Local and Far Field Stress-Analysis of Brittle Deformation in the Western Part of the Northern Calcareous Alps, Austria

Geol. Paläont. Mitt. Innsbruck, ISSN 0378–6870, Band 26, S. 109–136, 2003

LOCAL AND FAR FIELD STRESS-ANALYSIS OF BRITTLE DEFORMATION IN THE WESTERN PART OF THE NORTHERN CALCAREOUS ALPS, AUSTRIA

Hugo Ortner

With 25 figures and 1 table

Abstract Brittle deformation was analysed in 86 stations with in the western Northern Calcareous Alps (NCA). Six paleostress tensor groups were defined, which constrain the deformational history since onset of Alpine deformation and can be related to distinct stages of deformation in other parts of the orogen. The superposition of deformational events with stress tensors of the same geometry, but different ages was estblished. The evolution of the far field stresses (1st order stress), which are thought to be related to plate movements, was simple: Cretaceous NW-directed compression was followed by Paleocene/Eocene NNE-directed compression, which in turn was replaced by Oligocene/Miocene NNW-directed compression. Local stress fields (2nd order stress) complicate the picture: Cretaceous NW-compression was partitioned into NNW- directed compression in the NCA and W-directed compression in the Central Alps. Oligocene/Miocene NNW-directed compression was superposed by secondary stress fields related to lateral escape of crustal blocks during formation of the metamorphic core complex of the Tauern Window: N- to NNE-directed compression near large faults delimiting eastward moving blocks and E-directed extension. Third order stress fields are related to folding. The succession of these 3rd order stress fields is not systematic. Additionally, stress tensors related to Late Miocene E-W compression were recorded, which are related to collision in the Carpathian orogen.

1 Introduction This paper presents the results of an extensive field study on soft-sediment and brittle defor- The study of brittle deformation in polyphasely de- mation in the northwestern part of the Eastern formed orogens has three fundamental difficulties: (i) Alps. The results are compared to published data on the problem to separate inhomogeneous fault data brittle deformation of the Southern and Eastern sets into homogeneous subsets without previous kno- Alps, and some principles for interpretation of re- wledge of the shape of the involved stress tensors; (ii) sults of regional studies of brittle deformation the ages of paleostress tensors are usually poorly defi- are deduced. ned, in most cases only relative ages between indivi- dual deformational events can be defined; (iii) the interpretation of the deduced stress tensors either being 1.1 Geological setting equivalent to far-field stress parallel to the plate movement vector or representing local stress fields crea- 1.1.1 Deformation within the Northern Calcareous ted inside or near large shear zones. Timing of defor- Alps (NCA) mations can be established when investigating deformation in successively younger syntectonic sediments The thrust sheets of the NCA are the highest tec- (e.g. Kleinspehn et al., 1989) and soft-sediment defor- tonic units of the Austroalpine nappe complex. De- mation, occuring prior to complete lithification of the formation related to Alpine orogeny started in the rocks (e.g. Petit & Laville, 1987). Comparing brittle de- early Late Cretaceous, when the principal thrust formation in different parts of the orogen, e.g. in the architecture of the NCA was established by (N)NW- internal and external part or in the foreland- and the directed thrusting (Eisbacher & Brandner, 1996; Ort- hinterland directed wedge can help to distinguish bet- ner, 2001) in the foreland of the closure of the Meli- ween local and far-field stresses, because the different ata ocean (Froitzheim et al. 1994, 1996; Neubauer parts of the orogen may be affected by different pro- et al., 2000). Continental collision between the cesses, and the associated stress fields should only be (upper) Adriatic microplate and the European recorded in the respective part of the orogen. (lower) plate related to closure of the Penninic

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Fig. 1: Simplified geological overview of the Eastern Alps. Thick black lines delineate young (Oligocene and Miocene) faults. TN = Turba normal fault, EL = Engadine line, LF = Loisach fault, IF = Isartal fault, BN = Brenner normal fault, WP = Wipptal fault, SEMP = Salzachtal-Ennstal-Mariazell-Puchberg line, KLT = Königssee-Lammertal-Traunsee line. Numbers 1 – 4 indicate posi- ad2,2003 26, Band , tions of cross sections. N I oceans and subsequent post-collisional shortening + . was responsible for Eocene to Miocene deformation

within the NCA and accreted units of the Penninic - ocean (Flysch units), of the distal European margin (Helvetic units) and the allochthonous Molasse. In M E M! A M! the more internal parts of the Alpine chain, Oligo- E A cene and Miocene out-of-sequence thrusting and E A backthrusting led to exhumation of the Tauern Win- O dow accompanied by major orogen-parallel exten- Fig. 2: Interpretation of the TRANSALP deep seismic section sion (Lammerer & Weger, 1998), combined with a redrawn from TRANSALP Working Group (2002), according to generally E-directed flow of material. According to their "lateral extrusion model". Length of section = 300 km. Ratschbacher et al. (1991), the eastward moving delta started to prograde from the west (Unter- blocks were delimited to the north by major ENE- angerberg Fm.). Fluviatile sedimentation resumed in striking faults: from E to W these are the SEMP-line the Late Oligocene (Oberangerberg Fm.). Because of and several faults branching from it, the KLT-line the many similarities with deposits in the Molasse and the Inntal shear zone, that cut the NCA basin, the Oligocene of the Lower Inn Valley was obliquely (Fig. 1). Oblique backthrusting along the also termed „Inneralpine Molasse“ (Fuchs, 1976, Periadriatic line at the southern margin of the East- 1980; Ortner & Stingl, 2001). A detailed description ern Alps propagated into the footwall and towards of the sedimentary succession is found in Ortner & the south, leading to south-directed thrusting in the Stingl (2001). southern Alps until recent times, whereas orogen- parallel extension in the Eastern Alps came to an end in the Late Miocene (Peresson & Decker, 1997). 2 Deformational structures observed The TRANSALP seismic section (Fig. 2) shed new light on the 3D geometry of the Alps and the Inntal 2.1 Synsedimentary deformation in the Oligocene shear zone, revealing a major south-dipping thrust intra-Alpine Molasse deposits reaching the surface at the southern margin of the Inn Valley, which superimposes metamorphic Paleo- The depositional geometries of Lower Oligocene zoic units onto the Northern Calcareous Alps with a sediments were strongly influenced by synsedimen- throw of approximately 10km (Fig. 2; TRANSALP tary shearing (for a more detailed description, see Working Group, 2001). At the surface, the Inntal Ortner & Stingl, 2001). Bituminous marls of the shear zone appears to be a major subvertical ENE- Häring Fm. were deposited in decametric half gra- trending strike-slip fault. Much has been speculated bens along faults (Fig. 3a). Deposits of cobbles and about the sinistral offset of the Inntal fault, with boulders at the basin margins associated with sedi- estimates ranging from 75 km (Frisch et al., 1998) ment infilling and draping with bituminous marls to about 40 km (Ortner et al., 1999). Taking into are interpreted as scarp breccias. Towards the foot- account the possible thrust fault geometry of the wall, the breccias pass into intact rock without Inntal shear zone, part of the apparent sinistral off- sharp boundary. In some meters distance toward the set in map view might be produced by thrusting. basin, isolated blocks of up to 1/2 m size are found within the bituminous marls, derived from the footwall of the basin margin fault. Neptunian dykes 1.1.2 Synorogenic sediments filled with debris of Oligocene carbonates of the Werlberg Mb. all trend NW-SE (Fig. 3c). Locally, Late Eocene (Priabonian) deposits represent the slickensides are preserved on the walls of these oldest part of the Alpine Molasse basin on top of dykes, which indicate a dextral normal sense of the NCA with terrestrial to shallow marine deposits shear (Fig. 3b, faults with grey symbols). Therefore (Oberaudorf beds). After a short period of erosion dextral shearing along WNW trending faults was and/or non-deposition, Oligocene sedimentation active during deposition of the Häring and Paissl- started with limnofluviatile deposits (Häring Fm.). berg Fms. The depositional realm rapidly subsided to pelagic With an increase in sand-sized detritus shed into conditions (Paisslberg Fm.). In the Late Rupelian, a the basin, the Unterangerberg Fm. develops from

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20 30 30 20 "$( 12°11' '%(( ) %$ 12°12' a c Fig. 3: a) Geological sketch and cross section of the Dux syncline S of Kufstein, illustrating the half graben geometry of Rupelian basins. b) Faults from the Grattenberg locality. Two faults are sealed by neptunian dykes (grey symbols). c) orientations of neptunian dykes filled with Oligocene sediments. Top: uncorrected orientations. bottom: orientations after backtilting of bedding of overlying bituminous marls to horizontal. A possible interpretation as shear and tension joints is indicated.

the Paisslberg Fm. The Unterangerberg Fm. is domi- 2003, this volume). Hydroplastic slickensides (Petit nated by turbiditic sand/mud couplets. Deformation & Laville, 1987), indicating shearing in partly lithi- of these sediments started during deposition and fied sediment, are found on small ramps in sand- accompanied and outlasted diagenesis. Accordingly, stone beds, leading to stacking of sandstone beds deformational structures change depending on the (Fig 3a, b; Fig. 14d of Ortner & Stingl, 2003, this decreasing viscosity of the lithifying sediments and volume). The shear zone itself is characterized by an the increasing strain superimposed on the sedi- interval several meters thick, where bedding is over- ments. printed by a foliation and sandstone beds are dis- The oldest deformational structures are slump rupted and the fragments isoclinally folded (Fig. 14f folds at the base of sandstone beds. Fold axes of of Ortner & Stingl, 2003, this volume). The long axes slump folds are consistently orthogonal to flute and of the cuspate-lobate structures and intersection groove casts, which are NW-SE-trending. Commonly lineations of ramps with bedding trend NW-SE (Fig. the size of slump folds is approximately on the 3c-d), movement in the shear zone is top to SW. dimension of bed thickness. In contrast, structures Deformation continued after lithification and led to of soft sediment deformation are usually only a decametric folding above the shear zone with still fraction of bed thickness in size. In the hangingwall NW-trending fold axes. Locally, also E-W striking of a shear zone, deformational structures illustrate vertical shear zones with sandstone beds isoclinally increasing bedding-parallel shortening: Cuspate- folded around vertical axes arefound. lobate structures are found at the sharp lower inter- As demonstrated above, a major change in the face between sandstone and marl beds, but not on geometry of the stress field took place during the the gradational upper interface (Fig 3c, d; Fig. 14b Rupelian or near the boundary from Rupelian to of Ortner & Stingl, 2003, this volume). At locations Chattian. Before and during the Rupelian, NNW-SSE with more lithified sandstone beds, shortening is contraction was active, followed by NNE-SSW con- taken up by blind thrusts, but the horizontal top of traction from the latest Rupelian or Chattian the bed is preserved (Fig. 14d of Ortner & Stingl, onwards.

