States of Paleostress North and South of the Periadriatic Fault: Comparison of the Drau Range and the Friuli Southalpine Wedge

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Tectonophysics 637 (2014) 305–327

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Tectonophysics

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States of paleostress north and south of the Periadriatic fault: Comparison of the Drau Range and the Friuli Southalpine wedge

Esther Maria Bartel ⁎, Franz Neubauer ⁎⁎, Johann Genser, Bianca Heberer

Dept. Geography and Geology, University of Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria article info abstract

Article history: This study focuses on the analysis of structures and kinematics of a N–Sprofile along the axis of maximum short- Received 19 May 2014 ening of the European Eastern Alps. The area includes the southern Austroalpine unit in the north and the Received in revised form 16 October 2014 Southalpine unit, which is a part of the Adriatic indenter. The stratigraphically different units are separated by Accepted 22 October 2014 the Periadriatic fault, the major strike-slip fault within the Alps. In order to assess the kinematics of these units, Available online 30 October 2014 mainly fault-slip data from north and south of the Periadriatic fault were analyzed. We distinguish a succession fi – – – Keywords: of ve main kinematic groups in both units: (1) N S compression; (2) NW SE compression; (3) NE SW compres- σ – – Paleostress analysis sion, 3 changes gradually from subvertical to subhorizontal; (4) N S compression; and (5) NW SE compression. Orogenic wedge Our study reveals that the deformation sequence on either sides of the PAF is similar. The mean orientations of the Drau Range principal stress axes, however, show small, but consistent differences: The subhorizontal axes north of the Periadriatic fault Periadriatic fault plunge northward, in the south southward. A counterclockwise (CCW) rotation of the southern Southern Alps part in respect to the north is evident and in line with the well-known counterclockwise rotation of the Adriatic Adriatic indenter indenter as well as dextral displacement of the N-fanning stress-field along the Periadriatic fault. Opposing plunge directions are interpreted as a primary feature of the internal stress-field within an orogenic wedge further increased during ongoing compression. © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

1. Introduction SE-verging fold-and-thrust belt. Between the PAF and the Fella–Sava Fault (Fig. 1) units are steeply dipping towards the south. Structures The Periadriatic fault (PAF) is the main strike-slip fault of the south of the PAF are well investigated (Brime et al., 2008; Läufer, 1996; Alpine orogen and separates the Austroalpine unit and the indenting Läufer et al., 2001; Rantitsch, 1997; Venturini, 1990). Austroalpine units Southalpine unit in the Eastern Alps (Ratschbacher et al., 1991b; within the study area north of the PAF are characterized by intense Schmid et al., 1989)(Fig. 1a). Both units belong to the Adriatic micro- large-scale folding. Modern structural studies are scarce for the plate sensu lato. Structural studies on fault systems (e.g. San Andreas Austroalpine units north of the PAF, particularly for the Drau Range fault, Zoback et al., 1987; Morez fault zone, Homberg et al., 1997) and (DR) modern structural, kinematic and paleostress analyses are entirely 2-D distinct element modeling (Homberg et al., 1997)haveshown, missing. Paleomagnetic studies revealed largely similar counterclockwise that stress fields may systematically rotate around subvertical axes in (CCW) Neogene block rotations of about 20° in both the Austroalpine and the vicinity of such major crustal discontinuities and stress deviations Southalpine units (e.g. Márton et al., 2000; Mauritsch and Becke, 1987; may occur next to weak zones where crustal deformation occurs. In Thöny et al., 2006), but no conclusive data are available from the study this study, we pose the question whether the PAF represents an upper area itself. crustal stress perturbation, i.e. whether units on both sides of the PAF We provide paleostress reconstructions for Southalpine and record similar or distinct stress patterns and/or whether block rotation Austroalpine units N and S of the PAF based on a new dataset of brittle overprinted the stress pattern. fault kinematics. In conjunction with a compilation of existing structural The stratigraphical, structural and kinematic frameworks north data, we first recognize how many tectonic events occurred and second- and south of the right-lateral transpressional PAF differ significantly ly reconstruct the stress trajectories. The studied transect is located at (e.g. Tollmann, 1977; Fig. 1b): The Southalpine units represent a S- to the tip of the Adriatic indenter across the central part of a straight segment of the PAF between the Giudicarie Fault in the west and the western Karawanken Mts. in the east (Fig. 1). We selected the ⁎ Corresponding author. Tel.: +43 6462 4888. study area because of (1) the supposedly strongest shortening at least ⁎⁎ Corresponding author. Tel.: +43 662 8044 5401. E-mail addresses: [email protected] (E.M. Bartel), [email protected] in the Southalpine unit; (2) the straight segment of the Periadriatic (F. Neubauer). fault where stress reorientation by changing external conditions like

http://dx.doi.org/10.1016/j.tecto.2014.10.019 0040-1951/© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 306 E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327

Fig. 1. (a) Simplified tectonic model of the Eastern Alps (modified after Egger et al., 1999; Neubauer and Höck, 2000; Plenicar, 2009) showing major ca. orogen-parallel sinistral and dextral strike-slip faults and resulting east-directed lateral extrusion of the central Eastern Alps. Black box denotes the extent of (b) study area. E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327 307 plate motion and deformation structures along strike of the fault is stratigraphic evolution of a rift and a subsequent passive continental considered unlikely; and (3) the well known tilting of blocks north margin, with Permian continental red beds at the base followed by Mid- (Gailtal basement complex) and south (Carnic Alps basement) of the dle to Upper Triassic carbonate platform sediments, hemipelagic basinal fault, which implies rotation around a horizontal axis of pre-tilting sediments of Jurassic age and Cretaceous flysch sediments (Rantitsch, paleostress tensors. 2001 and references therein; Fig. 2a). The present study suggests an overall similar sequence of deforma- The Carnic Alps expose the basement of the Southalpine unit (Läufer tion stages but a slight and consistent CCW rotation of the Southalpine et al., 2001), paleogeographically sited at the southern border of the unit as well as an opposing tilt of the principal stress axes N and S of Variscan orogen, with south-directed tectonic transport (Rantitsch, this major tectonic lineament towards the foreland. These findings are 1997 and references therein). Along the PAF, the Variscan basement discussed in light of the tectonic evolution of the double-vergent orogen is exposed and overlain in the south by the southward dipping as well as correlated with the large-scale tectonic setting. Southalpine unit (e.g. Rantitsch, 1997). The Carnic Alps were overprinted by Variscan and Alpine orogenies (e.g. Brime et al., 2008) 2. Geological setting and represent a nearly continuous geological section ranging from Late Ordovician to Permian. In the Carnic Alps, two sedimentary The Adriatic microplate including the Southern Alps on its front megasequences can be chronologically distinguished (Fig. 2b): a acted and still acts as a rigid indenter, with ca. N-directed motion lower pre-Variscan and an upper post-Variscan cycle (Schönlaub and (Ratschbacher et al., 1991b and references therein) and an additional Heinisch, 1993). The pre-Variscan cycle represents a passive continental CCW rotation with respect to stable Europe (e.g. Márton et al., 2003; margin and subsequent, Early Carboniferous collision succession Soffel, 1972; Thöny et al., 2006). North of the PAF, part of the shortening (Rantitsch, 1997). Clastic sediments form the base and are followed by is accommodated by lateral extrusion and tectonic escape of the thick Devonian shelf margin carbonates of an extensional geodynamic Austroalpine units to the east (Ratschbacher et al., 1991b) along a environment and a contractional geodynamic environment, including corridor formed by the dextral PAF in the south and the sinistral a synorogenic flysch (Lower Carboniferous) as the youngest formation Innsbruck–Salzburg–Amstetten (ISAM), Salzach–Enns–Mariazell– (Mader et al., 2007). The post-Variscan cover unconformably overlies Puchberg (SEMP) and Mur–Mürz faults in the north (Fig. 1a). Eastward the folded and faulted Variscan basement. The deposition started in extrusion started in Oligocene times mainly on the ISAM and PAF faults the Late Carboniferous with intracontinental molasse sediments (Wölfler et al., 2011) and activity moved to the SEMP fault as the north- (Leppig et al., 2005). The Southalpine cover includes thick Permian ern border of the extruding block in Early Miocene. and Triassic carbonate-dominated successions followed by Jurassic to The PAF strikes approximately E–W for 700 km through the Alps Eocene mainly pelagic carbonates and Eocene flysch. In Mesozoic from NW Italy to NE Slovenia (Fig. 1a). There, the fault disappears times, during the opening of the Alpine Tethys, the pre-Southern Alps below Neogene sediments (Fodor et al., 1998). The amount of displace- were part of the passive continental margin of the Adriatic plate ment along the dextral reverse strike-slip fault is controversially (Bertotti et al., 1993; Carminati et al., 2010). Intensive back-thrusting discussed and estimates range from 100 km to 450 km (e.g. Haas affected the belt since Paleogene times and formed a S-verging orogenic et al., 1995). Local evidence of sinistral movements has been reported, structure (Castellarin and Cantelli, 2000, 2010) with no or very low- too (e.g. Nemes, 1996; Rathore and Becke, 1980). Partly, a vertical offset grade metamorphic overprint (e.g. Rantitsch, 1997 and references of several kilometers occurs (Läufer et al., 1997; Schmid et al., 1989; von therein). The thermal overprint of the Alpine orogeny reached high dia- Blanckenburg et al., 1998). Calc-alkaline intrusions were emplaced genetic to anchizonal metamorphic conditions (Brime et al., 2008). A K– along the whole fault during the Oligocene due to breakoff of the Ar age of ca. 100 Ma indicates a potential Cretaceous (eo-Alpine) origin Penninic slab (e.g. Rosenberg, 2004; von Blanckenburg et al., 1998). (Läufer, 1996). The pre-Alpine, Variscan metamorphic grade increases The N–Sprofile, selected for structural analysis, from Oberdrauburg from east to west, from the chlorite to almandine zone, and from (Austria) to Spilimbergo (Italy) includes the DR with Permian to south to north within the chlorite zone as well (Sassi et al., 2004 and -Mesozoic cover formations and the Gailtal crystalline basement north references therein). of the PAF (Figs. 1band2a). In the south the Southalpine unit with the Carnic Alps and its Permian to Cenozoic cover were investigated 3. Methodology of paleostress analysis (Figs. 1b and 2b). The transect runs parallel to the axis of maximum shortening of the Eastern Alps with present-day rates of more than Structural field analysis is based on the measurement of brittle defor- 3mm/a(Caporali et al., 2009). The present-day shortening particularly mation structures, mainly fault planes and associated slip indicators, within the Southern Alps generates thrust faulting along N-dipping struc- such as slickensides, striae and grooves (e.g. Angelier, 1994; Doblas, tures with episodic seismicity with magnitudes of up to Mw 6.6 (Burrato 1998; Gamond, 1983, 1987; Petit, 1987). The local slip direction on et al., 2008) and long quiescent conditions (Pondrelli et al., 2001). each individual shear plane reflects an ancient time-averaged stress The basement of the DR comprises metamorphic and unmetamorphic (Lacombe, 2012). Measurement and analysis of these slip directions on Paleozoic rocks, and the cover is composed of non-metamorphic mesoscale faults yield the principal stress axes and the contemporary re- Paleozoic and Mesozoic rocks (e.g. Tollmann, 1977; see stratigraphic gional deformation pattern. The major shortcomings of the paleostress log, Fig. 2a). The wedge-shaped Gailtal metamorphic complex dips analysis method are, among others, insufficient number of fault sets moderately to steeply to the north, and represents the original pre- and of overprint criteria formed during different deformation stages, Alpine basement of the sedimentary DR cover (Tollmann, 1977 and ref- largely varying fault orientations due to anisotropy like bedding, and ro- erences therein). From N to S within the Gailtal metamorphic complex, tation of paleostress axes of earlier deformation stages during subse- the metamorphic grade decreases from a garnet micaschist zone to quent folding or block rotation by wrenching (e.g., Fodor et al., 1998). almandine low-grade metamorphism (Heinisch, 1987) with phyllites The relative and absolute timing of deformation phases can be best of Silurian to Devonian age (Schönlaub, 1974). Investigations in the constrained by the presence of well-dated sedimentary and or magmatic Jenig Complex in the central Gailtal basement (Fig. 1b), yield a mini- rocks. However, for our study area, these types of rocks are missing ex- mum temperature of 570 °C, a maximum pressure of 0.4 GPa, Triassic cept for Oligocene tonalites, so the timing of the succession of deforma- to Jurassic mineral cooling ages and a very low-grade thermal overprint tion stages largely relies on such information from neighboring areas (T b 300 °C) of likely Cretaceous age (Schuster et al., 2001). In the (see Subsection 6.2). southern DR, the 4 to 5 km thick Permian to Mesozoic sedimentary for- For this study, all major steps, i.e. data collection, analysis and inter- mations (Tollmann, 1977) were deposited above an angular unconfor- pretation, which may all introduce errors (e.g. Sperner and Zweigel, mity onto the Gailtal metamorphic complex. The DR represents the 2010) were carried out by the first author (E. Bartel). Ca. 1000 fault 308 E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327

