balancing lateral orogenic float of the eastern alps
TRANSCRIPT
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.
D 2002 Elsevier Science B.V. All rights reserved.
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
0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
PII: S0040 -1951 (02 )00337 -2
* Corresponding author.
E-mail address: [email protected] (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
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
basin
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
H.-G. Linzer et al. / Tectonophysics 354 (2002) 211–237214
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.
H.-G. Linzer et al. / Tectonophysics 354 (2002) 211–237 215
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-
H.-G. Linzer et al. / Tectonophysics 354 (2002) 211–237216
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.
H.-G
.Linzer
etal./Tecto
nophysics
354(2002)211–237
217
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.
H.-G
.Linzer
etal./Tecto
nophysics
354(2002)211–237
218
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
(km)
Max. offset
(km)
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
H.-G. Linzer et al. / Tectonophysics 354 (2002) 211–237 219
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
H.-G. Linzer et al. / Tectonophysics 354 (2002) 211–237220
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,
H.-G. Linzer et al. / Tectonophysics 354 (2002) 211–237 221
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.
H.-G. Linzer et al. / Tectonophysics 354 (2002) 211–237222
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
H.-G. Linzer et al. / Tectonophysics 354 (2002) 211–237224
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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225
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
NCA.
H.-G. Linzer et al. / Tectonophysics 354 (2002) 211–237226
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
H.-G. Linzer et al. / Tectonophysics 354 (2002) 211–237228
imbricate fans with east-dipping normal and oblique–
normal faults.
5. Quantification and balancing lateral orogenic
float
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.
H.-G. Linzer et al. / Tectonophysics 354 (2002) 211–237 229
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.
H.-G. Linzer et al. / Tectonophysics 354 (2002) 211–237 231
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.
H.-G. Linzer et al. / Tectonophysics 354 (2002) 211–237 233
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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