Transpressional collision structures in the upper crust: the fold-thrust belt of the Northern Calcareous Alps

TECTONOPHYSICS ELSEVIER

Tectonophysics 242 (1995) 41-61

Transpressional collision structures in the upper crust: the fold-thrust belt of the Northern Calcareous Alps Hans-Gert Linzer, Lothar Ratschbacher, Wolfgang Frisch Institut fiir Geologic und Paliiontologie, Universitiit Tiibingen, D-72076 Tiibingen, Germany

Received 22 February 1993; revised version accepted 18 October 1993

Abstract

Two major structural events characterize the tectonic evolution of the Northern Calcareous Alps (NCA): (1) late-Early Cretaceous to Late Eocene NW-directed, dextral-transpressional stacking of nappes as an expression of the formation of the Austroalpine orogenic wedge; and (2) Miocene sinistral wrenching due to eastward lateral extrusion of crustal wedges along the central Eastern Alps. Three first-order detachment horizons are defined by differences of competence within areas of facies transition. Preexisting normal faults control the thrust architecture. Transpressional contraction in the NCA is indicated by: (a) NW-directed thrusting, oblique to both the long axis of the NCA and the edge of the orogenic foreland; (b) the occurrence of en-6chelon arrays of thrusts and folds laterally displaced by dextral strike-slip faults; the thrusts and strike-slip faults are kinematically connected to each other and dissect the NCA into rhomboidal blocks; (c) NE-directed extension parallel to fold and ramp axes and the internal strike of the NCA; and (d) clockwise rotation (>~ 30°) of the entire NCA around a vertical axis. Kinematic and dynamic analysis of mesoscale fault-striae data related to transpressional stacking indicates a NW-trend of contraction directions and, in general, a NE-trend of the extension directions, parallel to fold axes and branch lines of thrusts. Before rotation, the average contraction direction was probably parallel to the generally W-directed shear direction recorded in the crystal-plastically deformed central part of the Eastern Alps. Three balanced cross-sections across the NCA yield a minimum of 54-65% total shortening.

1. Introduction

Within the scope of studies of oblique collision in orogenic belts, this study presents a kinematic analysis of the early Alpine (late-Early Cretaceous to Late E o c e n e ; e.g., G a u p p and Batten, 1983; Tollmann, 1986, pp. 90-92; Kralik et al., 1987; T r o m m s d o r f et al., 1990) transpressive contraction structures of the N o r t h e r n Calcareous Alps (NCA). T h e N C A are part of the Austroalpine orogenic wedge and adjoin the Penninic ( R h e n o d a n u b i a n ) Flyschzone in the north and

the Austroalpine b a s e m e n t nappes in the south. It constitutes a multiply deformed, thin-skinned fold and thrust belt (Fig. la). It formes part of the Mesozoic continental margin o f the Apulian microcontinent with Triassic and Jurassic rifting and orogenic convergence as well as continental collision occurring from the late-Early Cretaceous to the Miocene (e.g., Frisch, 1979). Thrusting and crystal-plastic n a p p e internal flow toward the west during orogenic convergence are d o c u m e n t e d in the Austroalpine basem e n t n a p p e s (Fig. la; e.g., Ratschbacher, 1986;

0040-1951/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0040-1951(94)00152-9

"~ .f ~ l~

I /'7'~ ~ ..~

+-++"

.

4+--+4÷4+4 ~"g-"+"-+ + + + + +

Folded Molasse

major Neogene faults

Undeformed Molasse

strike-slipfaults (Northern Calcareous Alps)

Neogene basins

50

.

103k~n

"+++4+÷ ++4-44 -F4÷ 4 4 +÷

-- g +

~

.

displacement direction

windows

W Penhinic stretching lmeauon

Rhenodanubi~ Flysch

+ 4 4 4 4 4 + + + + ~ "~'r"t--.4..~4+4444++

• . . . . i:.'.'

53°+30°

Helveficum

~/~

c~3

and minimum principal stresses

mean orientation of maximum

STRESS ORIENTATION

~ Triassic-Jurassic sediments

+ #4-+4++44/~

majorthrusts (Northern Calcareous Alps)

o

"+ .4- + + +

+ +++44 +++

~ 139°+19°~ -

6l

80° • Gosau deposits ~

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~

÷ r ''--'+='-'~-'~/-'~'~ ~ 4

10 °

~

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= ~_~_~30 o ---~--gl 1 0 ~ =

STRESS RATIO

I ~h'.,-#N~,] l #/p~,7~

Bohemian basement

Bre

axis

of~

plunge

[~.

~o,

fault plane solution • P-ax/s,o'I o T-axis,~3

Northern Calcareous Alps

Quarzphyllit units

4--

. . . . ii

2

Grauwackenzone

Aus~oNpine basement

Sou~ Alpine umt

===============================

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~~*'%'$**$$$****÷÷..+** ........

~++÷÷÷÷÷++++÷+++÷++++÷+ ~++++++÷÷÷÷+÷++++÷+÷+++.+[

4~

Innsbruck

100 km

1 /nntal nappe Lechtal nappe 3 Allg~iu nappe

. . . . .

western NCA

central NCA 4 Dachstein nappe 5 Hatlsiatt nappe 6 Warscheneck nappe 7 Totengebirge nappe " 8 Staufe'n/H011engeb. nappe 9 Reichraming nappe 10 Ternberg nappe

Salzburg

eastern NCA 11 Schneeberg nappe 12 Miirzalpen nappe 13 G611 nappe i 14 lJnterberg nappe 115 Reisalpen nappe 16 Lunz nappe 17 Frankenfels nappe

generalized subdivision: Upper Juvavikum Lower Juvavikum Tirolikum Tirolikum Tirolikum Upper Bajuvarikum Lower Bajuvarikum

Wien

Fig. 1. (a) Tectonic sketch map of the Eastern Alps. The kinematic data from the central Eastern Alps are from Ratschbacher et al. (1989). Fault-plane solutions are calculated from principal stress directions derived from mesoscale fault-striae data. See Table 1 for parameters of deviatoric stress tensors and locations. Stress-orientation diagram gives directions (mean and standard deviation) of 0-~ to 0"3 (0"1 >~0"2 >/0"3, principal stresses) calculated from all stations. Stress ratio diagram plots calculated stress ratios R = (0"2- 0-3)(0"z- 0"3)-x (1, uniaxial extension; 0, uniaxial compression) versus plunge of the ~rI direction for the reverse, normal, and strike-slip faulting tectonic regimes; diagram modified from Oncken (1988). (b) The thrust systems of the Northern Calcareous Alps (NCA) modified from Tollmann (1976b).

