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

This study focuses on the analysis of structures and kinematics of a N-S profile along the axis of maximum shortening of the European Eastern Alps. The area includes the southern Austroalpine unit in the north and the Southalpine unit, which is a part of the Adriatic indenter. The stratigraphically different units are separated by the Periadriatic fault, the major strike-slip fault within the Alps. In order to assess the kinematics of these units, mainly fault-slip data from north and south of the Periadriatic fault were analyzed. We distinguish a succession of five main kinematic groups in both units: (1) N-S compression; (2) NW-SE compression; (3) NE-SW compression, σ3 changes gradually from subvertical to subhorizontal; (4) N-S compression; and (5) NW-SE compression. Our study reveals that the deformation sequence on either sides of the PAF is similar. The mean orientations of the principal stress axes, however, show small, but consistent differences: The subhorizontal axes north of the Periadriatic fault plunge northward, in the south southward. A counterclockwise (CCW) rotation of the southern part in respect to the north is evident and in line with the well-known counterclockwise rotation of the Adriatic 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.

Introduction

The Periadriatic fault (PAF) is the main strike-slip fault of the Alpine orogen and separates the Austroalpine unit and the indenting Southalpine unit in the Eastern Alps (Ratschbacher et al., 1991b, Schmid et al., 1989) (Fig. 1a). Both units belong to the Adriatic microplate sensu lato. Structural studies on fault systems (e.g. San Andreas fault, Zoback et al., 1987; Morez fault zone, Homberg et al., 1997) and 2-D distinct element modeling (Homberg et al., 1997) have shown, that stress fields may systematically rotate around subvertical axes in the vicinity of such major crustal discontinuities and stress deviations may occur next to weak zones where crustal deformation occurs. In this study, we pose the question whether the PAF represents an upper crustal stress perturbation, i.e. whether units on both sides of the PAF record similar or distinct stress patterns and/or whether block rotation overprinted the stress pattern.

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.

The stratigraphical, structural and kinematic frameworks north and south of the right-lateral transpressional PAF differ significantly (e.g. Tollmann, 1977; Fig. 1b): The Southalpine units represent a S- to SE-verging fold-and-thrust belt. Between the PAF and the Fella–Sava Fault (Fig. 1) units are steeply dipping towards the south. Structures south of the PAF are well investigated (Brime et al., 2008, Läufer, 1996, Läufer et al., 2001, Rantitsch, 1997, Venturini, 1990). Austroalpine units within the study area north of the PAF are characterized by intense large-scale folding. Modern structural studies are scarce for the Austroalpine units north of the PAF, particularly for the Drau Range (DR) modern structural, kinematic and paleostress analyses are entirely missing. Paleomagnetic studies revealed largely similar counterclockwise (CCW) Neogene block rotations of about 20° in both the Austroalpine and Southalpine units (e.g. Márton et al., 2000, Mauritsch and Becke, 1987, Thöny et al., 2006), but no conclusive data are available from the study area itself.

We provide paleostress reconstructions for Southalpine and Austroalpine units N and S of the PAF based on a new dataset of brittle fault kinematics. In conjunction with a compilation of existing structural data, we first recognize how many tectonic events occurred and secondly reconstruct the stress trajectories. The studied transect is located at 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 study area because of (1) the supposedly strongest shortening at least in the Southalpine unit; (2) the straight segment of the Periadriatic fault where stress reorientation by changing external conditions like plate motion and deformation structures along strike of the fault is considered unlikely; and (3) the well known tilting of blocks north (Gailtal basement complex) and south (Carnic Alps basement) of the fault, which implies rotation around a horizontal axis of pre-tilting paleostress tensors.

The present study suggests an overall similar sequence of deformation stages but a slight and consistent CCW rotation of the Southalpine unit as well as an opposing tilt of the principal stress axes N and S of this major tectonic lineament towards the foreland. These findings are discussed in light of the tectonic evolution of the double-vergent orogen as well as correlated with the large-scale tectonic setting.

Geological setting

The Adriatic microplate including the Southern Alps on its front acted and still acts as a rigid indenter, with ca. N-directed motion (Ratschbacher et al., 1991b and references therein) and an additional CCW rotation with respect to stable Europe (e.g. Márton et al., 2003, Soffel, 1972, Thöny et al., 2006). North of the PAF, part of the shortening is accommodated by lateral extrusion and tectonic escape of the Austroalpine units to the east (Ratschbacher et al., 1991b) along a corridor formed by the dextral PAF in the south and the sinistral Innsbruck–Salzburg–Amstetten (ISAM), Salzach–Enns–Mariazell–Puchberg (SEMP) and Mur–Mürz faults in the north (Fig. 1a). Eastward extrusion started in Oligocene times mainly on the ISAM and PAF faults (Wölfler et al., 2011) and activity moved to the SEMP fault as the northern border of the extruding block in Early Miocene.

The PAF strikes approximately E–W for 700 km through the Alps from NW Italy to NE Slovenia (Fig. 1a). There, the fault disappears below Neogene sediments (Fodor et al., 1998). The amount of displacement along the dextral reverse strike-slip fault is controversially discussed and estimates range from 100 km to 450 km (e.g. Haas et al., 1995). Local evidence of sinistral movements has been reported, too (e.g. Nemes, 1996, Rathore and Becke, 1980). Partly, a vertical offset of several kilometers occurs (Läufer et al., 1997, Schmid et al., 1989, von Blanckenburg et al., 1998). Calc-alkaline intrusions were emplaced along the whole fault during the Oligocene due to breakoff of the Penninic slab (e.g. Rosenberg, 2004, von Blanckenburg et al., 1998).

The N–S profile, selected for structural analysis, from Oberdrauburg (Austria) to Spilimbergo (Italy) includes the DR with Permian to -Mesozoic cover formations and the Gailtal crystalline basement north of the PAF (Figs. 1b and 2a). In the south the Southalpine unit with the Carnic Alps and its Permian to Cenozoic cover were investigated (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 3 mm/a (Caporali et al., 2009). The present-day shortening particularly within the Southern Alps generates thrust faulting along N-dipping structures with episodic seismicity with magnitudes of up to Mw 6.6 (Burrato et al., 2008) and long quiescent conditions (Pondrelli et al., 2001).

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, Venturini, 2006; and Schönlaub (1980). Major decollement horizons are indicated; representative field photos illustrate the stratigraphic evolution.

