Topographic Evolution of the Eastern Alps: the Influence of Strike-Slip Faulting Activity

Topographic evolution of the Eastern Alps: The influence of strike-slip faulting activity

Thorsten Bartosch1, Kurt Stüwe1, and Jörg Robl2 1INSTITUTE OF EARTH SCIENCES, UNIVERSITY OF GRAZ, 8010 GRAZ, HEINRICHSTRASSE 26, AUSTRIA 2INSTITUTE OF GEOGRAPHY AND GEOLOGY, UNIVERSITY OF SALZBURG, 5020 SALZBURG, HELLBRUNNERSTRASSE 34, AUSTRIA

ABSTRACT

We present the results of a numerical model that was used to investigate aspects of the landscape evolution of the Eastern European Alps in the Miocene. The model allows the consideration of strike-slip faulting, an inherent feature of the Miocene tectonics in the Eastern Alps, within a viscous medium. Mechanical deformation of this medium is coupled with a landscape evolution model to describe surface processes. For the input variables, the activity history of strike-slip faulting in the Eastern Alps was compiled from literature sources. The results present a major improvement in the predicted topographic development over earlier models in terms of the location and build-up of valleys and mountain ranges that form in response to the strike-slip faulting activity. Intramontane basin formation is predicted and the metamorphic dome of the Tauern Window evolves dynamically in the simulations, related to well-known east-west–striking strike-slip faults in the region. It is interesting that the metamorphic dome formation is predicted by the model without explicit consideration of the low-angle detachments bounding the dome in the west and east, suggesting that metamorphic domes can form in transpressional or strike-slip environments. The model underpredicts the mean elevation of the Eastern Alps by several hundreds of meters, which is interpreted in terms of an independent non-convergence-related event of the past 5 m.y. that has been inferred previously from other field data. Time-series analysis of elevations reveals a clear correlation between maximum height and the amount of strike-slip activity and a nonequilibrium state between uplift and erosion. We interpret this in terms of future topographical growth of the Eastern Alps.

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INTRODUCTION Royden et al., 1983). The Miocene lateral extru- the lateral extrusion is computed by plan-view sion of the Eastern Alps involved both compres- geometrical back rectification. This information The development of the Alpine orogeny is sional lateral escape tectonics and gravitational is then migrated into a finite element model and an ongoing multistage tectonic process between collapse of unstable crustal blocks (Ratschbacher validated with current fault locations. The results the stable European plate and the northeastward et al., 1991a, 1991b), and is thought to have been of this model are interpreted in terms of their moving and rotating Adriatic plate (Froitzheim intimately related to the early topographic evo- predictions for the evolution of topography in et al., 2008; Handy et al., 2010). The topographic lution of the Eastern Alps (Frisch et al., 1998; the Eastern Alps. evolution of this orogeny started after the final Frost et al., 2009; Linzer et al., 2002; Robl et subduction of the Penninic Ocean in the Eocene al., 2008b; Sachsenhofer et al., 2003; Wölfler et GEOLOGICAL SETTING and is ongoing (Hergarten et al., 2010). In the al., 2011). However, details of the topographic Miocene, the eastern part of this orogeny was evolution since the Miocene and the interaction According to Nocquet and Calais (2003), associated with large horizontal kinematics nor- of topography development with the strike-slip the tectonic processes in the Alps during the mal to the convergence direction, mostly accom- faulting are not very well known. past 30 m.y. have been dominated by a stable modated by orogen-scale strike-slip faults. This In this paper we expand on an early simpli- European plate, while the Adriatic plate is mov- so called lateral extrusion of the Eastern Alps fied modeling study (Robl et al., 2008b) by using ing ~5 mm/yr in a north to northwest direction, (Ratschbacher et al., 1991a, 1991b) was predom- a refined numerical modeling approach to pre- performing a counterclockwise rotation-like inantly facilitated by the rollback of a subduction dict aspects of the geomorphic evolution of the movement. However, the kinematic condition zone east of the Alps, which also opened the Eastern Alps since the late Oligocene. In particu- for the Adriatic plate motion is not uniformly Pannonian Basin (Horvath and Cloetingh, 1996; lar we consider the influence of temporal varia- described in the literature. Decker et al. (1994) tions of strike-slip faulting activity on the devel- and Ratschbacher et al. (1991b) assumed a linear opment of topography. In order to obtain the northward movement of a rectangular indenter, Thorsten Bartosch http://orcid.org/0000-0003 input variables for the model, a time history of while palinspastic reconstructions (Frisch et al., -1432-7330 relevant Miocene strike-slip activity is compiled 2000, 1998) imply more complex kinematics, *Bartosch: [email protected]; Stüwe: [email protected]; Robl: joerg.robl@ from literature sources and a geometrical recon- including translation and clockwise rotation to sbg.ac.at struction of fault geometry prior to the onset of the east of Bolzano (Fig. 1). Linzer et al. (2002)

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10° 11° 12° 13° 14° 15° 16° 17° A European Foreland Vienna 48° 48° Salzburg SEMP2 MM2 IN SEMP1 Innsbruck MM1 Graz 47° DAV PL 47° Austroalpine MV Klagenfurt PA Bolzano Pannonian Basin

46° 46° Southalpine 50 km

B Basin abbr. − FO FohnsdorfGÖGöriach GR Gröbming HF Hieflau Vienna KL KlagenfurtOBObdach PS Parschlug TM Tamsweg VB 48° VB Vienna WA Wagrain ssee−Lammertal 48° Salzburg Traunsee WB Wolfsberg König SEMP2 Northern Calcareous Alps GÖ MM2 IN GR HF WA PS SEMP1 FO Innsbruck MM1 Ahorn Katschberg Styrian block Brenner det. Gleinalm det. Tauern Window TM Graz OB 47° Jaufen 47° PL K Passeier DAV MV oralm Pusterta ZW WB KL PA Mauls l

Engadine e Bolzano Peio

Tonale Giudicari

46° 46° Major fault abbr. DAV Defreggen−Antholz−Vals IN Inntal MM1 Upper Mur valley MV Möll valley MM2 Mürz valley PA Periadriatic line SEMP1Salzach valley PL Pöls−Lavanttal SEMP2Ennstal−Mariazell

10° 11° 12° 13° 14° 15° 16° 17° Figure 1. The simulation area of the Eastern Alps. The southwest vertex of both maps constitutes the Euler pole of the Adriatic plate. (A) Topographic map of the Eastern Alps showing the seven major fault zones considered in this study (yellow lines) and the assumed boundaries of rheology contrast (green lines). (B) Map of the same region showing the location of the Tauern Window (TW), the Northern Calcareous Alps (NCA), the most important intramontane basins, and most regional names used in the text.