(D

I

4~

t,,a

bo

6

(.,,

44

H.-G. Linzer et al. / Tectonophysics 242 (1995) 41-61

Ratschbacher et al., 1989; Schmid and Haas, 1989; and references therein) along the Arosa suture zone (Ring et al., 1989) and in the Western Alps (e.g., Platt et al., 1989; Butler, 1992; and references therein). Convergence is frontal, i.e. perpendicular to the foreland in the Western Alps (Austroalpine and Penninic units onto the European foreland), and transpressive, i.e. oblique to the foreland in the Eastern Alps. In this paper transpression is considered as representing strike-slip shear, horizontal shortening, as well as vertical lengthening acting together across and along a finite deformation zone (e.g., Sanderson and Marchini, 1984). Large-scale movement of thrust sheets of the NCA toward the west to northwest, first recognized by Reis (1910) and Ampferer (1939), was historically ignored and fell into disuse due to the dictation of models of frontally outward thrusting in the Alps, which imply top-to-W displacement in the Western Alps and top-to-N displacement in the Eastern Alps (e.g., Heim, 1922; Kober, 1923). Reevaluation of original mapping as well as regional mapping for orogenic contraction estimates based on balanced cross-sections in the western (Linzer, 1989; Eisbacher et al., 1990), central, and eastern parts of the NCA (Linzer et al., 1990, 1991a) revealed non-cylindrical contraction structures on all scales. Characteristically, the structures (fold axes, thrusts) trend east-northeast to northeast, oblique to the overall trend of the NCA and the edge of the European foreland, and are disrupted by sets of dextral strike-slip fault zones trending westnorthwest; we (Linzer et al., 1991a, b) interpreted this structural geometry as being indicative of an oblique approach of the Austroalpine units toward the foreland. Another feature, so far unexplained by a tectonic model, is a palaeomagnetically documented rotation of >/30 ° of the entire NCA (Mauritsch and Frisch, 1978; Channell et al., 1992). In this paper we lay out the map-scale structure of the NCA as a starting point for a future mechanical analysis of oblique collision in the upper crustal sequence of the Eastern Alps. We outline the stratigraphy of the NCA emphasizing the major detachments. The macrostructure of the NCA is additionally delineated by structural

maps of its western, central, and eastern parts. Furthermore mesoscale fault-slip analysis is used to characterize distinct structures and to construct trajectories outlining the orientation of the principal stresses of Cretaceous to Early Tertiary shortening in the NCA. Three balanced and restored sections, based on surface, seismic, and well data characterize the subsurface structure of the NCA. Finally, we discuss the structural architecture of the NCA in terms of oblique convergence.

2. Tectonic stratigraphy and structural setting of the Northern Calcareous Alps A discussion of the complex stratigraphy and facies distribution of the NCA is beyond the scope of this work. We refer the reader to Tollmann (1976a) for a review, and to Fruth and Scherreiks (1982, 1984), Lein (1987), Faupl et al. (1987), Decker et al. (1987), and Channell et al. (1992) for recent contributions. In the following we present a generalized mechanical stratigraphy suitable for a map-scale analysis of the thrust and fold structures of the NCA. The NCA consist of a 3-5-km-thick P e r m o Mesozoic sedimentary succession (Fig. 2a). Incompetent claystone, marl, and evaporite form three major detachment levels (Fig. 2b). The basal detachment follows Upper Permian evaporitic and clayey rocks (Fig. 2a). In the western part of the NCA, the basal detachment steps down to the south into medium- to low-grade Variscan schists, gneisses, and phyllites (Otztal and Phyllitgneis units), in the eastern part into weakly metamorphosed Palaeozoic rocks (Grauwackenzone), all part of the Austroalpine basement. The major ramps are located within two competent Triassic carbonate complexes. The lower one (AnisianLadinian, Fig. 2a) is between 0.6 and 1.5 km thick and comprises thick-bedded reef-platform carbonates and, locally, basinal marly limestone. The upper one (Norian-Rhaetian, Fig. 2a) is 0.4-2 km thick and consists of bedded, supra- to subtidal dolomites in the northern and western parts of the NCA. In the southern and eastern parts, it encompasses well-bedded lagoonal and massive

H.-G. Linzer et al. / Tectonophysics 242 (1995) 41-61

reef limestones. Alternating siliciclastic to carbonitic Carnian black shale as well as evaporitic series separate the carbonate complexes and form the generally 100-m-thick middle detachment level (Fig. 2). Deformation of the upper carbonates is mainly by flexural-slip folding in contrast to the massive lower carbonates, which are generally thickened by imbrication. The Triassic shallow-water carbonates are overlain by Liassic to Lower Cretaceous open marine sedimentary rocks (Fig. 2). These thin-bedded marls and siltstones form the upper detachment level. Locally preserved erosional relics of synorogenic sediments consisting of the Rol3feld (late Valanginian to early Aptian; e.g., Faupl and Tollmann, 1979), Losenstein (Albian; e.g., Gaupp, 1983), and Branderfleck beds (Cenomanian to Coniacian; e.g., Weidich, 1984) were probably deposited along leading edges of major thrusts and indicate deposition in a deep-water environment (e.g., Faupl and Tollmann, 1979; Gaupp, 1983; Weidich, 1984;

45

Faupl and Wagreich, 1992). The deposits of the Gosau group (late Turonian to Early Eocene) record a change in sedimentation (Wagreich, 1991). High subsidence in the late Turonian (about 90 Ma) characterize pull-apart type basins with terrestrial and shallow-marine deposits. A second peak in subsidence led to deep-water turbidite sedimentation in the Santonian (about 85 Ma; in the western part of the NCA) to Maastrichtian (about 66 Ma; in the eastern part). The basal detachment of the NCA climbs up on ramp-flat systems from the basement to the middle detachment level (Fig. 2b). The southernmost (internal) nappe systems contain basement wedges (e.g., Eisbacher et al., 1990), the central nappe systems are detached within the PermoScythian series and the northern ones are generally sheared off at the base of the Carnian series. The formation and geometry of individual thrust sheets (Fig. lb), as revealed by the restored sections below, are primarily controlled by Triassic-

® group ian-Eocene (92-40 Ma)

A

Cretaceous

shale, radiolarite, clastic rocks z - Lower Cretaceous (210-90 Ma)

Jurassic

Carbonate lex 1-Rhaetian H0 Ma) -2000m) shale, sandstone, rite an (230-220 Ma) 100 m) r Carbonate plex an-Ladinian 230 Ma) -1500m) c sediments Jan (250-240 Ma) , evaporite r Permian (260-250 Ma)

Pe!an ~ ' ~

simplified stratigraphy (cross sections Fig. 5)

Fig. 2. (a) Generalized tectonostratigraphic column of the Northern Calcareous Alps. (b) General ramp-flat geometry: the tectonically lower and more northerly thrust systems are detached along higher stratigraphic levels than the tectonically higher and more southerly thrust systems.