The basement of the DR comprises metamorphic and unmetamorphic Paleozoic rocks, and the cover is composed of non-metamorphic Paleozoic and Mesozoic rocks (e.g. Tollmann, 1977; see stratigraphic log, Fig. 2a). The wedge-shaped Gailtal metamorphic complex dips moderately to steeply to the north, and represents the original pre-Alpine basement of the sedimentary DR cover (Tollmann, 1977 and references therein). From N to S within the Gailtal metamorphic complex, the metamorphic grade decreases from a garnet micaschist zone to almandine low-grade metamorphism (Heinisch, 1987) with phyllites of Silurian to Devonian age (Schönlaub, 1974). Investigations in the Jenig Complex in the central Gailtal basement (Fig. 1b), yield a minimum temperature of 570 °C, a maximum pressure of 0.4 GPa, Triassic to Jurassic mineral cooling ages and a very low-grade thermal overprint (T < 300 °C) of likely Cretaceous age (Schuster et al., 2001). In the southern DR, the 4 to 5 km thick Permian to Mesozoic sedimentary formations (Tollmann, 1977) were deposited above an angular unconformity onto the Gailtal metamorphic complex. The DR represents the stratigraphic evolution of a rift and a subsequent passive continental margin, with Permian continental red beds at the base followed by Middle to Upper Triassic carbonate platform sediments, hemipelagic basinal sediments of Jurassic age and Cretaceous flysch sediments (Rantitsch, 2001 and references therein; Fig. 2a).

The Carnic Alps expose the basement of the Southalpine unit (Läufer et al., 2001), paleogeographically sited at the southern border of the Variscan orogen, with south-directed tectonic transport (Rantitsch, 1997 and references therein). Along the PAF, the Variscan basement is exposed and overlain in the south by the southward dipping Southalpine unit (e.g. Rantitsch, 1997). The Carnic Alps were overprinted by Variscan and Alpine orogenies (e.g. Brime et al., 2008) and represent a nearly continuous geological section ranging from Late Ordovician to Permian. In the Carnic Alps, two sedimentary megasequences can be chronologically distinguished (Fig. 2b): a lower pre-Variscan and an upper post-Variscan cycle (Schönlaub and Heinisch, 1993). The pre-Variscan cycle represents a passive continental margin and subsequent, Early Carboniferous collision succession (Rantitsch, 1997). Clastic sediments form the base and are followed by thick Devonian shelf margin carbonates of an extensional geodynamic environment and a contractional geodynamic environment, including a synorogenic flysch (Lower Carboniferous) as the youngest formation (Mader et al., 2007). The post-Variscan cover unconformably overlies the folded and faulted Variscan basement. The deposition started in the Late Carboniferous with intracontinental molasse sediments (Leppig et al., 2005). The Southalpine cover includes thick Permian and Triassic carbonate-dominated successions followed by Jurassic to Eocene mainly pelagic carbonates and Eocene flysch. In Mesozoic times, during the opening of the Alpine Tethys, the pre-Southern Alps were part of the passive continental margin of the Adriatic plate (Bertotti et al., 1993, Carminati et al., 2010). Intensive back-thrusting affected the belt since Paleogene times and formed a S-verging orogenic structure (Castellarin and Cantelli, 2000, Castellarin and Cantelli, 2010) with no or very low-grade metamorphic overprint (e.g. Rantitsch, 1997 and references therein). The thermal overprint of the Alpine orogeny reached high diagenetic to anchizonal metamorphic conditions (Brime et al., 2008). A K–Ar age of ca. 100 Ma indicates a potential Cretaceous (eo-Alpine) origin (Läufer, 1996). The pre-Alpine, Variscan metamorphic grade increases from east to west, from the chlorite to almandine zone, and from south to north within the chlorite zone as well (Sassi et al., 2004 and references therein).

Methodology of paleostress analysis

Structural field analysis is based on the measurement of brittle deformation structures, mainly fault planes and associated slip indicators, such as slickensides, striae and grooves (e.g. Angelier, 1994, Doblas, 1998, Gamond, 1983, Gamond, 1987, Petit, 1987). The local slip direction on each individual shear plane reflects an ancient time-averaged stress (Lacombe, 2012). Measurement and analysis of these slip directions on mesoscale faults yield the principal stress axes and the contemporary regional deformation pattern. The major shortcomings of the paleostress analysis method are, among others, insufficient number of fault sets and of overprint criteria formed during different deformation stages, largely varying fault orientations due to anisotropy like bedding, and rotation of paleostress axes of earlier deformation stages during subsequent folding or block rotation by wrenching (e.g., Fodor et al., 1998).

The relative and absolute timing of deformation phases can be best constrained by the presence of well-dated sedimentary and or magmatic rocks. However, for our study area, these types of rocks are missing except for Oligocene tonalites, so the timing of the succession of deformation stages largely relies on such information from neighboring areas (see Subsection 6.2).

For this study, all major steps, i.e. data collection, analysis and interpretation, which may all introduce errors (e.g. Sperner and Zweigel, 2010) were carried out by the first author (E. Bartel). Ca. 1000 fault slip datasets were collected along a transect of strong N–S shortening from about 100 outcrops north and south of the PAF. In addition, available structural data from the literature were compiled and used for comparison. For data analysis every outcrop was treated separately, and only in cases of sparse data fault sets from neighboring outcrops with similar lithology and without fault zones in between were merged together.

For kinematic data visualization, processing and analysis, TectonicsFP, written by Reiter and Acs (Ortner et al., 2002) was used. First, the consistency of the data was controlled (Sperner and Zweigel, 2010): all striae of the raw data should lie perfectly on the respective fault planes, otherwise the data were corrected and misfits excluded. Fault-striae data are shown graphically in equal area, lower hemisphere stereonets as Angelier plots (Angelier, 1979). The fault-striae data enable calculation of the three principal stress axes σ1, σ2 and σ3 (σ1 ≥ σ2 ≥ σ3), and the stress ratio R (R = [σ2 − σ3]/[σ1 − σ3]). Three different numerical methods for calculation were used in this study: the PT-method (analysis of pressure and tension axes; Marrett and Allmendinger, 1990, Turner, 1953), the NDA (Numerical Dynamic Analysis, Spang, 1972) and the direct inversion (Angelier, 1979). The NDA method calculates the orientation of the principal stress axes from summation of individual tensors. The direct inversion method uses a least-square minimization of the shear stress component perpendicular to the observed slip (Sperner and Zweigel, 2010). Inhomogeneous datasets were separated on the basis of field observations (Fig. 3) and kinematic compatibility (e.g. Hancock, 1985, Lacombe et al., 2006) of the principal stress axes (calculated with the PT method) into homogenous partitions. The paleostress tensors for each cogenetic fault population were determined, preferably with the NDA method using a best-fit angle of internal friction calculated by the PT method.

