The Schneeberg Normal Fault Zone: Normal faulting associated with Cretaceous SE-directed extrusion...

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=Tectonophysics 401 (
The Schneeberg Normal Fault Zone: Normal faulting associated

with Cretaceous SE-directed extrusion in the Eastern Alps

(Italy/Austria)

Helmuth Sflvaa,T, Bernhard Grasemannb, Martin Thfnib,Rasmus Thiedec, Gerlinde Hablerb

aDepartment of Earth Sciences, University of Graz, Heinrichstrasse 26, A-8010 Graz, AustriabDepartment of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, AustriacInstitute of Geosciences, University Potsdam, Karl-Liebknechtstrasse 24/H25, 14476 Golm, Germany

Received 17 July 2003; accepted 15 February 2005

Available online 27 April 2005

Abstract

The Cretaceous eo-Alpine collisional event in the European Eastern Alps is generally accepted to induce W–NW-directed

thrusting both in basement and in sedimentary cover units. This study presents the first evidence of eo-Alpine W–NW directed

normal kinematics along the Schneeberg Normal Fault Zone, which separates eo-Alpine high-pressure rocks in a footwall

position from pre-Alpine basement rocks in a hanging wall position.

New Garnet Sm–Nd data indicate that exhumation of the high-pressure rocks along the normal fault zone started around 95

Ma ago and continued up to low greenschist/brittle conditions at 76 Ma, as indicated by a Rb–Sr age from a low temperature

mylonite.

The occurrence of pre-Alpine basement rocks both in the hanging wall and the footwall of eo-Alpine high-pressure rocks

suggests exhumation by extrusion processes. Despite the displacement or removal of parts of the lower portion of the high-pressure

unit by Tertiary strike-slip faults, eo-Alpine top-to-ESE thrusting, as expected for the structurally lower part of an extruding wedge,

was found at and below the base of the eo-Alpine high-pressure rocks. A Rb–Sr age of 77Ma from a greenschist facies mylonite in

this thrust shear zone shows the contemporaneity of deformation at the base and the top of the wedge.

The tectonic transport direction within the extruding wedge was E–SE, opposite to the W–NW direction so far reported for

the eo-Alpine event in the Eastern Alps. The contemporaneity of opposite tectonic transport directions during continental

subduction may be explained by a double-vergent wedge model with a narrow zone of ductile flow, where the high-pressure

rocks were exhumed.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Exhumation; High-pressure rocks; Extrusion; Eo-Alpine; Eastern Alps

0040-1951/$ - s

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T Correspondi

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2005) 143–166

ee front matter D 2005 Elsevier B.V. All rights reserved.

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ess: [email protected] (H. Sflva).

H. Solva et al. / Tectonophysics 401 (2005) 143–166144

1. Introduction

The European Eastern Alps cover Austria, north-

ern Italy, southern Germany and eastern Switzerland

(Fig. 1). They are composed of a series of nappes,

which can be derived from the Helvetic, the Penninic

and the Austroalpine realm. Paleogeographically,

from N to S, they were originally arranged in the

following way: the Helvetic realm, representing the

southern margin of the European plate, the narrow

North-Penninic oceanic trough, the Middle Penninic

continental slice, the narrow South-Penninic ocean

and the Austroalpine continental realm (e.g. Stampfli

et al., 1998). The complex evolution of the western

margin of the Neo-Tethys ocean led to Permian–

Triassic extension and related magmatism and LP-

metamorphism in the Austroalpine basement units

(e.g. Habler and Thoni, 2001; Schuster et al., 2001)

as well as the formation of a number of oceanic

troughs (e.g. Channell and Kozur, 1997; Stampfli et

al., 2001). One of these basins, the Meliata ocean

(Middle Triassic to Late Jurassic) and its continuation

to the west, the Hallstatt trough, separated the

Austroalpine terrane from the Africa-related South-

alpine realm (Channell et al., 1992). These basins

were successively closed from S–SE to N–NW

during Alpine collision.

The present study deals with the Austroalpine

realm, which experienced Cretaceous HP metamor-

phism, followed by Tertiary subduction of the

Penninic units and, finally, thrusting of both realms

onto the European continent.

It is generally accepted that the Early to Late

Cretaceous eo-Alpine event was caused by oblique

collision between the southern part of the Austro-

alpine and the Southalpine realm. The resulting

transpression caused W–NW directed thrusting in

the basement (e.g. Conti et al., 1994; Froitzheim et

al., 1994; Ratschbacher, 1987; Schmid and Haas,

1989) and in the sedimentary cover (Eisbacher and

Brandner, 1996; Eisbacher et al., 1990; Linzer et al.,

1995; Ratschbacher, 1986). During this eo-Alpine

event, the continental crust of the southernmost parts

of several Austroalpine basement sub-units was

subducted to maximum depths of ca. 60–70 km

and subsequently exhumed, creating an E–W striking

Eo-Alpine High-pressure Belt (Thoni and Jagoutz,

1993; Wagreich, 1995). Froitzheim et al. (1994)

noted the repeated change of convergence and

extension in the Austroalpine units during the Alpine

orogeny. The eo-Alpine thrusting stage was followed

by Late Cretaceous/Paleogene top-to-ESE directed

normal faulting and extension (Froitzheim et al.,

1994, 1997; Fugenschuh et al., 2000; Viola et al.,

2003).

Geochronologically and petrologically, the Eo-

Alpine High-pressure Belt is well constrained, while

the mechanics of its subduction and exhumation are

still debated. Tenczer and Stuwe (2003) concluded

from petrological and geochronological evidence that

high-pressure rocks from the eastern part of the Eo-

Alpine High-pressure Belt were exhumed by north-

ward extrusion between two blocks under convergent

settings. Kurz et al. (2002) proposed a model for the

same area whereby the high-pressure rocks were

exhumed towards N in the footwall of a lithosphere-

scale extensional shear zone. For the western part of

the high-pressure belt, no data concerning the

subduction/exhumation mechanism existed at all.

For this reason, we focused our study on the area

including the Texel Complex (Fig. 1), where eo-

Alpine eclogites have been previously described

(Hoinkes et al., 1991). This area is particularly suited

for detailed analyses, because high-pressure rocks are

well exposed and display clear structural relationships

with their host rocks. Moreover, preliminary inves-

tigations indicated some incompatibilities with top-to-

the-W thrusting, which is the generally accepted

tectonic transport direction in the Eastern Alps

inferred for the eo-Alpine event.

This study will present field evidence related to the

exhumation of high-pressure rocks in the western part

of the Eo-Alpine High-pressure Belt. It will show that

the Schneeberg Normal Fault Zone contributed to the

exhumation, but that regional extension is not a likely

exhumation mechanism. It will additionally provide

evidence that thrusting of the metamorphic Austro-

alpine basement during the eo-Alpine collision was

not only directed towards W–NW, but also towards

E–SE.

2. Geological setting

The large, E–W striking Austroalpine realm (Fig.

1) is divided into two subunits, (i) the cover unit of the

100 km

EW

Bregenz

Graz

Northern Calcareous Alps

ÖtztalComplex

Silv

retta

Compl

ex

Southalpine

Austroalpine PalaeozoicLower Austroalpine

Austroalpine Mesozoic

Penninic UnitsHelvetic Units

Austroalpine basement

EW Engadin Window

Eo-Alpine EclogitesEo-Alpine HP-belt

Tauern Window

CampoComplex

GF

Giudicarie FaultGF

Schobergruppe Nockberge Koralpe

Kreuzeckgruppe

Texel Complex

München Vienna

Bolzano/Bozen

A

G

IS

?

Bolzano/Bozen

Fig. 2

SNFZ

Sieggraben

VSZ

Vinschgau Shear ZoneVSZ

15°13°11°

47°47°

48°

47°

48°

Merano/Meran PohorjePeriadriatic LineSouthern Alps

Schneeberg Normal Fault ZoneSNFZ Vienna

Fig. 1. Tectonic overview of the Eastern Alps, redrawn after the new metamorphic map of the Alps (Frey et al., 1999). Eo-Alpine (Cretaceous) high-pressure rocks are found in a belt-

like arrangement near the southern limit of the Austroalpine basement. The rectangle indicates the location of Fig. 2.

H.Solva

etal./Tecto

nophysics

401(2005)143–166

145


Northern Calcareous Alps and (ii) the basement unit.

The Northern Calcareous Alps comprise unmetamor-

phosed Permo-Mesozoic, Tethys-derived sediments,

mostly carbonates. The Austroalpine basement unit

consists mainly of paragneisses, micaschists with

intercalations of orthogneisses and metabasites. It

shows a complex, polymetamorphic history (Thoni,

1999, for a review).

