Meso-cenozoic faulting and inferred palaeostresses in the Mons Basin, Belgium

Tectonophysics,

261

192 (1991) 261-271

Elsevier Science Publishers

B.V., Amsterdam

Meso-Cenozoic faulting and inferred palaeostresses in the Mons Basin, Belgium S. Vandycke a, F. Bergerat b and Ch. Dupuis ’ ’ National Fundfor Scientific Research, FacultP P&technique h Tectonique Quantitative, ’ ~~rutoire

Departement

de Mans, 9 rue de Houdain, 7000 Mom, Belgium de Geotectonique, II. R.A. 1315 C.N.R.S., Vniversite Pierre et Marie Curie, 4 place Jussieu, 75005 Paris, France

de GPologie Fondamentaie (Received

et Appliquee, May 4,199O;

Fact&

Po~yteehnique de Mans, 9 rue de chain,

revised version accepted

December

7000 Mom, Belgium

13, 1990)

ABSTRACT Vandycke, S., Bergerat, F. and Dupuis, Tectonophysics, 192: 261-271.

Ch., 1991. Meso-Cenozoic

faulting

and inferred

palaeostresses

in the Mons

Basin.

Fault slip analyses were carried out on Cretaceous to Lower Tertiary formations in the Mons Basin. The results were compared with the four main stages of subsidence. The successive palaeostresses were: (1) a N-S “ Weddian” extension, probably related to karstic dynamics; (2) a NW-SE extension during the late Campanian; (3) a NE-SW extension during the early Maastrichtian, characterized by a pull-apart process; and (4) N-S and E-W extensions that occurred during the Tertiary. The pull-apart mechanism can be observed in the “La Malogne” underground quarry, where synsedimentary tectonic features are particularly well developed. This is fairly consistent with the right-lateral motion along the “Nord-Artois” shear zone. The tectonic evolution of the Mons Basin is related to the geodynamic evolution of the North Sea and of the European Platform.

1. Introduction The

Mons

stratigraphical Early Tertiary position

displays

a Meso-Cenozoic

record ranging from Cretaceous to (about 70 Ma). It occupies a central

in the European

Sea, the Rhine (Fig.

Basin

Graben

1); distances

from

plate between

the North

and the English

Channel

the Mons

Basin

to the

Tethys and to the Atlantic Ocean are similar. In brief, the Mons Basin is a well defined, small and elongated area of subsidence in the vicinity of the Variscan front, between the Brabant massif to the north and the Ardennes massif to the south (Figs. 1 and 2). The sedimentary accumulation results from a continuous evolution in which synsedimentary extensional tectonics have taken place since at least the Campanian and lasted until the Maastrichtian. Large quarries and some outcrops enabled us to map and measure the faults. The sites of the ~~-1951/91/$03.50

D 1991 - Elsevier Science Publishers

Fig.

1. Situation

patterns Mons N.A.S.Z. Brabant Fourteens

of the Mons

of the Northwest Basin

(B)

and

=” Nord-Artois” Fault Zone; basin;

its

in the brittle platform.

structural

Shear

B = Brussels;

Zone;

Netherlands

tectonic

Insets:

framework S.B.F.Z.

M = Mons;

C. N. B. = Central

W.N.B. = Western

B.V.

Basin

European

The (A).

= South

B.F.B. = Broad

Netherlands basin.

basin;

262

S. VANDYCKE

readings graphical

are shown position

Our study

indicated

produced

of the sequence indicating

in Fig.

2 with

their

by a rapid

in Fig. 3.

a detailed

of faulting

with

at least two major

(2) The second

strati-

reconstruction

Upper

palaeostresses

the Mons

types of extensional

tectonics.

stage of subsidence

sedimentation

Cretaceous

of chalk

transgression.