Fig. 2. (a) Stratigraphic log of the Drau Range. (b) Stratigraphic log of the Carnic Alps and Southalpine Unit. The thickness of the stratigraphic units has been taken from Bauer and Schönlaub (1980); Carulli (2000); Discenza and Venturini (2002); Krainer (2007); Tollmann (1977); Unzog (1989); Venturini (1990, 2006);andSchönlaub (1980). Major decollement horizons are indicated; representative ﬁeld photos illustrate the stratigraphic evolution.

slip datasets were collected along a transect of strong N–S shortening principal stress axes σ1, σ2 and σ3 (σ1 ≥ σ2 ≥ σ3), and the stress ratio R from about 100 outcrops north and south of the PAF. In addition, avail- (R = [σ2 − σ3]/[σ1 − σ3]). Three different numerical methods for calcu- able structural data from the literature were compiled and used for lation were used in this study: the PT-method (analysis of pressure and comparison. For data analysis every outcrop was treated separately, tension axes; Marrett and Allmendinger, 1990; Turner, 1953), the NDA and only in cases of sparse data fault sets from neighboring outcrops (Numerical Dynamic Analysis, Spang, 1972) and the direct inversion with similar lithology and without fault zones in between were merged (Angelier, 1979). The NDA method calculates the orientation of the prin- together. cipalstressaxesfromsummationofindividual tensors. The direct inver- For kinematic data visualization, processing and analysis, TectonicsFP, sion method uses a least-square minimization of the shear stress written by Reiter and Acs (Ortner et al., 2002) was used. First, the consis- component perpendicular to the observed slip (Sperner and Zweigel, tency of the data was controlled (Sperner and Zweigel, 2010): all striae of 2010). Inhomogeneous datasets were separated on the basis of ﬁeld ob- the raw data should lie perfectly on the respective fault planes, otherwise servations (Fig. 3) and kinematic compatibility (e.g. Hancock, 1985; the data were corrected and misﬁts excluded. Fault-striae data are shown Lacombe et al., 2006) of the principal stress axes (calculated with the graphically in equal area, lower hemisphere stereonets as Angelier plots PT method) into homogenous partitions. The paleostress tensors for (Angelier, 1979). The fault-striae data enable calculation of the three each cogenetic fault population were determined, preferably with the E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327 309

Fig. 3. Examples of mesoscale faults observed in the field. (a) Sinistral slickenside with calcite steps (Alpiner Muschelkalk Fm., outcrop D2) on a strike-slip fault. (b) Subhorizontal slickenside with chlorite fibers on a strike-slip fault (Werfen Fm., outcrop D10). (c) Normal fault with calcite fibers (Alpiner Muschelkalk Fm., outcrop D2). (d) Pinnate fractures as kinematic indicator for the fault movement (Hochwipfel Fm., outcrop K37). (e) Mesoscale fault within the Kössen Fm. (outcrop D7). (f) Normal fault with gypsum overprinting oblique-slip fault and showing the relative age sequence of faulting in the Bellerophon Fm. (outcrop S2). (g) Relative age sequence in the Kössen Fm. (outcrop D7). (h) A reactivated fault plane with two distinguishable slickenside lineations constraining the relative age sequence (Hochwipfel Fm., outcrop K45).

NDA method using a best-fit angle of internal friction calculated by the PT information on the lithology, the method, the calculated results and method. field observations like visible chronological sequences. Stress fields Results are illustrated in Figs. 4–11. In a stress axis diagram, the with an insufficient amount of fault-striae evidence, e.g. the E–W exten- major and minor axes are overlain by a contour plot (Fig. 12)(Robin sion and compression, do not allow further processing. The results of and Jowett, 1986). For the σ1 axes the mean vector according to the main deformation stages of the two sections are first illustrated in- Wallbrecher (1986) with the 90% confidence cone and the spherical ap- dependently (N-section: Figs. 4–7, S-section: Figs. 8–11) and then jux- erture illustrate the error estimation (Fig. 13). taposed for comparison (Figs. 12 and 13). The chronological sequence As in many paleostress studies, we note method-inherent uncer- of the deformation stages is shown in the outmost right column of tainties on trend and plunges of paleostress axes because of insufficient Fig. 14 and is based on map studies and observations in the field like data after separation or uncertainties because of incorrect separation of cross-cutting relationships of lithological units (Fig. 3f), and faults fault-striae sets. We omitted data with less than four fault-striae sets (Fig. 3g) and reactivation of fracture planes with two distinguishable from a few outcrops, which could potentially constrain further deforma- slickenside lineations (Fig. 3h). Five mostly transpressive paleostress tion phases. All the remaining paleostress tensors have sufficient well- fields have been identified, and we describe them from old (D1)to constrained sets, except for D1. young (D5). Absolute time constraints are prohibited by the absence of sediments younger than Cenomanian. 4. Results 4.2. Map-scale structures of the Drau Range 4.1. Approach The description is based on eight regular 1:50,000 geologic map In this chapter, we describe the major structures and their relative sheets available from the Geologische Bundesanstalt (http://www. succession in detail. Literature and map interpretation precede the geologie.ac.at/produkte-shop/maps/)(Anderle, 1977; Kreuss et al., field study results. A comparison of results from the northern and 2006; Linner et al., 2013; Schönlaub, 1985, 1987, 1989, 1997, 2000). southern sections completes the chapter. This study is mainly based The western DR is bordered by faults and displays a trapezoidal shape. on fault-striae measurements and their paleostress assessment. Similar These include: The ENE-trending sinistral Drau fault and the dextral orientations of local stress tensors are assumed to represent a NW-trending Isel and Möll faults, with the Isel fault dextrally dislocating paleostress field of regional significance. The results of calculations of the Drau fault for about 6 km (Van Bemmelen and Meulenkamp, 1965) all fault-striae values are summarized in Tables 1 and 2 and include (Figs. 1b, 4–7). The DR is a synclinorium squeezed in between 310 E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327

Fig. 4. Tectonic map of the Drau Range: Typical structures of the deformation stage DN1 are emphasized. Stress-axis diagram plots all calculated principal stress axes σ1. Locations are plotted into the map, and the associated fault-slip data (Angelier plot, Angelier, 1979) and stress axes next to the map. Faults are drawn as great circles, striae as arrows pointing in the direction of displacement of the hanging wall (normal or reverse); double arrows indicate the shear sense on strike-slip faults. Confidence level of slip-sense determination in the field is expressed in the arrow-head style and follows usage for brittle faults (e.g. Ortner et al., 2002): full means large fault plane with well-developed sense of slip, open means reliable, half means rea- sonable, and without head indicates poor quality or undeterminable sense of slip. Black filled circles are the principal stress axes σ1, open squares σ2, grey filled triangles σ3. Black, thick arrows in the stereoplot show the direction of σ1. Dashed lines are the bedding planes or, when additionally labeled with sf, the foliation. Locations of data from the literature are plotted directly into the map; the orientation is shown with black (σ1) arrows. Legend for the map is shown in Fig. 1.