112 Geol. Paläont. Mitt. Innsbruck, Band 26, 2003 1-7 they develop prior to the throughgoing larger shear a zone (Mandl, 1988), which only connects older syn-

34° thetic Riedel shears. A kinematic interpretation was done to determine the orientation of compression and extension to be able to compare non-coaxial datasets with coaxial datasets. For doing this, the orientation of the kinematical axes was calculated a with the NDA method (Sperner et al., 1993), and 1-8 checked whether the compressional axis was located half way between Riedel and Antiriedel 34° shears, as these represent the local stress field within the shear zone. For coaxial datasets, the orientation of kinematical axes was calculated with the NDA method, using an angle of 30° between slip line and the compressional axis. The kinematical a a a a a a axes are considered to coincide with the principal 117 Data 84 Data stress axes !1, !2 and !3 (!1>!2>!3). The direct inversion method (Angelier & Goguel, 1979) proved 2 5/24 to be unreliable if fault planes were not distributed evenly in space, which was the case in many fault

251/03 24 /1 data sets. The analysis of all tectonic data was done with the software TectonicVB (Ortner et al., 2002). The results are presented in Table 1. a O Fault data were recorded inside and outside of Fig. 4: Structural data from the Angerberg area: a) Orientation major shear zones to evaluate the role of shear of hydroplastic slickensides in Unterangerberg Fm. b) Orienta- zones during distinct deformational events. Fault tion of hydroplastic slickensides after rotation of bedding to data sets were grouped according to the orienta- horizontal. c) Orientation of long axes of cuspate lobate folds (black symbols) and ramps (great circles) in Unterangerberg Fm. tions of their principal stress axes. The succession of d) Orientation of long axes of cuspate lobate folds (black symbols) and ramps (great circles) after rotation of bedding to hori- ) ! 4 !! ) 2 !! ) 55 !! T1 T2 T3 T4 T5 T6 T1 T2 T3 T4 T5 T6 T1 T2 T3 T4 T5 T6 zontal. e) Orientation of bedding planes Unterangerberg Fm. a a a T1 3 11 2 2 1 T1 2 1 1 1 T1 1 4 1 1 Black diamonds indicate bedding in folds considered to be T2 6 2 2 T2 4 1 1 T2 2 1 1 T3 1 3 3 1 2 T3 4 2 1 T3 14 1 3 1 1 related to Early Rupelian folding. f) Orientation of bedding T4 5 5 6 1 T4 2 2 1 1 T4 3 3 5 T5 1 4 1 2 T5 1 1 T5 1 3 1 1 planes Oberangerberg Fm. T6 4 4 1 1 T6 2 2 T6 2 2 1 1

) 1 !! ) !" 24 !! T1 T2 T3 T4 T5 T6 T1 T2 T3 T4 T5 T6 2.2 Postsedimentary brittle deformation a a T1 1 T1 1 T2 1 T2 2 T3 10 1 T3 11 2 2.2.1 Methods T4 1 1 1 T4 2 2 2 T5 1 1 T5 1 1 T6 1 1 T6 1 1 In the field, brittle faults (Petit, 1987), tension gashes, joints and stylolithes (Hancock, 1985) were Fig. 5: Discrimination matrices of different subsets of data. a) measured in sediments of known age. Geometrically All data. Cross cutting relationships are not systematic. b) All inhomogeneous datasets were split into homogene- data west of Brenner normal fault. Most of the data indicating ous subsets taking into account crosscutting and T3 (NNE-SSW-contraction) older than T1 (NNW-SSE contrac- overprinting relationships. Datasets with fault geo- tion) come from outcrops near the Engadine line. There, T1 is possibly related to the Domleschg phase of Froitzheim et al., metries indicative of non-coaxial deformation were 1994). c) all data east of the Brenner normal fault. Cross cutting interpreted to represent increments of shearing relationships indicating T3 older T1 are from localities, where (Woijtal & Pershing, 1991). Noncoaxial shearing can Oligocene or Cretaceous NNE-contraction predates Miocene be recognized, when a prinicipal displacement zone NNW-contraction. d) data from Oligocene rocks. e) data from and conjugate Riedel and Antiriedel shears are pre- Oligocene and Cretaceous rocks. c) and d) show, that the rela- sent, which are usually more abundant, because tive ages are not defined for all events.

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003 113 . L A B B G G GK !1 !23!

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(6/*-+ ')6477 .'88/'3/1/-4)+3+ 3 1 2 23 0/3+2'8/) /38+656+8'8/43 4 33/ 1 22 / 2 124/ 3 7/251+ 7.+'6 #3 3 1 2 23 0/3+2'8/) /38+656+8'8/43 4 13 / 3 2 / 0 22 / 7/251+ 7.+'6 #1 2 ')016/+*-6'(+3 " 4, 78'8+ 64'* ,642 C77+3 !95+1/'3/1/-4)+3+ 3 3000 2 03 0 1 / 1 / 4 2 0/ 3 596+ 7.+'6 #3 2 '/7+6;'1*-6'(+3 "% 96)..41>+3 .'88/'3/1/-4)+3+ 3 1 0 2 14 3 0/ 24 / 3 3/ 2 596+ 7.+'6 #1 30 /68 64'* 64A54/83+6'12 "" 96)..41>+3 3/7/'3/#6/'77/) 3 23 0 2 32 12 30/ 1 1 / 3 24/ 1 596+ 7.+'6 #3 3 23 0 2 32 3 / 20/ 22 300/ 2 596+ 7.+'6 #4 3 23 0 2 32 2 / 23 / 4 13/ 596+ 7.+'6 # 31 6>1+6 12 " 4, 337(69)0 3/7/'3/#6/'77/) 231010 240324 1 4/ 14 32 / 1 1/ 11 596+ 7.+'6 #2 32 !473+6;+- /3 D.1'9+6 1'22 " 4, 337(69)0 '63/'3/#6/'77/) 231 13 2402 4 1 / 1 33 / 1 1/ 11 596+ 7.+'6 #2 34 6')8/43 +/8+3 4, ).+30/). '8+ #6/'77/) 84 '8+ 96'77/) 2 413 2 13 1 4/ 1/ 1 23/ 0 596+ 7.+'6 #2 2 413 2 3 3 / 1 130/ 30 / 22 596+ 7.+'6 #3 2 413 2 11 / 10 12/ 20 342/ 596+ 7.+'6 # 2 413 2 1 / 33 2/ 2 2 / 1 596+ 7.+'6 #1 3 9'66= '10(69). " '* B6/3- '*/3/'3/#6/'77/) 3 4 2 3 3 33 / 1 3/ 40 231/ 44 596+ 7.+'6 #1 3 4 2 3 3 1 / 1 0/ 1 2/ 596+ 7.+'6 #4 3 4 2 3 3 11/ 4 2 1/ 102/ 12 596+ 7.+'6 #3 3 1/3*'9 " 4, !+/8 /2 %/30+1 !95+1/'3/1/-4)+3+ 3 4 3 2 0134 221/ 2 11 / 312/ 596+ 7.+'6 #3 3 %'77+6,'11-6'(+3 " 4, !'*,+1* '63/'3/#6/'77/) 343 2 2 13 31/ 40 201/ 4 2 / 596+ 7.+'6 #3 3 %C.1+6 6'(+3 4, 4.+3+/(+6- 46/'3/#6/'77/) 3 3 2 22 333/ 11 141/ 242/ 2 596+ 7.+'6 #1 40 %C.1+6 6'(+3 4, 4.+3+/(+6- !95+1/'3/1/-4)+3+ 3 3 2 22 1 / 1 11 / 2 / 4 596+ 7.+'6 #3 41 %C.1+6 6'(+3 " 4, /38+678+/3 !95+1/'3/1/-4)+3+ 3 403 2 10 213/ 3 334/ 3 123/ 596+ 7.+'6 #3 3 403 2 0/3+2'8/) /38+656+8'8/43 14 34 / 3 2/ 2 / 13 7/251+ 7.+'6 #1 43 +55+3'9 /8>;'3* 4, +3-,+1*+3'12 !95+1/'3/1/-4)+3+ 3 2 30 2 02 12 30/ 12 1 0/ 1 2 / 13 596+ 7.+'6 #3 4 '/7+6-+(/6-+ 9(/1B92778+/- 6988+3.D88+ '*/3/'3/46/'3 3 33 2 02 1 32 / 1 11 / 2 23 / 0 596+ 7.+'6 #1 4 '/7+6-+(/6-+ % 4, '/7+6-4).'12 '*/3/'3/#6/'77/) 3 2 1 1/ 2 2 / 3/ 33 596+ 7.+'6 #2 4 '/7+6-+(/6-+ ").3++0'6 3/7/'3/#6/'77/) 3 02 3 2 3 0/3+2'8/) /38+656+8'8/43 2 / 13 243/ 3 121/ 7/251+ 7.+'6 #3 3 02 3 2 3 0/3+2'8/) /38+656+8'8/43 14 / 1 2 / 4/ 2 7/251+ 7.+'6 #1 4 '/7+6-+(/6-+ 6+/*+-6'(+3 " 4, 6/+7+3'9 46/'3/#6/'77/) '3* '1+4)+3+ 3 0 2 0 42 !/-.8 /.+*6' 10 0/ 244/ 4 1/ 13 596+ 7.+'6 #2 4 '/7+6-+(/6-+ 4, %+-7).+/* 4).'12 3/7/'3/#6/'77/) 3 0 3 2 3 0/3+2'8/) /38+656+8'8/43 143/ 1 30 / 2 2/ 4 7/251+ 7.+'6 #1 0 "8'8+ 64'* ,642 6'27'). 84 6'3*+3(+6- 46/'3/#6/'77/) 33 3 2 1 1 11 13 / 1 43/ 22/ 11 596+ 7.+'6 #1 1 )43,19+3)+ 4, D.1('). '3* 6'3*+3(+6-+6 ).+ '8+ 6+8')+497 341 14 2 3 10 3 / 2 12 / 1 2/ 0 596+ 7.+'6 #3 2 %+78+63 +3* 4, '/7+68'1 /3 9,78+/3 '*/3/'3/#6/'77/) 3 41 2 3312 11 / 4 2 / 100/ 23 596+ 7.+'6 #3 3 ").'3>;B3*+ 9,78+/3 '*/3/'3/#6/'77/) 3 4120 2 34 1 / 31/ 0 / 596+ 7.+'6 #2 4 ?78+63*46,+6 /3-+6@ 4, '* B6/3- '*/3/'3/#6/'77/) '3* !95+1/'3 3 2 4 1 3/ 2 / 22 4 / 596+ 7.+'6 #1 3 2 4 #+37/43 -'7.+7 40 2 / 596+ 7.+'6 #4 +).8'1 157 4, "922/8 4, 988+045, '25'3/'3/'8+ 6+8')+497 1 3 23 2 0 /6+)8 3:+67/43 1 3 / 4 304/ 2 13 / 4 596+ 7.+'6 #3 Table 1: Locations and results of paleostress analysis. Dataset 23 from Reiter (2000), 39-43 from Gruber (1995), 34 and 57-61 from Sausgruber (1994) and datasets 81-84 from Prager pers. comm. (2003).