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).

Results are illustrated in Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11. In a stress axis diagram, the major and minor axes are overlain by a contour plot (Fig. 12) (Robin and Jowett, 1986). For the σ1 axes the mean vector according to Wallbrecher (1986) with the 90% confidence cone and the spherical aperture illustrate the error estimation (Fig. 13).

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 reasonable, 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) and white (σ3) arrows show the orientation. Explanation as in Fig. 1, Fig. 4.

Fig. 6

Tectonic map of the Drau Range: Typical structures of the deformation stage DN3 and DN3A; for explanation see Fig. 1, Fig. 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 Fig. 1, Fig. 4.

Fig. 8

Tectonic map of Friuli: Typical structures of the deformation stage DS1. Explanation as in Fig. 1, Fig. 4. Locations of external data are plotted directly into the map.

Fig. 9

Tectonic map of Friuli: Typical structures of the deformation stages DS2/5 and DS2/5A. Explanations as in Fig. 1, Fig. 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.

Fig. 10

Tectonic map of Friuli: Typical structures of the deformation stage DS3. For explanations see Fig. 1, Fig. 4. Locations of external data are plotted directly into the map.

Fig. 11

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

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 (Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 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.

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 aperture confidence 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. Bottom right: D1 shows the summarized results.

As in many paleostress studies, we note method-inherent uncertainties on trend and plunges of paleostress axes because of insufficient data after separation or uncertainties because of incorrect separation of fault-striae sets. We omitted data with less than four fault-striae sets from a few outcrops, which could potentially constrain further deformation phases. All the remaining paleostress tensors have sufficient well-constrained sets, except for D1.

Results

Approach

In this chapter, we describe the major structures and their relative succession in detail. Literature and map interpretation precede the field study results. A comparison of results from the northern and southern sections completes the chapter. This study is mainly based on fault-striae measurements and their paleostress assessment. Similar orientations of local stress tensors are assumed to represent a paleostress field of regional significance. The results of calculations of all fault-striae values are summarized in Table 1, Table 2 and include information on the lithology, the method, the calculated results and field observations like visible chronological sequences. Stress fields with an insufficient amount of fault-striae evidence, e.g. the E–W extension and compression, do not allow further processing. The results of the main deformation stages of the two sections are first illustrated independently (N-section: Fig. 4, Fig. 5, Fig. 6, Fig. 7, S-section: Fig. 8, Fig. 9, Fig. 10, Fig. 11) and then juxtaposed for comparison (Fig. 12, Fig. 13). The chronological sequence of the deformation stages is shown in the outmost right column of Fig. 14 and is based on map studies and observations in the field like cross-cutting relationships of lithological units (Fig. 3f), and faults (Fig. 3g) and reactivation of fracture planes with two distinguishable slickenside lineations (Fig. 3h). Five mostly transpressive paleostress fields have been identified, and we describe them from old (D1) to young (D5). Absolute time constraints are prohibited by the absence of sediments younger than Cenomanian.

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 regime	Method applied	σ1	σ2	σ3	R	Field observations
D1	Oberdrauburg to Zwickenberg	N46°45.41	E012°59.47	Alpiner Muschelkalk	Limestone	M. Triassic (Anisian)	27	187	D4E–W s	NDANDA	008/08072/33	268/52245/57	104/37340/03	0.60.4	YO	Cc
D2	Oberdrauburg to Zwickenberg	N46°45.35	E012°59.49	Alpiner Muschelkalk	Limestone	M. Triassic	22	15	D3	NDA	041/23	265/59	139/19	0.6		Cc
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, Liegensteig	N46°44.38	E012°58.51	Kössen	Shale + limestone	L. Triassic (Rhaetian)	11	10	D1	INV	020/06	112/17	272/72	0.2
D9	Oberdrauburg, Silberfall	N46°44.31	E012°57.87	Kössen	Shale + limestone	L. Triassic (Rhaetian)	12	8	D4	NDA	194/06	090/68	286/21	0.5	YO = res	Cc
D10	Gailbergsattel, Laas	N46°42.17	E012°58.35	Werfen	Shale	E. Triassic	17	96	D1D3	NDANDA	025/41037/23	290/06233/66	193/48129/06	0.50.5	O, Y = res	Qz, 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		YO = res	Cc
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	D2/5	NDA	328/11	111/77	237/08	0.5	OY = res	Qz
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 breccia	E. Permian	12	11	D2/5	NDA	137/17	027/48	239/37	0.5
D39	Lanz	N46°41.56	E013°01.08	Laas	Red sandstone with breccia	E. Permian	15a	89	D2/5D3	NDANDA	335/22024/01	097/52116/50	232/29293/40	0.50.5
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, micaschist		14	13	D3A	NDA	206/17	111/17	338/65	0.5		Chl
LD3	Tuffbad	N46°43.30	E012°45.95	Gailtal crystalline	Micaschist		18	116	D2/5D4	NDAPT	319/12013/34	102/75197/70	227/08119/23	0.5	Y1, Y2, Y3 O	Chl
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	YO = res
LD6	Eggenbach	N46°42.93	E012°42.56	Gailtal crystalline	Orthogneiss		16	115	D3D4	NDANDA	067/11005/09	241/79240/76	337/01097/12	0.60.5
LD7	Guggenberg to Lotteralm	N46°42.88	E012°42.93	Gailtal crystalline	Micaschist		12	8	D1	INV	352/07	259/19	101/70	0.5		Qz, Chl
LD8	Guggenberg to Lotteralm	N46°42.82	E012°43.13	Gailtal crystalline	Micaschist		16	744	D2/5D4E–W e	NDAPTNDA	138/13016/07258/56	025/59128/78011/14	234/27284/13109/30	0.40.5	O1, O2 Y1 (Y2 = res)
LD11	Ochsengarten	N46°44.28	E012°39.08	Alpiner Muschelkalk	Limestone	M. Triassic	24	96	D2/5D4	NDANDA	309/13196/04	199/55327/84	047/32105/05	0.60.5
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 O = Res	Qz
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, Rhaetian)	20	614	D3D4	NDANDA	239/14357/26	007/68193/63	144/16090/06	0.50.6
LD17	Lienzer Stadtweg	N46°47.66	E012°44.88	Schrambach		Cretaceous	29	9613	D2/5D3D4	NDAPTNDA	300/28216/01339/19	115/61115/64157/71	209/02304/07249/01	0.50.5	OY	Cc
LD20	Lienzer Stadtweg	N46°47.77	E012°44.35	Rotkalk	Limestone	Jurassic	17	68	E–W sE–W e	NDA NDA	265/10342/67	120/78163/23	356/07073/00	0.60.5		Cc
LD21	Tristacher lake	N46°48.46	E012°47.61	Buntsandstein	Buntsandstein	E. Triassic	21	684	D2/5D3D3A	NDANDANDA	302/00055/03032/26	044/90230/87140/33	212/00325/00272/46	0.50.50.6	YO	QzQzQz
LD22	Kornat to Mukulinalm	N46°42.91	E012°53.58	Partnach	Shale	M. Triassic (Ladinian)	9	44	D2/5NE–SW e	PTPT	331/16235/85	209/78116/04	238/03041/06			Cc
LD25	Kornat to Mukulinalm	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
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
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–Goggsteig	N46°47.74	E012°45.54	Schrambach	Shale	Cretaceous	4	4	D4	PT	183/06	302/72	092/30
LD44	Klammbrückl–Schwandthütte	N46°47.49	E012°44.12	Rotkalk	Limestone	Jurassic	9	7	D4	NDA	010/18	146/66	274/16	0.5
LD45	Lavant	N46°47.74	E012°50.27	Amlacher Wiesen	Marl	Cretaceous	12	48	D3D3A	PTNDA	243/01202/05	154/71292/05	329/19064/83	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	74	D1D3	NDAPT	013/39050/20	278/05293/52	182/51153/32	0.4
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

