development of the north viking graben: inferences from laboratory modelling

Sedimentary Geology, 86 (1993) 31-51 31 Elsevier Science Publishers B.V., Amsterdam

Development of the North Viking Graben: inferences from laboratory modelling

Jean-Pierre Brun * and Virginie Tron G~osciences Rennes, UPR 4661 CNRS, Campus de Beaulieu, 35042 Rennes Cedex, France

Accepted May 10, 1993

ABSTRACT

Brun, J.-P. and Tron, V., 1993. Development of the North Viking Graben: inferences from laboratory modelling. In: S. Cloetingh, W. Sassi, F. Horvath and C. Puigdefabregas (Editors), Basin Analysis and Dynamics of Sedimentary Basin Evolution. Sediment. Geol., 86: 31-51.

The North Viking Graben is a classic sedimentary basin in the North Sea that has been thoroughly studied over the last decade. Despite the amount and quality of seismic and well data now available, the processes of extension remain unclear and the relationship between sedimentation and deformatiorr is still a matter of debate. We present the results of small-scale laboratory modelling. The models are made with sand and silicone putty to simulate brittle and ductile behaviour on a crustal scale. The experiments were designed to study the effects of: (a) the degree of coupling between brittle and ductile layers; (b) the obliquity of extension; and (c) the reactivation of inherited crustal-scale structures. The results are used to discuss the symmetry of extensional processes on crustal and lithospheric scales, as well as the relationship between sedimentation and deformation. It is proposed that the asymmetry of faulting in the upper crust and the nearly symmetric thinning of the whole crust are not mutually exclusive. A model involving nearly pure shear on the lithospheric scale is used to explain to explain the development of an asymmetrical basin at shallow level. In addition, this model accounts for the nearly horizontal stratification of Lower Cretaceous sediments, the basal Cretaceous unconformity which caps many of the normal faults, and the localization of deformation to the west of the graben during the Lower Cretaceous.

Introduction

The Viking Graben, a target of intensive petroleum exploration, has been thoroughly studied over the last decade. Even though numerous deep seismic lines provide a fairly clear picture of the structure of the graben at a crustal scale (Beach, 1986; Beach et al., 1987; Gibbs, 1987; Klemperer, 1988; Holliger and Klemperer, 1989; Fichler and Hospers, 1990) the processes of extension remain uncertain. Some authors advocate simple shear (Beach, 1985, 1986; Beach et al., 1987; Gibbs, 1987, 1989) while others favour pure shear (Jarvis and McKenzie, 1980; Giltner, 1987;

* Corresponding author.

Badley et al., 1988; Klemperer, 1988; Klemperer and White, 1989; White, 1989, 1990). The former base their interpretation on the structural asymmetry demonstrated by seismic data; the latter on the compatibility of subsidence history with crustal thinning. The relationship between sedimentation and deformation is also debated because, for some authors, extension is restricted to the Jurassic (Beach et al., 1987; Badley et al., 1988; Gabrielsen et al., 1990; Marsden et al., 1990), whereas, for others, it continued into the Lower Cretaceous (Giltner, 1987; Klemperer and White, 1989; White, 1990; Ziegler, 1990).

In the present study, we first review the geological history and structural pattern of the northern end of the Viking Graben, then we present a mechanical interpretation based on laboratory experiments on layered brittle-ductile models.

0037-0738/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

32

Three effects have been studied separately: (1) variations of coupling between upper brittle crust and lower ductile crust; (2) effect of obliquity

.I.-P. B R U N A N D V, T R O N

between the direction of extension and the graben trend; (3) the reactivation of inherited crustal- scale structures. Following application of the re-

DEPTH

0krn-

(a)

6 2 °

6 0 °

2 ° 0 o 2 ° 4 ° 6 °

0 o 2 ° 4 ° 6 °

W-NW TAMPEN SPUR NORTH VIKING GRABEN HORDA PLATFORM

~ / ~ 1 1 ~ __ .~__~__ ; ~ ~ ~ - T : - - - ~ : - - - - - -

NSDP84-01

E-SE

~ I O k m

10

20

30 - - %-2- . . . . ~ - ~ - _ ~ ~- 30

" k 20 Km ~ .