Fig. 5. Tectonic map of the Drau Range: Typical structures of the deformation stage DN2/5; inserts show detail maps of cross-cutting relationships. Locations of data from the literature are plotted directly into the map, black (σ1)andwhite(σ3) arrows show the orientation. Explanation as in Figs. 1 and 4. E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327 311

Fig. 6. Tectonic map of the Drau Range: Typical structures of the deformation stage DN3 and DN3A; for explanation see Figs. 1 and 4. External data are plotted directly into the map.

Fig. 7. Tectonic map of the Drau Range: Typical structures of the deformation stage DN4; Insert shows detailed map of cross-cutting relationships. External data are plotted directly into the map. Explanation as in Figs. 1 and 4. 312 E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327

subparallel to the PAF are common in the northern and southernmost parts of the DR (Figs. 1b, 4–7). In the south, the WNW-trending South Margin (SM) fault separates the Gailtal crystalline from the Permotriassic cover of the DR and is traceable for several tens of kilometers. North and parallel to the SM fault runs the DRSM (Drau Range South Margin) fault and these faults are interpreted to form a positive ﬂower structure (Schmidt et al., 1993). In the westernmost Lienz Dolomites, the DRSM fault incorporates major shear lenses of metamorphic basement rocks (Fig. 6a) (Schmidt et al., 1993; Schönlaub, 1997, 2000). Schmidt et al. (1993) interpreted the metamorphic basement lenses as exotic horses or strike-slip duplexes after Woodcock and Fischer (1986), which are laterally dislocated and shunted from the west. In the westernmost DR, a NW–SE striking dextral fault juxtaposes the DR Permian to Mesozoic formations to the Gailtal crystalline basement (Fig. 7a) (Schönlaub, 2000). In the whole DR, several NW- trending dextral and NE-trending sinistral strike-slip faults dislocate E–W striking strike-slip faults (Figs. 5a, b, 6a, and 7a). In the south of the DR several tectonic lamellae of Mesozoic cover are included in the crystalline basement (Fig. 5a) (Schönlaub, 1985; Tollmann, 1977). For example, the ~10 km long Pittersberg span is bordered by the DRSM and SM faults, shows doubling of the stratigraphic units, argues for at least 10 km sinistral displacement in E–W direction and is additionally dislocated by conjugated shears in particular by sinistral NW–SE striking faults (Schönlaub, 1985, 1987).

4.3. Map-scale structures of the Southalpine unit

The description of the South section is based on several Austrian and Italian geological and structural geological maps of the Carnic Alps and Friuli (Avigliano et al., 2007a,b; Bigi et al., 1990; Brandner, 1980; Cantelli et al., 1960; Feruglio et al., 1925; Gortani et al., 1925; Schönlaub, 1985, 1987, 1997, 2000; Venturini, 1990; Venturini et al., 2002; Zenari et al., 1927). N–S shortening affected the Southern Alps and resulted in S- vergence, strongest thickening and basement uplift of the Carnic Alps in the northernmost sectors of the Southalpine nappe stack. For the area between the Austrian–Italian border and Udine, Venturini (1990) estimated the total N–S shortening during the Alpidic orogeny at about 25% with respect to the original extent. About 50 km west of our study area, Schönborn (1999) calculated at least 50 km shortening of the lower crust and upper mantle below the eastern Southern Alps since the Late Miocene based on a balanced cross section. Structures are distinguished into Alpine E–W striking south-verging thrusts and, more to the east, the fault systems of the Dinarides with NW–SE oriented thrusts and strike-slip faults such as the Idrija fault (Fig. 1b; Slejko Fig. 8. Tectonic map of Friuli: Typical structures of the deformation stage D . Explanation S1 – as in Figs. 1 and 4. Locations of external data are plotted directly into the map. et al., 1999). The E W striking south-verging thrusts are common and are dislocated by NE-, NW- and N-trending strike-slip faults or thrusts subvertical crystalline basement units exposed in the south and north. (Figs. 1band8a). Important NE–SW striking thrusts are the Val The metamorphic Gailtal basement is located south of the DR. In the S Bordaglia fault, Tramonti–Verzegnis fault and the But-Chiarsó fault it is cut off by the Gailtal fault, an approximately W-trending (N280°) (Fig. 9). The Val Bordaglia fault consists of a bundle of subvertical faults segment of the dextral PAF (Fig. 1). The DR is characterized by several (Sassi et al., 1995), which forms a positive flower structure near Forni kilometers long, E–W striking tight high-amplitude folds, which plunge Avoltri (Venturini, 1990) and separates areas of very-low-grade and about 40° to the W within the central sector of the DR (Figs. 1b, 4–7) low-grade metamorphism (Brime et al., 2008; Sassi et al., 1995, 2004). (Schlager, 1963). The main anticlines are located along the central axis NE-trending thrusts are dislocated by NW–SE and N–S striking strike- of the range with synclines and subordinate minor anticlines on both slip faults (Fig. 9b). NW-trending thrusts prevail in the SE of Friuli sides. The folds are dislocated and deflected by numerous steep strike- near Udine e.g. Udine fault (Fig. 10) and are also found in the Carnic slip faults. An example is the E–W striking Amlacher Wiesen syncline Alps, where they are dislocated by NW–SE striking faults (Fig. 10a). near Lienz, where its northern fold limb is cut-off by an E–W striking WNW to NW trending dextral faults are located more in the east, fault (Linner et al., 2013; Schmidt et al., 1993) and the western part of e.g. the Idrija fault, dislocated E–Wstrikingthrustfaults(Fig. 1b) and ex- the fold is dragged to the WSW subparallel to the neighboring Drau tend over more than 120 km to Croatia (Gosar, 2007). In the NE of Friuli fault (Linner et al., 2013; Linner et al., 2013)(Fig. 4a). In the central sec- the Tröpolach–Camporosso fault cut off E–W striking thrusts and is tion of the Lienz Dolomites, the dextral NW–SE striking Oberalpl fault truncated by the Fella–Sava fault, which is a dextral ~E–Wstrikingre- dislocated and deflected an E–W striking anticline with Jurassic forma- verse strike-slip fault (e.g., Bartel et al., 2014; Fig. 1b). Dextral NW–SE tions (Fig. 5b). More to the west the same anticline is sinistrally and sinistral, NE–SW striking strike-slip faults are common in whole dislocated and partly deflected by an ENE-trending fault subparallel to Friuli, whereupon the dextral NW-trending faults often are transpressive the Drau fault (Fig. 6a). WNW–ESEtoE–W striking strike-slip faults and dislocated the NE–SW striking faults (Fig. 11). E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327 313

Fig. 9. Tectonic map of Friuli: Typical structures of the deformation stages DS2/5 and DS2/5A. Explanations as in Figs. 1 and 4. Insert shows detail map of horsetail structures of deformation stage DS5. Locations of external data are plotted directly into the map. Fault-slip data of outcrops with two different ages and the fault-slip data for DS2/5A are accentuated.

4.4. Field study results corresponding outcrops displays each deformation stage of the northern section (Figs. 4–7). Figs. 8–11 illustrate the Friuli area in the same

For each deformation stage of regional signiﬁcance, we used all σ1 manner, and the values of all calculated paleostress axes are shown in principal stress axes to calculate a mean vector based on the method of Table 1(N) and Table 2 (S), respectively. Results of compiled published mean vectors of lineations or poles of planes in the three-dimensional fault-striae data of the area are additionally charted in the maps domain after Wallbrecher (1986). We compared the orientations of the (Figs. 4–11) as orientations of the paleostress axes (σ1 ± σ3). Composite mean vectors based on our measurements with orientations of the diagrams (σ1, σ3) of all deformation stages are juxtaposed in Fig. 12. mean vectors from published data (Discenza and Venturini, 2002; Fritz Deformation stage D1: Thrust compression σ1 is subhorizontal N–S, et al., 1990; Läufer, 1996; Nemes, 1996; Sprenger, 1996; Venturini and σ3 subvertical. Carulli, 2002). Since the results are similar, robustness of our data is as- The oldest paleostress tensor, for which fault-striae data are sparse, sumed and we proceeded with a combination of the entire data set. belongs to N–S thrust compression (DN1,DS1). The relative age sequence in the north and south sections are mostly DN1 (Fig. 4): In the northern part fault-striae data indicating thrust in agreement and thus we combined their description. An overview compression were found in outcrops with variable lithologies ranging map of the western DR including locations and stereoplots of the from Gailtal crystalline basement up to Cretaceous formations. The 314 E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327

In both sections, the tensors appear in the basement up to the cover of Cretaceous age.

DN2/5 (Fig. 5): In our dataset the deformation stage is represented by a mean vector of 320/08, (318/09 including data from the literature).

DS2/5 (Fig. 9): The mean vector of this deformation phase in the south is 131/12 (135/11 respectively). Differentiation in the ﬁeld between the two similarly oriented tensor sets of different ages was possible in two outcrops: Outcrop K17 is located in the Southalpine basement (shale, Lower Carboniferous) and outcrop S53 (limestone with shale in- tersection, Upper Triassic) in the Southalpine cover. The stereoplots are shown in Fig. 9 and the relative age sequence is based on visible reactivation of slickenside planes in the ﬁeld (Fig. 3h). More details on the relative chronology of fault-striae data are given in Table 2.