Multiple use of reactivated slickensides.

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 regime	Method applied	σ1	σ2	σ3	R	Field observations
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	565	D3D4E–W e	INVNDANDA	041/08006/25356/47	157/72237/54183/43	308/16108/24089/04	0.30.50.5		Cc
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	1778	D2/5NW–SE eE–W e	NDANDA NDA	328/05164/73231/46	116/84044/09346/22	237/03312/15093/35	0.50.50.6	Y2Y1,O2,(O1 = res)	Cc
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
K17	Plöckenpass, S-side (I)	N46°35.88	E012°56.73	Hochwipfel	Shale	Carboniferous	27a	14945	D2/5D2/5N–S eNW–SE e	NDANDAPTNDA	305/01325/10082/66219/80	212/78086/71284/45041/10	035/12232/16009/01311/00	0.50.20.4	O1, O2Y3, Y4O3,O4,Y1,Y2
K18	Plöckenpass, S-side (I)	N46°35.56	E012°56.43	Devonian limestone	Limestone	Devonian	18	95	D2/5NW–SE e	PTPT	132/03346/49	343/77233/03	037/09128/34
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	46	D4NW–SE e	NDANDA	006/09106/58	245/72219/13	098/15316/28	0.70.6
S43	Zuglio–Sezza–Fusea (I)	N46°26.51	E012°59.77		Limestone		8	44	D4D2/5A	PTPT	184/14319/01	285/80051/11	093/12224/71
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	55	D2/5NE–SW e	NDANDA	155/09070/61	261/59326/08	060/29232/28	0.50.5
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	887	D1D2/5D2/5	NDANDANDA	184/30123/40121/17	092/04241/30307/72	355/59356/36211/02	0.50.505	OY
S56	Lago Verzegnis (I)	N46°22.83	E012°57.87	Forni	Dolomite	L. Triassic (Norian)	20	77	D2/5NW–SE e	PTNDA	123/05184/56	217/51040/28	035/14301/17	0.5
S57	Str Verzegnis–San Francesco (I)	N46°20.97	E012°55.43	Soverzene	Dolomite	L. Jurassic	7	7	D1	NDA	342/03	249/41	075/49	0.5
S60	Str Clauzetto–Almandis (I)	N46°13.35	E012°54.19			Cretaceous	14	54	D2/5D3	INVPT	142/43041/46	315/47260/43	049/03154/20	0.2
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	44	NE–SW eE–W t	PTPT	246/46276/21	061/44358/17	338/01091/66

Multiple use of reactivated slickensides.

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, Kurz et al., 1996, Läufer (1996), Peresson and Decker (1997), Venturini (1990).

Map-scale structures of the Drau Range

The description is based on eight regular 1:50,000 geologic map sheets available from the Geologische Bundesanstalt (http://www.geologie.ac.at/produkte-shop/maps/) (Anderle, 1977, Kreuss et al., 2006, Linner et al., 2013, Schönlaub, 1985, Schönlaub, 1987, Schönlaub, 1989, Schönlaub, 1997, Schönlaub, 2000). The western DR is bordered by faults and displays a trapezoidal shape. These include: The ENE-trending sinistral Drau fault and the dextral NW-trending Isel and Möll faults, with the Isel fault dextrally dislocating the Drau fault for about 6 km (Van Bemmelen and Meulenkamp, 1965) (Figs. 1b, Fig. 4, Fig. 5, Fig. 6, Fig. 7). The DR is a synclinorium squeezed in between subvertical crystalline basement units exposed in the south and north. The metamorphic Gailtal basement is located south of the DR. In the S it is cut off by the Gailtal fault, an approximately W-trending (N280°) segment of the dextral PAF (Fig. 1). The DR is characterized by several kilometers long, E–W striking tight high-amplitude folds, which plunge about 40° to the W within the central sector of the DR (Figs. 1b, Fig. 4, Fig. 5, Fig. 6, Fig. 7) (Schlager, 1963). The main anticlines are located along the central axis of the range with synclines and subordinate minor anticlines on both sides. The folds are dislocated and deflected by numerous steep strike-slip faults. An example is the E–W striking Amlacher Wiesen syncline near Lienz, where its northern fold limb is cut-off by an E–W striking fault (Linner et al., 2013, Schmidt et al., 1993) and the western part of the fold is dragged to the WSW subparallel to the neighboring Drau fault (Linner et al., 2013, Linner et al., 2013) (Fig. 4a). In the central section of the Lienz Dolomites, the dextral NW–SE striking Oberalpl fault dislocated and deflected an E–W striking anticline with Jurassic formations (Fig. 5b). More to the west the same anticline is sinistrally dislocated and partly deflected by an ENE-trending fault subparallel to the Drau fault (Fig. 6a). WNW–ESE to E–W striking strike-slip faults subparallel to the PAF are common in the northern and southernmost parts of the DR (Figs. 1b, Fig. 4, Fig. 5, Fig. 6, Fig. 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, Schönlaub, 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, Schönlaub, 1987).