4O

NSDP84-01 BCU

W-NW E-SE DEPTH TAMPEN SPUR / NORTH VIKING GRABEN HORDA PLATFORM

J okra7 [ ~ - - - - _ _ _ 0 km

1 0 10

20 20

30 30

40 40

Fig. 1. (a) Structural map of the northern part of the North Sea with location of the study area and deep seismic lines NSDP84-01,

02 and 03. (b) Line drawing and interpretation of NSDP84-021. At the graben axis, the Moho is drawn according to Pinet (1989) (1)

and Klemperer (1988) (2). (3) = mantle reflection.

D E V E L O P M E N T OF T H E NORTH VIKING GRABEN: INFERENCES FROM LABORATORY MODELLING 33

sults to the Northern Viking Graben, it is argued that the structural asymmetry of the brittle upper crust is compatible with nearly symmetrical crustal-scale thinning, and that sedimentation and deformation continued in the region from the Jurassic to the Lower Cretaceous.

The Northern Viking Graben

Crustal-scale structure

The northern end of the Viking Graben (Fig. la) has been studied through well and seismic data provided by Elf Aquitaine Production and available deep seismic lines. The oil industry seismic data used in this study come mainly from two series of regional seismic lines (NNST84-06 to 12,

and 17 and 18; NVGT88-05 to 15, and 19 to 21. See location in Fig. 2a).

The deep seismic lines NSDP84-01 and 04, which were acquired and processed by GECO AIS, have been interpreted by many authors (Gibbs, 1983, 1984, 1987, 1989; Beach, 1985, 1986; Beach et al., 1987; Klemperer, 1988; Holliger and Klemperer, 1989; Klemperer and White, 1989). Figure lb shows a line drawing of identified reflectors, plotted on a depth-converted section of NSDP84-01 (Pinet, 1989). As shown by many other deep seismic images of extensional basins, this line is characterised by a relatively transparent upper crust and a highly reflective and layered lower crust. The lower crust is inhomoge- neous and varies along section, being rich in short and nearly horizontal reflectors, but also contain-

(a) 2°oo ' 3-oo' 4'oo'

~ ~ ~ 1

~ u ' ~ I /SOGN SPUR ~ t~u^J /

Fig. 2b

2*00 ' 3"O(Y 4000 '

Fig. 2. (a) Grid of commercial seismic lines used in this study. (b) Geological cross-sections interpreted from lines NVGT88-09 and 08 (see location in Fig. 2a). BCU = Basal Cretaceous Unconformity.

34 J P. B R U N A N D V. T R O N

l l i l i

_ ~ ~ =~ ._o

' =

D i D I I I

o

o

e-

e-/

.Q

DEVELOPMENT OF THE NORTH VIKING GRABEN: INFERENCES FROM LABORATORY MODELLING 35

2 o 0 0 ' 3 o 0 0 ' 4 o 0 0 '

MORE

62°00'

TAMPEN SPUR j)

SOGN SPUR

i ; i i l ; i ¸ ; !

CENTRAL VIKINGiIMiL

/ /

/ /

/ HORDA PLATFORM ]

o

I I a

2°00 ' 3"00 ' 4"00 '

"61°00'

TIME INTERVAL BASE CRETACEOUS - TOP TURONIAN

I I 0 msTW'r ~ 400-800 msTWT ~ 1200-1600 msTWT

0 -400 msTWT ~ 800 -1200 msTWT ~ • 1600 msTWT

Fig. 3. (a) Thickness contours of the lower Cretaceous measured in two-way travel times (TROT) on seismic sections, from the BCU of the top Turonian. (b) Map of faults usually identified below the BCU. Thick lines = major faults. (c) Map of deep structures and crust thicknesses.

