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.
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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
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(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 about 500to 2000m es exposed on the south side of the Inn River val thick which interfinger locally with thinly bedded ley (fig. 1 b) are still poorly understood. Field evi nodular Muschelkalk Iimestone and regionally dence so far suggests that the large and internally with calcareous mudrock of the much thinner complex Lechtal sheet of the NCA is in primary Partnach Formation." The overlying incompetent but sheared contact with frontal slivers of the Raibl Formation therefore varies from about 100m Silvretta complex in the west and with phyllitic above massive Wetterstein Formation to 750 m rocks of the Paleozoic Grauwacken complex in above the Partnach Formation. lt consists of clas the east (FRANK, 1987; LAUBSCHER, 1989; EIS tic and/or carbonate cycles including an upper BACHER et al., 1990; ROCKENSCHAUB, 1990). The evaporitic interval which constitutes the detach Allgäu sheet therefore is considered to be a lower ment horizon for the overlying Hauptdolomit For Lechtal imbricate, while the Inntal sheet is prob mation. The Hauptdolomit Formation is a compe ably a trailing upper imbricate of the Lechtal tent but weil bedded dolostone unit, about 1500 m sheet (Fig. 1 b ). Because the Ötztal complex lies thick, which, towards the top, changes into calcar structurally on top of the southeastem Silvretta eous Plattenkalk facies, bituminous Seefeld facies complex (THöNI, 1981; Se & HAAs, 1989) or calcareous mudrock of the Kössen Formation. we presume that movement of the Ötztal base All these serve as upper detachments for the thin ment was kinematically relatedi-IMID to motion of and bedded Jurassic-Cretaceous succession above. For deformation within the Inntal sheet. Alternatively, the sake of convenience the uppermost Triassie a correlation of the Ötztal complex and the Lech Kössen shale-lirnestone couplet has been grouped
tal sheet would be possible too. The sedirnentary · with the Jurassic-Cretaceous succession in the contact of the lnntal sheet with the Grauwacken sketch maps. Jurassic..:cretaceous strata are as thin zone, a variscan deformed, low-grade metamor as 100 m in platformal settings and as thick as phic basement complex, along the lower Inn val 1000 m in basinal settings. A thin radiolarian chert ley area supports this interpretation by one of the unit, the Ruhpolding Formation, serves as conven authors (R.B.). The increasing grade of metamor ient structural marker. Within the NCA, Gleavage phism in the Ötztal cover-basement complex im developed only in anchimetamorphic argillaceous plies that a thick thrust mass must have existed on units of the southernmost outcrop belt. top of the Ötztal complex towards the southeast. The Quartz Phyllite zone along the Inn valley (Fig. 1 b) is a sub-Grauwacken low-grade meta morphic sliver of unknown provenance, but pos Geometry of the lnntal sheet sibly was originally situated in front of the NCA platform (TOLLMANN, 1976). . The ENE-trending frontal thrust trace of the lo Within the NCA the development of flexural bate Inntal sheet can be mapped for about 100 km slip folds, blind or emergent thrusts, folded sole along strike (fig. 1). To the east it is deflected thrusts, and high-angle transfer structures was sharply to the southeast and disappears in the Inn controlled by the behaviour of two mechanically valley, while to the west erosion has created a dominant carbonate units of Triassie age: the large E-plunging semi-window near the Inn valley Muschelkalk-Wetterstein interval and the Haupt defining the main body of the sheet. The sheet is dolomit Formation (Fig. 1 c ). Basal detachment of about 20 km wide in the east and 10 km wide in the NCA occurred along the 100 to 300m thick the west and towards the area beyond it is repre-