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, Avigliano et al., 2007b, Bigi et al., 1990, Brandner, 1980, Cantelli et al., 1960, Feruglio et al., 1925, Gortani et al., 1925, Schönlaub, 1985, Schönlaub, 1987, Schönlaub, 1997, Schönlaub, 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 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 (Figs. 1b and 8a). Important NE–SW striking thrusts are the Val Bordaglia fault, Tramonti–Verzegnis fault and the But-Chiarsó fault (Fig. 9). The Val Bordaglia fault consists of a bundle of subvertical faults (Sassi et al., 1995), which forms a positive flower structure near Forni Avoltri (Venturini, 1990) and separates areas of very-low-grade and low-grade metamorphism (Brime et al., 2008, Sassi et al., 1995, Sassi et al., 2004). NE-trending thrusts are dislocated by NW–SE and N–S striking strike-slip faults (Fig. 9b). NW-trending thrusts prevail in the SE of Friuli near Udine e.g. Udine fault (Fig. 10) and are also found in the Carnic Alps, where they are dislocated by NW–SE striking faults (Fig. 10a). WNW to NW trending dextral faults are located more in the east, e.g. the Idrija fault, dislocated E–W striking thrust faults (Fig. 1b) and extend over more than 120 km to Croatia (Gosar, 2007). In the NE of Friuli the Tröpolach–Camporosso fault cut off E–W striking thrusts and is truncated by the Fella–Sava fault, which is a dextral ~ E–W striking reverse strike-slip fault (e.g., Bartel et al., 2014; Fig. 1b). Dextral NW–SE and sinistral, NE–SW striking strike-slip faults are common in whole Friuli, whereupon the dextral NW-trending faults often are transpressive and dislocated the NE–SW striking faults (Fig. 11).

Field study results

For each deformation stage of regional significance, we used all σ1 principal stress axes to calculate a mean vector based on the method of mean vectors of lineations or poles of planes in the three-dimensional domain after Wallbrecher (1986). We compared the orientations of the mean vectors based on our measurements with orientations of the mean vectors from published data (Discenza and Venturini, 2002, Fritz et al., 1990, Läufer, 1996, Nemes, 1996, Sprenger, 1996, Venturini and Carulli, 2002). Since the results are similar, robustness of our data is assumed and we proceeded with a combination of the entire data set.

The relative age sequence in the north and south sections are mostly in agreement and thus we combined their description. An overview map of the western DR including locations and stereoplots of the corresponding outcrops displays each deformation stage of the northern section (Fig. 4, Fig. 5, Fig. 6, Fig. 7). Fig. 8, Fig. 9, Fig. 10, Fig. 11 illustrate the Friuli area in the same manner, and the values of all calculated paleostress axes are shown in Table 1(N) and Table 2 (S), respectively. Results of compiled published fault-striae data of the area are additionally charted in the maps (Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11) as orientations of the paleostress axes (σ1 ± σ3). Composite diagrams (σ1, σ3) of all deformation stages are juxtaposed in Fig. 12.

Deformation stage D: Thrust compression σ1 is subhorizontal N–S, σ3 subvertical.

The oldest paleostress tensor, for which fault-striae data are sparse, belongs to N–S thrust compression (DN1, DS1).

D (Fig. 4): In the northern part fault-striae data indicating thrust compression were found in outcrops with variable lithologies ranging from Gailtal crystalline basement up to Cretaceous formations. The calculated mean vector of the maximum compressive stress axis for the north is 006/17. Including published data yields an only slightly shifted maximum principal stress axis (005/14).

D (Fig. 8): In the southern part, evidence for thrust compression is found from the Paleozoic basement up to Triassic cover formations. Our data yields 176/25 for the mean vector and 178/02 including compiled data, i.e. the declination values are very similar whereas the inclination varies.

D: In both cases (all data or exclusively data of this study), a CCW rotation of the southern part and a difference between plunges north and south of the PAF is recognizable. Including the additional data an average CCW rotation of 7° and a tilt of 16° are calculated. Within the DR a trend of dip was observed: near the PAF the calculated principal stress axes dip at a lower angle than in outcrops more distant to the PAF.

Deformation stages D both indicate compression with subhorizontal NW–SE orientation of σ1, and subhorizontal NE–SW orientation of σ3 (Fig. 5, Fig. 9). The deformation stages D2/5 represent thrust compression with a subhorizontal NW–SE striking σ1-axis, and a subvertical σ3-axis.

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

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

D (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 intersection, 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.

D: A difference of 20° in inclination and 3° CCW rotation of the paleostress axes is observed.

Deformation stage D represents strike-slip compression with a subhorizontal NE–SW σ1-axis and a subhorizontal NW–SE σ3-axis. Deformation stage D 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.

D (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).

D (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).

D: A CCW rotation (1 to 16°) and a tilting between 13 and 19° is visible between northern and southern sections.

During deformation stage D, compression with a subhorizontal N–S oriented σ1-axis and a subhorizontal E–W σ3-axis prevailed.

D (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).

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

D: A rotation of 6° CCW of the orientation of the paleostress axes and a change of the inclination of 12° are visible.

Comparison of kinematics north and south of the PAF

The relative sequences of deformation stages in both units separated by the PAF are alike; therefore, we suggest a coeval evolution of DN1 and DS1, DN2 and DS2, et cetera. However, small differences in the orientations of the paleostress axes between north and south exist and persist throughout all deformation stages (Fig. 12, Fig. 13). We are aware that the differences are very small nevertheless we found them in all deformation stages regardless of the observed set of data (exclusively data of this study, exclusively published data, and merged data), indicating robustness of our data. We assume that the two trends: (1) mainly a plunge of σ1 towards the foreland on either side of the PAF (Fig. 13) and (2) a CCW rotation of the southern major paleostress axes (σ1) compared with the northern part (Fig. 13) are real and not a random result.