0

lO

20

30 km

HJndelan

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I Tegestal

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•: . . : . : ~ ? , .

' 0

Inntal nappe

Lechtal nappe

-

~

___-_-z2

I

deep well

~

Penninic units

~ southern margin of the I.-'I Northern Calcareous Alps ~ QuarEphyllit units i....... 1 I' .' ." ." i' ." I Austroalpine r ." ." .. - - i basement nappes strike-slip fault anticline sync/ine ~ dip symbols ~/.,.-\j "

thrust

~ ~ J

~:~]carnien

.-.....:zz:::z==::=z=z_-

U. Cretaceous (Gosau)

-

.-:

Jurassic , L. Cretaceous

-

-

~

~

l

-

.' ." ..' ~ n n S b r l ] c k i _ = - _. . . .z.- - ~

~

:?sd

Fig. 3. Tectonic maps and mesoscale fault-slip data of the Northern Calcareous Alps. Stereonets visualize fault-striae data: faults are drawn as great circles, arrows indicate the slip direction of the hanging wall, and head styles express degree of confidence of the slip-sense determination (full, certain; open, reliable; half, unreliable; without head, very unreliable slip-sense determination), l to 3 = maximum, intermediate, and least principal stress directions, respectively.

Vaduz

i

+

NCA western area

@

,..,,.

I

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e~

-

-

--'2.

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:::--:-:---<---i~ fi~ui;i~;~---.. X:_--'.-:.-:.----::.::-I~.:Z-.-:':.--~':

;-,

O taufen- HNlengebirgenappe O Totengebirgenappe O arschenecknappe O Berchtesgaden/ Dachsteinnappe O Hallstattnappe

N C A central area

1~

2 Kasbargflat

n

0

10

4 Kirchdorf

Fig. 3 (continued).