392 Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995 sented by several erosional klippen (e.g. Krabach Thiersee syncline (NAGEL, 1975; CHANNELL et al., klippe in Fig. 1). Within the broadly synclinal Ion 1992). The Achental thrust structure probably nu tal sheet a westward decrease in the proportion of cleated along a roughly NE-striking synsedimen Muschelkalk-Wetterstein carbonates and a west tary normal fault zone of Jurassie age (CHANNELL ward rise in the position of the sole thrust from et al., 1990) and propagated from below the over within the basal Haselgebirge-Reichenhall inter tumed forelimb of the Unutz hangingwall anti val to the Raibl and upper Hauptdolomit Ievels is cline into tightly folded Jurassic-Cretaceous strata expressed in a change of wavelengths of subsidi of the Thiersee syncline. There the m- to 100 m ary fold-thrust structures from about 5 km in the scale folds developed by stratal detachment above east to around 3 km in the west. High-angle trans the Hauptdolomit Formation. The Achental thrust, fer faults within the Iontal sheet are also spaced exposed east of Achenkirch, is. associated with progressively closer towards the west. Near the semipenetrative shear veins in Lower Cretaceous Ion valley proximity of pre-Mesozoic basement argillaceous footwall. Calcite fibre lineations indi and pronounced facies changes coincide with cate a mean N- to NNW-directed motion perpen backthrusts within both the Iontal and Lechtal dicular to the general ENE-striking thrust trace sheets (see below). The Ötztal, Silvretta, and (fig. 2, diagram 3). The thrust disappears andlor at Grauwacken basement complexes thus represent least los�s displacement both to the east and west northward tapering backstops for both Iontal and within a strike distance of about 15 km. A strike Lechtal sheets (EISBACHER et al., 1990). Since the projection of steeply dipping to overtumed Ju Iontal sheet appears to have been detached along rassic-Cretaceous strata (e.g. Ruhpolding chert the trailing portion of the Lechtal sheet, progres marker) into the area beneath the .central thrust sive detachm�nt, high-angle faulting, intemal de segment indicates minimum forward thrust dis formation, and heteroaxial overprinting are dis placement of about 7 km. This motion was ac cussed by proceeding from the Achensee area on compailied by propagation of and dextral dis the east to tbe westemmost exposures of the sheet. placement along NW-striking high-angle transfer faults. Some rotation around local vertical axis is also suggested by paleomagnetic data from Lower Achensee area Jurassie redbeds (CHANNELL et al., 1990). To the west the Unutz- Achental structure is truncated The Achensee area encompasses the trailing by the dextral Pertisau transfer fault which also edge of the Lechtal sheet east of the Iontal thrust offsets the Ebner backlimb thrust by about 6 km. trace (fig. 1). The area is dominated by the NW Along the transfer faults displacement is accom verging composite Unutz anticline - Achental modated between the Unutz- Achental structure thrust structure that is segmented by NW striking on the east and the Iontal thrust on the west. The high-angle transfer faults, the most prominent of northwesterly strike of the eastemmost segment of which are the Issalm, Kramsach and Pertisau the Iontal thrust tracesuggests that it also originat faults (fig. 2). The Unutz-Achental-structure re ed as a dextral transfer fault. presents an arrested stage of fold-thrust detach Throughout the Achensee area there are numer ment along the trailing portion of the Lechtal ous superimposed folds with axial surfaces orient sheet, a situation quite similar to that which prob ed at relatively high angles to the trend of the� ably preceded detachment of the Iontal sheet to Unutz- Achental structure (fig.. 2). They are par the west. The SW-plunging Unutz anticline is ticularly prominent in the upright limb of the cored by competent Wette�tein and Hauptdolomit Thiersee syncline. Their geometry also deter carbonates with overlying Jurassic-Cretaceous stra mined the lobate erosional geometry of the ta changing from relatively thin platformslope fa- Achental thrust trace. Superimposed contraction . cies in the Rofan fold train of the hangingwall to appears to have been substantial in Achental han considerably thicker basinal facies in the adjacent gingwall strata as well and is indicated by strongly
Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995 393 I ( 1) (2}
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RAI BL FORMATION !NNTAL THRUST - ( KARNIAN --�: MUSCHEL KALK- WETTERS TEIN -470 2J' )10Schwaz PARTNACH FORMATION � (ANISIAN-l LADINJAN) � . HASELGEBIRGE w REICHENHALL FORMATION ( PERMIAN-ANISIAN) ·.� Pi\\ 0 11°1.5' L= l '"O::;':J�::-.-::c_r.:::,.======l