Discussion

In the following discussion we first establish a time frame for the deformation events, then we correlate our results with the large-scale tectonic framework of the study area and finally discuss the variations of paleostress axis orientation N and S of the PAF.

Timing of deformation events

Correlating individual deformation stages with absolute rock ages or lithology is fraught with difficulty because Cenozoic sediments are lacking within the study area. The relative timing is clearer, but even there discrepancies still exist for the Southalpine unit as indicated by various paleostress reconstructions (Fig. 14) (Caputo et al., 2010, Discenza and Venturini, 2002, Fodor et al., 1998, Läufer, 1996, Venturini, 1990, Venturini, 2006, Venturini and Carulli, 2002). In the following, we consider published age constraints from our study area as well as from neighboring regions with a similar structural evolution in order to confine a time frame for the deformation events.

Deformation stage D1, N–S thrust compression

E–W striking folds resulting from the oldest deformation event were reported for the southwestern Lienz Dolomites (Brandner and Sperling, 1995) and for the Kreuzeck–Goldeck Complex north of the DR (Amann et al., 2002). The youngest formation included in the core of the folds in the DR is the Amlacher Wiesen Formation of Barremian to Albian age (Blau and Grün, 1995), thus, deformation stage DN1 must be of Albian age or younger. From a regional point of view and similar kinematics, D1N folds could be related to similarly oriented Late Oligocene to Early Miocene folds within the Tauern window, which affect Oligocene amphibolite- to greenschist grade isotherms of the Tauern window (e.g., Hoinkes et al., 1999, Kurz et al., 1998). Peak metamorphism was reached at ~ 27 Ma (Rb–Sr white mica, Reddy et al., 1993), and the Sonnblick dome was formed by NNE-directed overthrusting between 27 and 29 Ma (Inger and Cliff, 1994). Later local clockwise rotation of ca. 20–30° of the eastern Tauern window compared to similar structures in the central and western Tauern window has been suggested (Schmid et al., 2013 and references therein). We consider that deformation stage D1 is the main stage of regional folding in the study area, responsible for E–W trending folds in the Drauzug and initial southward tilting of the basement exposed within the Carnic Alps along the PAF.

Deformation stage D2, NW–SE compressionand dextral shear along PAF

Mylonitization ages (not necessarily representing D2 dextral shear) along the PAF must postdate tonalite intrusions (ca. 33 to 28 Ma; Diener, 2002, Rosenberg, 2004) and range from 32 to 28 Ma (Läufer et al., 1997, Mancktelow et al., 2001, Müller et al., 2001, Pomella et al., 2011, von Gosen, 1989). These ductilely deformed tonalites likely constrain a major phase of dextral shearing, which continued to brittle deformation. Similar deformation ages of ~ 28 Ma were found along the Möll Valley fault within the Tauern Window (Th–Pb allanite, Cliff et al., 1998; U–Pb allanite & titanite, Inger and Cliff, 1994) where they are interpreted as the maximum age of dextral shearing. Based on Rb–Sr dating of mylonite (25.3 ± 2.9 Ma, 20.7 ± 2.3 Ma) Glodny et al. (2008) suggest continuous or at least pulsed active ductile deformation for the Möll Valley fault within the Tauern Window. NNW-trending dikes synkinematic to the Isel fault (Amann et al., 2002) yield K–Ar ages of 36 ± 1 Ma (Müller et al., 1992).

A dextral slip of 350–550 km along the PAF near the Pannonian basin at the eastern continuation is assumed to have accumulated since the Early Oligocene (Haas et al., 1995, Tari, 1994) although the exact timing remains unclear. The conjugate dextral SEMP fault includes sedimentary basins formed at ca. 18 Ma (Ratschbacher et al., 1991b). From all arguments given above, we assume a duration of dextral D2 slip between ca. 28 and 18 Ma representing the main stage of activity of lateral extrusion of the Eastern Alps (Ratschbacher et al., 1991a, Ratschbacher et al., 1991b). We note some potential temporal overlap with the D1 deformation event, which has to be resolved by future work.

Deformation stage D3, NE–SW compression

In the Southern Alps a N20°–30° compression with subvertical σ3-axis is reported for Late Chattian to Early Miocene (Caputo et al., 2010 and references therein) and is associated with trends of regional folding in the Dinarides (Castellarin et al., 1998). Mainly the northern sector of the Southern Alps was affected (Caputo et al., 2010 and references therein). Fission-track ages ranging from 22 to 16 Ma (Monegato et al., 2010) indicate exhumation. In the Tauern Window, N10°–30° trending subvertical gold quartz veins (Feitzinger and Paar, 1991) yield white mica and adularia Ar–Ar ages of ~ 18 to 12 Ma (Handler and Neubauer, 2001), where white mica ages of 18 to 16 Ma are representative for vein opening during deformation stage D3.

Deformation stage D4, N–S compression

For the Serravallian to Early Tortonian (ca. 16 to 8.5 Ma), a compressional stage with approximately N160°–170° orientation prevailed in the southern parts of the Southalpine unit (e.g. Caputo et al., 2010 and references therein; Castellarin et al., 1998, Castellarin et al., 2006). New thrust systems produced large amounts of shortening and propagated progressively to the foreland (Caputo et al., 2010 and references therein). Strong regional uplift and exhumation in the Tortonian is confirmed by detrital apatite fission-track ages (Dunkl et al., 1996, Monegato et al., 2010, Zattin et al., 2003, Zattin et al., 2006). The tectonic load of the rapidly growing Southalpine fold-and-thrust belt produced flexural folding of the Adriatic lithosphere and the development of a foredeep basin (Caputo et al., 2010 and references therein).