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estimated a linear north-northeast translational Tauern Window between 17 and 5 m.y. Their data Eastern Alps from the somewhat stronger South- displacement of the Adriatic plate with almost are consistent with increased sedimentation in alpine unit over a length of ~700 km (Robl and no rotation. For the present-day, Nocquet and the Molasse basins starting ca. 18 m.y., as docu- Stüwe, 2005b). In the target region, it consists Calais (2003) estimated an exclusively coun- mented by Kuhlemann et al. (2002). of several faults (locally named the Tonale, Giu- terclockwise rotation of the Adriatic plate at a Evidence has emerged that a renewed surface dicarie, Mauls, and Pustertal). Based on fault rate of φ˙ = 0.52°/m.y. with the Euler pole being uplift event, uplifting the Eastern Alps by at least gouge and pseudotachylite dating, Müller et al. located close to Milan, Italy. 500 m, commenced ca. 7–4 m.y. (Legrain et al., (2001) revealed variable activity for individual The geomorphological evolution of the Alps 2014; Wagner et al., 2010, 2011). This event is parts of the fault. Two time frames, from 32 to 29 that accompanies this kinematic frame is the sub- now well documented at the eastern edge of the m.y. for the North Giudicarie (thrusting), Tonale, ject of much debate, but the generally accepted Alps where glacial carving during the past 2 m.y. and Pustertal, and 22 to 16 m.y. for the Tonale, onset of topography development is the begin- was absent, but it appears to have also affected Pustertal, Jaufen, and Passeier faults are most ning of subsidence in the Alpine foreland basins both the northern and southern Molasse (Genser striking (Table 1). Zwingmann and Mancktelow (Linzer et al., 2002). This started ~30 m.y. ago et al., 2007) and may therefore be much wider (2004) dated illites in mylonites in the Mauls and was largely synchronous with the final sub- spread than is currently documented. The event fault as 16 m.y. and also considered the nature of duction of the Penninic Ocean, the head-on col- appears to be of large wavelength and unrelated individual fault segments of the PA to be nonco- lision of the Adriatic and European continental to fault activity. While it has been argued that the eval. Additional minor times of activity that are plates, and the formation of a new plate boundary young uplift of the Alps may correlate with ero- shown in Figure 2 are listed in Table 1 for the PA between the two, the Periadriatic line (Fig. 1). sional unloading due to glacial erosion (Cham- and all other faults discussed in the following. Frisch et al. (1998) argued that topography pagnac et al., 2007), the event commenced ear- The DAV extends for ~80 km in an east-west development in the Eastern Alps began from a lier than the glaciation periods. However, the direction to the south of the Tauern Window and flat lowland topography in the Oligocene. Late event correlates with a dramatic spike in the north of the PA (Pustertal, Mauls faults) (Fig. 1). Oligocene alluvial fans, which were fed from sedimentation rates in the basins surrounding It constitutes the southern border of alpine meta- the Northern Calcareous Alps (NCA), can only the Alps that started ca. 5–4 m.y. (Kuhlemann, morphism and shows dominant sinistral and sub- be found to the west of the Inntal-Brenner line. 2007; Kuhlemann et al., 2001, 2002). ordinate normal faulting kinematics (Müller et The NCA east of the IN were not uplifted at al., 2000). Pseudotachylite dating revealed two that time. This is documented by remnants of ACTIVITY OF STRIKE-SLIP FAULTING IN major phases of activity at 33–29 m.y. and 26 conglomerates and sands (Augenstein Forma- THE EASTERN ALPS m.y. (Müller et al., 2001) (Table 1). Wölfler et al. tion) deposited by braided river systems, today (2011) inferred eastward extrusion of the Aus- located at planation surfaces as high as 2000 m. In the Eastern Alps, strike-slip faulting is troalpine unit with major displacements along The source of these deposits is interpreted to be intimately related to the development of topog- the IN, DAV, and PA from eastward-moving a hilly (moderate elevation) low metamorphic raphy (Frisch et al., 1998; Frost et al., 2009; activity patterns and kinematic settings of the area to the south of the eastern NCA. Linzer et al., 2002; Ratschbacher et al., 1991a, faults. In general, few time constraints are avail- Following the onset of topographic uplift, 1991b; Robl et al., 2008b; Sachsenhofer et able for faulting of the DAV and all other faults there is limited evidence about the details of al., 2003; Wölfler et al., 2011). We therefore between the Tauern Window and the PA. the geomorphic evolution of the Eastern Alps provide a summary of the Miocene strike-slip To the south of the Tauern Window there is between 30 m.y. and today. Nevertheless, it is activity here. For simplicity we consider only a network of fault zones with varying kinematic known that intramontane basins formed during seven major faults, the Periadriatic (PA), Defreg- sense (Fig. 1). The dextral MV, which shows an the main phase of lateral extrusion between 18 gen-Antholz-Vals (DAV), Möll valley (MV), offset of ~2.5 km, constitutes the eastern margin; and 15 m.y. (Dunkl et al., 2005; Sachsenhofer Salzach-Ennstal-Mariazell-Puchberg (SEMP), it strikes along 80 km in a northwest direction et al., 2003). These basins formed along major Inntal (IN), Mur-Mürz valley (MM), and Pöls- and cuts into the Tauern Window at its southeast- strike-slip faults at the time of subsidence in the Lavanttal (PL) (Fig. 1). All relevant activity ages ern end. According to Wölfler et al. (2011), the Pannonian Basin and contain terrestrial sediments. are collated in Table 1. Other than these mostly main vertical kinematic activity (exhumation) Legrain et al. (2014) documented that U-He ages Miocene strike-slip faults, there are two north- of this fault zone was between 27 and 25 m.y. from the Koralpe range are ca. 16 m.y., indicat- south–striking shallow-angle detachments that and a second activity pulse occurred at 21 m.y. ing that the tilting and early uplift of this range are largely synchronous and bound the Tauern Wölfler et al. (2011) suggested that their data can occurred in connection with this activity. Simul- Window to the west and east: the Brenner line be interpreted in terms of warping of isotherms taneously, the tectonic Tauern Window (TW) was and the Katschberg line (Fügenschuh et al., 1997; and therefore with surface uplift that occurred exhumed between the low-angle Brenner and Genser and Neubauer, 1989). Both also play a in connection with this exhumation event. Time Katschberg detachments and the high peaks of role in the topographic evolution of the Eastern constraints about horizontal activity are not well the Tauern range were uplifted (Fügenschuh et al., Alps, but we show here that they formed dynami- known. The activity time frame was extended to 1997; Wölfler et al., 2011, 2012). In the Samar- cally in response to the strike-slip zone activity 31–22 m.y. by Inger and Cliff (1994). tian (12.7–11.6 m.y.) the Tauern Window was and need not be explicitly considered. The SEMP has a total length of ~400 km, opened and from 10 m.y. onward NCA debris The PA bounds the Eastern Alps to the south extends approximately in the west-east direc- becomes important in Molasse sedimentation in and is the most important large-scale fault sys- tion, and constitutes the second-largest tectonic the Eastern Alps, indicating substantial surface tem of the Alps (Fig. 1). It is interesting that structure in the Eastern Alps. It is thought to have uplift (Frisch et al., 1998). Overall, the low- the southern boundary of alpine metamorphism accumulated ~60 km of sinistral strike-slip dis- temperature geochronological data of Wölfler does not follow the PA, but is located north of placement (Urbanek et al., 2002). For much of its et al. (2012) are consistent with a more or less it within the Austroalpine, partly along the length, it separates the Mesozoic NCA from crys- linear history of final exhumation and topography DAV fault (Müller et al., 2001). However, the talline basement of the middle Austroalpine units development for much of the region east of the PA separates the rheologically somewhat softer to the south and formed a number of pull-apart