Basin

and

for synsedimentary

expressed

in the Campanian

filled with flint material 2. Relationships between tectonics and subsidence

the nodular

in the Mons Basin

chalk.

flint

main

tinguished

stages

(Dupuis

of subsidence

and Vandycke,

(1) The first stage corresponds

can

be dis-

earliest

1989): to the Hastings

facies and to the Albo-Cenomanian deposits which occupy a special position along the northern flank of the basin required

(Fig. 4). During

for the subsidence

sion took place rapidly ping

in

this stage, the space

sponding

was created

(details

by the

This

subsiding

NE-SW

mesotectonic

chalk where faults are interstratified

horst

stage

and by

trending

zones.

has

graben

to synsedimentary in section

Cretaceous

is characterized

quarries and

with in the

while the regres-

the latest

Tertiary.

underground

The first is clearly

which is conneected

stage occurred during

to the

The sea invaded tectonics

normally

(3) The third Four

was marked related

the area beyond.

evidence

ET AL.

Map-

indicated

pattern

faulting

a

corre-

phenomena

4). Stratigraphical

control

en-

dissolution of Visean evaporites, which are concealed within the Palaeozoic basement. This in depth dissolution phenomenon may be related to

ables the identification of two main periods of activity. The first period is the Maastrichtian, the second postdates the Danian. The Maastrichtian

the Early-Middle Jurassic horst (Patijn, 1963; Van

event is consistent with pull-apart origin of which has been attributed

tectonics, the to the activity

coutere, 1989). The Mesozoic uplift caused the erosion of the Carboniferous sedimentary cover of

of the “Nord-Artois”

(Vandycke

the Brabant high, as a result of which the Visean limestone and anhydrite were exposed. This pro-

sponds

cess was enhanced

during

a period

levels;

the

by the Early

eustatic sea levels but Cretaceous transgression in Turonian times.

Fig. 2. Locality

uplift of the Brabant den Haute and Ver-

Cretaceous

low

ceased when the Upper occurred, approximately

map of the measured

Shear

al., 1988). The post-Danian to the fourth

(4) The fourth major

Zone

event probably

stage described

evidence

high is the

corre-

below.

stage of subsidence of relatively

et

took place eustatic

sea

sedimentary

thickening of the Upper Cretaceous subsiding zones. However, during this stage subsiding axes,

sites in the Mons Basin. Hatched

area

= Palaeozoic

basement;

KF. = Variscan

front.

MESO-CENOZOIC

FAULTING

AND

INFERRED

PALAEOSTRESSES

IN THE

MONS

263

BASIN

3. Small-scale tectonic analysis of faults in the &Ions Basin; palaeostress field r~ons~on during the Late Cretaceous and Tertiary

trending NNW-SSE, probably appeared. The zones of subsidence probably resulted from synsedimentary tectonics related to another geodynamic stress field. To date, there is insufficient tectonic information about the Tertiary to define the tectonic mechanisms related to this fourth stage (work in progress).

Cretaceous and Tertiary formations of the Mons Basin have been affected by several generations of faulting and thus represent a polyphase deforma-

-7

Chrono -

and

position

Lithostratigraphy

measured

of the sites

LUTETIAN I

-45

Mont-PaniSel YPAESfAN

Mom -en-

-53

T

P&Pie

Mont-Hirtbu / /,/f”//-/‘/’ //‘Erquelinnes.

4

Leval,

‘/ I f-k’,/ ED;

12 <

Grandgl,se.Bla(on.Angre

THANETIAN -59 (MONTIAN) DANIAN -65 MAASTRICHTIAN -72

Nouvelles

CAMPANJAN

Obourg

...........

I

8

HE

Chalks TrtwPres

9 <3 7

2-3

6

. . . . f . .. . . . . . . . HG

-63 SANTONIAN -66 66

CONlAClAN

MaisiPres

J 1

Rabots.Fortes-To,ses.Meur,Bre

TURONIAN

d

- 91 CENOMANIAN -9s

ri VRKONlIl AL BIAN

-107 APTIAN -112

....... .. ::‘/// ‘/i/:

various matn

conglomerates

hiatus

continental

HG

induraw surfaces

ED

erosional surfaces unconformities

and deposits

and

Fig. 3. Position of the measured sites in relation to the chrono- and lithostratigraphic succession of the Mons Basin (after Dupuis and Vandycke, 1989). Chrono- and lithostratigraphy are established mainly after Odin (1982), Dupuis and Robaszynski (1986) and Robaszynski and Christensen (1989). Numbers in the last column refer to Fig. 2.