36 J.-P. BRUN AND V. TRON

ing some inclined reflectors that extend locally into the mantle. Beneath the Horda Platform and the deepest part of the graben, reflectors are scarce or even totally lacking. The position of the Moho, taken as the base of the reflective lower crust, is generally in good agreement with gravity modelling (Holliger and Klemperer, 1989). How- ever, the absence of reflectors in places along the graben axis leads to some uncertainty about the form of the Moho in this region. Alternative interpretations include a dome-shaped Moho (2 in Fig. lb) culminating at less than 21 km (Hol- liger and Klemperer, 1989), or a rather flat Moho (1 in Fig. lb) extrapolated on the basis of ESP data from outside the profile (Pinet, 1989). The Moho deepens on either side of the graben axis,

extending to 30 km beneath the Horda Platform, which corresponds to the unstretched part of the Norwegian craton.

Although the crustal-thickness map compiled by Klemperer (1988, Fig. 8a) shows only slight asymmetry of crustal thinning in the studied area, the structure of the upper crust of the graben is clearly asymmetric (Fig. 2b). The western border comprises large blocks which are tilted toward the west, but the central part, making up the Viking Graben proper, shows numerous faults that dip either east or west, but always with small offsets. The eastern border shows a continuous curvature of pre-Cretaceous bedding from the Horda Platform to the centre of the graben--this feature could be linked to a regional scale roll-

(b) 2"oo' s"oo' 4'oo'

2*OO' 3°O0 ' 4*O0'

Fig . 3 ( c o n t i n u e d ) .

62*00'

61"00'


2*00' 3*00' 4000 '

M O R E BAS IN

T A M P E N

NORTH" .

G R A B E N "

S, OGN G R A B E N

• t

S O G N S P U R

• 62*00'

- 61000 '

~'P~. BLd'N'. . . . . " . . /'. " / ~ " +~++'

• o . + ÷ . . . . + + +

r ÷ + +

2000 ' 3*00'

Crustal thickness (from Klemperer, 1988) F-?t I I FT-1

+ + + + I- + + + + +

k 4. + ÷ + + + + + ,~

4- ÷ + ÷ 4- + + H.ORD.A P.L.~,TFORMF

l

. + • • . I , t . • i

0 20KM' [4 . + + + +

++ [+ ~ + [+ ~ + + + +

÷

÷ + C

4*00"

Eastward dipping reflectors

< 21 km ~ below the Moho between 21 and 26 km between 26 and 31 km / at the base of the Upper crust

Fig. 3 (continued).

over anticline accommodated by conjugate sets of small-offset normal faults. This structural asymmetry, which is also observed on the NSDP lines (Fig. lb), is well displayed on the fault map of the study area (Fig. 3b). Most faults affecting the Jurassic are cut by the Basal Cretaceous Uncon- formity (see BCU on Fig. 2b); only a few re- mained active during Lower Cretaceous sedimentation. The en-echelon pattern of Lower Creta- ceous basins (Fig. 3a) could therefore be linked to a topography inherited from Jurassic deforma-

tion, with extension localized along the western border.

Sedimentation and deformation

The sedimentation history of the Northern Viking Graben area, as inferred from well an d seismic data, can be summarized as follows (Gabrielsen et al., 1990; Marsden et al., 1990; Ziegler, 1990; Figs. 2b and 4).

