GeolCarp_Vol51_No5_281

GEOLOGICA CARPATHICA, 51, 5, BRATISLAVA, OCTOBER 2000

281300

RESULTS OF 2D BALANCING ALONG 20° AND 21°30 LONGITUDE

AND PSEUDO-3D IN THE SMILNO TECTONIC WINDOW:

IMPLICATIONS FOR SHORTENING MECHANISMS

OF THE WEST CARPATHIAN ACCRETIONARY WEDGE

MICHAL NEMÈOK

1,2

*, JÁN NEMÈOK, MAREK WOJTASZEK

, LÍVIA LUDHOVÁ

, RICHARD

A. KLECKER

, WILLIAM J. SERCOMBE

, MIKE P. COWARD

and

J. FRANKLIN KEITH, JR.

Department of Geology, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BP, UK

Institute for Geology, University of Würzburg, Pleicherwall 1, D-970 70 Würzburg, Germany

Institute of Geological Sciences, Jagiellonian University, ul. Oleandry 2A, 30-063 Kraków, Poland

Department of Mineralogy and Petrology, Faculty of Science, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovak Republic

Amoco Prod. Co., P.O.Box. 4381, Houston, TX 77210, USA

Earth Sciences and Resources Institute, University of South Carolina, Columbia, SC 29208, USA

(Manuscript received March 15, 2000; accepted in revised form June 20, 2000)

Abstract: The restoration of structures along two balanced cross sections through the West Carpathian accretionary

wedge and the pseudo-3D restoration in the Smilno tectonic window area shows that various defined units are parts of

the Magura and Silesian sedimentary successions. The shortened Magura and Silesian successions were detached at

the base of the Upper and Lower Cretaceous sediments, respectively. The interpretation of the structural and sedimen-

tological data places the Magura depositional area as the southwestern neighbour of the Silesian depositional area.

Both areas were shortened during the Upper EoceneOligocene. The Magura area was shortened strongly owing to

the collision between the Alpine orogen and the European Platform. The Silesian area was shortened gently due to the

subduction of the oceanic plate attached to the European Platform. The Magura Unit was thrust over the Silesian

sediments much later during the Miocene as an out-of-sequence oblique thrust. The Miocene shortening of the Magura

Unit and the oblique closure of the Silesian portion of the basin caused a significant contribution to the orogen strike-

parallel sinistral strike-slip faulting in the deformation of the accretionary wedge. The general shortening mode was a

piggy-back process. Thrust geometries were created by both the fault-bend and fault-propagation folding. The fre-

quent out-of-sequence thrusting is caused by the friction/erosion interplay. Variations in friction along the basal thrust

include low friction, documented by subhorizontal veins with vertically grown fibers and long thrust sheets, medium

friction, indicated by the duplexing, and high friction indicated by antiformal stacks. Basement steps along pre-

existing rifting-related normal faults caused complications in the wedge geometry. The step perpendicular to the

tectonic transport caused the development of the antiformal stack, the oblique step caused the sinistral transpression.

Key words: West Carpathian accretionary wedge, 2D and pseudo-3D structural balancing, deformation mechanisms,

basin restoration.

Introduction

The Outer West Carpathian accretionary wedge (Fig. 1) was a

focus for numerous studies in the sixties and seventies that pre-

sumed a fold and thrust-belt character (e.g. Mahe¾ 1973). The

few recent balanced cross section projects (e.g. Roure et al.

1993; Roca et al. 1995) also assume that thrusting was a domi-

nant mechanism. These balancing campaigns frequently used

large scale map data (e.g. from Poprawa & Nemèok 1989) and a

limited number of bore holes, but their objectives did not in-

clude studying the deformation mechanisms in detail.

Thrust sheets of the wedge comprise the fill of several ba-

sins. These basins include the Early Cretaceous rifts that

evolved on a passive margin of the European Platform (e.g.

widziñski 1948; Ksi¹¿kiewicz 1960, 1962a, 1965, 1977),

the Upper Cretaceous-Paleocene basins formed by basin in-

version of earlier rifts (e.g. Suk et al. 1984 and references

therein; Malkovský 1987; Schröder 1987) and the Eocene-

Oligocene deep foreland basin (e.g. Poprawa & Nemèok

1989 and references therein; Winkler & l¹czka 1992).

This paper introduces regional balanced cross sections

along Krakow-Zakopane and Bochnia-Kroscienko transects

and the pseudo-3D balancing in the Smilno tectonic window

area. Both transects and area balancing are based on detailed

field data, seismic, magnetotelluric, bore hole data and data

from published geological maps. The paper presents a calcu-

lation of the shortening and restoration of the original basin

width along regional cross sections. The main aim of the pa-

per is to use the data produced by balancing to examine

mechanisms of the wedge deformation in detail. Time period

names and related radiometric ages used for the Neogene and

older periods are taken from the time-scale for the Central

Paratethyan Neogene (Vass et al. 1987; Rögl 1996) and time-

scale of Haq & van Eysinga (1998), respectively.

*Present address: EGI, University of Utah, 423 Wakara Way, Salt Lake City, U.S.A.

RESULTS OF 2D BALANCING AND PSEUDO-3D IN THE SMILNO TECTONIC WINDOW 283

Methods

Regional transects were located to be parallel to the tectonic

transport. They cross the whole accretionary wedge, being

pinned on the platform. Both profiles and a key area were

mapped and checked in the field (Figs. 1, 2), using available

1:50,000 Polish and Slovak Geological Survey maps. The aver-

age density of field-check locations is one per each km of the

profile. At each location, a GPS location was made, measure-

ments of bedding attitude, sediment transport and structural fea-

tures were taken, and samples for biostratigraphic analysis were

taken. Most of available bore hole data and reflection seismic

profiles were taken from the Amocos confidential data package

acquired together with an exploration concession.

In order to avoid difficulties during the 2D balancing, pinch-

ing and swelling facies and laterally changing facies were

grouped in larger sequences to obtain suitable layered packag-

es (Figs. 3, 4). Marginal facies and series related to potential

intra-basinal highs belonging to the Subsilesian Nappe were

grouped together with their basinal equivalents belonging to

the Silesian Nappe (Fig. 3) in these packages. Sediments be-

longing to the Grybów, Obidowa-Slopnice and Dukla Units

were grouped as in the case of the Silesian Nappe. Because the

balancing has shown that they are the southern continuation of

the Silesian sediments, they are enclosed as the southern part

of resulting layers. This results in the sedimentary succession,

which will be called the Silesian Succession in this paper.