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394 Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995 NW-plunging folds in Hauptdolomit Formation early NW-SE-oriented contraction and associated along the Achensee shore (fig. 2). Pre-existing propagation of NW-striking dextral transfer faults NW-striking high-angle transfer faults were local from the trailing edge of the Lechtal sheet com ly reactivated as reverse structures. In suitably menced in mid-Cretaceous time at about 100 Ma. oriented stratalpanels bordered by faults.the hetero Pronounced subsidence along high-angle transfer axial contraction thus produced folds whose plung faults occurred during the Late Cretaceous, bet es were clearly determined by pre-existing stratal ween 90 and 80 Ma. A superimposed NE-SW dip (fig. 2, diagrams 1 and 2). oriented contraction probably caused stratal short Timing of initial detachment and heteroaxial ening and uplift of the deposits after 80 Ma. Final superposition of fold-thrust structures in the ly, the NE-striking Ernbach fault along the Inn Achensee area are constrained by rernnants of River valley truncated a11 structures of the NCA syntectonic clastics. Thus, the youngest deformed and borders a lower Oligocene clastic basin strata in the Thiersee syncline are Albian calcare- (STINGL, 1990). The superimposed contraction . ous mudrocks which locally are interbedded with therefore is crudely constrained between about 80 conglomeratic channel deposits composed of and 30 Ma. The approxirriate timing of. the two Iimestone clasts derived from a nearby Lower deformation events and the role of high-angle Cretaceous shallow water platform (WEIDICH, transfer faults within the trailing Lechtal sheet is 1984). Therefore, closure of the syncline and relevant with respect to the timing of initial de emergence above sea level of anticlinal structures tachment, emplacement, and deformation of the probably commenced by late Albian time. Upper Inntal sheet to the west. Cretaceous ( Coniacian-Santonian) nonmarine marine Gosau .Group strata of the Brandenberg Basin rest unconformably on Triassiestrata down to the level of the Wetterstein Formation and Inntal sheet straddle the high-angle Issalm transfer fault. De tailed biostratigraphic work of HERM et al. (1979) a) Eastern Inntal sheet shows that the trace of the Issalm fault separates a shallow-water carbonate-clastic facies. domain of In the easternmost Inntal sheet open W- to SW Coniacian age on the northeast from a turbiditic plunging folds are truncated by NW-strikinghigh deeper water clastic facies of Coniacian-Santonian angle faults and by the NW-striking lateral border age on the southwest. Depositional relief of about of the Inntal thrust sheet (fig. 3). The main ENE 500 m, inferred from the paleoecology of the two striking frontal thrust is overlain by multiple NW domains (HERM, 1992), is still reflected by the directed imbricates of competent Muschelkalk structural relief of the basal unconformity. Post Wetterstein strata. Intemally, the geometry of the Gosau deformation of the Brandenberg strata pro sheet is govemed by the major Mieming anticline duced a NW-trending anticline-syncline pair Autal syncline pair which is also offset by several (fig. 2). Similar trending folds were observed in WNW- to NW-trending dextral transfer faults and other rernnants of the Gosau Group a few kilome by the Seefeld axial depression (fig. 3). Most of tres to the north. We therefore propose that the the dextral high-angle transfer faults do not extend
Geologie sketch map for the trailing Lechtal sheet in the Achensee area, east of the Inntal thrust trace (see fig. 1). Note the major NE- to ENE-trending Unutz-Achental-Thiersee structures and related NW-striking highangle transfer faults, one of which controlled the location of the syndeforrnational Upper Cretaceous Brandenberg basin. Superimposed WNW-trending folds whose plungeFig. 2: and geometry deterrninedthe eventual erosional outcrop pattem are also prevalent on the mesoscopic scale as shown by ster eonet diagrams for dominant stratal dips (squares) and asymmetric folds (vergence by circle around the fold axis dot) in the areas west (a) and east (b) of Achenkirch. Diagram (c) illustrates stratal dips (squares) ahd trend of shear vein lineations (arrows) in argil laceous footwall rocks below the Achental thrust east of Achenkirch.