Four main tectonometamorphic events have been

distinguished:

(i) First, mainly magmatic event (610–420 Ma) is

present in all basement units and characterized

by large granitic intrusions as well as some

MORB-type basic rocks.

(ii) The Variscan (Hercynian) event (375–310 Ma)

is characterized by medium- to high-grade and

local high-pressure metamorphism and intense

deformation. It is the dominating event in many

parts of the basement units.

(iii) The Permian–Triassic LP-event (290–220 Ma)

is characterized by widespread intrusion of

pegmatites, but is not present within all base-

ment units.

Meran/Merano

S. Leonhard

Texelspitz e

A

B

10 km5

GF

PF

BF

Hohe Kreuzspitz e

GF

Jaufen line

Periadria

ticfault

Fartleis

mylonitezo

ne

PejoFault

PF

BF

Hohe Weisse

Timmelsjoch/Passo Rombo

Penninic

units

Southalpine(non-metamorphic rocks)

CD

BD

AC

EH6699

RT99120VIIIRT99120III

OH6697

11° 11.5°

46.6°

46.8°EH0102

Ötztal-StubaiComplex

CampoComplex

Mauls-

Pense

rjoch

Complex

SNFZ

Fig. 2. Detailed tectonic map of the study area (located in Fig. 1) with the

(SC), Texel Complex (TC), Mauls–Penserjoch Complex and Campo Comp

the profile C–D, on which geochronological samples for the Rb–Sr metho

for the Sm–Nd isotope system are indicated with a star and those analyze

limits of the Schneeberg Normal Fault Zone (SNFZ) are also indicated.

(iv) The Alpine event is further subdivided into

the eo-Alpine sub-event (pressure-peak at

about 105–95 Ma), with eclogite facies rocks

in a southern, E–W oriented zone (Fig. 1),

the meso-Alpine sub-event leading to a

metamorphic overprint mainly of the Penninic

realm (Tauern Window; pressure-peak at

about 60–40 Ma) and the neo-Alpine sub-

event related to strike-slip movements (since

about 20 Ma).

In the study area, the Austroalpine basement

consists of several subunits, representing coherent

single blocks separated by ductile and brittle shear

zones or discriminated on the basis of lithological

differences (Fig. 2).

The five subunits covered by the present study are

lithologically similar, but differ strongly in the degree

of eo-Alpine overprint. From NW to SE (Figs. 1 and

2), these are

(a) the Otztal–Stubai Complex (OSC) consisting

mainly of paragneisses, with intercalated micas-

chists, orthogneisses and metabasites. The OSC

Italy/Austria border

Austroalpine basement mainly Variscan overprintwithBrenner Mesozoics (metamorphic Austroalpine cover units)

Texel Complex (TC, mainly eo-Alpine overprint)Texel Complex with eo-Alpine high-pressure rocksSchneeberg Complex (SC, showing only eo-Alpine imprint)Laaser Serie (part of the SC with basement rocks andintercalations of white marbles)

Giudicarie Fault (Tertiary sinistral strike-slip fault, southernpart of the GF-PF fault system)Passeier Fault (Tertiary sinistral strike-slip fault, northern partof the GF-PF fault system)Brenner Fault (Tertiary brittle/ductile normal fault, delimiting theTauern Window to the W)

Contours of the synoptic cross section in Fig. 13Rb-Sr biotite sample profile (Tab. 1)

Schneeberg Normal Fault Zone (SNFZ)

tectonic units Otztal–Stubai Complex (OSC), Schneeberg Complex

lex. The location of cross section A–B of Fig. 3 is shown, as well as

d on biotite were taken (Table 2). The localities of samples analyzed

d for the Rb–Sr isotope system with a circle. The lower and upper

H. Solva et al. / Tectonophysics 401 (2005) 143–166 147

has a polymetamorphic history (e.g. Purtscheller

and Rammlmair, 1982; Thoni, 1980; Van Gool et

al., 1987). PT-conditions during the Variscan

imprint reached amphibolite (Hoinkes et al.,

1997) and locally eclogite facies conditions

(Miller and Thoni, 1995). The OSC is charac-

terized by chemically discontinuously zoned

garnets and pseudomorphs of eo-Alpine fine-

grained white mica aggregates after coarse-

grained Variscan staurolite and aluminosilicate

(Hoinkes et al., 1987).

(b) the Schneeberg Complex (SC; Fig. 2) is litho-

logically notably different from the surrounding

basement rocks. In clear contrast to the OSC, it

shows chemically single-phase zoned eo-Alpine

garnets and lacks evidences of pre-Alpine

mineral content.

The main and internal part (between 2.5 to 3 km

thick), represented by the Monotone Serie (Fig.

3) of Mauracher (1981), comprises garnet

micaschists, with few intercalations of meter-

scale quartzite layers. The narrow 200–400 m

thick, S–SE and N–NW border zones of the SC

are characterized by alternating layers of marble,

amphibolite, micaschist, quartzite and calcschist,

whose thickness is less than a few tens of meters.

These alternating layers are called Bunte Serie

(heterogeneous series; Fig. 3) by Mauracher

(1981).

The Laaser Serie (Mauracher, 1981; Figs. 2 and

3) has traditionally been separated from the SC

applying lithological and petrographical criteria.

In this study, both units are discussed together,

because they share the same tectonic history.

Lithologically, the Laaser Serie comprises sev-

eral 10–100 m thick marble layers with inter-

calations of micaschists and calcschists at the

base of the SC.

The sediments are interpreted to be of Paleozoic

age and have been regarded as a separate,

autochthonous unit within the OSC (Hoinkes et

al., 1987). In this study, the term Schneeberg

Complex defines a tectonic unit, separated from

the OSC and strongly affected by the eo-Alpine

tectonometamorphic event.

(c) the Texel Complex (TC) is lithologically similar

to the OSC. In contrast to the OSC, it is

characterized by a pervasive eo-Alpine overprint

under amphibolite and eclogite facies conditions

(Solva et al., 2001; Spalla, 1990).

Eo-Alpine eclogites occur in the entire area of

the TC between the Laaser Serie in the N and W,

the Passeier Fault in the E and the Spronser

valley in the S (Fig. 2).

In order to emphasize the differences, we

introduce the term Texel Complex for basement

rocks SE of the Schneeberg Complex (Fig. 2),

pervasively affected by eo-Alpine metamor-

phism, formerly attributed to the OSC and the

Campo Complex.

(d) The Campo Complex consists of polymetamor-

phic basement rocks, similar to those of the OSC

and, typically, Permian intrusives. Granites and

abundant pegmatites yield ages of about 260 Ma

(Solva et al., 2003). The Campo Complex is

separated from the OSC in the N by the E–W

striking eo-Alpine Vinschgau Shear Zone

(Schmid and Haas, 1989). The main tectonome-

tamorphic imprint is, like in the OSC, Variscan

and characterized by amphibolite facies meta-

morphic conditions. A strong eo-Alpine over-

print is restricted to the northern part within the

Vinschgau Shear zone.

(e) The Variscan Mauls-Penserjoch Complex is

lithologically and geochronologically similar to

the Campo Complex (Frank et al., 1977, 1987;

Spiess, 1995). When the Tertiary left-lateral

offset along the N–S striking Passeier Fault

(Viola et al., 2001) is taken back, the Mauls–

Penserjoch and the Campo Complex represent a

single coherent unit.

According to this new terminology, the OSC, the

Campo Complex and the Mauls–Penserjoch

Complex are hardly affected by the Alpine

event, while the SC and the TC are part of an

eo-Alpine shear zone.

In general, eo-Alpine metamorphic conditions in

the OSC increase from NW to SE (Purtscheller

and Rammlmair, 1982; Thoni, 1983) reaching a

maximum within the Texel Complex, but being

weak SE of it in the Campo Complex (see data in

Frank et al., 1987).