Some of Magura and Silesian layers, result of grouping, are di-

achronous, progressively younger toward the foreland and to-

ward the east. Resulting sequences were named according to

their age relationship to main evolutionary stages of the re-

gion, known from papers documenting rifting (e.g. Michalík &

Soták 1990; Michalík 1990, 1991; Ksi¹¿kiewicz 1960, 1962a,

b, 1965, 1977; widziñski 1948; Sikora 1976; Rakús et al.

1990; Roth 1973; Jiøíèek 1981, 1982; Malkovský 1987; Han-

zlíková & Roth 1965), basin inversion (e.g. Malkovský 1979,

1987; Betz et al. 1987; Bachman et al. 1987; Schröder 1987;

Suk et al. 1984; Lamarche et al. 1999; Ksi¹¿kiewicz 1954,

1960, 1977; Nemèok 1971), Eocene relative tectonic quies-

cence (e.g. Ksi¹¿kiewicz 1957, 1960; widziñski 1948) and

the youngest accretionary wedge stage, that is syn-tectonic

deposition fed directly by the advancing thrust belt (e.g.

widziñski 1948; Ksi¹¿kiewicz 1957, 1960; Ksi¹¿kiewicz &

Leko 1959; Roth 1973; Rakús et al. 1990). Balancing, de-

scribed later, have proved this division to be correct, as it was

in cases of earlier balancing studies (Roure et al. 1993; Roca et

al. 1995).

After the described preparation, all data readings were pro-

jected into profiles, together with bore hole, shallow reflec-

tion seismic and magnetotelluric data. The regional balanced

Fig. 2. The geological base map of the Smilno tectonic window used for the pseudo-3D balancing (modified after Nemèok 1990). The

area is located in Fig. 1b.

284 NEMÈOK et al.

cross sections were constructed using the Paradigm Geo-

physical GeoSec2D software. The pseudo-3D balancing in

the Smilno tectonic window was made manually along three

short cross sections. Structures interpreted in these cross sec-

tions and the surface were projected on to horizontal sections

at 0 and 500 m altitude. Sediments from regional balanced

cross sections were restored to their original undeformed

state. The shortening was calculated from the comparison of

the deformed and the undeformed state and the strain rate

was determined from the shortening divided by the related

time period in seconds.

Basin fill

Silesian Succession

The oldest continuous unit of the Silesian Succession

above the basal décollement is the ValanginianHauterivian

aged rifting-related sequence (Fig. 4). The basal parts of this

sequence have a Tithonian age in the area further to the west

of our profile. The succession in profile comprises shale,

sandstone and limestone with a cumulative average thickness

of 420 m. It unconformably overlaps its basement. The pale-

ocurrent data from sandstone indicate a transversal sediment

transport, coming from NW-SE striking intra-basinal highs

and basin margins (Ksi¹¿kiewicz 1962b). This oldest rifting-

related sequence is conformably overlaid by the Barremian-

Aptian rift-related sequence comprising mostly shale, which

was occasionally deposited below Calcium Compensation

Depth (CCD). This sequence is also on average 420 m thick.

It is conformably overlaid by the Albian-Cenomanian sedi-

ments which are also related to the rifting. They form a thin

rhythmic distal flysch, on average 280 m thick. It was fre-

quently deposited below CCD and has a longitudinal sedi-

ment transport.

The Turonian sequence related to the inversion of earlier

rifts is conformable in some places and unconformable in

other places, lying on rift related sequences. It consists of on

average 280 m thick flysch sediments (Fig. 4), occasionally

deposited below CCD. The Turonian sequence is overlain by

a Senonian-Paleocene sequence related to the continuing ba-

sin inversion. The contact, in places conformable, in places

unconformable, is characterized by a change from thin to

thick rhythmic flysch (Ksi¹¿kiewicz 1960, 1962b). The NW-

SE striking sediment transport was transversal, from margins

and intra-basinal highs.

This basin-inversion related sequence is conformably

overlaid by the Eocene distal flysch sequence with several

sandstone bodies (Fig. 3). It is 0.45 to 1 km thick (Fig. 4).

The youngest parts of the Silesian Succession are formed

by Oligocene syn-tectonic sediments that frequently uncon-

formably overlie the older sequences described above. They

consist of rhythmic flysch and sandstones on average 2.1 km

thick (Fig. 4). Their sediment transport was transversal,

mainly from the south where the ancestral Carpathian oro-

genic belt existed (see also Ksi¹¿kiewicz 1960, 1962b).

Major competent strata in the Silesian stratigraphic succes-

sion (Fig. 4) are the Senonian-Paleocene thick-rhythmic fly-

sch sequence and the Oligocene flysch and sandstone se-

quence. The secondary competent strata is the Turonian

flysch sequence. Incompetent sequences comprise the Val-

anginian-Hauterivian shale, sandstone and limestone se-

quence, the Barremian-Aptian shale sequence, the Albian-

Fig. 3a. Example of grouping of sedimentary formations of the Magura Nappe (modified after Geroch et al. 1967) into the layered-cake

sequence required by the balancing. Explanation in text.

RESULTS OF 2D BALANCING AND PSEUDO-3D IN THE SMILNO TECTONIC WINDOW 285

Cenomanian thin-rhythmic flysch sequence and the Eocene

pelagic and distal flysch sequence.

Magura Succession

The oldest continuous unit of the Magura Succession above

its basal décollement is the Senonian-Danian sequence related

to an inversion of the Early Cretaceous rifts (Fig. 4). It com-

prises shale, sandstone and flysch facies and is on average 700

m thick. Measured sediment transport from the NE is transver-

sal (see also Ksi¹¿kiewicz 1962b). Older sediments are

present only locally, and are as old as Albian-Cenomanian

(e.g. Miík et al. 1985; Oszczypko 1992).

The Senonian-Danian sequence is conformably overlaid by

the Thanetian-Ypresian pelagic and distal flysch sequence,

that also contains several sandstone bodies (Fig. 3). It is on av-

erage 500 m thick (Fig. 4) and has a longitudinal paleotrans-

port direction, with the exception of the uppermost portion.

The youngest parts of the Magura Succession are formed

by mostly unconformably lying late Ypresian-Priabonian

syn-tectonic sediments. They are on average 1.25 km thick

(Fig. 4) and comprise a thick rhythmic flysch and sandstone.

Their sediment transport was transversal.

The major competent strata in the Magura stratigraphic

succession (Fig. 4) are the Senonian-Danian flysch sequence

and the Lutetian-Priabonian flysch and sandstone sequence.

The incompetent unit is the Thanetian-Ypresian pelagic and

distal flysch sequence.

Balanced regional cross section 1

The cross section is pinned on the East European Platform

and ends at the contact with the Pieniny Klippen Belt (Fig. 5).