114 Geol. Paläont. Mitt. Innsbruck, Band 26, 2003 . L A B B G G GK !1 !23!

55 %#(3!, ,02 /& 4--)3 /& 433%+/0& !-0!.)!./!3% 1%3!#%/42 1 3 5 236260 13 0/ 1 26 / 156/ 0 041% 2(%!1 2 56 %#(3!, ,02 1)2#(,23%)' /& 433%+/0&(<33% !-0!.)!./!3% 1%3!#%/42 1 5 46 2354 12 1 / 2 6/ 5 26 / 14 041% 2(%!1 2 1 5 46 2354 / 62 143/ 20 240/ 1 041% 2(%!1 4 5 /&!. 6%23%1. &!#% /& ..438 %!1,7 !3% 1%3!#%/42 254126 265 0 1 1/ 2 261/ 2 33/ 6 041% 2(%!1 2 5 /&!. %!23%1. &!#% /& ,)#+%.+/0& %!1,7 !3% 1%3!#%/42 252210 266334 32 / 0 5 / 234/ 0 041% 2(%!1 1 60 /&!. 4,5%1%1 !($ /& ..438 %!1,7 !3% 1%3!#%/42 256163 26 2 12 346/ / 21 / 041% 2(%!1 2 61 /&!. %23,!,- )%$%1,%'%1 %!1,7 !3% 1%3!#%/42 255 6 2 0413 6 40/ 5 131/ 15 2 1/ 3 041% 2(%!1 3 255 6 2 0413 153/ 0 63/ 11 246/ 041% 2(%!1 1 62 %#(3!, ,02 /& !.!4%1 <33% !-0!.)!./!3% 1%3!#%/42 1 0261 234521 162/ 4 0/ 20 263/ 6 041% 2(%!1 1 63 )33%1"%1'3/"%, /& ,4$%.8 !1.)!./1)!22)# 113452 22615 10 1 / 10 225/ 5 110/ 5 041% 2(%!1 3 113452 22615 5 15 / 23 251/ 35 / 64 041% 2(%!1 2 64 $)13 1/!$ !#1/22 433%12"%1' !1.)!./1)!22)# 112 4 22 236 )1%#3 .5%12)/. 3 / 2 206/ 61 305/ 4 041% 2(%!1 3 112 4 22 236 5 3 / 10 30 / 1 210/ 041% 2(%!1 3 65 )33%16!,$ /& ,4$%.8 !1.)!./1)!22)# 11301 225 5 5 15/ 11 150/ 3 2 2/ 11 041% 2(%!1 3 11301 225 5 6 163/ 2 5/ 2/ 041% 2(%!1 1 66 /& !(.%.+!-- 24--)3 .%!1 %433% .)2)!./1)!22)# 1 3220 25 6 4 134/ 21 41/ 2 2/ 6 041% 2(%!1 1 1 3220 25 6 4 12 136/ 33 / 10 24 / 4 041% 2(%!1 4 6 3%-0%,*/#( /& ..2"14#+ !$).)!./1)!22)# 233 243 4 5 321/ 1 230/ 2 133/ 1 041% 2(%!1 1 233 243 4 250/ 6 3 / 1 132/ 041% 2(%!1 6 6 1,"%1' 0!22 - 140 4 22116 342/ 5 1 / 3 2/ 2 041% 2(%!1 1 140 4 22116 1 6/ 14 343/ 4 5/ 5 041% 2(%!1 3 6 400%1-/23 0!13 /& !33%."!#( .%!1 !6).!,0% %1-/2#73()!. 16051 223 4 15 1 1/ 23 0/ 3 304/ 41 041% 2(%!1 3 0 ,)12#( - 155 5 223565 5 216/ 1 115/ 31 331/ 52 041% 2(%!1 3 1 :.$,+;0& /& 1,"%1' 0!22 .)2)!. 3/ !$).)!./1)!22)# 13 30 222132 0/ 1 264/ 0/ 11 041% 2(%!1 2 2 /.3,!38%1 1<#+% - 1 4 23 21 24 13 342/ 1 16 / 0 3/ 1 041% 2(%!1 1 1 4 23 21 24 15/ 16 1 / 3 105/ 0 041% 2(%!1 3 3 42#(,). - 1 5 2 21 002 1 / 2 0/ 4 2 0/ 14 041% 2(%!1 2 1 5 2 21 002 10 321/ 11 203/ 66 55/ 20 041% 2(%!1 1 4 /& 4..%, ). #(.!..%1 ,!-- !$).)!./1)!22)# 1535 2 224263 +).%-!3)# ).3%101%3!3)/. 10 150/ 11 2 5/ 1 5 / 11 2)-0,% 2(%!1 1 5 /& #(.!..%1 ,!-- !3% 1%3!#%/42 153326 224 5 1 1/ 11 312/ 2 / 12 041% 2(%!1 2 6 !3 23!3% 1/!$ /& 31%.'%. %1-/2#73()!. 162345 220 32 / 12 1 3/ 6 32 / 12 2)-0,% 2(%!1 1 /& 3!.8 .%!1 !.$%#+ %1-/2#73()!. 166152 223554 10 213/ 4 10 / 4 315/ 3 041% 2(%!1 3 166152 223554 12 1 6/ 22 62/ 5 2 5/ 20 041% 2(%!1 3 !,&!8%1 ,- /& !41!#( )!222)#/!1,7 41!22)# 2564 25 516 6 352/ 1 1 / / 0 041% 2(%!1 4 2564 25 516 5 1 5/ 1 62/ 62 2 0/ 1 041% 2(%!1 3 -0%,2"!#( 41!22)# 25 64 2 12 2 10 332/ 1 1 / 66 6 / 12 041% 2(%!1 1 0 $)13 1/!$ /& !33%."%1' !$).)!./1)!22)# 341 6 2563 )1%#3 .5%12)/. 105/ 64 256/ 22 351/ 11 041% 2(%!1 6 1 %1.0!9 1/!$ !3 /3%, %1.23%). /1)!./1)!22)# 1 6 42 2453 3 +).%-!3)# ).3%101%3!3)/. 13 1 3/ 33 14/ 56 2 3/ 0 2)-0,% 2(%!1 3 1 6 42 2453 3 12 34 / 33 24 / 10 144/ 53 041% 2(%!1 2 1 6 42 2453 3 10 / 10 13/ 1 22 / 6 041% 2(%!1 5 2 ;-%16%' !"/5% /3%, %1.23%). /1)!./1)!22)# 1 353 246315 5 1 6/ 11 343/ 6/ 2 041% 2(%!1 2 1 353 246315 14 2/ 6 23 / 1 332/ 10 041% 2(%!1 6 1 353 246315 0/ 13 26 / 6 152/ 4 041% 2(%!1 2 4 !4(%2 !, %1.0!22 /1)!./1)!22)# 1 66 5 24 1 10 53/ 56 252/ 61 14 / 041% 2(%!1 3

Table 1 continued

events younger than Oligocene was established in T3: NNE-SSW contraction defined by conjugate Oligocene sediments using cross cutting relation- strike slip faults and, less abundant, by thrust faults. ships with the help of a discrimination matrix (Fig. The only major shear zone in the investigated area, 5d); this succession corresponds to the scheme by the Inntal shear zone was active during this event. Peresson & Decker (1997a). Cross cutting relation- T4: E-W extension defined by conjugate normal ships in rocks older than Oligocene were not sys- faults. The direction of extension actually varies tematic (Fig. 5a-c). between WNW-ESE and WSW-ENE. T5: E-W compression defined by conjugate strike slip faults and thrust faults. 2.2.2 Deformational events and their ages T6: NNW-SSE extension defined by conjugate normal faults. The observed stress tensors from old to young are: In the next paragraphs the characteristics, T1: NNW-SSE contraction defined by conjugate regional distribution and ages are discussed in more strike slip faults and, less abundant, by thrust faults. detail: On the large scale, thrusting within the NCA, of the NCA onto Penninic units and of the Alpine nappe stack onto the Molasse basin took place under 2.2.2.1 T1 (NNW-SSE-compression; Fig. 6) NNW-SSE contraction. T2: N-S contraction defined by conjugate strike The Oligocene deposits in the Inn valley are slip faults and, less abundant, by thrust faults. folded with WSW-trending axes on hecto- to kilo-

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003 115 0

2 27 7 61 8427-1 8428-2 *() $ 34 54 58 18 45 50 41 47 21 66 1 22 c) 20 !# 67 1 112-15 ( " 112-16 5 6 7 79 D !1 32 D ! " 62 $$('*! 3 ) 112-17 b) a) () 61 72 " () 74 68 76 73 /).)" # ( " "+')) (#$) *$ ) ) # ( ! (#$) *$ ) - ) 1 - ! ! 1 3 %"(( .%$ ' %/*++ $&&( ')%*( (-$%'%$ (. $$('*! *').&-"" ) !1 di ec i : "+) .%$ $$)"/ '%" $&&( ( #$)'- %+' (/).)" *.) '-,! .%$ )'*() )-& )$(%'( "-(/A'%( .%$ )"/B *+' $&&( ""' %$ ( -"" )$ ( .%$ ()' ! (" & )-& )$(%'(

