GeolCarp_Vol55_No4_281

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THE TROFAIACH BASIN (EASTERN ALPS): GEOPHYSICAL AND STRUCTURAL STUDY

GEOLOGICA CARPATHICA, 55, 4, BRATISLAVA, AUGUST 2004

281298

THE ARCHITECTURE OF THE TROFAIACH PULL-APART BASIN

(EASTERN ALPS): AN INTEGRATED GEOPHYSICAL AND

STRUCTURAL STUDY

WILFRIED GRUBER

, REINHARD F. SACHSENHOFER

, NICOLA KOFLER

and KURT DECKER

Institute of Water Resources Management, Joanneum Research, Roseggerstraße 17, 8700 Leoben, Austria

Author for correspondence: Tel: +43 3842 47060 2241, Fax: +43 316 876 9 2241, E-mail: wilfried.gruber@joanneum.at

Department of Geological Sciences, University of Leoben, 8700 Leoben, Austria

Rohöl-Aufsuchungs Aktiengesellschaft, Schwarzenbergplatz 16, 1015 Wien, Austria

Institute of Geology, University of Vienna, Althanstrasse 14, 1090 Wien, Austria

(Manuscript received April 4, 2003; accepted in revised form October 2, 2003)

Abstract: Numerous sedimentary basins including the Trofaiach Basin were formed along wrench corridors during the

Miocene lateral extrusion of the Eastern Alps. Because of its rhomboidal outline, a pull-apart mechanism was proposed

for the Trofaiach Basin already in the 1980s. However, the internal basin architecture is still widely unknown. To get a

better insight into basin formation during continental extrusion, the Trofaiach Basin was studied integrating different

geophysical techniques (gravity, seismics, magnetics), digital elevation models, microtectonic and maturity data. Basin

formation is related to the EW trending Trofaiach strike slip fault, which enters the basin at its eastern tip. The northern

basin margin is controlled by a terminating branch of this fault, while the main movement was transferred through the

basin along subvertical faults in the central basin and along its southern rim. (Oblique) normal faults define the western

basin margin. The basin depth reaches a maximum of 800 to 900 m. A fluvial and shallow lacustrine environment was

interpreted from seismic facies and borehole data. Clinoform geometries and petrographic evidence indicate sediment

supply mainly from the South. Localized coal seams developed in different stratigraphic positions. Water depth probably

did not exceed 50 m. Deep lacustrine environments resulting from high subsidence rates are characteristic for many pull-

apart basins, but were not established in the Trofaiach Basin. Several erosional events are part of the evolution of the

basin. An early erosional phase followed southward tilting of the oldest basin fill and uplift of basement rocks north-west

of the basin. A second event caused a major erosional unconformity in the central basin. Finally, related to post-Middle

Badenian compression, more than 1 km of strata have been eroded.

Key words: Miocene, strike-slip tectonic, seismic, gravity, magnetic.

Introduction

The Miocene evolution of the eastern part of the Eastern Alps

is controlled by lateral extrusion and movement of crustal

blocks towards the east (Neubauer 1988; Ratschbacher et al.

1991; Decker & Peresson 1996). The movements occurred

along sinistral north-east and dextral south-east trending

strike-slip faults. A series of pull-apart basins and half-grabens

formed along the Noric Depression, one of the major sinistral

strike-slip zones (e.g. Neubauer et al. 2000; Fig. 1).

Mainly because of former coal mining activity, the geology

of some Miocene basins along the Noric Depression is fairly

well known. In contrast, very little is known about the archi-

tecture of the Trofaiach Basin, which does not host economic

coal deposits (Weber & Weiss 1983). The main sources of in-

formation are coal mines from the turn of the 19

and 20

centuries (Baumgartner, Gimplach; for location see Fig. 2)

and three 400 to 550 m deep wells drilled between 1902 and

1951. In spite of the poor knowledge of the basin architecture,

the Trofaiach Basin was the first basin within the Eastern Alps

for which a pull-apart mechanism was proposed (Nievoll

1985; Neubauer 1988). The main arguments for such a mecha-

nism were the observed sinistral displacements along the

Trofaiach Fault and the rhombic shape of the basin.

In the present paper different geophysical methods (reflec-

tion seismics, gravimetry, magnetics) together with structural

geological investigations are used to reveal the architecture of

the basin. Prior to this, unpublished information on the three

deep wells and on some recently drilled shallow wells will be

presented. Data on the thermal maturity of the basin fill are

used to estimate the thickness of eroded rocks. Apart from re-

gional aspects, the investigation should provide better insights

into the evolution of small-scale intramontane basins along

strike-slip faults.

Geological setting

The Miocene Trofaiach Basin is about 13 km long and up to

5 km wide (Fig. 2). The pre-Neogene basement is formed by

the Upper Austroalpine Greywacke Zone, which is composed

entirely of Paleozoic rocks (Neubauer et al. 1994). Phyllite

and limestone are the prevailing lithotypes with porphyroids,

metabasites, and higher grade metamorphic rocks occurring as

well. Variscan-Alpine metamorphic rocks of the Middle

Austroalpine Unit (gneiss, mica schist, quartzite) are exposed

in the southern and eastern study area (Fig. 2).

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GRUBER, SACHSENHOFER, KOFLER and DECKER

The Trofaiach Basin formed at the western end of the EW

trending left lateral Trofaiach Fault (Vetters 1911), which is

part of a system of en-echelon strike-slip faults along the

Noric Depression (Metz et al. 1978, 1979). The 30 km long

Trofaiach Fault was active in Late Cretaceous times as a duc-

tile shear zone and was reactivated as a brittle shear zone dur-

ing Miocene times (Neubauer et al. 1995; Nievoll 1985).

Due to limited exposures, the basin fill and its internal

structure are poorly known. According to Petrascheck (1924),

flat lying reddish conglomerates interlayered by clay and red

paleo-soil north-west of Trofaiach represent the oldest rocks.

Except for this area, the basin fill generally dips towards the

south-east and is dominated by shale, sandstone, and thin con-

glomeratic layers. Tuffitic horizons are also present. The bed-

ding dips at higher angles (up to 30°) along the north-western

basin margin and is lower (about 10°) near the fault controlled

southern margin of the basin. Gently westward dipping sand-

stones were observed in the basin centre (Hoefer 1902a).

Thin SSE dipping (~25°) coal seams occur in a deep strati-

graphic position near Baumgartner (lower seam) and in a

higher position near Gimplach (main seam; Hoefer 1902a).