Deformation stage D5 NW–SE compression

Polinski and Eisbacher (1992) and Nemes et al. (1997) connected the Serravallian (ca. 13 Ma) to Pliocene formation and destruction of the Klagenfurt basin north of the PAF with NW–SE compression along the Möll fault. Recently, based on (U–Th)/He ages of 11 to 6 Ma from Periadriatic plutons from the eastern PAF, Heberer et al. (2014) proposed a similar age of activity. For the Southern Alps, Messinian–Pliocene NW–SE compression and younger Pliocene–Pleistocene N–S compression are described (Caputo et al., 2010 and references therein). Based on a very detailed litho- and chronostratigraphic record of the sedimentary units Caputo et al. (2010) investigated pebbles along the frontal thrust system in Tortonian to Pleistocene deposits and distinguish four events with the main compression oscillating between NW and NNW and ages between Late Tortonian and Early to Middle Pleistocene. In summary, these data constrain a second phase of NW–SE compression and dextral shear along the PAF between maximum 11 Ma to Quaternary.

Deformation stages D2 to D5 mostly include strike-slip compressional shortening with subhorizontal σ3 and σ3 principal stress axes, in which the effects of large-scale folding seem to be minor. Consequently, no major tilting of stress axes of deformation stages D2 to D5 has to be taken into consideration.

Deformation and motion of Adria

Since the benchmark paper of Platt et al. (1989), the deformation pattern of the Eastern Alps is generally related to the changing motion direction of the Adriatic microplate. Recent reconstructions of the E–W-trending Southern Alps to the SE-trending Dinarides by Ustaszewski et al. (2008) and Handy et al. (2014) further relate the changes of kinematics to a change of the subduction polarity during the collision stage. Polarity changed from earlier Eocene to Miocene subduction of the European plate underneath the Eastern Alps to the later late Early Miocene to Pliocene subduction of Adria underneath the easternmost Southern Alps and Dinarides leading to oroclinal bending in the Friuli area. Furthermore, Adria rotated in a CCW manner since Oligocene although details still remain to be solved (Márton et al., 2011 and references therein). In summary, the stress field is primarily governed by the generally northward motion of the Adriatic microplate, concurrent CCW rotation as well as its ca. rectangular shape and potentially by the change of subduction polarity.

Due to the northward motion of Adria, the main post-collisional deformation stage within the Eastern Alps is the Late Oligocene to Early Miocene east-directed lateral extrusion (e.g., Ratschbacher et al., 1991a), for which the Periadriatic fault formed the southern boundary of the extruding wedge. The succession of deformations in the northern and eastern sectors of the Eastern Alps includes some additional deformation stages, which are not or only weakly expressed in the study area. These include a stage of E–W extension (E and NE of the Tauern window) and shear reversal along the major confining faults (mainly SEMP fault) of the extruding wedge (Peresson and Decker, 1997) (Fig. 14).

Interpretation of paleostress patterns

Based on our fault slip data sets N and S of the PAF, we suggest a coeval evolution of the Austroalpine and Southalpine units under the same kinematic and paleostress framework. However, the orientations of the principal stress axes for the succession of all deformation events are not identical: North of the PAF the principal stress axes σ1 mainly plunge to the north, south of the PAF to the south. Also, a CCW rotation of the southern major stress axes compared with the northern part is evident (Fig. 13).

Different theoretical scenarios could explain the current state of the angular misalignment of σ1 axes between the north and the south (Fig. 15): (1) Uniform radial stress regimes (Fig. 15a, b). Radial paleostress patterns are common, e.g. in the Western Alps and Jura Mountains (Grünthal and Stromeyer, 1992, Homberg et al., 1999, Homberg et al., 2002). There, the observed stress field is perpendicularly arranged to the mountain arc, an effect of pushing an indenter from the south to the north (Grünthal and Stromeyer, 1992, Homberg et al., 1999, Homberg et al., 2002). Fig. 15a adjusts this regional scenario with a fanning out to the north. A dextral lateral offset, as generated along the PAF, leads to an apparent clockwise rotation of the stress-axes σ1 of the southern block compared to the north — a result, which is not reproduced by our data. (2) A radial stress pattern fanning out to the south, combined with the dextral offset, would theoretically produce a CCW rotation (Fig. 15b), but is kinematically incompatible with the Adriatic indenter pushing from the south. (3) An identical pattern of the paleostress axes would be derived by a uniform stress regime, a later dislocation along the dextral PAF and CCW rotation of the southern block (Fig. 15c). We suggest that the slight but consistent CCW rotation of σ1 axes resulted from (4) a combination of late-stage CCW rotation of the southern units as well as dislocation along the PAF within a north-fanning stress regime (Figs. 15d and 16). The two contrary effects attenuate each other. This scenario agrees with the Adriatic indenter pushing from the south and the current CCW rotation of the Adriatic microplate. Numerous studies have described Cenozoic CCW rotation of Adria, although the exact timing and amount of rotations remain a matter of debate (e.g. Márton et al., 2000, Márton et al., 2011, Ustaszewski et al., 2008, van Hinsbergen et al., 2014). Based on paleomagnetic constraints Late Oligocene to Miocene CCW rotation was reported from both the Eastern (e.g. Thöny et al., 2006) and Southern Alps (e.g. Márton et al., 2011). A second step of CCW rotation between about 6 and 4 Ma affected the easternmost Eastern Alps (Márton et al., 2000), NW Croatia and NE Slovenia (Márton et al., 2003) as well as the Styrian and Vienna basins (Scholger and Stingl, 2002). GPS derived motion yields a current CCW rotation rate of 0.297 ± 0.116°/Ma and a present-day Euler pole near Turin for the Adriatic microplate (Weber et al., 2010). Our study suggests that differential rotation affected the Southalpine and Austroalpine units. However, it remains to be demonstrated, whether the Periadriatic fault or the southern front of the Southalpine units represents the border between differently rotated units.

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.

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).

Finding an explanation for the observed difference in plunge north and south of the PAF is even more difficult. The question arises, whether we are dealing with a primary or secondary (e.g. a late-stage tilting) feature. Such tilting might be explained by an evolving fold and thrust belt system, which is characterized by ramp geometries (wedging in the TRANSALP profile, Fig. 17a) and displays an overall wedge-shaped geometry. For the study area, this would imply that tilting postdates D5, which started in Miocene times, as all tensors D1 to D5 plunge to the foreland.

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, μi — coefficient of internal friction, μb — 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.