DN2/5 versus DS2/5: A difference of 20° in inclination and 3° CCW rotation of the paleostress axes is observed.

Deformation stage D3 represents strike-slip compression with a subhorizontal NE–SW σ1-axis and a subhorizontal NW–SE σ3-axis. Deformation stage D3A is a substage with thrust compression represented by a subhorizontal NE–SW σ1-axis and a subvertical σ3-axis. The deformation stages D3 and D3A are distinct, the tensors show a subhorizontal NE–SW oriented maximum σ1-stress axis, and σ3 changes gradually from a subvertical to a subhorizontal NW–SE orientation.

DN3 (Fig. 6): In the DR, we found evidence for the deformation stage DN3 from the Gailtal crystalline basement up to Cretaceous formations. When D3 and D3A are distinguished, the mean vector for D3 is 045/10 (with published data 045/08); otherwise it is 041/08 (including published data 042/05).

DS3 (Fig. 10): In the South, D3 occurs within Devonian to Cretaceous formations. We found D3 = 215/05, including published data the mean vector is 223/11 (D3), and 221/11 (D3 +D3A). DN3 versus DS3: A CCW rotation (1 to 16°) and a tilting between 13 and 19° is visible between northern and southern sections.

During deformation stage D4, compression with a subhorizontal N–S oriented σ1-axis and a subhorizontal E–W σ3-axis prevailed. DN4 (Fig. 7): In the north, we found fault-striae data indicating deformation stage D4 in outcrops ranging from the Gailtal crystalline basement up to Cretaceous rocks. The calculated mean vector is 004/09 (004/08 with published data).

DS4 (Fig. 11): In the south D4 was found in outcrops including Devo- nian to Cretaceous formations and the deformation stage is represented by a mean vector of 181/04 (178/04 with published data).

DN4 versus DS4: A rotation of 6° CCW of the orientation of the Fig. 10. Tectonic map of Friuli: Typical structures of the deformation stage D . For expla- S3 paleostress axes and a change of the inclination of 12° are visible. nations see Figs. 1 and 4. Locations of external data are plotted directly into the map.

5. Comparison of kinematics north and south of the PAF calculated mean vector of the maximum compressive stress axis for the The relative sequences of deformation stages in both units separated north is 006/17. Including published data yields an only slightly shifted by the PAF are alike; therefore, we suggest a coeval evolution of DN1 and maximum principal stress axis (005/14). DS1,DN2 and DS2, et cetera. However, small differences in the orienta- DS1 (Fig. 8): In the southern part, evidence for thrust compression is tions of the paleostress axes between north and south exist and persist found from the Paleozoic basement up to Triassic cover formations. Our throughout all deformation stages (Figs. 12 and 13). We are aware that data yields 176/25 for the mean vector and 178/02 including compiled the differences are very small nevertheless we found them in all defor- data, i.e. the declination values are very similar whereas the inclination mation stages regardless of the observed set of data (exclusively data of varies. this study, exclusively published data, and merged data), indicating ro-

DN1 versus DS1: In both cases (all data or exclusively data of this bustness of our data. We assume that the two trends: (1) mainly a study), a CCW rotation of the southern part and a difference between plunge of σ1 towards the foreland on either side of the PAF (Fig. 13) plunges north and south of the PAF is recognizable. Including the addi- and (2) a CCW rotation of the southern major paleostress axes (σ1) tional data an average CCW rotation of 7° and a tilt of 16° are calculated. compared with the northern part (Fig. 13) are real and not a random Within the DR a trend of dip was observed: near the PAF the calculated result. principal stress axes dip at a lower angle than in outcrops more distant to the PAF. 6. Discussion

Deformation stages D2 and D5 both indicate compression with subhorizontal NW–SE orientation of σ1, and subhorizontal NE–SW ori- In the following discussion we ﬁrst establish a time frame for the de- entation of σ3 (Figs. 5 and 9). The deformation stages D2/5 represent formation events, then we correlate our results with the large-scale tec- thrust compression with a subhorizontal NW–SE striking σ1-axis, and tonic framework of the study area and ﬁnally discuss the variations of a subvertical σ3-axis. paleostress axis orientation N and S of the PAF. E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327 315

Fig. 11. Tectonic map of Friuli: Typical structures of the deformation stage DS4; for explanation see Figs. 1 and 4. Locations of external data are plotted directly into the map.

6.1. Timing of deformation events in the central and western Tauern window has been suggested (Schmid et al., 2013 and references therein). We consider that deformation stage

Correlating individual deformation stages with absolute rock ages or D1 is the main stage of regional folding in the study area, responsible for lithology is fraught with difﬁculty because Cenozoic sediments are lack- E–W trending folds in the Drauzug and initial southward tilting of the ing within the study area. The relative timing is clearer, but even there basement exposed within the Carnic Alps along the PAF. discrepancies still exist for the Southalpine unit as indicated by various paleostress reconstructions (Fig. 14)(Caputo et al., 2010; Discenza and 6.1.2. Deformation stage D2,NW–SE compressionand dextral shear along Venturini, 2002; Fodor et al., 1998; Läufer, 1996; Venturini, 1990, 2006; PAF

Venturini and Carulli, 2002). In the following, we consider published Mylonitization ages (not necessarily representing D2 dextral shear) age constraints from our study area as well as from neighboring regions along the PAF must postdate tonalite intrusions (ca. 33 to 28 Ma; with a similar structural evolution in order to conﬁne a time frame for Diener, 2002; Rosenberg, 2004) and range from 32 to 28 Ma (Läufer the deformation events. et al., 1997; Mancktelow et al., 2001; Müller et al., 2001; Pomella et al., 2011; von Gosen, 1989). These ductilely deformed tonalites likely

6.1.1. Deformation stage D1,N–S thrust compression constrain a major phase of dextral shearing, which continued to brittle E–W striking folds resulting from the oldest deformation event were deformation. Similar deformation ages of ~28 Ma were found along reported for the southwestern Lienz Dolomites (Brandner and Sperling, the Möll Valley fault within the Tauern Window (Th–Pb allanite, Cliff 1995) and for the Kreuzeck–Goldeck Complex north of the DR (Amann et al., 1998;U–Pb allanite & titanite, Inger and Cliff, 1994) where they et al., 2002). The youngest formation included in the core of the folds in are interpreted as the maximum age of dextral shearing. Based on Rb– the DR is the Amlacher Wiesen Formation of Barremian to Albian age Sr dating of mylonite (25.3 ± 2.9 Ma, 20.7 ± 2.3 Ma) Glodny et al.

(Blau and Grün, 1995), thus, deformation stage DN1 must be of Albian (2008) suggest continuous or at least pulsed active ductile deformation age or younger. From a regional point of view and similar kinematics, for the Möll Valley fault within the Tauern Window. NNW-trending

D1N folds could be related to similarly oriented Late Oligocene to Early dikes synkinematic to the Isel fault (Amann et al., 2002)yieldK–Ar Miocene folds within the Tauern window, which affect Oligocene ages of 36 ± 1 Ma (Müller et al., 1992). amphibolite- to greenschist grade isotherms of the Tauern window A dextral slip of 350–550 km along the PAF near the Pannonian basin (e.g., Hoinkes et al., 1999; Kurz et al., 1998). Peak metamorphism was at the eastern continuation is assumed to have accumulated since the reached at ~27 Ma (Rb–Sr white mica, Reddy et al., 1993), and the Early Oligocene (Haas et al., 1995; Tari, 1994) although the exact timing Sonnblick dome was formed by NNE-directed overthrusting between remains unclear. The conjugate dextral SEMP fault includes sedimenta- 27 and 29 Ma (Inger and Cliff, 1994). Later local clockwise rotation of ry basins formed at ca. 18 Ma (Ratschbacher et al., 1991b). From all ar- ca. 20–30° of the eastern Tauern window compared to similar structures guments given above, we assume a duration of dextral D2 slip between 316 E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327

Fig. 12. Comparison of main deformation stages N and S of the PAF; fault patterns, summarized stress-axis diagrams and typical structures for the paleostress phases D1 to D4 are shown. The central column outlines the compressional field with characteristic structures. Faults and folds are drawn as lines; double arrows indicate shear sense on strike-slip faults; filled triangles thrust faults; open squares fold axes. Dark filled arrows in the stereoplot show the direction of σ1; light gray arrows σ3. On either side summarized stress-axis diagrams are displayed; each measurement (Figs. 4–11) is represented by two associated symbols; the principal stress axis σ1 as filled circle; σ3 as cross; our own measurements are in black, data from literature compilation is in grey. Contour plots (Robin and Jowett, 1986) overlay the stress axis plots, the areas with the highest density σ1-axes are dark-colored, areas with low- density are bright; N0 — number of outcrops of the same stress regime. ca. 28 and 18 Ma representing the main stage of activity of lateral extru- and Neubauer, 2001), where white mica ages of 18 to 16 Ma are repre- sion of the Eastern Alps (Ratschbacher et al., 1991a,b). We note some sentative for vein opening during deformation stage D3. potential temporal overlap with the D1 deformation event, which has to be resolved by future work. 6.1.4. Deformation stage D4,N–Scompression For the Serravallian to Early Tortonian (ca. 16 to 8.5 Ma), a compressional stage with approximately N160°–170° orientation prevailed in