264

S. VANDYCKE

ET AL.

slip, indicating remobi~tion of the fault during later tectonic events. For example, some NE-SW trending faults involving the Campanian chalks show three generations of slickenside lineations. The first is dip-slip, whereas the other two are oblique-slip (Fig. 5D, E). (2) Some fault sets are synsedimentary, especially in the Ciply Phosphatic Chalk. The continuity of some stratigraphic horizons above these faults provides the relative ages (Vandycke et al., 1988). The synsedimentary character is confirmed by detailed cross-sections of the “La Malogne” quarry (Fig. 6) where five successive fault mechanisms have been recognized in the Ciply Phosphatic Chalk and in the Saint-Symphorien and Ciply calcarenites (see next section). (3) The absence of a fault set above a given formation provides a reliable stratigraphic standard. For example, NE-SW trending normal faults commonly involve the Campanian chalks but are

tion. Incidentally, the same tectonic event is characterized in each formation by different brittle behaviour (Vandycke and Bergerat, 1989). Consequently, before analysing the structures, it is necessary to study the geometry of the fracture pattern as well as the chronological criteria of each formation, in order to distinguish between the different synchronous and successive brittle fracture patterns.

The chronology of the different tectonic events is well established in the Mons Basin (Vandycke and Bergerat, 1989). Three types of criteria are applied to distinguish between the different fault sets which characterize successive tectonic events: (1) Many fault planes show two or three generations of slickenside lineations. The first is generally dip-slip or strike-slip, the next usually oblique

SW

NE

St Ghlslam

S

N

oouvraln

Turonian

I

0.5

b complety

dissolved

replaced

by

= maximum

ikm

anhydrjte

collapse

breccla

volume

_d ‘j

rogenous

- 3000m

karstified = no

Fig. 4. Cross-section (redrawn correlates

heterogeneous

association

anhydrite

collapses

= scattered

effects

the Mons Basin and its Palaeozoic

1977 and Leclercq,

with the almost

of

tuber

surface

through

after Delmer,

dissolutton

complete

basement

showing

1980). Vlb to V3c = Visean divisions;

dissolution

of the evaporites,

of rocks with dissolution The thud zone contains

but without

only karstic

replaced

collapsed

structures

the three presumed

by collapse anhydrite

in anhydrite

zones of deep karstification

w.4 to WC = Westphalian breccias.

beds, the increase rocks without

divisions.

The second

in volume

subsidence.

The first zone

zone consists

of an

is heterogeneous,

MESO-CENOZOIC

FAULTING

AND

Fig. 5. Some characteristic circles)

examples

and slickenside

five- (q),

four- (q)

INFERRED

PALAEOSTRESSES

of fault-slip

lineations;

equal-area

and three- (ua) pointed

to the sites mentioned

in Fig. 2. (A) N-S

de

Ctply

de

Saint-Symphorien

(calcarenite 2 Tuffeau

stars, directions

of compression

extension;

phosphatde

Lower Craie

Maastrichtian

. .

. phosphatzc

c3

belemnlte

(C) NW-SE (F) E-W

compression

numbers

and NE-SW

data as

are relative

extension;

(D)

extension.

Ciply

(chalk)

nodules

pecten indurated

surface

and tectonic

systems and

de

Spiennes

CD) in Fig. 7A. The present synsedimentary

as large black arrows;

Campanran

a

Fig. 6. Stratigraphic

extension;

(E) N-S extension;

of fault planes (great from fault-slip

chalk)

b

n

and extension

projections

axes computed

Maastrtchtian de

Upper

c.

(b) palaeostress

calcarenlte)

(phosphatic

d. ti

(B) NW-SE

extension;

Basin. (a) Stereographic

Maastrichtian

3. Crate

L

in the Mom

lower hemisphere;

BASIN

-Danian)

f phosphatic Upper

data analyses

MONS

projection,

NE-SW

1.Tuffeau

IN THE

cross-sections

appearance

were active during

the Danian

in the “La Malogne”

underground

quarry.

of the faults results from several Maastricht&n

in which

the Early Maastrichtian some

reactivated

of the previous

(e.g. faults faults

after the Danian

See location

and Tertiary

ZIZ, V). Another

are reactivated

(e.g. faults

(e.g. faults Z, VIZ,VIZZ).