Geol. Paläont. Mitt. lnnsbruck, Bd. 20, 1995 395 into the Lechtal sheet beyond the Inntal thrust NNE-directed motionof theInntal sheet by at least front. The trailing edge of the sheet is character about 4 to 8 kmis suggested by Muschelkalk-Wet ized by the major S-directed Halltal backthrust terstein klippen and by the northnortheasterly over and related imbricates which, in the past, have tumed footwall panel including the earlier formed been interpreted as parts of the footwall below a Eng backthrusts. Along the original thrust front 'freely floating' Inntal sheet (TOLL , 1976). near Mittenwald the NE-striking sinistralIsar trans However, the Halltal backthrust clearly cuts up fer fault of the footwall accommodated the advanc sectionfrom west to east and progressivelyMANN deeper ing Inntal thrust, the Jura-Cretaceous footwall out footwall imbricates contain a higher proportion of crop was closed and backthrusts were tilted into a basinal Partnach mudrock facies in place of Wet vertical orientation. West of the Isar fault zone, the terstein platform carbonates (fig. 3). Also, a south Zugspitzebackthrust, continued to move in a south ward rising basement surface below the basal southwesterly direction and backtilted the frontal evaporitic Haselgebirge-Reichenhall detachment, Inntal thrust. Thus, NNE-directed motion of the indicated by refraction seismic interfaces with ve Inntal sheet in the east, sinistral displacement trans locities of up to 6 km s-I near the Inn valley (AN fer in the centre, and south-southeasterly-motion of GENHEISTERet al., 1972), suggests a basement con the Zugspitze backthrustin the west a11 accommo trolled backthrust zone within the southernmost dated NNE-SSW-oriented contraction amounting Inntal sheet. Partnach mudrock facies also domi to about 5 to 10 km. This pattem of contraction was nates imbricated panels of Triassie strata above superimposed onto NW-directed structures which the Grauwacken zone basement exposed south of had developed in the Inntal sheet during earlier the Inn River. thrust motion. Superimposed deformation also ac Because the eastem end of the Inntal thrust stri centuated the Seefeld axial depression andaccounts kes into the Inn valley a potential footwall cutoff for the bulbous plunging geometry of the Mieming structure for the lnntal front is difficult to define. anticline. . However, the complex structure in steeply N-dip ping to overtumed Wetterstein stratain the Lechtal sheet east of Schwaz indicate that detachment of b) WesternIontal sheet Wetterstein-Muschelkalk strata probably occurred along tr�nd to the west, somewhere in the vicinity Towards its westem up-plunge termination of the present Inn valley. This would imply a both hangingwall folds and related branch thrusts NNW- to NW-directed thrust motion of at least in the Inntal sheet are cut and offset by NW-strik 20 km. In the footwall east of the main Inntal ing high-angle transfer faults (fig. 4). Dextral off thrust trace massive Wetterstein facies also chang sets on individuaLhigh-angle faults are generally es northerly into Partnach mudrock (DONOFRIO et small, tend to decrease from southeast to north al., 1980) and we therefore infer that most of the west, and most of them do not seeVI to extend be Triassie footwall below the Inntal sheet consists of yond the frontal lnntal thrust. Between the major Wetterstein-Hauptdolomit carbonate facies. Dur Telfs transfer zone and the syntectonic Muttekopf ing progressive contraction the Eng- and Zugspit Gosau basin distributed dextral separation of the ze backthrust zones in the footwall carried the Tarrenz synclinal axial surface and the SSE-dip Wetterstein-Partnach facies transition southward ping Raibl Formation is in the order of 10 to over Jura-Cretaceous strata which constitute the 20 km. Towards the trailing edge of the Inntal immediate footwall of the Inntal thrust sheet (sec sheet high-angle transfer faults separate stratal tians AB and CD in fig.3). panels with variable structural vergence (fig. 5). Post-thrust emplacement deformationof the Inn Near the locality of Imst, shortening is accommo tal sheet probably continued within a changed field dated within the strongly NW-vergent Tarrenz of contraction and resulted in the presently exposed syncline, by the SSE-vergentTschirgant backfold hangingwall-footwall pattem. Towards the east thrust structure (NIEDERBACHER, 1982), and by
