Structural synthesis of the Northern Calcareous Alps, TRANSALP segment

Tectonophysics 414 (2006) 225 – 240 www.elsevier.com/locate/tecto

Structural synthesis of the Northern Calcareous Alps, TRANSALP segment Jan H. Behrmann a,*, David C. Tanner b,1 a

Geologisches Institut, Universita¨t Freiburg, Albertstr. 23b, 79104 Freiburg, Germany b Geologisches Institut, RWTH-Aachen, Wu¨llnerstr 2, 52056 Aachen, Germany

Received 10 June 2004; received in revised form 21 November 2004; accepted 4 October 2005 Available online 19 December 2005

Abstract The Northern Calcareous Alps (NCA) are the site of very large top-to-north convergent movements during Cretaceous–Tertiary Alpine mountain building. To determine the amount of shortening, the depth of detachment and the style of deformation, we retrodeformed an approximately 40  40 km area comprising the Lechtal and Allga¨u Nappes. On the basis of all available geological data and processed sections of the TRANSALP reflection seismic experiment, coherent 3D models were constructed by splining lines from N–S cross-sections. Integration of 3D kinematic modeling and field data shows the following. The structure of the Lechtal Nappe is controlled by the Triassic Hauptdolomit. Four main thrusts link to a detachment at 2–6 km depth below sea level. Shortening estimates vary, from 25% (east) to 42% (west). Additional contraction is accommodated by folding. In the east the subjacent Allga¨u Nappe can be traced about 10 km down-plunge, and is shortened by about one third. In the western part the downplunge width is at least 15–20 km, with restorable shortening of one third. The triple (Inntal, Lechtal, Allga¨u Nappes) NCA nappe system was moved uniformly N–S to produce laterally heterogeneus shortening of 40–90 km or 50–67%. We suggest that the NCA are underlain by substantial amounts of buried Molasse sediments and/or overthrust units of Helvetic and RhenoDanubian Flysch, indicating post-Eocene N–S shortening of at least 55 km. Restored to an initial configuration, the basin topography of the NCA reveals strong E–W thickness variations of the Triassic Wettersteinkalk and Hauptdolomit platform carbonates. Such variations may pertain to N–S trending growth faults, which were important precursors to later Jurassic extension of the Austroalpine passive margin. Such structures are unlikely to be seen in the conventional N–S cross-sections, but form an essential geometrical and mechanical element in later, convergent mountain building. D 2005 Elsevier B.V. All rights reserved. Keywords: Northern Calcareous Alps; Structure; Deformation; Thrusting; Kinematics; Palaeogeography

1. Introduction The Alps are a classical orogen resulting from largescale convergence and collision of the Eurasian and * Corresponding author. Tel.: +49 761 203 6495; fax: +49 761 203 6496. E-mail address: [email protected] (J.H. Behrmann). 1 Present address: Geowissenschaftliches Zentrum Go¨ttingen, Universita¨t Go¨ttingen, Goldschmidtstr. 3, D-37077 Go¨ttingen, Germany. 0040-1951/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2005.10.018

Adriatic continental plates after subduction of intervening oceanic basins (Dewey and Bird, 1970; Oxburgh, 1972) along a complex movement trajectory (Platt et al., 1989). In the Eastern Alps (Fig. 1a) the general Cretaceous–Tertiary kinematic signature appears to be dextrally transpressive (e.g. Ratschbacher, 1986; Ring et al., 1989; Behrmann, 1990), with a major net component of N–S overthrusting and shortening (e.g. Tollmann, 1963) that has been recognized since the discovery of the Tauern Window as expression of

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Fig. 1. (a) Tectonic sketch map of the Eastern Alps around TRANSALP (modified from TRANSALP Working Group, 2002) showing major thrusts, transcurrent faults and detachments and location of the reflection seismic traverse. (b) Interpretative tectonic cross-section along TRANSALP (modified after TRANSALP Working Group, 2002). (c) Un-interpreted depth-migrated part of the TRANSALP reflection seismic traverse between the northern Alpine thrust front at CDP 2300 and the Inn valley at CDP 4000, showing the reflection signature to 14 km depth. See Fig. 2 for location. An integrated interpretation is presented in Fig. 9, and discussed later in the text.

nappe tectonics (Termier, 1904). Palaeogeographic reconstructions (e.g. Blundell et al., 1992) constrain three major crustal domains: the Helvetic–European foreland realm to the North, the Penninic oceanic domain in a central position, and the Austroalpine–Adriatic realm to the south. Results of recent crustal-scale

reflection seismic experiments (Pfiffner et al., 1997; TRANSALP Working Group, 2002) have revealed large-scale tectonic superposition of Austroalpine– Adratic crust over the Pennine and Helvetic–European crusts, involving effective duplication of crustal thickness, and formation of a bi-vergent orogenic wedge