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TABLE 1. SYNOPSIS OF TEMPORAL ACTIVITIES AND KINEMATICS OF GEOLOGICAL STRUCTURES OF THE EASTERN ALPS FROM VARIOUS LITERATURE RESOURCES Name Activity* Type Remark Reference (m.y.) Tauern window 24–22, comp. uplift Kuhlemann (2007) 18–15, comp. uplift Kuhlemann (2007) 31.2 ± 0.4 sinistral Olperer fault Glodny et al. (2008) 26.7 ± 1.2, 21.5 ± 0.8 sinistral Greiner fault Glodny et al. (2008) 19.8 ± 0.4 sinistral Ahrntal fault Glodny et al. (2008) 31–30 slightly sinistral ductile, eclogite zone Glodny et al. (2008) 5 uplift Carpathian slab break off 10–7 m.y.Wölfler et al. (2012) Defreggen-Antholz-Vals 46.1 ± 3.1 sinistral Zinsnock area Mancktelow et al. (2001) 33.4 ± 1.9, 30.1 ± 4.6 sinistral Müller et al. (2000) 32–30, 25.5 ± 0.9 sinistral Müller et al. (2000) Engadine late Oligocene sinistral 4 (southwest)–20 (northeast) km Linzer et al. (2002) Inntal 33–23 sinistral 34–48 km, thin skinnedLinzer et al. (2002) Königssee-Lammertal-Traunsee uplift, sinistral 10 km, thin skinned, uplift 0.7–2 km Linzer et al. (2002) Mur-Mürz valley 17–13 pull–apartsedimentationDunkl et al. (2005) 17 sinistral change to normal faultingReinecker (2000) 17–14 sinistral major subsidence Ratschbacher et al. (1991b) Möll valley 31–22 no uplift localized deformationInger and Cliff (1994) 25.3 ± 2.9, 20.7 ± 2.3 Dextral Glodny et al. (2008) 27–25, 21 uplift, dextral 2.5 km Wölfler et al. (2011) Periadriatic line 32–29, 18–13 dextral 60 km Wölfler et al. (2011) 29.88 ± 0.29 sinistral Jaufen PasseierMüller et al. (2000) 32, 21, 16 dextral TonaleMüller et al. (2001) 32–29 thrustingNorthern Giudicarie faultMüller et al. (2001) 17 sinistral PasseierMüller et al. (2001) 32–30, 21–17 sinistral Jaufen Müller et al. (2001) 27 dextral MaulsMüller et al. (2001) 22–20 dextral Pustertal Müller et al. (2001) 20.1 ± 2.2 sinistral Speikeboden Mancktelow et al. (2001) (Periadriatic splay fault) 23.5–19 dextral Tonale fault Zwingmann and Mancktelow (2004) 16 dextral Mauls fault Zwingmann and Mancktelow (2004) 9–6 dextral (Tonale), Centovally, Simplon Zwingmann and Mancktelow (2004) 17 sinistral Northern Giudicarie fault Pleuger et al. (2012) Pöls-Lavanttal 18–16, 14–12 dextral 8–10 km Reinecker (2000) 11–5.5 dextral 8–10 km Reischenbacher and Sachsenhofer (2013) Ahorn shear zone 12–7 sinistral ductile, 2 km wide, part of SEMP Rosenberg and Schneider (2008) 15.7 ± 1.3 sinistral ductile, part of SEMP Glodny et al. (2008) 19.15 ± 0.03, 17.04 ± 0.3 sinistral Stillup valley Schneider et al. (2007) Salzach 35–28 sinistral 60 km, Salzach (35–28 m.y.), Urbanek et al. (2002) Hieflau (17 m.y.) Ennstal-Mariazell-Puchberg 23–12 sinistral lateral extrusion main phase Ratschbacher et al. (1991b) 17–14 sinistral Dunkl et al. (2005) 18–15 sinistral intramontane basinsLinzer et al. (2002) 118–9 ka sinistral flowstone deformation Plan et al. (2010) Zwischenberg-Wöllatratten Oligocene sinistral Wölfler et al. (2012) *Ages in m.y., except for last (118–9 ka) Note: comp.—compression; SEMP—Salzach-Ennstal-Mariazell-Puchberg fault.

basins (e.g., Wagrain, Gröbming, Hieflau) during 35 and 28 m.y. According to Ratschbacher et (Linzer et al., 2002). The fault dies out in the Oligocene to Miocene activity. Many sinistral al. (1991b) the main activity time frame of the Molasse foreland basin to the east. The IN is one strike-slip faults branch from the SEMP toward SEMP, which correlates with the main lateral of several strike-slip faults (e.g., the Königssee- the northeast (Decker et al., 1994; Ratschbacher extrusion phase, is late Oligocene to middle Mio- Lammertal-Traunsee fault) cutting the NCA in et al., 1991b). The Salzach valley part of the cene (23–12 m.y.). However, Dunkl et al. (2005) northeast directions related to lateral extrusion fault cuts into the northwestern part of the Tauern and Decker et al. (1994) documented intramon- and north-south shortening of the NCA during Window and changes into the ductile, as much as tane basin sedimentation ages between 18 and the second alpine orogeny (Frisch et al., 1998). 2 km wide, Ahorn shear zone, which dies out ~15 14 m.y. that were used in this study. The sinistral MM is also known as the Noric km east of the Brenner normal fault (Frost et al., The sinistral IN commences as the Enga- Depression. The fault extends for ~250 km from 2009; Rosenberg and Schneider, 2008). Urbanek dine fault in the west and accumulated ~31–50 the eastern end of the Tauern Window to the et al. (2002) published mylonitic marble ages km of strike-slip displacement between 32 and south of the Vienna Basin (Fig. 1). Together with for the SEMP line that indicate activity between 23 m.y. within a thin-skinned tectonic setting the PL it forms a conjugate strike-slip fault set

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Abbr. Fault name Time history of strike-slip activity from literature Time in Ma before recent35343332313029282726252423222120191817161514131211109 876543210 PA Periadriatic fault DAV Defreggen-Antholz-Vals SEMP1 Salzach-Ennstal fault SEMP2 Ennstal-Mariazell-Puchberg fault MM1Mur-Mürztal fault – west nnnn MM2Mur-Mürztal fault – east PL Pöls-Lavanttal fault MV Möll-valley fault In Inntal fault Time sequence of simulation runs Simulation time in Ma 012345678910 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 run #0 123 run #1 4 56 run #2 78910 run #3 11 12 13 run #4 14 15 16 record numbers of stored solution run #5 17 18 19 run #6 20 21 22 23 run #7 24 25 26 27 28 29 30 31 32 33 run #8 34 35 36 37 38 39 Time sequence of strike-slip activity simplified Time in Ma before recent35343332313029282726252423222120191817161514131211109 8 76543210 PA Periadriatic fault DAV Defreggen-Antholz-Vals SEMP1 Salzach-Ennstal fault SEMP2 Ennstal-Mariazell-Puchberg fault MM1Mur-Mürztal fault – west MM2Mur-Mürztal fault – east PL Pöls-Lavanttal fault MV Möll-valley fault In Inntal fault

Figure 2. Summary of the major Miocene strike-slip faulting activity in the Eastern Alps (Abbr.—abbreviation). The top panel shows the activity from the literature as detailed in Table 1; shaded boxes indicate active (open) and white boxes indicate inactive (closed). The symbol n indicates normal faulting. The middle panel shows how the Tertiary evolution was discretized into nine selected time slices for the simulations. During each time slice, no changes in fault activity are assumed. The bottom panel shows the simplified activity history as used for the nine time slices of the simulations.

pushing the Styrian block to the east. The MM is evidenced by paleolandscapes today above ca. 14–12 m.y., and between 11 and 5.5 m.y. formed at the beginning of lateral extrusion in ~1100 m above sea level. This paleolandscape Observed seismic events, which indicate recent early to middle Miocene time (Linzer et al., was uplifted to its present elevation after 5 m.y. activity of this fault, were not taken under con- 2002; Sachsenhofer et al., 2003). A series of (Wagner et al., 2010, 2011). sideration for this analysis. pull-apart basins formed along the Mur-Mürz The PL is a dextral fault system consisting of valley (Reinecker, 2000; Strauss et al., 2001). several parts (Fig. 1). It extends north-northwest THIN VISCOUS SHEET AND LANDSCAPE Their sedimentary records allow the dating of for ~150 km and separates the Saualpe from the EVOLUTION MODELING its activity. Dunkl et al. (2005) gave an activity Koralm complex. Both parts, the northern Pöls period of 17–13 m.y., consistent with a sedi- fault with a slip accumulation of ~8 km and the In order to model the influence of strike-slip mentary date of 16 m.y. and a slip accumulation southern Lavanttal fault with a slip displacement faulting on topography development in the East- of 30 km by Linzer et al. (2002). Wölfler et al. of 10 km, are dying out in the Fohnsdorf Basin ern Alps, we have expanded an existing finite (2011) suggested a kinematic separation of the and cross the MM in this zone (Frisch et al., element code that is based on a viscous sheet MM into two segments west and east of the 2000). From the sediment record in small associ- formulation. The thin viscous sheet formula- PL system. Both are supposed to have initiated ated pull-apart basins (Wolfsberg and Obdach), tion was introduced by England and McKenzie with strike-slip activity before 16 m.y. Afterward its activity was Karpathian to mid-Pannonian (1982, 1983) and assumes a two-dimensional only the eastern part keeps its lateral tectonics, (Reischenbacher et al., 2007; Reischenbacher sheet that deforms according to a generalized while the western part changes into a normal and Sachsenhofer, 2013). Due to drill core age power law rheology:

fault, extending the Katschberg detachment and dating results from cataclasites and fault gouges, 1 1 n causing the hanging-wall Gurktal block to slip the age of its activity is well known, compared ij = BE ij, (1) in southeast direction (Wölfler et al., 2012). Sur- to the less-well-known activity of the conjugate

face uplift of the entire region south of the Noric MM. According to Kurz et al. (2011), Reinecker the deviatoric stress τij = σij – pδij, the strain rate

