalen Bereich der Inntal-Hangendscholle fort. Das Abtauehen der präexisitierenden Faltenstrukturen wird dadurch verstärkt, und es entstehen neue Sets von Fal­ km · ten mitabtauchenden Faltenachsen innerhalb der mit Störungen begrenzten SchichtstapeL Steilstehende Querstörungen spielten daher sowohl bei der polyphasen Krustenverkürzung der Nördlichen Kalkalpen, als auch bei den Krustendehnungen eine wesentliche Rolle. / 389 Introduction Teetonic setting and mechanical stratigraphy of the Northern CalcareousAlps (NCA) Fold-thrust belts and accretionary wedges are thought to grow by forward and downwardpropa­ The arcuate Austroalpine accretionary wedge gation of defonnation, which, within more or less (fig. 1) originated during W- to NNW-directed de­ constant fields of regional contraction, results in tachment of both pre-Mesozoic crystalline base­ wedge-shaped cross sections (ÜRIEL & ARM­ ment and Mesozoic platfonnal and basinal strata, STRONG, 1965; BALLY et al., 1966; PRlCE, 1981; 3 to 4 km thick, along the northwestem sector of BOYER & ELLIOTI, 1982; DAHLEN et al., 1984). the .convergent Adriatic plate margin (DIEfRICH, Geometrie details of individual structures within 1976; F , 1987; LAUBSCHER, 1988). Syndefor­ fold-thrust wedges arecontrolled mainlyby varia­ mational Upper Cretaceous clastics deposited on tions in stratal competence, thickness, facies, and defonnedRANK carbonate strata within the NCA are basement configuration prior to the onset of con­ generally referredto as Gosau Group (FAUPL et al., traction (LAUBSCHER, 1965, 1981; DAHLSTROM, 1987; LEISS, 1988). They correlate roughly with 1969, 1970; THOMAS, 1990; GIDSETTI & V , turbiditic successions of adjacent slope arid deep 1988; HARrusoN & BALLY, 1988; McCLAY et al., sea environments (GAUPP, 1982; GAUPP & BAT­ 1989; CAS & PICOTTI, 1990; DARDEAUEZZANI & TEN, 1983; WEIDICH, 1984; WINKLER, 1988; BER­ G CIANSKY, 1990; HUMAYON et al., 1991; ScHöN­ NOULLI & WINKLER, 1990). In Paleogene time the BORN, 1992).TELLARIN Recently, the interaction of major Austroalpine thrust sheets with a basal. carpet of high-angleRA faults with growing foldthrust struc­ ophiolitic melange and slivers of basement were tures in arcuatethrust belts or accretionarywedges emplaced over distal European crust, parts of has been recognized as significant by BENVENUTO which developed into the Penninie and Helvetic & PRlCE (1979), SCHMIDTet al. (1988), NAMSON & basement-cover nappes below the relatively stiff DAVIS (1988), McDOUGALL & KHAN (1990), PEI­ Austroalpine lid(FRISCH, 1979; W EL & FRISCH, et al. (1990), and BITIERLI (1990) among oth­ 1989; LAUBSCHER, 1988; STAMPFLI & MAR ­ ers. In such settings the propagating networks of ER, 1990). Subsequent crustal stackingAIB below this high-angleZI-IEN faults that truncate or interfer with lid induced major Neogene uplift in eastem THALSwit­ fold-thrust structures range in scale from local ex­ zerland (HURFORD et al., 1989; PF'IFFNER et al., tension fracturesand tear faults to major strike-slip 1990), and caused eastward tilting and erosional or convergent transfer faults that relay contraction retreat of the westemmost Austroalpine thrust fromone segment of a fold-thrust belt to another. complexes including the sedimentaryNCA. Along In westem Austria the accretionary wedge of their westem up-plunge termination the NCA the Northem Calcareous Alps (NCA) displays a about 50 km wide and consist of the sedimentary bewildering pattem of high- and low-angle faults Allgäu, Lechtal, and Inntal sheets which are exposedare which developed during latest Mesozoic-Paleo­ in E-plunging synclinal semi-klippen and anticlinal. gene detachment, stacking and final motion of the semi-windows (AMPFERER, 1932; ToLLMANN, 1976). Austroalpine crustal thrust plates towards the To the east the NCA sole thrust dips below sea southem continental margin of Europe (TOLL­ level and has been intersected at a depth of about 1976). Accessibility and outcrop permit a 6 km in thepetroleum exploration well VorderrissI reasonable appraisal of the significance of high­ (fig. l b, BACHMANN & MÜLLER, 1981). Overall angleMANN, structures during the protracted but poly­ shorteningwithin the westem NCA wedge is rough­ phase-heteroaxial defonnation of sedimentary ly 60% (EISBACHER et al., 1990). ·Coal rank and il­ strata at relatively shallow crustal levels. To docu­ lite crystallinity studies within the NCA suggest ment and understand some of the baffling structu­ that thethree main thrustsare warm-over-cold dis- ral relationships we have selected the highest . continuities and that there is a general north-to­ NCA-structure, the Inntal thrust sheet, exposed south increase of paleotemperatures (KRUMM, no�h of the Inn valley, Tirol (fig. 1) . 1984; KRUMM et al., 1988; PETSCHICK, 1989). In- 390 Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995 0München M" .""",. flOOO500 Q . ...... (c) 0 KM 50 ORM PLAiF ( b) A L P u E R N S \) 1 n AUSTROALPINE W E DGE D. Sedimentary Permo-Mesozoic � Pre-Permian metamorphic basement - Arosa ophiolitic melange -+-- (a) Teetonic framework of the northwestem Adriatic plate margin as exposed within the Alps. (b) Present outcrop pattem of the allochthonous Austroalpine cover and basement units of the frontal Adriatic plate which is soled Fig.l:by ophiolitic melange. The transported Late Cretaceous metamorphic isograds for stilpnomelane, epidote, chlorite, garnet, stau­ rolite and the occurrence of eclogite within the Ötztal sheet are after FRANK et al. ( 1987). The main thrust sheets of the Northern Calcareous Alps (NCA) are outlined roughly as is their relationship to the trailing basement units. (c) Sedimentary formations and principal detachment horizons within the NCA wedge. Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995 391 crease of Late Cretaceous regional metamorphism Permo-Triassie shale-evaporite units of the Hasel­ continues southeasterly into basement-cover com­ gebirge-Reichenhall interval.which commonly is plexes located south of the NCA (PuRTSCHELLER pervasively brecciated along major thrust ramps. & RAMML , 1982; HüiNKES et al., 1982) . The Muschelkalk-Wetterstein interval above the The structural relationships between the sedi­ basal detachment consists of massive to thick -bed­ mentary NCAMAIR and the basement-cover complex­ ded reef or platformcarbonates