(AB and

of the cross-sections motions.

system

Strike-slip

appears

and normal

between

the early

ZZ, V, VI).Some of them

are

S. VANDYCKE

266

never

found

in the Ciply

the Saint-Symphorien overlie

chalk

and Ciply calcarenites

the Campanian

Vandycke

phosphatic

or in which

chalks (see table 1, Fig. 8;

and Bergerat,

(1) A N-S tions

acterized Wealdian

1989).

“Wealdian”

equivalent by

E-W

After

mapping

geometrical fault tensors

and

system

of successive palaeostress and

data

collection,

chronological

is analysed

1984, 1990). The succession is reconstructed

including

information,

in terms

by the direct inversion

fields

each

of palaeostress

method

(Angelier,

of palaeostress

fields

1

Wealdian

formation.

ably related tion;

2

which

Dupuis

microfaults

90-loo”,

dynamics

(2) A NW-SE

extension

NE-SW

faults

with

in the Lower overlie

the

is prob-

(see previous

sec-

1989). is characterized

(azimuth

(Fig. 5B). Their

1

the

are observed

immediately

and Vandycke,

Late Campian

In

In fact, this faulting

to karstic

is char-

faulting.

(Fig. 5A). This fault system is absent marls

(in forma-

facies),

normal

striking

Turonian

ing normal

as follows:

normal

clay, conjugate

very small offsets, 3.2. Reconstruction

extension,

to the Hastings

ET AL.

40-60°)

by trend-

offset is gener-

2

Fig. 7. Palaeostress analysis data from the “La Malogne” underground quarry (after Vandycke et al., 1988). (A) Horst and graben structures. Hatched areas = grabens (phosphatic chalk thickness: 4-6 m); white areas = horsts (phosphatic chalk thickness: 1-2 m). North of faults VIII and IX, another graben corresponds to another underground quarry presently inundated). (B) Entire dip-slip fault population (a) and strike-slip fault population (b), Legend of diagrams as for fig. 5, computation of palaeostress tensors, unweighted (axes as simple symbols), weighted (axes as circles).

MESO-CENOZOIC

FAULTING

261

AND INFERRED PALAEOSTRESSES IN THE MONS BASIN

trend N130-140 *. This strike-slip faulting, which only involves the base of the Ciply Phosphatic occurred during the Early Chalk, probably Maastrichtian. (4) NE-SW extension also occurred during the Early Maastrichtian, resulting in a complex synsedimentary normal fault system trending NlZO140’ and N160-170’ (Fig. 7 Ba). This system probably developed in conjunction with the E-W strike-slip (see next section and Vandycke et al., 1988). (5) The NE-SW extension continued until the end of the Late Cretaceous and during the Early Tertiary, but is characterized by a single set of normal faults, trending N120-140” (Fig. 5D).

ally difficult to measure, except when they displace indurated surfaces. Offsets are in the order tens of centimetres to metres. In the Upper Campanian Spiennes Chalk and Nouvelles Chalk, the faults are filled in with flint, sometimes in continuity with subhorizontal flint beds in the chalk. This flint coating along the faults fixes slickenside lineations and provides evidence for a motion that predates the silicification phenomenon. (3) A strike-slip condition is indicated by NESW extension and NW-SE compression, with right-lateral, N90-110’ trending strike-slip faults (Figs. 5C and 7 Bb). These faults are often associated with secondary faults (Riedel fractures) which TABLE 1

Pataeostress tensor computations from fault-slip data analysis in the Mons Basin. The columns contain, from left to right: N = number of the site (see location on Fig. 2); La = latitude; Lg = longitude; Age (see details in Fig. 3); NF = number of fault slip data; SF = stress field characteristics; trend and plunge of the principal stress axes (in degrees); Q,= ratio (a* - u3)/(q - uj), average angle between computed shear stress and observed slickenside lineation (indegrees);(a)Tertiaryfilling; (b)synsedimentary strike-slip faulting. N

La

LP

1

50"20

3945

2 3

50"28

4"02 to 3-58

1

NF

SF

al

a2

Wealdian

21

N-S Ext.