Sedimentation started in the Permian-Triassic

38 J.-P. B R U N A N D V. T R O N

during an extensional episode that is only poorly recorded (Fig. 2b). Sedimentary deposits are lo- cated within half-grabens above tilted blocks made up of the Caledonian basement. The Lower and Middle Jurassic is thin and directly overlies the basement in the northeast. Two intra-Jurassic markers are classically identified on commercial seismic lines: the top of the Dunlin Formation (Toarcian-Pliensbachian) and the top of the Stat- fjord Formation (Hettangian-Sinemurian). The Brent Formation (Upper Jurassic) unconformably overlies the top of the Dunlin Formation; the main phase of deformation started during the Callovian-Oxfordian. Tilted blocks can be identified beneath an intra-Kimmeridgian unconformity that is easily mistaken for the BCU when the Kimmeridgian clays are too thin to be identified in seismic sections. These clays are marine and rich in organic matter, suggesting a restricted basin, probably deep-water, environment. They occur as lenticular sedimentary bodies bounded by normal faults and up to few hundred metres thick. The base of the Lower Cretaceous forms the most striking unconformity (BCU) observed in seismic sections from the North Sea. It is locally strongly erosive, especially on top of the Tampen Spur tilted blocks (Fig. 4), and it overlies many of the faults cutting the Jurassic formations. Neocomian sediments, encountered drillholes on top of the tilted blocks are interpreted as a con- densed sequence. Variations in stratigraphic thickness from the Lower Cretaceous to the upper Turonian (Fig. 3a) reflect the topography of the basal unconformity (e.g. three en-echelon sub-basins within the graben). The sedimentation rate was low during the Neocomian, but acceler- ated during the Albian-Aptian (Zervos, 1987; Nelson and Carny, 1987). Continuing subsidence probably led to an increase of water depth during most of the Cretaceous (Marsden et al., 1990). Toward the east, the important development of chalk deposition during the Coniacian-Maas- trichtian produced a succession which unconformably covered the marly limestones of the Turonian. All these formations display onlaps on the eastern border of the graben, which was totally filled by the end of the Cretaceous. During the Tertiary sedimentation rates exceeded subsi-

dence rates with the result that water depths shallowed to present-day values (100-200 m).

Oblique extension

The en-echelon pattern of Lower Cretaceous basins within the North Viking Graben (Fig. 3a) suggests that the stretching direction was not perpendicular to the graben trend. A qualitative analysis of the fault map (Fig. 3b), based on the experimental investigation of oblique extension of Tron and Brun (1991), leads to the same conclu- sion. Within a graben, with a mean trend of N30 °, the secondary normal faults are oriented N170 ° to N10 °. A zone of high fault density trending roughly N-S, i.e. oblique to the graben trend, occupies the central part of the graben. Faults are often strongly curved a n d / o r arranged into sinistral en-echelon sub-systems at various scales.

The statistical analysis of fault trends displayed in Fig. 3b gives a multimodal asymmetric distribution with three well defined peaks at N05 °, N25 ° and N45 °. To take into account the magnitude of these faults, their lengths have been weighted by fault offsets TWT-estimated is measured on seismic lines using the offset of the Dunlin marker (Fig. 2b) and interpolated from line to line over the entire grid (Fig. 2a). Esti- mated offsets vary from less than 0.1 s to more than 1.0 s TWT. The highest values are obtained from the east-dipping faults that border the graben to the west. These faults contribute mostly to the second peak of the distribution, that oriented at N25 °. The N05 ° peak corresponds mainly to the west-dipping faults at the eastern border of the graben.

Comparable multimodal and asymmetrical dis- tributions of normal fault trends have been obtained in experimental studies of oblique extension (Tron and Brun, 1991). However, to draw a relationship between the distribution of fault trends obtained for the Northern Viking Graben and those obtained in the experiments, it is nec- essary to establish the strike of the deformation zone.

A zone of dipping reflectors is observed to extend from the base of the upper crust to the upper part of the mantle (see 3 in Fig. lb) on

D E V E L O P M E N T O F T H E N O R T H V I K I N G G R A B E N : I N F E R E N C E S F R O M L A B O R A T O R Y M O D E L L I N G 39

ILl

to to to to o~ o~ to co 0 ~ 0.1 O~ ~ tO ~O I ~

Z n l ea

tw L0

¢5 Z

to tO to to to t/~ to t o

e~ t'q

.9

O

~D

.=.