Depression and east of the PL is likely to be con- (2000), and Reischenbacher and Sachsenhofer tensor is ij, the second invariant of the strain rate nected to the fault activity in the Miocene and (2013), the fault was active ca. 18–16 m.y., tensor E = kl kl , and B is a material constant;

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σij and p = σii/3 are the stress tensor and the initially in Houseman and England, 1986; Robl lines into parts that are turned on and off during

lithospheric pressure, and δij is the Kronecker and Stüwe, 2005a, 2005b; Robl et al., 2008b; successive time increments according to geologi- symbol. The indices i, j = 11()3 represent the Stüwe et al., 2008). cal knowledge. This applies in particular to the Cartesian coordinates. Double indices within a Following Robl and Stüwe (2005b; see also SEMP and MM faults. Between adjacent fault term imply a summation according to the Ein- Stüwe, 2007), the topographic height h (initial tips a small gap (1–5 km) is placed, because of

steinian summation rule. The thin viscous sheet height h0) is calculated from lithospheric layer the finite element program requirements. This 0 model based on this rheology has successfully thickness sl, the initial thickness sl , and initial limitation introduces some additional resistance 0 been used to describe the India-Asia collision crust thickness sc using: against free strike slip and therefore inhibits the zone (England and Houseman, 1986) and the lateral extrusion in general. For the conjugate 0 0 m c Alps (Robl and Stüwe, 2005b) and is briefly h = ()sl sl sc + h0 (4) MM and PL fault set around the Fohnsdorf Basin summarized here. c in particular, we decided to inhibit the MM fault The static momentum equation forms the when isostatic compensation is assumed. In because for the past 16 m.y. its activity is known basis of the mechanical modeling and is given by: Stüwe et al. (2008) this model was expanded to be different between both sides of the PL. by coupling a landscape evolution model with p ij + = g , (2) the thin sheet model; for their erosion model Initial and Boundary Conditions x x i3 j i they followed the common assumption that the

with gravity acting in −z direction and p being erosion rate he of a given spatial location is pro- For the modeling, a region between long 9.1° negative for compression. Neglecting all vertical portional to the upstream drainage area A and and 17.5° and lat 45.36° and lat 48.5° is cho- gradients of deviatoric stress and assuming isos- the square of slope dh/dL; L is the distance along sen (Fig. 1) and assumed to be in plane geom- tasy, the thin viscous sheet formulation is derived a river channel, as implemented in many land- etry. The lower left corner of this region is near by averaging across the lithosphere (England scape evolution models (e.g., Hergarten et al., the Euler pole of the Adriatic plate at 45.36°N, and McKenzie, 1983; Houseman and England, 2010; Robl et al., 2008a; Stüwe, 2007; Stüwe et 9.10°E (Nocquet and Calais, 2003) and is here 1986). After normalization this results in: al., 2008; Wobus et al., 2006). This assumption assumed to correspond to the Cartesian coor- may be written as: dinate system origin. The region largely corre-

2 sponds to that chosen in Robl and Stüwe (2005b) s2 u u u u dh dh nA c = i + j + 2 1 + 2 i, j =h 1,=2n e = eA . (5) so that the results can be directly compared. Also r x x x x x x x e dt dL i j j i i 1 2 following these earlier studies, we consider three In this equation e is a constant erosion rheological distinct regions with different vis- s2 u u u u nA c = i + j + 2 1 + 2 i, j = 1, 2n. (3) parameter, chosen to be e = 2500 m–1s–1. cosities, that we name European foreland (eu), r x x x x x x x i j j i i 1 2 In order to describe brittle faulting within Austroalpine (aust), and indenter (adr) (green the viscous medium, we assume the strike-slip polygons in Fig. 1). The dimensionless viscos- In this equation u / x u / x /2 faulting model of Barr and Houseman (1996), ity contrast parameters are chosen to / ij = ()i j + j i ηadr,aust,eu 1 which allows free slip along a fault line with η = 1.5:1:3. The Argand number is chosen as 1 aust is the strain rate tensor, Es /u n the a vertical fault plane and only transfers normal A = 1 and n = 1, which results in the best fit in = ()l 0 r stresses across the fault. In the finite element terms of the width of the orogenic wedge and code, the faults are implemented as two parallel kinematics that match today’s global positioning c g c 1 sl mesh lines. The nodes on both sides move inde- system velocity field and principal stress orienta- m dimensionless viscosity and Ar = 1/n . pendently according to the different stress fields. tion (Robl and Stüwe, 2005b; Robl et al., 2008b). Bu()0 /sl Therefore, the local amount of slip along a fault The exponent choice n = 1 reduces Equation 1

The Argand (Ar) number with the normaliza- line depends on the actual stress field caused to a linear Newton fluid rheology. According to

tion constants u0 and sl (velocity and initial by the deformation process. In other words, slip Barr and Houseman (1996) this rheology cor- lithosphere thickness) summarizes all mate- and topography along the faults form implicitly relates to that of olivine at high temperatures rial constants and can be interpreted as a ratio by the model. As the model is two-dimensional, and low deviatoric stresses and may therefore be

of vertical and horizontal stresses. For Ar > 1 the fault model is considered to be valid only appropriate for the alpine lithosphere as a whole. the orogeny is dominated by gravitational col- for deep-rooted faults, which also cut through In Robl and Stüwe (2005b) and Robl et al.

lapse, whereas for Ar < 1 a kinematic control the most competent upper mantle lithosphere. (2008b) it was shown that the Nocquet and Cal- by boundary condition is given. The thin vis- In order to account for temporally varying fault ais (2003) findings can be used as a constant cous sheet assumption reduces the indices to activity after the deformation is already in prog- southern boundary condition for the indenter

i, j = 11()2 Z representing the x1 and x2 plan- ress, we divide the simulation time into time seg- for the past 30 m.y. if one restricts the model- view coordinates only. Equation 3 constitutes a ments, during which a certain set of faults is ing area to east of the Euler pole. In the present two-dimensional partial differential equation for active (Fig. 2). Additional code adaptions were work the best fit is found for an indenter length the velocity field, which can be solved by Galer- required in order to handle large deformation of 400 km. Along this length the angular veloc- kin finite element approximation. For all n ≠ 1 along faults and to enable the control and/or ity ω = 0.52°/m.y. is interpolated into a local this nonlinear scheme is solved iteratively with change of the fault state when a subsequent simu- velocity using recalculated after each step. Although Equa- lation run is started based on a previous deforma- η x tion 3 is strictly two-dimensional in plan view, tion. The actual implementation does not allow v()xk[]m = ()3.6, 0.5 T mm /yr 400 a crustal thickness distribution sc(x, y) can be the handling of fault propagation. However, in calculated for each time step from the continu- view of the geologically limited resolution of by linear approximation of the motion between ity equation (not shown here). These calcula- fault activity, there is no need to consider their the initial and the finite deformation state after tions are done using a finite element code (used growth. Nevertheless, we segmented some fault 30 m.y. of rotation; x is the lateral coordinate of