210.86

089.2

SantonianCampanian

10s

NW-SE

360.77

NW-SE Camp. NE-S Ext. NE-SW

ASa

a3

@

a

359.4

0.4

7

232.8

140.10

0.7

32

155.15

314.74

063.5

0.9

29

082.77

315.8

043.10

0.3

17

N-S Ext.(a)

080.78

280.11

190.4

0.4

12

E-W Ext.'

204.80

035.10

305.2

0.7

32

Ext.

Ext.

4

50-24

3047

Coniacian SantonianCampanian

15

5

50-26

4"13

SantonianCampanian

2

not enough faults to stress tensor computations

6

50-25

3-53

Campanian

7

not enough faults to stress tensor computations

7

50"25

4-01

Campanian

46

NW-SE NE-SW

Ext. Ext.

309.71 157.11

218.0 309.17

128.11 042.8

0.3 0.3

9 13

8

50025

4'00

Campanian

52

NW-SE NE-SW NW-SE NE-SW

Ext. Ext. Comp. 1 Ext.

350.80 111.76 140.07

227.5 316.13 265.78

136.8 224.6 048.10

0.2 0.4 0.5

19 11 26

9

50025

3-69

Campanian

1

10

50"26

3-54

MaastrichtianDanian Maastrichtian only

2-45

to

3-56

not enough faults to stress tensor computations

not enough faults to stress tensor computations NR-SW

Ext.

015.74

132.07

223.14

0.2

14

NE-SW NW-SE NE-SW

Ext. Comp Ext.*)(b)

079.75 150.13

322.07 257.53

230.13 051.34

0.3 0.4

16 13

11

50-25

3-56

MaastrichtianDanian

4

not enough faults to stress tensor computations

12

50030

3940

Thanetian

2

not enough faults to stress tensor computations

1

1

S. VANDYCKE

268

Some

of these

Phosphatic

normal

Chalk

(Early

Saint-Symphorien trichtian), (Middle

Danian).

(metres

trend

N90-100

extensional

the Tertiary,

cut these

o and

trending

These normal N170-180

faulting.

these faults display probably Thanetian

Each palaeostress and relationships

significant fault sets because

formations

of inferred

during

filling, that at the Early

palaeostress

the Mons

which

in some cases,

a sandy or argillaceous in age. This suggests

throughout

and

O, respectively

However,

least some of the faults developed Tertiary. The succession

N-S

exact age is unknown,

were taken in Mesozoic predate

stress fields de-

fault sets exhibit

or more).

(Fig. 5E,F). Their

consistent

the

Maas-

Other faults, however,

The two normal

offsets

clearly

and

(Late

units (Fig. 6).

during

readings

the Ciply

Maastrichtian)

Calcarenite

(6) Two successive E-W.

involve

but are sealed by the Ciply Calcarenite

three lithologic veloped

faults

fields is

Basin (Table

1).

was reconstructed individually between contemporaneity and

fault-slip

data

set contains

measurements

per square

divided

into

subsets

of respectively

fault

slip data pass

single

average

weighted

were

thus

fault

was 7A):

determined.

surface

During

at

each

of stress-tensors

weighting

and

(Vandycke

identical

station,

as in accuracy

of measurements

terms

of resolved

fault

were made

then

and demonstrate

in fault geometry

influence

a

compared

et al., 1988). Results

that variations cantly

a

we considered

to the size of the visible

without

with each other

the quarry

A to F (Fig.

the data

surface. Computations

are almost

metre);

(5

86, 72, 92, 93, 52 and 17

through

relative

with and

412 measurements

six sub-areas,

second

ET AL.

the quality

once more

and size as well do not

of data

palaeostress

signifi-

collection

(Angelier,

in

1984,

1990). By a simple fault population

analysis of the geometry in the “La Malogne”

tectonic

can be identified

events

correspond

to

(1)

which

strike-slip

Maastrichtian-Vandycke

of the quarry, (Fig.