The lower seam is accompanied by sapropelic shale with

numerous shells. The main seam consists of two coal beds

each about 1 m thick separated by a shaly and sandy succes-

sion. Hoefer (1902a) mentioned an upper seam a few hun-

dred meters above the main seam. Thin coal seams in even

higher stratigraphic positions are known to exist in different

parts of the Trofaiach Basin.

A tuff layer in borehole A5 (252 m above sea level) was

dated using the zircon fission track technique and yielded an

age of 17.3±1.2 Ma (I. Dunkl, pers. comm.). This indicates

that the basin fill has a similar age to that of neighbouring

Karpatian (late Early Miocene) to Badenian (early Middle Mi-

ocene) basins along the Noric Depression (e.g. Sachsenhofer et

al. 2000; Weber & Weiss 1983).

Borehole data

Lithologs of the boreholes Dirnsdorf, Trofaiach 1,

Trofaiach 2, KB13 and A5 are shown in Fig. 3. Unfortu-

nately none of the boreholes was further investigated by

wireline logs.

Very little is known about the 150 m deep borehole

Dirnsdorf, which drilled through two thin seams at 27 m

(0.65 m thick) and 48 m (0.45 m) depth. Although the drill site

is located only about 300 m from the western basin margin, it

did not reach the basement.

Trofaiach 1 investigated the sedimentary sequence beneath

the main seam horizon near Gimplach (Hoefer 1902a). It

Fig. 1. Sketch map of the study area together with Neogene faults (modified after Decker & Peresson 1996 and Neubauer et al. 2000). The

insert shows the study area within the Eastern Alps. The Noric Depression is indicated as a sinistral strike-slip corridor bounding an eastward

extruding wedge.

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THE TROFAIACH BASIN (EASTERN ALPS): GEOPHYSICAL AND STRUCTURAL STUDY

Fig. 2. Simplified geological map of the study area showing well locations and former coal mines within the Trofaiach Basin.

drilled mainly shale, coaly shale, several thin layers of sand-

stone and reached the phyllitic basement at a depth of 450 m.

Trofaiach 2 was planned to investigate the succession above

the main seam and penetrated shale, some coaly shale, a

seam 0.3 m thin in 196 m depth and several layers of sand-

stone and conglomerate (Hoefer 1902). Gas eruptions (meth-

ane?) were reported at depths of 268 m (conglomerate) and

280 m (shale; Weber & Weiss 1983).

Boreholes KB1 to KB3 are located near the basin centre and

drilled grey sandstone, siltstone and claystone. A number of

fault planes, either with horizontal slickenside lineation or

with steeply dipping vertical lineations, were penetrated in

KB1 and KB2.

Borehole A5 is located in the narrow Laintal Valley and is

555 m deep (Lackenschweiger 1951). Nevertheless the base-

ment was not reached, clearly indicating steep basin margins.

The sedimentary succession is dominated by shale, calcareous

shale and sandstone. In comparison to boreholes Trofaiach 1

and 2 in the central basin, the proportion of sand is higher in A5.

Two tuff horizons more than 1 m thick and a few thin layers of

coal and coaly shale occur as well. Conglomerates near the base

of the borehole were interpreted as representing a basal con-

glomerate. The drilled rocks are generally gently dipping (10

20°), but are steeper in a heavily faulted zone between 437 and

452 m depth. A steeply dipping calcite filled fault was pen-

etrated in 430 m depth (see insert in Fig. 3). Left lateral motion

was reconstructed from fibrous calcite observed in drill cores.

Thermal maturity

Vitrinite reflectance of surface samples and samples from

wells A5 and KB13 were determined following established

procedures (Taylor et al. 1998). Results are presented as ran-

dom reflectance values (R

). Additional data were taken from

Sachsenhofer (1989).

Vitrinite reflectance in well A5 decreases upwards from

about 0.55 to 0.40 % R

. In spite of the rather short depth in-

terval and the scatter of the data (Fig. 4), the reflectance trend

was used to estimate the thickness of eroded rocks. In a first

attempt linear and logarithmic trend lines were calculated and

extrapolated to a surface value of 0.2 % R

. The results sug-

gest that rocks 1000 m to 1500 m thick were removed above

the youngest outcropping sediments. The basin modelling ap-

proach was also applied, using PetroMod 1D software (Yalcin

et al. 1997) and the subsidence history shown in the insert of

Fig. 4. A good fit is obtained assuming a paleoheat flow of

85 mW/m

and erosion of rocks 1200 m thick. Note that the

short depth interval results in a considerable uncertainty of the

heat flow estimate (±25 %). However, an elevated heat flow

in the Trofaiach area fits well with paleoheat flow maps for

Miocene times (Sachsenhofer 2001).

Along a surface profile in the central Trofaiach Basin,

vitrinite reflectance increases from 0.32 % R

in the north-

west (Baumgartner) to 0.41 % R

in the south-east (Fig. 4).

Even higher values occur in the Leoben Basin. Within the

284

GRUBER, SACHSENHOFER, KOFLER and DECKER

Trofaiach Basin the oldest sediments are exposed at the north-

western basin margin and are characterized by the lowest

reflectivity. This suggests that the south-eastward increase in

maturity is controlled by lateral heat flow variations rather

than by the stratigraphic position of the rocks (see also

Sachsenhofer 2001).

Geophysical investigations

A published Bouguer gravity map of Styria includes the

Trofaiach Basin (Winter 1993), but is not detailed enough for

this study. Therefore, in a first step, the resolution of this sur-

vey had to be improved by measuring additional gravity sta-

tions. Preliminary results of the gravity survey were used to

find the best location for two reflection seismic profiles. Addi-

tional gravity data were acquired along the main seismic pro-

file. Finally, a magnetic survey was conducted. The primary

aims of the latter were to support the interpretation of the

gravity data and to track the strike of fault planes interpreted

in the seismic section.

Reflection seismic lines

Data acquisition and processing

In May 2000 two shallow reflection seismic lines were ac-

quired. The locations of the seismic sections are shown in Fig.

Fig. 3. Lithostratigraphic profiles of boreholes in the Trofaiach Basin (after Hoefer 1902a,b; Petrascheck 1924; Lackenschweiger 1951). For

location see Fig. 2. Insert shows a photograph of fibrous calcite indicating post sedimentary sinistral movements along the Trofaiach Fault.

2. The NNWSSE directed profile TR0001 is 4 km long. It

starts about 250 m south of the northern basin margin, crosses

the depocenter of the basin, and ends at the southern basin

margin. Spacing of the receiver stations was fixed to 10 m and

an accelerated drop weight was the seismic source.