The major problem in reconstructing the original

tectonic situation was caused by post-Cretaceous

deformation related to the Giudicarie Fault

System (Muller et al., 2001; Viola et al., 2001),

Pfelders valleySeeber valley

Passeier valley

D4 structures (lower greenschist facies)

D5 structures (brittle-ductile transition)

D2 structures (amphibolite facies,) TC

D3 structures (amphibolite facies,) SC

dominantly Pre-Alpine structures

Texel Complex (eo-Alpine high-pressure rocks)

Schneeberg Complex (~SNFZ)

D1 pinch and swellrefolded by D2 folds

Passe

ier Fa

ult

VariscanBasement

sigma clastSCC' fabricflanking structureshear bandpseudotachylitesense of movement

D3 structures (upper greenschist facies)

trace of D1 structures (high-pressure facies)

Symbols

Deformation stages

N

D4 fold axes

D4 stretching lineationD4 mylonitic foliationD4

N

D3 fold axes

D3 stretching lineationD3 mylonitic foliationD3

N

D2 fold axesD3 crenulation axes

D1 mylonitic foliation

N

D3 fold axes

D3 stretching lineationD3 mylonitic foliation

N

N

D5 slickenlinesD5 fault plane

N

D1 stretching lineation

D1 stretching lineation



D3

D2 D1

D5

D1

D3

N

D3

D2 axial plane cleavage

ÖSC91±4^

93±5*

90±3 (E)

white mica Rb-Sr age (million years)garnet Sm-Nd age (million years)

biotite Rb-Sr age (million years)

78±1 (S)77±1

N

fold axes

metamorphic layering

axial plane cleavagestretching lineation

1k m

NNW

Variscan

A (NW) B (SE)

H: Hoinkes et al., 1991E: Exner et al., 2002

garnet micaschist EH6697

* garnet micaschist RT99120III

^ garnet micaschist RT99120VIII

S: Sölva et al., 2001

Gurgler valley

80±1 (S)

85±1

98±3"

73±1 (H)153±2 (H)

173±1 (T)

T: Thöni, 1980

white mica K-Ar age (million years)

Monotone Serie

Bunte Serie

Bunte Serie

Laaser Serie

D4 axial plane

Timmelsjoch

axesfoliation

foldD3D3 stretching lineation

mylonitic

Fig. 3. Synoptic cross section A–B (located in Fig. 2). Stereoplots show orientation data of planar and linear fabrics for the corresponding deformation stage of the eo-Alpine shear

zone. The distribution of the deformation stages indicates that deformation localization increases and active displacement migrates towards structurally higher levels with decreasing

temperature.

H.Solva

etal./Tecto

nophysics

401(2005)143–166

148

normal shear zone

NW SE

50 cm

folia

tion

(334

/42)

Fig. 4. Meter-scale greenschist facies normal shear zone within the

lowermost parts of the OSC with strong eo-Alpine overprin

indicating top WNW sense of shear.


which cuts the Texel Complex in the E (Fig. 2).

We were, however, able to overcome this

problem by using complementary data from the

southern part of the Texel Complex and the

northeastern part of the Campo Complex W of

Merano/Meran, where the original tectonic

arrangement of units was only subordinately

disturbed by Tertiary faults.

3. Structural data

Structural field data from the Otztal–Stubai Com-

plex, the Schneeberg Complex and the Texel Complex

are presented for different deformation stages together

with a synoptic cross section A–B (Fig. 3). The

location of the cross section is shown in Fig. 2.

It is important to note that the deformation stages

discussed here are discrete steps of a continuous

process, preserved in different parts of the shear zone

(Fig. 3).

3.1. Otztal–Stubai Complex

The main foliation, represented by a metamorphic

layering, is attributed to the Variscan event. Large

orthogneiss and metabasite bodies, concordant with

this foliation, mark km-scale folds with a steep fold

axis (see Van Gool et al., 1987). Rarely, a N to NE

trending mineral lineation, associated with the main

foliation, was observed. This mineral lineation has

been folded together with the foliation. Up to 1 km

above the SC, coarsening of the mineral grains,

especially of micas, indicates increasing eo-Alpine

recrystallization (Hoinkes et al., 1987). Pre-Alpine

structures were thus increasingly overprinted and

obliterated approaching the SC and disappear at the

contact with the SC.

About 1–2 km above the SC, open to close folds

with horizontally E–W oriented axes start to tighten.

These folds are interpreted to be eo-Alpine by Van

Gool et al. (1987). The axial planes of these folds are

upright in the NW, changing to shallow dip angles in

the immediate vicinity of the SC. The axial plane

foliation becomes more intense towards the SC/OSC

contact and is pervasive within a few hundreds of

meters next to the SC. The increase in strain is

correlated with an increase in eo-Alpine metamor-

phism towards the SC/OSC contact. Close to the SC,

the axial planes of these folds were partly reactivated

in shear bands and cm- to m-scale normal shear zones

with a stretching lineation plunging towards NW,

giving a normal, top-to-NW sense of shear (Fig. 4).

Blasts of fine-grained white mica and chlorite within

the shear zones indicate greenschist facies conditions

during deformation; this is correlated to the eo-Alpine

deformation event in the SC (see Section 3.2.).

3.2. Schneeberg Complex

New and existing structural and petrological data

(Habler et al., in press; Solva et al., 2001) suggest 5

deformation stages, which are discrete events along a

continuous exhumation path.

3.2.1. Deformation stage D1

Deformation stage D1 is defined by a mylonitic

foliation (metamorphic layering) with a well-devel-

oped stretching lineation plunging towards NW–

t


WSW, present throughout the SC. Locally, isoclinal

folds with fold axes parallel to the D1 stretching

lineation are present. These folds do not invert the D1

shear sense and are accordingly related to D1. The

general shear sense is top-to-W, indicated by asym-

metric clasts with stair stepping geometries, asym-

metric boudinage and flanking fold structures

(Grasemann and Stuwe, 2001; Passchier, 2001). From

petrological investigations, deformation stage D1

operated close to the pressure peak in the SC at 8–

10 kbar and 550–600 8C (Habler et al., in press;

Konzett and Hoinkes, 1996).


Deformation stage D2 is present only in the lower

part of the SC (see Fig. 3). It is characterized by close

folds with N-trending axes, which refolded D1-related

structures. Towards the top of the Laaser Serie, the

D2 axial plane foliation becomes a mylonitic foliation

with an E–W trending stretching lineation, geometri-

cally and kinematically identical to the D1 foliation

and lineation.


Deformation stage D3 is characterized by S-

vergent fold trains with NW–WSW oriented fold axes

and N dipping axial planes in the central part of the

SC (Fig. 3), overprinting D1 mylonites. The average

wavelength of D3 folds is 800 m.

Microscopic observations on garnet (Fig. 5a)

affirm the relative succession of D1 and D3.

Petrological investigations (Habler et al., in press)

have shown that D3 still took place at amphibolite

facies conditions.

Towards the border of the SC, D3 folds become

isoclinal and the D3 axial cleavage evolves into a

pervasive mylonitic foliation with a pronounced

stretching lineation, plunging towards NW–WSW

associated with top-to-W shear sense indicators (Fig.

6). Within D3 shear zones, D3 fold axes are parallel to

the D3 stretching lineation and the shear sense is not

inverted, suggesting contemporaneous activity of D3

folds and non-coaxial deformation.

While deformation continued in the external parts

of the SC to lower PT-conditions, it terminated at

amphibolite facies conditions in the more internal

parts (Habler et al., in press), where folding is the

main deformation mechanism. In this part, pervasive

static mineral growth occurred still at amphibolite

facies conditions.


Deformation stage D4 is characterized by locally

present open folds with NW-plunging fold axes (Fig.

3). Chlorite, white mica and sometimes biotite define

a rarely evolved axial plane cleavage suggesting

greenschist facies conditions during folding (see

Solva et al., 2001). In the internal parts of the SC,

single m-scale normal shear zones with chlorite on the

shear plane are locally present similar to Fig. 4. Their

shear band geometry and kinematics imply a top-to-

NW sense of shear.

The main D4 structure is a shear zone in the lower

part of the SC (Laaser Serie; Fig. 3), bound to a few

hundreds of meters thick marble layer and extending

from the Hohe Weisse in the SW to the Hohe

Kreuzspitze in the NE (Fig. 2). Blasts of fine-grained

white mica and chlorite, but no biotite in the

mylonites indicate lower greenschist facies condition.

The D4 mylonitic foliation reactivated D3 planar

fabrics, the stretching lineation plunges towards

WNW–NW (stereoplot in Fig. 3). Kinematic indicators

consistently give top-to-NW sense of shear (Fig. 5b).


Deformation stage D5 corresponds to a 2 km-thick

zone at the SC-OSC boundary just above the SC (near

Timmelsjoch; Fig. 2) showing distributed brittle strain

with distinct cataclasite zones, slickensides and

pseudotachylites (Fig. 3). The general dip of all

planar structures is NW, slightly steeper than the

general dip angle of the ductile foliation in the SC at

the northwestern limit (40–708 versus 25–458). Thelineation trends towards NW. Different kinematic

indicators like secondary foliation (SC) structures,

Riedels, oriented slickenfibers or stylolites indicate a

top-to-NW sense of shear.