Fig. 6: Orientations of !1 (black arrows) of the tensor group T1 (NNW-SSE compression). Numbers next to arrows refer to Table 1 or to Decker et al. (1993) if >100. Inset a) orientations of !1 and !3 of all stress tensors of group T1 and orientation analysis of !1 (black rose diagram) and !3 azimuths (grey rose diagram). Inset b) orientation of compressional and extensional axes of earthquakes in the Alps and orientation analysis of P-axes (black rose diagram and T-axes (grey rose diagram). Data from Pavoni (1977)(circles) and Sleijko et al. (1987)(diamonds). Inset c) schematic representation of orientation of faults related to tensors of group T1.

metric scale (Fig. 4e & f). Conjugate dextral WNW- compression, as described above (Fig. 4e,f). It did and sinistral N-striking faults are associated with also affect the Molasse basin at the northwestern folding and usually postdate folds. In all localities margin of the Eastern Alps (brittle data published where T1 tensors were found in Oligocene rocks, by Decker et al., 1993; Schrader, 1988), which is NNW-compression led to formation of strike slip clearly dominated by brittle structures related to faults. In rocks older than Eocene, both strike slip NNW-SSE shortening (T1), orthogonal to the faults and thrust faults are found, in some cases in large fold trains of the allochthonous Molasse the same station. In contrast to Peresson & Decker with regional WSW-ENE trending fold axes. The (1997a), who observed older thrust faults crosscut sediments of the allochthonous Molasse are Oli- by strike slip faults in the eastern part of the NCA, gocene in age, and therefore the age of the faults no systematic succession could be deduced in the must be younger than Oligocene. Also in the western part of the NCA. One reason is that D1 is Flysch and Helvetic zones, D1 stuctures were fre- heterogeneous: quently found. • NW to NNW-directed contraction was active in • The orientation of P- and T-axes of earthquakes the western part of the NCA during the Late Cre- in the Alps (Pavoni, 1977; Sleijko et al., 1987; taceous, as demonstrated by Ortner (2001) in inset b in Fig. 6) parallel to !1 and !3 directions synorogenic deposits of the Muttekopf area. of T1 suggests, that NNW-directed contraction is There, NW-striking dextral cross faults were still active. However, in the Oligocene deposits active during NW-directed thrusting and folding investigated, faults belonging to the T1 tensor (Fig. 6a,b; Eisbacher & Brandner, 1996), predating group are systematically older than other faults Late Cretaceous NNW-directed thrusting associ- (Fig. 5d). ated with folding of synorogenic sediments (l.c.; Fig. 7). When assuming the activity of cross faults during • NNW-directed shortening was active during Cretaceous thrusting, a conceptual problem arises. deposition of Rupelian sediments on top of the Analysing brittle structures, usually an angle of 30° NCA, as described in the chapter „Synsedimentary between fault plane and compressional axis is used. deformation“. However, if interpreting NW-striking faults as cross • Post-Oligocene (Early Miocene) deformation faults, the angle between compressional force and caused folding and subsequent strike slip faulting fault plane is near 0° (Mandl, 1988). Therefore, most in Oligocene sediments during NNW-directed of the cross faults associated with NW-directed

116 Geol. Paläont. Mitt. Innsbruck, Band 26, 2003 ! 5 ( - ) ! 1 20# " 0/1 62 11 D a 66 b

61/26

Ceace ae h g ! 1 20# " 0/1 ! 4 ( - " ) 62 c d Fig. 8: The effect of tilting of strike slip datasets belonging to 55° tensor T1. Top: anticlockwise rotation about a ENE-trending fold axis. If rotated more than ~60°, datasets will be interpreted to belong to tensor group T5 (E-W compression). Bottom: clock- wise rotation about a ENE-trending fold axis. If rotated more than ~60°, datasets will be interpreted to belong to tensor Ceace ed h fa group T4 (E-W extension). e f 7 7 more difficult, because tilted strike slip faults will be interpreted to be related either to E-W-extension or to E-W compression, if tilted to the opposite direc- 80° tion (Fig. 8).

2.2.2.2 T2 (N-S compression) Ceace ed h fa N-S compression was recorded in many stations Fig. 7: Examples of Cretaceous deformation and the effect of in the western NCA, the Flysch, Helvetic and later tilting. a) fold axis of fold sealed by Late Cretaceous sedi- Molasse zones (Fig. 9). Both conjugate thrust and ments near Hanauer Hütte, Lechtal Alps. b) fault data set meas- strike slip faults were observed. Where thrust faults ured near the contact between Lechtal and Allgäu nappe near Reutte. c) Thrust faults from Late Cretaceous Gosau sediments were observed in Oligocene sediments, they are associated with soft sediment deformation before backtilting transitional to T3 (see below), otherwise conjugate and d) after backtilting. d) Thrust faults from the Triassic rocks NNW-striking dextral and NNE-striking sinistral before and d) after rotating with bedding back to horizontal faults were observed. No cross cutting relationships around the regional fold axis. between thrust faults and strike slip faults were recorded. The age of N-S compression must be post- thrusting will be attributed to NNW-SSE contrac- Oligocene, as Upper Oligocene sediments in the tion, and only in the few cases, when the age of the Molasse basin and in the Inneralpine Molasse are fault and its relation to a thrust plane can be affected by this deformation. However, for older exactly defined, the interpretation will be correct. rocks, also a Late Campanian to Late Maastrichtian However, in this study NW- and NNW-directed age of N-S compression must be considered, as sug- compression are both part of the same tensor group gested by Ortner (2001). T1, and the problem is avoided. Nappe thrusting was postdated by folding. Fault data sets related to Late Cretaceous nappe thrusting 2.2.2.3 T3 (NNE-SSW-contraction) should be tilted, except where the thrust plane is still horizontal. A few datasets of thrust type do An event showing NNW-SSE contraction was show this geometry and can only be interpreted if found in the NCA (Fig. 10). The western Flysch, Hel- rotated with bedding to a horizontal position (Fig. vetic and Molasse zones were not affected by this 7c-f). The identification of rotated cross faults is event according to Decker et al. (1993). In a few

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003 117 0 2 8428-4 53

34 *() $ 8428-3 2 1 46 48 113-3 112-13 57 21 111-5 8428-1 20 !# 113-1 113-6 82 11 112-14 81 112-15 55 31 !1 45 D! 113-13 56 ! $$('*! 3 113-1 32 63 b) a) 112-16 ! !! 140 73 2 ( ) ( ! !# # 71 68 75 /).)" (#$) *$ ) " ! $"! ( "+')) ! ! " ! !# ) 1 (#$) *$ ) ! ! ) 3 %"(( .%$ ' %/*++ $&&( ')%*( (-$%'%$ (. $$('*! *').&-"" ) "+) .%$ $$)"/ '%" $&&( ( #$)'- %+' (/).)" *.) '-,! .%$ "-(/A'%( .%$ )"/B *+' $&&( ""' %$ ( -"" )$ ( .%$

Fig. 9: Orientations of !1 (black arrows) of the tensor group T2 (N-S contraction). Numbers next to arrows refer to Table 1 or to Decker et al. (1993) if >100. Inset a) orientations of !1 and !3 of all stress tensors of group T2 and orientation analysis of !1 (black rose diagram) and !3 azimuths (grey rose diagram). Inset b) schematic representation of orientation of faults related to tensors of group T2.

0 26 30 37 52 8428-8 61 8427-1 *() $ 28 22 47 34 78 43 51 23 16 1 20 c) 20 !# ! 84 31 3 10 ( ! !# ! " ! 13 1 41 D! 55 12 ! ! " ! !# 81 $$('*! 3 ! ) 7 64 65 3 b) ! a) ! ! !! 63 ( ) 72 ! ! # 70 6 77 /).)" $. (#$) *$ ) ! $"! ( "+')) ! ) 1 (#$) *$ ) ! 3 . 3 %"(( .%$ ' %/*++ $&&( ')%*( (-$%'%$ (. $$('*! *').&-"" ) "+) .%$ $$)"/ '%" $&&( ( #$)'- %+' (/).)" *.) '-,! .%$ "-(/A'%( .%$ )"/B *+' $&&( ""' %$ ( -"" )$ ( .%$

Fig. 10: Orientations of !1 (black arrows) of the tensor group T3 (NNE-SSW contraction). Numbers next to arrows refer to Table 1 or to Decker et al. (1993) if >100. Inset a) orientations of !1 and !3 of all stress tensors of group T3 and orientation analysis of !1 (black rose diagram) and !3 azimuths (grey rose diagram). Inset b) Schematic representation of deformed stress trajectories across the Inntal shear zone. Map scale Riedel shears within the shear zone lead to reorientation of the stress trajectories. Inset c) schematic representation of orientation of faults related to tensors of group T3.

cases conjugate reverse faults were observed, much NCA: The Isartal and the Loisach fault systems more common are conjugate N-striking dextral and (Fig. 1). Offset across the faults is in the range of NE-striking sinistral faults. Due to their age and 2 km for the Isar fault and probably in the same geometries two groups of faults must be distin- range across the Loisach fault. Both run into a guished: zone of S-directed backthrusting (Eisbacher & • West of Innsbruck, usually conjugate fault sets, Brandner, 1996). Following observations suggest as described above, are found. On map scale, two a Paleocene/Eocene age of D3 west of Innsbruck: sinistral fault systems can be traced in SW-direc- - In the Muttekopf area, subhorizontal NNE- tion from the front to the central part of the directed thrusts crosscut folded Upper Maas-

118 Geol. Paläont. Mitt. Innsbruck, Band 26, 2003 "!! 71-1 2 $ 55 15 $ $ 6 $ between 120 and 65 Ma (Thöni, 1981; Pet- schick, 1989). Rb/Sr ages from micas grown in the E-W-foliation show ages of 65 Ma (Fer- reiro-Mählmann, 1994), suggesting a latest 6 %$# %$# # Cretaceous/Early Tertiary metamorphic event 70 5 $ "!! L! ! ! ! ( connected to formation of the foliation. The $ quartz fibres probably formed at the brittle-

! ductile transition during cooling in the Paleo- $ 70- 1 $ #$"$ 5 cene/Eocene. %$# 7 • East of Innsbruck structures related to Oligocene/ #% " %! 5 . . "## $ . %"## Miocene NNE-SSW compression are superim- " #&$ 4 7 &$ # ' posed onto Paleocene/Eocene structures formed %$# I! under compression in the same direction. In the Unterinntal Molasse, at the localities, where soft ) 4 sediment deformation is observed, a mid-Oligo- 5 2 3 1 cene age for D3 can be deduced , and a post-Oli- 6 F L )( A gocene age, where Oligocene sediments are