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(Fig. 1b). In the NCA nappe stacking and associated folding is known to be polyphase, with early movements as early as Upper Jurassic, mid-Cretaceous largescale thrusting, and late compressive movements in the Tertiary (e.g. Mandl, 2000). Apart from the effects of nappe stacking, other important facts to consider in palaeogeographic reconstruction, and restoration of Alpine deformation at any scale are as follows. Firstly, the effects of Paleogene to Neogene east–west stretching of the orogen due to eastward tectonic escape (Frisch et al., 1998; Ratschbacher et al., 1991), resulting in substantial structural modifications of the nappe architecture by strike-slip faulting, and formation of normal detachment faults (e.g. Behrmann, 1988). Secondly, possible effects of passive continental margin development in the Austroalpine–Adriatic domain during the Triassic and Jurassic need attention. These are especially well seen and can be reconstructed where the Alpine convergent deformation is absent or weak, i.e. in the South Alpine Block (e.g. Bernoulli et al., 1990; Bertotti et al., 1993). In regions with nappe architecture, such as the Austroalpine, few reconstructions exist (SE Switzerland; see Froitzheim and Manatschal, 1996; Manatschal and Bernoulli, 1999). Further east, in the TRANSALP segment of the Northern Calcareous Alps (NCA), such quantitative evidence has not been provided to date. If nappe stacking deformation is removed correctly, and if pre-nappe and post-nappe topologies of the NCA are properly assessed, then depth and geometry of the basal detachment to the NCA can be defined. As a result, the volume of allochthonous tectonic units (Helvetic and Rheno-Danubian Flysch nappes) between the NCA and the European crystalline basement and its sedimentary cover may be assessed. Therefore, the main objectives of our paper are: 1. to report the results of an integrated 3D retro-deformation exercise of the Northern Calcareous Alps in the TRANSALP segment, 2. to discuss implications of retro-deformation on the depth extrapolation of structures, especially the geometry and depth of the basal detachment of the NCA, and the rocks of the Northern Alpine thrust front, and 3. to image the thickness distribution of the main reefbuilding carbonate rocks in order to constrain palaeogeographic interpretations. We focus on the TRANSALP segment of the NCA north of the lower Inn valley (Tyrol, Austria; TRANSALP Working Group, 2002) as new reflection seismic

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data (Fig. 1c) have become recently available in addition to a large surface geological and drillhole dataset there. We will first discuss part of the section in detail, and then concentrate on an interpretation from geological data. 2. The TRANSALP reflection seismic image, Northern Calcareous Alps and substratum Fig. 1c shows a depth-migrated version of the TRANSALP reflection seismic traverse between the northern Alpine thrust front at CDP 2300 and the Inn valley at CDP 4000 down to 14 km depth, giving an image of the reflection signature. We do not add detailed tectonic or line-drawing interpretation at this point in order not to prejudice the reader. In the north, between CDP 2300 and 2700, the well-stratified clastic sediments of the foreland and folded Molasse (see also Roeder and Bachmann, 1996) can be recognised to depths of 4–5 km. This configuration of the strata is well constrained by well log data of the Miesbach I drill hole (Mu¨ller, 1978) and interpretations of high-resolution seismic data from commercial surveys (e.g. Thomas et al., 2002; Berge, 2002). The transparent zone underneath, which can in fact be traced well below the frontal chains of the Alps to CDP 3200 at 6–9 km depth, is probably made up of the Cretaceous–Paleogene rocks of the sub-Molasse autochthonous sediments, and the tectonically dislocated units of the Helvetic realm (e.g. Oberhauser, 1995) and the Rheno-Danubian Flysch Nappe (e.g. Hesse, 1982; Schnabel, 1992). The most obvious seismic feature in the whole section is the 1–2 km broad band of reflections at 5–6 km depths in the north. It can be followed down a large ramp at CDP 2500 to a depth of 9–10 km, and then is horizontally traceable almost to the southern termination of the NCA substratum near the Inn valley. One clear candidate to produce this reflective zone is the Triassic–Jurassic epicontinental sediment series of the European foreland, which can be traced more than 100 km updip to the north beneath the prism-shaped Molasse basin to the present surface outcrop in the Franconian Jura north of the Danube River. The lowermost and seismically almost transparent unit underlying this reflective zone is interpreted as the European crystalline basement. The tectonic units of the NCA, occurring south of the frontal thrust with a surface outcrop at CDP 2800, do not offer as much detailed structural resolution in terms of reflectors as one might have expected. There is no information to 2 km below sea level, and underneath this depth, the clearest signature is provided by a sequence of hinterland (south) dipping reflections between CDP