3 Kasbergramp

20

~~~i---_]_ ~ ~ i ~ i ~ ~ thrust ~----~____.]stri ke-slifault p r anticline,syncline ~..~},~ldip.symbols deepwell

C,eta=oos
8 Hengstpass

~

eenninic units

~Grauwackenzone

[~Carnian p~"_'~'..~"]southernmarginof the I~"- tNodhernCalcareousAlps ~ Quarzphyllunits it

~Jurass,c - L.Cretaceous

//--
6 Pyhrnpass 7 Pyhrnpass- Gosau

Gmunden

30 km

5 Steyrling

I

t~

"x.

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pi0 - sneqnal\l oz

Guno! - sneynaN oz

~(panu!]uo3) wlp!aN

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- U.IIV la6Jag

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6JX,WUV -

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w-wa

GJaquayaJa 6

11.-G. Linzer et al. / Tectonophysics 242 (1995) 41-61

Jurassic rifting (e.g., Vecsei et al., 1988; Channell et al., 1992). Ramps developed in areas of competence contrast caused by preexisting normal faults change in thickness and facies transition. In the western NCA (Fig. 3a), for example, the Ladinian platform-basin transition is located between the Inntal nappe and the Lechtal nappe (Kraus and Schmidt-Thom6, 1967). There, the approximately 800-m-thick Ladinian carbonate platform of the Inntal nappe was thrust over the about 100-mthick Ladinian basin claystone and marly siltstone of the southern Lechtal nappe. In the central NCA (Fig. 3b), the transition from well-bedded dolostones to massive limestones of the Norian carbonate series between the H611engebirge and Totengebirge nappe probably controlled ramp formation. In the eastern NCA (Fig. 3c), stacking of the Lunz nappe developed by reactivation of a synsedimentary normal fault indicated by an extreme change in thickness of Carnian rocks.

3. Structural map of the Northern Calcareous AIps

A structural map of the NCA (Fig. 3) was compiled from geological maps of the Austrian and Bavarian geological surveys, our own mapping along traverses, and the structural analysis of fault zones in key areas. The map contains the following stratigraphic markers: the Carnian beds, the Jurassic-Lower Cretaceous beds, and the Gosau group. 3.1. T h e w e s t e r n a r e a

The western area of the NCA consists of three major thrust sheets: the Allg~iu, Lechtal, and Inntal nappes (Fig. 3a). The generally ENE-trending thrust and fold structures are cut by a set of dextral and sinistral strike-slip faults. The NEstriking sinistral faults are Miocene structures and are associated with the eastward lateral extrusion deformation stage of the NCA (Ratschbacher ct al., 1991a, b). They prograded from the foreland (molasse basin) into the NCA and show decreasing displacement from north to south. The northern margin of the Inntal nappe is, for exam-

49

pie, displaced by a set of sinistral strike-slip faults west of the Vorderriss well (Fig. 3a) and south of Garmisch-Partenkirchen (Fig. 4a) (Linzer, 1989); these faults disappear toward the south. The sinistral set is superimposed on a set of dextral strike-slip fault zones. Along major dextral strike-slip zones, such as the Tells fault zone west of Innsbruck with a displacement of 7.5 km (Fig. 4a), fold axes are dextrally dragged with variable plunging axes constituting axial depressions and culminations (Linzer, 1989; Eisbacher and Brandner, 1993). The northern edge of the Inntal nappe is stepwise displaced by a set of dextral strike-slip faults (Figs. 4a, 4b). First-order faults show displacements up to 1.5 km and are arranged en 6chelon in distances between 2 to 4 km (Fig. 4a). Parallel to these first-order faults, narrowly spaced, second-order faults disrupt, also stepwise, the Inntal nappe with dextral displacements of a few metres. Some of the first-order faults form horse-tail structures, i.e. they fan out when approaching thrust boundaries (e.g., Fig. 4b). These strike-slip faults have no continuation in the footwall represented by the Lechtal nappe: consequently, the two thrust sheets were mechanically decoupled. NW-directed thrusting of the Inntal nappe onto the Lechtal nappe is facilitated by incompetent marls of the upper detachment zone in the Lechtal nappe. Mesoscale fault-striae data in Jurassic marls below the sole thrust of the Inntal nappe indicate NW-directed contraction along obliqueslip faults (station Tegestal, Fig. 3a). The structurally high thrusts are open-folded, indicating refolding during foreland progradation of thrusting. Along strike the folds and thrusts form axial culminations and depressions outlined by an alternating, but generally easterly plunge of the structural grain. The culminations and depressions are, at least locally, arranged en 6chelon and are particularly well outlined by the erosional trace of the Lechtal nappe which forms half windows and klippens (Fig. 3a). Thrusts and backthrusts within the southern Lechtal nappe are laterally connected by dextral strike-slip faults (Fig. 4b) forming a kinematically connected system of strike-slip and thrust duplexes (Linzer, 1989).

50

H.-G. Linzer et al. / Tectonophysics 242 (1995) 41-61

A n obviously rheologically controlled difference in the structural architecture occurs between the Inntal and Lechtal nappes. T h e f o r m e r has a rigid core due to the thick-bedded reefplatform carbonates of the lower carbonate complex. N W - d i r e c t e d convergence is a c c o m m o d a t e d by thrusting as well as slip along dextral strike-slip faults. T h e latter comprises the b e d d e d u p p e r c a r b o n a t e complex and the d o m i n a n t structures are folds, often with variable axial orientation and axial culminations and depressions along their

trend. W e interpret both the curved fold traces as well as the en-6chelon culminations and depressions as an effect of a c c o m m o d a t i o n of dextral strike-slip along WNW-striking faults in the more i n c o m p e t e n t rocks of the Lechtal nappe. 3.2. T h e c e n t r a l area

T h e central area of the N C A consists of the S t a u f e n / H 6 1 l e n g e b i r g e , Totengebirge, Warscheneck, B e r c h t e s g a d e n / D a c h s t e i n and Hallstatt

® I

Garmisch -Patenkirche~l~r ' ~

~

..~,

]

, n o , a na00e

Lechtal nappe @

AIIg~iu nappe U. Cretaceous (Gosau) Jurassic - L. Cretaceous

r

®

"

Carnian

,,

Quarzphyllit unit

©,

basement nappes ,~f----] thrust ~ ~

strike-slip fault anticline, syncline

dip-symbols

~

dip, dip direction

(altitude) in m

1 km

Fig. 4. (a) Structures of the central Inntal and Lechtal thrust systems. (b, c) Restoration of movement along dextral strike-slip faults to minimize mass flow out of the plane of the cross-sections. Modified from Linzer (1989).

H.-G. Linzer et al. / Tectonophysics 242 (1995) 41-61