Data processing was performed on a Unix based worksta-

tion by using the Focus/Disco software package and followed

standard procedures for land seismic data. Wave equation mi-

gration was performed on line TR0001 using a velocity model

slightly faster (105 %) than the stacking velocity. The final

display of time sections refers to a datum plane of +650 m

above sea level.

Seismic interpretation

The PC based SeisX software package was used for geo-

logical interpretation. Interpreted horizons are denoted in

capital letters (AH), faults are labelled by numbers (15).

The stacking velocity was used for rough depth estimations.

Interval velocities calculated from stacking velocities are

2100 to 2500 m/s for upper complex rocks, 2700 to 3200 m/s

for lower complex rocks and above 3800 m/s for basement

rocks.

On line TR0001 good information above the pre-Miocene

basement is provided only along the northern part of the

transect, where it is represented by a southward dipping

baselap surface. The phyllitic basement is characterized by

low reflectivity. The position of the top of the basement is fur-

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THE TROFAIACH BASIN (EASTERN ALPS): GEOPHYSICAL AND STRUCTURAL STUDY

ther constrained by exposures near the northern end of the

seismic line and by well Trofaiach 1, which penetrated the

basement at a depth of 450 m. In contrast to the north-western

basin margin, the southern margin is formed by steep faults.

This interpretation is supported by exposures of basement

rocks south of station 480. The interpretation of the basin con-

figuration between stations 320 and 460 is ambiguous. The

absence of reflections below 500 m suggest a small horst at

station 380 which separates horizontal reflections of the

depocenter in the north from southward dipping reflectors in

the south. However, the exact basement depth is unclear. The

Miocene basin fill is subdivided into a lower complex and an

upper complex by horizon E.

The seismic facies of the lower complex differs greatly

along the transect. At the northern transect southward dipping

reflectors generally exhibit a high amplitude. The deepest re-

flectors terminate against the basement in a baselap relation.

Reflectors below horizon B show lapouts on either side. This

reflection geometry indicates sediment transport oblique to

the transect. Conglomerates, shales and coaly shales in the

deepest part of well Trofaiach 1 suggest deposition in a

fluvio-deltaic environment. Horizons C and D enclose a

northward prograding sedimentary package characterized by

downlaps. Clinoform geometries indicate a water depth in the

order of 50 m. Sediments below an elevation of 500 m in

Trofaiach 1 which show a coarsening upward trend possibly

represent this interval. Several northward dipping listric faults

((1) in Fig. 5) displace the basement and the lower part of the

Fig. 4. Vitrinite reflectance in the Trofaiach and Leoben Basins. Insert shows vitrinite reflectance in well A5. Linear and logarithmic trend

lines are extrapolated to a surface value of 0.2 % R

. Its elevation suggests erosion of rocks 1000 m or 1500 m thick. A basin model, as-

suming the subsidence history shown and a heat flow of 85 mW/m

, results in 1200 m of missing rocks.

basin fill south of station 175. The faults cannot be traced to

the surface and were obviously only active during early stages

of basin formation. Despite faulting and significant lateral

variations in the thickness of single horizons, the thickness of

the complex between the top of the basement and horizon D is

quite uniform. This suggests that the top of the basement was

not yet tilted during deposition of the rocks beneath horizon

D. Thereafter, normal faulting along the prominent fault 2 ro-

tated the lower part of the lower complex. The accommoda-

tion space created by faulting was filled by sediments

baselapping against horizon D. It cannot be decided whether it

is an onlap relationship or the downlap of northward

prograding clinoforms. In the latter case, the topset area is dis-

located by fault system 3. The upper part of the lower com-

plex is characterized by a lower dip angle than that below ho-

rizon D. This is due in part to faulting along the listric fault 2.

However, an effect of fault 3, which might have re-rotated the

higher part of the lower complex, cannot be excluded. Along

the central and southern part of the seismic section, facies in-

terpretation of the lower complex is problematic due to the ab-

sence of continuous reflectors. Perhaps the rather chaotic low

amplitude reflectors result from intense strike-slip faulting.

Horizons F and G define the base of the sequences with north-

ward prograding clinoforms near the southern basin margin. It

cannot be decided, whether these horizons correlate with hori-

zons C and D at the northern basin margin.

Before deposition of the upper complex, major erosion

caused erosional truncations between stations 330 and 380

286

GRUBER, SACHSENHOFER, KOFLER and DECKER

and created a prominent reflector (horizon E). The resulting

relief was filled up by the upper complex onlapping the ero-

sional surface. Reflectors are generally characterized by a low

continuity and moderate amplitude. Upper complex sediments

dominated by shale and sand were drilled by boreholes KB1

3 and Trofaiach 2. Thin coal seams are present in outcrops and

KB boreholes, suggesting a similar depositional environment

to that during deposition of the lower complex. Fault systems

4 and 5 reach the surface and displace horizon E and upper

complex sediments. Obviously, these fault systems are

younger than faults systems 1 and 2. Tectonic data from bore-

holes KB1 and 2 suggest a major strike-slip component of

fault system 4. Fault system 5 corresponds to the south-eastern

basin margin.

Line TR0002 is of bad data quality due to unfavourable po-

sitioning parallel to the main fault zone. Some information

comes from the uppermost 200 m, where reflectors of low co-

herency show slight westward dips. Further detailed interpre-

tations are impossible.

Fig. 5. Migrated seismic time section (above) and interpretation of the NNWSSE profile TR0001. Nonlinear depth scale is estimated from

stacking velocities and refraction analyses. Well projection distances are 630 m for Trofaiach 1 and 1100 m for Trofaiach 2. Faults are refer-

enced by numbers, main horizons by capital letters.

Gravity survey

Data acquisition and processing

250 gravity stations were measured using a LaCoste-

Rhomberg gravity meter model G 374. Data collection was

performed in a single loop measurement with multiple instru-

ment readings. The altitude of the gravity stations was deter-

mined by differential-GPS surveys with an elevation error

ranging from 1 to 10 cm.

Data processing started with tidal corrections, which indi-

cated small long-term drift rates. Subsequently the Bouguer

corrections and terrain corrections to a reference spheroid

were made. Fays and Bouguer corrections were done for a

correction level of +600 m and simply a constant reduction

density of 2670 kg/m

. In order to represent the data graphi-

cally the irregularly spaced grid must be interpolated. In

geostatistical analyses of the processed data an anisotropy for

the directional angle of 33° with an axis ratio of 1 : 2 was ob-

287

THE TROFAIACH BASIN (EASTERN ALPS): GEOPHYSICAL AND STRUCTURAL STUDY

served. A Kriging algorithm utilized these results in a

variogram model, which was used for gridding.

