balancing lateral orogenic float of the eastern alps

Balancing lateral orogenic float of the Eastern Alps

Hans-Gert Linzer a,*, Kurt Decker b, Herwig Peresson c,Rudi Dell’Mour c, Wolfgang Frisch d

aRohol-Aufsuchungs AG, Schwarzenbergplatz 16, A-1015 Vienna, AustriabInstitut fur Geologie, Universitat Wien, Althanstr. 14, A-1090 Vienna, Austria

cOMV-AG, Gerasdorfer Str. 151, A-1210 Vienna, AustriadInstitut fur Geologie, Universitat Tubingen, Sigwartstr. 10, D-72076 Tubingen, Germany

Received 18 January 2002; accepted 24 June 2002

Abstract

Oligocene to Miocene post-collisional shortening between the Adriatic and European plates was compensated by frontal

thrusting onto the Molasse foreland basin and by contemporaneous lateral wedging of the Austroalpine upper plate. Balancing

of the upper plate shortening by horizontal retrodeformation of lateral escaping and extruding wedges of the Austroalpine lid

enables an evaluation of the total post-collisional deformation of the hangingwall plate. Quantification of the north–south

shortening and east–west extension of the upper plate is derived from displacement data of major faults that dissect the

Austroalpine wedges. Indentation of the South Alpine unit corresponds to 64 km north–south shortening and a minimum of

120 km of east–west extension. Lateral wedging affected the Eastern Alps east of the Giudicarie fault. West of the Giudicarie

fault, north–south shortening was compensated by 50 to 80 km of backthrusting in the Lombardian thrust system of the

Southern Alps. The main structures that bound the escaping wedges to the north are the Inntal fault system (ca. 50 km sinistral

offset), the Konigsee–Lammertal–Traunsee (KLT) fault (10 km) and the Salzach–Ennstal–Mariazell–Puchberg (SEMP) fault

system (60 km). These faults, as well as a number of minor faults with displacements less than 10 km, root in the basal

detachment of the Alps. The thin-skinned nature of lateral escape-related structures north of the SEMP line is documented by

industry reflection seismic lines crossing the Northern Calcareous Alps (NCA) and the frontal thrust of the Eastern Alps.

Complex triangle zones with passive roof backthrusts of Middle Miocene Molasse sediments formed in front of the laterally

escaping wedges of the northern Eastern Alps. The aim of this paper is a semiquantitative reconstruction of the upper plate of

the Eastern Alps. Most of the data is published elsewhere.

Keywords: Orogenic float; SEMP; Eastern Alps

1. Introduction

The concept of orogenic float describes the mechan-

ical separation of a complexly deformed orogenic

wedge from the underlying lithosphere (Oldow et al.,

1989,1990). Reflection seismic profiles define the

geometry of detachment horizons, and balanced

cross-sections based on seismic data have been used

for the last three decades to estimate orogenic contrac-

tions (Bally et al., 1966). Balanced cross-sections are

typically constructed normal to the general strike to

PII: S0040 -1951 (02 )00337 -2

* Corresponding author.

E-mail address: hans-gert.linzer@rohoel.at (H.-G. Linzer).

www.elsevier.com/locate/tecto

Tectonophysics 354 (2002) 211–237

obtain the best fit to the geometry of structures, but in

many cases shortening and extension are oblique or

even parallel to the orogenic trend. Thus, estimation of

orogenic contraction with the aid of balanced cross-

sections reflects only a part of the total orogenic short-

ening. Contractional structures as well as folds and

thrusts are coeval with strike–slip faults which produce

an apparent volume loss in a 2D restoration. This

volume loss related to displacements out of the vertical

section, caused by lateral orogenic float, is balanceable

by 2D horizontal (map-scale) restorations of displace-

ments of strike–slip faults.

The Eastern Alps represent a thrust wedge that is

composed of the Austroalpine nappe system and

formed mainly by pre-collisional ESE to WNW stack-

ing in Cretaceous times and syn-collisional north–

south shortening in Paleogene times (Ratschbacher,

1986; Ring et al., 1989; Decker et al., 1993; Froitz-

heim et al., 1994, 1996; Linzer et al., 1995). Collision

of the Adriatic plate with Europe is documented by

the basal foreland unconformity in middle Eocene

times in the west (Salzburg area) and in late Eocene

to Oligocene times in the east (Bohemian spur)

(Nachtman and Wagner, 1987; Wessely, 1987; Stei-

ninger et al., 1988). Incipient collision with the Euro-

pean Plate is dated by the youngest sedimentary rocks

of Middle Eocene age (47 Ma) of the ‘‘Ultrahelvetic’’

nappes. The detachment and thrusting of the distal

European crust was synchronous with collision and

foreland subsidence. The Zentralgneiss cores and the

Lower Schieferhulle of the Tauern window show their

metamorphic pressure peak in late Eocene times

indicating their maximum depth of burial (42 Ma;

Blanckenburg et al., 1989; Froitzheim et al., 1996).

The collision of the Adriatic and the European plates

caused frontal and lateral wedging in the colliding

upper plate: frontal wedging corresponds to imbrica-

tions of the foreland basin, lateral wedging describes

the movements of strike–slip fault blocks as indicated

on the geological map (Fig. 1). Laterally escaping

wedges formed along the dextral Periadriatic and

sinistral Engadine lines. The Engadine line was active

in late Oligocene times (3–20 km displacement;

Trumpy, 1977) and probably continued to northeast

in the Inntal fault with an assumed total displacement

in the order of 75 km (Frisch et al., 1998). At the

frontal Alpine thrust in the northeast, the maximum

displacement of the Inntal fault is documented by the

offset of Molasse imbricates and the Rhenodanubian

flysch.

In this paper, we discuss the post collisional lateral

float of the Eastern Alps which is linked to extreme

orogen-parallel extension in the central part of the

Eastern Alps and to stacking in the northern Eastern

Alps. The Austroalpine basement complex was

strongly reactivated by Oligocene–Miocene lateral

eastward extrusion of wedges of Austroalpine base-

ment and cover nappes due to the indentation of the

South Alpine block (Ratschbacher et al., 1991a,b).

The eastward extruding Austroalpine wedge caused

unroofing of the Tauern window (Frisch et al., 1998)

by orogen-parallel extension and low-angle ductile

normal faulting (Selverstone, 1988; Ratschbacher et

al., 1991a,b). The Austroalpine nappe complex in the

inner part of the wedge east of the Tauern window is

detached along the contact to the Penninic units

(Ratschbacher et al., 1990, 1991a,b; Becker, 1993).

The detachment can be traced by reflection seismic

Fig. 1. The Salzachtal–Ennstal–Mariazell/Puchberg (SEMP) fault system line forms the northern border of the eastwards extruding central

Eastern Alps. The Upper Austroalpine nappes north of the SEMP line were dismembered into wedges by NE striking splays of the SEMP line:

(1)=Karwendel wedge; (2)=Kaiser –Watzmann wedge; (3)=Dachstein wedge; (4)=Warscheneck wedge; (5)=Haller Mauern wedge;

(6)=Reichraming wedge; (7)=Weyer Arc structure; (8)=Otscher wedge; (9)=Goller wedge; (10)=Wienerwald wedge; (11)=Schneeberg

wedge; (12)=Hochschwab wedge; (13)=‘‘Styrian wedge’’; (14)=Hochreichart wedge; (15)=Saualpe wedge; (16)=Mirnock wedge;

(17)=Kreuzeck wedge; (18)=Hochgall wedge.