396 Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995 ..... Vl ::> a:: ..... w I� w..... ljz � f-Vl� t. ;-;;:::;��:/21 � 1 I
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::::> ..... w �-� _,w "' z 0 w� z- �oz :::EZ <>:4 :Z:;::::E� •cro ..,_,z :So<( <>=-'4 4"--' ><:r -'<..>Z w �- z4 :ü��UJ44 w:r"' Vl w�� "'a:z V'l�UJ 44 4wn. :::En.- :rcr- äi�� � Wi . ����I � z z 0 ;::: 4 :::E4� o- :::E _,4 er er- z o� o o� cr u.� �w _,z ��O..a_ mo: ::::>a.. - "' w 1/ll< ::::><>: 04 �:::E > 4"' "' �z 4 uJ Geologie sketch map of the eastem Inntal sheet. Note the ENE-trending fold thrust structures within the Inntal sheet which are truncated by the NW-striking eastem segment of the Inntal sole thrust and by other high-angle dextral transfer faults. Superim posed heteroaxial contraction with SSW-directed motion of the Zugspitze backthrust and NNW -directed motion with overtuming of Fig.the Eng 3: footwall backthrusts is illustrated by the two cross sections. These motions were accommodated by sinistral-convergent dis placement transfer amounting to about 5 to 10 and including the SW-propagated Isar fault and others which cut the earlier NW striking faults. km Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995 397 1.7030'+ 11°oo· OUATERNARY RAIBL FORMATION THRUST FAULT D - I KARNfAN I (OVERTURNEOJ MUSCHELKALK- WETTERSTEIN ANTICLINE, N + SYNCLINE � i!�;�,!;�:�;,�;"Z�� HASELGEBIRGE- _T' ATION (-l-) V �i������-�����:�) la "'ui \._.- V/l HAUPTDOLOMIT FORMATION BASEMENT '\ LL__Lj IUPPER TRIASSICJ - B � N +f.7°os· KM 10 11028' Geologie sketch map of the western Inntal sheet Note that ENE-trending f9lds and the frontal thrust are displaced by numer ous NW-striking transfer faults which appear to have propagated in a northwesterly direction. A diving footwall fold is outlined by the Ruhpolding chert marker east of Madau. Erosional remnants of the Upper Cretaceous clastic Muttekopf basin are Jocated along Fig.a major 4: dextral transfer fault and in front of the Larsenn klippe, whose emplacement could have occurred as part of the fürward mo tion of the Ötztal sheet and thus may have enhanced local tectonic subsidence. Erosion along the Starkenbach footwall backthrust has exposed a potential footwall cutoff for Hauptdolomit Formation in the frontal Inntal sheet and the Silvretta basement units which are characterized by retrograde semi-penetrative Stretching lineations (shown as arrows) and indicate dextral shear. emplacement of the Larsenn klippe. Loading by mediate Jurassic-Cretaceous footwall suggest a the Larsenn thrust may have aided local subsi broadly NNW- to NW-directed thrust detachment dence of the Murtekopf basin along a major high- · and emplacement of the Inntal sheet), as inferred angle transfer fault (fig. 4). To the west transfer by TOLLMANN (1976). There is strong evidence faults are seen to cut footwall strata below the Inn that contraction of footwall strata below the Inntal tal sheet. They also merge with W-trending serni sheet began while forward motion of the sheet penetrative dextral shear zones within the steeply was still going on. Near the locality of Madau (fig. dipping transition zone between the Lechtal sheet 4) the footwall geometry of the Jurassie Ruhpold and Silvretta basement rocks. The prevailing ing marker, frrst mapped by ÄMPFERER (1932), NNW- to NW-vergence of subsidiary folds both demonstrates that during development of the in hangingwall Hauptdolomit strata and in ·the im- Sax er thrust -anticline in the footwall the J urassic- 398 Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995 LorPffl ,,c#'OTZTAL 0COMPLEX � © E:;;) r��'i��? RI4ATION z :����� - :l ATIOH � �:'fC� A "'''''�I =�:��!� F�KÄ:;:.��'rTEIN- (ANISIAN-LADINIANl ••• E ro ·1 ��� �:CEOUS) 144>�I �:.���C:, .!'