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2800 and 3800, which can be traced to depths of up to 6 km. Candidates to cause these reflections are the south dipping contacts between Triassic reef carbonates (see below) and shaly–marly interbeds, or tectonic nappe and imbricate contacts. 3. Structural and stratigraphic setting, western NCA In the western NCA, nappe architecture is expressed by an edifice of at least three major thust units (e.g. Tollmann, 1976; Eisbacher et al., 1990; Eisbacher and Brandner, 1996). In the TRANSALP segment (Fig. 2) the lowermost and northernmost unit is the Allga¨u

Nappe (e.g. Richter, 1984), tectonically overlain by the structurally complex Lechtal Nappe. Strata of the Allga¨u Nappe were intersected in the drillhole Vorderriss 1 (Bachmann and Mu¨ller, 1981), documenting its downdip extension underneath the Lechtal Nappe for at least 15 km at 11830V Eastern longitude (Fig. 2). The highest of the three nappe units is the Inntal Nappe, which can be traced along strike of the NCA for about 100 km (Eisbacher and Brandner, 1996). It overlies the Lechtal Nappe as a coherent, rootless thrust sheet to the west of the Inntal Fault (Fig. 2), and is in direct contact with the Paleozoic basement rocks of the Graywacke Zone (e.g. Schramm, 1979) to the southeast of the Inntal

Fig. 2. Geological sketch map of the studied area (modified after Tanner et al., 2003), showing major structural and stratigraphic elements, the location of the TRANSALP line with CDP positions, and locations of the depth-extrapolated cross-sections 1 8 used in our study.

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Fault. The general kinematics of the three nappes appears to be that of large-scale south-over-north thrusting and associated folding (Tanner et al., 2003; Auer and Eisbacher, 2003) in the TRANSALP segment of the NCA. There is evidence in studies of other parts of the NCA (e.g. Linzer et al., 1995, 1997; Peresson and Decker, 1997) to suggest that some of the movement may have been in a more NW direction. However, the internal structure of the NCA in the TRANSALP segment is not retrodeformable in NW–SE oriented shear (see Tanner et al., 2003, and discussion below). Most of the overthrust movements within the NCA have occurred in the Upper Cretaceous, prior to deposition of the Gosau Group clastic sediments (e.g. Faupl et al., 1987; Wagreich and Faupl, 1994). The Gosau sediments mark a major period of tectonically induced subsidence at the Northern Austroalpine margin (Wagreich, 1995). Tertiary reactivation of thrusts and attenuation of folds in connection with the final emplacement of the NCA over the Rheno-Danubian Flysch Nappe, the Helvetic units, and the Molasse certainly caused some modifications of the tectonic edifice, a feature that is particularly well seen in the folding of the Zo¨ttbachalm Gosau deposits (see Fig. 2 of Eisbacher and Brandner, 1996) in our area of investigation. Later structural modifications were induced by large strike-slip movements, mainly sinistral, in the Tertiary (Eisbacher and Brandner, 1996). The most prominent of these structures is the Inntal Fault (see Fig. 2), with sinistral displacement on the order of 40 km (Ortner et al., 1999, 2002). Thus, structural coherence of the NCA nappe stack is more or less maintained north and south of the Inn valley, but any restoration of deformation of the NCA as a whole would have to deal with correct removal of the movements along the Inntal Fault and related structures elsewhere. Fig. 3 shows a synopsis of the Mesozoic stratigraphy and facies, which is developed in all three major thrust units. For monographic reading, we refer to Tollmann (1976). There are marked variations in stratigraphic thickness between and in the units that reflect local and regional facies variations between carbonate reef platforms and pelagic or hemipelagic basinal areas. However, everywhere the stratigraphic sequence is continuous from the Permo-Triassic to the Lower Cretaceous strata (Fig. 3). The basement of parts of the NCA is assumed to be the Graywacke Zone (Schramm, 1979); this contact is exposed on the southern side of the Inn River (see Fig. 3). The strata were deposited on the southeastern margin of the Tethys Ocean (Gawlick et al., 1999; Mandl, 2000). In the Middle Triassic a sequence of pelagic carbonates developed above the