In our region, the subsurface is formed by a large-scale orogenic wedge, and two alternative models of the deep subsurface structure for the TRANSALP profile (~ 100 km west of the research area) have been suggested: the Crocodile and Extrusion models (Fig. 17a, Gebrande et al., 2002). Both models are consistent with the ALP01 profile (Brückl et al., 2007), less than 50 km to the east of our study area. The Alpine orogen is a rare case of a double-vergent orogen with two foreland basin systems (e.g. Naylor and Sinclair, 2008). The PAF separates the stable Europe-verging Eastern and Western Alps from the Africa-verging Southern Alps (Castellarin et al., 2006). The critical taper theory can be used for both wedges independently as Carrapa (2009) demonstrated especially for the Alpine orogeny. The retro-wedge below the Southalpine unit is younger, dominantly of Miocene age and at most 10 to 15 km thick (Schmid et al., 2004). South of the PAF exhumation increased towards the PAF, as indicated by the exposure of the Paleozoic basement in the Carnic Alps. North of the PAF, exhumation of 15 km occurred within the Tauern Window with rates up to ~ 4 mm/a since ~ 20 Myrs (Fügenschuh et al., 1997, Genser et al., 1996). The main pulse of exhumation took place between 20 and 15 Ma (Fügenschuh et al., 1997, Genser et al., 1996), and minor exhumation has been active since 15 Ma (fission-track ages; Luth and Willingshofer, 2008). Underneath the Tauern Window, the subordinate northward thrusting of a lower crustal wedge and the bulk of the shortening in the axial zone led to a mainly stationary situation near the retro-wedge, with an almost fixed location of exhumation and deformation since Oligocene times (Rosenberg and Berger, 2009). We consider the pro- and retro-wedge as long-term stationary features and the influence of the lateral displacement along the PAF for the geometry of the wedge as negligible, since the strike-slip fault is straight, large-scale and perpendicular to the orogenic wedge in this segment of the Eastern Alps. Theoretically, we expect an axial zone between the pro- and retro-wedge for a double-vergent wedge (Fig. 17b). A zone of horizontal σ1 is lacking along our profile or the entire zone between PAF and the Tauern window represents the axial zone. The southern part shows a large variability of the southwards dip of the bedding planes, and our calculated plunge angles of σ1 are in contrast less variable. This observation is contrary to a pure secondary tilting, unless the whole area was tilted homogeneously after the youngest deformation.

An initial (i.e. primary) plunge of σ1 might be attributed to an overburden pressure depending on friction along the basal decollement inside the wedge. The geometry and physical characteristics of the pro- and retro-wedge of double-vergent wedges (Fig. 17b) correspond for each detached wedge to the features of a Coulomb wedge (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, as well as the angle of the wedge taper (α), depend on the coupling of the detachment, and hence on the basal friction coefficient (μb). High basal friction generally increases the wedge taper, low basal coupling reduces the angle (Davis et al., 1983). Provided that the slightest cohesion existed at the top surface of the wedge depending on friction along the basal decollement, overlying weight within a wedge deflects the orientation of the stresses in the subsurface. The plunge of the principal stress axis (σ1) changes from subhorizontal to parallel to the local topography taper (Davis et al., 1983), in larger depths this effect increases and the plunge of σ1 is steeper. For uniform and noncohesive wedges the angle between σ1 and the surface, ψ0, and the angle between σ1 and the basal detachment, ψb, are constant (Dahlen, 1984, Lallemand et al., 1994) (Fig. 17). The angles are both dependent on the basal friction coefficient (Davis et al., 1983, Graveleau et al., 2012), 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 et al., 2013). Studies exemplify for Taiwan ψb = 12° (Dahlen, 1984) and for the Aleuten ψb = 14° (Lallemand et al., 1994). These correlate to an additional plunging of 3° (Taiwan), 9° (Aleuten) and a general dip of the principal stress axis σ1 of 6° (Taiwan) and 12° (Aleuten). For the Southalpine retro-wedge, a strong coupling of the Adriatic foreland and the Southalpine is suggested (Willingshofer and Cloetingh, 2003). In Taiwan published slickensides data reveal a similar plunge of σ1-axes to the foreland (Chang et al., 2003, Deffontaines et al., 1997, Lacombe et al., 1997, Lacombe et al., 1999). Recent earthquake data also show that a majority of σ1-axes dip to the foreland (Angelier et al., 2009, Mozziconacci et al., 2009). The similar general dip of earthquake σ1-axes to the foreland is visible in our investigation area. In Friuli, the earthquake σ1-axes dip to the S in most cases (Bressan et al., 1998, Pondrelli et al., 2001, Reinecker and Lenhardt, 1999, Slejko et al., 1999), north of the PAF mainly to the N (Reinecker and Lenhardt, 1999). Similar variable plunge directions have also been detected by paleomagnetic studies. The paleomagnetic data of the northern area show almost exclusively an inclination to NW, whereas both NW and SE directions are found within the Southalpine unit (Márton et al., 2011, Thöny et al., 2006 and references therein).

In summary, we suggest that a combination of the two models proposed above produced the observed variable plunge, i.e. a secondary late-stage tilting event further increased the initial plunge of σ1, which is an effect of wedge geometry and basal friction.

Conclusions

The paleostress analysis of fault-striae data from the blocks north and south of the Periadriatic fault allows the following major conclusions:

Kinematic analyses of brittle fault datasets allow defining five successive tectonic, mostly compressive regimes. These include (1) N–S compression, (2) NW–SE compression, (3) NE–SW compression, σ3 changes gradually from subvertical to subhorizontal, (4) N–S compression and (5) NW–SE compression and show a systematic shift of the principal stress σ1 over ca. 90° between Oligocene and Late Miocene/Pliocene.

Deformation stages on either side of the PAF show large similarities. The mean orientations of the principal stress axes (σ1) are, however, significantly different:

(a) A CCW rotation of the principal stress axes of the southern block in respect to the north is evident.

(b) The principal stress axes in the northern block show mainly a northward plunge, and in the south a southward plunge.

Our results are consistent with the published CCW rotation of the Adriatic microplate. However, the magnitude of stress field rotation indicated by our dataset seems to be lower (~ 20° vs. 7°). This discrepancy is resolved by dextral displacement along the PAF within a N-fanning stress field. Thus, units S of the PAF were formerly located further to the east, where stress directions were accordingly oriented more to the east.

Opposing plunge directions of σ1 to the foreland are a consequence of a primary initial plunge of σ1, typical for the internal stress-field of an orogenic wedge and a further increase of this effect during continuing compression.