AF=Ahrtal fault; AnF=Annaberg fault; BL=Brenner line; EL=Engadine line; GoF=Gostling fault; HoF=Hochstuhl fault; InF=Inntal fault;

IsF=Iseltal fault; KL=Katschberg line; KLT=Konigssee –Lammertal –Traunsee fault; LoF=Loisach fault; LS=Lower Schieferhulle;

MoF=Molltal fault; PeF=Pernitz fault; PLF=Palten–Liesing fault; PoF=Pols fault; PyF=Pyhrn fault; RTS=Radstadt thrust system;

RW=Rechnitz window; SaF=Salzsteig fault; TF=Telfs fault; WeF=Weyer fault; WGF=Windischgarsten fault; Z=Zell pull-apart structure;

ZC=Zentralgneiss core.

GoT=Goriach basin; PaT=Parschlug basin; SeT=Seegraben basin; FoT=Fohnsdorf basin; ObT=Obdach basin; WiT Wiesenau basin;

StT=St. Stefan basin.

A–AV=section Fig. 5; B–BV=section Fig. 7; C–CV=section Fig. 8.

H.-G. Linzer et al. / Tectonophysics 354 (2002) 211–237212

H.-G. Linzer et al. / Tectonophysics 354 (2002) 211–237 213

lines into the western Pannonian Basin (Tari and

Horvath, 1995). The wedge is bordered in the south

by the Periadriatic line and in the north by the

Salzach–Ennstal–Mariazell–Puchberg (SEMP) line

(Linzer et al., 1991).

The WSW–ENE oriented SEMP transform fault

extends for 400 km across the Eastern Alps. The fault

trends subparallel to the regional orogenic strike from

the western part of the Tauern window (Figs. 1 and 2)

to the Vienna Basin and crosses all Austroalpine

tectono-stratigraphic units. It is composed of the

Salzach fault, which forms the northern limit of the

Tauern window, the Ennstal fault, which runs along

the southern margin of the central NCA, and the

Mariazell–Puchberg line in the eastern segment of

the NCA. The recognition of the SEMP line as a

single continuous major strike–slip zone resulted

from analyses of the kinematic structures along nappe

boundaries of the southernmost nappes of the NCA

(Ratschbacher et al., 1991a,b; Decker et al., 1994a,b;

Linzer et al., 1995). The SEMP line provides a unique

opportunity to study an indentation-linked strike–slip

fault at different crustal levels exhumed along one

single fault zone (Fig. 1). The east–west striking fault

zone separates the Austroalpine and Penninic units of

the central Eastern Alps, which experienced substan-

tial orogen-parallel extension during the Oligocene

and Miocene, from the cover nappes of the northern

Eastern Alps (Northern Calcareous Alps, Rhenodanu-

bian flysch and Helvetic units; Fig. 1). The fault zone

forms the lateral ramp of west- and east-directed

detachment faults that accounted for the tectonic

exhumation of the Tauern metamorphic dome during

the Oligocene to the Middle Miocene. Deeper struc-

tures of the fault were exhumed in the west by the late

Neogene uplift of the Tauern window. Ductile defor-

mation structures adjacent to the Tauern window in

the west change eastward into a narrow zone

deformed under ductile–brittle transitional conditions,

whereas the central and eastern segments show brittle

deformation that is distributed over a broad shear zone

in the southern part of the Northern Calcareous Alps

(NCA). This zone is composed of extensional and

compressional flower structures (Lowell, 1972; Wil-

cox et al., 1973; Sylvester, 1988), strike – slip

duplexes (Woodcock and Fischer, 1986) and en-eche-

lon splay faults that are partially linked to major

thrusts (Fig. 1).

2. Regional setting

2.1. Northern Eastern Alps and the Molasse foreland

The sequences north of the SEMP line are part of a

thin-skinned fold-thrust belt with the following main

tectono-stratigraphic units (Fig. 2; from base to top):

the autochthonous European basement and the

Molasse foreland basin, Molasse imbricates, the Hel-

vetic cover nappes that were stripped from the Euro-

pean margin (lower plate), the South Penninic and

North Penninic (Rhenodanubian flysch) nappes with

partially ophiolitic remnants of the Penninic Ocean,

and the Austroalpine cover nappes that derive from

the upper colliding plate (compare sections by

Wessely et al., 1993).

The Austroalpine cover nappes consist of weakly

metamorphosed Paleozoic sequences (Innsbruck

quartz phyllite, Grauwacken zone) and the unmeta-

morphic Permo-Mesozoic sequences of the Northern

Calcareous Alps (NCA) which are partially in strati-

graphic contact with the Paleozoic units. The NCA are

composed of 3–5 km thick sequences of a Permo-

Mesozoic passive margin (Tollmann, 1976; Lein,

1987). Competent Triassic platform carbonates alter-

nate with incompetent marls and evaporite series

forming major detachment horizons and well-defined

seismic reflectors. Shortening in the NCA which

occurred in separate phases during the Cretaceous

and the Eocene is between 55% and 65% (Eisbacher

et al., 1990; Linzer et al., 1995). The Gosau Group

represents Upper Cretaceous to Eocene sedimentary

rocks deposited on the Austroalpine nappe system.

The lower part consists of freshwater to shallow

marine sedimentary rocks and the upper part of

deep-water sedimentary rocks that are related to a

sudden deepening of the whole NCA (Ampferer et al.,

1918; Decker et al., 1987; Faupl and Wagreich, 1992;

Wagreich, 1995).

The Penninic (Rhenodanubic) flysch north of the

NCA marks the main Alpine suture zone between the

Austroalpine units of the upper plate and the units that

were derived from the subducted margin of the Euro-

pean plate. The upper flysch nappes are composed of

an ophiolite-bearing melange with Lower to Upper

Cretaceous sedimentary rocks assigned to the South

Penninic ocean, which was overthrust during the

Cretaceous (Arosa and Ybbsitz flysch nappes; Schna-

bel, 1979; Decker, 1990; Ring et al., 1988). The

underlying North Penninic units of the Rhenodanubic

flysch (Early Cretaceous to Early Eocene; Schnabel,

1992) were overthrust to the north in the Early to

Middle Eocene (Decker et al., 1993). The Rhenoda-

nubic nappes display regular ramp-flat geometries

with flats following marl formations and ramps cut-

ting through sandstone-rich flysch.

The Helvetic nappes are composed of Upper Creta-

ceous to Middle Eocene clastic rocks that overly

Jurassic passive margin sedimentary rocks of the

subducted European continental margin. The young-

est sedimentary rocks of the uppermost Helvetic units

date the onset of continental collision at about 47 Ma

(Decker and Peresson, 1996). The Helvetic nappes are

thrust over imbricates of Oligocene to Lower Miocene

allochthonous Molasse units.

The autochthonous units of the European margin

are composed of sequences that are similar to the

Helvetic nappes. Jurassic passive-margin sediments

are unconformably overlain by Late Cretaceous to

Eocene clastic rocks. These sequences form promi-

nent reflections that are traced on seismic reflection

lines and proved by deep wells; they dip southward

under the Alpine nappes into the region below the

trace of the SEMP line (Wachtel and Wessely, 1981;

Hamilton, 1989; Wessely et al., 1993). Eocene rocks

were drilled in the Berndorf well, more than 30 km

south of the Alpine thrust front (Wachtel and Wessely,

1981). The Molasse foreland basin is composed of

Oligocene to Miocene sequences (Bachmann et al.,

1987; Bachmann and Muller, 1991; Nachtman and

Wagner, 1987; Wessely, 1987). Thrusting of the

Molasse toward the European foreland continued until

about 17 Ma (age of the youngest overthrust sedi-

mentary rocks). The post-collisional shortening

between 47 and 17 Ma (Decker and Peresson, 1996)

therefore interfered with orogen-parallel motion along

the SEMP line and with orogen-parallel detachment

faulting in the area of the Tauern window south of the

SEMP line.