���RI4ATIOH I ,...... , I ® - JURA-CRETACEOUS YARISCAN 8ASE14ENT . Three partial sections which displaY,'thevariable vergence and structural style of individual stratal panels separated by high angle transfer faults within the trailing portion of the westem Inntal sheet (for location see fig.4). Fig. 5: Cretaceous strata along the anticlinal crest were a11 footwall stnictures (EISBACHER et al., 1990). extended below the moving Inntal sheet and rolled Other NE-striking sinistral faults which propagat forward into a NNW-diving anticline filling the ed from the Lechtal sheet southwesterly cut both core of an adjacent syncline before motion of the the Inntal thrust front and the earlier developed hangingwall was impeded by footwall backthrust NW-striking dextral transfer faults. Near the Star ing along the Burkopf and Starkenbach back kenbach backthrust which is a cold-over-warm thrusts. The latter facilitated erosion of . an E paleogeothermal discontinuity (KRUMM et al., plunging semi-window revealing the probable 1988) cleavage in argillaceous units was refolded footwall cutoff of Hauptdolomit strata at a dis into E-W-trending 'retrograde' kink bands. tance of about 20 kmto the southeast of the frontal Inntal thrust trace. Thus, NW-directed thrust dis placement of at least about 20 km, similar in mag c) Timingof contractionand displacementtrans nitude to the eastem Inntal sheet, can be inferred fer within the Iontal sheet for the westem part of the sheet. However, dis placement by thrusting was accompanied by the Timing of detachment and intemal deformation equally significant longitudinal segmentation of of the Inntal sheet are relatively weil constrained. the thrust sheet along dextral transfer faults. These Jurassic-Cretaceous footwall strata in the Puitental seemingly propagated into the direction .of thrust zone near the Zugspitze backthrust contain mafic sheet motion which was accompanied by some dikes which intruded at about 100Ma and suggest footwall contraction below the sheet. Backthrust a broadly extensional framework (ThOMMSDORFF ing along the Burkopf structure probably occurred et al., 1990). Locally derived olistostromes of within a slightly changed field of contraction and probable Cenomanian age are overridden by the was transferred from the broadly SSW-directed westem Inntal sheet near Madau, while on top of Zugspitze thrust via the SW-propagating sinistral the sheet syndeformational Gosau Group of Conia Loisach fault and caused significant tightening of cian to Maastrichtian age (about 90 to 70 Ma) was Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995 399 0 km 10 Approximate restoration of the incipient Inntal thrust and trailing Ötztal complex relative to the LechtalSilvretta complexes. Vitrinite reflectance values Rmax in the Raibl Formation (after 1989) and transported metamorphic isograd are also re stored and demonstrate that NW-directed thrust and distributed dextral transfer displacements in the Inntal sheet during the Late Fig. 6: die'üicebus amounted to about 40 and that they increased towards the west. About 10 of NE-directed motion is implied for the superimposed contraction event. PETSCHICK, km deposited unconformably on Hauptdolomit Forma stretching lineations that indicate oblique dextral tibn in the synclinal Muttekopf basin (LEISS, 1988; motion. The WNW- to NW-striking high-angle ÜRTNER, 1992, 1994) ). faults which propagated fromthe Inn valley into This basin contains coarse mass flow deposits the Inntal thrust sheet probably created the relief with huge limestone slabs of Kössen reef facies for sedimentary source areas and transfer fault ba and fining-upwards cycles of turbiditic sandstones sins, before continued contraction and backthrust and conglomerates containing detritus derived ing elevated:. the entire Inntal edifice above sea from pre-Triassie basement (WOPFNER, 1954) and level duringlatest Cretaceous-Paleogene time. ultramafic source areas (DIEfRICH & FRANZ, A palinspastic model for the Inntal sheet in 1976). Since WNW-directed thrusting of the Ötztal which about 20 km of NW-directed thrusting, 10 basement complex has been dated also at about 90 to 20 km of distributed dextral shear along high to;70 Ma (THöNI, 1988; Sc & HAAs, 1989), it angle NW-striking faults, and about 5 to 10 km of is probable that emplacement and deformation of NNE-directedsuperimposed thrusting restoredare the Inntal sheet was driven HMIDby thrust motion of the shows an original NE-trending thrust front with Ötztal complex onto· the Silvretta complex. The the realigned coal rank values of PETSCHICK small Larsenn klippe (fig. 5) could have been part (1989) for the Raibl