In the contour map of the Bouguer anomaly (Fig. 6a) the val-

ues range between 25.9 mGal and 9.6 mGal. The correlation

between gravimetric and topographic structures indicates that

surface effects predominate the anomaly field. For further struc-

tural enhancement of the anomalies a regional field was sub-

tracted from the Bouguer data. This regional field was defined

by a simple first order polynomial. The obtained residual

anomaly map is not shown, since it displays similar features to

the Bouguer anomaly map. The horizontal gradient of the

Bouguer anomaly is mapped in Fig. 6b. The gradient of the

Bouguer anomaly highlights zones of maximum lateral changes

in gravity and thus traces the boundary between bodies of dif-

ferent densities. This can be caused either by fault related base-

ment relief or by anomalies within the basement or basin fill.

The basin structure was modelled along seismic line

TR0001 using the residual gravity field for calibration. The

model calculation was carried out using the program GM-SYS.

The main goal of the model is to check whether the basin

structure interpreted from the seismic line is in accordance

with observed gravity data.

Interpretation

Gravity field

The Bouguer anomaly outlines the overall shape of the ba-

sin as an elongated flat bottomed trough (Fig. 6a). The central

parts of the basin are characterized by the most negative val-

ues (26 mGal to 22 mGal). The basin depth appears to be

constant in the western and central parts and shallower in the

narrow eastern part.

The gradient map (Fig. 6b) yields information on the loca-

tion of faults. A WSWENE trending zone with very high gra-

dients follows the steeply dipping south-eastern basin margin

fault. Another zone with a high gradient represents a parallel

lineament north of the basin margin. The gradient along this

fault increases towards the south-west. This is probably be-

cause the lineament is a central basin fault in the east, which

grades into the basin margin in the south-west. Both faults

were interpreted along seismic line TR0001. The southern

fault corresponds to fault system 5, whereas the northern fault

was labelled fault system 4.

Due to a low number of gravity stations in the north-eastern

study area, the trace of the Trofaiach Fault in the Laintal area

is hardly reflected in the gravity maps. However, the continua-

tion of the Trofaiach Fault west of Trofaiach is visible as a

roughly E-W striking zone with a high horizontal gradient.

Note that the western segment of the Trofaiach Fault is not ex-

actly the continuation of the fault forming the northern basin

margin in the Laintal area. This suggests subtle displacements

of this major strike-slip fault.

Two NESW oriented segments with high gradients occur

at the north-western basin margin. Because of a low density of

gravity stations in this area (Fig. 6a), the exact position of the

southern segment is poorly constrained. The most eastern po-

sition theoretically possible is shown in Fig. 6b. Even in this

case, the pattern of gravity anomalies suggests that the western

basin flank is displaced along an EW trending line.

Model results

The gravimetric model along seismic line TR0001 is shown

in Fig. 7. Bench marks of the basin geometry are a shallow

dipping north-western basin flank constrained by seismic

data, borehole Trofaiach 1 and a nearly vertical southern basin

margin.

Laboratory measurements of 15 phyllitic basement rocks

yielded an average density of 2690 kg/m

. The average den-

sity of Paleozoic carbonates beneath the Styrian Basin is

2770 kg/m

(Sachsenhofer et al. 1996). Considering these re-

sults, a model density of 2720 kg/m

was used for the mainly

phyllitic basement. Hussain & Walach (1980) determined in

situ density values for Miocene sandy marls (2520 kg/m

) and

sandstones (2630 kg/m

) in the nearby Fohnsdorf Basin (see

Fig. 1 for location of Styrian Basin and Fohnsdorf Basin). On

the basis of this work, and in order to underestimate rather

than to overestimate basin depth, a density of 2520 kg/m

was

adopted for the deeper parts of the Miocene basin fill and

24502480 kg/m

for the shallower parts of the basin fill.

Upward decreasing densities of the sedimentary basin fill

can be related to lower compaction rates of shallower layers.

Both, the 2450 kg/m

body in the northern part and the

2480 kg/m

in the southern dip parallel to the bedding planes

and are structured by the main faults (Fig. 7). The high gravity

gradient near profile meters 2000 and 3900 suggests a shallow

depth for a body with significantly lower density. This body

might partially represent upper complex rocks.

The deeper structure of the southern part of the basin is

more speculative, so two alternative models with comparable

residual error were tested. Model A follows the seismic inter-

pretation where the basement is characterized by a horst.

Model B incorporates a broad depocenter, a sub-horizontal

basement top, but a higher density of the sedimentary sequence.

Magnetic survey

Data acquisition and processing

The total magnetic intensity (TMI) was measured along 7

profiles (Fig. 8) with an overall length of 21.5 km. Readings

were made at a station spacing of 30 m using a Geometrics

G816/826 proton magnetometer. The readings were repeated

three times and subsequently averaged at every station. Sta-

tions close to powerlines, buildings, known pipes and those

with larger fluctuations than ±2 nT were rejected. Station

coordinates were obtained to within 5 m by using a portable

GPS receiver. Data processing included corrections for diur-

nal variations recorded at a base station every hour and for

latitude (0.0026 nT/m), longitude (0.001 nT/m) and eleva-

tion (0.02 nT/m) referred to the base station (Militzer & We-

ber 1984).

Kriging, using variogram analysis, provided a TMI database

to derive a contour map (Fig. 8). Profile spacing is large com-

pared to the station intervals so that only regional features can

be interpreted on the contour map. Interpretation of high fre-

quency anomalies can only be performed along a measured

profile.

In addition, a 2.5 D modelling technique was used to derive

models with bodies causing the main magnetic anomalies

289

THE TROFAIACH BASIN (EASTERN ALPS): GEOPHYSICAL AND STRUCTURAL STUDY

Fig. 7. 2.5 D gravity modelling along the profile shown in Fig. 6.

turbed only by small-scale negative anomalies of 10 to 30 nT

amplitude and 100 to 300 m wavelength (Fig. 9). Benson &

Mustoe (1995) report that negative anomalies may be due to

oxidation and alteration of magnetite and secondary deposi-

tion of limonite by ground water moving through fault zones.

This interpretation agrees with the observation that the south-

ern basin margin fault is characterized by negative anomalies.