Normal faults in the basement and subthrust section below the

wedge detachment fault have been interpreted from available

reflection seismics (profiles 5-3-73K, 5-1-78K, 5A-1-78K)

and magnetotelluric data (e.g. Rylko & Tomas 1995). There

was no evidence regarding their inversion found in outcrop

during this study. The mining activity in the Wieliczka area

has documented evidence of the inversion (Poborski & Jawor

1989) in addition to the field studies of Cretaceous basins in

the Bohemian Massif (e.g. Malkovský 1979, 1987; Betz et al.

1987; Bachman et al. 1987; Schröder 1987). Jurassic subthrust

sediments cut by these faults do not indicate any distinct thick-

ness changes along our profile (Fig. 5). Faults are overlain by

Fig. 3b. Example of grouping of sedimentary formations of the Silesian, Subsilesian Nappes into the layered-cake sequence required by

the balancing (modified after Geroch et al. 1967). Explanation in text.

286 NEMÈOK et al.

undeformed Neogene molassic sediments (Fig. 5). A small

proportion of these sediments is accreted into a wedge. Upper

Badenian sediments incorporated in the frontal part of the

wedge indicate the age of the last thrusting. Behind these Bad-

enian sediments, a wedge is formed by the 3.515 km wide

Silesian thrust sheets. The first thrust sheet of the wedge in di-

rection from the platform has an unconformable contact be-

tween the Oligocene and underlying Senonian-Paleocene sedi-

ments (Figs. 5, 6a). Its whole Eocene pelagic and distal flysch

sequence is eroded off (Fig. 6a). A similar unconformity is

present in the thrust sheet penetrated by the Tokarnia bore hole

(Figs. 5, 6b). The restored cross section provides evidence for

syndepositional thrusting of this sheet during the Oligocene

(Fig. 6b,c). Silesian thrust sheets are mostly deformed by fault-

bend folding (sensu Suppe 1983) associated with the post-Oli-

goceneLate Badenian development of the Carpathian accre-

tionary wedge. The calculated shortening is 75 km, the original

basin width is 131 km and the strain rate 8.8

The deformation of the Magura Nappe is different, being

characterized by fault-propagation folding (sensu Suppe &

Medvedeff 1984) (Fig. 5). Sheets are 4.512 km wide. Fault

tips are usually present in the Eocene pelagic and distal fly-

sch sequence. Unconformably lying and unfolded Middle

Sarmatian transgressive facies of the Orava-Nowy Targ Ba-

sin (Cieszkowski 1992; Nagy et al. 1996) provides the upper

time limit for the wedge deformation. The calculated short-

ening is 20 km, original basin width 64 km and strain rate

1.1

. An unrealistic thickness of the Thanetian-

Ypresian pelagic and distal flysch sequence in the balanced

cross section near Chabowka bore hole indicates a capability

of balancing to find a mapping error in the survey map. The

same is indicated in the restored cross section, which indi-

cates a correct thickness in the neighbouring thrust sheets.

Balancing and restoration does not provide any direct evi-

dence about the pre-Neogene shortening. However, the age

of the youngest Magura sediments in its various parts indi-

cate that the initial shortening of the Magura sedimentary

succession took part during the late Eocene and Oligocene.

The ages of the youngest Magura sediments further indicate

a piggyback sequence of thrusting. Numerous observations

of deformation bands, which were formed prior to Eocene

sediments cementation, in the Krynica and Raèa Nappe of

the Magura nappe system serve as further evidence for pre-

Neogene shortening (e.g. wierczewska & Tokarski 1998;

Tokarski & wierczewska 1998).

The proximal half of the wedge in the restored cross section

indicates a strike-slip component of the movement along

thrusts because of the mismatch of restored sheets. The re-

stored Silesian Basin geometry shows that lithofacies of the

Subsilesian Unit, mapped separately in available survey maps,

are either marginal facies of the Silesian Basin or facies of its

intra-basinal highs. This restored geometry also shows that the

basin originated in the Lower Cretaceous as a system of NW-

SE trending horsts and grabens associated with the Early Cre-

taceous rifting that acted in the European Platform (e.g. Zie-

gler 1982; Malkovský 1987; Ksi¹¿kiewicz 1977).

Balanced regional cross section 2

The cross section is pinned on the European Platform and

ends at the contact with the Pieniny Klippen Belt (Fig. 7).

There have been several normal faults interpreted from the

available reflection seismic data below the wedge. Their

thrust reactivation is not visible from our cross section. Au-

tochthonous Neogene sediments seal these normal faults and

unconformably overlie the Upper Cretaceous and Jurassic

sediments. Both the Jurassic and Cretaceous sediments are

preserved in a rift, the fill of which was not incorporated into

the Outer Carpathian accretionary wedge (Fig. 7).

The upper Badenian molassic sediments are the youngest

sediments of the wedge. They are present in its frontal parts,

where they unconformably overlie the Senonian-Paleocene

Fig. 4. Simplified lithostratigraphic column showing average

thickness values for the Silesian and Magura Basin fill. Note that

age limits for diachronous accretionary prism sedimentary se-

quence is given by onlap and end in the proximal and distal part of

the wedge, respectively. The upper age limit of the underlying

Eocene sequence is adjusted to this.

RESULTS OF 2D BALANCING AND PSEUDO-3D IN THE SMILNO TECTONIC WINDOW 287

Fig.

egional

balanced

and

restored

cross

section

The

location

the

profile

shown

Fig.

1b.

Thick

gray

and

black

lines

dicate

the

detachment

faults

the

Magura

Unit

and

Outer

Carpathian

accretionary

wedge,

respectively.

omment

apping

error

indicates

the

area

where

balancing

indicates

error,

issing

facies,

the

geological

survey

map.

Further

explanation

text.

288 NEMÈOK et al.

sequence related to the basin inversion. Missing sediments

indicate at least 0.6 km of the Lower Miocene erosional re-

moval of the Silesian sediments before the molasse was de-

posited (Fig. 7). Molassic sediments are also present in the

frontal parts of the wedge as they were accreted to its base

(Fig. 7). Frontal thrust sheets of the wedge, comprised of the

Silesian Basin fill, are 2.86.7 km wide. They were made by

the fault-propagation folding. The décollement is located in

the middle Badenian shale sequence with gypsum, like in

profile 1. Subhorizontal veins with the fibrous gypsum are

present close to the décollement (Fig. 8a). Fibers grew verti-

cally in the direction of the minimum compressive stress

(Fig. 8b), indicating the fluid overpressure along the décolle-

ment. The data on the timing of this growth come from the

location Bochnia (location 107) where sub-vertical fibers are

sigmoidally bent (Fig. 9). Their bending indicates that their

growth was coeval with the wedge displacement, which is

determined to be toward the northeast direction (Fig. 9c).