faulting

7)

(Early

et al., 1988)

and

(2)

succession were confirmed at each site by superposition of slickenside lineations and/or by the

synsedimentary normal faulting and Early Tertiary). With regard

presence Bergerat,

faulting mode, palaeo-stress tensor computations were carried out (1) in each sub-area A to F (Fig.

of inherited 1989).

faults

(Vandycke

and

7A) on the Early Maastrichtian

(Maastrichtian to the dip-slip

population

(faults

Note the absence of compressional tertiary events (mostly strike-slip conditions), such as the

which do not affect

N-S compression (Late Eocene) and the NW-SE compression (Mio-Pliocene), which have been ob-

top of the Ciply Phosphatic Chalk), and (2) on the entire Maastrichtian and Early Tertiary popula-

served throughout the West European platform (Letouzey and Tremolieres, 1980; Letouzey, 1986; Bergerat, 1985, 1987). We shall return to this point

in both cases consistent

in due time.

u, being subvertical.

tion of normal extension

pull-apart process

faults,

represents

at the

with a NE-SW

are

trending

Us), the compressional

stress

In the second case (the entire

of normal an average

to several tectonic

surface

(Fig. 7 Ba). The results

(horizontal

population 4. The Early Maastrichtian

the indurated

faults),

the

stress tensor

stress

tensor

corresponding

events. This stress tensor is very

A more detailed study was carried out in the “La Malogne” abandoned underground quarry exploited by the room and pillar method (site 20,

similar to the Early Maastrichtian one. Clearly, the direction of extension remained rather conthe

Early

Fig. 2). In this quarry, synsedimentary tectonic events which occurred during the Maastrichtian

Tertiary. The strike-slip Early Maastrichtian population consists essentially of NNW-SSE

fault and

are particularly well developed (Fig. 6). This area is characterized by horst and graben structures trending N120-130 o and N160-170 o (Fig. 7A).

NW-SE right-lateral strike-slip faults which are characterized by horizontal ur and u, axes, trend-

Approximately ten large faults or fault zones have been systematically surveyed on about 120 rock pillars in this underground quarry. The entire

sistent

during

the

Maastrichtian

and

ing NW-SE and NE-SW respectively (Fig. 7 Bb). The analysis of normal and strike- or obliqueslip faulting was dealt with separately, for the sake of clarity. Computations yielded well defined

MESO-CENOZOIC

FAULTING

AND

INFERRED

tensors which correspond

to two stress-fields

in both cases, horizontal Permutations a vertical

trending

between

PALAEOSTRESSES

NE-SW

plane trending

NW-SE.

faulting

belong

with show

strike-slip the

movements

lineations)

lique- and dip-slip Qualitative

and oblique-

(some

of

two

or reg-

fault

planes

and progressed

of

towards

ob-

observations

E-W

and quantitative

trending

anal-

the structarea, with

right-lateral

strike-

slip faulting and associated NW-SE to NNW-SSE trending dip-slip faulting, during Early Maastrichtian times. This model lateral

strike-slip

is consistent

motion

Mons tural

with a right-

along the “Nord-Artois”

Shear Zone and the pull-apart

development

of the

Basin (Vandycke et al., 1988). This strucevolution is also consistent with Late

Maastrichtian

and Tertiary

times reactivating

are

normal

the pre-existing

faulting, normal

somefaults.

trends. the

normal

Mons

Basin

faulting

along

rifting

Sea (Ziegler,

During

period

Sea

abated

during

the Late

Palaeocene

Hercynian

with extensional

in

activity

and

tectonics

clearly pre-

1975, 1982, 1987).

of basin

in the Broad

inversion

(subNether-

Fourteens

basin

(Van Wijhe, 1987) as well as in the Sole Pit basin (Van Hoorn, 1987) (Fig. l), major tensional faults were reactivated sional

by compressional

stresses.

Small-scale

dent

along

west

Netherlands

(Roos

the margins

and

Broad

1983).