6.1.3. Deformation stage D3,NE–SW compression the southern parts of the Southalpine unit (e.g. Caputo et al., 2010 and In the Southern Alps a N20°–30° compression with subvertical σ3- references therein; Castellarin et al., 1998, 2006). New thrust systems axis is reported for Late Chattian to Early Miocene (Caputo et al., 2010 produced large amounts of shortening and propagated progressively and references therein) and is associated with trends of regional folding to the foreland (Caputo et al., 2010 and references therein). Strong re- in the Dinarides (Castellarin et al., 1998). Mainly the northern sector of gional uplift and exhumation in the Tortonian is confirmed by detrital the Southern Alps was affected (Caputo et al., 2010 and references apatite fission-track ages (Dunkl et al., 1996; Monegato et al., 2010; therein). Fission-track ages ranging from 22 to 16 Ma (Monegato Zattin et al., 2003, 2006). The tectonic load of the rapidly growing et al., 2010) indicate exhumation. In the Tauern Window, N10°–30° Southalpine fold-and-thrust belt produced flexural folding of the trending subvertical gold quartz veins (Feitzinger and Paar, 1991) Adriatic lithosphere and the development of a foredeep basin (Caputo yield white mica and adularia Ar–Ar ages of ~18 to 12 Ma (Handler et al., 2010 and references therein). E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327 317

Fig. 13. Comparison of the fault-striae data of the DR (top) and Friuli (below). The mean vector of Wallbrecher (1986) with 90% conﬁdence is calculated from the stress-axis plots and shown graphically and numerically for each deformation stage and area. The stereoplot (lower hemisphere) shows the mean vector as cross, the apertureconﬁdence cone as inner circle and the spherical aperture as a larger circle. The differences between the plunge angles and the orientation have been calculated. Bottom left: The three numeric values are exemplarily shown for DS1.Bottomright:D1 shows the summarized results.

6.1.5. Deformation stage D5 NW–SE compression European plate underneath the Eastern Alps to the later late Early Polinski and Eisbacher (1992) and Nemes et al. (1997) connected Miocene to Pliocene subduction of Adria underneath the easternmost the Serravallian (ca. 13 Ma) to Pliocene formation and destruction of Southern Alps and Dinarides leading to oroclinal bending in the Friuli the Klagenfurt basin north of the PAF with NW–SE compression along area. Furthermore, Adria rotated in a CCW manner since Oligocene the Möll fault. Recently, based on (U–Th)/He ages of 11 to 6 Ma from although details still remain to be solved (Márton et al., 2011 and refer- Periadriatic plutons from the eastern PAF, Heberer et al. (2014) ences therein). In summary, the stress ﬁeld is primarily governed by the proposed a similar age of activity. For the Southern Alps, Messinian– generally northward motion of the Adriatic microplate, concurrent CCW Pliocene NW–SE compression and younger Pliocene–Pleistocene N–S rotation as well as its ca. rectangular shape and potentially by the change compression are described (Caputo et al., 2010 and references therein). of subduction polarity. Based on a very detailed litho- and chronostratigraphic record of the Due to the northward motion of Adria, the main post-collisional de- sedimentary units Caputo et al. (2010) investigated pebbles along the formation stage within the Eastern Alps is the Late Oligocene to Early frontal thrust system in Tortonian to Pleistocene deposits and distin- Miocene east-directed lateral extrusion (e.g., Ratschbacher et al., guish four events with the main compression oscillating between NW 1991a), for which the Periadriatic fault formed the southern boundary and NNW and ages between Late Tortonian and Early to Middle of the extruding wedge. The succession of deformations in the northern Pleistocene. In summary, these data constrain a second phase of NW– and eastern sectors of the Eastern Alps includes some additional defor- SE compression and dextral shear along the PAF between maximum mation stages, which are not or only weakly expressed in the study area. 11 Ma to Quaternary. These include a stage of E–W extension (E and NE of the Tauern win-

Deformation stages D2 to D5 mostly include strike-slip compressional dow) and shear reversal along the major conﬁning faults (mainly shortening with subhorizontal σ3 and σ3 principal stress axes, in which SEMP fault) of the extruding wedge (Peresson and Decker, 1997) the effects of large-scale folding seem to be minor. Consequently, no (Fig. 14). major tilting of stress axes of deformation stages D2 to D5 has to be taken into consideration. 6.3. Interpretation of paleostress patterns

Based on our fault slip data sets N and S of the PAF, we suggest a 6.2. Deformation and motion of Adria coeval evolution of the Austroalpine and Southalpine units under the same kinematic and paleostress framework. However, the orientations Since the benchmark paper of Platt et al. (1989), the deformation pat- of the principal stress axes for the succession of all deformation events tern of the Eastern Alps is generally related to the changing motion direc- are not identical: North of the PAF the principal stress axes σ1 mainly tion of the Adriatic microplate. Recent reconstructions of the E–W- plunge to the north, south of the PAF to the south. Also, a CCW rotation trending Southern Alps to the SE-trending Dinarides by Ustaszewski of the southern major stress axes compared with the northern part is et al. (2008) and Handy et al. (2014) further relate the changes of kine- evident (Fig. 13). matics to a change of the subduction polarity during the collision stage. Different theoretical scenarios could explain the current state of the

Polarity changed from earlier Eocene to Miocene subduction of the angular misalignment of σ1 axes between the north and the south 318

Table 1 Results of paleostress analysis north of the Periadriatic fault. Abbreviations: No. — outcrop identification, A — Austria, I — Italy, N [all] — total number of fault-striae in the outcrop, N [D] — number of fault-striae sets of the same stress regime, σ — main stress axis, R — stress ratio R = (σ2 − σ3)/(σ1 − σ3), E. — Early, M. — Middle, L. — Late, D — deformation stage, e — extension, t — thrust, s — strike-slip compression, NDA — numeric dynamic analysis, INV — inversion method, PT — pressure–tension method, Y/O — young/old field observations, res — residual (slickenside is not classifiable), Cc — calcite crystal fibers, Qz — quartz crystal fibers, Chl — chlorite crystal fibers.