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the model and (3.6, –0.5)T means the transpos- In order to extract a surface elevation h from that cannot be controlled by the presented sim-

ing of a vector. The indenter velocity setting Equation 4 from the thin sheet layer thickness sc, ulation, within the time frame of 32–30 m.y. only holds exactly for straight rotational axes we assume that the initial lithosphere layering (Mancktelow et al., 2001). 0 0 through the Euler pole. Note that the southern is a buildup of s c = 35-km-thick crust and a s l boundary condition of our model is assumed to = 100-km-thick lithosphere with the densities Modeling Miocene Faulting 3 3 be located substantially south of the present-day ρc = 2700 kg/m and ρm = 3300 kg/m for the In order to insert the relevant fault activity

plate boundary at the PA (Fig. 3). Thus, the rhe- lithospheric mantle. From Ar = 1 and with the into the model, we have coarsened the Miocene

ology contrast at the PA (with the region south of chosen normalization u0 = 500 km/100 m.y. = fault activity collated in Table 1 (top panel of it being substantially stronger than the Eastern 5 mm/yr, the matrix viscosity is (η = 1) B ≈ 3 Fig. 2) into a scheme of nine consecutive time Alps) imposes an effective internal boundary at · 1023 Pa·s. A flat topography of 100 m above slices during which there is no change in fault- the PA, so that the model is robust toward crude sea level (the hilly surface at 30 m.y.; Frisch et ing activity (middle panel in Fig. 2). The run for

approximations of the southern model bound- al., 1998) is assumed as initial condition for h0 each time slice uses the final results of the prede- ary. The western boundary is fixed to allow slip so that all modeled topography, drainages, and cessor run as its initial condition and faults are in the north-south direction, but no westward morphologies, except the initial fault locations, activated (also called open) or deactivated (also motion. The eastern boundary is set to have an are dynamically developed during the model called closed). The assumed constant strike-slip open boundary condition, which reflects the free runs. The choice to start the simulation 30 m.y. activity within each run is shown in the bottom extrusion constraint in Robl et al. (2008b) (see ago is reinforced by a reversal of the PA fault panel of Figure 2. For this, some adaptations of Fig. 2B middle trace). kinematic from sinistral to dextral, a property the fault activity were necessary.

spatial distribution of crustal features 0.7 A 0.6 European foreland 233km SEMP2 147km 0.5 SEMP1 IN MM2 0.4 MM1 Figure 3. The model boundary Austroalpine conditions. See Figure 2 for fault DAV PL 0.3 MV abbreviations. (A) The spatial distribution of dominant strike-slip faults and PA rheology contrasts within the model 0.2 boundary (outer box) based on normalized coordinates using length lateral distance y ′ [normalized] scale normalization D = 500 km, u rheo. today 0 0.1 faults today = 5 mm/yr (velocity). The Paleogene Indenter pal. rheo. initial model for the simulation after pal. faults rectification is shown using dotted 0 lines for the faults and dashed lines for 00.2 0.40.6 0.811.2 the viscosity contrast line. The Adriatic longitudinal distance x′ [normalized by D = 500 km] plate Euler pole is located at the lower left boundary vertex. The two double model boundary conditions arrows measure the distance between 0.7 the foreland and the Paleogene Peri- adriatic fault (PA). (B) The boundary u′ = 0,u′ = 0 x y B conditions of the model (see text). The 0.6 northern edge is stiff, while the southern edge approximates the rotational 0.5 indenter and a free slip condition to u = 0 = 0 x′ σ′x the east of it. The western boundary allows free movement in the north- 0.4 σ′y = 0 σ′y = 0 ′ [normalized] south direction. The eastern boundary y is open (free of stresses). 0.3 velocity profile acting on boundary: 0.2 u′x = [0, - 0.1], u y′ = [0, 0.72] lateral distance

0.1 σ′x = 0 15.6° σ′y = 0 0 00.2 0.4 0.6 0.811.2 longitudinal distance x′ [normalized by D = 500 km]

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Following Wölfler et al. (2011) it is assumed TABLE 2. RECTIFICATION TRANSFORMATION to uplift sharply at both viscosity contrasts. that the western part of the SEMP fault shows OF STRIKE-SLIP FAULTS 30 MY While areas close to fault tips of the IN, MV, an early activity between 35 and 28 m.y. After Fault −φback T and DAV show increased uplift and subsidence name (°) a phase of inactivity the complete fault line (km) due to strain accumulation, this can hardly be becomes active again in 18–14 m.y. Therefore, In 1–5.8 (−5, 0)T observed for the SEMP1, which is inactive at 25 this fault is divided into two branches (Fig. 1): SEMP1 3.4–6.4 (−5, 0)T m.y. A pulse of uplifting strain rate in the Tauern the Salzach-Ennstal branch in the west (SEMP1) SEMP2 0–3.3 (−5, 0)T Window close to the later Großglockner loca- and the Ennstal-Mariazell-Puchberg branch in DAV 6.5–7.7 (−20, −5)T tion and Nockberge is observed due the active the east (SEMP2). For a similar reason the MV 5.1–7.2 (−30, −10)T MV (see Fig. 5). T MM is segmented. According to Wölfler et al. MM1 1.5–4.4 (−35, −10) In the early Miocene (20 m.y.) there is a MM2 0–1.5 (−35, −15)T (2011), a time frame from 18–17 m.y. is derived PL 2.2–3.8 (−35, −10)T phase of relative silence in terms of strike-slip in which both parts of the fault are active, and PA 15.6 (0, 0)T activity, and a uniform elevation increase occurs. between 16 and 13 m.y. only the eastern seg- The highest elevations, to 912 m, can be found Note: Angles −φ back describe a back rotation; ment (MM2) is assumed to be active. vector T describes a translation. See text for fault between the southern end of the MV fault and A problem with the placement of strike-slip names. the PA. The topographic mean height of the faults in the model geometry is that their loca- orogenic wedge reaches 289 m. Only the PA tion and shape prior to Miocene shortening is active, which results in high uplift rates to would need to be known. We approximate this 3A two north-south–directed distances between the north of Bolzano and west of Klagenfurt. by rectification of today’s fault maps (exclud- the Paleogene PA and the foreland boundary are The MV and IN became inactive and the Tauern ing PA) and plate boundaries by back rotation measured as 147 km at the PA knee and 233 km Window uplifted moderately, which can best be around the Euler pole with the angle: around the Fohnsdorf Basin. Linzer et al. (2002) seen by small vertical strain rates. Erosion starts max gave ~156 km and 230 km for these distances, to form valley systems that reflect roughly the DP()ft , 0 < DP()ft < 150 km back ()Pft = 150 km which comes close to the suggested rectification locus and geometry of the Drau, Inn, Enns, and 0, otherwise performed here. Adige valley systems. max The implicit assumption of the two-dimen- In the middle Miocene (15 m.y.) the activity DP()ft , 0 < DP()ft < 150 km back ()Pft = 150 km , (6) sional model is that the modeled faults cut the pattern propagates further eastward. Most faults 0, otherwise entire lithosphere. It is interesting that Brückl close to the Tauern Window except the SEMP1 et al. (2010) observed from mantle tomography are inactive. The vertical strain rate of this time with the normal distance D in kilometers data that structural discontinuities in the litho- frame shows a clear stagnation in the central