The other location inside the wedge close to its décollement

is at Zglobice (location 106). It shows the shale duplexing,

and sandstone boudinage within a shale horizon, E-W strik-

ing fold axes and randomly oriented gypsum veins indicating

fluid overpressure (Fig. 10). This location is also deformed

by a set of normal faults made by N-S extension roughly

parallel to the regional compression (Fig. 10). Further back

in the accretionary wedge, the Lakta bore holes 1, 3 and 27

allowed the determination of an unconformable contact of

the Oligocene sediments with older sequences in the frontal

part of the thrust sheet (Figs. 7, 11). The restored image of

this thrust sheet (Fig. 11) indicates syndepositional thrust-

ing coeval with and postdating a 1.1 km deep erosion of pre-

Oligocene sequences. The rear of this thrust sheet is folded

adjacent to the Magura sole thrust in its hanging wall. The

immediate contact of the Magura and Silesian Units here is

made by the Zegocina sinistral transpressional strike-slip

fault zone. It brings a large portion of the oldest sediments to

the surface in the form of the strike-slip duplexes. These du-

plexes comprise both marginal and basinal facies of the Sile-

sian Basin fill. The zone is formed above a large NE-SW

striking normal fault in the autochthonous basement that is

oblique to the cross section. 2.116.3 km wide buried Sile-

sian thrust sheets form duplexes underneath the Magura

thrust (Fig. 7). They are formed by fault-bend folding. The

exception from the foreland-vergent duplex system is the

Slopnice antiformal stack and the most proximal parts of the

wedge. The Slopnice antiformal stack is formed by four

sheets of various length, which are cut at the base of the

Lower Cretaceous, Upper Cretaceous and Eocene. The over-

lapping ramp anticlines of the stack do not have coincident

trailing branch lines and the Magura sole thrust above them

is corrugated. The complex structure of the stack rules out its

sequential development (e.g. Boyer & Elliott 1982; Butler

1982). The proximal parts of the wedge have subvertical and

overturned thrust faults. The calculated shortening of the

Silesian Basin fill is 80 km (58 %). The restored basin width is

of 137 km. The calculated horizontal strain rate is 8.9

These values are similar to those from profile 1.

The shortening of the Magura Basin fill is 42 km (50 %)

and results in an original basin width of 83 km and a strain

rate of 1.7

. These values are different from those in

profile 1 and will be discussed later. The Magura Unit shows

evidence of the prevalent fault-bend folding. The Magura

thrust sheets in the cross section are 3.612.1 km wide. The re-

stored balanced cross section (Fig. 7) shows that the average

length of thrust sheets in the frontal half of the unit is compa-

rable with the average length of thrust sheets in the frontal half

of the Silesian Unit. The Magura sole thrust is composed of a

deformed zone up to 100 m thick. This zone varies in compo-

sition according to formations juxtaposed in the footwall and

hanging wall. It either contains sandstone blocks of various

sizes in a highly deformed shale fault gouge or it is formed by

the tectonic breccia in a sandy matrix. The out-of-sequence

movement of the Magura thrust is best documented by the

Fig. 6. a Zoom on the frontal thrust sheet from regional bal-

anced cross section 1 from Fig. 5. Explanation in Fig. 5. b

Zoom on the thrust sheet penetrated by the Tokarnia IG 1 bore hole

from regional balanced and restored cross section 1 from Fig. 5. Ex-

planation in Fig. 5. c Cartoon illustrating tectonic scenario lead-

ing to missing of the Eocene sequence in the anticlinal area of the

thrust sheet from Fig. b. Subsequent stages 14 indicate a pre-

shortening Silesian Valanginian-Eocene sedimentary package (1),

its detachment (2), thrust sheet formation (3) and syntectonic ero-

sion in the anticlinal area and contemporaneous and subsequent

Oligocene deposition (4).

290 NEMÈOK et al.

lower-middle Badenian molasse sediments penetrated by the

Zawoja bore hole between the Magura and Silesian Units

(Moryc 1989), 55 km to the west of our cross section.

The Smilno tectonic window area

Local cross sections 1, 2 and 3 through the Smilno tectonic

window are pinned on the northeastern boundary of the stud-

ied area and end at the southwestern boundary (Figs. 2, 12).

The structure of the area is made by a Magura Unit thrust over

the Silesian duplexes. The Magura basal thrust zone is formed

by a brecciated zone several hundred meters thick which is

penetrated by the bore holes Smilno-1 and Zborov-1. Both

bore holes found hydrocarbon accumulations in this brecciated

zone (Leko et al. 1987; Wunder et al. 1991). The average

strike of thrusts and fold axes in the area is NW-SE. The

Magura Unit is, except thrusts and folds, deformed by two

NE-SW striking sinistral strike-slip faults mapped by Nemèok

(1990). The western one is located between local cross sec-

tions 1 and 3, and the eastern one runs through the local cross

section 2. As shown by our balancing, the eastern strike-slip

Fig. 8. a Great circle and pole diagram of subhorizontal extensional veins filled by gypsum at location Bochnia (loc. 107) in a lower

hemisphere stereonet. The veins are formed in the middle Badenian shale of the Wieliczka Formation. b Scatter diagram of gypsum

fibers vertically grown in veins, which are shown in a.

Fig. 9. a Scatter diagram of gypsum fibers sigmoidally grown in subhorizontal extensional veins at location Bochnia (loc. 107) in a lower

hemisphere stereonet. Numbers at points refer to certain fiber, the black dot indicates the orientation of the initial growth and the white dot

shows the orientation of the late growth. b Arrows indicate the change in orientation from the early stage of growth to the late stage of

growth. c Arrows indicate the accretionary wedge advance trajectories determined from the sigmoidal growth of gypsum fibers.

RESULTS OF 2D BALANCING AND PSEUDO-3D IN THE SMILNO TECTONIC WINDOW 291

fault ends in reality to the west of the local cross section 2. The

area to the NE of its tip is deformed only by the folding and

thrusting. The different northeastward displacement of the

eastern and western parts of the Magura thrust sheets here are

transferred by the sigmoidal bend of folds and thrusts, present

to the northeast of this tear fault. The thrust sheets to the east

of this transfer zone are located further northeastward than the

same sheets present to the west of the bend. However, the

shortening along cross sections to the east and west of the

transfer zone does not differ significantly. The shortening

along the western local cross section 1, calculated for the situ-

ation prior to the out-of-sequence thrusting above the uplifted

Silesian sheet which is exposed in the window, is about 9.6 km

(43.8 %). The shortening along the eastern local cross section

2 is about 13.1 km (51.6 %). The difference is the geometry of

structures involved and the mechanism of shortening. The

Magura thrust sheets, made of the Cretaceous-Paleocene basin

inversion sediments underneath Eocene sediments in the local

cross section 1, are much shorter than the Magura thrust sheets

comprised of both the Cretaceous-Paleocene basin inversion

Fig. 10. a Shale duplexing at the location Zglobice (loc. 106), situated 28 km to the east from regional profile 2, formed by the Upper

Badenian Chodenice Formation that comprises sand/sandstone with shale intercalations. b Sandstone boudines inside the shale hori-

zon. Scatter diagram shows the orientation of pinch-out lines. c Scatter diagram of all fold axes from the location. d Detail of ran-

domly oriented extensional veins filled by fibrous gypsum in the shale horizon. e Great circle diagram of normal faults deforming the

location with the stress state calculated by the program of Hardcastle & Hills (1991). Arrows show the direction of the extension.