No. Station Longitude Latitude Formation Lithology Age of stratigraphic unit N [all] N [D] Stress Method σ1 σ2 σ3 R Field observations regime applied ..Bre ta./Tcoohsc 3 21)305 (2014) 637 Tectonophysics / al. et Bartel E.M. D1 Oberdrauburg to N46°45.41 E012°59.47 Alpiner Muschelkalk Limestone M. Triassic (Anisian) 27 18 D4 NDA 008/08 268/52 104/37 0.6 Y Cc Zwickenberg 7 E–Ws NDA 072/33 245/57 340/03 0.4 O D2 Oberdrauburg to N46°45.35 E012°59.49 Alpiner Muschelkalk Limestone M. Triassic 22 15 D3 NDA 041/23 265/59 139/19 0.6 Cc Zwickenberg D7 Gailbergsattel, N-side N46°43.94 E012°58.16 Kössen Shale + limestone L. Triassic (Rhaetian) 15 9 D4 NDA 353/02 086/60 262/29 0.4 D8 Gailbergsattel, N46°44.38 E012°58.51 Kössen Shale + limestone L. Triassic (Rhaetian) 11 10 D1 INV 020/06 112/17 272/72 0.2 Liegensteig D9 Oberdrauburg, N46°44.31 E012°57.87 Kössen Shale + limestone L. Triassic (Rhaetian) 12 8 D4 NDA 194/06 090/68 286/21 0.5 Y Cc Silberfall O = res D10 Gailbergsattel, Laas N46°42.17 E012°58.35 Werfen Shale E. Triassic 17 9 D1 NDA 025/41 290/06 193/48 0.5 O, Y = res Qz, 6 D3 NDA 037/23 233/66 129/06 0.5 Chl D13 Gailbergsattel, N-side N46°44.16 E012°58.12 Kössen Shale + limestone L. Triassic (Rhaetian) 12 7 D3 PT 054/32 271/48 161/25 Y Cc O = res D24 Gailbergsattel, N-side N46°44.37 E012°58.10 Kössen Shale + limestone L. Triassic (Rhaetian) 10 10 D3 PT 038/13 291/59 141/40 Cc D25 Gailbergsattel, S-side N46°42.67 E012°58.06 Alpiner Muschelkalk Limestone M. Triassic (Anisian) 5 5 D3 PT 023/25 248/67 291/03 Cc D28 Jaukenalm N46°40.57 E013°03.98 Gröden Sandstone, conglomerate E.M. Permian 9 7 D NDA 328/11 111/77 237/08 0.5 O Qz 2/5 – Y = res 327 D32 Gaibergsattel N46°43.98 E012°58.16 Kössen Shale + limestone L. Triassic (Rhaetian) 7 7 D4 NDA 196/09 340/80 105/06 0.5 Cc D37 Lanz N46°41.74 E013°00.86 Laas Red sandstone with E. Permian 12 11 D2/5 NDA 137/17 027/48 239/37 0.5 breccia a D39 Lanz N46°41.56 E013°01.08 Laas Red sandstone with E. Permian 15 8 D2/5 NDA 335/22 097/52 232/29 0.5 breccia 9 D3 NDA 024/01 116/50 293/40 0.5 LD1 Tuffbad N46°43.51 E012°46.21 Gröden Sandstone Permian 8 5 D1 PT 165/01 084/16 267/71 LD2 Tuffbad N46°43.35 E012°46.11 Gailtal crystalline Paragneiss, migmatite, 14 13 D3A NDA 206/17 111/17 338/65 0.5 Chl micaschist LD3 Tuffbad N46°43.30 E012°45.95 Gailtal crystalline Micaschist 18 11 D2/5 NDA 319/12 102/75 227/08 0.5 Y1, Y2, Y3 O Chl 6 D4 PT 013/34 197/70 119/23 LD4 Tuffbad N46°43.04 E012°46.15 Gailtal crystalline Granite, micaschist 17 15 D4 NDA 180/12 064/65 274/22 0.5 LD5 Eggenbach N46°43.09 E012°42.60 Gailtal crystalline Micaschist 18 14 D4 NDA 012/03 176/87 282/01 0.6 Y O = res LD6 Eggenbach N46°42.93 E012°42.56 Gailtal crystalline Orthogneiss 16 11 D3 NDA 067/11 241/79 337/01 0.6 5 D4 NDA 005/09 240/76 097/12 0.5 LD7 Guggenberg to N46°42.88 E012°42.93 Gailtal crystalline Micaschist 12 8 D1 INV 352/07 259/19 101/70 0.5 Qz, Lotteralm Chl LD8 Guggenberg to N46°42.82 E012°43.13 Gailtal crystalline Micaschist 16 7 D2/5 NDA 138/13 025/59 234/27 0.4 O1, O2 Y1 (Y2 = res) Lotteralm 4 D4 PT 016/07 128/78 284/13 0.5 4 E–We NDA 258/56 011/14 109/30 LD11 Ochsengarten N46°44.28 E012°39.08 Alpiner Muschelkalk Limestone M. Triassic 24 9 D2/5 NDA 309/13 199/55 047/32 0.6 6 D4 NDA 196/04 327/84 105/05 0.5 LD13 Gürberbach N46°43.75 E012°38.49 Gailtal crystalline Micaschist 29 16 D1 NDA 005/09 096/06 221/79 0.1 Y1,Y2,Y3,Y4,Y5,Y6 Qz O = Res LD14 Gürberbach N46°43.77 E012°38.30 Gailtal crystalline 4 4 D3 NDA 218/00 123/85 308/05 0.5 LD15 Gürberbach N46°43.54 E012°38.01 Gailtal crystalline Micaschist 6 4 D4 NDA 174/11 303/74 082/13 0.5 LD16 Lienzer Stadtweg N46°47.43 E012°45.70 Kössen Shale + limestone L. Triassic (Norian, 20 6 D3 NDA 239/14 007/68 144/16 0.5 Rhaetian) 14 D4 NDA 357/26 193/63 090/06 0.6 LD17 Lienzer Stadtweg N46°47.66 E012°44.88 Schrambach Cretaceous 29 9 D2/5 NDA 300/28 115/61 209/02 0.5 O Cc 6 D3 PT 216/01 115/64 304/07 0.5 Y 13 D4 NDA 339/19 157/71 249/01 LD20 Lienzer Stadtweg N46°47.77 E012°44.35 Rotkalk Limestone Jurassic 17 6 E–Ws NDA NDA 265/10 120/78 356/07 0.6 Cc 8 E–We 342/67 163/23 073/00 0.5 LD21 Tristacher lake N46°48.46 E012°47.61 Buntsandstein Buntsandstein E. Triassic 21 6 D2/5 NDA 302/00 044/90 212/00 0.5 Y Qz 8 D3 NDA 055/03 230/87 325/00 0.5 O Qz 4 D3A NDA 032/26 140/33 272/46 0.6 Qz LD22 Kornat to N46°42.91 E012°53.58 Partnach Shale M. Triassic (Ladinian) 9 4 D2/5 PT 331/16 209/78 238/03 Cc Mukulinalm 4 NE–SW e PT 235/85 116/04 041/06 LD25 Kornat to N46°42.50 E012°53.85 Gröden/Werfen Sandstone/shale Permian/Triassic 8 7 D2/5 NDA 146/04 250/74 056/15 0.4 Mukulinalm LD28 Mussen near lot N46°42.20 E012°55.72 Gröden Sandstone, conglomerate E.M. Permian 11 7 D2/5 NDA 326/19 180/67 060/12 0.5 Qz LD37 Thur–Klammbrückl N46°47.55 E012°43.60 Rotkalk Limestone + shale layer Jurassic 20 16 D4 NDA 191/11 082/60 287/28 0.5 Qz LD38 Lienzer Stadtweg N46°47.61 E012°45.41 Kössen Shale L. Triassic (Rhaetian) 8 8 D4 NDA 356/41 173/49 265/02 0.5 ..Bre ta./Tcoohsc 3 21)305 (2014) 637 Tectonophysics / al. et Bartel E.M. LD41 Galitzenklamm N46°47.86 E012°44.68 Hauptdolomite Dolomite U Triassic (Norian) 6 4 D4 PT 346/21 109/59 252/08 LD42 Klammbrückl– N46°47.74 E012°45.54 Schrambach Shale Cretaceous 4 4 D4 PT 183/06 302/72 092/30 Goggsteig LD44 Klammbrückl– N46°47.49 E012°44.12 Rotkalk Limestone Jurassic 9 7 D4 NDA 010/18 146/66 274/16 0.5 Schwandthütte LD45 Lavant N46°47.74 E012°50.27 Amlacher Wiesen Marl Cretaceous 12 4 D3 PT 243/01 154/71 329/19 8 D3A NDA 202/05 292/05 064/83 0.5 LD47 Lavant N46°47.72 E012°50.39 Amlacher Wiesen Marl Cretaceous 20 20 D3A NDA 201/15 292/04 036/75 0.5 O1, O2, Y, Y2 Cc LD49 Kraßbauer N46°44.88 E012°56.50 Kössen Limestone, banked L. Triassic (Rhaetian) 14 6 D4 NDA 019/20 126/39 268/44 0.4 Cc LD53 Lavant N46°47.68 E012°49.87 Amlacher Wiesen Marl Cretaceous 14 7 D1 NDA 013/39 278/05 182/51 0.4 4 D3 PT 050/20 293/52 153/32 LD54 Lavant N46°47.64 E012°49.89 Amlacher Wiesen Marl Cretaceous 4 4 D4 INV 344/17 136/70 252/09 0.0 LD56 Lavant N46°47.74 E012°50.05 Amlacher Wiesen Marl Cretaceous 13 9 D3 NDA 059/00 149/45 328/45 0.6 O, Y LD57 Dolomitenhütte N46°47.12 E012°47.23 Kössen Limestone, banked L. Triassic (Rhaetian) 7 7 D4 NDA 008/24 200/66 100/04 0.5 LD58 Dolomitenhütte N46°47.31 E012°47.10 Steinplattenkalk Limestone L. Triassic (Norian) 6 4 NW–SE s INV 323/47 149/42 056/03 0.5 LD59 Tristacher lake N46°48.52 E012°48.47 Tristach Komplex Micaschist 4 4 D3 PT 035/14 271/70 147/56 – LD64 Iselberg Pass N46°49.99 E012°50.64 Kreuzeck crystalline Micaschist 11 4 D2/5 PT 329/04 212/74 061/13 327 a Multiple use of reactivated slickensides. 319 320 ..Bre ta./Tcoohsc 3 21)305 (2014) 637 Tectonophysics / al. et Bartel E.M.

Table 2 Results of paleostress analysis south of the Periadriatic fault. Abbreviations: No. — outcrop identification, A — Austria, I — Italy, N [all] — total number of fault-striae in the outcrop, N [D] — number of fault-striae sets of the same stress regime, σ — main stress axis, R — stress ratio R = (σ2 − σ3)/(σ1 − σ3), E. — Early, M. — Middle, L. — Late, D — deformation stage, e — extension, t — thrust, s — strike-slip compression, NDA — numeric dynamic analysis, INV — inversion method, PT — pressure–tension method, Y/O — young/old field observations, res — residual (slickenside is not classifiable), Cc — calcite crystal fibers, Qz — quartz crystal fibers, Chl — chlorite crystal fibers.

No. Station Longitude Latitude Formation Lithology Age of stratigraphic unit N [all] N [D] Stress Method σ1 σ2 σ3 RFieldobservations regime applied