69-1 15 $ 77-2 10 $ affected by brittle faulting. The fact that the NCA east of Innsbruck were affected by Paleocene/Eocene, Oligocene and Mio- 10

) #$ " ! $! cene NNE-contraction, which was alternating with ! ! " !' " ! ! ( 1 ! ! !) &! %$# 3 %$# NNW-contraction makes it evident, that cross cut- 69-2 6 $ 77-1 12 $ 2 ting relationships cannot be systematic. It is still true, that most of the cross-cutting relationships indicate T3 younger than T1 (see Peresson & Decker, ) #$ - $! # ! % (! ) -- ! "!" ( 1997a), however many indicate the opposite (Fig. 5). ! !) 2 D$/L!! !' As it is not possible to distinguish between faults Fig. 11: Structural data from the Arlberg area illustrating NNE- related to Paleocene/Eocene deformation and those directed contraction. Geological sketch modified from May & related to Oligocene/Miocene deformation in rocks Eisbacher (1999). Inset "Muttekopf": Schematic N-S cross sec- older than Oligocene/Miocene, the reversed succes- tion across the Muttekopf Gosau outcrop showing post-Maas- trichtian NNE-SSW contraction and associated foliation com- sions may be a result of Oligocene/Miocene T1 bined with brittle faulting. Inset "Dawinalm": a) formation of faults overprinting Paleocene/Eocene T3 faults. pull-aparts and b) folding with steep axes associated with bedding-parallel shear in Permoscythian rocks (diagram 3) and formation of s-c-structures in underlying phyllitic rocks (diagram 2.2.2.4 T4 (E-W extension) lower right). Inset "Stanz": bedding parallel shear in Permo- scythian rocks (diagram 2) and at the thrust beween Phyllitgneis The T4 tensor group is inhomogeneous, with zone and Permoscythian rocks (diagram 1). extension directions ranging from WNW to WSW (Fig. 12; see also Peresson & Decker, 1997a). In most trichtian to ?Paleocene sediments (Fig. 11, stations either conjugate normal faults or tension Inset „Muttekopf“; Ortner, 2001; the socalled gashes are found. In the Unterinntal Molasse, fold „Deckelklüfte“ of Niederbacher, 1982). axes in Upper Oligocene sediments consistently - In the Arlberg area, quartz fibres on steeply plunge to the WSW. Therefore the large scale nor- south dipping bedding planes in Permoscythian mal faults are probably E-dipping. West of Inns- rocks (diagrams 1,2,3 of Fig. 11) and on simi- bruck, ENE-WSW extension is locally found to be larly oriented foliation surfaces indicate NNE- older than or alternating with T1, as observed in the SSW contraction. Shearing along the faults led Molasse basin (see Fig. 13 of Decker et al., 1993) to folding of bedding and foliation planes with and in Cretaceous sediments of the Muttekopf area subvertical axes and to formation of cm-size (Fig. 13). The direction of extension in most of these pull-aparts between shear planes (Fig. 11, inset data sets is parallel to the regional fold axis and is „Dawinalm“). K/Ar ages from the area lie therefore interpreted to be related to extension

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003 119 0 4 8428- 8427-4 - +,"' 30 8428-6 35 54 8428-1 111-5 113-3 78 1 20 66 113-6 20 $& 112-14 112-15 4 5 !1 22 ! 56 ''+*-$ 3

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Fig. 12: Orientations of !3 (black arrows) of the tensor group T4 (E-W extension). Numbers next to arrows refer to Table 1 or to Decker et al. (1993) if >100. Inset a) orientations of !1 and !3 of all stress tensors of group T4 and orientation analysis of !3 azimuths (grey rose diagram). Inset b) schematic representation of orientation of faults related to tensors of group T4.

orthogonal to the direction of maximum compres- 6 54 40 sion.

2.2.2.5 T5 (E-W compression)

Throughout the investigated area, an event was recorded that reverses the movement sense of most - 0,'+"(' *%, ,( "(' .', of the major faults (Fig. 14). The analysis of these 66 15 66 12 data sets consistently shows WNW-ESE compression. In outcrops, where no older steep faults are found, conjugate top-to-west or top-to-east thrust faults formed. Faults related to the T5 tensor are restricted to outcrop scale and do not create map- scale offsets. The age of this deformation cannot be constrained in the investigated area (except younger

56 35 56 than Oligocene), however in the eastern part of the Eastern Alps it is also found in Middle Miocene rocks of the Ennstal Tertiary and the Vienna basin and has a Late Miocene age (Peresson & Decker, 1997b).

- 0,'+"(' *%, ,( *,(-+ (%"' 2.2.2.6 T6 (NNW-SSW extension)

Fig. 13: Examples of data sets representative of E-W extension In many outcrops, a conjugate WSW-striking set (T4). a) E-W extension recorded by fault planes, Dalfazer Alm, of normal faults is found, locally with offsets in the Rofan and by b) tension gashes in Oligocene carbonates, Bad order of 100m (Fig. 15). Faults related to the T5 ten- Häring. c-f) poles of planes (white circles) and regional fold axis sor group are restricted to the Inn Valley and the (black circle; left side) and fault data sets with !3 (black triangle) parallel to the fold axis. In this case, spreading of material parallel area immediately N of it west of Kufstein, and are to the fold axis is interpreted. c) and d) from Triassic rocks at found in the Flysch and Helvetic zones. This event is Hahnenkamm near Reutte, e) and f) from Drischlsteig near heterogeneous, because beside a Late Miocene age Muttekopf. of activity as proposed by Decker et al. (1993), pos-

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Fig. 14: Orientations of !1 (black arrows) of the tensor group T5 (E-W contraction). Numbers next to arrows refer to Table 1 or to Decker et al. (1993) if >100. Inset a) orientations of !1 and !3 of all stress tensors of group T5 and orientation analysis of !1 (black rose diagram) and !3 azimuths (grey rose diagram). Inset b) schematic representation of orientation of faults related to tensors of group T5.

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Fig. 15: Orientations of !3 (black arrows) of the tensor group T6 (NNW-SSE extension). Numbers next to arrows refer to Table 1 or to Decker et al. (1993) if >100. Inset a) orientations of !1 and !3 of all stress tensors of group T6 and orientation analysis of !3 azimuths (grey rose diagram). Inset b) schematic representation of orientation of faults related to tensors of group T6.

sible Cretaceous normal faulting during subsidence 25 of Fig. 16). The principal displacement zones in of Gosau basins (Wagreich & Decker, 2001) and Oli- these datasets deviate from the orientation of the gocene normal faulting, as discussed below, must be trace of the faults in map view, indicating that the taken into account. master planes on outcrop scale are Riedel planes to the map scale faults. Therefore the maximum com- 2.2.3 Activity of the Inntal shear zone pressive stress within the shear zone is N-S to NNW-SSE directed. Datasets from within the shear Faults related to the T3 tensor group are domi- zone can be distinguished from T2 datasets also nant in the Inn valley area, as T3 tensors are associ- indicating N-S compression by their simple shear ated with shearing at the Inntal shear zone. Data- geometry and from the position of the stations, sets from the T3 tensor group in this area often where data were measured, on map scale Riedel show simple shear geometry (see diagrams 16, 19, shears (diagrams 22,29 of Fig. 16).

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1 " - - 2 19 " ee f g d 38 Weee 10 +- af ad2,2003 26, Band , Fig. 16: Structural sketch of the Inntal Molasse and fault data sets illustrating shearing in the Inntal shear zone. Numbers next to diagrams refer to table 1. Inset a) shows the anostomosing character of the shear zone. The lozenge shaped blocks within the shear zone experienced either uplift (+) or subsidence (-) during shearing. Inset b) mechanical model for movement near the bend in the Inntal shear zone. Black arrow indicates assumed direction of movement parallel to western part of Inntal fault. PDZ = principal displacement zone. Further explanations in the text. " $

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Fig. 17: Reconstruction of the pre-Miocene geometry of the Inntal. Due to the bended geometry of the Inntal shear zone, shortening orthogonal to the fault must increase toward the east. As the southern branch of the fault ends south of the Kaisergebirge, no major lateral offset was accomodated there.

2.2.3.1 Geometry of the Oligocene Inntal shear zone 2.2.3.2 Geometry of the Miocene Inntal shear zone

From the begin of the latest Early Oligocene, In contrast, Miocene sinistral shearing is gener- debris from the eroding Alps was not transported ally combined with S-directed oblique thrusting across the Inntal shear zone toward the N, only the southwest of Kufstein and the Kaisergebirge, partly alluvial fan east of Kufstein did bring clasts from onto Oligocene sediments (Fig. 16, diagram 86; Gru- Australpine basement rocks into the Molasse basin ber, 1997). This is probably an effect of a bend of (Ortner & Sachsenhofer, 1996; Brügel, 1998; Ortner the Inntal shear zone east of Kufstein from a strike & Stingl, 2001). Sediments were channelized along direction of 54° to 71° (Fig. 16). Immediately east of the Inntal shear zone to the Chiemgau fan. The Oli- the bend, a important component of shortening gocene age of the swell is also proven by Upper Oli- orthogonal to the fault (transpression) is indicated gocene conglomerates in contact with Triassic rock by curved fault planes changing alongstrike from north of the Inntal shear zone (Zerbes & Ott, 2000, thrust to strike-slip geometry (Fig. 16, diagram 25). cross section 14 on plate 2; Ortner & Stingl, 2003, Towards the east, the southdirected thrusts disap- this volume), that seal the relief created aong the pear and are replaced by N-striking dextral faults Inntal line. Therefore Oligocene activity of the Inntal that dominate over ENE-striking sinistral faults and shear zone was combined with a normal throw of do show map-scale offsets (Fig. 16, inset a). This the southern block, as visible in Fig. 18 (see below). transition coincides with the eastern end of the The data discussed in section 2.1 do not allow to Wetterstein carbonate platform. If the southern decide, whether a shear zone with large offset block moved parallel to the western part of the Inn- existed in the Oligocene or not, but only confirm the tal shear zone, movement across the bend will lead existence of NE-SW compression in the Oligocene. to major space problems. Therefore, near the south-