292 NEMÈOK et al.

and Eocene pelagic/distal flysch sediments in the local cross

section 2. The former are 1.63.4 km wide and the latter 3.4

7.6 km wide. The Magura sheets with the Cretaceous-Pale-

ocene sediments in the local cross section 1 are formed by the

fault propagation and fault-bend folding. Their ramps end or

tip at the base of the Eocene sequence which is partly de-

tached. The separate movement of the Eocene sequence con-

tinued by the out-of-sequence thrusting, that is indicated by

the complex geometry of the underlying Cretaceous-Paleocene

sheets in the area of the bore hole Zborov-1. It is also indicated

by an apparent 4.3 km extension of the Magura Cretaceous/Pa-

leocene sequence above the uplifted Silesian sheet exposed in

the window. This Silesian sheet is formed by the fault bend

folding and placed at the top of the Smilno antiformal stack.

The out-of-sequence build-up of the stack and the separate

movement of the detached Eocene Magura sequence from its

underlying Cretaceous/Paleocene sheets has created the seem-

ing extension above the stack. The out-of-sequence thrusting

above the Smilno antiformal stack is also indicated by the out-

of-sequence folding of the syncline located in front of the

stack and out-of-the-syncline thrust (sensu McClay 1992)

(Fig. 12). The out-of-the-syncline thrust indicates the top-to-

northeast displacement along the Magura thrust. The short

Magura thrust sheet formed by the Cretaceous/Paleogene sedi-

ments in the local cross section 2 also indicates the out-of-se-

quence thrusting. The remaining sheets in the cross section are

formed in the piggy-back thrusting sequence. Unlike sheets in

the local cross section 1, all thrust sheets in the local cross sec-

tion 2 are deformed by the fault-bend folding.

Local cross section 3 shows thrust sheets which are sepa-

rated by a sinistral strike-slip fault from sheets described

along local cross sections 1 and 2. This cross section indi-

cates the smallest shortening in this area. It is about 3.9 km

(24 %). The whole cross section shows only two sheets

formed by the fault-bend folding. The northern sheet is very

wide, roughly 12.6 km, in the profile. The southern thrust

sheet is roughly 3.6 km wide in the cross section, but contin-

ues towards the SW. It is cut and displaced by a dextral

strike-slip fault.

Thrust structures laterally change over short distances

(Fig. 2). Two open anticlines with the interlimb angles of

Fig. 11. Zoom on the thrust sheet penetrated by the Lakta bore holes from regional balanced and restored cross section 2 from Fig. 7. Ex-

planation in Fig. 7. Figure shows missing Upper Cretaceous to Eocene sequences in the anticlinal area of the thrust sheet, indicating syn-

tectonic erosion in the anticlinal area predating and coeval with Oligocene deposition.

Fig. 12. Local profiles across the Smilno tectonic window area

(area located in Fig. 1b). Location of profiles is indicated in Fig. 2.

Explanation in text. Small line perpendicular to bedding symbol

shows the direction toward older stratigraphy. Note that not all

bedding symbols are parallel to balanced solution. They were ei-

ther ignored after calculation of kink bands, which honored the av-

erage value, or ignored after a check of parasitic folding.

▲

294 NEMÈOK et al.

125° and 145° in the Magura Unit cut by the local cross sec-

tion 3 in the surroundings of the Zborov-1 bore hole merge

into a single anticline in the tectonic window area. This anti-

cline has an acute interlimb angle. Its axial plane dips to the

southwest. This tight anticline becomes open to the east of

the window, having an interlimb angle of 130°. Similar later-

al variations in the strike, geometry and number of thrust

sheets can be observed to the NE and SW of this structure

(Fig. 2).

Interpretation and discussion

Palinspastic implications from regional cross sections

The position of the Magura detachment fault in the Upper

Cretaceous sediments in regional cross sections is much high-

er in the stratigraphic column than the position of the Silesian

detachment fault, which is cut in the lowermost Lower Creta-

ceous sediments. It is physically impossible for a propagating

detachment fault to jump down in the piggyback succession

(see e.g. Mandl 1988). On the contrary, it tends to propagate

upwards in the succession (e.g. Boyer & Elliott 1982; Suppe

1985). If the sediments of the Magura Unit were deposited to

the south of the Silesian sediments, their décollement should

propagate either in the Lower Cretaceous or older sediments,

as it is implied from the mentioned physical laws and staircase

geometry of the décollements in other orogenic belts (e.g.

Rich 1934; Bally et al. 1966). The higher stratigraphic position

of the Magura detachment than the Silesian detachment, thus,

indicates that the Magura succession was not deposited to the

south of the original position of the Silesian Succession as is

generally accepted in the literature (e.g. Rakús et al. 1990).

Sediments of the Magura and Silesian Nappes have to be orig-

inally southwestern and northeastern neighbours, respectively

(Fig. 13a,b; Morley 1996). This determination of their posi-

tions takes into account the Neogene northeastward oblique

thrusting of the Magura Unit (e.g. Nemèok et al. 1998), sinis-

tral transpression in the western part of the Central Carpathian

Paleogene Basin (CCPB) and compression in the eastern part

of the CCPB (e.g. Nemèok et al. 1996), paleomagnetic decli-

nation data (Fig. 1a; Túnyi & Kováè 1991; Krs et al. 1977,

1982, 1991, 1993; Koráb et al. 1981; Pãtrascu et al. 1994;

Márton & Márton 1989), which indicate larger counterclock-

wise mass rotation in the western West Carpathian accretion-

ary wedge than in its eastern part. Sediment transport data col-

lected by numerous authors for the Magura sequences (e.g.

Ksi¹¿kiewicz 1962b; Rakús et al. 1990; Fig. 13c) also indicate

a northern source that was originally interpreted as the cordil-

lera between the Magura and Silesian Basins. Accepting the

northeastward transport of the Magura Nappe (Figs. 13a,b)

this sediment source could only be the Bohemian Massif, lo-

cated north of the original position of the Magura depositional

area (Fig. 13a). When the Magura Nappe got to its present po-

sition by oblique out-of-sequence thrusting, transported pale-

ocurrent indicators in their present position indicate the pres-

ence of a non-existing cordillera (Fig. 13c). This mistakingly

interpreted cordillera was used earlier (e.g. Rakús et al. 1990

and references therein) as the justification for the separation of

the Silesian Basin from the Magura Basin during the Eocene

Oligocene and gives them original pre-thrusted positions as

northern and southern neighbours.