G1 Mauthen Klamm (A) N46°39.56 E012°59.41 Gailtal crystalline Greenschist 6 4 D3 PT 205/01 358/64 118/02 K1 Plöckenpass, N-side (A) N46°39.00 E012°58.36 Hochwipfel Shale Carboniferous 31 25 D4 NDA 173/16 046/64 269/20 0.5 O (Y = res) Chl, Qz K2 Plöckenpass, N-side (A) N46°38.69 E012°58.14 Eder Limestone Devonian–E. Carboniferous 10 7 D2/5A NDA 146/16 248/36 037/50 0.5 Cc K3 Plöckenpass, N-side (A) N46°38.60 E012°58.09 Eder Limestone Devonian–E. Carboniferous 6 4 D3 PT 020/27 234/52 113/08 Cc K5 Plöckenpass, N-side (A) N46°37.28 E012°56.98 Findenig Limestone E.M. Devonian 17 5 D3 INV 041/08 157/72 308/16 0.3 Cc 6 D4 NDA 006/25 237/54 108/24 0.5 – 5 E We NDA 356/47 183/43 089/04 0.5 – 327 K7 Plöckenpass, N-side (A) N46°37.20 E012°56.98 Cellon Limestone E.M. Devonian 19 13 D4 INV 005/23 161/65 271/09 0.5 O1,O2,Y1,Y2 Cc K8 Plöckenpass, border (A, I) N46°36.21 E012°56.68 Cellon Limestone E.M. Devonian 35 17 D2/5 NDA 328/05 116/84 237/03 0.5 Y2 Cc 7 NW–SE e NDA NDA 164/73 044/09 312/15 0.5 Y1,O2,(O1 = res) 8 E–We 231/46 346/22 093/35 0.6 K15 Plöckenpass, E-slope (A) N46°36.46 E012°56.84 Devonian limestone Limestone Devonian 9 8 D2/5 NDA 119/14 276/75 028/06 0.5 Cc a K17 Plöckenpass, S-side (I) N46°35.88 E012°56.73 Hochwipfel Shale Carboniferous 27 14 D2/5 NDA 305/01 212/78 035/12 0.5 O1, O2 9 D2/5 NDA 325/10 086/71 232/16 0.2 Y3, Y4 4 N–Se PT 082/66 284/45 009/01 0.4 O3,O4,Y1,Y2 5 NW–SE e NDA 219/80 041/10 311/00 K18 Plöckenpass, S-side (I) N46°35.56 E012°56.43 Devonian limestone Limestone Devonian 18 9 D2/5 PT 132/03 343/77 037/09 5 NW–SE e PT 346/49 233/03 128/34 K19 Plöckenpass, N-side (A) N46°39.42 E012°58.93 Hochwipfel Shale Carboniferous 4 4 E–W e INV 025/68 190/21 282/05 0.5 K20 Würmbach, querry (A) N46°39.23 E013°01.29 Eder Limestone Devonian–E. Carboniferous 10 7 D2/5 NDA 314/11 193/70 047/17 0.5 Cc K24 Plöckenpass, S-side (I) N46°35.92 E012°56.65 Hochwipfel Shale Carboniferous 7 6 E–W s PT 099/03 059/85 190/05 K25 Plöckenpass, S-side (I) N46°35.92 E012°56.77 Hochwipfel Shale Carboniferous 16 12 D2/5 NDA 121/12 337/75 213/08 0.4 K26 Plöckenpass, S-side (I) N46°35.87 E012°56.68 Hochwipfel Shale Carboniferous 9 9 D2/5 NDA 126/00 218/82 036/08 0.4 K27 Plöckenpass, S-side (I) N46°35.92 E012°56.67 Hochwipfel Shale Carboniferous 13 11 E–W s NDA 264/25 061/63 170/09 0.5 K33 Plöckenpass, S-side (I) N46°35.90 E012°56.75 Hochwipfel Shale Carboniferous 21 18 D2/5 NDA 129/13 235/50 028/37 0.5 O, Y K36 Sigilletto–Collina (I) N46°34.60 E012°48.24 Hochwipfel Shale Carboniferous 12 9 D1 NDA 201/41 097/16 351/45 0.5 K46 Zollner, Geotrail (A) N46°36.14 E013°04.37 Bischofsalm Shale E.L. Devonian 4 4 E–W t NDA 279/08 011/13 158/74 0.5 K50 Weidenburg–Zollner (A) N46°37.29 E013°03.98 Bischofsalm Shale E.L. Devonian 6 4 D2/5 PT 122/30 315/65 214/03 K51 Weidenburg–Zollner (A) N46°37.45 E013°04.00 Bischofsalm Shale E.L. Devonian 6 6 D2/5 NDA 143/19 266/58 044/25 0.5 K53 Weidenburg–Zollner (A) N46°37.74 E013°04.09 Bischofsalm Shale E.L. Devonian 8 8 D3 NDA 232/18 016/69 138/12 0.4 K55 Plöckenpass (A) N46°39.21 E012°58.65 Hochwipfel Shale Carboniferous 11 11 E–W e NDA 200/65 350/22 085/11 0.5 K57 Plöckenpass, S-side (I) N46°35.94 E012°56.68 Hochwipfel Shale Carboniferous 7 6 D2/5 NDA 123/12 282/77 032/05 0. 5 S11 Zuglio-Fielis (I) N46°28.05 E013°00.60 Degano Limestone L. Triassic (Carnian) 4 4 D4 NDA 167/11 338/79 077/02 0.4 S17 Mt. Zonoclan (I) N46°30.08 E012°55.60 Werfen Shale E. Triassic (Olenekian) 6 6 N–S e NDA 086/81 265/09 355/00 0.5 Cc S25 Noiarris bridge (I) N46°29.59 E012°59.88 Bellerophon Shale Permian 4 4 E–W s PT 277/09 200/68 014/39 S28 Arta–Cabia (I) N46°28.07 E013°01.79 Werfen Shale E. Triassic (Induan) 7 6 NW–SE e NDA 213/68 052/21 319/06 0.5 S40 Zuglio-Str Fielas (I) N46°27.92 E013°01.27 Schlern Limestone M.L. Triassic 10 4 D3 PT 223/30 023/53 108/29 S42 Zuglio–Sezza–Fusea (I) N46°27.12 E013°00.14 Schlern Limestone M.L. Triassic 15 4 D4 NDA 006/09 245/72 098/15 0.7 6 NW–SE e NDA 106/58 219/13 316/28 0.6 S43 Zuglio–Sezza–Fusea (I) N46°26.51 E012°59.77 Limestone 8 4 D4 PT 184/14 285/80 093/12 4 D2/5A PT 319/01 051/11 224/71 S44 Zuglio–Sezza–Fusea N46°26.41 E012°59.66 Degano Marl, turbidite L. Triassic (Carnian) 6 4 D4 PT 156/26 002/51 255/12 S45 Fusea–Butteo (I) N46°26.39 E012°58.40 Siera Marl M. Triassic (Anisian/Ladinian) 5 5 D2/5 PT 118/29 250/50 014/23 S47 Caneva di Tolmezzo N46°24.88 E013°00.44 Limestone 6 6 N–S e NDA 293/59 063/21 162/22 0.5 S48 Fusea–Butteo (I) N46°25.35 E012°59.38 Siera Marl M. Triassic (Anisian/Ladinian) 5 5 D4 PT 017/20 245/59 116/23 S50 Fusea–Pesmolet (I) N46°26.83 E012°59.5 Degano Limestone L. Triassic (Carnian) 8 8 D1 PT 162/28 064/05 303/57 S51 Butteo (I) N46°26.19 E012°57.84 Siera Marl M. Triassic (Anisian/Ladinian) 13 5 D2/5 NDA 155/09 261/59 060/29 0.5 5 NE–SW e NDA 070/61 326/08 232/28 0.5 S52 Plugano–Vinaio (I) N46°26.53 E012°57.40 Dürrestein Limestone L. Triassic (Carnian) 8 5 E–W e NDA 072/76 171/02 262/14 0.5 S53 Runchia (I) N46°26.68 E012°57.90 Degano Limestone L. Triassic (Carnian) 29 8 D1 NDA 184/30 092/04 355/59 0.5 8 D2/5 NDA 123/40 241/30 356/36 0.5 O 7 D2/5 NDA 121/17 307/72 211/02 05 Y 305 (2014) 637 Tectonophysics / al. et Bartel E.M. S56 Lago Verzegnis (I) N46°22.83 E012°57.87 Forni Dolomite L. Triassic (Norian) 20 7 D2/5 PT 123/05 217/51 035/14 7 NW–SE e NDA 184/56 040/28 301/17 0.5 S57 Str Verzegnis–San N46°20.97 E012°55.43 Soverzene Dolomite L. Jurassic 7 7 D1 NDA 342/03 249/41 075/49 0.5 Francesco (I) S60 Str Clauzetto–Almandis (I) N46°13.35 E012°54.19 Cretaceous 14 5 D2/5 INV 142/43 315/47 049/03 0.2 4 D3 PT 041/46 260/43 154/20 S63 Tolmezzo–Illego (I) N46°26.41 E013°02.48 Schlern Dolomite L. Triassic (Norian) 5 4 D4 PT 009/06 255/72 099/10 S64 Tolmezzo–Illego (I) N46°25.10 E013°02.99 Schlern Dolomite L. Triassic (Norian) 4 4 D4 PT 009/06 225/86 274/10 S65 Tolmezzo–Illego (I) N46°24.71 E013°02.68 Schlern Dolomite L. Triassic (Norian) 10 4 NE–SW e PT 246/46 061/44 338/01 4 E–Wt PT 276/21 358/17 091/66 a Multiple use of reactivated slickensides. – 327 321 322 E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327

Fig. 14. Comparison of published paleostress data in the Southalpine and Austroalpine units. Data sources: Caputo et al. (2010), Discenza and Venturini (2002), Kurz et al. (1994, 1996), Läufer (1996), Peresson and Decker (1997), Venturini (1990).

(Fig. 15): (1) Uniform radial stress regimes (Fig. 15a, b). Radial 2002). Fig. 15a adjusts this regional scenario with a fanning out to the paleostress patterns are common, e.g. in the Western Alps and Jura north. A dextral lateral offset, as generated along the PAF, leads to an ap-

Mountains (Grünthal and Stromeyer, 1992; Homberg et al., 1999, parent clockwise rotation of the stress-axes σ1 of the southern block 2002). There, the observed stress ﬁeld is perpendicularly arranged to compared to the north — a result, which is not reproduced by our the mountain arc, an effect of pushing an indenter from the south to data. (2) A radial stress pattern fanning out to the south, combined the north (Grünthal and Stromeyer, 1992; Homberg et al., 1999, with the dextral offset, would theoretically produce a CCW rotation

Fig. 15. Schematic sketches of various scenarios explaining differences in rotation of σ1 N and S of the PAF (younging from the left to the right). (a) Dextral displacement along the PAF within a N-fanning stress regime leads to CW rotation of the southern units. (b) Dextral displacement within a S-fanning stress regime leads to CCW rotation of the southern units. (c) Dextral displacement within a uniform stress regime and late-stage CCW rotation of the southern units as well as (d) dextral displacement within a N-fanning stress regime and late-stage CCW rotation of the southern units. E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327 323

(Fig. 15b), but is kinematically incompatible with the Adriatic indenter which started in Miocene times, as all tensors D1 to D5 plunge to the pushing from the south. (3) An identical pattern of the paleostress foreland. axes would be derived by a uniform stress regime, a later dislocation In our region, the subsurface is formed by a large-scale orogenic along the dextral PAF and CCW rotation of the southern block wedge, and two alternative models of the deep subsurface structure for (Fig. 15c). We suggest that the slight but consistent CCW rotation of the TRANSALP proﬁle (~100 km west of the research area) have been sug-