2.2. Central Eastern Alps

The Austroalpine nappe complex (AA) is com-

posed of polymetamorphic basement complexes,

weakly metamorphosed Paleozoic cover sequences

of the Innsbruck and Landeck quartz phyllite, the

Grauwacken zone, and Permo-Mesozoic passive-mar-

Fig. 2. Generalized tectono-stratigraphic cross-section of the Eastern Alps, Molasse foreland basin and European basement. Numbers indicate

deformation and denudation ages in Ma.

gin sequences (equivalent to the NCA). The basement

complexes (e.g., Otztal complex), the Paleozoic meta-

sedimentary rocks of the Grauwacken zone and the

Innsbruck quartz phyllite show penetrative Variscan

deformation and Alpine shear zones (Satir and Mor-

teani, 1978; Neubauer et al., 1995). Top to WNW

stacking of the Austroalpine nappe complex in Creta-

ceous times (Ratschbacher, 1986) was followed by

N–NW directed thrusting in the Early Tertiary (Ring

et al., 1989; Decker et al., 1993). The Austroalpine

basement nappes of the central Eastern Alps, south of

the SEMP line, were affected by Oligocene–Miocene

orogen-parallel extension and normal faulting along

east and west dipping low-angle detachment faults

(Selverstone, 1988; Genser and Neubauer, 1989;

Ratschbacher et al., 1991a,b; Mancktelow et al.,

1995; Tari, 1996). East and west of the Tauern

window, the Austroalpine is detached at the contact

with the Penninic units. The Penninic sequences

formed a metamorphic dome that was tectonically

exhumed in Oligocene to Early Miocene times (Frisch

et al., 1998).

The Tauern window is composed of metasedi-

mentary rocks and granitoids subdivided into three

units: the Zentralgneiss core (ZC), the Paleozoic–

Mesozoic Lower Schieferhulle (distal European

crust) and the Mesozoic Upper Schieferhulle, which

represents transitional and oceanic sequences of the

Penninic realm (Frisch, 1974, 1977, 1980a,b; Frisch

et al., 1987). Alpine metamorphism in the Tauern

window indicates a pressure peak of about 10–14

kbar at a temperature of about 550 jC during

prograde metamorphism (Selverstone, 1988; Selver-

stone et al., 1991; Brunsmann et al., 2000). HP-

metamorphism is only poorly constrained by geo-

chronological data which indicate a range between

about 60 and 30 Ma (Paleocene to Oligocene; Cliff

et al., 1985; Blanckenburg et al., 1989; Zimmermann

et al., 1994; review by Genser et al., 1996). Blue-

schist facies phengites from the Schieferhulle of the

southern Tauern window revealed 39Ar–40Ar ages

between 36 and 32 Ma, which are dated as crystal-

lization ages (Zimmermann et al., 1994). Whole-rock39Ar–40Ar ages from Lower Austroalpine units in

the northern part of the Tauern window in the range

of 51–37 Ma (Dingeldey et al., 1997) are also

assumed to date peak metamorphic conditions.

Christensen et al. (1994) reported Rb/Sr ages from

zoned garnets from the Schieferhulle. Final garnet

growth, which corresponds to the thermal peak

conditions, occurred at about 30 Ma. By time cali-

bration of P–T paths, Christensen et al. (1994) argue

that rapid decompression of the Schieferhulle related

to extensional shearing which occurred at about 35

to 30 Ma (Early Oligocene). K–Ar-ages of white

mica, which reflect cooling ages and time–temper-

ature paths, show that cooling postdates the HP-

event and subsequent decompression (Cliff et al.,

1985; Blanckenburg et al., 1989). K–Ar data for

white mica indicate the onset of cooling at about

28–25 Ma (Late Oligocene) for the higher structural

units and 20–17 Ma (Early Miocene) for the internal

part of the window (Zimmermann et al., 1994, and

references therein). The decompression and exhuma-

tion of the metamorphic rocks resulted from orogen-

parallel extension and tectonic unroofing of the

Tauern window by detachment faulting along the

Brenner fault (Selverstone, 1988; Ratschbacher et al.,

1991a,b; Mancktelow et al., 1995) and the Katsch-

berg line (Genser and Neubauer, 1989). As the first

stages of decompression may have occurred under

nearly isothermal conditions due to advective heat

transport, cooling ages only give minimum ages for

the onset of tectonic unroofing which must have

occurred between about 30 Ma (youngest HP-ages)

and about 27 Ma (onset of cooling). Dating of

transpressional deformation of mylonitic marbles

along the Salzachtal fault show Ar/Ar ages between

28 and 35 Ma (Urbanek, 2001). These time brackets

also constrain the onset of movement along the

SEMP strike–slip system which we regard as the

lateral ramp of the major detachment faults exhum-

ing the Tauern window.

East of the Tauern window, the major detachment

cuts through Austroalpine units and accounts for the

Miocene exhumation of Penninic units in the Rechnitz

metamorphic core at the transition of the Eastern Alps

to the Pannonian basin system (Tari et al., 1996). This

easternmost detachment can be traced on reflection

seismic lines into the western Pannonian Basin over a

distance of about 150 km (Tari and Horvath, 1995).

The Penninic sequences below the detachment are

composed of dismembered ophiolite sequences (Kol-

ler and Pahr, 1980; Pahr, 1984) and metasedimentary

rocks comparable to the Schieferhulle of the Tauern

window. The Penninic rocks record polyphase meta-

Fig. 3. Loisach (LoF) and Inntal (InF) strike–slip systems of the western part of the northern Eastern Alps. Dashed lines indicate displacements along strike–slip faults.

KLT=Konigssee–Lammertal –Traunsee fault; OT=Otztal thrust; TF=Telfs fault; IT=Inntal Tertiary. Location in Fig. 1.

.Linzer

etal./Tecto

nophysics

354(2002)211–237

Fig. 4. The Salzach–Ennstal segments of the SEMP line and displacement transfer structures in the NCA: NE-ward escaping Dachstein wedge shows at its border older

transpressional and younger transtensional features. A–AVreflection seismic line Fig. 5; GT=Grobming Tertiary; KLT=Konigsee–Lammertal –Traunsee strike–slip system;

MZ=Mandling Zug; SaF=Salzsteig fault; WT=Wagrain Tertiary; Z=Zell pull-apart structure. Location in Fig. 1.

.Linzer

etal./Tecto

nophysics

354(2002)211–237

morphism including ocean floor metamorphism, a

high-pressure event (330–370 jC, > 8 kbar) and a

second thermal overprint that indicates rapid decom-

pression (390–430 jC, lower than 3 kbar; Koller,

1985). Geochronologic data indicate the onset of

cooling at about 22–19 Ma (Early Miocene; Koller,

1985; (Tari et al., 1996), which may be slightly

younger than the onset of cooling in the Tauern

window.

2.3. Intraorogenic basins of the Eastern Alps

Oligocene to Miocene intraorogenic pull-apart and

transtensional basins of the Eastern Alps facilitate the

separation and dating of tectonic events. Oligocene

(ca. 33–23 Ma) sedimentary rocks occurring along

the Inn Valley (Inntal Tertiary, IT: Fig. 3) allowed a

detailed analysis of Tertiary tectonic events (Ortner

and Sachsenhofer, 1996). Early to Middle Miocene

(Karpatian to Badenian; ca. 17–15 Ma) clastic sedi-

mentary rocks are situated along the Salzachtal and

Ennstal segments of the SEMP fault (Wagrain, WT:

Fig. 4, Grobming, GT: Fig. 4, and Hieflau, HB: Fig.