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clastics of Permo-Skythian age. In the Ladinian, there is a first marked differentiation into reef (Wetterstein Limestone) and basinal facies (Partnach beds) (Fig. 3), leading to marked stratigraphic thickness heterogeneity. The main stratigraphic unit of the area is the Norian Hauptdolomit (HD), a massive, coarsely laminated dolomite body (Zankl, 1967; Fruth and Scherreiks, 1984) between 700 and 2500 m thick. Above the HD, there is a differentiation into reefal and basinal limestones again, before the shallow water platform development is terminated in the Jurassic (e.g. Gawlick et al., 1999), to give way to widespread deep-water deposition at low sedimentation rates in the Jurassic and Lower Cretaceous. Above a tectonic unconformity the Upper Cretaceous Gosau Group (Fig. 3) consists of coarse conglomerates, sandstones, marls and limestones (Sanders, 1998). The western NCA was subject to no more than anchimetamorphism (i.e. less than 200 8C), although illite crystallinity data have shown that the thermal overprint of the Inntal Nappe is slightly higher than that of the Lechtal and Allga¨u Nappes (Kralik et al., 1987). 4. Results of 2D and 3D kinematic modelling We restored the pre-deformation geometry of the Allga¨u and Lechtal Nappes, using 2D and 3D techniques, respectively. A total of 8 cross-sections were made through both nappes (1–8, Fig. 2) using information from geological maps (e.g. Schmidt-Thome´, 1953; Ganss and Doben, 1984; Eisbacher and Brandner, 1996). Each of the sections was created using the principles line-length balancing and minimum displacement (Woodward et al., 1989). A constant template method, as commonly used in structural balancing (Woodward et al., 1989), was not used because of the very large thickness changes within the carbonate layers. At least in the Lechtal Nappe, thickness changes within the Hauptdolomit were easier to constrain because of its large outcrop area. Six bstratigraphicQ complexes (see also Fig. 3) were used for the construction. These are (from oldest to youngest): 1. Hochfilzen beds, Alpine Buntsandstein and Reichenhall Beds; 2. Alpine Muschelkalk and Partnach Beds; 3. Wetterstein Limestone; 4. Raibl Beds; 5. Hauptdolomit, Plattenkalk, Ko¨ssen Beds, Upper Rha¨tian Limestone; 6. Liassic red limestone and cherts, Cretaceous Aptychen Beds and Neocomian Marl.

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Fig. 3. Generalized stratigraphic column of the NCA nappes (after Tanner et al., 2003).

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Fig. 4. Balanced cross-sections of the Allga¨u and Lechtal Nappes (along section lines 2, 5 and 7, Fig. 2), showing lateral variation in thrust style. Hauptdolomit is shaded grey. Small numbers refer to thrusts. See text for discussion.

As examples, Fig. 4 shows the balanced 2D crosssections 2, 5 and 7 through the Allga¨u and Lechtal Nappes. These form part of the eight sections (Fig. 2)

that were used to construct the complete 3D structural model on which the retro-deformation is based. Two perspective views of the model are seen in Fig. 5,

Fig. 5. Two views of the complete 3D structural model of the Lechtal Nappe. (a) Downward view from the ESE shows the approximately cylindric geometry of the fault-fold package in the E of the study area, consisting of Achental anticline, Achental Thrust, Thiersee Syncline and the northward fault-fold packets. (b) View from the W shows structure dominated by the conjugate pair of Thrust 4 and Backthrust 5, enveloping large upright fold structure of the thickest part of the Hauptdolomit. See text for further discussion.

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depicting the strong contrast in structural style between the east and the west of the study area. This is well seen in comparing the three sections in Fig. 4. A more complete representation and discussion of the data used from the Lechtal Nappe can be found in Tanner et al. (2003). Here, we concentrate on sections 2, 5, and 7, as along-strike differences in structural style are best represented in this way. The leading horse block of the Allga¨u Nappe contains only Jurassic and Cretaceous strata, the so-called bLower Border SliceQ (see Auer and Eisbacher, 2003). In the east (section 2) the Allga¨u Nappe is composed of four fault-bounded units, two of which form a duplex below the Lechtal Nappe. In section 5 the duplex is modified to a simple thrust and this structure is at all not present in section 7. Typically, the longest thrust slice of the Allga¨u Nappe is that which is in contact with the Lechtal Nappe and exposed at the present-day surface. We estimate that the present-day trailing edge of the Allga¨u Nappe is therefore between 7 and 10 km behind the frontal outcrop. The main constraint for this is the fact that the Vorderriss borehole (Bachmann and Mu¨ller, 1981) (Fig. 2) intersected rocks of the Allga¨u Nappe at depth after transecting the tapered shape of the Lechtal Nappe. In the east the main structure of the Lechtal Nappe is the Achental Thrust (Thrust 3, sections 2 and 5; Fig. 4). It forms a ramp/flat/ramp structure (also seen in Fig. 5a) which causes a hanging-wall anticline and syncline structure to form, the later being the Thiersee Syncline (Eisbacher and Brandner, 1996). The anticline to the north is caused by the duplexing of the Allga¨u Nappe underneath (see section 2 in Figs. 4 and 5a). This is a structure that is less and less well developed along strike westward (compare sections 2,5 and 7; Fig. 4). Towards the west, as shown by section 5 (Fig. 4), the hanging wall anticline and footwall syncline next to Thrust 3 are tight to isoclinal recumbent folds. To the