Two Silesian thrust sheets in Figs. 5, 6, 7, 11 indicate the

initial thrusting in the Silesian depositional area to be as ear-

ly as the Oligocene. This date of deformation indicates that

Silesian sediments underwent initial shortening significantly

earlier than when they were overthrust by the Magura Nappe

in the Early-Middle Miocene. Magura out-of-sequence over-

thrust is proved by the presence of the lower-middle Bade-

nian molassic sediments penetrated between the Magura and

Silesian Nappes by the Zawoja 1 bore hole (Moryc 1989) lo-

cated about 30 km to the west of the regional profile 1.

The restored Silesian sedimentary succession (Figs. 5, 7)

shows that all ramps and flats of neighbouring thrust sheets

match with each other. We observe thrust sheets mapped as

belonging to the Grybow, Obidowa-Slopnice and Dukla

Units as close neighbours originally. These thrust sheets

form the southern continuation of the sedimentary succession

present in the Silesian Unit mapped at surface. Their position

in the southern marginal parts of this over 130 km wide basin

with complex morphology justifies their slightly different fa-

cies. Restoration shows that they belong to the same basin

(Figs. 5, 7). The restoration of the Subsilesian Unit also

shows that it belonged to the same basin. Subsilesian facies

are present in the northern margin and intra-basinal highs of

the basin when restored in the cross section.

A relatively young, Early-Middle Miocene, out-of-sequence

Magura overthrust is required to bring the Magura accretion-

ary wedge together with the ALCAPA unit (sensu Csontos

1995) of the Inner Carpathians from its ancestral Eocene-Oli-

gocene position in the present Eastern Alpine area (Fig.

13a,b). The Magura accretionary wedge thus moved eastward

together with the ALCAPA and formed the ancestral accre-

tionary wedge of the Carpathian orogen overriding the sub-

ducting oceanic slab attached to the European Platform. This

eastward movement of the Outer Carpathian wedge involves,

apart from thrusting, a distinct sinistral strike-slip component

parallel to the strike of the wedge. It is documented in the

southwestern parts of the restored Silesian Basin where the

ramps and flats of neighbouring thrust sheets do not match ex-

actly (Figs. 5, 7). They require balancing perpendicular to the

cross section, which indicates strike-slip displacement. In-

deed, a field check of some of them has shown sinistral strike-

slip faulting parallel to the strike of the wedge. Orogen-paral-

lel sinistral strike-slip faulting is also visible easily in the map

of Kulka et al. (1985), indicated by NE-SW striking sinistral

Riedel shears splaying off the Pieniny Klippen Belt into the

wedge. Thrusting and sinistral strike-slip data in support of

this same mechanism are known from the wedge areas further

to the west (Nemèok et al. 1998). Further structural evidence

of partitioned deformation include orogen-perpendicular

thrusting and folding (e.g. Roca et al. 1995; Mahe¾ 1973) and

large-scale orogen-parallel sinistral strike-slip faulting (e.g.

Royden 1985; Royden et al. 1982; Marko et al. 1991). Larger

sinistral rotation of paleodeclination data along the western

margin of the ALCAPA and smaller sinistral rotation of these

data along the northeastern margin (Túnyi & Kováè 1991; Krs

et al. 1977, 1982, 1991, 1993; Koráb et al. 1981; Pãtrascu et al.

RESULTS OF 2D BALANCING AND PSEUDO-3D IN THE SMILNO TECTONIC WINDOW 295

1994; Márton & Márton 1989; Fig. 1a) also support this mech-

anism. Other evidence from the Inner Carpathians about the

eastward movement of the ALCAPA unit is discussed in detail

by Csontos et al. (1992).

Thus the only plausible interpretation is to place the original

Eocene position of the Magura sediments to the southwest of

the Silesian sediments, in the area in front of the ancestral

Eastern Alps-Western Carpathians. The Magura Unit then be-

comes a part of the Middle Eocene-Oligocene accretionary

wedge of the Eastern Alpine-Carpathian orogen, which is

known to be a coherent structural domain during this time

(Royden & Báldi 1988). In such a case the Magura Unit can be

correlated with the Rhenodanubian flysch of the Eastern Alps,

as suggested earlier (e.g. Laubscher & Bernoulli 1982; Toll-

mann 1989). This further implies that the deposition of the

Magura sedimentary succession happened from the Early Cre-

taceous to the Eocene, as based on analogy with the deposition

of the Rhenodanubian flysch (e.g. Faupl 1975; Prey 1980).

This would be in accordance with documented rare occurrenc-

es of the Albian-Santonian sediments in the Magura Unit (e.g.

Miík et al. 1985; Oszczypko 1992). This timing of the deposi-

tion is in accordance with the situation in the Silesian part of

the basin, where the sedimentation started from the Lower

Cretaceous (e.g. Geroch et al. 1967) and lasted longer than in

the sedimentary succession, which became the Magura Unit

(Fig. 4). The correlation of the Magura and Rhenodanubian

flysch further implies that the Magura sediments underwent

their initial shortening in the middlelate Eocene due to colli-

sion in their ancestral position, as based on the Rhenodanubian

flysch data (e.g. Decker et al. 1993 and references therein).

This is in accordance with our data and available data (Eliá et

al. 1990; Stráník et al. 1993), which show that the youngest

Magura sediments in its western part are of middleupper

Eocene age. The age trend of youngest sediments along our

cross section, from the middle Eocene sediments in the south

to the upper Eocene sediments in the north, indicates a piggy-

back sequence of thrusting. A part of the Rhenodanubian-

Magura sediments later became the northeastern part of the

wedge, known as the already mentioned ALCAPA block (sen-

su Csontos 1995), which extruded eastward during the Mi-

Fig. 13. a Late Oligocene regional geological setting in the Alpine-Carpathian-Pannonian area (modified after Morley 1996). Thick arrow

indicates convergence in the Alpine area. b Early Miocene regional geological setting in the Alpine-Carpathian-Pannonian area (modified

after Morley 1996). Thick arrow indicates convergence in the Carpathian area. Note that the advance of the Magura Unit to its new position

requires its oblique advance. It is in accordance with predominant sinistral strike-slip faulting along its western part and predominant thrust-

ing in its eastern part (e.g. Nemèok et al. 1998), larger paleomagnetically-indicated counterclockwise mass rotations in its western part and

smaller counterclockwise mass rotations in its eastern part (e.g. Túnyi & Kováè 1991; Krs et al. 1977, 1982, 1991, 1993; Koráb et al. 1981;

Pãtrascu et al. 1994; Márton & Márton 1989). c Sediment transport data for the Lutetian (modified from Rakús et al. 1990).