nappes (Fig. 3b). As in the western area, the folds and thrusts are cut by a set of dextral and sinistral strike-slip faults. In the central area, the sinistral faults are generally parallel to the trend of the main structural features such as fold axes and thrust boundaries. Two major sinistral strike-slip fault zones cross the mapped area between G m u n d e n and Liezen: the Traunsee fault (Decker and Jarnik, 1992) in the north with a displacement of about 10 km and the SEMP (Salzach-Ennstal-Mariazell-Puchberg) fault in the south (Fig. 3b). Both fault zones are of Miocene age (Ratschbacher et al., 1991b); the age being given by syntectonic sedimentation along releasing structures (e.g., Vienna and Hieflau basins; e.g., Royden, 1985). The SEMP-Iine is the main sinistral strike-slip fault zone bounding the eastwardly extruding wedges of the central Eastern Alps in the north (Ratschbacher et al., 1991b; Linzer et al., 1991b). It extends 400 km W S W E N E from the central Tauern window in the west to the Vienna basin in the east and forms an orogen-parallel strike-slip zone now exposed from deep crustal to surface levels. Displacement decreases from 60 km in the west to about 20 km in the east. Strike-slip displacement occurred along broad, en-6chelon shear zones formed under amphibolite to upper greenschist metamorphism in the western part. In the central part, it constitutes a ductile-brittle fault zone concurring with decompression and cooling of rocks in the Tauern window, dated between about 25 and 13 Ma (von Blanckenburg et al., 1989). It anastomoses across the cover thrust belt of the NCA in the eastern part. Three large-scale dextral strike-slip fault zones segment the central NCA: the Lammertal, Wolfgangsee, and Windischgarsten fault systems. The Wolfgangsee and Windischgarsten faults contain windows exposing the footwall of the NCA, the Flyschzone, which otherwise lies in a depth of about 2 km in this area (Hamilton, 1989). Assuming that the rocks of the Flyschzone were brought up along lower structures by oblique slip, and using the 8 ° to 10° plunge of striation along the main strand of the Wolfgangsee fault, the horizontal displacement was at least 10-15 km. The B e r c h t e s g a d e n / D a c h s t e i n nappe front is dis-

51

placed by about 30 km by the Lammertal fault system. Fine-grained mica in fault gouge of an Upper Triassic limestone dated along the southern part of the Lammertal fault zone yielded R b - S r and K - A r ages between 96 and 104 Ma (Kralik et al., 1987). Thrusting oblique to the overall strike of the NCA is documented on the ramp-fiat system of the Totengebirge nappe: fault-striae data in the surrounding of the main thrust in the GriJnau area indicate NW-directed thrusting on SSE-dipping planes in the ramp area (Fig. 3b, station Kasberg ramp), and NW-directed displacement on SE-dipping planes in the flat area (Fig. 3b, Kasberg fiat). Similarly, all other stations also record N W - S E contraction. 3.3. T h e eastern area

The eastern section of the NCA consists of the Frankenfels, Lunz, Reisalpen, Unterberg, G611, Miirzalpen, and Schneeberg nappes (Fig. 3c). Miocene lateral extrusion of crustal blocks of the central Eastern Alps affected the eastern part of the NCA most severely (Ratschbacher et al., 1991b). The SEMP-Iine truncated the eastern n a p p e systems, eastwardly displacing the Mfirzalpen and Schneeberg nappes. NE-striking faults, branching off the main strand of the SEMP-Iine to the north, displace older thrusts, folds and strike-slip faults (Linzer et al., 1991b). The Cretaceous-Early Tertiary transpressional structures are preserved in large blocks between the sinistral strike-slip fault zones. NE-trending folds and thrusts are displaced dextrally by strike-slip faults. Displacements along large dextral faults, e.g. the Hochwart fault, reaches 10 km (Fig. 3c). The generally NE-trending folds are bent into an E - W - t r e n d when approaching the dextral strike-slip faults, e.g. large-scale fold axes in the Lunz and the western G611 nappes. The thrusts at the base of the western and central part of the G611 nappe are stepwise displaced by dextral strike-slip faults (Fig. 3c). The sole thrust of the G611 nappe in the east (south of Mitterbach well) splits to the west in a set of imbrications and folds; there the dextral strike-slip faults act

52

H.-G. Linzer et al. / Tectonophysics 242 (1995) 41-61

as displacement

transfer

between

and folds.

thrusts

structures

("tear

sense

faults")

in the raw data

of homogeneous sive deformations ferentiated

4. Fault-slip analysis and palaeostress trajectories

and

lateral ("fault-striae

to roughly

characterize

scale fault systems. tion,

sense

chronology

of

data") on an outcrop the geometry

Fault

slip,

and

attitude,

( F i g . 3, T a b l e

slip

and

to as stations

1). O v e r p r i n t i n g

in t h e f i e l d , d a t a c l u s t e r i n g ,

lated

of the map-

and determined

field at 24 localities referred

deformation

scale

criteria

The

incorporation

its

and

ages into deformation

below

and incompatible

slip

to Early

of faulting

structures

Miocene

of Miocene

here we present

of sedimentary

crop-scale

and

details on sub-

characterization

dating

data re-

Tertiary is b a s e d

rocks

well

dated

the

of out-

map-scale

fault zones. Cretaceous

to Early Tertiary

affected

pre-Gosau

predominantly

deforon

of different

and the correlation with

difscale

Cretaceous-Early

paper)

We will present

in the

observed

(this

elsewhere;

succesWe

on a regional

to reflect

to the Cretaceous

mation.

striae orienta-

polyphase

were measured

extrusion.

during

all s t a t i o n s .

subsets

convergence

set separation

In the following, we use faults with slickenside lineations

at nearly

them

the superposition

sets formed

two major

interpret

Tertiary

indicate

data

faulting

rocks. Where

Table 1 C r e t a c e o u s - E a r l y Tertiary contraction in the Northern Calcareous Alps: location of stations and parameters of the deviatoric stress tensor No.

Sites

E long. N lat.

Method

Nr.

o.t

o.2

o3

R

F (0)

l 2 3 4 5 6 8 13 15 t7 18 19 20 20 21 22 23 24

Tegestal Kasberg-flat Kasberg-ramp Kirchdorf Steyrling Pyhrnpass Hengstpass Altenmarkt Wentneralm Hinterwildalpen Greith GuBwerk Neuhaus-old Neuhaus-young Neidfleck Vorstadtau Hinterleiten Annaberg

10o14 ' 47o20 ' 13058 ' 47°47 ' 13°58 ' 47°46 ' 14°08 ' 47°51' 14°09' 47°48 ' 14o17 ' 47o36 ' 14o26 ' 47°42 ' 14040 ' 47044' 14°51 ' 47°39 ' 14°55 ' 47039 , 15013' 47043 ' 15018 ' 47045 ' 15°11 ' 47°47 ' 15011' 47°47 ' 15°00' 47°53 ' 15°03' 48001 ' 15004 ' 47052 ' 15023 ' 47053 '

inversion iteration iteration dihedra iteration dihedra iteration iteration dihedra dihedra iteration dihedra dihedra dihedra dihedra dihedra iteration dihedra

21-21 34-30 19-16 10-10 19-16 16-16 12-11 36-26 19-16 05-05 32-29 06-06 19-19 31-31 21-17 17-15 10-08 10-10

341 10 130 00 148 20 127 45 350 10 119 30 150 10 330 10 097 04 334 22 290 10 350 12 196 12 342 30 317 07 305 05 332 70 339 03

072 05 040 06 268 54 334 42 245 56 325 57 278 74 212 70 239 85 215 50 192 39 081 05 004 78 186 58 053 43 212 40 155 20 086 78

189 78 220 84 047 28 231 14 086 32 216 12 058 12 063 17 007 03 079 32 032 50 194 77 105 02 078 11 220 46 041 50 065 01 249 12

0.2 0.6 0.3

06 11 11

0.1

21

0.7 0.1

08 12

0.3

14

0.7

13