between a point Pft at a recent fault and the unro- spheric mantle appear to mimic the location of Eastern Alps, consistent with extensional exhu- tated viscosity contrast line, which largely fol- several of the Miocene faults mapped on the mation of the Tauern Window at this time, as lows the PA, and a pure translational component, surface. Therefore, we believe that our assump- documented by geochronological investigations which corrects for the predominantly east-west tion is justified. of the bounding detachments at Brenner and displacement due to lateral extrusion. The maxi- Katschberg (Fügenschuh et al., 1997). Strong mum angle φmax is determined by experiment and RESULTS and spatially extended subsidence occurs around is chosen as φmax = −15.6°/2, ∀x > 0 ∧ x < 425 Vienna, predicting the early formation of the km, and changing to φmax = −15.6°/4, ∀x >425 The simulation run shown in Figure 4 Vienna Basin. Localized strong subsidence can km, which means an unsteady transition at the describes the model evolution of topography also be observed at locations that roughly corre- PL in an east-west direction (rotational decou- of the Eastern Alps for 8 different time steps, late with intramontane basins along the PA, PL, pling). Figure 3A shows the actual location of starting 30 m.y. ago and ending with a predic- and SEMP. In general, due to the activation of the regarded fault system, while the approxi- tion of the future in +5 m.y. Figure 5 shows the the conjugate fault set of PL and MM2, a mod- mated situation 30 m.y. ago is shown by dotted vertical strain rates as a proxy for uplift rate erate and localized uplift of the Styrian block to lines. The literature reports different palinspastic for the same time steps. In very general terms, the west of the PL (Gleinalm, Koralm) and to the reconstruction models (Frisch et al., 2000, 1998; the Miocene strike-slip faulting activity caused southeast of the Mürz valley is observed. Figure Linzer et al., 2002), but here we follow the Noc- uplift near transpressional ends of faults and 4 shows a continuous rise of mean elevation to quet and Calais (2003) indenter back rotation subsidence near transtensional ends of faults, 374 m and a strong uplift, with elevations close assumption, which is assumed to have been con- thereby facilitating drainage development in to 800 m north to the SEMP2 near Schladming stantly acting during the past 30 m.y. (Robl and these regions and dissection of the Eastern Alps (Dachstein region). Valley incisions of the Sal- Stüwe, 2005b; Robl et al., 2008b). The applica- into individual blocks. At 30 m.y. the vertical zach-Ennstal, the Pöls-Lavanttal, and faintly in tion of Equation 6 results in a drastic reduction of strain rate in Figure 5 indicates strongest uplift the Mur-Mürz valley become visible. the rotational influence with increasing distance north of the eastern end of the PA but little uplift In late Miocene time (10 m.y.) most strike- to the indenter, which dies out east of the Fohns- for the Tauern Window region, despite the activ- slip activity, except the PL line and SEPM1, has dorf Basin. By experiment a vector T is chosen ity (open state) of the DAV and SEMP1 faults, died out (Fig. 2). In general, uplift rates in Fig- individually for each fault line in order to get but inactive (closed state) MV fault. ure 5 seems to be continuously distributed and the best fit between the final simulation and the At 25 m.y. (after 5 m.y. of indentation), the sharply bounded by the rheology contrasts near today’s fault geometry. All rectification parame- highest elevations are found close to the indenter the PA and at the European foreland over the ters are given in Table 2. A steady increase of lat- tip (west to the Klagenfurt Basin), in the Tauern orogen to the west of the PL fault. Subsidence eral extrusion from 5 km in the east to 35 km in Window adjacent to the MV and the DAV (close still occurs around Wolfsberg and in the Seckau the west at the PL is observed. The rotation angle to Mount Großglockner location) and to the east area, but the subsidence close to the Fohnsdorf is greatest at locations close to the PA. In Figure of the Inn valley. The entire Eastern Alps starts Basin seems to have terminated, because the

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30 Ma, h : 100 m, h : 100 m 25 Ma, h : 175 m, h : 390 m 300 km mean max mean max

200 km 2 2

100 km 1 1 km km 0 0 0 km

20 Ma, h : 289 m, h : 812 m 15 Ma, h : 374 m, h : 1191 m 300 km mean max mean max

200 km 2 2

100 km 1 1 km km 0 0 0 km

10 Ma, h : 453 m, h : 1211 m 5 Ma, h : 517 m, h : 1396 m 300 km mean max mean max

200 km 2 2

100 km 1 1 km km 0 0 0 km

Today, h : 562 m, h : 1642 m Wien +5 Ma, h : 589 m, h : 1897 m 300 km mean max mean max Salzburg

Schladm. Kapfenberg Innsbruck Krimml 200 km Graz Lienz Wolfsberg 2 2 Klagenfurt Bolzano 100 km 1 1 km km 0 0 0 km 0 km 200 km 400 km 600 km 0 km 200 km 400 km 600 km Figure 4. Simulation results of the topographic elevation development of the Eastern Alps during the past 30 m.y. until +5

m.y. The fault locations (black) and rheology contrast boundaries (green) are in dotted lines. The hmean and hmax are the mean and maximum elevations of topography inside the dotted box, which was chosen inside the model boundaries to avoid boundary condition effects.

MM fault system is inactive and the Styrian m is reached. At the PA, erosion built up a broad previous steps. The maximum height, which can block is further elevated in the Koralm and valley, which largely denudes the Southalpine. be found at the PA west of Klagenfurt, increases Gleinalm region, consistent with observations During the remainder of the model evolution to 1897 m. This happens without any strike-slip of Wölfler et al. (2011), Reischenbacher and no more faulting is active and uplift proceeds activity. Sachsenhofer (2013), and Strauss et al. (2001). continuously. The mean elevation continuously In Figure 4 the mean height climbs to 453 m, increases to 562 m, while the maximum height DISCUSSION but the peak height has barely increased to 1211 reaches 1642 m. m west of Klagenfurt close to the PA. In the The future projection of topography devel- The results of the forward modeling approach Pliocene (5 m.y.) the tectonic setting hardly opment to +5 m.y. from present, assuming no to predict the topographical evolution of the changed; still only the PL is active. A mean ele- more active faults, shows a further increase of Eastern Alps show that the Miocene strike-slip vation of 517 m and a maximum height of 1396 mean height to 589 m, which is comparable to faulting is profoundly and intimately related to

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Figure 5. Vertical strain rate at the same time steps as in Figure 4. Vertical strain rate is used as a proxy for compression (red) and extension (green).

topographic uplift and drainage geometry. Most 6C corresponds to the bottom left panel in Fig- the development of topographic features like changes in mean height or the dissection of the ure 4 (i.e., for temporally variable faulting, as valleys and mountains. The simulation without model mountain range into individual mas- discussed here). Similar to the results shown in any faults (Fig. 6A) hardly reflects any features sifs occur during the time steps at which faults Ratschbacher et al. (1991a, 1991b) and Robl of the Eastern Alps except the buildup of a drain- are most active. In order to illustrate this fur- et al. (2008b), we observe that the activity of age, which correlates with the valley systems ther, Figure 6 shows a comparison of the topo- strike-slip faulting has a strong influence on north of Bolzano. However, the open faulting graphic evolution of the model after 30 m.y. of the development of topographic elevations. In run (Fig. 6B) resolves features like several of deformation (i.e., today) between three differ- general, the increase of strike-slip faulting activ- the Miocene intramontane basins or the Glock- ent assumptions about the activity of faulting. ity (in terms of number of faults and activity ner massif, but hardly reproduces the uplift of Figure 6A shows the result of a model evolu- duration) decreases the rate of elevation gain in the Tauern Window, especially its western end. tion without any active faults; in Figure 6B, all the orogen. Strike-slip faulting appears to be a In contrast, the temporal faulting run (Fig. 6C) seven faults zones discussed are active during major process for vertical strain accumulation exhibits most intramontane basins and the uplift the entire evolution of the model run, and Figure and increased erosion, which is responsible for of the Tauern Window reasonably well.