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-14 3400 3600 3800 4000 4200 4400 4600 4800 85 0 5 100 105 110 115 120 Fig. 18: Bottom: Depth-migrated TRANSALP section between km 85 and 120. Top: Geological interpretation of seismic transsect. Trace of section: 2 in Fig. 1.

ern margin of the carbonate platform, SE-directed 2.2.4 Thrust movements across the Inntal shear thrusts developed . Further east, less rigid units were zone: interpretation of the TRANSALP line segmented into narrow units delimited by N-S- striking dextral faults, and shortening was trans- The TRANSALP deep seismic line crosses the Inn ferred to the south (Fig. 16, inset b). valley near the westernmost outcrops of Oligocene Within the Inntal shear zone, the Oligocene sedi- rocks. A general interpretation of the TRANSALP mentary succession was disrupted into lozenge seismic line was published by the TRANSALP work- shaped flakes and offset by ENE-striking sinistral ing group (2002) (Fig. 2). They proposed a sub-Tau- faults. Under the assumption, that all lateral move- ern ramp, cutting from the middle part of the Euro- ments were horizontal and not combined with pean upper crust straight to the surface in the Inn thrusting, as shown by the brittle data, the post-Oli- valley, facilitating uplift and exhumation of the Tau- gocene offset can be reconstructed by bringing ern window. Near the surface, the NCA are over- together the south-dipping northern basal contact thrust by its basement. This upper part of the sub- of the Oligocene to older rocks. Depending on the Tauern ramp is renamed to Brixlegg thrust. The seg- sequence of activity of different branches of the ment of the TRANSALP seismic line across the Inn Inntal shear zone, offsets between 22 and 30 km valley was interpreted in detail by Ortner et al. can be reconstructed (Fig. 17). (2003), to exactly define the place where the Brix-

124 Geol. Paläont. Mitt. Innsbruck, Band 26, 2003 Fig. 19: Compilation of coalification data (Oligocene rocks: Ortner & Sachsenhofer, 1996; NCA: Petschick, 1989 and Reiter, 2000; well Vorderriß: Hufnagel et al., 1981), illite crystallinity data (Kralik et al., 1987), apatite and zircon fission track ages (1Fügenschuh et al., 2000; 2Grundmann & Morteani, 1985) and a K/Ar isochrone age (3Krumm, 1984) from the NCA and southerly adjacent areas. legg thrust reaches the surface and to evaluate tim- represent a major thrust of the Greywacke zone and ing and amount of displacement. its sedimentary cover, the NCA and other basement In the seismic section (Fig. 18), a contrast in units onto rocks of the NCA, the Brixlegg thrust. reflectivity is observed across a plane dipping to the The NCA in the hangingwall of the Brixlegg south from the southern margin of the Inn valley. thrust experienced anchimetamorphic conditions Strong reflectivity is found below the plane, and the during the Cretaceous (Rmax 3-4,5%; Reiter, 2000; strongly reflective rocks continue to the north into Krumm, 1984; Fig. 19). Assuming no metamorphism the Northern Calcareous Alps. The pattern of reflec- in the footwall units of the thrust, a maximum ver- tions is comparable to the pattern created by the tical offset of 7-8km can be deduced, which is in marls and dolomites of the Raibl Fm. found further line with the interpretation of the TRANSALP line in north in the section (compare Auer & Eisbacher, Fig. 18. Oligocene deposits overlie both the footwall 2003). Therefore a zone of imbricates or a very thick and the hangingwall of the thrust. On the hanging- continuous section consisting of a succession of wall, Oligocene sediments are widespread east of marls, dolomites and limestones is interpreted below Kufstein and around Bad Häring, but disappear the contrast. The weakly reflecting rocks above are about 10 km east of the TRANSALP line. According part of the metamorphic basement of the Austroal- to Kralik et al. (1987), anchimetamorphic conditions pine unit, which in the section from top to bottom south of the Inntal shear zone are found as far to are the Greywacke zone, the Kellerjoch Gneiss nappe the east as Kössen and Reit im Winkl (Fig. 19). The and the Innsbruck Quartzphyllite nappe (Fig. 18). anchimetamorphic rocks are overlain by Oligocene The contacts between these units dip gently to the rocks that experienced only diagenetic conditions north, as does the foliation. The Greywacke zone is (Rr ~ 0,4%; Ortner & Sachsenhofer, 1996). Therefore in sedimentary contact with the Tirolic unit of the most of the offset across the thrust is sealed by Oli- NCA. The contrast in reflectivity is interpreted to gocene sediments and must have a Paleocene/

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003 125 Eocene age, as the thrust cuts Cretaceous nappe Jurassic sediments (see Wächter, 1987; Channell et boundaries. However, the presence of solid bitumen al., 1990, 1992; Lackeschewitz et al., 1991; Böhm et in horsts within the basin indicates that some of the al., 1995; Hebbeln et al., 1996). Up to now, no sediments reached the oil window, probably below analysis of brittle Jurassic faulting was done in the the tip of the Brixlegg thrust (Ortner, 2003; Fig. 18). NCA. Jurassic deformation was characterized by Minor (post-) Oligocene thrust movements are normal faulting leading to development of horsts therefore interpreted. Towards the west, the young and grabens, which in some way were connected to uplift related to the Brixlegg thrust increases, as strike slip faults, however the exact geometry of seen on geologic map, because successively deeper Jurassic faulting is not known in the investigated units of the Cretaceous nappe stack are exposed in area. A tentative reconstruction was given by Chan- the hangingwall. Apatite fission track dating in the nell et al. (1990), with NNE-strikung normal faults area south and east of Innsbruck revealed a phase connected to ENE striking sinistral. At the western of uplift around 14 Ma (Fügenschuh et al., 1997; margin of the Austroalpine unit in eastern Switzer- Grundmann & Morteani, 1985) that might indicate land, deformed Jurassic normal faults are well docu- the time of young thrusting (Fig. 19; see also Figs. mented and are NNE-striking and both east and 22,23), whereas Zircon fission track data scatter west-dipping in paleogeographic reconstructions between 69 and 35 Ma. (Fügenschuh et al., 1997). (Froitzheim & Manatschal, 1996). There, normal According to these authors, this means that the faults were active throughout the Jurassic (l.c.). rocks started to cool below 300° in the Early Paleo- Recently, Gawlick et al. (1999) proposed Late Juras- cene. This is possibly related to Paleocene/Eocene sic thrusting in the central part of the NCA based on uplift due to thrusting at the Brixlegg thrust. a stratigraphic analysis and conodont alteration In cross section, the Brixlegg thrust is offset by index (CAI) data, however no structural data were several south-side-up sinistral faults, which are part given to support the hypothesis. of the (Miocene) Inntal shear zone. This is the reason, that south of the Inn valley the footwall of the Brixlegg thrust cannot be observed at the surface 3.1 Eoalpine thrusting (Fig. 18). At depth, the Paleocene/Eocene Brixlegg thrust must be deformed by the Miocene uplift of Thrusting in the Eastern Alps during the early the Tauern window, however the Miocene activity of Late Cretaceous is usually interpreted to be related the thrust must be rooted in an antiformal stack to closure of the Meliata ocean, which was then below the Tauern window (Fig. 18; see also Fig. 22; located somewhere in the southeast of todays East- Lammerer & Weger, 1998). ern Alps (see paleogeographic reconstructions by The sub-Tauern ramp must be connected to the Schmid et al., 1997 and Stampfli et al., 1998). The Alpine basal thrust, because during uplift of the Adriatic plate, to which the Eastern Alps belong, Tauern window in Oligocene/Early Miocene times, was in a lower plate position at this time; the upper the frontal Alpine thust was still active, as pointed plate is still under dispute. According to Dallmeyer out by Lammerer & Weger (1998). et al. (1998), thrusting propagated from the top to the base of the Cretaceous nappe stack in the eastern central part of the Eastern Alps between Ceno- 3 Discussion manian (ca. 95 Ma.) and the Maastrichtian (ca. 75 Ma.). In the western central part of the Eastern Alps, As the NCA represent the topmost and northern- thrusting started later (Turonian; see review in most part of the Austroalpine nappe stack, they did Froitzheim et al., 1997; Trupchun phase). West- not experience significant metamorphic overprint directed tectonic transport for this time span is well except along their southern margin. Therefore all established (e.g. Froitzheim et al., 1994; Neubauer, deformational events since passive margin forma- 1987) in the Central Alps, even for the Greywacke tion adjacent to the Penninic ocean should be pre- zone (Ratschbacher & Neubauer, 1989), which is in served and documented by brittle structures. As sedimentary contact with the Tirolic nappe unit of Jurassic deformation was accompanied by sedimen- the NCA. West-directed tectonic transport of nap- tation, Jurassic structures were usually recon- pes or slices within the NCA was reported only structed using contrasting facies and thicknesses of locally (e.g. Unnutz fold: Ortner, 2003), and nappe