14017 ' 47°37 ' 14023 ' 46°44 ' 14°31' 46044' 14031' 46°45 ' 14034' 46043 , 14055 ' 47o40 ' 14051' 47°39 '

inversion iteration iteration iteration iteration inversion dihedra

19-19 13-11 67-59 26-23 10-09 19-19 16-15

308 334 300 140 326 342 284

038 03 154 50 032 09 230 42 146 60 241 54 183 48

200 244 162 050 236 078 023

0.48 0.7 0.1 0.1 (1.2 0.12

15 19 17 15 15 10

Gosau deposits 7 Pyhrnpass 9 Breitenberg 10 Saigerin 11 Berger Aim 12 Unterlaussa 16 Torstein 25 Bachler

01 40 10 00 30 I)8 10

87 00 77 48 00 35 40

For methods used to calculate reduced stress tensors see text. No. = location n u m b e r in Fig. 3. Nr. = n u m b e r of fault measurements and n u m b e r of m e a s u r e m e n t s compatible with the solution, o-t-o. 3 = azimuth (first number) and plunge (second number) of the principal stress axes. T h e stress ratio R = (o-2 - 0"3)(o-~ - o-3) ~ (1 is uniaxial extension, 0 is uniaxial shortening). The fluctuation F gives the average angle between the measured striae and the orientation of the calculated theoretical shear stress.

H.-G. Linzer et al. / Tectonophysics 242 (1995) 41-61

deformation truncated Gosau deposits (see e.g. Table 1), structures can be related to basin geometry (Wagreich, 1991, 1995-this issue; Eisbacher and Brandner, in press, and unpublished data); Wagreich (1991), for example, related the deposits of the stratigraphic older succession of the Gosau group to pull-apart basins formed along the WNW-trending strike-slip faults. Lateral extrusion structures deform the Gosau rocks on a regional scale; deformation is concentrated along E N E - s t r i k i n g first-order, and N E - s t r i k i n g second-order map-scale structures such as the SEMP-Iine and faults branching off toward the northeast. We also use the fault-striae data to study the state of palaeostress associated with brittle deformation (Table 1 for parameters of the deviatoric stress tensor). For assumptions and critical analysis of methods of kinematic and dynamic analysis of faults, the reader is referred to review papers (e.g., Angelier, 1984; Allmendinger et al., 1989). We stress that several assumptions are not strictly met in the analysis of the NCA in terms of stress due to the strong deformation and the inherited heterogeneity of its rock pile. For convenience, however, we will refer in the following to stress instead of strain and displacement and will not interpret our data in a rigorous, quantitative way. We use graphic and numeric methods to derive principal stress orientations. The quality and the quantity of field data determined the selection of the method used for calculation. The P - T dihedra method (see e.g. Allmendinger et al., 1989) was used with scarce data and where the outcrop conditions prohibited a careful analysis of fault and striae characteristics. The rest of the stations were analyzed numerically either by the direct inversion method (Angelier, 1979) or the grid search method (Gephart, 1990; Hardcastle and Hills, 1991). The data base is available from the authors. The raw data are plotted in Fig. 3. Fig. la depicts the orientation of maximum (o-l) and minimum (o-3) compressive stresses. The average o-1 trend is 139_+ 19° (mean and standard deviation), that of o-3 is 53 _ 30 °, parallel to the general strike of branch lines and ramps. Seventeen out of 26 solutions have o-3 trending subhorizon-

53

tal and N E - S W ; the average o-3 trend of these stations is 54 _+ 27 °. In addition to stress orientation, the computation of the reduced stress tensor determines the ratio R, which expresses the relationship between the magnitudes of the principal stresses. Extreme values of R correspond to stress ellipsoids with o-2 = o-3 (R = 0) or o-1 = o-2 (R = 1). Most ratios (Table 1) show low R values (mean R = 0.31), which indicate that o-2 and o-3 had similar magnitudes. Numerous NW-trending, steeply dipping tension gashes (an example is shown for a hybrid strike-slip-thrust solution in Fig. 3c, station Berger Alm) indicate N E - S W extension. Several of the fault-striae data characterize distinct structural geometries (see Fig. 3). In the Grfinau area of the Totengebirge nappe (Fig. 3b), for example, the mesoscale fault pattern gives a thrust solution with subhorizontal o-~, subvertical o-3, and R = 0.6 for a flat area (station Kasberg flat). The ENE-trending ramp area gives a hybrid strike-slip-thrust solution with R = 0.3 (station Kasberg ramp), indicating thrusting over an oblique ramp. A station within the Lunz nappe, just below the Unterberg nappe (station Altenmarkt), demonstrates oblique-slip thrusting along bedding planes kinematically connected with dextral strike-slip faulting. Fault-striae data collected along several of the map-scale dextral strike-slip fault zones (Figs. 3b, 3c; stations Steyerling, Hengstpass, Hinterwildalpen, Greith, Neidfleck, Vorstadtau, Hinterleiten) are strike-slip solutions with °'3 subhorizontal and NE-trending. At all stations, N W - S E compression is documented, emphasizing the obliquity of the Cretaceous-Early Tertiary stress field to the E - W trend of the European margin (Fig. la). Note also that there is no significant difference between stations analyzed in pre-Gosau and Gosau rocks. The general NW-trend of the or1 directions shows anticlockwise deflection to more easterly trends for stations along the sinistral SEMP fault zone (Fig. la). In the eastern NCA, the SEMP-Iine forms a wide belt of anastomosing fault zones enclosing elliptically to circularly shaped blocks in map view. We speculate that anticlockwise rotation of subcircular blocks within the distributed sinistral shear zone of the SEMP-line (compare

54

14.-G. Linzer et al. / Tectonophysics 242 (1995) 41-61

Beck, 1976) has passively rotated the fault-striae data of the C r e t a c e o u s - E a r l y Tertiary deformation (preserved in the interior of the blocks).

5. Balanced cross-sections

Balanced cross-sections were constructed almost perpendicular to the trend of large-scale fold axes and parallel to the local average maxim u m compression direction. In transpressional fold and thrust belts lateral mass transport, out of the plane of section, complicates balancing. For balancing sections across the NCA, we selected areas within blocks between large-scale strike-slip faults, thus minimizing lateral mass loss. Where it was unavoidable to cross strike-slip faults by the section plane, lateral mass loss was compensated by restoring the fault displacements. This is demonstrated for the western cross-section in Figs. 4b and 4c (see also Linzer, 1989). The displacements along the strike-slip faults were reversed and the bedding dips were projected from the restored (western) block into the section plane. The sections are line-length balanced using the competent carbonate m e m b e r s as marker horizons. Local strain analysis using tension gashes points to finite strains of ~< 5%, thus to a high degree of conservation of line length and stratigraphic thickness in those units. The western section (Fig. 5a) trends N W - S E across the Lechtal and Inntal nappes. The basal detachment was projected from the Vorderriss and Hindelang exploration wells (Fig. 3a) into the section (Bachmann and Mfiller, 1981; Mi~ller et al., 1988). The northern part of the Lechtal nappe exhibits open folds and several internal imbrications. The central part shows strong shortening, which is recorded by a duplex zone with SE-directed stacking. The total bed length in this area is 15 km and the section length is about 5 km. The shortening in the incompetent, mostly evaporitic Carnian rocks is recorded by an increase in thickness to up to 2 km in a triangle zone. The Inntal nappe consists of an open syncline with a NW-directed internal thrust and three SE-directed backthrusts with a total displacement of over 7 km. The backthrusts probably originated