Model results

Several profiles cross the southern basin margin. Model

bodies with a susceptibility contrast of about 0.003 SI units

relative to the basin fill have to be considered to fit the ob-

served magnetic data at the southern parts of profiles 6, 4 and

3. The body at profile 6 reaches the surface and correlates ex-

cellently with exposures of porphyroids at the southern end of

this profile (Fig. 2). The model bodies at profiles 6 and 4 have

vertical northern edges indicating truncation by steep faults,

which cause small negative anomalies. Towards the east (pro-

file 2), the models suggest the presence of a structured body

with a relative susceptibility of 0.002 SI units, representing a

shallow, faulted basement. The faulted basement probably ex-

tends eastwards to the southern part of profile 1 characterized

by strong variations in the magnetic field. A distinct TMI de-

crease at the very southern end of profile 1 indicates a body

with a lower susceptibility (0.002) than the basin fill. Com-

parable low susceptibilities were measured by Ströbl (pers.

comm.) in carbonates exposed at the southern end of profile 1

and at the western basin margin (Fig. 2).

effect of a giant positive anomaly (+700 nT, 8 km wave-

length) located north of the Trofaiach Basin (Gössgraben

anomaly according to Ströbl, pers. comm.). Within the study

area, major, mostly positive anomalies define areas with shal-

low or outcropping basement, whereas a generally smooth

magnetic pattern occurs in the central basin. The smooth char-

acter of the magnetic field within the Trofaiach Basin is dis-

Fig. 8. Map of total magnetic intensity in nT measured along the profiles shown. Arrows indicate local negative anomalies interpreted as

fault zones, + and signs indicate regional positive and negative anomalies.

290

GRUBER, SACHSENHOFER, KOFLER and DECKER

Information about the northern and western basin margins

is provided by profiles 7, 5 and 1. High susceptibilities (0.024

and 0.016) have to be assumed for shallow bodies along pro-

file 7. These bodies are probably related to metabasitic rocks,

which are exposed east of the profile, north-east of Trofaiach.

This interpretation points to a shallow basement depth and in-

dicates that the main fault forming the northern basin margin

is located close to the southern end of profile 7. A more

gradual southward increase in basement depth is suggested by

profile 5. The trough in the TMI curve at profile meter 1500

may indicate a fault. The positive anomaly in the north of pro-

file 1 is best matched by two adjacent bodies with suscepti-

bilities of 0.004 and 0.009, respectively.

Structural investigations

Methods

A digital elevation model (DEM) was used to map linear

morphological features likely to depict brittle faults, and to re-

veal the geometry of major structures, which cannot be

mapped in the field due to limited and poor outcrops in the ba-

sin and along its margins. The DEM is shown in Fig. 10a as a

shaded relief map illuminated from the north-west. The azi-

muth and length of 244 linear features were identified and the

azimuthal distribution of the structures, weighted for their

length, is presented in a rose diagram (Fig. 10b).

Fig. 9. A perspective view of relative total magnetic intensities along seven profiles is shown in the upper part. Arrows point to anomalies

with a short wavelength interpreted as fault zones. Modelled bodies causing the observed long wavelength anomalies are shown in the lower

part. Numbers denote relative susceptibility in SI units. See Fig. 8 for position of profiles.

292

GRUBER, SACHSENHOFER, KOFLER and DECKER

of the relative chronology of deformational events. Bending

of striation, superimposed striations on fault surfaces as well

as cross-cutting relationships between faults were used for es-

tablishing this chronology. The homogeneity of separated

data sets was checked by the computation of pT-axes (Turner

1953). The results were plotted in stereographic projections of

fault patterns in Schmidt net, lower hemisphere.

Interpretation

In the rose diagram (Fig. 10b) several groups of lineament-

directions can be identified. In the first part of this section, E

W to NESW striking faults which form the margins of the

Trofaiach Basin are discussed. Microstructural data from

these faults are plotted in Fig. 11.

An important group of morphological lineaments trends

roughly WE (ca. 265085). The map-scale Trofaiach Fault

in the Laintal area is one of these lineaments. Microstructural

data are not available from the Trofaiach Fault in the Laintal

area because of bad outcrops. However, Nievoll (1985) found

ample evidence for sinistral movements along the central and

eastern sector of the 30 km long fault. Evidence for sinistral

strike-slip faulting is also present at the margins of the

Trofaiach Basin, where a displaced thrust fault indicates dis-

placements in the order of 10 km (Fig. 11). This distance

roughly agrees with Vetters (1911) estimate, which recon-

structed a displacement of 12 km along the central Trofaiach

Fault. West of Trofaiach, the Trofaiach Fault is hidden, but

gravimetric data suggest a 4 km continuation westwards

(Fig. 6b). The main structure of the Trofaiach Fault is not ex-

posed, but accompanying minor faults show cataclasite and

reddish or brownish kakirite. Left lateral WE trending linea-

ments also form important structures with duplexes in carbon-

ates along the southern part of the western basin margin

(Fig. 10; sites 1318 in Fig. 11). The WE directed linea-

ments are not observed within the Trofaiach Basin, where

they are covered by Miocene rocks.

A second group of lineaments strikes ca. WSWENE (235

055 to 255075; Fig. 10). Left-lateral faults in this group are

mostly interpreted as Riedel shears of the sinistral Trofaiach

Fault, formed during the initial stage of basin formation. Rep-

resentative faults are excellently exposed in a quarry north of

Trofaiach (site 1; Fig. 11). Other representatives of this group

are found in the south-eastern margin of the Trofaiach Basin

(see also Figs. 7, 9). In the Laintal area, limestone along the

fault is covered by a massive fault breccia. In the south-west-

ern basin two branches of the strike-slip fault confine an east-

ward tilted basement wedge. Numerous small scale WSW

ENE to WE directed strike-slip faults and dip-slip faults run

parallel to the main structure. Some outcrops show small

scarps, others up to 1 m thick brownish kakirite and

cataclasite. Sinistral faults are also documented in Miocene

sediments drilled from the boreholes KB12 and A5 (Fig. 3).

As the cores are not oriented, the direction of sinistral strike-

slip movements cannot be safely determined. However, the

predominance of WSW-trending lineaments within the south-

Fig. 11. Selected tectonic data for the pull-apart phase of the Trofaiach Basin. Fault planes are plotted in Schmid net, lower hemisphere pro-

jection. Great circles and points denote fault planes and slicken lines, respectively. Double arrows indicate sense of shear.

293

THE TROFAIACH BASIN (EASTERN ALPS): GEOPHYSICAL AND STRUCTURAL STUDY

ern Trofaiach Basin suggests that the cored faults are related

to this group of lineaments. This indicates that the faults re-

mained active after the deposition of the Miocene rocks.

NE-striking lineaments with strike directions between 225

045 and 235055 occur at the western basin margin (Fig. 10).