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most important primary differences between model and observed topography include the No faults, hmean: 567 m, hmax: 1715 m Wien A 300 km absence of topography west of Innsbruck and the unusual high topography in the region Salzburg between Klagenfurt and Lienz (Fig. 7). Both Hochtor Schladm. Kapfenberg shortcomings are easily explained: the lack of Innsbruck Krimml Speikkogel topography in the western part of the model 200 km Grossglockner Ankogel Graz region (Fig. 7) is a consequence of our boundary Wildspitze Lienz Wolfsberg conditions, which do not consider convergence Klagenfurt there. Because uplift is exclusively driven by Bolzano convergence, erosion according to Equation 5 100 km 2 causes substantial incision of drainages toward the west, where base level remains at low ele- 1 vations. Therefore, the modeled topography

km 0 should not be compared in the transparently 0 km shaded region in Figure 7. Correspondingly, the modeled high plateau region between Lienz and With faults, hmean: 485 m, hmax: 1604 m Wien B 300 km Klagenfurt (Fig. 7B) is the consequence of maximum convergence in this model region. The Salzburg absence of such topography in the Eastern Alps Hochtor Schladm. Kapfenberg is interpreted to be caused by substantial glacial Innsbruck Krimml Speikkogel erosion in the Klagenfurt region (well known in 200 km Grossglockner Ankogel Graz this region), and by soft lithologies such as the Wildspitze Lienz Wolfsberg Gurktal nape system in the region. Remnants Klagenfurt of highly elevated plateau-like paleolandscapes Bolzano outside the glacially carved and soft lithologies 100 km 2 are found in the region (e.g., Nockberge), similar to those featured in the model. 1 Despite these two major discrepancies

km 0 between model and nature, there are also many 0 km aspects of the topography of the Eastern Alps that are well reflected by the model. In the south Temporally variable faulting, hmean: 562 m, hmax: 1642 m Wien C 300 km a drainage system has formed in the model that resembles the Adige drainage, and a series of Salzburg west-east–striking valley form in the region of Hochtor Schladm. Kapfenberg the PA. In the north, west-east–running drain- Innsbruck ages akin to the Salzach, Enns, and Inn Rivers 200 km Krimml Speikkogel Grossglockner Ankogel Graz form in the model. In the eastern model region, Wildspitze Lienz Wolfsberg pull-apart basins formed along the major faults, Klagenfurt and asymmetric mountain ranges, like the Bolzano Koralpe, develop. The highest elevations are

100 km 2 located in the central model region, where the Tauern range is located. Differences between 1 model and mountain range can be used to infer

km 0 interesting aspects of the uplift history of the 0 km range that depart from the simplistic model 0 km 100 km 200 km 300 km 400 km 500 km 600 km boundary conditions. Figure 6. Comparison of the final stage of the model simulation (for today) for three different end For example, the valley incision in the western part of the SEMP is underpredicted in the members of faulting activity during the past 30 m.y. The hmean and hmax are the mean and maximum elevations of topography inside the dotted box. (A) No faulting during the entire simulation for 30 simulation. One explanation is that several splay m.y. (B) All seven faults considered in this study are active (open) for the entire simulation. (C) Tem- faults of this system are not considered in our porally variable fault activity as shown in Figure 2, bottom left panel. model. Furthermore, the decision to model the SEMP as two separated parts probably inhibits a potential valley incision. Correspondingly, Comparison with the Topography of the evolution. This is particularly exciting as we the Inn valley also appears underdeveloped in Eastern Alps reemphasize that the model assumes extremely the simulation. This raises the question if the simplistic boundary conditions and starts with Engadine fault also needs to be regarded as a Figure 7 shows that many of the primary a flat topography so that all valleys and ranges prolongation of the IN, or if there are times of morphological features of the Eastern Alps are dynamically formed by the model. However, activity that are still unknown. However, the are reproduced by the model after 30 m.y. of there are also some major discrepancies. The IN fault is supposed to be part of a low-angle

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Evolution of Orogenic Heights and the Tauern Window SRTM, hmean: 954 m, hmax: 3682 m Wien A 48° To give further insight to the topographical Salzburg evolution of the orogen, the mean and maximum Hochtor elevations of the simulation area for the three Schladm. Kapfenberg Innsbruck Krimml Speikkogel runs in Figure 6 are shown in Figures 8A and 8B. Grossglockner 47° Ankogel Graz The mean elevation shows a stable, more or less Wildspitze Lienz Wolfsberg linear growth over time for the closed and the Klagenfurt temporally varying fault activity runs, consistent Bolzano with interpretations of thermochronology. How- 3 ever, the open faulting run shows a slight expo- 46° 2 nential decrease of growth and an ~5% reduced 1 final mean height. This can be interpreted as the

km 0 orogen still not being in equilibrium with the erosion process, and has strong potential for fur- 10° 12° 14° 16° ther topographical growth, which is consistent with the interpretations of Hergarten et al. (2010) and Robl et al. (2015) (see the animations in 1 Temporally variable faulting, hmean: 562 m, hmax: 1642 m Wien B Data Repository Figs. S1–S5 ). The slight depar- 300 km ture from linear topography growth for the vari- Salzburg able faulting run at 15 m.y. corresponds to the Hochtor time of maximum faulting activity. Thus, Figure Schladm. Kapfenberg 8A confirms the correlation of strike-slip activ- Innsbruck 200 km Krimml Speikkogel ity with the decreasing mean elevation of the Grossglockner Ankogel Graz Wildspitze orogen. The relationship between elevation and Lienz Wolfsberg time is different for the maximum peak heights Klagenfurt Bolzano (Fig. 8B), which are mostly observed around 100 km 2 the southern end of the MV fault and around the PA west of Klagenfurt. Strike-slip faulting 1 results in a substantial increase of maximum heights, compared to an orogeny without fault- km 0 0 km ing (Fig. 8B) until ca. 15 m.y. Then maximum peak height stagnates completely due to erosion, 0 km 100 km 200 km 300 km 400 km 500 km 600 km while the mean elevation (large-scale uplift) is Figure 7. Topographic map of the Eastern Alps in comparison with the predicted topography of the still continuously growing. Therefore, the relief model simulation. The h and h are the mean and maximum elevations of topography inside mean max (i.e., the difference between maximum and mean the dotted box. (A) Plotted from the STRM (Shuttle Radar Topography Mission) digital elevation model (CGIAR-CSI Consultative Group on International Agricultural Research–Consortium for Spatial elevation) is predicted to be largest in the middle Information) (2014; http://srtm.csi.cgiar.org/), 250 m. (B) The modeled topography for temporally Miocene, which may be supported by the peak variable faults, as in Figure 4. 1 GSA Data Repository Item 2017083, Figure S1: Evo- lution of topography and drainage network in a zone of continental collision computed for model bound- detachment fault (Pfiffner, 2010) and therefore change, indicating that the erosion parameter in ary conditions akin to the Eastern European Alps as described herein. The model is somewhat simplified does not fit the modeling constraints in Barr and the model results shown is not the reason for this from the figures in the article and consider only four Houseman (1996). Other shortcomings can also underprediction. We interpret the underpredic- major faults. Formation of topography over time. Note be explained by regional differences between tion of mean topography by almost 400 m (Fig. that faults and the rheology contrast between major model and local geology. Nevertheless, despite 7) to be the consequence of a large-wavelength, tectonic units cause a spatially complex uplift pattern. several shortcomings of the simplistic fault deep-seated uplift event that is unrelated to Figure S2: Development of the drainage system color coded for steepness index ksn. Note the response geometry, the model presents a major improve- convergence and therefore independent of our time between topography formation and fluvial inci- ment over earlier models in Robl et al. (2008b). model boundary conditions. It is interesting that sion in the center of the orogen. Figure S3: Perspective An interesting aspect of the model results is a similar amount of uplift was suggested from view showing topography formation by plate conver- that both the mean elevation and the maximum field evidence by Legrain et al. (2014), Wagner gence and crustal thickening, topography destruction by fluvial incision and lateral extrusion confined by peak height are underpredicted by several hun- et al. (2010), and others to have occurred during orogen-scale strike-slip faults. Figure S4: Perspective dreds of meters. In order to test if this is due to an a renewed uplift event of the Eastern Alps in the view showing erosion rates on an evolving orogen in overestimation of the assumed erosion rate, we past 4 m.y. Atmospheric upwelling (Lippitsch et a zone of plate convergence. Figure S5: Perspective reduced the erosion parameter, e, from 2500 to al., 2003), glacial erosion (Sternai et al., 2012), view showing the computational mesh color coded for 1500 m–1s–1 for the temporally variable faulting elastic rebound (Baran et al., 2014), and climate erosion rates during the evolution of a mountain range driven by indenter tectonics. Available at http://www run and compared it with the results shown here. factors (Willett, 2010) are under discussion for .geosociety.org/datarepository/2017, or on request Only a small elevation gain is achieved by this an explanation of this event. from [email protected].