from out-of-sequence thrusting of the 0tztal basement nappe due to Miocene indentation (Ratschbacher et al., 1991b). The restored section is levelled on the Triassic-Jurassic boundary (Fig. 5a). A g r a b e n h o r s t - g r a b e n structure is documented by thickness variations of the N o r i a n - R h a e t i a n carbonate complex with a thick sequence in the area of the Inntal nappe, low thickness in the buried, southern Lechtal nappe, and again a thick sequence in the northern Lechtal nappe. The Anisian-Ladinian carbonate complex shows a facies transition from the Ladinian reef platform of the Inntal nappe to the Ladinian basin sequence with clay, marly limestone, and marly siltstone of the southern Lechtal nappe. The total shortening amounts to 65%, calculated from a minimum bed length of 85 km to a section length of 32 km. The central cross-section (Fig. 5b) trends N W SE across the Staufen/H611engebirge, Totengebirge, and Warscheneck nappes. The section line crosses the Miocene Traunsee fault system in its northern part; the H611engebirge nappe north of the Traunsee fault is not balanceable. The location of the basal detachment and the interpretation of the deep structure is calibrated by the Grfinau well (Fig. 3b; Hamilton, 1989). The Totengebirge nappe shows NW-directed internal imbrication in the central part, and a backthrust in the southern part; minimal thrusting over the Staufen/H611engebirge nappe is 8 km. The Totengebirge and Warscheneck nappes are separated by a Miocene normal fault with a displacement of 2 km. The normal fault is kinematically connected to the SEMP fault zone, which marks the southern end of the profile. The Warscheneck nappe shows SE-directed internal backthrusts. The restored section shows facies transitions expressed by thickness changes between the Norian dolomites and limestones. The total shortening across the central section is 54%, calculated from a minimum bed length of 96 km to a section length of 44 km. The eastern cross-section (Fig. 5c) trends N W - S E across the entire width of the NCA. The depth to the basal detachment is projected from the Mitterbach 1 well into the section (Fig. 3b; Hamilton, 1989). The open-folded northern Lunz

2~

Molasse

-.,,,

. . . . . . . . . . .

:=,,

l~

.,]';

-~

.11 . g ............ . ,

::+...~ . -

'-- . . . . . . . .

--~-__;-==-=

Inntal nappe

~

- - =-==_-=-

+++

-6

.2

)

Otztal basement 3E complex 2

. . . . . . . . . . . .

~

/

Allg~iu nappe

~ ~ f

It

....."3

i ' : 'i,:~"

~

p

I-+++++'l Quarzphyllit Austroalpine units basement

Inntal nappe

.ower Carbonate Complex Scythian Ladinian Anisian Upper Permian

.......

restored section 65% shortening

Zig. 5. Balanced cross-sections across the Northern Calcareous Alps. For location of sections see Fig. 3.

Lower CretaceousLiassic

S-Lechtal nappe

~-~

5pper Carnian 2arbonate 7omplex ~orian-Rhaetian

" -'~*'-~~'~

N-Lechtal nappe

10km

----.-~"-'~

f.rll

i~i~!iii!~::::;' i i::i:!i::i::!::~:: ~:::::ii::!::! i::~:: ::!::i!::i::::::!::i :: ::!::::!::::::!:::: ::!i:~i:::~i:::::: ::::::ii::::!! ::::~:: ::::::::::::::::::::::::: i!::::::::!:::::::::::: :::::::: :::::::::::::::::::::::::: ::i::::i:::::::::: :::::::::::::::: i:::::::::::::::: i::::i:::: ::::::i:::::::::: :::::: ::::::!:: ; ~ ; : . . . : : ~

...............................

Helvetic nappes Cretaceous and Flysch Jurassic (foreland)

Sip symbols, ~hmst

-6-': ¢in

-2" , .......

/Allg~iu nappe _/-~ Lechtal nappe

NW P

~, 0-

a~

I

10km

HNlengebirge nappe

Cretaceous Jurassic

Serpentinite

ti:!:!::i:i;':!'-:i:]~::i:i:i;:iI

:::ii!::::i::ill IIIIIHIIIIIIIIIIII~ I~iii~ii~l Molasse Helveticnappes Flysch Gosau group

I

~_I-~ '

Liassic

Fig. 5 (continued).

Complex Norian-Rhaetian

Carbonate

I ?

Warscheneck nappe

f

Quarzphyllit or Ladinian Anisian UpperPermian Grauwackenzone

Totengebirge nappe

---------~mo~,ooo

~

11 Upper 11 Carnian ELowerCarbonateComplex~!it Scythian

Lower Cretaceous-

dolostone

r

SEMP-line S E

~ ~ _ ~ o ~

-6 km

~_

-6 km

~

~ 4 W a r s c h e n e c k nappet~

-2

~

Totengebirge nappe

-2'

~'i~ '°u

~ ~

. 0

e®

~ , ; ,,o,,

-4F-----H611engebirge nappc

Traunsee fault

O,

lip symbols, hrust

B

~,

NW

I

i.

~/

/

/

and Flysch

" .......

Upper Catalan Carbonate Complex Norian-Rhaetian

Lunz nappe" - ' r ' ' - ' ' ~ - ~

10 k m

Eretaceous2iassic

:::!:::i:::i:::;:::t IIIIIIIIIIIIIIIIIIIIH Lower Molasse Helveticum

~rankenfels ~enfi nappe

31"1

-6-

~'~

........................................................

.2" •

.

qW

......... . . ,_~,'~..~_.'..' ~,~L:

. ............

:ig. 5 (continued).

3ranwackenzone

km

'-6

,-4

. -2

c

Austroalpine basement

.

dip symbols, thrust

Mi dtirzalpen nappe

section 60% shortening

GOll nappe

LowerCarbonateComplex Scythian Ladinian Anisian UpperPermian

Unterberg nappe

"::":-iiiiiiiiiililliJilllllllliiiiJ-li~lii~lii~lill~m~.+. + -'l- ,,'1- -I- + + "l-. . . . . . . . . . . ~llllillliilillliilllllllJ~

- - ~ ~ - - - - - - - _ _ ~ r e d

,~,'.'.','.'.'.

Frankenfels nappe Lunz nappe----------~-~---- Unterberg-----~-4-------G611 n a p p e - - - - - q l ~ - Mtirzalpen nappe----~ nappe SEMP-line SE 2

I

e~

L

i

58

H.-G. Linzer et al. / Tectonophysics 242 (1995) 41-61

nappe is divided into two parts by a thrust with a displacement of 2 km. The southern part consists of open-folded Carnian rocks with an average (probably tectonically exaggerated) thickness of 800 m of evaporitic and clastic series and local coal seams. It contrasts to the 100-200 m of Carnian rocks in the north and 30-50 m in the south. The restored section depicts a Carnian graben in this region. The Unterberg and G611 nappes show NW-directed imbrications of the Norian carbonate complex with little internal deformation. The SEMP fault system displaces the Miirzalpen nappe eastward by at least 20 km. Internal structures of the Miirzalpen nappe are in general SE-directed backthrusts. The restored section is levelled at the Carnian-Norian boundary. An increased thickness of the Middle Triassic carbonates indicates increased subsidence in the southern part. The total shortening is 60%, given by a minimum bed length of 120 km to a section length of 50 km.

6. Discussion and conclusions