126 Geol. Paläont. Mitt. Innsbruck, Band 26, 2003 was characterized by NNE-SSW compression. In the L C NCA, the Late Cretaceous synclines and anticlines are systematically offset by sinistral NE-trending faults. In the Central Alps, a dramatic change in NP NCA NCA transport direction took place since the Cretaceous, from W-directed to NNE-directed. The Austroalpine P orogenic lid (in the sense of Laubscher, 1983) expe- M P CA rienced not much internal deformation during the Cenozoic (Froitzheim, 1994), and also in the NCA only Cretaceous to post-Cretaceous sediments

ran por direc ion clearly do show folding due to Paleocene/Eocene contraction (Fig. 11; Fig. 2 in Eisbacher & Brandner, 1996). In other areas, continuing growth of prexist- Fig. 20: Palinspastic model of the Alps in the Late Cretaceous ing fold structures has to be assumed. Paleocene/ following Schmid et al., (1997). Within the Eastern Alps, strain Eocene sinistral faulting across NE-striking faults partitioning at approximately E-W striking faults caused diverging thrust directions in the Central Alps (CA) and Northern Cal- cannot be related to possible continued extension in careous Alps (NCA). Orogenic collapse leading to activity of the central part of the Eastern Alps, as suggested by major normal faults in the hinterland of active thrusting is Froitzheim et al. (1996), because they end in S- delimited to the north by the same set of faults. NP = North directed (Isar- and Loisach faults; Eisbacher & Penninic ocean, SP = South Penninic ocean. Brandner, 1995, 1996) or N-directed thrust faults (Brandenberg; Ortner, 1996). Moreover, near the transport ssociated with folding was toward the NW transition from the thick-skinned Central Alps to the (e.g. Eisbacher & Brandner, 1996) and NNW, as thin-skinned NCA, a major thrust became active shown in this study. Ortner (2003) proposed a sys- (Brixlegg thrust; BT in Fig. 21) and in its western tem of approximately E-W-striking faults along or continuation the Stanzertal fault in the Arlberg area near the southern margin of the NCA, across which (May & Eisbacher, 1999), which were the locus of WNW-directed thrusting could be partitioned into major Paleocene/Eocene thrust activity (see section NNW-directed thrusting in the north and W- 2.2.1). directed thrusting in the south (Fig. 20). Onset of major out-of-sequence thrusting and The basement units of the Eastern Alps were therefore thickening of the orogenic wedge com- affected by extensional collapse caused by bined with sinistral faulting at the orogenic front overthickening of the crust during preceding contraction. Major ESE-directed detachments were active (Silvretta unit: Froitzheim et al. 1997; Ötztal P /E unit: Fügenschuh et al., 2000; Gleinalm: Neubauer MP NP et al. 1995) during west directed transport in the ran por direc ion Campanian and Maastrichtian. This deformational NCA phase is not found in the NCA. Instead, Campanian B and Maastrichtian contraction was oriented N-S CA (Ortner, 2001). A possible reason for the change in the direction of compression in the NCA is the onset of extensional collapse in the central Alps, however the mechanism of strain partitioning is not clear at the moment. Late Cretaceous deformation is summarized in Fig. 20. Fig. 21: Palinspastic model of the Alps in the Paleocene/Eocene modified from Schmid et al., (1997). Due to oblique collision with the distal European margin, sinistral NE-striking faults 3.2 Paleocene/Eocene deformation propagate from the front into the nappe stack and out-of- sequence thrusts within the Alpine wedge develop (BT = Brix- According to Eisbacher & Brandner (1996) and legg thrust). CA = Central Alps, NCA = Northern Calcareous Froitzheim (1994) Paleocene/Eocene deformation Alps, NP = North Penninic ocean, MP = Middle Penninic unit.

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0 km --/11 --/35 --/16 /10 10/13 7/13 /15 13/67 24/ 1 13/24 + . -5 #&"%(%'' &%#!* #$## BT +- &( (#! &%) AA PGL -10 A &(% &( + A+)*(&#'!% )$%* ABT . #,*! )#!) + !##(*# &( . -15 &+* (% A#') E+(&'% )$%* -20 0 km 10 20 30 40 50 *( & )*!&% !''*#- !((+%% ( ! . 23) Fig. 22: N-S Cross section Innsbruck-Franzensfeste drawn using the results of the TRANSALP seismic section. Apatite and zircon fission track ages are indicated by symbols with numbers: white circles – Fügenschuh et al. (1997), black circles –Mancktelow et al., (2001). LN = Lechtal nappe, AA = undifferentiated Austroalpine basement, IQP = Innsbruck Quartzphyllite nappe, ABT = Alpine basal thrust, BT = Brixlegg thrust, PGL = Pustertal-Gailtal line. Trace of section: 2 in Fig. 1.

could possibly be related to oblique collision with (Ratschbacher et al., 1991; Fügenschuh et al., the most distal part of the European margin. This is 1997; Frisch et al., 2000), which was contempo- for example documented in the duration of flysch raneous with stacking of horses and exhumation sedimentation in the Rhenodanubic Flysch, which of the Tauern Window (Lammerer & Weger, 1998) ends in the Maastrichtian in the west (Vorarlberg; and major backthrusting along the Periadriatic Tollmann, 1985) and in the Early Eocene further east line. (Salzburg; Egger, 1992). Paleocene/Eocene deforma- • formation of a peripheral foreland basin at the tion is summarized in Fig. 21. end of the Eocene. These processes are the frame, in which the brittle processes in the NCA must be seen. Especially 3.3 Oligocene/Middle Miocene deformation east of orogen-parellel extension in the Tauern window is the Brenner normal fault seen as a controlling factor on brittle deformation in the NCA and is therefore dicussed here in some The main processes within the Alps controlling detail: Oligocene/Miocene deformation are: •collision of the Adriatic plate with the European margin and accretion of flakes of this margin into 3.3.1 Oligocene/Early Miocene the Alpine edifice. The European distal margin was overthrust after the Bartonian (late Middle Following Fügenschuh et al. (1997), exhumation Eocene), as the youngest sediments in the Ultra- of the Tauern window took place in two stages: helvetic unit at the Alpine front have this age Prior to 13 Ma, only the Penninic Tauern Window (Hagn, 1981). Sedimentation on the Helvetic was in the footwall of the Brenner normal fault, and margin lasted until the Priabonian (Late Eocene; Austroalpine units north of it were in the hanging- l.c.). At the same time, basement slices were wall, so that most of the relative movement inbe- scraped off the European margin to form an tween must have been transferred into the northern imbricate stack in the area of todays Tauern Win- margin of the Tauern window, and further east into dow, which was tilted to the south as the basal the SEMP-line Fig. 1). Moreover, the roof of the Tau- thrust propagated downward (Fig. 22). ern window must have been relatively flat, as it is • ductile orogen-parallel stretching below the still today (see cross sections of Thiele, 1976). Due Brenner and Katschberg normal faults to the drag of the eastward moving Penninic units a

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Fig. 23: E-W Cross section Wipptal-Fieberbrunn drawn using the results of the TRANSALP seismic section. Apatite and zircon fission track ages are indicated by symbols with numbers: white circles – Fügenschuh et al. (1997), black circles – only apatite fission track ages, Grundmann & Morteani (1985). Trace of section: 3 in Fig. 1. secondary stress field developed north and on top of 3.3.2 Middle Miocene the Brenner normal fault, leading to a reorientation of the maximum compressive stress from NNW-SSE Via the Wipptal fault (Fig. 1), the approximately to NNE-SSW, as outlined already by Decker et al. N-S trending Brenner normal fault is connected to (1994) for Miocene times. At the Inntal shear zone, the sinistral ENE-striking Inntal shear zone to the no major sinistral offset due to exhumation of the north. This is a result of Miocene activity of the Tauern window is expected, but rather NNE-directed Brenner normal fault after 13 Ma (Fügenschuh et contraction combined with sinistral shearing, as al., 1997), when the Innsbruck Quartzphyllite unit observed (see section 2.1). was part of the footwall of the Brenner normal The normal fault component across the major fault, which was then extending to the Inn valley in sinistral faults cannot be understood in this sce- the north and called Wipptal fault. Vertical offset nario. On orogen scale, the latest Eocene and Early across the Wipptal fault was in the range of 4-5 km Oligocene was the time, when the Alpine foreland (l.c.), leading to a sinistral offset across the Inntal basin subsided rapidly (e.g. Zweigel et al. 1998) due fault of about 8.5 km, assuming a dip of the Wipptal to flexure of the European plate. As shown by Ort- fault of 30°, or 9 km assuming a dip of 25°. Exhu- ner & Stingl (2001), on top of the NCA a piggy back mation of the Innsbruck Quartzphyllite was not only basin formed, which was part of the Molasse basin due to east-directed upward movement below the during the Oligocene. Strong subsidence in the area Wipptal fault, but also due to synchronous thrusting of the NCA requires that most of the load was at the Brixlegg thrust and other related thrusts (Fig. located to the south. The load was most probably 23). Thickening of units is needed to fill up the created by out-of-sequence thrusting and thicken- space created by uplift of the Innsbruck Quartzphyl- ing of the Alpine wedge in todays Tauern Window lite nappe. after increase of friction at the base of the wedge Sinistral shearing along the Inntal shear zone in related to the arrival of buoyant European continen- Middle Miocene times is part of the lateral extru- tal lithospere in the subduction zone. In the Molasse sion in the sense of Ratschbacher et al. (1991). The basin, a system of approximately WNW-ESE-striking TRANSALP cross section (Fig. 18) offers the opportu- normal faults formed, sealed during sedimentation nity, to examine the 3D-geometry of lateral extru- in the Molasse basin, which are both hinterland and sion in its westernmost part. Ductile E-W extension foreland-dipping (e.g. Figs. 2, 4 and 5 of Bachmann in the Tauern window took place between the S- et al., 1982; Zweigel et al., 1998; plate 1 of Brix & dipping Alpine basal thrust and/or the Brixlegg Schultz, 1993) and formed due to bending stresses thrust in the N and the N–dipping oblique dextral during flexure of the lower plate (see p. 203 in Price backthrust of the Pustertal-Gailtal line due to & Cosgrove, 1990). A possible nucleus of the Inntal strong compression in front of the indenter of the shear zone could be such a hinterland-dipping nor- Southern Alps (Fig. 22). Sinistral faulting across the mal fault, rooted in the European plate. Inntal shear zone delimited the eastward movement

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003 129 Middle Mi cene tionships between outcrop scale faults belonging to datasets related to NNW-contraction and NNE-con- A BA traction are not systematic as well (Fig. 5b), sug-

gesting another phase of NNE contraction in this 20-30 ! ""& " area as well. Ratschbacher et al. (1991) proposed