south, Thrusts 4 and 5 (Fig. 4) begin to develop, with displacements of 1500 m. The duplex anticline is less well developed compared to section 2. In section 7, Thrust 3 has minimal displacement, the major compressional structures are Thrusts 4 and 5 which form steep forward- and (partially overturned) back-thrust, respectively. A small splay thrust (Thrust 2; Fig. 4) is present at the front of the nappe, where the Hauptdolomit is less than 1 km thick. To construct the 3D-model of the area, faults and stratigraphic horizons from the eight sections (for location see Fig. 2) were splined together into surfaces (see Tanner et al., 2003; for method). However, the structure of the Allga¨u Nappe is too variable along strike to correlate fault blocks and maintain strata continuum over the 4 km interval between faults. As a result, it was only possible to restore each 2D section of the Allga¨u Nappe separately. The 3D model of the Lechtal Nappe is shown in Fig. 5. Since Tanner et al. (2003) describe in detail the fault geometries, along-strike displacement of strata on the faults, and the methods and results of the retro-deformation, only a discussion of the results will be given here. Fig. 6 shows the restored sections of the Allga¨u and Lechtal Nappes. Note that because the Lechtal Nappe was restored geometrically in three dimensions, which maintains continuum between all layers, it was not possible to completely flatten the strata. This is a source of error, which is due to the failure of the algorithms used to completely restore some structures. This way, internal shortening within the Lechtal Nappe is typically underestimated by about 6% (Tanner et al., 2003). The Inntal Nappe forms the highest and most-hinterland nappe of the NCA. Its base is depicted in the model view in Fig. 5b. In principle, the Inntal Nappe has the same stratigraphy as the other nappes (see Fig. 3), but the Hauptdolomit is either greatly reduced in thickness or not present at all due to erosion. Strata

Fig. 6. Restored sections 2, 5 and 7 through the Allga¨u Nappe (2D restoration) and Lechtal Nappe (3D restoration). Hauptdolmit is shaded grey. Small numbers refer to thrusts.

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the model (Fig. 7b, easting 4475), the surface remains above 2500–3000 m below sea level. This effect is due to folding of the whole nappe structure at this point. This effect was probably caused by locking-up of the Karwendel Syncline (which is the overturned tight fold seen in the centre of Fig. 5b) where the Hauptdolomit is developed to its greatest thickness (more than 2500 m). As a result of the mechanical jamming of the thrust system here, most of the movement is taken up by Thrust 5, assuming the role of a backthrust (right in Fig. 5b).

younger than the Hauptdolomit were either never deposited or have been eroded since (Eisbacher et al., 1990). Along strike of the NCA, the Inntal Nappe extends for approximately 80 km from west of Landeck to east of Innsbruck. Because of the small outcrop length perpendicular to strike, large errors for the Inntal Nappe are to be expected. Eisbacher et al. (1990) proposed that the Inntal Nappe was internally shortened by 8–10 km, i.e. 36–45%. These figures are very much in line with the shortening values of the Allga¨u and Lechtal Nappes (see Table 1 and discussion below). From the kinematic modeling it is possible to extrapolate the depth to detachment for the Allga¨u and Lechtal Nappes. Depth maps of the sole thrust surfaces of the Allga¨u and Lechtal Nappes are shown in Fig. 7. The Allga¨u Nappe is soled by a relatively simple listric surface (Fig. 7a), which is steepened in one place (at easting 4480) to form a base approximately 500 m deeper than in the adjoining areas. In part, this may be an artifact resulting from flaws in downplunge depth extrapolation. However, around easting 4480, there are large thickness gradients for the Triassic Carbonates (at least in the overlying Lechtal Nappe; cf. Fig. 8), so that thrust sheet topology could cause a geometrical effect in the base. At the hinterland (southern) termination the detachment is found at about 3000 m in the east, and more than 5500 m below sea level in the west. The base to the Lechtal Nappe (Fig. 7b) is more complex and structurally better constrained by the 3D model. The general geometry is also that of a listric surface steepening to the north, but there are important modifications. Firstly, to the east, there is a rise and trough between CDP 2900–3300 of the seismic section. This is caused by antiformal duplexing of the underlying Allga¨u Nappe (cf. Fig. 4, sections 2 and 5; Tanner et al., 2003, Fig. 4). South of this antiform, the Lechtal Nappe detachment surface dips steeply to depths greater than 3500 m to the east and west, but in the middle of

5. Shortening estimates Minimum North–South shortening in the Allga¨u and Lechtal Nappes can be visually estimated by comparing the deformed and retro-deformed cross-sections 2, 5 and 7 (Figs. 4 and 6). A more quantitative approach, however, is documented in Table 1. All the shortening values given there are derived from the structural models, and are based on application of a minimum displacement criterion (Woodward et al., 1989). While the undeformed Allga¨u thrust sheet length varies along strike between 25.2 km and 32.7 km (see Table 1), the initial thrust sheet lengths of the Lechtal Nappe show much higher variability. The eastern four crosssections (1–4) are between 27.3 and 31 km long. After passing a sharp discontinuity the western sections (5–8) have lengths between 57.9 km and 65.6 km. Obviously this goes along with a change in structural style, which is probably best seen by comparing the two views of the structural model in Fig. 5. The change is at the western termination of the Thiersee Syncline, just west of the TRANSALP profile. When comparing undeformed lengths to deformed thrust sheet lengths (Table 1) it can be seen that the Allga¨u Nappe has a rather constant downdip length of 13.5–20 km, and total internal shortening (Dl) is on the

Table 1 Results of the restoration of the Allga¨u and Lechtal Nappes Section

Allga¨u Nappe (A.N.)