Formation (fig. 6). A NW- to ofthefrontal cover ofthe Ötztal complex. Numer WNW:..directed motion of at least about 40 km for ous semi-penetrative and/or chlorite-coated brittle the sheet amounts roughly to the thrust overlap of shear zones in both Silvretta and Ötztal basement Ötztal over the Silvretta complex, as inferred by rocks display striations and semipenetrative Sc & HAAs (1989). HMID 400 Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995 (a) fb) (a) Model for the development of erosional source areas and small syndeformational clastic basins adjacent to high-angle transfer faults within growing foldthrust belt or accretionary wedges, as displayed by the Brandenberg and Muttekopf basins of the west Fig.ern 7: NCA. The strong overprint by intragosauian normal faulting is not shown in this model. (b) The effect of heteroaxial contraction on pre-existing folds, thrust faults, and high-angle transfer fauJts as displayed by structures within and adjacent to the Inntal sheet of the NCA wedge. Transfer faultbasins in accretionary settings rection of regional contraction and since fold thrust structures along several high-angle faults Although uplift and erosion down to the Ievel show variable vergence it can be inferred that the of Triassie formations seem to have accompanied shear stresses along the faults were low. Relative initial fold-thrust detachment along the trailing vertical displacements along the transfer faults edge of the Lechtal sheet and within the Inntal thus could have led to the juxtaposition of high sheet foundering of short-lived syntectonic basins standing anticlinal sedimentary source areas and occurred soon thereafter. Although the Upper Cre low lying synclinal depositional sinks. High-gra taceous Brandenberg and MuttekopfGosau basins dient sedimentary transport could occur across developed in synclines which are located behind .and low-gradient transport along high-angle majot thrusts they are also located adjacent to faults. The developing basins therefore contained . high-angle transfer faults. Since the high-angle elements of both piggyback basins (ORI . & faults appear to have propagated roughly in the di- FRIEND, 1984) and strike-slip pull-aparts (CR.ow- Geol. Paläont. Mitt. Innsbruck, Bd. 20, 1995 40 1 ELL, 1974) depending on the local angle between sheet, continuing shortening appears to have oc the strike of the transfer fault and the general di curred within a changed NNE-SSW-oriented field rection of contraction. The bewildering sedirnen of contraction, resulting in complex hangingwall tary assemblages rangjng from fluvial conglomer footwall interference and in the southwesterly ates and small bioherms to deep water turbidites, propagation of new NE-striking high-angle sinis the highly variable sedimentary provenance, and tral faults. This second structural event was pos the short subsidence history which was followeo , sibly also responsible for the downward change in by irregular uplift are possibly all characteristic för · the orientation of stretching lineations from transfer fault basins in other accretionary settings · WNW-ESE to N-S within the Arosa melange as well. Subsidence is probably strongest near re near the westem NCA-sole thrust (RING et al., leasing bends of transfer faults and need not have 1988) and could have been related to early Paleo been accompanied by large strike-slip displace gene (?) irnpingement of the NCA-wedge against ments (see also Wo L, 1991). The resulting the European continental margin. structural pattem in the sedimentary ftll of transfer The presence of both high-angle and low-angle basins reflects both synandRRAL post-depositional con faults substantially eased heteroaxial overprinting. ditions. Fig. 7 is a model for the initial develop Thus, inclined stratal panels, confined laterally by ment and subsequent deformation of a transfer high-angle faults, responded to the changed pat basin within a changing field of contraction as vis- tem of contraction individually and thereby the / ualized for the example of the Brandenberg basin 'corrugated iron' effect of pre-existing fold trains within the trailing Lechtal sheet of the NCA. was reduced. In other area pre-existing stratal dips The southem margin of the Brandenberg basin were