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2000 Temporal Temporal 600 Open Open Closed Closed 1500

400 1000

500 Mean height [m] 200 ABMaximum height [m] 0 −30 −20 −10 0 −30 −20 −10 0 Time [Ma] Time [Ma]

1500 SEMP1 1500 With DAV TW CDWith DAV & ZW IN PA 0 Ma 1000 1000 5 Ma

10 Ma

15 Ma Height [m ] 500 500 20 Ma Height [m ]

25 Ma 30 Ma 0 0 0 100 200 300 50 100 150 200 250 300 Profile length [km] Profile length [km]

Figure 8. Predicted evolution of mean and maximum topography in the Eastern Alps and the Tauern Window profile. See Figure 2 for fault abbreviations. (A) Evolution of mean elevations of the topography measured within the 100 m contour in comparison for the simulation with open, closed, and temporally varying strike-slip faulting activity. (B) Maximum elevation of the orogen for the same three simulations. Note that maximum elevation stagnates during the peak period of lateral extrusion. (C) Profile through the Tauern Window (black line in inset), showing an over-broadened valley system along the Periadriatic fault (PA) and underpredicted valley incision along the Salzach-Ennstal-Mariazell-Puchberg fault (SEMP). The Tauern Win- dow starts to exhume ~20 m.y. ago. (D) Two more scenarios for the topographic evolution of this profile with the Defreggen-Antholz-Vals fault (DAV) excluded and Zwischenberg-Wöllatratten fault (ZW) included. An increase of faulting at the southern end of the Tauern Window drastically broadens the Tauern range (TA).

in the sedimentation rates observed for that time profile across the window, as shown in the inset south of the Tauern Window. In the Eastern Alps, (Kuhlemann et al., 2002). During the last 12 m.y. for the model run with temporally variable fault this region is characterized by a large number of the model evolution, the maximum elevations activity (cf. Figs. 1 and 8C). The profile loca- of small fault zones (Wölfler et al., 2012). In begin to rise again and this process is predicted tion crosses the model location where Figure 5 order to test this hypothesis, we added an addi- to continue for the next few million years unless shows that crustal extension occurs at 15 m.y., tional fault zone (Zwischenberg-Wöllatratten faulting activity commences again (cf. with the consistent with the exhumation of the window fault) into the model in this region and explored +5 m.y. snapshot in Fig. 4). at this time. It is seen that the major topographic model evolutions with this and the nearby We conclude this study by investigating features of the Eastern Alps are reproduced by DAV faults active versus them being inactive the model predictions for the topographic evo- the model. High elevations occur inside the (Fig. 8D). The results in Figure 8D show that lution of the Tauern Window region in more Tauern Window, and valleys have incised along the overall height of the Tauern Window varies detail, because this region has the highest peaks, the bounding fault lines. The southern Alps (the only slightly, which indicates that internal splay exhumes the deepest crustal level rocks of the region south of PA) are somewhat lower, while faults of the Tauern Window might be the con- Eastern Alps (Fig. 8C, 8D), and its exhumation the NCA north of the IN fault are characterized trolling factor. However, the extent of the Tauern history is not only subject to an active discus- by high relief. The largest differences between range depends on the DAV and Zwischenberg- sion about, e.g., subduction polarity changes and the model and Eastern Alps are the underpre- Wöllatratten faulting. The most extended and deep-crust collision structures (Kissling et al., diction of elevation (as discussed here) and the elevated Tauern range is gained when DAV and 2006; Lippitsch et al., 2003), but also has impli- much too prominent incision of the valleys Zwischenberg-Wöllatratten fault are introduced cations for the formation of metamorphic domes around the PA. We interpret this prominent val- into the model. in general. Figure 8C shows the time history ley incision around PA to be the consequence of In summary, it may be said that the exhuma- of the topography along a northwest-southeast our simplified assumption of the fault geometry tion of the Tauern Window and the elevation

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of the Tauern range are largely controlled by surface uplift event that currently affects the Fritz, H., Dallmeyer, D.R., Wallbrecher, E., Loizenbauer, J., Hoinkes, G., Neumayr, P., and Khudir, A.A., 2002, Neopro- the activity of the Miocene fault lines that eastern end of the Alps independent of conver- terozoic tectonothermal evolution of the central Eastern bound the region to the north and south. The gence. It is also consistent with the inference Desert, Egypt: A slow velocity tectonic process of core low-angle detachments that bound the Tauern of Hergarten et al. (2010) that the Eastern Alps complex exhumation: Journal of African Earth Sciences, v. 34, p. 137–155, doi:10.1016/S0899-5362(02)00014-3. Window to the west and east (Brenner normal are morphologically immature and still growing. Froitzheim, N., Plasienka, D., and Schuster, R., 2008, Alpine fault; Fügenschuh et al., 1997, and Katschberg Exhumation of the Tauern Window and the tectonics of the Alps and Western Carpathians, in McCann, normal fault; Genser and Neubauer, 1989) form associated uplift of the Tauern range in the cen- T., ed., The geology of Central Europe, Volume 2: Mesozoic and Cenozoic: London, Geological Society, p. 1141–1232. passively only on its eastern end in response ter of the Eastern Alps are strongly linked to Frost, E., Dolan, J., Sammis, C., Hacker, B., Cole, J., and to the fault activity (see also Fig. 5 at 15 m.y.) west-east–striking slip faults to the north and Ratschbacher, L., 2009, Progressive strain localization and need not be considered explicitly in the south. The low-angle detachments bounding in a major strike-slip fault exhumed from midseismo- genic depths: Structural observations from the Salzach- model. This interpretation has implications for the window to the west and east (Brenner and Ennstal-Mariazell-Puchberg fault system, Austria: Jour- the exhumation of metamorphic core complexes Katschberg faults) develop passively in response nal of Geophysical Research, v. 114, B04406, doi:10 .1029 /2008JB005763. in general. Originally, core complexes were only to the strike-slip faulting. Our model predicts Fügenschuh, B., Seward, D., and Mancktelow, N., 1997, Ex- thought to be exhumed in extensional environ- that metamorphic domes need not form exclu- humation in a convergent orogen: The western Tauern ments (Lister and Davis, 1989). Later studies sively in extensional settings, but may be related window: Terra Nova, v. 9, p. 213–217, doi:10.1046/j.1365 -3121.1997.d01-33.x. showed that core complexes are also exhumed in to transpressional or strike-slip regimes. Genser, J., and Neubauer, F., 1989, Low angle normal faults at transpressional (Fritz et al., 2002) or strike-slip the eastern margin of the Tauern window (Eastern Alps): environments (Meyer et al., 2014). The current ACKNOWLEDGMENTS Mitteilung der Österreichischen Geologischen Gesell- schaft, v. 81, p. 233–243. We thank L. Evans, Monash University, for her support in mechanical considerations show that low-angle Genser, J., Cloetingh, S.A., and Neubauer, F., 2007, Late oro- implementing advancements in the fault functionality into genic rebound and oblique Alpine convergence: New detachments exhuming metamorphic domes the finite element code used here; H. Fritz, University of Graz, constraints from subsidence analysis of the Austrian Mo- for discussions about faulting history in the Eastern Alps; and may form passively in response to slip zones lasse basin: Global and Planetary Change, v. 58, p. 214– G. Houseman, University of Leeds, for his explanations of bounding the domes, and therefore widen the 223, doi:10.1016/j.gloplacha.2007.03.010. the thin sheet formulation. We also thank two anonymous Glodny, J., Ring, U., and Kühn, A., 2008, Coeval high-pressure number of tectonic regimes in which core com- reviewers, and A. Weil for editorial handling. metamorphism, thrusting, strike-slip, and extensional plexes may form. shearing in the Tauern Window, Eastern Alps: Tectonics, REFERENCES CITED v. 27, TC4004, doi:10.1029/2007TC002193. 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