_o

e~ r/3

E k-,

.=. ¢-

O

o

'5

O

d~

4 0 J . -P. B R U N A N D V. T R O N

both deep seismic lines NSDP84-01 and 04 (Fig. la). When plotted on a map, these define a zone striking roughly N20-N30 ° and dipping to the ESE (Fig. 3c). This zone is rooted in the mantle beneath the eastern border of the graben and emerges among the tilted blocks of Tampen Spur. Although most authors agree that the mantle portion of this zone is a shear zone (Gibbs, 1984, 1987; Beach, 1985, 1986; Klemperer, 1988; Pinet, 1989), there is disagreement about the age of deformation. Gibbs (1984, 1987), Beach (1985, 1986), Beach et al. (1987) and Klemperer (1988)

have postulated a normal-sense shear zone that was activated during Mesozoic extension. Pinet (1989), on the other hand, considered the lack of Moho offset along the zone as evidence for Cale- donian thrusting. A mixed origin~react ivat ion of a Caledonian thrust zone by normal movement during the Mesozoic--has been advocated by Gibbs (1987) and Pinet and Colletta (1990). This zone is also identified in the upper crust on commercial seismic lines (Fig. 4), where it is seen to be not only roughly parallel to the major normal faults that define the tilted blocks of

Fault orientation in northern Viking Graben

N 1 2 0 ° N 1 5 0 ° NO* | 3 0 ° N 6 0 ° N g 0 °

S e n e s t r a l

p E x t e n s i o n

D e x t r a l

e--

o

"o

e- r a

N:. "':~'.!~:i

I ! - 9 0 ° . 6 0 ° - 3 0 ° 0 ° 3 0 ° 6 0 ° 9 0 °

V . D °

Fig. 5. Distribution of fault orientation in the North Viking Graben (see fault map in Fig. 3b) compared to the distribution of fault orientations obtained in laboratory experiments of oblique extension (Tron and Brun, 1991). The best fit between field and

experimental data predicts a stretching direction trending nearly E-W.

D E V E L O P M E N T OF T H E N O R T H VIKING GRABEN: INFERENCES FROM LABORATORY MODELLING 41

Tampen Spur, but also to the Moho rise at the graben axis (Fig. 3c). These lines of evidence all suggest that the N20-N30 ° orientation of the Northern Viking Graben is inherited from pre-existing lithospheric-scale structures (probably Caledonian) that were reactivated during Meso- zoic extension.

Taking the N20-N30 ° direction as the strike of the deforming zone, the distribution of fault trends has been adjusted to those obtained from the experiments (Fig. 5). The best fit gives a stretching direction at nearly 60 ° to the graben direction (i.e. trending E-W).

Analogue modelling

Preuious work and working hypotheses

The experiments on brittle-ductile models presented here are an attempt to explain the structure and history of the Viking Graben in terms of a simple lithospheric-scale process. They account for all the major features discussed previously, including: (1) the structural asymmetry of the graben and the nearly symmetrical crustal thinning; (2) the intense and widely distributed Jurassic faulting, as well as the weak and localized Lower Cretaceous faulting; (3) the direction of extension which, at 090 ° , is oblique on the N30 ° graben direction.

In a layered brittle-ductile system, an asymmetrical graben results when displacements at external boundaries are themselves asymmetrical (Allemand et al., 1989). It is now widely accepted that continental lithosphere with normal crustal thickness (around 35 km), a stable geotherm and a strain rate of 10-15 s- 1 can be represented by a four-layer type strength profile (e.g. Sawyer, 1985). Two high-strength zones correspond to the middle crust and the upper part of the lithosphere mantle while two low-strength zones correspond to the ductile lower crust and the lower ductile part of the lithosphere mantle. Such a multilayer is modelled in the laboratory using sand to represent the high-strength brittle layers and silicone putties to represent the low-strength ductile layers. The principles of modelling and scaling for sand-silicone models of the continen-

tal lithosphere are described in detail by Davy and Cobbold (1991). Extension experiments have been carried out using four-layer models floating on a low-viscosity fluid which represents the asthenosphere. These experiments have demonstrated that the high-strength layer representing the upper part of the mantle undergoes localized deformation and boudinage that tends to develop asymmetrically at high-amplitude deformation (Beslier and Brun, 1991; Brun and Beslier, submitted). Experiments in which a two-layer brittle-ductile model is deformed above a basal asymmetric velocity discontinuity provide a sim- plified and technically convenient way to investi- gate the various mechanical effects of asymmetrical extension of a brittle-ductile continental crust (Allemand et al., 1989) and, in particular, the development of asymmetric grabens (Allemand and Brun, 1991; Tron and Brun, 1991). The same technique was used in the present study.