At the upper boundary of the D4 shear zone within

the lower part of the SC, ductile fabrics are reactivated

by small-scale cataclasite zones and pseudotachylites

with similar kinematics and geometries (Fig. 5c).

3.3. Texel Complex

To take into account the difference in pressure-

peak conditions and structures across the TC (Fig. 2),


the Complex has been subdivided into a structurally

higher, eclogite-bearing and a structurally lower,

eclogite-free part.

3.3.1. Eclogite-bearing part of the TC

3.3.1.1. Deformation stage D1. Deformation stage

D1 in the TC is contemporaneous with eo-Alpine

high-pressure metamorphism and characterized by a

SE–NW to WSW–ENE oriented stretching lineation

(Solva et al., 2001). Shear sense in D1 mylonites is

difficult to determine, because D1 structures are

intensely overprinted by D2 and/or D3. However, in

some outcrops, a top-to-ESE shear sense can be

deduced from eclogite facies shear bands (Fig. 7),

SCC’ structures and shearband boudins (Goscombe

and Passchier, 2003). The foliation generally dips 258towards NNW.


D2 is characterized by close folds with mostly N

plunging fold axes. Competent lithologies preserve

the original E-vergence of D2 folds. The average

wavelength of first order D2 folds is about 1 km.


D3 corresponds to intense folding with subhorizontal

E–W trending axes and N-dipping axial planes

occurring in distinct, a few hundreds of meters thick

shear zones (Fig. 3).


D4 is characterized by locally present open folds with

NW–SE oriented axes and steep axial planes (Solva et

al., 2001).

3.3.2. Eclogite-free part of the TC

The structural pattern in the lower part of the TC

is similar to that of the eclogite-bearing part except

for the top-to-ESE shear sense. Top-to-SE shear

sense indicators and amphibolite to greenschist

facies conditions for the eo-Alpine deformation

events have already been reported by Spalla (1990,

1993).

D2 folds (N–S axes) refold the main, mylonitic

foliation with a E–W oriented stretching lineation. D2

folds disappear towards the lower part of the TC,

where close to isoclinal D3 folds become the main

structure with their axes parallel to the D3 stretching

lineation. Consistent top-to-ESE shear sense indica-

tors (Fig. 5d) in different limbs of D3 folds indicate

that folding and stretching were contemporaneous.

The intensity of eo-Alpine strain decreases towards

SE, visible from Permian pegmatites in the lower TC

and the Campo Complex S of it (compare Fig. 2).

Crosscutting relationships show that Variscan struc-

tures in the Campo Complex are preserved (Fig. 8b).

The southern limit of the strong eo-Alpine imprint

(i.e. the southern limit of the TC) coincides with a

jump in K–Ar and Rb–Sr mica ages between the TC

and the Campo Complex (Frank et al., 1987 and

references therein).

4. The Schneeberg Normal Fault Zone

Eo-Alpine lower-grade metamorphic rocks in the

OSC occur in a structurally higher position than

higher-grade metamorphic rocks in the SC and high-

pressure rocks in the TC to the SE. This arrangement

(Fig. 3) and the W–WNW-directed sense of shear

within all units imply that high-pressure rocks from

the TC were exhumed relative to the OSC along a

northwesterly dipping normal fault zone, the Schnee-

berg Normal Fault Zone (SNFZ). It represents an

about 4.5 km thick fault zone, which was active from

pressure-dominated amphibolite facies (ca. 1.0 GPa at

ca. 600 8C, Konzett and Hoinkes, 1996) to brittle

conditions. The shear sense during all deformation

stages is top-to-WNW. Due to strain localization with

decreasing temperature, the SNFZ is characterized by

a complex distribution of high-, medium and low

temperature shear zones, which were active at differ-

ent times (Fig. 3).

Considering existing petrological and geochrono-

logical data (Habler et al., in press; Hoinkes et al.,

1991; Konzett and Hoinkes, 1996; Solva et al., 2001;

Spalla, 1993) and kinematic and geometric similarities

of the deformation stages D1–D5 in the TC and SC, a

continuous and coherent eo-Alpine deformation

(exhumation) path can be deduced.

The hanging wall position of the brittle D5 shear

zone (Fig. 3) in the SNFZ is typical for normal faults

in general, showing a decreasing temperature gradient

from the footwall into the fault zone towards the

hanging wall (compare Sibson et al., 1979). In the


SFNZ, rheologically bweakQ lithologies led to defor-

mation localization at low temperature (D4) in a more

internal position of the fault zone (Fig. 3).

trace of D3axial plane

bio

matrix: white micaquartz, plagioc

trace of D1 foliationN

a

white mica

quartz

matrix: fine-grai

quartz clast

NW

b

Contemporaneous folding and shearing with fold

axes parallel to the stretching lineation, as reported for

all ductile deformation stages in the SNFZ, indicate

500 µm

garnet

tite

,lasebiotite,

trace of D1 foliation

S

inclusion-free garnet rim

100 µmned calcite

calcite fibers+minor quartz

SE

calcite: SPO

NWNW

shear bands

ultracataclasite/pseudotachylite

normal shear zone

1000 µm

c

SESE

white mica

Matrix: quartz, plagioclase, white mica

sigma clast: plagioclase

EW

250 µm

plagioclase sub-grains

d

Fig. 5. (a) The garnet core of the garnet micaschist sample RT99200III (Sm–Nd age of 90.9F4.1 Ma, Fig. 9a) is post- to synkinematic relative

to D1. The outermost, almost inclusion-free part of the garnet is postkinematic relative to D1, but predates the D3 crenulation, present only in

the matrix. The chemical zoning of the garnet is continuous (Habler et al., in press). (b) Quartz grains forming sigma-clasts in a D4 marble

mylonite from the lower part of the Schneeberg Complex (Laaser Serie, sample EH4501). Note the very small grain size (10–50 Am) of calcite

in the matrix. The shape-preferred orientation (SPO) of these grains gives consistently top-to-NW sense of shear. (c) Brittle pseudotachylite/

cataclasite layer in orthogneiss sample EH3801 offset by a D4 ductile extensional shear zone indicating top-to-NW sense of shear. (d)

Plagioclase clasts in an eo-Alpine, orthogneiss-derived mylonite from the lower Texel Complex (EH1801). Feldspar grains form mantle–core

structures. A top-to-E sense of shear can be derived from white mica-rich strain caps and stair stepping geometries.


shear band (quartz+omphacite)

Fig.7b

W E

reverse-draga-type flanking fold

asymmetricfold

WNW ESE

Fig. 6. An A-type reverse-drag flanking fold (Grasemann and

Stuwe, 2001) in a coarse-grained marble from a D3-related shear

zone within the lower Schneeberg Complex (Laaser Serie). The

top-to-W shear sense is constrained by the asymmetry of dm-scale

folds.


shortening perpendicular to the shear zone boundary.

Consequently, the SNFZ is a stretching normal fault

(Means, 1989).

garnet-rich layer

a

mylonitic foliation

shear bandomphacite

garnet

quartz

amphibole

rutile 1 mmb

W E

Fig. 7. Eclogite facies D1 normal-drag shear bands and SCC’

fabrics indicate top-to-E shear sense in the lower parts of the high-

pressure unit of the Texel Complex. The dominant minerals in the

shear bands are quartz, omphacite and white mica. (a) Macroscopic

view of asymmetric shearband boudinage (Goscombe and Passch-

ier, 2003), indicating top E sense of shear. (b) Microscopic detail of

the neck of the boudinage in panel a.

5. Chronology of deformation events

In order to constrain the age of the D1–D5

deformation stages, geochronological methods with

different closure temperatures have been applied to

minerals linked to deformation and certain metamor-

phic conditions. The Sm–Nd isotopic system in garnet

with a high closure temperature (N600 8C, Scherer etal., 2000; Thoni, 2002; Van Orman et al., 2002) gives

insights into the early stages of the cooling history.

The white mica Rb–Sr system records crystallization

or cooling ages at medium temperatures (500F50 8C,Jager et al., 1967). White micas from metasedimen-

tary rocks are often unsuitable for Rb–Sr dating

because of low Rb/Sr ratios. Therefore, only orthog-

neiss samples have been analyzed in this study. The

Rb–Sr system in biotite has a lower closure temper-

ature (300F50 8C, Jager et al., 1967), thus providinginformation on later cooling stages and low-temper-

ature tectonometamorphic events.

In combination with age data from the literature,

the geochronological results from this study constrain

the exhumation path of the SC and TC during the eo-

Alpine event.