Late Oligocene

European Foreland

Thrust front (Jura)

Thrust front (Molasse Basin)

Carpathian thrust front

Outer Carpathian flysch

Helvetic zone

Magura flysch

Alcapa

Tisza-

Dacia

Penninic zone

Austro-Alpine thrust sheets

Flysch

PKB

Thrust front

(Balkans)

200 km

Vardar

Skole

Silesian

Subsile

sian

Dukla

Magura

PKB

Central Carpathian

Paleogene Basin

Emergent land

Thick continental

crust

Siliciclastic turbidites

Calciclastic turbidites

Thin continental

(oceanic) crust

Fault

Facies boundary

Eocene (Lutetian)

(53 - 39 Ma)

Early Miocene

European foreland

Overthrusting of

Austro-Alpine thrust sheets

largely accomplished by

Lower Miocene times

Extensive strike-slip

deformation and

basin formation (Balkans)

Thrusting in Southern Alps

Extensive strike-slip

deformation in Dinarides

Carpathian thrust front

Outer Carpathian flysch

Alcapa

Tisza-

Dacia

200 km

Magura flysch

Bohemian

Massif

296 NEMÈOK et al.

ocene (e.g. Ratschbacher et al. 1991). The exact timing of the

extrusion and the detailed geometry of the wedge is not firmly

established. The extrusion of the Central Eastern Alps has

been put into the broad late OligoceneMiocene (3014 Ma)

interval (Ratschbacher et al. 1991). Since only the southeast-

ern part of the shortened Rhenodanubian-Magura sediments

became part of the extruding wedge together with the Central

Eastern Alps, this would require erosion of the upper (south-

ern) sheets, which had to be detached at the Lower Cretaceous

level. Such a Lower Cretaceous stratigraphic level of the déc-

ollement is reported from the uppermost Rhenodanubian du-

plexes (Decker et al. 1993). A 34 km thick missing section of

Magura burial, which would comprise missing thrust sheets

detached at Lower Cretaceous level, might be indicated by ab-

normaly high outcrop vitrinite reflectance data from the Mora-

vian part of the Magura Unit (Francù 1997, pers. commun.).

The younger age of the extrusion than in the Central Eastern

Alps is indicated in the Rhenodanubian flysch. The structural

data (Decker et al. 1993) indicate a Miocene age. The West

Carpathian data also indicate the younger timing of the AL-

CAPA block movement. They include paleomagnetic evi-

dence for the Eggenburgian-Karpatian progressively decreas-

ing counterclockwise mass rotation (Túnyi & Kováè 1991)

and the Karpatian pull-apart opening of the Vienna Basin

(Royden 1985), both along the northwestern boundary of the

ALCAPA block, and an existence of lower-middle Badenian

molassic sediments between the Magura and Silesian Units

(Moryc 1989) along the northeastern boundary of the ALCA-

PA block.

The earlier collisional shortening of the sediments of the

future Magura Unit explains why the calculated strain rate

for the Magura Unit is higher than the strain rate calculated

from the Silesian Unit sediments, which were shortened only

due to subduction. Other evidence for large out-of-sequence

movements of the Magura thrust is the Silesian thrust sheet

from Fig. 6b. It was thrust during the deposition of Oli-

gocene sediments (Fig. 6c). It was formed before the Mi-

ocene Magura Unit thrust over the Silesian Unit, during the

time when deposition had finished in most parts of the ances-

tral Magura depositional area and the Magura Unit was in-

volved in the Oligocene collision in its ancestral Alpine posi-

tion to the southwest of the Silesian sediments. Deposition

continued only in few remnant depressions (e.g. Nemèok

1961; Cieszkowski & Olszewska 1986).

Deformation of the wedge

Both regional cross sections (Figs. 5, 7) indicate that the

West Carpathian accretionary wedge was shortened in gener-

al piggy-back mode. However, the buttressing effect of pre-

existing normal faults with greater throw caused two out-of-

sequence thrusts in the regional profile 1, one to the south of

the Obidowa IG 1 and the other near the Trzebunia 2 bore

hole (Fig. 5) and several more along the regional profile 2,

one called the Zegocina transpressional zone and the remain-

ing ones in the rear third of the wedge (Fig. 7). The sinistral

Zegocina transpressional zone is formed in the Silesian Unit

in front of the Magura thrust by the displacement of a wedge

that encountered an oblique buttress in the basement. Defor-

mation brings the oldest wedge sediments to the surface in

the form of small duplexes.

Early normal faults are related to the Lower Cretaceous

rifting (e.g. Roure et al. 1993). None of them in our profiles

indicates their inversion, observed in adjacent areas (e.g.

Ksi¹¿kiewicz 1977; Malkovský 1979, 1987; Schröder 1987).

However, an out-of-sequence thrusting caused by the inver-

sion of basement structures is known from the Wieliczka

area (Poborski & Jawor 1989), to the east of the frontal parts

of our regional profile 1. Neogene molasse sediments overlie

normal faults with smaller throw to the extent that they do

not form any buttresses (Fig. 7).

Normal fault buttresses acted as frictional force concentra-

tors during the shortening, reducing the critical width of the

thrust sheet behind them, as observed in physical or numeric

models (Nieuwland 1997, pers. commun.; Mandl 1988).

Areas with low basal shear stress had wide thrust sheets.

Such Silesian sheets are present in the regional cross section 2

(Fig. 7). They are 8.516.3 km wide, while the average width

is about 6 km. The first of them climbed over the buttress,

which was far to the south from the present position of the

Slopnice stack, if we restore it to its original position. Both the

abruptly increased frictional force along its toe fault and the

weight of the toe caused the out-of-sequence thrusting of the

sheet No. 3 behind it. This one, apparently with very low basal

shear stress, is by far the widest sheet. It has no rifting-related

sediments and is much thinner than surrounding sheets. The

reduced thickness should, however, result in a shorter length

of sheets than in the case of surrounding thicker sheets (see

e.g. Boyer 1995). Under these circumstances its length is even

more anomalous.

Accreted in the wedge, the above mentioned sheets ad-

vanced toward the foreland until they encountered a normal

fault with a stratigraphic omission of 2.3 km in the Slopnice

area. Its increased friction caused the antiformal stack devel-

opment. The out-of-sequence thrusting here is indicated by

Neogene molasse sediments between sheets penetrated by

the Lesniowka 2 well (Fig. 7). The increased friction in the

antiformal stack drove the out-of-sequence movement of the

third sheet behind it. This sheet was cut, together with its

Magura roof thrust, forming a breaching thrust (sensu Mc-

Clay 1992). Any larger out-of-sequence movement of this

sheet was cancelled by the localized erosion above the anti-

formal stack (Fig. 7), which reduced the weight there and

drove a new out-of-sequence thrust. Similar complex short-

ening/erosion interplay can be determined at the Smilno anti-

formal stack (Figs. 2, 12).