The widely distributed and/or localized char- acter of deformation in the brittle crust depends on coupling between brittle and ductile layers, which can be expressed as the ratio of bulk brittle strength to bulk ductile strength (Allemand, 1988). In a two-layer model where the thickness and rheology of the brittle and ductile layers are kept constant, the coupling varies only as a function of strain rate. At high strain rates, the strength of ductile layers, i.e. the product of strain rate and viscosity, is high. Because the strength of brittle layers is independent of strain rate, the coupling increases with strain rate. Laboratory experiments (Allemand, 1988) show that an increase in the coupling gives more dispersed faulting in brittle layers, and that lowering the coupling tends to localize the faulting. In the case of the Viking Graben, this suggests that coupling between the upper brittle and lower ductile crust has decreased from Jurassic to Lower Cretaceous times.

The obliquity between the direction of extension and the graben trend gives rise to a fault pattern characterized by the asymmetrical distribution of fault trends (Tron and Brun, 1991). For the North Viking Graben, a comparison between field and experimental data indicates a stretching direction at 60 ° to the graben direction, i.e. E-W stretching of the N30°-trending graben (Fig. 5).

42 J . - P B R U N A N D V. T R O N

0 t-" o

c-

"0 0 ~ ~"~= .-~ = ~

~ 9 I,

o n c-

E

_1

0

<

,<

E

E 0 0

~ a

. < >


SEDIMENTATION BETWEEN PHASE 1 AND PHASE 2

181 1

0.5

0.0

1t• ~ ~ ~ S A N D 0.5

0.0

t 0.5

0.0

SlUCONE

THINNING

F o.25

fi i: °

V,D.

SEDIMENTATION DURING PHASE 2

(b), t 0.5

0.0

13 =1.4

TOTAL

0.5

0.0

SAND

1 0.5

0.0

SILICONE

0 3 cm

THINNING iX 0.75 0.5 0.25

f 0.75

V.D. [~ =1.44

Fig. 7. Cross-section of two models showing the effects of a decrease in bri t t le-ducti le coupling. (a) Sedimentation between the

strongly coupled phase 1 and the weakly coupled phase 2. (b) Progressive sedimentat ion during the weakly coupled phase 2. Curves

show variations of thinning (1 / f l ) along section for the total model, the sand layer and the silicone layer, fl values indicated on

cross-sections are the mean values of extension measured between graben border faults. 109 = velocity discontinuity.

44 .t.-P. B R U N A N D V, ' I R O N

Experimental procedure

The laboratory work consisted of three differ- ent types of experiment designed to study the effects of: (1) a decrease in brittle-ductile coupling during extension; (2) obliquity of 60 ° between the stretching direction and the graben trend; (3) a pre-existing dipping zone of weakness in the upper brittle crust.

Two-layer britt le-ductile models were con- structed with sand and silicone putty using a 2 /1 thickness ratio for a total thickness of 3 cm. The sand is characterized by an angle of friction of 30 ° and negligible cohesion; the silicone putty has a viscosity of 10 4 Pa s at 20°C (Fig. 6).

The three types of experiments were carried out using an apparatus comprising a fixed and rigid basal plate above which a thin mobile plate was pulled at constant rate (between 1.0 and 15.0 cm/h) . The end of the mobile plate induces an asymmetric velocity discontinuity (VD) at the base of the model (Fig. 6). For experiments aimed at modelling oblique extension, the VD is at an angle of about 60 ° to the direction of displacement.

Eighteen experiments were performed to test the combined effects of variations of bri t t le- ductile coupling, the obliquity of extension and the existence of a preexisting zone of weakness. Because of limited space, only four are described here.