Lechtal Nappe (L.N.)

A.N. and L.N.

l 0 (km)

l 1 (km)

Dl (km)

l 0 (km)

l 1 (km)

Dl (km)

Total shortening (%)

1 2 3 4 5 6 7 8

n.d. 24.5 24.8 21.8 22.8 32.0 27.1 33.7

n.d. 12.3 12.9 12.2 12.8 21.0 18.3 22.6

n.d. 12.2 11.9 9.6 10.0 11.0 8.8 11.1

27.3 27.4 31.0 30.9 57.9 59.6 65.6 64.7

18.7 19.3 22.3 23.0 34.0 34.5 39.6 41.0

8.6 8.1 8.7 7.9 23.9 25.1 26.0 23.7

31.5 (L.N. only) 39.1 36.9 33.2 42.0 39.6 37.5 35.4

Shortening values are based on the Base Hauptdolomit in the Lechtal Nappe and Top Hauptdolomit in the Allga¨u Nappe.

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order of 10 km. The lack of surface outcrop of the Allga¨u Nappe introduces a certain lack of constraints regarding thrust sheet length and shortening. By working with a minimum displacement criterion, however, and by accepting the evidence for downdip extension from the Vorderriss borehole (Fig. 2; cf. Bachmann and Mu¨ller, 1981), our interpretation appears to be a viable and conservative one. Without supporting field or subsurface evidence Auer and Eisbacher (2003, Fig. 16) suggest that the Lower Border Slice of the Allga¨u Nappe extends below the complete Allga¨u Nappe, whereas we do not (Fig. 4, section 2). The interpretation of Auer and Eisbacher (2003) would increase thrust

sheet length and shortening values for the Allga¨u Nappe by about 10 km. The deformed thrust sheet lengths of the Lechtal Nappe still depict the change in structural style at the western termination of the Thiersee Synform, but in a less dramatic way. The eastern four cross-sections (1–4) are between 18.7 and 23 km long. The western sections (5–8) have lengths between 34 km and 41 km. The internal total contraction of the Allga¨u and Lechtal Nappes is 30.7–33.2% in the east, and 37.5–42.3% in the west (Table 1). As discussed, this difference depicts a significant primary and secondary along-strike heterogeneity of the nappe edifice due to differences in architecture of the strata, the

Fig. 7. Depth of detachment maps for (a) Allga¨u Nappe and (b) Lechtal Nappe, as extrapolated from the kinematic modeling results.

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Fig. 8. Maps of thickness distribution for the Hauptdolomit (left) and Wettersteinkalk (right) stratigraphic layers in the Lechtal Nappe, as derived from the retrodeformed rock volumes. Note interpreted positions of normal growth faults where gradients in stratigraphic thickness of the units are very steep. Barbed lines show positions of downfaulted hanging wall blocks.

mechanics and/or the kinematics of the whole fold– thrust belt. Internal contraction as a whole is broadly similar to estimates from the Inntal Nappe (36–45%; Eisbacher et al., 1990), which were obtained by evaluation of balanced cross-sections further to the west, where outcrop preservation is more complete. In order to obtain a rough estimate for total N–S shortening of the NCA in the TRANSALP segment, we consider the undeformed N–S lengths of the thrust slices. These are approximately: Allga¨u Nappe = 22– 34 km; Lechtal Nappe = 27–65 km; Inntal Nappe = 31– 36 km. In view of the present downdip width of the NCA of 40–45 km, the range of N–S shortening is 40– 90 km, or 50–67%, depending on the choice of section in the studied rock volume. 6. Implications for palaeogeographic reconstruction Retro-deformation in three dimensions offers the chance to view the modeled initial distribution of sedimentary thicknesses of key statigraphic units. This is shown in map view in Fig. 8 for the Wettersteinkalk and Hauptdolomit units of the Lechtal Nappe. Immediately obvious are steep gradients in thickness along N–S to