The thrust sheet width was also influenced by sediment

thickness. It is visible in the restored frontal part of the Sile-

sian sedimentary succession (Fig. 7) how the thickness and

width of the first six thrust sheets progressively increase

from north to south. This relationship is frequently modified

by complex basin floor morphology. The Silesian detach-

ment fault propagated inside the Lower Cretaceous shale and

carbonate formations. Frontal Silesian sheets were later

thrust above autochthonous Neogene sediments, where the

décollement indicates a very low friction. This is shown by a

RESULTS OF 2D BALANCING AND PSEUDO-3D IN THE SMILNO TECTONIC WINDOW 297

minor accretion of Neogene sediments to the wedge (Fig. 7).

It is also indicated by fact that each rear sheet overrides only

the rear portion of its frontal neighbour (Fig. 7), as observed

in the low-friction sand-box models (Nieuwland 1997, pers.

commun.). As shown by subhorizontal gypsum veins with

vertical fibers (Fig. 8a,b), the low friction can be attributed to

cycles of the overpressure along the décollement. The syn-

tectonic fiber growth is documented by their sigmoidal bend-

ing (Fig. 9a,b) coeval with advance of the accretionary

wedge (Fig. 9c). The overpressure developed in compart-

ments, which developed along the basal thrust when fluids

were periodically trapped. Each overpressure cycle triggered

a new episode of hydraulic fracturing and fluids moved along

the décollement further toward the foreland, as shown by re-

cent ring-shear experiments and deep sea drilling data (e.g.

Knipe 1993; Brown et al. 1994). Parts of the basal thrust be-

tween overpressured compartments behaved as asperities,

supporting the wedge against its collapse, as proved by Lay

et al. (1982). Local extensional collapses in the wedge in the

area of grown morphology and reduced basal friction are in-

dicated by normal faulting (Fig. 10).

Low friction is also indicated along the Magura thrust,

which mostly juxtaposes rather thicker rhythmic, competent

flysch sequences of the Magura and Silesian Units. The over-

lying Magura sheets have the same lengths as Silesian sheets

in the wedge front (Figs. 5, 7), where the existence of the flu-

id overpressure is proved by vein data (Figs. 8, 9, 10). The

character of the Magura thrust is, however, different from the

character of the basal thrust in the frontal parts of the wedge.

It is formed by the thick brecciated zone, which varies in

thickness up to several hundred meters. The proven occur-

rence of hydrocarbons in the breccia (Leko et al. 1987;

Wunder et al. 1991) can explain the reduced friction by the

potential for an increased fluid pressure. The reduced friction

thus could be caused by migrating hydrocarbons that pre-

served the high porosity of the breccia until now.

The initial basal friction underneath the Magura Unit, be-

fore it was thrust over Silesian sheets, was not low. This is

indicated by its deformation by fault-bend folding in its fron-

tal parts (Fig. 7), similar to medium-friction sandbox models

(Nieuwland 1997, pers. commun.). The underlying Silesian

thrust sheets indicate variations in the basal friction from

place to place. The fault-bend folding, as observed in sand-

box models (Nieuwland 1997, pers. commun.), indicates a

moderate friction and the antiformal stack indicates a high

friction.

The erosion of the wedge varies from place to place. The

total Neogene-Quaternary erosion, visible mainly in the re-

stored cross section through the Magura sedimentary succes-

sion, progressively increases towards the hinterland, reach-

ing a maximum value of 2.7 km (Fig. 7). The locally

accelerated erosion modifies this general trend, for example

up to 2.2 km above the Slopnice antiformal stack (Fig. 7).

The Smilno area shows that thrust and fold geometries

change laterally over short distances (Figs. 2, 12). These

changes are accommodated by the shorter strike-slip fault or

sigmoidal structure and longer strike-slip fault in the case of

smaller and larger difference.

Conclusions

1) The calculated shortening of the Silesian sedimentary suc-

cession is 7580 km, the original basin width is 131137 km

and strain rate 8.88.9

. The calculated shortening of

the Magura sedimentary succession is 2042 km, the original

basin width 6483 km and strain rate 1.11.7

2) Both successions were northeastern and southwestern

neighbours, the Magura succession located to the south of

the Bohemian Massif and the Silesian succession to the east

of it. The Magura Unit was shortened first during the

EoceneOligocene and then emplaced as a nappe by a rela-

tively young, EarlyMiddle Miocene, oblique out-of-se-

quence thrusting. This thrusting comprised a distinct strike-

parallel sinistral strike-slip component.

3) The shortening of the Silesian sedimentary succession

started already in the Oligocene, before the emplacement of

the Magura Nappe. The Grybow, Obidowa-Slopnice and

Dukla Units comprise facies which were deposited in the

southern parts of the Silesian depositional area. Facies

present in the Subsilesian Unit were deposited in intra-basi-

nal highs and the northern margin of the same basin. The ma-

jor deformation of the Silesian sedimentary succession is co-

eval with the emplacement of the Magura Nappe.

4) The general thrusting mode of the Outer Carpathian

wedge is piggy-back. Out of-sequence thrusts were caused

by the basement inversion that influenced the sequence of

thrusting above them, the buttressing effect of the pre-exist-

ing structures, which locally increased basal friction and thus

influenced the thrusting behind them, and the interaction of

basal friction and localized erosion. A basement step perpen-

dicular to the tectonic transport of the overlying wedge

caused the development of an antiformal stack, while an ob-

lique step caused the sinistral transpression.

5) The frontal parts of the basal thrust of the West Car-

pathian accretionary wedge experienced periods of the over-

pressure caused by migrating fluids. The Magura thrust also

indicates the decreased basal friction, caused by increased flu-

id pressure, most probably due to migrating hydrocarbons.

6) The West Carpathian accretionary wedge indicates lat-

eral changes in the structural style over short distances.

Acknowledgements: The work has been carried out with the

financial support of the Amoco Prod. Co., Houston, and the

follow up work was supported by the Alexander von Hum-

boldt Fund and Slovak Geol. Survey Project MZP-513/96.

MN wishes to thank Piotr Krzywiec, Nestor Oszczypko,

Marek Cieszkowski, Zbygniew Paul, Antoni Tokarski, Jim

R. Plomer, Andrzej l¹czka, Gary A. Taylor and Dick Nieu-

wland for help and valuable discussion. The authors wish to

thank JN who cannot see the final result.

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