The large-scale structural analysis of the NCA reveals two major structural sets: (a) folds and thrusts, kinematically connected with dextral strike-slip fault zones, and (b) sinistral strike-slip fault zones. The structural types are connected, respectively, with predominantly Cretaceous (pre-Gosau) thrusting and dextral strike-slip faulting as an expression of the formation of the Austroalpine orogenic wedge, and Miocene sinistral wrenching associated with eastward lateral extrusion of crustal wedges along the central Eastern Alps. The age of the stacking event is constrained by radiometric dating of fault gauge (e.g., Kralik et al., 1987) and the deposition of synorogenic sediments (Rogfeld, Losenstein, Branderfleck beds; see above). Gosau sediments seal major thrust planes, but also are synsedimentary to thrusts and dextral strike-slip faults (e.g., Wagreich, 1991; Eisbacher and Brandner, 1993). Miocene extrusion is dated by fault-related basins, e.g. the Vienna pull-apart structure (see Royden, 1985).

Several features are common to the Cretaceous-Early Tertiary structures. The trend of ramp and fold axes is predominantly east-northeast to northeast. This trend deviates by > 20 ° from the overall strike of the NCA and the general trend of the Eastern Alps. WNW-striking, dextral strike-slip fault zones are a prominent feature of the NCA on all scales. These faults form an en-dchelon pattern, transecting the NCA into a number of rhombohedral crustal blocks. The dissection of the NCA into rhombohedral blocks may reflect their oblique approach toward the foreland within a distributed dextral shear zone spanning the width of the Austroalpine realm. Figure 6a visualizes the geometric model. Consider the NCA atop the Austroalpine stack, which as a whole is translating toward the westnorthwest, oblique to the foreland (see displacement directions in Fig. la). Contraction along the frontal Austroalpine wedge caused partitioning of deformation. In the basement, oblique convergence was accommodated by crystal-plastic flow and thrusting toward the west-northwest and coeval folding with fold axes parallel to the transport direction (e.g., Ratschbacher, 1986). In the mechanically layered Permo-Mesozoic succession of the NCA, oblique convergence was principally achieved by partitioning of deformation into thrust and fold structures and high-angle transfer faults. Earlier slowing of foreland-directed motion caused convergence to be accommodated by the propagation of the western segments of the NCA along WNW-striking dextral transfer faults. This is possibly due to the northeastward narrowing of the Penninic units in the eastern Eastern Alps, a n d / o r the necessity of the frontal Alpine orogenic wedge to climb up the lateral foreland ramp. The implied truncation of the NCA into blocks is particularly well displayed by the offset of the major facies boundaries in the NCA (Fig. 6b). The displacement along the transfer faults allows for the narrowing of the deformed area and implies clockwise rotation of the NCA (Fig. 6a); the latter readily explains the palaeomagnetically detected rotations. The difference between the average displacement directions of the Austroalpine basement and the NCA (Fig. la) reflects the rotation of the NCA and further im-

H.-G. Linzer et aL / Tectonophysics 242 (1995) 41-61

plies decoupling between the basement and cover along one or several weak horizons. Decoupling occurred most probably along the basal detachment of the NCA, along the Permian evaporitic rocks a n d / o r the Grauwackenzone. During the deformation history of the NCA, dextral strike-slip faults may have performed different functions: they either accomplished rigid translation between differential blocks of the NCA ("transfer" faults) a n d / o r acted as tear faults with vertical and horizontal offsets and accommodating displacement differences between thrust and fold structures. Vertical partitioning in the Austroalpine unit during orogenic contraction reflects rheological layering with dominantly crystal-plastic flow in the basement and dominantly brittle failure in

59

the NCA. Within the NCA, vertically variable strain rates are indicated, for example, by the termination of transfer faults at sole thrusts. This testifies to weak vertical mechanical coupling along incompetent horizons within the NCA and between the NCA and the basement. The map-scale structural analysis of the NCA reveals a transpressional strain field during stacking. Transpression is indicated by the following characteristics: (a) A NW-directed thrust propagation direction, oblique to both the overall strike of the NCA and the orogenic foreland. (b) The occurrence of thrusts and folds kinematically connected with dextral strike-slip faults. Characteristically the strike-slip faults and the trend of the fold and thrust structures enclose an

+i+iiiiiiiiiiii+ii+!+!!!i++,iiiS ++!!i ............................... + ii"i +++!!+i++i+i+J+++i++!i ¸

JurassicNorth

/ A

/"

//

I /

I/ P

Q

JurassicNorth

100 km

facies

.,~_.~.~~

backreeffacies

reeffacies

facies

~,

Wien

0

paleodeclination (Channellet al. 1992)

Fig. 6. (a) Geometric model for the formation of the major structures of the Northern Calcareous Alps (NCA). Oblique convergence was achieved by partitioning of deformation into thrust and fold structures and high-angle transfer faults, indicating horizontal shortening, vertical lengthening and transcurrent slip. Transcurrent motion is accommodated by propagation of the western segments of the N C A along WNW-striking dextral transfer faults. The displacement along the transfer faults allows a narrowing of the deformed area and implies a clockwise rotation of the NCA; the latter readily explains the palaeomagnctically detected clockwise rotations of >/30 ° (see b). (b) The truncation of the N C A into blocks, implied by the oblique convergence model, is particularly well displayed by the offset of the major facies boundaries.

60

H.-G. Linzer et al. / Tectonophysics 242 (1995) 41-61

E-facing acute angle. The strike-slip faults are interpreted as transfer faults. The fold, thrust, and strike-slip fault structural systems indicate partitioning of convergence into horizontal shortening and vertical lengthening (due to the thrusts and folds) and transcurrent slip (due to the transfer faults). (c) NE-directed fold-axes-parallel extension, subparallel to the internal strike of the NCA.

Acknowledgements Construction of structural maps and balanced cross-sections benefited from discussions with K. Decker (University of Vienna), R. Dell'mour and G. Wessely (IDMV, Vienna), G. Eisbacher (University of Karlsruhe), C. Morley and G. Weir (AMOCO, Houston, Texas), F. Moser (University of Tiibingen), A. Ruttner (Wien) and G. Tari. OMV allowed us to study unpublished maps. We were funded by the Deutsche Forschungsgemeinschaft (Ra442/2). S.M. Schmid and an anonymous reviewer provided helpful comments and J. Nebelsick improved the English text. All this is gratefully acknowledged.

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