6). The sedimentary rocks of the Hieflau basin repre-

sent a coarsening upward sequence of clastics of

Ottnangian to Karpatian age (ca. 18–17 Ma) and are

subsequently deformed (Ampferer, 1921; Winkler,

1928; Wagreich et al., 1997; Frisch et al., 1998).

The formation of pull-apart basins along the Mur–

Murz fault system (Goriach, GoT, Parschlug, PaT,

Seegraben, SeT, Fohnsdorf, FoT: all Fig. 1) and along

the Lavanttal fault (Obdach, ObT, Wiesenau, WiT, St.

Stefan, StT: all Fig. 1; Steininger et al., 1989) has

been related to the onset of strike–slip faulting. The

Early Miocene fluviatile – limnic lignite-bearing

sequences of the basins are overlain by Early Bade-

nian (ca. 16 Ma) marine sedimentary rocks (Steininger

et al., 1989).

Table 1

Fault Marker Min. offset

Max. offset

Reference

Engadine fault Austroalpine floor thrust, Oberhalbstein 3 20 Trumpy, 1977

Engadine fault S Penninc ophiolites, Oberhalbstein Schmid and Froitzheim, 1993

Inntal fault S margin of Calcareous Alps 34 48 this paper

Inntal fault Paleozoic Landeck– Innsbruck quarz phyllite 31 50 this paper

Loisach fault system Floor thrust of Calcareous Alps,

Wetterstein fm

10 15 Linzer et al., 1995

KLT Floor thrust of Calcareous Alps 7 5 Decker et al., 1994a,b

KLT S margin of Calcareous Alps 10 15 Decker et al., 1994a,b

Salzsteig fault–Warscheneck thrust Overthrust Gosau group 6 this paper

Pyhrn fault–Haller Mauern thrust Dachsteinkalk Fm., Hallstatt limestones 28 30 this paper

Palten–Liesing fault Greywacke unit, Wolz micaschists 8 8 this paper

Weyer fault Floor thrust of Calcareous Alps 1.5 Decker et al., 1994a,b

Gostling fault Gams Gosau group,

Floor thrust of Calcareous Alps

7 1.5 Linzer et al., 1995;

Decker et al., 1994a,b

Annaberg fault Eocene Hochwart fault 2

Pernitz fault Helvetic windows,

Carnian sedimentary rocks

(Raibl Fm.), S margin of Calcareous Alps

11.5 12 Decker 1996

Molltal fault Austroalpine basement, triassic cover 20 45 this paper

Iseltal fault Kreuzeck crystalline 15 20 this paper

Hochstuhl fault Periadriatic fault 20 20 Polinski and Eisbacher, 1992

Pols fault Wolz micaschist 12 Decker et al., 1994a,b

Lavanttal fault Koralm–Saualm crystalline,

Periadriatic fault

12 12 this paper

Ennstal fault (SEMP line) Mafic dykes of the quarzphyllite, 60 70 this paper

Mariazell–Puchberg fault

(SEMP line)

Carnian sedimentary rocks (Raibl Fm.) this paper

Mur–Murz line Austroalpine sedimentary rocks 28 33 this paper

3. Kinematic data: Tauern window and central

Eastern Alps

The nappe system of the Tauern window (Fig. 1)

forms a large-scale crustal antiform with WSW–

ENE-striking foliations. The stretching lineations in

the western TW show variable axial plunge (Frisch,

1980a,b; Reicherter et al., 1993): the stretching

lineations in the southern area plunge to the WSW,

the lineations in the northern area to ENE. In both

areas a general left-lateral shear is observed. Major

shear zones occur along the margins of the Zentral-

gneiss core (ZC) and the contact with the Lower

Schieferhulle (LS). The shear zones are subparallel

to the general strike and merge with the SEMP line.

Left-lateral shear sense is documented in S–C fab-

rics, dynamically recrystallized quartz and s-type

clasts in mylonitic gneisses, feldspar clasts (d-type

porhyroclasts) and asymmetric boudins. Asymmet-

ric quartz c-axis fabrics indicate sinistral rotation

(Reicherter et al., 1993; Hermann, 1989). The folia-

tions in the eastern TW show a general WNW–ESE

trend, whereas the stretching lineations plunge to

the ESE (Becker, 1993). This is also observed in

the Austroalpine Radstadt thrust system (RTS, Fig.

1). The RTS was separated by a low-angle detach-

ment fault from the Penninic series in the footwall.

Both show common mineral cooling ages of about

16 Ma which in the footwall of the detachment

fault are related to uplift and cooling (retrograde

path) and in the hangingwall of the detachment fault

to prograde heating due to subsidence caused by

normal faulting (Becker, 1993). The normal faults

which are related to the low-angle detachment fault

have a general east to southeast dip and foliation

parallel stretching increases from about 6% in the

west to 87% in the southeast (Becker, 1993). Exten-

sion in the RTS is compensated in a broad zone of

deformation with major displacement along the Pen-

ninic/Austroalpine detachment. Top-to-the-east

extension south of the RTS is concentrated on the

Katschberg line, which forms the major detachment

between the Austroalpine and Penninic units (Genser

and Neubauer, 1989; Ratschbacher et al., 1991a,b).

The eastwards extruding Austroalpine units were

dismembered in horizontal wedges (Fig. 1; Nos.

13–18) by right-lateral splays of the Periadriatic

line (Fig. 1; Isltal fault, Molltal line, Hochstuhl

fault, Lavanttal fault). Right-lateral displacements of

stratigraphic markers range between 45 and 12 km

(Table 1).

4. Kinematic data: northern Eastern Alps

The northern Eastern Alps were dismembered by a

set of northeast trending left-lateral strike–slip faults

in horizontal, northeastward escaping wedges (Fig. 1;

Nos. 1–12) which were floating on the basal detach-

ment of the Eastern Alps. The escaping wedges show

internal extensional features, e.g., the Zell pull-apart

structure (Z) of wedge 3 as well as contractional

structures, e.g., the Weyer thrust between wedge 6

and 7. The strike–slip faults are forming displacement

transfer structures between the eastwards extruding

central Easter Alps (extrusion=effect of gravitational

collapse and lateral escape, see Ratschbacher et al.,

1991a,b) and the northeastwards escaping northern

Eastern Alps.

4.1. Engadine– Inntal fault system and related

displacement transfer structures

The Engadine line (Fig. 1; EL) shows an increas-

ing displacement from 4–5 km in the southwest to 20

km in the area of the Engadine window (Trumpy,

1977; Schmid and Froitzheim, 1993). Farther to the

northeast, we speculate that the Engadine line is

covered by the out-of-sequence thrust of the Otztal

complex (Linzer et al., 1995) and joins the Inntal

fault (Fig. 1; InF). The out-of-sequence thrust of the

Otztal complex (Fig. 3; OT) turns into the WNW

trending dextral Telfs strike–slip fault (Fig. 3; TF)

in the upper Inn Valley along which 10 to 15 km

displacement has occurred when comparing the inter-

nal structures and marker horizons of the Inntal

nappe. Left-lateral displacement of the Inntal fault is

indicated by 58 km total offset of stratigraphic

markers (middle Triassic clastic beds) along the

southern margin of the Northern Calcareous Alps

(NCA) and 55 km offset of the Tirolean/Bavarian

nappe boundaries (Fig. 3; Table 1). The Inntal fault

has in its northeastern continuation 65 km displace-

ment of the Lower/Upper Bavarian nappe bounda-

ries (Fig. 4). Internal structures of the Helvetic and

Rhenodanubian flysch nappes and the flysch/Molasse

boundary were displaced 48 km (Egger, 1997). The

Inntal fault dies out in the Molasse foreland basin. The

Engadine–Inntal fault system shows, from southwest

to northeast, increasing displacements up to the NCA/

flysch boundary and decreasing displacements farther

to the northeast.