5.1. New Sm–Nd data

Table 1 presents new Sm–Nd data for garnet

fractions and whole rock splits of three samples:

RT99120III and RT120VIII from the SC and OH6697

from the TC.

Sample RT99-120-III is a garnet mica schist from

the Monotone Serie (Seeber Valley; Fig. 2). Four

fractions (a strongly magnetic leached garnet fraction

(grt str. magn. R), its leachate, the corresponding

unleached garnet magnetic fraction (grt n.l.) and the

whole rock (wr)) have been analyzed. By including all

four data points in one regression calculation, a mean

age of 90.9F4.1 Ma and an initial qNd of�11.2F0.6

Permian pegmatite

paragneiss

SSW ENE

eo-Alpinemylonitic foliation

D1

D3 fold

a

Permian pegmatite

paragneiss

pre-Permianfoliation

NW SE

b

Fig. 8. Permian pegmatites from the eastern lower Texel and the

upper Campo Complex. (a) The pegmatite from the lower Texel

Complex was strongly affected by eo-Alpine ductile deformation.

Close folds with E–W axis fold a mylonitic foliation (D1) with an

E–W oriented stretching lineation. (b) The pegmatite from a more

southern position within the Campo Complex cuts the pre-existing

dominant foliation in the paragneiss host rock.


were obtained (Fig. 9a). This regression does not fulfill

the conditions of a best-fit isochron (MSWD=7.3).

Different combinations of internal two-point regression

give minimum and maximum ages of 89.4F1.7 and

96.3F3.2 Ma, respectively.

The garnet micaschist sample RT99-120-VIII was

collected near Timmelsjoch (Fig. 2) in the Bunte Serie

(see Fig. 3) and is in a structurally higher position than

sample RT99-120-III. From this sample, three leached

residues of two differently magnetic garnet fractions

(grt 1MF/1 R, grt 1MF/2 R and grt 2MF R), four

leachates (L), one unleached garnet magnetic fraction

(grt 1MF n.l.) and the whole rock (wr) have been

analyzed (Table 1). By considering all analyzed data

points (n =9; Table 1) in one single regression

calculation, a mean Sm–Nd age of 93.1F4.7 Ma

and an initial qNd value of �9.9F1.4 (Fig. 9b) were

obtained. Again, the data exhibit considerable stat-

istical scatter along the regression line (MSWD=64).

Minimum and maximum two-point regression ages as

calculated from different combinations of data points

(Fig. 9b) range between 86F6.1 Ma (grt 1MF L-grt

1MF n.l.) and 100.6F1.3 Ma (grt 1MF/2-wr).

Both garnets from the SC (RT99-120 III and RT99-

120 VIII) exhibit a continuous chemical zoning,

which developed during one single tectono-metamor-

phic event. The garnets grew syn- to postkinemati-

cally relative to D1 in the SC (Habler et al., in press).

The ages obtained by the analyses (90.9F4.1 Ma and

93.1F4.7 Ma, for the Monotone Serie and the Bunte

Serie, respectively) represent the time of garnet

growth at and immediately after the pressure peak

(compare Habler et al., in press). Thus, the mean ages

of 90.9F4.1 Ma and 93.1F4.7 Ma from both

samples are congruent with D1, representing exhu-

mation shortly after the pressure peak. The age and

textures of the analyzed garnets are very similar to

those used by Habler et al. (in press), who derived the

same structural relationships of garnet growth and

deformation stages.

Sample OH6697 is a garnet micaschist from the

Kalm valley in the eclogite bearing part of the TC

(Fig. 2). Three garnet fractions of different grain sizes

(Grth: 0.15–0.3, handpicked, Grtc R: 0.15–0.42,

handpicked and leached, and Grtf R: 0.1–0.125, fine

fraction, magnetically cleaned and leached) together

with the leachate of Grtf R (Lf) and Grtc R (Lc) and

the whole rock have been analyzed (Table 1). All six

data points define an age of 92F15 Ma (Fig. 9c).

Given the relatively small spread in Sm/Nd (overall

range of 147Sm/144Nd is 0.1–0.48; Table 1), the

uncertainty is large and, in addition, the high MSWD

value of 22 indicates considerable excess scatter of the

data beyond analytical errors. It is probable that the

Sm–Nd budget of Grtc R, both leachates and to a

minor amount also Grth are influenced by very small

LREE-rich inclusions, which were not completely

equilibrated isotopically during garnet growth. The

relatively high 147Sm/144Nd ratio of Grtf R gives

confidence that this fraction is almost inclusion-free,

thus making Grtf R the most reliable data point for the

garnet of sample OH6697. Therefore, the date of

98.2F2.7 Ma is taken as a relevant age, correlating

well with the time of garnet crystallization in sample

OH6697.

Table 1

Sm–Nd results for three garnet micaschist samples from the SC (RT99120III, RT99120VIII) and the eclogite-bearing TC (OH6697)

Sample (analyzed fraction) Abbreviation Rock type Tectonic unit Grain size

[Am]

Sm

[ppm]

Nd

[ppm]

147Sm/144Nd 143Sm/144Nd F2j

RT99-120 III (Monotone Serie SC)

Whole rock wr Garnet Monotone S. 1.65 8.18 0.1219 0.512018 0.000004

Garnet strong magnetic fraction, not leached grt n.l. Micaschist SC 164–420 2.88 2.24 0.7783 0.512423 0.000012

Garnet strong magnetic fraction, residue grt str. magn. R 164–420 2.13 0.48 2.6588 0.513523 0.000009

Garnet strong magnetic fraction, leachate Leachate 5.56 19.0 0.1769 0.512044 0.000003

RT99-120 VIII (Buntie Serie SC)

Whole rock wr 8.17 43.0 0.1150 0.512039 0.000004

Garnet 1 magnetic fraction, not leached grt 1 MF n.l. Garnet Bunte Serie 160–420 1.46 0.99 0.8923 0.512541 0.000010

Garnet 1 magnetic fraction/1, residue grt 1MF/1 R Micaschist SC 1.08 0.24 2.6902 0.513627 0.000015

Garnet 1 magnetic fraction/1, leachate L 1/1 3.84 10.8 0.2155 0.512180 0.000005

Garnet 1 magnetic fraction/2, residue grt 1MF/2 R 1.04 0.33 1.8833 0.513203 0.000009

Garnet 1 magnetic fraction/2, leachate L 1/2 18.7 53.0 0.2139 0.512150 0.000004

Garnet 2 magnetic fraction, residue grt 1 MF R 0.69 0.28 1.4993 0.512896 0.000008

Garnet 2 magnetic fraction, leachate L 2 7.00 21.0 0.2017 0.512105 0.000006

grt 0.16–0.42, leachate L 10.0 29.0 0.2084 0.512179 0.000015

OH6697 (eclogite-bearing part TC)

Whole rock wr Garnet Eclogite-bearing 5.08 28.3 0.1085 0.511907 0.000003

Garnet handpicked grth Micaschist TC 160–420 4.29 12.0 0.2160 0.511976 0.000006

Garnet fine, residue grtf R 100–120 2.82 3.53 0.4833 0.512148 0.000005

Garnet fine, leachate Lf 39.7 194 0.1239 0.511935 0.000003

Garnet coarse, residue grtc R 160–420 3.69 6.54 0.3410 0.512037 0.000009

Garnet coarse, leachate Lc 12.5 55.8 0.1351 0.511935 0.000013

H.Solva

etal./Tecto

nophysics

401(2005)143–166

156

0,5132

0,5134

0,5128

0,5130

0,5124

0,5126

143 N

d/14

4 Nd

0,4 1,2 2,0

0,0

0,0 2,8147Sm/144Nd

147Sm/144Nd

wr

wr

leachate

L1/1

grt n.l.

grt 1MF n. l.

grt, str. magn. R

grt 1MF/1

RT99-120IIIgarnet micaschist

Monotone SerieSchneeberg Complex

RT99-120VIIIgarnet micaschist

Bunte SerieSchneeberg Complex

Age = Ma90.9 ± 4.1Initial Nd/ Nd =

0.511946 ± 0.000031Nd (91 Ma) = –11.2

MSWD = 7.3 (n = 4)

143 144

∈

Age = Ma93.1 ± 4.7Initial Nd/ Nd =

0.512008 ± 0.000038Nd (93 Ma) = –9.9

MSWD = 64 (n = 9)

143 144

∈

grt 2MF

grt 1MF/2

0,8 1,6 2,4

LL1/2

L2

a

b

0,51215

0,51205

0,51195

0,1

wr

Lc grth OH6697garnet micaschist

eclogite-bearing partTexel Complex

wr-grtf Rage = MaInitial Nd/ Nd =

0.511837 ± 0.000004(98) = -13.1

98.2 ± 2.7143 144

∈ Nd

grtf R

0,2

all data pointsage = MaInitial Nd/ Nd =

0,511850 ± 0.000027Nd (91) = -13.1

MSWD = 22 (n = 6)

91 ± 15143 144

∈

0,3 0,4 0,5 0,6

grtc R

Lf

c

Fig. 9. Sm–Nd isochron diagrams of three garnet micaschist samples from the SNFZ and the Texel Complex. RT99120III (a) and RT99120VIII

(b) were sampled in the central and the upper part of the SC, respectively, while sample OH6697 (c) is from the eclogite-bearing part of the TC

(Fig. 2). Based on microstructural and chemical data, the age results correlate to final pressure-peak and initial exhumation (D1) conditions in

both SC and TC.