After

creased tectonic activity during inversion axes were apparently Maastrichtian ments

and Danian

accelerated (Laramide

again phase)

faults

are evi-

of the strongly

and

Srnits,

and transpres-

thrust

inverted

Fourteen

basins

a period

of de-

which the major overlapped by

chalks, inversion during

move-

the Mid Palaeo-

in the Broad

Fourteens

basin (Fig. 1) during the Late Cretaceous Early Tertiary (Lake and Karner, 1987). At

the

same

time,

in

the

the Late

into a stress field characterized

then

along

N-S

and

uplift

is evident in the Sole Pit area (Van Hoorn, 1987). There is also evidence for inversion in the Wessex

approximately

and

in

inversion

in the west and central

and

the

and lasted until

(Ziegler,

dominating (Fig. 8). Successive extensions occurred along NW-SE and NE-SW trends during Cretaceous,

to the end of

rifting

gradually

phase

phase)

basins

char-

N40-60”

identified

the Maastrichtian

the initial

and

1975, 1982, 1987).

the Late Cretaceous,

North

occurred

lands

the Campanian

basin (Van Wijhe, 1987). No such Laramide

The succession of faulting and inferred palaeostress fields in the Mons Basin shows a rapid tectonic evolution during the Late Cretaceous and the Tertiary,

in the

by

Cretaceous

the

during

They may be closely related

North

cene 5. Conclusions

processes

documented

acterized

During

movements.

269

BASIN

Extensional

began

generations

yses provide good information about ural evolution of the “La Malogne” approximately

in

These relation-

process probably

superposition

slickenside

u3 axes.

to the same tectonic

ime. In fact, the pull-apart

MONS

with,

u, and a, axes occurred

ships suggest that both strike-slip dip-slip

IN THE

E-W

slip fault system

trending

(Early

Mons

and the

Basin,

right-lateral

Maastrichtian)

the

strike-

developed

by NE-SW

exten-

E-W trends during the Tertiary. Synsedimentary palaeostress fields during the Late Cretaceous are

clearly

characterized by inversions between the principal axes of stress tensors; first, between u2 and Us

bounded by normal faults, trending N120-140” and N160-170“. The entire structure involves a

(NW-SE followed by NE-SW extensions), then between uI and o, (pull-apart followed by NE-SW extension) (Fig. 8).

pull-apart

Our study provides, for the first time, detailed information on the mechanisms of the Late Mesozoic-Early Cenozoic tectonics near the Belgian-French border, south of the North Sea Basin. The tectonic evolution characterized in the Mons

the “Nord-Artois” Shear Zone (Fig. 1; Colbeaux 1974; Colbeaux et al., 1977). The extensional character of the tectonic stress field apparently persisted at least until the Eocene.

Basin may be partly related to the global namic evolution of Northwest Europe.

geody-

sion

and

NW-SE

related

compression.

to the horst

process

during

and

This

pattern

graben

is

pattern

the Early Maastrichtian.

On a larger scale, this right-lateral strike-slip fault may be related to a large right-lateral shear zone:

The N-S Late Eocene known

and NW-SE compressional phases, and Mio-Pliocene respectively, well

in the

neighbouring

areas

to the

south

_

-

APTIAN

ALBIAN

re,

r

exhibits

. .

.

_

a fast evolution

.,,

and

E

_

_

-

s

-,

subsidence

_

^

trends

.^

.A.

-

and lithological

GEOMETRY

.

record.

-

-

Later,

characterized

.

various

7

-

the tectonic

by inversions

^.

OF

stress

-t>

._

-

stress field becomes

of principal

succession

d

.

-

entirely

-

-7

-

that tectonics

Note extensional.

axes. These

--

coincide

.”

e

._.

-

changes

.”

_-.

1

_

in both

deformatton

z In ul

: 5

I

9

?

Subsidence steps

of brittle with drastic

the succession

+f

TENSORS EVOLUTION

indurated surfaces

Basin.

HG

?

PALAEOSTRESS

in the Mons

FAULTS AND

conglomerates

and the stratigraphic

.... ......