The Inntal fault forms the western margin of

wedge 2, which was thrust over the imbricated

Molasse foreland basin. The seismic reflection line

A–AV(Fig. 5; for location see Fig. 4) across the

frontal hinterland vergent wedge shows complex

triangle structures of the imbricated part of the

Molasse foreland basin (Wagner et al., 1986). The

line crosses the Oberhofen well northeast of Salzburg

and shows the folded and truncated early Miocene

sequences at the southern margin of the Molasse

foreland basin.

4.2. Konigsee–Lammertal–Traunsee (KLT) displace-

ment transfer structures of the Salzach and Ennstal

fault segments

The 110-km-long sinistral KLT converges with

the sinistral Salzachtal–Ennstal fault north of the

Tauern window across the Zell pull-apart structure

(Fig. 3). The fault crosscuts the entire Calcareous

Alps (Fig. 4) and is characterized by a two-stage

development encompassing the propagation of a

curved fault which changes strike direction from

N70jE in the south to N30jE in the north, and the

overthrust of the central part of the fault by the

northern tip of the Dachstein nappe. The southern

fault segment encompasses a number of restraining

bends with sinistral transpressive deformation. Fault

patterns depict contractional strike–slip duplexes,

positive flower-structures and high-angle Riedel

shears accompanying the main fault. Rocks within

duplexes and flower-structures are uplifted between

700 and 2000 m. The shear zone links up with

several subparallel northeast– southwest striking,

non-transpressive sinistral faults in the northern part

of the NCA. The sinistral offset along the fault

system is estimated at 7–10 km from the offset of

the southern and northern margin of the NCA

(Decker et al., 1994a,b). These values match the

7.4 km offset which was computed by the modelling

of transpressional strain at the Rigaus restraining

bend (Fig. 4). Transpression modelling of the con-

vergent duplex array further west allowed the com-

putation of the minimum depth of detachment of

these duplexes which accordingly lies about �2 km,

i.e., well within the Calcareous Alps, probably coin-

ciding with the sole thrust of the Tirolic Nappe unit

(Decker et al., 1994a,b).

4.3. The SEMP line and linked structures

4.3.1. Salzach fault

The Salzachtal fault forms the western segment of

the SEMP line and runs along the northern edge of

the Tauern window (Figs. 3 and 4). The Salzach

fault is probably rooted in sinistral shear zones, e.g.,

Ahrtal fault (Fig. 1; AT) through amphibolite to

upper greenschist grade Penninic rocks. The Salzach

fault forms a 50- to 100-m-wide, clearly defined,

steeply dipping mylonitic shear zone in the west.

Eastward, the fault shows a change of ductile to

brittle deformation (Hermann, 1989). Ductile–brittle

deformation structures are indicated in extensional

crenulation cleavage structures. Brittle faults devel-

oped on foliation planes indicate transpression (obli-

que left-lateral thrusting). East plunging striations are

overprinted by left-lateral WNW plunging striations

indicating transtension (oblique left-lateral exten-

sional strike–slip faults). In both cases, the contrac-

tional axes show northeast orientations (Hermann,

1989).

4.3.2. Ennstal fault

The Ennstal fault segment of the SEMP line runs

from the Wagrain Tertiary (WT; Fig. 4) in the west

along the Mandling–Zug (MZ; Fig. 4) to the Hieflau

basin (HB; Fig. 6). This segment cuts the upper

crustal level and shows increasing widening of the

fault zone from west to east. In this segment, the fault

is slightly curved, which produced both extensional

and compressional strike–slip duplexes.

West of Admont, the northeast striking Pyhrn

(PyF) and Salzsteig (SaF) faults branch off from

the major fault (Fig. 6). These faults are kinemati-

cally linked to northeast-directed thrusts. East of

Admont, the major fault zone cuts into the NCA

and branches into a broad zone of large scale strike–

slip duplexes that are 2 to 6 km wide and about 10

km long. The Ennstal fault forms the central segment

of the SEMP line with the largest displacement,

Fig. 5. Seismic expression of the frontal wedge of the Eastern Alps. Location in Fig. 4. (A) NE–SW section (courtesy of Rohol-Aufsuchungs,

Vienna; well Oberhofen 1, see Wagner et al., 1986). (B) Geologic interpretation.

Fig. 6. The Ennstal segment of the SEMP line, displacement transfer structures in the NCA and orientation of reflection seismic lines: B–BVreflection seismic line Fig. 7; C–CVreflection seismic line Fig. 8; Ai=Admont imbricates; AnF=Annaberg fault; BF=Barntal flower structure; BS=Bosenstein complex; GoF=Gostling fault; HB=Hieflau basin;

HM=Haller Mauern; PLF=Paltental –Liesing fault; PyF=Pyhrn fault; SaF=Salzsteig fault; W=Warscheneck; WB=Weyer Arc; WeF=Weyer fault. Location in Fig. 1.

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indicated by the 60 km left-lateral offset of the

boundary between quartz phyllite units and the

Grauwacken zone (Fig. 1).

4.3.3. Mandling–Zug structure

The Mandling–Zug (MZ; Fig. 4) is situated

between quartz phyllite units and the Grauwacken

zone and consists of Triassic limestone dolomites

(Mandl, 1987). The MZ represents a fragment of

the NCA that is bordered by branches of the

central segment of the SEMP line, forming an

extensional strike–slip duplex. Its position within

the Grauwacken zone and partly between the Grau-

wacken zone and the quartz phyllite indicates that

the area was originally covered by the NCA. Fault

striae data indicate an older compressional and a

younger extensional strike–slip event (Linzer et al.,

1995, 1997). The major fault runs along the north-

ern margin of the MZ and shows sinistral trans-

tension.

4.3.4. Warscheneck, Haller Mauern, Bosenstein and

Eisenerz–Hochschwab structures

The Warscheneck (W; Fig. 6) and Haller Mauern

(HM; Fig. 6) represent up to 4000 m thick competent

blocks of Triassic carbonates that were bordered by

left-lateral splays of the SEMP line and thrust to the

northeast (Decker et al., 1994a,b). The sinistral

Salzsteig fault (SaF; Figs. 4 and 6) merges into the

northeast-directed thrust of the Warscheneck nappe

which exhibits a minimum thrust distance of 6 km

over the Late Cretaceous Gosau Group. For the

Pyhrn fault (PyF; Fig. 6) a minimum of 28 km

left-lateral motion is estimated from offset slices of

the Dachstein nappe. To the northeast, the fault

merges into the northeast-directed thrust plane of

the Haller Mauern nappe (HM; Fig. 6). The entire

sinistral offset of the Pyhrn fault is compensated in

the Haller Mauern nappe by northeast-directed

thrusts and southwest-directed backthrusts in the

Admont imbricates (AS; Fig. 6). South of the main

strand of the Ennstal fault, a block of Austroalpine

basement (Bosenstein complex, BS; Fig. 6) was

indented into the Grauwacken zone and the NCA.

These basement units were moved between the Pyhrn

fault and the NNW-striking Pols fault (PoF; Fig. 6)

which shows 12 km of dextral displacement. The

translation path which is computed from the strike of

the Pyhrn and Pols fault and from the displacements

along the faults matches the thrust direction of the

crystalline units onto the Grauwacken zone. The

Eisenerz–Hochschwab wedge (EHW; Fig. 6) east

of the Bosenstein complex is bordered in the north

by the Ennstal branch of the SEMP line and in the

south by the dextral Palten–Liesing fault (PLF; Fig.