They coincide with SSE- (sites 1719) and SE-dipping (sites

1215) oblique-normal faults (Fig. 11). This pattern suggests

that the EW directed Trofaiach strike-slip fault in the north

bends into an oblique-normal fault at the western basin mar-

gin. Simultaneously, the NE-striking normal faults are dis-

placed by sinistral EW trending strike-slip faults.

Whereas the above faults are kinematically and geometri-

cally related to the extensional phase of basin formation

within the framework of Miocene lateral extrusion, the role of

lineaments discussed below is ambiguous.

A prominent group of morphological lineaments trends

280100 to 310130 (Fig. 10) sub-parallel to major valleys.

Scarps in the basement west of the Trofaiach Basin, as well as

thick kakirites and cataclasites are related to such faults. Al-

though not detectable in geophysical studies, this group forms

conspicuous lineaments in the Trofaiach Basin. SE trending

faults are frequently observed within the north-eastern Eastern

Alps (e.g. Peresson & Decker 1997) and are generally related

to Late Eocene/Early Miocene dextral movements. A conju-

gate sinistral fault system strikes N to NE. During Late Mi-

ocene times E-W compression reactivated several of these

faults with opposed shear sense (Peresson & Decker 1997).

This may explain the observed evidence for both sinistral (e.g.

sites 6, 7) and dextral (sites 1416, 19) SE trending faults

within the basement rocks (Fig. 12). Within the Miocene ba-

sin fill, dextral NE and sinistral SE trending faults fitting the

Late Miocene stress pattern are expected. However, a sketch

of the Gimplach mine (Fig. 12) shows dextral SE (~310 to

130) and sinistral NE (200 to 020) trending faults. This fault

pattern suggests N-S (NNWSSE) compression.

Discussion

Basin architecture

The northern margin of the basin is formed by the roughly

E-W striking sinistral Trofaiach Fault (Fig. 13). Interpretation

of the DEM (Fig. 10) and of the gravimetric (Fig. 6) and mag-

netic data (Figs. 8, 9) suggests that the fault controlled north-

ern basin margin is split from east to west into three segments,

with each western segment slightly displaced towards the

north. The exact position of the main fault along magnetic

profile 7 (Fig. 9), which crosses the Trofaiach Fault north-

west of Trofaiach, is ambiguous. The magnetic survey sug-

gests that the main fault is located at the southern edge of a

modelled body, whereas a high gravimetric gradient suggests

a slightly more northern position. Changing the relative mag-

netic susceptibility of the model body (e.g. to 0.024 corre-

sponding to the highest observed value) does not solve the

problem. Perhaps there are two faults resulting from the over-

Fig. 12. Selected tectonic data for faults pre- and postdating the main pull-apart phase of the Trofaiach Basin. Fault planes are plotted in

Schmid net, lower hemisphere projection. Great circles and points denote fault planes and slicken lines respectively. Double arrows indicate

sense of shear. The insert is a sketch map of the fault pattern reported from the Gimplach coal mine.

294

GRUBER, SACHSENHOFER, KOFLER and DECKER

lap of the western and central segments of the Trofaiach Fault.

However, because of thick Quaternary cover this hypothesis

cannot be checked. The Trofaiach Fault disappears about

6 km west of Trofaiach. Indications for a westward continua-

tion of the fault exist neither at the northern end of seismic

line TR0001, nor within the exposed basement. NWSE

trending sinistral strike-slip faults north of the Trofaiach Fault

(e.g. site 1 in Fig. 11) are interpreted as Riedel shears.

The south-eastern basin margin is accompanied by two par-

allel ENEWSW striking faults, which include an angle of

about 20° with the Trofaiach Fault. The southern fault forms

the main southern margin of the basin. This rather straight

margin is interrupted by a northward protrusion of basement

rocks south of Trofaiach, splitting the basin margin into two

sectors. The eastern sector is clearly visible in the DEM. The

western sector is visible in the DEM, the gravimetric and the

magnetic survey. The seismic line TR0001 shows that the

fault comprises several steep branches (fault system 5 in

Fig. 5). The northern ENEWSW striking fault is visible in

the gravimetric survey, but its westernmost part is also visible

in the DEM. The fault separates Miocene rocks in the north

from basement rocks in the south along its western part, and

grades eastwards into a steep central basin fault (fault system

4 in Fig. 5). Both parallel faults enclose a basement block,

which is tilted towards the ENE.

Gravimetric and structural data suggest that the north-west-

ern basin margin is formed by south-eastward dipping normal

and oblique-normal faults. These faults are displaced by EW

trending strike-slip faults. Tilted Miocene rocks, but no south-

eastward dipping normal faults, are visible at the northern end

of seismic line TR0001.

An ENEWSW striking gravity low is located in the western

part of the basin between the Trofaiach Fault, the north-western

basin margin and the northern ENEWSW striking fault

(Fig. 13). Traditionally this feature would be interpreted as the

depocenter of the basin. However, the model in Fig. 7 suggests

that at least at its eastern end, the gravity low may be the result

of shallow low density bodies within the basin fill. Along the

strike, the axis of the gravity low is laterally displaced.

According to seismic data and the gravity model, the basin

depth is in the range of 800 to 900 m. Borehole A5 shows that

the preserved thickness of Miocene rocks in the narrow east-

ern part of the basin in the Laintal area is at least 550 m. This

implies steep marginal faults.

Seismic line TR0001 provides information on the internal

architecture of the basin. Along the northern part of this line

the rocks dip in southward directions (~30°) and are horizon-

tal or dip slightly southward along the central and southern

part. Southward tilting of the northern part is related to fault-

ing along fault system 2 (see Fig. 5). A major unconformity

separates rocks from a lower and an upper complex.

Basin evolution

Inferences about the evolution of the Trofaiach Basin fill

based mainly on borehole data (Fig. 3) and on information

from seismic line TR0001 (Fig. 5) are presented in this sec-

tion. However, the reconstruction is fragmented because of

the limited availability of sample material and the lack of in-

terpretable reflectance patterns in some parts of the seismic

transect. The latter is due at least in part to a result of intense

faulting.

Sedimentation in the Trofaiach Basin commenced with the

deposition of coarse-grained clastic rocks. Conglomerates

were drilled in boreholes Trofaiach 1 and A5 and are also pre-

served in outcrops north of the Trofaiach Fault. The basal

conglomerates in borehole Trofaiach 1 are overlain by coaly

and calcareous shale. Seismic patterns along the northern part

of line TR0001 suggest that transport directions perpendicular

to the seismic line prevailed during the early stages of basin

evolution. Coaly shales in Trofaiach 1 and a thin coal seam

with sapropelic shale found at the former exploration mine

near Baumgartner indicate a shallow lacustrine or a marginal

fluvial depositional environment.