PRINCIPAL AXIS PALAEOSTRESS TENSOR

TECTONICS:

subsidence

the Late Cretaceous,

to the intermittent

during

in relation

Bracquegnles Cat1lbn

Rabots,Fortes-To~ses.MeuliOre

Saint -Vaast

BOlSde Id HousslPre

Lithostratigraphy

of the tectonism

field evolution

CENOMANIAN

TURONIAN

SANTONlAN

CAMPANIAN

DAN/AN

Fig. 8. Palaeostress

65

WONTIAN)

THANETIAN

YPRESIAN

LUTETIAN

Chrono -

;

7

6

MESO-CENOZOIC

FAULTING

AND

INFERRED

PALAEOWRESSES

IN THE

(Letouzey and Tremolieres, 1980; Letouzey, 1986; Bergerat, 1985, 1987), are apparently not represented in the Mons Basin. This could imply that the Mons Basin may have been outside the area influenced by the compressional effects of the Africa-Eurasia collision.

MONS

271

BASIN

Lake, S.D. and Karner, inversion Leclercq,

tectonics.

Letouzey,

J., 1986. Cenozoic

foreland

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around

B.R.G.M., Odin,

Angelier,

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Res., 89: 5835-5848. of field data in fault tectonics

stress. III. A new rapid inversion

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J. Int., 103: 363-376.

F., 1985. Deformations

1982.

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Patijn,

R.J.H.,

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a

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aminifera

in

the

Mijnbouw,

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dune

in block

Sci., Paris,

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J.P., Beugnies,

A., Dupuis,

Somme,

J., 1977. Tectonique

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et le Nord

C., Robaszynski,

de blocs

de la France.

dans

Ann.

F. and

le Sud de la

Sot. Gtol.

Nord,

97: 191-222. Delmer,

Saint-Ghislain. Dupuis,

Ch.

Quaternary

Le Bassin

du Hainaut

Prof. Pap., Serv. Gtol. and

Robaszynski,

deposits

F.,

in and around

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et le sondage

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the Mons Basin, docu-

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S., 1989. Tectonique

Un modele

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original

Sot. Gtol.

pour

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In:

Petroleum

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62: 75-82. C., 1989. Apatite

fission

uplift of the Brabant

F., 1989. Analyse

cassantes

struction

des

Tertiaire.

Ann. Sot. Gtol. S.,

contraintes

massif:

pal&o-champs

Bergerat,

J.

de contraintes

au

Crttad-

Belg., 112 (2): 469-478. and

Implications

la zone de cisaillement

microtectonique

dam le Bassin de Mons. Recon-

Dupuis,

C.,

a la limite Crttace-Tertiaire

Geol.,

Van Hoom,

1988.

Paleo-

dans le Bassin de

cinematiques.

Nord-Artois.

B., 1987. Structural

CR.

et karstificadu

la subsidence

Belg., 112 (2): 479-487.

evolution,

style of the Sole Pit inversion.

Relations Acad.

avec

Sci., Paris,

timing and tectonic

Tectonophysics,

137: 239-

284. Van Wijhe, D.H.,

Ch. and Vandycke,

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S. and Bergerat,

Vandycke,

and

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23: 2-19. Dupuis,

Vandycke,

North

Upper

areas.

Rijers (Editors),

Geol. Mijnbouw,

Mons (Belgique).

A., 1977.

Ciply

K/13-A

and T.J.A.

The

belemnites

and

P., and Vercoutere,

track evidence preliminary

zone de cisaille-

1159-1161. Colbeaux,

Kaaschieter

Van den Haute,

time of Africa-Eurasia

of

Harmignies

1989.

Roos, B.M. and Smits, B.J., 1983. Rotliegend J.P.H.

van

van Brabant.

chalks of the Mons Basin,

of the Southeastern

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in de ondergrond

van het massief W.K.,

Areas.

at the

I.

Christensen,

Onshore

platform

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Geology

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in Stratigraphy.

Carboon

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Paleo-stress

with plate tectonic

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europeenne.

la plate-forme

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de con-

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This work was partly supported by the C.G.R.I. in Belgium and the M.A.E. in France. The manuscript has benefited from constructive reviews by M. Friedman and anonymous referees.

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and

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(1989),

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geo-