10 ! ""%& " NNW to NW contraction west of the Brenner line A D during lateral extrusion. The Domleschg phase of EW Froitzheim et al. (1994) probably represents this . $ contraction which may also account for some brittle "%'$ " A ' $ " structures in the NCA. 100 ! The main Oligocene/Miocene structures south of &( ' & """ "#) !"&&#" the NCA are the Engadine Window, another window $ %&$%% # %&$%% !#(!"& # # % of Penninic units within the Austroalpine basement, and the Engadine line (Fig. 1), which was active dur- Fig. 24: Palinspastic reconstruction of the Eastern Alps and ing exhumation of the Engadine window (Froitzheim Southern Alps for Middle Miocene during orogen-parallel exten- & Schmid, 1993; Schmid & Haas, 1989; Pfiffner & sion and sinistral shearing at major faults with directions of Hitz, 1997). Like the Tauern window, the Engadine local stress indicated in different regions of the Alps. Dashed window was exhumed by the stacking of European line S of the Inntal line indicate hypothetical faults needed to basement slices during the Late Oligocene/Early transfer sinistral offset from the Tauern window northern margin to the Inntal line, because of the eastward increasing offset, Miocene. The Engadine line acted as normal fault at indicated by numbers north of the Inntal line. the the eastern side of the window and offset the Ötztal basement unit against the Penninic units by 4 of the units to the south of the Inntal shear zone km in response to passive backthrusting above a toward the N. Helvetiv basement slice actively thrusting toward As shown in this paper above, during the Oligo- the NW (Hitz & Pfiffner, 1996). Therefore no cene/Miocene NNW-SSE contraction and NNE-con- regional extension is needed for normal faulting traction related to sinistral faulting were alternating across the Engadine line. Synchronous uplift at the in the southern part of the NCA, controlled by proc- western block at the southern end of the Engadine esses in the metamorphic core complex of the Tau- led to block rotation along the Engadine line. As the ern window. NNW-SSE contraction is observed in pole of rotation was located below todays earth sur- the NCA, when thickening was dominating over E- face, sinistral movements are observed in the cen- W-extension in the Tauern Window metamorphic tral part of the Engadine line (Schmid & Froitzheim, core complex. This contraction corresponds to the 1993). far field stress as in the Molasse basin (Schrader, Probably more relevance for deformation within 1988) and in the Southern Alps (Castellarin & Can- the NCA has the Turba phase at the western margin telli, 2000; Fig. 24). NNW-SSE contraction prevailed of the Eastern Alps. The Turba normal fault (Fig. 1) during times of orogen parallel extension, in the downthrows the Lower Austroalpine Platta nappe Eastern Alps. When extension dominated over con- against Penninic flysch units approximately 2 km to traction, a secondary stress field with NNE-SSW the east (Nievergelt et al., 1996). Weh & Froitzheim direction was imposed on all units to the north (Fig. (2001) described a northern continuation of the 24). Turba normal fault, the Gürgaletsch shear zone, which ends in the Prättigau half-window. Phases of extension in the Central Alps could lead 3.3.3 Oligocene/Middle Miocene deformation west to the modification of the stress field (NNW-SSE of the Brenner normal fault contraction) in the hangingwall of the normal faults, where extension will prevail and to the north of the West of the Brenner normal fault, Oligocene/ extending region, where extension must be delimited Miocene NNE-compression did overprint structures by steep faults against stable regions. Both events related to Paleocene/Eocene deformation only could lead to a (re)activation of sinistral E(NE)-strik- weakly, and no map-scale sinistral faults with large ing steep faults in the NCA and therefore to creation offset are known. However, the crosscutting rela- of a secondary NNE-SSW oriented contraction.

130 Geol. Paläont. Mitt. Innsbruck, Band 26, 2003 1 2 inverted Cenozoic rift graben structure (Fig. 18). Its ( ) CA CA formation and growth after the end of major thrusting at the Alpine basal thrust could well have led to / normal faulting above, caused by extension above a neutral surface during flexural folding. This type of faulting is restricted to the locus of the fold in the subsurface (see Fig. 15). In other places, normal / E faulting might be older and for example related to Early Oligocene normal faulting during flexure of the European plate (see section 3.3.1) or Cretaceous

L normal faulting as described by (Wagreich & Decker, C 1991).

Fig. 24: Attempt to correlate deformational structures in the 4 Conclusions NCA and in the Central Alps. Data from Central Alps taken from Froitzheim et al. (1997) and Lammerer & Weger (1998). NCA = Brittle deformation recorded in the western part of Northern Calcareous Alps, CA = Central Alps. the NCA can be related to distinct events of Alpine orogeny: 3.4 Late Miocene • Eoalpine thrusting (after Hauterivian) was responsible for initial nappe stacking in the NCA Late Miocene E-W compression is found in various and created folds with NE-trending axes and locations within the investigated area and was also NW-striking cross faults. Subsequent NNW-SSE described by previous authors (e.g. Müller-Wolfskeil contraction led to folding of thrusts and forma- & Zacher, 1984). Peresson & Decker (1997b) relate tion of many of the main folds within the NCA. this stage of deformation in the eastern part of the Strain partitioning across (hypothetic) approxi- NCA to continental collision in the eastern Carpathi- mately E-W-striking shear zones might be ans. This event reactivates many older fault planes, responsible for diverging thrust directions in the but does not create map scale structures. NCA (top to NW to NNW) and Central Alps (top to W). Deformation took place in a lower plate position in relation to closure of the Meliata- 3.5 Post-Miocene Hallstatt ocean. Strain partitioning created local stress fields NNW-SSE extension is mainly found in and near on both sides of shear zones. Therefore both E- the Inn Valley. Similar normal faults have been W-compression in the Central Alps and NNW- related by Decker et al. (1993) and Peresson & SSE compression in the NCA have to be consid- Decker (1997a) to gravitational collapse due to ered as local stress fields. The true orientation of ongoing uplift of the Alpine chain. An alternative the far field stress cannot be deduced from field model is related to the interpretation of the TRANS- data, but must lie between the orientations of ALP seismic section. The basal reflections below the compression in NCA and Central Alps. Molasse basin, which are related to the Jurassic- • Paleocene/Eocene deformation led mainly to fold Cretaceous sedimentary succession of the auto- growth of prexisting folds and formation of new chthonous Mesozoic, can be traced horizontally to folds with WNW-trending axes. Some larger NE- the south under the NCA until some point below the striking faults propagagated from the front of the Inn valley in a depth of 8 to 9 km (Fig. 18). Near the Alpine wedge toward the south. A first set of end of the throughgoing reflections, a similar set of major thick-skinned out-of sequence thrusts like reflections, frequently interrupted, downlap on the the Brixlegg thrust developed. The reason was latter and continue via a open anticline into well- oblique collision with the distal European margin. defined reflections, which clearly represent the During the Paleocene/Eocene, no major differ- autochthonous Mesozoic in a depth of 12 to 14 km. ences regarding the direction of compression The structure inbetween is interpreted to be an between different units in the Alps could be

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003 131 observed. Therefore, NNE-compression is thought Decker, C. Tomek and O. A. Pfiffner. Many thanks to to be represent the far field stress. A. Gruber and F. Reiter for assistence in the field •Post-collisional Oligocene/Miocene deformation and to Ch. Prager, who supplied unpublished fault was characterized by major out of sequence data. thrusting and thickening in the central part of the Alps combined with orogen parallel extension and eastward escape of crustal blocks. Eastward movement took place between N- and S-dipping References out-of-sequence thrusts. The analysis of brittle data in the NCA showed, that the process must Angelier, J. & Goguel, J. (1979): Sur une méthode simple have been episodic. There, alternating phases of de détermination des axes principaux des contraintes NNW-SSE compression and sinistral shearing at pour une population de failles.- C. R. Acad. Sci. Paris, the northern margin of eastward escaping blocks 288, 307 - 310, Paris. were recorded. The tensors indicating NNW-SSE- Auer, M. & Eisbacher, G. H. (2003): Deep structure and directed compression are regarded to represent kinematics of the Northern Calcareous Alps (TRANS- the far field stress, whereas a secondary stress ALP profile).- Int. J. Earth Sci., 92, 210-227, 20 Abb., field with NNE-SSW oriented compression was Stuttgart. created by the drag of eastward moving blocks Bachmann, G. H., Dohr, G. & Müller, M. (1982): Explora- during phases of dominant extension. NNW- tion in a classic thrust belt and its foreland: Bavarian directed compression is still active, as the orien- Alps, Germany.- AAPG Bull., 66, 2529-2542, 11 Abb., tation of compression axes of earthquakes indi- Tulsa. cate (Fig. 6b). Böhm, F., Dommergues, J. - L. & Meister, C. (1995): Brec- cias of the Adnet Formation: indicators of a Mid-Lias- As shown above, the direction of far field stress (1st sic tectonic event in the Northern Calcareous Alps order stress) in the western part of the Eastern Alps (Salzburg/Austria).- Geol. Rundsch., 84, 272-286, 12 changed in a simple way: 1) NW-directed compres- Abb., Stuttgart. sion in the Late Cretaceous, 2) NNE-directed com- Brix, F. & Schultz, O. (1993): Erdöl und Erdgas in Öster- pression in the Paleocene/Eocene and 3) NNW- reich.- Veröff. Nat.-hist. Mus. Wien, 19 (2. Auflage), directed compression in the Oligocene/Miocene (Fig. 668 p., 200 Abb., 62 Tab., 17 Taf., Horn (Berger). 25). The complex deformational history as shown by Brügel, A. (1998): Provenances of alluvial conglomerates the analysis of fault planes above is the result of from the Eastalpine foreland: Oligo-/Miocene denuda- creation of local stress fields by strain partitioning tion history and drainage evolution of the Eastern and faulting during orogen-parallel extension (2nd Alps.- Tübinger geowiss. Arb., 40, 168 p., 65 Abb., 10 order stress). These stress fields can be expected to Tab., Tübingen. follow each other systematically. On outcrop scale, Castellarin, A. & Cantelli, L. (2000): Neo-Alpine evolution the formation of structures related to 2nd order of the Southern Eastern Alps.- Journal of Geodynam- stress is accompanied by local stress fields (3rd ics, 30, 251-274, 10 Abb., Oxford. order stress). Mechanisms creating 3rd order stress Channell, J. E. T., Brandner, R., Spieler, A. & Smathers, N. P. include shearing within shear zones with large off- (1990): Mesozoic paleogeography of the Northern Cal- set, spreading of material parallel to fold axes dur- careous Alps - evidence from paleomagnetism and ing folding or extension above a neutral surface facies analysis.- Geology, 18, 828 - 831, 5 Abb., Boul- during flexural folding. These stress fields will not der. show any systematic succession of events. Channell, J. E. T, Brandner, R., Spieler, A. & Stoner, J. S. (1992): Paleomagnetism and paleogeography of the Northern Calcareous Alps (Austria).- Tectonics, 11, 792 5 Acknowledgements - 810, 20 Abb., 3 Tab., Washington. Dallmeyer, R. D., Handler, R., Neubauer, F. & Fritz, H. The author was supported by the Austrian Sci- (1998): Sequence of thrusting within a thick-skinned ence foundation (Project P13566-TEC). The ideas tectonic wedge: Evidence from 40Ar/39Ar and Rb/Sr presented in this papers were developed and modi- ages from the Austroalpine nappe complex of the fied by discussions with F. Reiter, R. Brandner, K. Eastern Alps.- Journal of Geology, 106, 71-86, Chicago.

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