6). The total displacement along the SEMP line of 60

km is partitioned to a broad zone of strike–slip

duplexes, each of them is displaced several kilo-

meters (Fig. 6).

4.3.5. Tertiary of Hieflau and Barental flower-

structure

Lower Miocene (Karpatian, ca. 16 Ma) clastic se-

dimentary rocks near Hieflau (HB; Fig. 6) are

situated south of the Ennstal fault segment (Wagreich

et al., 1997). Structures related to sinistral shearing

along the fault are composed of east–northeast

striking sinistral faults, northeast-striking subvertical

tension gashes, and southwest-directed thrust faults

that dip towards the main Ennstal fault. These

deformed sedimentary rocks provide the only

straightforward evidence for Miocene (post-Karpa-

tian, <17 Ma) fault activity along this segment of the

SEMP line (Peresson and Decker, 1997a,b). East of

Admont, the SEMP line forms the boundary of the

Juvavic nappe complex (Fig. 6). Fault geometry

defines a positive flower structure (Barental, BF;

Fig. 6) that is bound by convex up, oblique–reverse

sinistral faults which dip southeast and northwest

towards the center of the structure. In the center of

the flower structure, uplifted Permian and Lower

Triassic evaporites and shales are sheared between

Upper Triassic carbonates. Deformational structures

are dominated by subvertical sinistral faults in the

carbonates and by S–C fabrics and shear bands in

the shaly formations.

4.3.6. Mariazell–Puchberg fault

The Mariazell–Puchberg fault (Spengler, 1931)

forms the eastern segment of the SEMP line. The

total displacement along the SEMP line in the eastern

segment of the NCA is about 40 km, distributed on a

broad strike–slip zone and indicated by the offset of

Carnian, Upper Jurassic and Gosau Group type

sedimentary rocks. Characteristic features of the east-

ern segment are NNE-striking splays of the main

Fig. 7. Seismic expression of the ‘‘post-Gosau’’ thrust of the Weyer Arc structure. Location in Fig. 6. (A) N–S section (courtesy of OMV–AG, Vienna). (B) Geologic interpretation.

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fault that transect the NCA into wedges. A part of

the displacement is transferred to the north by NNE-

trending sinistral strike–slip faults with decreasing

displacements from south to north, e.g., the Weyer

fault (WeF; Fig. 6) with a 1.5-km offset and the

Gostling fault (GoF; Fig. 6) with a displacement of 5

km in the south, 2 km in the central part and 1.5 km

in the north (Linzer et al., 1995). Both faults exhibit

a north–south striking releasing segment in the

central part of the NCA. Transtension along these

segments is compensated by east- to ENE-directed

normal faults that dip towards the main fault. In

addition, the Gostling fault system is composed of

several arrays of extensional strike–slip duplexes

that are up to 5 km long (Fig. 6). Northeast-directed

thrusting south of the SEMP line in the Gosau

sedimentary rocks is similar to that of the Weyer

Arc structure. The SEMP line ends at the eastern end

of the NCA in the Vienna basin (Fig. 1).

4.4. Weyer Arc structure

One of the most prominent features of the NCA is

the Weyer Arc structure (Weyerer Bogen, WB; Fig.

6). The WSW–ENE general strike direction of the

Cretaceous thrusts and folds of the Frankenfels and

Lunz nappe system gradually changes counterclock-

wise in a 90j arcuate structure to the NNW–SSE.

This arcuate structure was thrust over the Reich-

raming nappe system (Fig. 8) where folds and thrusts

show general east–west trends and are covered by

Late Cretaceous to Paleocene Gosau sediments

(Faupl, 1983). The formation of the arcuate structure

was initiated with the separation of the Reichraming

and the Frankenfels–Lunz nappes along a major

northwest-trending dextral strike–slip fault with 10

km minimum offset (Decker et al., 1994a,b). Middle

Eocene Helvetic marls that were sheared along this

fault provide the upper time bracket for this defor-

mation. Subsequent thrusting of the Frankenfels–

Lunz nappe system over the Reichraming nappe

system reactivated this shear zone as a reverse fault.

Thrusting followed a complex translation path that

curved from (initial) top-to-west to (final) top-to-

south. Increments of this path are depicted by calcite

fibers on the thrust planes. Changing slip directions

are accompanied by a 90j counterclockwise rotation

of the overriding nappe (Decker et al., 1994a,b).

West-directed thrusts also splay into the footwall and

transect the older structures of the Reichraming

nappe. The well Unterlaussa 1 (Fig. 8) 6 km east

of the outcrop trace of the footwall thrust of the

Weyer Arc, penetrated the Frankenfels–Lunz system,

the Gosau Group overlying the Reichraming nappe,

and terminated within Late Triassic carbonates of the

Reichraming nappe. The well proves that there was a

minimum of 6 km of west-directed thrusting of the

Weyer Arc structure. The Weyer Arc structure is

transected by the NNE-striking sinistral Weyer fault

(Fig. 6, WeF) that branches off from the Ennstal fault

and by very prominent east directed normal faults

that partially reactivate the older west-directed thrust

planes. These normal faults compensate divergent

movement along a large-scale, north–south striking

releasing bend of this major sinistral strike–slip

fault.

Reflection seismic lines cross the Gosau Group

from north to south (Line B–BV; Fig. 7) and from

east to west (Line C–CV; Fig. 8). The Gosau beds

seal most of the internal thrust systems of the NCA.

Line B–BVshows a nice example of a ‘‘post Gosau’’

thrust (Wenger thrust; Fig. 7): Triassic dolomites

were thrust to the north onto Eocene–Paleocene

Gosau beds along an east–west to ESE striking fault.

This thrust shows a similar orientation to the War-

scheneck thrust and was reactivated (indicated by

Calcite fibers; Decker and Peresson, 1996) by thrust-

ing to the northeast, but we do not believe that this

motion added much to the 10 km total offset along

the thrust. The east–west oriented reflection seismic

line C–CVcrosses the Weyer Arc structure (Fig. 8).

The Gosau beds run up to 15 km to the east below

Triassic carbonates and shales. The structure is veri-

fied by the Unterlaussa well, which reached the

Gosau beds at a depth of about 2000 m. The Gosau

beds are about 1000 m thick and are cut by a

probably north–south striking normal fault along

which a displacement of about 1000 m occurred

and which roots in the sole thrust of the NCA. A

similar normal fault was observed north of the

Warscheneck block (Linzer et al., 1995). The ‘‘post

Gosau’’ thrust of the Weyer Arc is cutting the normal

fault and is rooted, as is the normal fault, in the sole

thrust of the NCA. This clearly indicates the thin-

skinned nature of the laterally escaping wedges of the

Fig. 8. Seismic expression of the post-Gosau thrust of the Weyer Arc structure. Location in Fig. 6. (A) E–W section (courtesy of OMV–AG, Vienna). (B) Geologic interpretation.

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4.5. Vienna basin and the transition to Pannonian

basin system

The Upper to Middle Miocene sedimentary rocks

that unconformably cover the Alpine nappes in the

Vienna basin and its surroundings allow establishment

of a detailed chronology of deformation events at the

Alpine–Carpathian transition (Fodor, 1995; Decker,

1996). Structural analysis indicates two major tectonic

stages in the Early to Middle Miocene evolution of the

basin and the surrounding units.