NE–SW trending zones in both units. As the Wettersteinkalk in the area represents reefal shallow-water carbonate with no basinal facies (Partnach Beds) present, and the Hauptdolomit (Zankl, 1967; Fruth and Scherreiks, 1984) was most likely formed in a very shallow-water, lagoon-like environment, a tectonic origin for large, abrupt thickness variations is the most straightforward interpretation. Thus, the thickness distribution patterns shown in the maps in Fig. 8 can be seen to depict a network of Triassic growth faults (barbed solid lines), with the barbs showing the positions of the downfaulted blocks. For the Hauptdolomit the eastern part of the area shows the presence of a N–S trending tectonic horst approximately at the position of the TRANSALP section, flanked by normal growth faults with throws up to 2000 m. A similar horst-andgraben feature is seen in the Wettersteinkalk map (Fig. 8, right), offset about 12 km further to the west. Although throws of the interpreted growth faults here are less (up to approximately 1000 m), the inferred tectonic horst exactly depicts the position of the large transfer zone between the Thiersee and Karwendel Synclines, between Lines 4 an 6 (Fig. 2), where the Jurassic– Cretaceous strata are preserved from erosion in the

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present outcrop pattern. In the deformed models (Fig. 5), this is the zone where there is a fundamental change in the fold–thrust architecture of the Lechtal Nappe. We suspect, therefore, that positions of the Triassic growth faults were instrumental in localizing the folds and thrusts in the later contractional deformation of the NCA. In the large-scale structural and palaeogeographic context, extensional structures of similar trend are known from the South Alpine Block (Bechsta¨dt et al., 1978; Bernoulli et al., 1990; Bertotti et al., 1993) and the Austroalpine (Bechsta¨dt et al., 1978; Froitzheim and Manatschal, 1996; Manatschal and Bernoulli, 1999). Our analysis indicates that the NCA nappes around TRANSALP contain a comparable structural heritage from Mesozoic passive margin development at the northern border of the Adriatic Plate. Presence of major extensional structures in the Austroalpine of the larger area had been indicated by earlier workers (e.g. Sarnthein, 1967; Brandner, 1984; Schuster et al., 2001), but it has not been detected as clearly and quantified, as seen in Fig. 8. 7. Discussion The results of the inverse kinematic modeling of deformation in the NCA have some important bearings on the large-scale interpretation of the Northern thrust front of the Eastern Alps. In the depth-converted part of the TRANSALP reflection seismic traverse (Fig. 1c) it is obvious that a considerable volume of tectonically displaced rocks (Molasse, Helvetic units, Rheno-Danubian Flysch) must exist between the base of the NCA nappes and the autochthonous sedimentary cover of the

European crystalline basement, well defined by the ramp-like structure to the north, and the strong, subhorizontal reflector at about 8–10 km depth. Fig. 9 shows our interpretation, in which the NCA thrust wedge is underlain by a 1.5–3 km thick section of the Rheno-Danubian Flysch Nappe and the Helvetic allochthonous units. In principle this is compatible with early interpretations of large-scale tectonics of the Northern Alps (e.g. Brinkmann, 1936). Surface outcrop of these units at the northern margin of the Alps is continuous (Fig. 2), and several geological arguments exist to support the downdip extension of the deep-water clastics of the Flysch Nappe, possibly as far south as the Inn River, with a palaeogeographic continuation into the Penninic units of the Tauern Window. Firstly, Flysch units and tectonic equivalents (e.g. the Pra¨tigau Schists) are present downdip along the whole Pennine–Austroalpine contact in Eastern Switzerland (e.g. Ring et al., 1989). Secondly, there are several small tectonic windows exposing Flysch units beneath the NCA nappes further east far south of the thrust front (e.g. Brinkmann, 1936; Tollmann, 1976, 1985). The fact that these windows are small, highly deformed, and probably incorporated into the tectonic hanging wall by Tertiary imbrication (see e.g. Peresson and Decker, 1997) makes an estimation of thickness and volume of the Flysch units there very difficult, but is a clear indication of their presence beneath the NCA. Thirdly, Flysch and Helvetic units were intersected in exploratory drillholes (e.g. Ku¨pper, 1968, Tollmann, 1985) Results from locations far south of the NCA thrust front in eastern Austria, away from TRANSALP, show that thickness of the Flysch (and Helvetic) units

Fig. 9. Interpreted NCA segment of the TRANSALP reflection seismic line between CDP 2300 in the undeformed Molasse foreland and CDP 4000 at Inn Valley. Structure of the Allga¨u and Lechtal Nappes are derived from kinematic modelling. The European Autochthonous, and the overlying Helvetic and Flysch Nappes are line-drawing interpretation.