σ1 axes resulted from (4) a combination of late-stage CCW rotation of gested: the Crocodile and Extrusion models (Fig. 17a, Gebrande et al., the southern units as well as dislocation along the PAF within a north- 2002). Both models are consistent with the ALP01 profile (Brückl et al., fanning stress regime (Figs. 15dand16). The two contrary effects 2007), less than 50 km to the east of our study area. The Alpine orogen attenuate each other. This scenario agrees with the Adriatic indenter is a rare case of a double-vergent orogen with two foreland basin systems pushing from the south and the current CCW rotation of the Adriatic (e.g. Naylor and Sinclair, 2008). The PAF separates the stable Europe-verg- microplate. Numerous studies have described Cenozoic CCW rotation ing Eastern and Western Alps from the Africa-verging Southern Alps of Adria, although the exact timing and amount of rotations remain a (Castellarin et al., 2006). The critical taper theory can be used for both matter of debate (e.g. Márton et al., 2000, 2011; Ustaszewski et al., wedges independently as Carrapa (2009) demonstrated especially for 2008; van Hinsbergen et al., 2014). Based on paleomagnetic constraints the Alpine orogeny. The retro-wedge below the Southalpine unit is youn- Late Oligocene to Miocene CCW rotation was reported from both the ger, dominantly of Miocene age and at most 10 to 15 km thick (Schmid Eastern (e.g. Thöny et al., 2006) and Southern Alps (e.g. Márton et al., et al., 2004). South of the PAF exhumation increased towards the PAF, 2011). A second step of CCW rotation between about 6 and 4 Ma as indicated by the exposure of the Paleozoic basement in the Carnic affected the easternmost Eastern Alps (Márton et al., 2000), NW Alps. North of the PAF, exhumation of 15 km occurred within the Tauern Croatia and NE Slovenia (Márton et al., 2003) as well as the Styrian Window with rates up to ~4 mm/a since ~20 Myrs (Fügenschuh et al., and Vienna basins (Scholger and Stingl, 2002). GPS derived motion 1997; Genser et al., 1996). The main pulse of exhumation took place be- yields a current CCW rotation rate of 0.297 ± 0.116°/Ma and a tween 20 and 15 Ma (Fügenschuh et al., 1997; Genser et al., 1996), and present-day Euler pole near Turin for the Adriatic microplate (Weber minor exhumation has been active since 15 Ma (fission-track ages; Luth et al., 2010). Our study suggests that differential rotation affected the and Willingshofer, 2008). Underneath the Tauern Window, the subordi- Southalpine and Austroalpine units. However, it remains to be demon- nate northward thrusting of a lower crustal wedge and the bulk of the strated, whether the Periadriatic fault or the southern front of the shortening in the axial zone led to a mainly stationary situation near the Southalpine units represents the border between differently rotated retro-wedge, with an almost fixed location of exhumation and deforma- units. tion since Oligocene times (Rosenberg and Berger, 2009). We consider Finding an explanation for the observed difference in plunge north the pro- and retro-wedge as long-term stationary features and the influ- and south of the PAF is even more difficult. The question arises, whether ence of the lateral displacement along the PAF for the geometry of the we are dealing with a primary or secondary (e.g. a late-stage tilting) fea- wedge as negligible, since the strike-slip fault is straight, large-scale and ture. Such tilting might be explained by an evolving fold and thrust belt perpendicular to the orogenic wedge in this segment of the Eastern system, which is characterized by ramp geometries (wedging in the Alps. Theoretically, we expect an axial zone between the pro- and retro-

TRANSALP proﬁle, Fig. 17a) and displays an overall wedge-shaped ge- wedge for a double-vergent wedge (Fig. 17b). A zone of horizontal σ1 is ometry. For the study area, this would imply that tilting postdates D5, lacking along our proﬁle or the entire zone between PAF and the Tauern

Fig. 16. Chronological evolution of the stress patterns formed during D2 to D5. We assume a N-fanning stress regime, predominantly dextral movement along the PAF with possibly sinistral shear reversal during D3 and passive rotations of former stress patterns (broken lines). Motions for Adria–Europe convergence were taken from Caputo et al. (2010). 324 E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327

Fig. 17. Possible origins of the differences of the plunge of σ1 between north and south. (a) Two models of the orogenic wedge (Gebrande et al., 2002). Abbreviations: DM — Dolomite mountain, PAF — Periadriatic fault, NCA — Northern Calcareous Alps, TW — Tauern Window, AC — Adriatic crust, AS — Adriatic sediments, EC — European crust, ES — European sediments, PO — remnants of Penninic ocean. (b) Model of a double-vergent wedge (McClay and Whitehouse, 2004). (c) Model of a convergent single-vergent wedge (bulldozer wedge) with the internal stress distribution (Davis et al., 1983), α — surface slope, β — basal dip, μ i — coefﬁcient of internal friction, μ b — coefﬁcient of basal friction, σ1 — maximum principal stress,

σ3 — minimum principal stress, ψ0 — angle between σ1 and the surface slope, ψb — angle between σ1 and the basal decollement.

window represents the axial zone. The southern part shows a large vari- 1999), north of the PAF mainly to the N (Reinecker and Lenhardt, ability of the southwards dip of the bedding planes, and our calculated 1999). Similar variable plunge directions have also been detected by pa- plunge angles of σ1 are in contrast less variable. This observation is con- leomagnetic studies. The paleomagnetic data of the northern area show trary to a pure secondary tilting, unless the whole area was tilted homo- almost exclusively an inclination to NW, whereas both NW and SE di- geneously after the youngest deformation. rections are found within the Southalpine unit (Márton et al., 2011;

An initial (i.e. primary) plunge of σ1 might be attributed to an over- Thöny et al., 2006 and references therein). burden pressure depending on friction along the basal decollement In summary, we suggest that a combination of the two models pro- inside the wedge. The geometry and physical characteristics of the posed above produced the observed variable plunge, i.e. a secondary pro- and retro-wedge of double-vergent wedges (Fig. 17b) correspond late-stage tilting event further increased the initial plunge of σ1, which for each detached wedge to the features of a Coulomb wedge is an effect of wedge geometry and basal friction. (Fig. 17c, Davis et al., 1983). Analysis of such single-vergent wedges (bulldozer wedges) delivers insight into their internal stress-field (Fig. 17c; Davis et al., 1983). The geometry and the length of a wedge, 7. Conclusions as well as the angle of the wedge taper (α), depend on the coupling of the detachment, and hence on the basal friction coefficient (μ b). High The paleostress analysis of fault-striae data from the blocks north and basal friction generally increases the wedge taper, low basal coupling south of the Periadriatic fault allows the following major conclusions: reduces the angle (Davis et al., 1983). Provided that the slightest cohe- sion existed at the top surface of the wedge depending on friction 1) Kinematic analyses of brittle fault datasets allow defining five along the basal decollement, overlying weight within a wedge deflects successive tectonic, mostly compressive regimes. These include the orientation of the stresses in the subsurface. The plunge of the prin- (1) N–S compression, (2) NW–SE compression, (3) NE–SW com- σ cipal stress axis (σ1) changes from subhorizontal to parallel to the local pression, 3 changes gradually from subvertical to subhorizontal, topography taper (Davis et al., 1983), in larger depths this effect in- (4) N–S compression and (5) NW–SE compression and show a σ creases and the plunge of σ1 is steeper. For uniform and noncohesive systematic shift of the principal stress 1 over ca. 90° between wedges the angle between σ1 and the surface, ψ0, and the angle be- Oligocene and Late Miocene/Pliocene. tween σ and the basal detachment, ψ , are constant (Dahlen, 1984; 1 b 2) Deformation stages on either side of the PAF show large similarities. Lallemand et al., 1994)(Fig. 17). The angles are both dependent on The mean orientations of the principal stress axes (σ1) are, however, the basal friction coefficient (Davis et al., 1983; Graveleau et al., 2012), significantly different: and in cases of low basal friction where ψ0 is only a few degrees, an increase of the basal friction equally increases ψ0 and the total dip (Ruh (a) A CCW rotation of the principal stress axes of the southern block et al., 2013). Studies exemplify for Taiwan ψb = 12° (Dahlen, 1984) in respect to the north is evident. and for the Aleuten ψb =14°(Lallemand et al., 1994). These correlate (b) The principal stress axes in the northern block show mainly a to an additional plunging of 3° (Taiwan), 9° (Aleuten) and a general northward plunge, and in the south a southward plunge. dip of the principal stress axis σ1 of 6° (Taiwan) and 12° (Aleuten). 3) Our results are consistent with the published CCW rotation of the For the Southalpine retro-wedge, a strong coupling of the Adriatic fore- Adriatic microplate. However, the magnitude of stress field rotation land and the Southalpine is suggested (Willingshofer and Cloetingh, indicated by our dataset seems to be lower (~20° vs. 7°). This dis- 2003). In Taiwan published slickensides data reveal a similar plunge of crepancy is resolved by dextral displacement along the PAF within

σ1-axes to the foreland (Chang et al., 2003; Deffontaines et al., 1997; a N-fanning stress ﬁeld. Thus, units S of the PAF were formerly Lacombe et al., 1997, 1999). Recent earthquake data also show that located further to the east, where stress directions were accordingly a majority of σ1-axes dip to the foreland (Angelier et al., 2009; oriented more to the east. σ Mozziconacci et al., 2009). The similar general dip of earthquake 1- 4) Opposing plunge directions of σ1 to the foreland are a consequence axes to the foreland is visible in our investigation area. In Friuli, the of a primary initial plunge of σ1, typical for the internal stress-ﬁeld earthquake σ1-axes dip to the S in most cases (Bressan et al., 1998; of an orogenic wedge and a further increase of this effect during con- Pondrelli et al., 2001; Reinecker and Lenhardt, 1999; Slejko et al., tinuing compression. E.M. Bartel et al. / Tectonophysics 637 (2014) 305–327 325

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