Clinoform geometries between seismic reflectors C and D

indicate a later deltaic system, which prograded with apparent

northern directions into an approximately 50 m deep lake.

Sandy layers in borehole Trofaiach 1 at an elevation of 500 m

may represent the sandy topmost part of the prograding delta.

The rocks below reflector D are affected by northward dip-

ping faults (fault system 1) south of borehole Trofaiach 1,

which were only active during the early phases of basin evo-

lution. Because of poor seismic resolution, the continuation of

the rocks south of fault system 1 remains unclear.

The thickness of the tilted Miocene deposits below seismic

reflector D is about 250 m. Remarkably, the thickness does

not decrease northwards (Fig. 5). This suggests either a steep

fault north of seismic line TR0001, which is not observed, or,

more likely that the early basin continued northwards across

the basin margin which exists today. It cannot be decided

whether the Trofaiach Fault was active during these early

stages. Tilting of the northern part of the basin occurred after

formation of reflector D and resulted in uplift of the pre-Mi-

ocene basement along the north(west)ern basin margin, a

steep (30°) southward dip of Miocene rocks and significant

erosion of these rocks north(west) of the present-day basin

margin. The mechanisms which caused this inversion are

poorly understood, because no indications for syndepositional

compression or transpression (reverse faults or folds) were

found in outcrops. In any case, basal Miocene rocks north of

Trofaiach are in a horizontal position, indicating that tilting

did not occur north of the Trofaiach Fault.

The sedimentary sequence between seismic reflectors D

and E is dominated by fine-grained clastic rocks and overlies

reflector D with an onlap or with a downlap relation. The

thickness of the sedimentary sequence increases southwards

and the dip angles decrease upwards (Fig. 5). Both features

are consequences of synsedimentary normal faulting along

listric fault 2.

Thereafter, major erosion incised a channel-like structure in

the central basin. Sand, silt and clay of the upper complex

onlap the erosional surface. Cross bedding and local coaly

interlayers indicate a fluvio-deltaic facies. Sediments in bore-

holes KB13 are extremely rich in detrital mica. The mica is

most probably derived from Middle Australpine micaschists

suggesting sediment transport from eastern or southern direc-

tions. Displacements of seismic reflector E and steeply dip-

ping normal and strike-slip faults in shallow drillholes KB13

testify that faulting continued during and after deposition of

upper complex rocks.

295

THE TROFAIACH BASIN (EASTERN ALPS): GEOPHYSICAL AND STRUCTURAL STUDY

Maturity data indicate that the preserved rocks represent

only a part of the entire basin fill. Rocks more than 1000 m

thick were removed by erosion. This erosion is probably re-

lated to compressional events, which affected the Trofaiach

Basin during post-Middle Badenian times.

A roughly N-S compressional event fragmented the coal

seam in the Gimplach mine (Fig. 12). Similar fault geometries

are known from the Fohnsdorf Basin and are related to post-

Middle Badenian movements along the Lavanttal Fault

(Sachsenhofer et al. 2000; Strauss et al. 2001). On the other

hand, the reconstructed compression also agrees with the

present-day stress-field (Reinecker & Lenhardt 1999). An ad-

ditional E-W compressional event is suggested by the en-ech-

elon arrangement of the sectors of the Trofaiach Fault (Fig.

13) which does not fit with Early/Middle Miocene sinistral

movements, and by the protrusion of basement rocks at the

south-eastern basin margin along dextral NNE and sinistral

NW trending faults. The latter event fits well with Late Mi-

ocene E-W compression across the Alpine-Pannonian realm

(Decker & Peresson 1996; Peresson & Decker 1997).

Basin models

Classical pull-aparts form at major oversteps between dis-

continuous master faults and at releasing bends on

throughgoing faults (Aydin & Nur 1982; Mann et al. 1983;

Sylvester 1988). Both mechanisms require the existence of a

master fault at both sides of the pull-apart structure. Exten-

sional structures in strike-slip regimes also develop at the end of

strike-slip faults (Deng & Zang 1984). In this case the basin

subsides along splay faults (Willemse & Pollard 1998) and nor-

mal faults generated by extension parallel to the main fault.

The Trofaiach Basin is clearly related to the sinistral

Trofaiach Fault. However, a continuation of this fault or a

Fig. 13. a Summary of important features of the Trofaiach Basin derived from geophysical and geological investigations. b Fault pat-

tern at the termination of a single strike-slip fault (modified after a sandbox model; Fig. 7c in Basile & Brun 1999). c Fault pattern at the

non-overlapping overstep of two strike-slip faults (modified after a sandbox model; Fig. 10b in Dooley & McClay 1997).

296

GRUBER, SACHSENHOFER, KOFLER and DECKER

separate master fault at the south-western end of the Trofaiach

Basin in the Liesing Valley are not as obvious. Therefore, it

was first tested whether the geometry of the Trofaiach Basin

can be explained by movements along the Trofaiach Fault

alone. Basin formation at the end of a strike-slip fault was

modelled by Basile & Brun (1999). Resulting fault patterns

are sketched in Fig. 13b. Applied to the Trofaiach Basin, the

uplift of basement rocks north-west of the basin and block tilt-

ing towards the basin centre could be explained by the forma-

tion of a marginal ridge contemporaneous to the horse tail

splay at which the adjacent basin subsided. At the western end

of the northern branch, strike-slip displacement is partly trans-

ferred into NESW strike. Such behaviour was also found in

the analogue model by Basile & Brun (1999), in outcrop scale

(Kim et al. 2001), and on major intraplate strike-slip zones

(Storti et al. 2001). However, apart from the north-western ba-

sin margin, there is no evidence for EW trending strike-slip

faults bending into NS directed normal faults. Moreover,

about 10 to 14 km of left-lateral motion occurred along the

Trofaiach Fault. This was only partly compensated by the for-

mation of the Trofaiach Basin, which is not even 1000 m

deep. Thus, most of the strike-slip movement must have

passed through the basin and continued in the west. These are

major arguments that the Trofaiach Basin represents a classi-

cal pull-apart structure.