Similar to garnets from the SC (samples RT99-120

III and RT99-120 VIII), those from the TC (sample

OH6697) grew syn- to postkinematically relative to

D1 under eclogite facies conditions (Hoinkes et al.,

1991; Solva et al., 2001). Therefore, the age of

98.2F2.7 Ma may also be correlated with the eo-

Alpine pressure peak and initial exhumation of high-

pressure rocks.

5.2. New Rb–Sr data

Rb–Sr results have been obtained for nine whole

rock and biotite concentrates (Table 2). The samples

were collected over a continuous section across the

northern Monotone Serie, the Bunte Serie and the

lowermost part of the OSC, in order to reveal possible

age variations across the brittle shear zone (D5) at the

boundary SC/OSC.

Biotite–whole rock (bt–wr) age results from nine

metapelite samples (Table 2) of the profile C–D

(Fig. 2) range between 73.7F0.7 and 85.3F0.9

Ma. This variation exceeds analytical uncertainties.

A systematic trend was not observed, although ages

in the upper part of the section (OSC) are older

than ages from the lower part. In general, the

results are concordant with literature data on a

regional scale (Solva et al., 2001; Thoni, 1999 for

review).

Table 2

Rb–Sr results for an orthogneiss sample (EH6699), a marble sample (EH0102) and 10 biotite-whole rock pairs of micaschist samples taken on a

profile across the OSC/SC boundary (see Fig. 2)

Sample Rock type Tectonic unit Grain size

[Am]

Rb

[ppm]

Sr

[ppm]

87Rb/86Sr 87Sr/86Sr F2j Two-point

isochron

age [Ma]

0T190 wr Micaschist OSC 200 79.1 7.345 0.73243 0.00008 85.3F0.9

01T190 bt Point C on profile 150–400 431 1.92 706.1 1.57968 0.00002

RT99120XI wr Micaschist OSC 128 75.7 4.913 0.72692 0.00007 81.9F0.8

RT99120XI bt 150–400 398 3.40 353.6 1.13269 0.00049

RT99120IX wr Paragneiss OSC 87.7 168 1.515 0.71964 0.00005 82.9F0.8

RT99120IX bt 150–400 418 3.67 342.6 1.12131 0.00035

RT99120VIII wr Garnet–micaschist Bunte Serie 119 326 1.057 0.71302 0.00006 83.7F0.8

RT99120VIII bt SNFZ [uppermost] 150–400 436 8.06 159.3 0.90128 0.00013

RT99120V wr Mica-rich calcschist Bunte Serie 108 148 2.114 0.72248 0.00007 84.7F0.8

RT99120V bt SNFZ 150–400 412 2.68 469.2 1.28463 0.00016

RT99120VI wr Garnet–micaschist Bunte Serie 60.7 245 0.716 0.71181 0.00004 77.2F0.8

RT99120VI br SNFZ 150–400 261 3.63 213.2 0.94472 0.00009

RT99120IV wr Garnet–micaschist Bunte Serie 78.8 116 1.973 0.72165 0.00005 73.7F0.7

RT99120IV bt SNFZ 150–400 385 4.03 284.4 1.01690 0.00009

RT999120III wr Garnet–micaschist Monotone Serie 202 219 2.681 0.73402 0.00006 79.9F0.8

RT999120III bt SNFZ 150–400 475 3.28 440.7 1.23156 0.00015

RT99120II wr Garnet–micaschist Monotone Serie 199 146 3.953 0.73285 0.00008 77.7F0.8

RT99120II bt SNFZ 150–400 536 3.75 434.2 1.20799 0.00015

RT99120I wr Garnet–micaschist Monotone Serie 149 139 3.111 0.73339 0.00007 77.0F0.8

RT99120I bt SNFZ [point D] 150–400 486 4.46 326.8 1.08741 0.00040

EH6699 wr Orthogneiss mylonite Campo Complex 125 249 1.451 0.71740 0.00249 77.1F0.8

EH699 wm NE part 160–300 258 251 29.83 0.74851 0.00025

EH0102 cal Marble mylonite Laaser Serie 100–200 0.18 115 0.005 0.70968 0.00000 76.0F0.7

EH0102 wm SNFZ 100–200 294 3.96 220.0 0.94739 0.00001

The uppermost sample in the table (01T190, OSC) represents point C on the profile in Fig. 2, sample RT99120I (Monotone Serie, SC) point D.


The results show that the part of the OSC in the

immediate hanging wall of the SC was also affected

by the eo-Alpine event at least up to the closing

temperature of biotite for the Rb–Sr system (see Fig.

14 in Thoni and Hoinkes, 1987).

Marble sample EH0102 was taken from the main

lower greenschist facies D4 shear zone in the lower

part of the SC. The average grain size of the

carbonate matrix is ca. 50–150 Am, where large

white micas (N1 mm) form mica fishes. Quartz

behaved brittlely during deformation, pointing to

temperatures during deformation at or below 300 8C(e.g. Stipp et al., 2002; Stockhert et al., 1999).

Another generation of fine grained white micas is part

of the mylonitic matrix and shows an average grain

size similar to calcite. These fine mica grains were

mechanically separated from the coarse fraction and

analyzed together with the isotopic-exchange partner

calcite. By considering the low temperature condi-

tions during dynamic recrystallization and deforma-

tion-induced growth of the analyzed minerals

together with the closure temperature of the Rb–Sr

system in white mica (ca. 500 8C, Jager et al., 1967),the result of 76.0F0.7 Ma from the calcite (cal)–

white mica (wm) pair is interpreted to represent the

crystallization and deformation age of the low-

temperature stage D4 in the SNFZ. The age is in

agreement with Zircon fission track ages of 70 to 75

Ma from the same area (Elias, 1998).

The orthogneiss mylonite sample EH6699 was

taken in the structurally lower, eclogite-free part of the

TC (Fig. 2) from a D3 shear zone. The Rb–Sr result of

77.1F0.8 Ma from a whole rock-white mica (wr-wm)

pair reveals that deformation and metamorphism in

this part of the TC correlate with the TC north of it.

The brittle–ductile behavior of feldspar clasts (Fig.

5d) in the orthogneiss mylonite indicates temperature

conditions close to 500 8C (Tullis and Yund, 1985)

during deformation. Considering the closure temper-

ature of the Rb–Sr system in white micas (ca. 500 8C,


Jager et al., 1967), this Rb–Sr age represents a

crystallization age and gives the age of D3 in the

lower part of the TC.

6. Discussion

6.1. Geometry and kinematics of the exhumation of

the eo-Alpine high-pressure rocks

Almost all published data concerning the eo-

Alpine event in the Eastern Alps indicate W to NW

directed thrusting (e.g. Eisbacher and Brandner, 1996;

Ratschbacher, 1986; Schmid and Haas, 1989; Viola et

al., 2003) and exclude major W–NW directed normal

faulting. In contrast, we present a model where eo-

Alpine metamorphic rocks were extruded in the

opposite direction, namely towards E to SE.

It is well known that underplating/subduction

induces exhumation of deeply buried rocks during

convergence (see Platt, 1993 and references therein).

In all proposed models, the unit containing high-

pressure rocks is delimited by a thrusting shear zone at

the base and a normal fault at the top of the wedge.

Accordingly, high-pressure rocks from the TC should

be delimited by two shear zones with opposite

kinematics. In our model, the SNFZ represents the

hanging wall normal fault of the extruding wedge, in

which the eo-Alpine eclogites are exhumed (Fig. 10a).

Coeval thrust kinematics should be found in the

structurally lowermost parts of the TC southeast of the

high-pressure rocks.