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can be very variable locally. A good description and discussion of this feature is found in Tollmann (1985; pp. 112 ff.) At the Berndorf 1 location the NCA are floored by only 270 m of Flysch, underlain by Molasse and gneisses of the Bohemian Massif. However in drillhole Mitterbach 1, the NCA are floored by at least 600 m of Flysch, and the interpretation of Tollmann (1985) suggests that these formations may be as thick as 2 km here. The drillhole results, therefore, cannot be used to prove or disprove our hypothesis, which is solely based on the results of the retro-deformation and the interpretation of the TRANSALP section. An interpretation somewhat similar to our view has recently been proposed for the TRANSALP segment by Auer and Eisbacher (2003, Fig. 15), although their model implies a smaller volume of Rheno-Danubian and Helvetic beneath the NCA base due to a larger downdip extension of the Allga¨u Nappe (see discussion above) and occurrence of Austroalpine basement rocks of the Graywacke Zone north of River Inn. For the subsurface geology of the TRANSALP segment proper we suppose that local tectonics of the mega-shear zone consisting of Helvetic units and Rheno-Danubian Flysch at the base of the NCA may be more complex than the large-scale picture suggests. For example only the Helvetic units were intersected in the drillhole Vorderriss I in the expected tectonic position beneath the base of the NCA (Bachmann and

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Mu¨ller, 1981). We think that heterogeneous and high strains there may have locally lead to extreme thinning and/or tectonic excision of units at a scale smaller than the resolution limit of our volume model. The Cretaceous–Palaeogene sediments of the Helvetic form a thrust sheet, or a collage of several tectonic units reflecting different palaeogeographic origins (e.g. Hagn, 1960), overriding the frontal imbricate stack of the foreland Molasse sediments (Fig. 9). One way to test the interpretation in Fig. 9 is to attempt a balanced cross-section of the Molasse and Helvetic units along the TRANSALP traverse as presented in Fig. 10. South of a pin line at CDP 2300 the Molasse sediments can be retrodeformed quite straightforward using the folded marker beds (Baustein Beds and Upper Eocene beds) that are well visible in the seismic section and were intersected in the Miesbach I drillhole (Mu¨ller, 1978). In our reconstruction we follow the interpretations of Thomas et al. (2002) and Schwerd and Thomas (2003), rather than that of Berge (2002), who proposed a triangle structure, as the latter does not seem to provide the same quality of fit between reflector signature and dip of beds. Triangle structures, however, are present in the deformed Molasse further west along strike, where they have been documented by Mu¨ller et al. (1988). Total N–S shortening of the Molasse along the TRANSALP line is about 10 km (Fig. 10). Reconstruction of the Helvetic can only be done as a rough estimation yield-

Fig. 10. Balanced cross-section of the Molasse Units and the Helvetic nappe at the Northern Alpine thrust front; (a) deformed state, (b) restored state. Restoration is quantitative for the Molasse and qualitative for the Helvetic. See text.

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ing minimum shortening, as too little is known about the total length of the thrust sheet, and several thrust slices may be present (Hagn, 1960). The most important control, however, is given by the fact that the Helvetic units were intersected in the deepest part of the Vorderriss drillhole (Fig. 2) (Bachmann and Mu¨ller, 1981). If moved back to an initial position (Fig. 10) another minimum 15 km of shortening are added for the Helvetic. Removing the carapace of the Rheno-Danubian Flysch Nappe (Fig. 9) to an initial position south of the Helvetic adds another minimum shortening of 30 km. Thus, total post-Eocene N–S shortening between the Inn Valley at CDP 4000 and the northern margin of the Alps at CDP 2300 is probably at least 55 km. As cross-sectional area was preserved, the section balancing shows that the interpretation in Fig. 9 is in principle viable for all tectonic units beneath the NCA nappes and their downdip extension. 8. Conclusions Kinematic modeling of the TRANSALP segment of the Northern Calcareous Alps (NCA) in three dimensions allowed us to test a depth-extrapolated volume of a fold-and-thrust belt for retro-deformability, and provide a quantitative base for the following conclusions regarding NCA structure and palaeogeography, and that of the Northern Alpine thrust front as a whole. 1. The amount of N–S shortening within the NCA is heterogeneous along strike, and varies between 40– 90 km or 50–67%. The NCA are probably underlain by substantial volumes of buried Molasse sediments and/or overthrust Helvetic and Rheno-Danubian Flysch units, with post-Eocene N–S shortening of at least 55 km. 2. The structure of the NCA, and especially that of the Lechtal Nappe, is controlled by the Triassic Hauptdolomit. There, four main thrusts link to a detachment at 2–6 km depth below sea level. Shortening estimates vary, from 25% (east) to 42% (west). Additional contraction is accommodated by folding. In the east, the subjacent Allga¨u Nappe can be traced about 10-km down-plunge. In the west, downplunge width is about 15–20 km. The Allga¨u Nappe is internally shortened by about one third. 3. In restored initial configuration the basin topology of the NCA reveals strong E–W thickness variations of the Triassic Wettersteinkalk and Hauptdolomit platform carbonates. These variations probably reflect the presence of syn-sedimentary faults, which were important precursors to later, Jurassic extension of

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