(1) Northwest-directed thrusting and convergent

wrenching along ENE-striking fault zones during

final foreland imbrication that lasted up to the

Karpatian (ca. 17 Ma). The deformation is dated

by shallow piggy-back basins (Egerian–Early Kar-

patian, ca. 22–17 Ma) in the northern part of the

Vienna basin and in the Western Carpathians. These

basins are characterized by WSW to east-trending

axes (Sauer et al., 1992) paralleling convergent

wrench faults that formed the basin boundaries

and by synsedimentary thrusting and wrench fault-

ing (Kovac et al., 1989; Marko et al., 1991). The

ENE-trending sinistral wrench faults are correlatable

with shear zones in the easternmost Calcareous

Alps and the flysch zone (Fig. 9). These wrench

zones are composed of convergent strike–slip

duplexes of about 1 to 5 km length. Several major

faults display convex-up geometries and oblique–

reverse–slip vectors typical for convergent flower-

structures. The most important shear zones coincide

with a narrow zone in the Rhenodanubian flysch

where Helvetic sequences from the substratum were

squeezed upwards into an array of convergent

duplexes. Left-lateral shear-zones follow the north-

ern margin of the Goller nappe and link up with

the Mariazell–Puchberg and Hohe Wand fault (Fig.

9) in the southern part of the Calcareous Alps

(Linzer et al., 1995, 1997). At the Hohe Wand

range, Triassic sequences were thrust over folded

and overturned Upper Cretaceous sedimentary rocks

of the Gosau Group (Fig. 9). Within the convergent

structures at the northern margin of the Goller

Nappe, sequences from the deep stratigraphic levels

of the Calcareous Alps (Permian evaporites, Upper

Triassic marls) and from the Helvetic units which

underlay the Calcareous Alps were uplifted. The

presence of Helvetic sequences and the absence of

deeper overthrust Molasse units indicate that the

convergent faults root in the floor thrusts of the

Rhenodanubian flysch and the Calcareous Alps.

Sinistral offsets cannot be estimated as the faults

parallel regional strike. In the Western Carpathians,

sinistral convergent wrenching is time constrained

by faulted and thrust Eggenburgian to Karpatian

rocks (Marko et al., 1991).

(2) During late Early Miocene to Middle Miocene

(Late Karpatian to Pannonian, ca. 17–8 Ma), NNE-

trending sinistral fault zones transsected the older

convergent faults. Sinistral faults extended from the

central Eastern Alps into the outer Western Carpathi-

ans and formed the boundary of the extruding West

Carpathian/west Pannonian wedge. In the Vienna

basin area, fault patterns depict NNE-oriented exten-

sional duplexes (Decker, 1996). Duplexes are

delimited by arrays of northeast striking sinistral faults

and by NNE-striking normal faults which are arranged

in left-stepping enechelon patterns (Kroll and Wessely,

1993). Duplexing associated with substantial horizon-

tal extension and normal faulting on NNE-striking

faults was the main mechanism accounting for the

rapid subsidence in the Middle Miocene Vienna pull-

apart basin. Growth strata show that normal faulting

occurred from the Karpatian to the Pannonian (from

17–8 Ma). Around 9 Ma, rift-type basement subsi-

dence reached up to 5.8 km (Wessely, 1988; Kroll and

Wessely, 1993). Paleostress data as well as basin

modelling results indicate that deformation during that

time was restricted to the uppermost 10–12 km of the

crust and that both strike–slip and normal faults root

in the Alpine floor thrust (Royden, 1985; Peresson and

Decker, 1997a,b). In the Calcareous Alps, NNE-trend-

ing fault zones crosscutting older ENE-striking

wrench faults are composed of arrays of divergent

strike–slip duplexes that are a few hundred meters to

about 1 km long. These divergent fault systems are

characterized by up to 1.5 km wide zones of intense

faulting and fracturing. Three deformation zones, each

accounting for 3.5 to 4.5 km of sinistral offset, occur at

a regular spacing of about 3 km adjacent to the Vienna

basin (Fig. 9). Duplexes are composed of down-

thrown Upper Cretaceous Gosau formations that are

sandwiched between Triassic units and fault-bounded

Early Pannonian conglomerates, which argues for

Miocene deformation ages. To the south, the NNE-

striking deformation zones terminate in extensional

imbricate fans with east-dipping normal and oblique–

normal faults.

5. Quantification and balancing lateral orogenic

Quantification of the lateral orogenic float is

approached by the assumption of minor internal defor-

mation of the previously defined horizontal wedges.

This assumption is supported by the strain analysis in

the central Eastern Alpine lid (Becker, 1993), which

shows a dramatic increase of deformation approach-

ing the Penninic/Austroalpine detachment. Calcula-

tion of extension in the upper part of the Austroalpine

nappe stack revealed stretching of up to 20%, whereas

close to the major detachment zone the stretching

reached 87%. Major deformation occurred only along

the basal detachment and along the strike–slip wedge

boundaries. The structural analysis of the northern

Fig. 9. Displacement transfer structures in the eastern segment of the NCA and the Vienna basin. AnF=Annaberg fault; PeF=Pernitz fault.

Location in Fig. 1.

Eastern Alps indicated major deformation along

wedge boundaries by strike–slip faulting and north-

east-directed stacking. It is assumed that the internal

deformation of the wedges is less than 20% of the total

deformation, similar to the central Eastern Alps. The

difference in the structural style of the central and

northern Eastern Alps is the higher amount of denu-

dation, ductile deformation and the basement involve-

ment in the central Eastern Alps versus the thin-

skinned and brittle deformation in the northern East-

ern Alps.

Retrodeformation of the Alpine wedges was car-

ried out by restoration of their original position by the

displacement data of the wedge-bounding faults. The

displacement vectors show on map scale the amount

of displacements and their north–south and east–west

components. The directions were oriented subparallel

to the strike of the faults (Fig. 10). Each wedge was

removed to its pre-Miocene position depending on

direction and amount of displacement (Fig. 10, Table

1). The present locations of the wedges and the

displacement vectors of the wedge boundaries were

indicated on the upper geologic map (Fig. 10). The

lateral displacement of the wedges is indicated by the

respective displacement vector diagrams (Fig. 10,

upper diagram), which show the north–south compo-

nents of shortening and east–west components of

extension. The north–south components of the dis-

placement vectors north or south of the SEMP line

add to the amount of the total displacement of the

indenting South Alpine block. The lower map shows

the retrodeformation of the wedges according to their

respective displacements. Parts of the Eastern Alps

were uncovered by restoring the wedges (Fig. 10;

black areas), indicating regions of compression.

The north–south shortening was calculated along

three lines between the northern margin of the NCA

and the Periadriatic line (Fig. 10; lines 1–3). The

north–south lines show total shortening between 61

and 64 km, which corresponds to the 65 km north–

south component of the indenting South Alpine block

as well as to the north–south components of the

displacement vectors north and south of the SEMP

line. The 120 km east–west movement of the central

Eastern Alps resulted from restoring the wedges. The

starting point for calculation was the pre-Miocene

east–west length of 255 km of the Eastern Alps

between the Brenner line (Fig. 10; BL) and a pin-line

across the Styrian basin. The present distance of 375

km between the Brenner line and the pin-line indicates

120 km of west–east extension.

6. Discussion and conclusion

The concept of orogenic float describes deforma-

tion of the upper plate in the hangingwall of a basal

detachment zone of a fold-thrust belt (Oldow et al.,

1989, 1990). The basal detachment of the Eastern

Alps runs from the frontal triangle zone, the contact

with the Molasse foreland basin (Fig. 5) below the

Northern Calcareous Alps (Figs. 7 and 8), and

steepens below the central Eastern Alps into the distal

European basement. Schematic north–south sections

(Fig. 11) indicate the thin-skinned character of the

northern Eastern Alps and the thick-skinned character

of the central Eastern Alps during the Oligocene and

Miocene post-collisional deformation stages of the

Eastern Alps. These differences of the position of

the detachment level produce the different structural

styles of the post-collisional stages of the Eastern

Alps: lateral escape of the northern Eastern Alps and

lateral extrusion of the central Eastern Alps.