This thrust shear zone is present in the lower,

eclogite-free part of the TC. There, the footwall

position of the Variscan basement and the consistent

top-to-ESE shear sense document thrusting kine-

matics, which place higher-grade rocks of the TC

over the lower-grade Campo Complex.

The present model is in agreement with the

extrusion mechanism recently postulated for eo-

Alpine high-pressure rocks in the E-part of the Eo-

Alpine High-pressure Belt (Tenczer and Stuwe, 2003).

Exhumation of deeply buried rocks to the surface

implies removal of the material resting above (Eng-

land and Molnar, 1990). The removal of exhumed

rocks is either tectonic or erosional. For the proposed

model with east-/southeastward extrusion of the HP

rocks, erosion should generate sediments, which are

transported towards the foreland. In fact, some

sequences in Southalpine Cretaceous flysch deposits

show significant south- or southeastward transport of

clastic sediments (Bernoulli and Winkler, 1990;

Bichsel and Haring, 1981) coeval with the eo-Alpine

exhumation of rocks in the TC. Increased detrital

sedimentation started approximately at the same time

(95 Ma, Bichsel and Haring, 1981) as the onset of

exhumation of high-pressure rocks in the Texel

Complex (Sm–Nd garnet ages in Fig. 9). Unfortu-

nately, no geochronological information on detrital

minerals from these sediments exists so far and

therefore the basement-related detritus cannot defi-

nitely be related to the extrusion of eo-Alpine high-

pressure rocks.

Indications of a S–SE-directed tectonic transport in

an eo-Alpine collision zone were published by

Doglioni and Bosselini (1987), who proposed a model

of N over S imbrication of units situated north of the

Lombardian and Bergamascian flysch basins. Accord-

ing to these authors, rocks from the northern units,

including the basement units relevant to the present

study, were eroded and transported southward into the

flysch basins.

The presence of top-to-NW normal faulting does

not fit a simple model with a general top-to-W or NW

thrusting direction. The position of the Schneeberg

Normal Fault Zone and the high-pressure rocks of the

Texel Complex near the rear of a collision zone with

contemporaneous west-directed tectonic transport

along the eo-Alpine Vinschgau Shear Zone (Schmid

and Haas, 1989; see Fig. 1) would geometrically fit a

doubly-vergent wedge model (Willett et al., 1993).

The Texel Complex would represent a part of the

retro-wedge, while the Vinschgau Shear Zone occu-

pied a pro-wedge position (Fig. 10b). Such a model is

in principle capable to explain the contemporaneous

presence of opposite exhumation directions and SE-

directed subduction of the Austroalpine realm as

postulated by Froitzheim et al. (1996).

Geometrically, the kinematics of the reconstructed

extrusion zone allow two general scenarios: In one

case the limit of the hanging wall buttress is more or

less parallel to the subducting plate (Fig. 10c), like

e.g. in the extruding wedge model (Burchfiel et al.,

1992) or the channel flow model (Shreve and Cloos,

1986). In the other case, the subducting plate forms an

obtuse angle with the upper contact of the wedge (Fig.

Variscan basement(Campo Complex,

Mauls-Penserjoch Complex)

Texel Complex

SNFZWNW ESE

~ 290 Ma ~ 290 Ma~ 80 Mac.h.*

Variscan basement(Ötztal-Stubai Complex)

10 km5

Gurgler Tal Passeiertal

UltentalHigh-pressurerocks

TCCampo Complex

ÖSC

WNW ESE

Silvretta

Erosion

WNW ESE

ÖSCSilvretta Campo ComplexTC

Erosion

b) Geometric model: obtuse angle between lower plate and upper wedge contact

a) Schematic geometry and kinematics in the study area

c) Geometric model: acute angle between lower plate and upper wedge contact

Vinschgau Shear Zone

SNFZ

SNFZ

Fig. 10. (a) Schematic model of the suggested tectonic settings exhuming high-pressure rocks within an extruding wedge. The SNFZ represents

the upper boundary and the lower part of the Texel Complex is the lower, thrusting boundary of the wedge. Heat is advected from depth by the

extruding rocks and the hanging wall, i.e. the OSC, is conductively heated (*area marked with c.h.). Age data in the model indicate cooling

below 300 8C. Panels b and c represent interpretative geometric models, which can explain kinematics and geometry of the TC in a subduction

zone environment. Subduction of the Austroalpine basement units (Silvretta unit and/or OSC) in panel b is directed towards the E; in panel c, the

Campo Complex and Mauls–Penserjoch Complex represent the lower plate and are subducted towards the W under the OSC.



10b), representing a doubly-vergent wedge. In a brittle

model, a ductile wedge could form locally, when

deformation localization and local heat advection

occurred. The documented rapid increase in sedimen-

tation in the Lombardian flysch basins south of the

high-pressure rocks (Bernoulli and Winkler, 1990;

Bichsel and Haring, 1981) at about 95 Ma indicates

that erosion in the Austroalpine basement realm was

concentrated in the south (or southeast), which could

have caused deformation localization in the south-

eastern part of the wedge (Fig. 10).

However, the exact geometry of the whole

subduction/collision zone, especially the crucial angle

between lower plate and the upper contact of the

extruding wedge is still conjectural.

From existing geochronological and petrological

data (e.g. Purtscheller et al., 1987), it is evident that

large parts of the hanging wall of the SNFZ (about 5

km from the OSC/SC boundary along a NW–SE

section) suffered a significant thermal overprint

during the eo-Alpine event (Fig. 10a). SNFZ-related

deformation in the OSC, however, is restricted to a

narrow, about 1 km thick zone above (i.e. NW of the

OSC/SC contact). The thermal overprint was pre-

viously explained by a bdome-likeQ heat-flux during

the eo-Alpine event (Purtscheller and Rammlmair,

1982; Thoni, 1983), causing a thermal imprint in the

SE (i.e. the Texel Complex) and decreasing temper-

ature conditions towards the NW. However, this

model contradicts the P-data from the TC and the

SC.

An alternative explanation for the eo-Alpine

thermal overprint could be local tectonic heat advec-

tion, as the extruding wedge of the TC may have

conductively heated the hanging wall i.e. the OSC.

Similar effects were reported from other shear zones

(e.g. Grasemann and Mancktelow, 1993; Grujic et al.,

1996). At least qualitatively, the actual eo-Alpine

metamorphic field gradient in the OSC seems to be

the result of heat advection during the eo-Alpine

collision/subduction event.

6.2. The Eo-Alpine High-pressure Belt (EHB)

Several occurrences of eo-Alpine high-pressure

rocks (from E to W) are distributed along the entire

southern margin of the Austroalpine basement (Fig. 1;

Thoni, 1999) and possibly also in the western

Languard-Campo unit (Gazzola et al., 2000). Thoni

and Jagoutz (1993) emphasized the strikingly similar

tectonic position and age of all eclogite occurrences

and introduced the idea of an Eo-Alpine High-

pressure Belt.

The lack of post-Variscan oceanic crust in the

central and western parts of the EHB could be

explained either by a complete tectonic and/or

erosional removal of formerly present oceanic rocks

or by a primary intracratonic collisional setting

without the involvement of oceanic crust. The only

evidence of subducted Permian MORB-type basic

rocks has been reported from the eastern part of the

EHB (Miller et al., 1988; Miller and Thoni, 1997).

They are interpreted as remnants of a small

Mesozoic oceanic basin linked to the Meliata ocean

(Miller and Thoni, 1997 and references therein),

subducted in Late Jurassic times (Faryad and Henjes-

Kunst, 1997; Kozur, 1991). A continuation of the

Meliata ocean to the west (Hallstatt trough) in the

position of the EHB has been postulated by different

authors, based on paleomagnetic (Channell and

Kozur, 1997) and sedimentological evidence (Chan-

nell et al., 1992; Pober and Faupl, 1988; Von

Eynatten and Gaupp, 1999; Wagreich et al., 1999).

In this case, the eo-Alpine underplating/subduction

zone may continue further west and cause formation

and exhumation of HP/LT rocks in the Texel Com-

plex. A modern equivalent can be found at the eastern

margin of the Arabian plate, where ocean–continent

subduction of the Arabian lithosphere is described

from the southern part in Oman, while the northern

part of the same collision zone is characterized by a

continent–continent collision with underthrusting of

the Arabian plate beneath the Zagros mountain belt

(e.g. Ravaut et al., 1997).