translated directly into plunge of newly de is formed by . a strong intragosauian normal fault veloping folds. Also, pre-existing high-angle stri with an offset of about 1 km, along which mega ke-slip faults accommodated reverse displace breccias accumulated (B R & ÜRTNER, ment. New transfer faults nucleated preferably 1995). It is assumed by one of the authors (R.B.) along favourably oriented axial surfaces of pre-ex that strong-subsidence in a RANDNENW-SE oriented field isting flexural-slip folds and propagated south of distension overprinted older basin structures. westward thus transferring displacement onto The normal fault is lacking in the earlier version of major backthrusts which impeded further thrust the basin model in fig. 7. motion, but accommodated shortening at deeper Ievels of the NCA wedge. High-angle faults and progressive heteroaxial fold-thrust development Conclusions Progressive contraction of the mechanically Within the NCA accretionary wedge low-angle heterogeneaus Permo-Mesozoic sedirnentary suc fold-thrust detachment and high-angle transfer cession within the frontal Austroalpine accretion faulting interacted to accommodate both initial ary wedge was achieved by partitioning of region motion and post-emplacement deformation of the al deformation onto fold-thrust and high-angle Iontal thrust sheet of the Northem Calcareous transfer structures. This partitioning frrst accom Alps. Thus,- early NW-directed thrust motion modated NW -SE-oriented contraction along an amounted to about 20 km and was accompanied arcuate belt of sedirnentary strata with associated by distributed high-angle dextral shear that stacking of crystalline thrust sheets between about. amounted to about 10 to 20 km and affected both 95 and 80 Ma (see also FRANK, 1987; ScHMill & advancing hangingwall and overridden footwall HAAs, 1989; RATSCHBACHER et al., 1989; THöNI, strata. Since high-angle transfer faults formed the 1988). As displayed by the structure of the Iontal border between thrust panels with different struc- 402 Geol. Paläont. Mitt. lnnsbruck, Bd. 20, 1995 tural vergence theresulting structural relief within Columbia. - Canadian Petroleum Geology Bulletin, the detached lnntal sheet and along the trailing 27, 360-394. Lechtal sheet led to subsidence of syntectonic BERNOULLI, D. & WINKLER, W. (1990): Heavy mineral Upper Cretaceous transfer fault basins. Orogen assemblages from Upper Cfetaceous South- and Aus parallelextensional faultingoverprinted the earlier tmalpine flysch sequences (Northern Italy and Soutb nappe pile and the supposed structural relief and ern Switzerland): Source terranes and palaeotectonic accommodated Gosau basin sedimentation (in the implications. - Eclogae geol. Helv., 83, 287-3 10. view of one of the authors- R.B.). During super BITTERLI,T. (1990): The kinematic evolution of a classi imposed NNE-SSW-oriented heteroaxial contrac cal Jura fold: a reinterpretation based on 3-dimen tion pre-existing high-angle faults eased develop sional balancing techniques (W eissenstein Anticline, ment of new plunging folds in tilted but laterally Jura Mountains, �witzerland): Eclogae geol. Helv., fault-bounded stratal panels and induced SW-di 83, 493-5 11.. rected propagation of sinistral transfer faults and BaYER, S.M. & ELLIOTT, D. (1982): Thrust systems: backthrusting within footwall strata by about 5 to Amer. Ass. Petrol. Geol. Bull., 66, 1196-1230. 10 km. Heteroaxial structural overprinting was BRANDNER, R. & ÜRTNER, H. (1995): Polyphase basin probably related to latest Cretaceous(?)-Paleogene formation and inversion in the western Northern Cal impingement of the NCA-wedge against the outer careous Alps. - Abstracts, 'Workshop on Alpine European platform and terminated deposition Geology', 87-88, Basel. within the transfer fault basins. Transfer fault ba CASTELLARIN, A. & PICOTTI, V. (1990): Jurassie tectonic sins such as those hosting the Upper Cretaceous framework of the eastern border of the Lombardian Gosau Group within the westem NCA aretypical basin. - Eclogae geol. Helv., 83, 683-700. ly heterogeneaus in composition, variable in sedi CHANNELL, J.E.T., BRANDNER, R., SPIELER, A. & SMATH ment provenance and short-lived. In other accre ERS, N.P. 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