The Oligocene stage is characterized by the for-

mation of the deep Molasse foreland basin due to the

flexural load of the Eastern Alps, the involvement of

parts of the foreland basin and of distal parts of the

European basement into Alpine deformation (Nacht-

man and Wagner, 1987; Wagner, 1996; Froitzheim et

al., 1996). Major structural features were the Periadri-

atic line and the Engadine–Inntal fault system. Total

displacement along the Periadriatic line is assumed to

be 400–500 km (Tari et al., 1995 and discussion

therein). A part of the large-scale displacement along

the Periadriatic line is transferred by lateral wedging in

Oligocene times to the north by the left-lateral Enga-

dine– Inntal fault system. Displacement increases

from 4 to 5 km offset of Penninic ophiolites in the

Fig. 10. Balancing lateral orogenic float of the Eastern Alps: Retrodeformation of the Alpine wedges was carried out by restoration of their

original position by the displacement data of the wedge-bounding faults. Displacement vectors show on map scale the amount of displacements

and their north–south and east–west components.

southwest to the offset of Silvretta and Otztal markers

in the Austroalpine cover nappes (Schmid and Froitz-

heim, 1993; Trumpy, 1977). It is speculated that the

Bergell intrusion took place due to the lateral move-

ment of the Otztal block between the Engadine–Inntal

fault system and Periadriatic lines. Lateral wedging

was synchronous with backthrusts in the southern

Eastern Alps (Werling, 1992; Muller et al., 1996;

Heitzmann, 1987) and imbrications in the foreland

basin. The deep marine sequences of the foreland

basin show along the southern margin of the basin

imbrications, which were covered by late Oligocene to

early Miocene sequences. The Oligo–Miocene se-

quences were redeformed due to Miocene indentation

of the South Alpine block.

The Miocene stage is characterized by ongoing

lateral extrusion of the central Eastern Alps, deep

denudation and lateral escape of the northern Eastern

Alps. The result of the crustal thickening in the central

Eastern Alps is lateral extrusion of the Austroalpine

lid above the Tauern crustal anticline (Ratschbacher et

al., 1991a,b). The east–west extension is related to

updoming of the Tauern complex and caused an

exhumation by unroofing of the Austroalpine units

to the east (Frisch et al., 1998). The unroofing of the

Tauern window in Miocene times was accompanied

by two low-angle detachment faults, the Brenner line

in the west (Selverstone, 1988) and the Katschberg

line in the east (Genser and Neubauer, 1989). Unroof-

ing and lateral extrusion were linked to a complex

system of normal faults and thrust faults, depending

on the geometry of the strike–slip faults which bound

the indenting and extruding/escaping wedges. Normal

faulting west of the Tauern is compensated by out of

sequence thrusting (OT; Fig. 3) and indentation of the

Otztal complex onto the NCA (Linzer et al., 1995).

The NCA were displaced 15 km to the northwest

along the Telfs fault (TF; Fig. 3) in the Inn valley.

Internal offsets in the NCA show displacements of 7.5

km indicating a decreasing displacement to the north-

west (Linzer et al., 1995). Low-angle normal faults

east of the TW stretch the Austroalpine nappes up to

150% towards the Pannonian Basin (Ratschbacher et

al., 1991a; Becker, 1993; Tari and Horvath, 1995;

Tari, 1996; Frisch et al., 1987). The northern limita-

tion of the extruding wedge is formed by the SEMP

line, which consists of an imbricational fan of ductile

shear zones, forming the restraining bend in the west

(Tauern window). The SEMP line shows in its central

segment 60 km of major left-lateral displacement

accompanied by splays like the Konigsee–Lammer-

tal–Traunsee fault, Weyer fault and Gostling fault

with displacements between 2 and 28 km. The splays

of the SEMP line are connected with pop-up struc-

tures like the Warscheneck and Haller Mauern block

which have their counterpart in the Weyer Arc thrust

with a horizontal shortening of 15 km. Further in the

east, releasing bends formed generally north–south

striking sets of normal faults. The SEMP line ends in

the Vienna basin. Its kinematic link to the Vienna

basin is not clear.

Normal faulting east of the Tauern complex is

compensated by 60 km of right-lateral strike–slip

movement along the Periadriatic line in the south

(mean value computed from displacements of Austro-

alpine units north of the Periadriatic line; Molltal,

Iseltal, and Hochstuhl faults, Table 1) and 60 km left-

lateral strike–slip movement along the SEMP line in

the north (offset of the Innsbruck and Ennstal quartz

phyllite units; Ratschbacher et al., 1991a,b). The

extension north and east of the Tauern complex is

compensated by the lateral escape of wedges of the

NCA toward the northeast. The wedges are bound by

left-lateral strike–slip faults with displacements in the

order of several km (Fig. 10; Table 1). Fig. 10 shows

the displacement vectors of major faults in the Eastern

Alps during Miocene times. The orientation of the

pre-extrusion convergence vector was calculated by

analysis of 39 thrust related (T2a: 001/03) and 86

strike–slip related (T2b: 358/01) stations in the NCA

(Peresson and Decker, 1997a,b). It is assumed that the

indentation vector of the South Alpine and the con-

tractional axis calculated from fault–slip analysis

have the same north–south orientation.

Calculated displacement vectors lead to a bal-

anced block-kinematic model: 80 km indentation of

Fig. 11. Kinematic model of post-collisional deformation of the Eastern Alps: 80 km displacement of the Periadriatic line was transferred to the

Eastern Alps and caused 64 km of north–south shortening and 120 km of east –west orogen parallel extension. Schematic north–south sections

indicate the thin-skinned character of the northern Eastern Alps and the thick-skinned character of the central Eastern Alps during the Oligocene

and Miocene post-collisional deformation stages of the Eastern Alps.

the South Alpine block (Fig. 11, striped area) into

the Eastern Alps is compensated by 80 km short-

ening of the Lombardian back-thrust system west of

the Giudicarie line (Schonborn, 1992). Indentation

east of the Giudicarie line is transferred by 80 km

of strike–slip displacement along the Giudicarie line

to the northeast and compensated by lateral extru-

sion of the central Austroalpine and of lateral

escape of blocks of the NCA. The 80 km displace-

ment of the Periadriatic line was transferred to the

Eastern Alps and caused 64 km of north–south

shortening and 120 km of east–west orogen parallel

extension. A regional north–south transect across

the Swiss and Italian Alps west of the South alpine

indenter shows post-collisional (Oligocene to recent)

total north–south shortening of 119 km (Schmid et

al., 1996) between the European foreland and the

upper colliding plate of the Alps. The 64 km

north–south shortening which resulted from balanc-

ing the lateral float took into account only the

deformation of the upper colliding plate. Taking

into account a similar total post-collisional short-

ening for the Eastern Alps as it was calculated for

the eastern Swiss Alps, an additional 55 km of

north–south shortening in the lower colliding plate,

has to be compensated for.

Acknowledgements

We greatly acknowledge the critical and helpful

reviews of H.G. Ave Lallemand, J.S. Oldow, G.

Schoenborn, J. Selverstone, and P. Vrolijk. H.-G.

Linzer thanks A. Bally, G. Tari and J. Flinch for

stimulating discussions at Rice University and W.

Nachtmann, RAG, and W. Zimmer, OMV, for the

courtesy of reflection seismic lines. The improvement

made by R. Derksen, A. Hierl-Linzer, C. Hochwald to

the manuscript is greatly acknowledged. This work

was partly financed by the German Science Founda-

tion (DFG project Li 575).

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balancing lateral orogenic float of the eastern alps

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