7. Conclusions

High-pressure rocks from the western part of the

Eo-Alpine High-pressure Belt (Texel Complex, East-

ern Alps) were exhumed within a ca. 15 km broad eo-

Alpine high strain zone striking SW–NE. The 4–5 km

broad Schneeberg Normal Fault Zone represents the

structurally upper part of this shear zone. It separates

pre-Alpine basement rocks (Otztal–Stubai Complex)

in the hanging wall from eo-Alpine high-pressure


rocks in the footwall. This is in accordance with

slightly older Rb–Sr ages in biotite samples from

above the normal fault.

In the Schneeberg Normal Fault Zone, ductile

deformation from pressure-dominated amphibolite

facies to lower greenschist facies conditions is

characterized by contemporaneous shearing and fold-

ing with fold axes parallel to the stretching lineation,

suggesting shortening perpendicular to the shear zone

boundary and the direction of stretching.

Deformation localization within the Schneeberg

Normal Fault Zone increased with decreasing temper-

ature, with the coldest and youngest event concen-

trated in the uppermost part of the shear zone.

Deformation during early exhumation was homoge-

neously distributed over the whole shear zone.

Subsequent deformation under general shear condi-

tions was partitioned into flattening deformation in the

more internal part and simple shear deformation in the

external parts of the wedge.

According to new Sm–Nd results on garnets from

the Schneeberg Normal Fault Zone and the eclogite-

bearing Texel Complex, exhumation started around 95

Ma ago. A calcite–white mica Rb–Sr age of 76 Ma

from a lower greenschist facies mylonite within the

normal fault zone indicates that exhumation was

active for about 25 Ma, exhuming eclogite facies

rocks to shallow levels of the crust.

Eo-Alpine mylonites in a footwall position rela-

tive to the high-pressure rocks show a top-to-ESE

thrust sense of shear, placing eo-Alpine high-pressure

rocks on top of pre-Alpine basement rocks (Campo

Complex, Mauls–Penserjoch Complex). Together

with the Schneeberg Normal Fault Zone, this

thrusting shear zone formed an extruding wedge,

which extruded eo-Alpine high-pressure rocks of the

Texel Complex under general shear conditions

towards E–SE.

Kinematics and geometry of the Texel Complex fit

two general tectonic models:

(a) the down-going lower plate and the upper wedge

contact formed an obtuse angle during ESE-

directed subduction (Fig. 10b) and

(b) the down-going lower plate and the upper

wedge contact formed an acute angle or were

almost parallel during WNW-directed subduc-

tion (Fig. 10c).

In (a), the Texel Complex represented the ductile

retro-wedge during ESE-directed subduction of the

Austroalpine basement units. This geometry would

explain contemporaneous collision, subduction and

opposite thrusting directions observed in the study

area, while (b) implies WNW-directed subduction of

the Campo and Mauls–Penserjoch Complex, but does

not explain the contemporaneous existence of W- and

E-directed thrusting and exhumation. Consequently,

with the present knowledge of the configuration with

ESE-directed subduction and an obtuse angle, (a) is

the most applicable model.

Acknowledgements

We would like to thank the FWF for founding the

present study through the projects P13227-Geo and

P15474-GEO. Monika Jelenc was a great help in

preparing the geochronological samples. Ulli Exner,

Gerhard Wiesmayr and the Structural Processes

Group Vienna are thanked for fruitful discussions.

The editor, Jean-Pierre Burg, and the reviewers,

Giulio Viola and Bernhard Fqgenschuh, significantlyimproved the quality of the manuscript by their

extensive revision.

Appendix. Sample preparation and analytical

techniques for geochronology

Sample preparation: After cleaning and crushing,

different grain size fractions (b0.15, 0.1–0.2, 0.15–

0.3, and 0.3–0.42 mm) were extracted by sieving.

Mica and carbonate concentrates (0.15–0.3 mm) have

been prepared using a vibrating table and a magnetic

separator. Further repeated grinding of the mica

concentrates in an agate mill (using alcohol), sieving

and careful magnetic purification helped to remove

inclusions and intergrowths.

Garnet was concentrated from defined sieve

fractions using a Frantz magnetic separator. For

samples RT99-120 III and RT99-120 VIII, a grain

size of 0.071–0.125 mm was extracted by sieving and

then split into two magnetic fractions: 1 MF (1st

magnetic fraction) is the more Fe-rich fraction,

whereas 2 MF (2nd magnetic fraction) represents the

weaker magnetic (Fe-poorer) fraction. The sample


splits were subsequently checked carefully under a

binocular microscope for contaminating mineral

phases. Before leaching and decomposition, garnet

samples were repeatedly rinsed, using acetone and

deionised water. Leaching experiments for both

samples were performed using 5.8 N or 2.5 N HCl,

respectively, at 70–90 8C for a time span between 20

and 60 h; residues (R) and leachates (L) were then

spiked and processed separately. For the garnet

fraction Grth of sample OH6697, a grain size of

0.15–0.3 mm was extracted, rinsed in acetone and

finally handpicked under a binocular. A second garnet

fraction (Grtf) was leached. For that purpose, the same

sieve fraction (0.15–0.3 mm) was crushed in a mortar

to reduce the grain size to 0.1–0.125 mm, which was

then cleaned at the magnetic separator. The steps of

the leaching procedure follow those described for the

other samples. For the third fraction (grtc) clean

garnets of a grain size between 0.15 and 0.45 mm

were carefully handpicked and than leached.

Sample weights for whole rocks and mineral

separates used for dissolution varied between 100 and

300 mg. Sample digestion and element separation for

Sm and Nd closely followed the procedure described

by Thfni and Jagoutz (1992). Sr and Rb were

determined from separate aliquots using an HF/HNO3

mixture for dissolution. Rb, Sr, Sm and Nd concen-

trations were determined by isotope dilution (ID) from

separate aliquots of the sample (HCl) solution, using a87Rb-84Sr and a 147Sm-150Nd spike, respectively. Total

procedural blanks were b100 pg for Sm, b150 pg for

Nd and b1 ng for Rb and Sr. Rb and Sr concentration

(ID) and Sr isotope composition (IC) samples were

loaded on single Ta filaments and measured on a

MICROMASSM30machine. Sm and Nd ID as well as

Nd IC samples were measured as metals from a Re

double filament, using a VG FINNIGAN MAT262

mass spectrometer. 87Sr/86Sr and 143Nd/144Nd ratios for

the NBS987 (Sr) and the La Jolla (Nd) international

standards during the course of this work were

0.71014F3 and 0.51185F1, respectively. Errors for

the 87Rb/86Sr and the 147Sm/144Nd ratios are taken as

F1%, or smaller, based on iterative sample analysis

and spike recalibration. Isochron calculation follows

Ludwig (1999), and isochron results are based on the

proposed uncertainties for the 147Sm/144Nd ratio. Ages

are based on decay constants of 1.42�10�11 a�1 for87Rb and 6.54�10�12 a�1 for 147Sm. A linear

evolution of Nd isotopes is assumed throughout

geological time; the following DM (Depleted

Mantle) parameters were used: 147Sm/144Nd=0.222,143Nd/144Nd=0.513114 (Michard et al., 1985).

Leaching technique applied for Sm–Nd analysis:

Most of the isotopic results for garnet of the present

study have been obtained by applying the leaching

technique (e.g., DeWolf et al., 1996; Zhou and

Hensen, 1995). This method of sample preparation/

digestion focuses on the elimination of undetected

high-LREE micro-inclusions in handpicked minerals,

especially garnet. One expected accompanying effect

of such an acid treatment is a considerable increase of

the Sm/Nd ratio in the leached garnet (Thoni, 2002,

for review). Such a bleaching effectQ is evident also in

the present case (Table 1). The result are fairly high

Sm/Nd ratios, thus suggesting the conclusion that the

garnet residues (R) are obviously almost bcleanQ, i.e.,largely free of high-LREE inclusions (such as apatite,

epidote, etc.). Furthermore, Sm/Nd ratios and element

concentration data of the leachates for the different

sample splits analysed show clearly that a consider-

able amount of the Sm and Nd budget in the

unleached garnet fractions was controlled by LREE-

rich inclusions (Nd concentrations of 19-53 ppm in

the L fractions from the SC samples; see Table 1). On

the other hand, the Sm/Nd ratios of unleached garnet

magnetic fractions for each of the two samples from

the SC analysed (fractions labeled bn.l.Q in Table 1)

are also sufficiently high to allow fairly precise two-

point age calculation, either relative to the corre-

sponding whole rock or the data points of the

leachates. These unleached samples support the

results obtained from the leaching experiments.

However, perfect equilibration of Nd isotopes

between garnet host, matrix minerals and/or inclu-

sions has not been achieved during garnet crystal-

lization, as shown by the relatively large MSWD

values in all three samples, when including all points

in isochron calculations.

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