Possibly, the westward continuation of the Trofaiach Fault

is hidden below the Quaternary cover of the generally NW

SE trending Liesing Valley. Sinistral displacements along the

Liesing Valley were postulated by Ratschbacher et al. (1991)

and Neubauer et al. (1995). Perhaps, a short WNWESE trend-

ing sector of this valley at the south-western tip of the Trofaiach

Basin (Fig. 1) indicates sinistral displacement of the Liesing

Valley. Alternatively, younger NWSE strike-slip movement

may have disrupted or overprinted the Trofaiach Fault in the

Liesing Valley. Accepting that the Trofaiach Fault continues

into the Liesing Valley, the structure of the Trofaiach Basin

(Fig. 13a) shows striking similarities to fault patterns found by

analogue modelling (Hempton & Neher 1986; Dooley &

McClay 1997; Fig. 13c). The similarities include the positions

of the main strike-slip faults, which grade into (oblique) normal

faults, the array of Riedel shears, and a strike-slip fault crossing

the basin, which in the case of the Trofaiach Basin, links the

Trofaiach Fault in the north with a (speculative) strike-slip fault

in the Liesing Valley. The main movement is probably trans-

ferred along this cross-basin fault zone.

Fig. 14 provides a schematic perspective view of the

Trofaiach Basin, showing the basin as a pull-apart structure.

The second cross-section from below is based on seismic line

TR0001. All other sections are more speculative.

Comparison with other basins along the Noric Depression

Basins along the Noric Depression formed as pull-apart ba-

sins (Aflenz Basin, Parschlug Basin), as asymmetric pull-

Fig. 14. Perspective view of N-S cross-sections through the Trofaiach Basin. Cross-section 2 is based on seismic line TR0001. The other

cross-sections are more speculative.

297

THE TROFAIACH BASIN (EASTERN ALPS): GEOPHYSICAL AND STRUCTURAL STUDY

apart basins (e.g. Leoben Basin), or are characterized by a

pull-apart phase and a halfgraben phase (Fohnsdorf Basin;

Neubauer et al. 2000; Strauss et al. 2001; see Fig. 1 for loca-

tion of basins). Despite these differences, the sedimentary se-

quence of these basins is quite similar and typically comprises

(from bottom to top) fluvial sediments, a single thick coal

seam, a sapropelite, and fine-grained lacustrine/brackish

clastics, which grade upwards into coarse-grained clastic sedi-

ments. This sequence reflects very high subsidence rates dur-

ing the early stages of basin evolution, resulting in the forma-

tion of a deep lake and its subsequent filling. Within this

framework, the Trofaiach Basin is unique. The basal con-

glomerates may represent a fluvial phase, and the fine-grained

sediments partly represent lacustrine deposits. However, fre-

quent coal layers in different stratigraphic positions indicate

that the lake was probably shallow. This is an indication that

the subsidence rates in the Trofaiach Basin were smaller than

in other basins along the Noric Depression preventing the for-

mation of a deep lake. Subsidence rates in the Trofaich Basin

were probably also lower than in the Parschlug Basin, al-

though both basins are controlled by the Trofaiach Fault

(Sachsenhofer et al. 2001). Perhaps the lower subsidence rates

in the Trofaiach Basin indicate that deposition of the pre-

served sediments in the Trofaiach Basin commenced before

the main faulting activity.

Another difference concerns the development of a cross-ba-

sin fault zone in the Trofaiach Basin, which is not apparent in

other basins along the Noric Depression. For example, seis-

mic lines in the Fohnsdorf and Aflenz Basins (Sachsenhofer et

al. 2000; Gruber et al. 2002) show that faulting in these basins

was restricted to the basin margins. This probably reflects dif-

ferences in strike-slip displacements along the principle fault

zone. The overall displacement is about 4 km in the case of

the relatively large Fohnsdorf Basin (Strauss et al. 2001),

probably even less in the Aflenz Basin, and 10 to 14 km in the

Trofaiach Basin. We speculate that only in the Trofaiach Ba-

sin an important part of the total displacement had to be trans-

ferred along a cross-basin fault zone.

Most basins in the western and central Noric Depression,

including the Trofaiach Basin are controlled by EW trending

faults. ENEWSW trending faults, which are postulated by

classical escape models, dominate only in the eastern Noric

Depression.

Conclusions

The Trofaiach Basin formed during the Early/Middle Mi-

ocene lateral extrusion of the eastern parts of the Eastern Alps

at the western termination of the sinistral Trofaiach Fault. The

basin geometry suggests that it represents a classical pull-

apart structure. It occupies a left step between the Trofaiach

Fault with 10 to 14 km of total displacement and a (poorly

constrained) strike-slip fault in the Liesing Valley.

The basin is bordered in the north by en-echelon segments

of the Trofaiach Fault and in the south(east) by near-vertical

ENEWSW trending faults. The western basin margin is

formed by NESW trending (oblique) normal faults. Total

displacement along the Trofaiach Fault was only partly com-

pensated by basin formation. Part of the displacement was

transferred along a cross-basin fault.

The western Trofaiach Basin is about 800 to 900 m deep.

Basin depth in the narrow Laintal area exceeds 550 m. Ac-

cording to borehole data and seismic facies, the basin fill is

dominated by fluvial and shallow lacustrine deposits. As far

as it can be reconstructed, the water depth only reached a

maximum of 50 m. In contrast to other basins along the Noric

Depression, deep lacustrine environments were not estab-

lished, indicating that subsidence rates in the Trofaiach Basin

were probably lower than in other basins along the Noric De-

pression. A major erosional unconformity (seismic reflector

E) within the basin fill separates a lower complex from an up-

per complex.

The preserved basin fill represents only part of the original

deposits. At least three erosional events occurred during the

evolution of the Trofaiach Basin. A first phase is related to

tilting of the northern part of the basin along transect TR0001

and uplift of basement rocks north-west of the basin. A deep

channel-like structure was eroded into lower complex sedi-

ments during a second erosional event. Finally, rocks more

than 1 km thick were removed during a third phase. The latter

event is related either to post-Middle Badenian N-S or to Late

Miocene E-W compression.

The main differences from other basins along the Noric De-

pression include the presence of a cross-basin fault, more ero-

sional events, coaly layers within the entire basin fill, and the

absence of deep lacustrine depositional environments. This

shows that basin forming mechanisms changed significantly

along different segments of the main wrench corridor within

the Eastern Alps.

Acknowledgments: We wish to thank the seismic field crew

for their encouragement and the colleagues at Joanneum Re-

search for critical comments on seismic processing and inter-

pretation. We are grateful to G. Walach sen. for supervising

the gravity survey and to R. Scholger who helped interpret

magnetic data. D. Reischenbacher is acknowledged for cheer-

fully supporting field work. I. Dunkl kindly provided unpub-

lished FT ages. Linguistic improvement introduced to the

manuscript by Ch. Wohlfahrt is also gratefully acknowl-

edged. This study was funded under Grant P 14025-Tec by

the Austrian Science Foundation. Parts of the seismic survey

were funded by VALL.

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