Effects of uariations of the brittle-ductile coupling

A first type of experiment was carried out with a velocity discontinuity perpendicular to the direction of displacement (Fig. 6). Experiments were performed in two successive phases. In the first, the displacement rate was 15 cm/h , giving high strength to the silicone layer (as shown in the strength profile in Fig. 6) which resulted in strong coupling between brittle and ductile layers. Dur- ing the second phase, the displacement rate was decreased by more than an order of magnitude (to 1 c m / h ) thus conferring a very low strength to the silicone layer and resulted in weak bri t t le- ductile coupling.

Giltner (1987), Klemperer (1988), Klemperer

and White (1989), Faure (1990) and Ziegler (1990) give widely varying estimates of the amount of bulk stretching in the North Viking Graben, i.e. 19-80 km for the Jurassic, and 2.5-10 km for the Lower Cretaceous. However, the amount of stretching estimated for the Cretaceous is invari- ably lower than that for the Jurassic. In order to visualize structural effects in the experiments, the amount of stretching was set at 4.0 cm for phase 1 and 2.0 cm for phase 2. Since the sedimentation rate was extremely low during the Jurassic, no sediment was added during phase 1. Two types of sedimentation were modelled during phase 2: either the basin was filled during a single discrete episode between phase 1 and phase 2 (Fig. 7a) or it was progressively filled during phase 2 (Fig. 7b).

Phase 1 extension produced an asymmetric basin defined by two sets of normal faults with opposing dips that converge towards the velocity discontinuity at the base. Above the basal mobile plate, the normal faults delimit tilted blocks whose envelope is nearly horizontal or has a shallow dip towards the basin axis. On the opposite side, the envelope of faulted blocks define a long wavelength roll-over anticline.

During phase 2, the normal faults which facili- tated formation of the roll-over anticline became inactive; this did not occur on the basin axis during experiments in which sedimentation was modelled in a single discrete event (Fig. 7a). Deformation and faulting were localized above the velocity discontinuity and progressively mi- grated toward the centre of the basin. Progressive sedimentation during phase 2 resulted in a fan- like pattern that can be compared to sedimentary wedges associated with normal growth faults (Fig. 7b). However, rather than being concentrated along a single fault, the displacement was accommodated between several.

Curves representing the extent of vertical thinning of the layers (1//3) show that in both types of experiments, thinning of the upper brittle layer is clearly asymmetric, especially with progressive basin infilling during phase 2 (Fig. 8b). By contrast, thinning of the lower ductile layer is nearly symmetric, showing horizontal thinning gradients which are stronger than in the upper brittle layer.


The overall thinning of the two-layer system ~s only weakly asymmetric, i.e. the concentrated and symmetric thinning of the lower ductile layer tends to control the asymmetric thinning of the upper brittle layer.

Effects of oblique extension

The second type of experiment was carried out with a basal velocity discontinuity trending at an angle of 60 ° to the displacement direction (Fig. 8). The experimental procedure was identical to the model shown in Fig. 7b, i.e. displacement rates of 15.0 and 1.0 cm/h during phases 1 and 2, and progressive sedimentation during phase 2.

Phase 1 gives an asymmetric basin whose gen- eral morphology is comparable to those formed in the previous experiments, but with en-echelon fault patterns (Fig. 9a). As in previous experiments, the basin shoulder opposite to the basal mobile plate shows a long wavelength flexure or roll-over anticline. The faults that accommodate this flexure dip toward the basin axis (Fig. 10) and are at a mean angle of 13 ° to the basin axis, or 73 ° to the stretching direction (Fig. 9). This value corresponds to the major peak in the distribution of fault orientations previously obtained by Tron and Brun (1991) for an obliquity of 60 ° in the stretching direction. The faults that delimit tilted blocks above the basal mobile plate have strongest offsets and a mean 27 ° obliquity to the basin axis or a mean 87 ° obliquity, i.e. nearly perpendicular, to the stretching direction (Fig. 9a). This difference in fault orientation on each side of the basin is explained by a 13-15 ° sinistral rotation of the blocks situated above the basal mobile plate. Strain is moderate in the roll-over anticline, as demonstrated by small fault offsets, with faults maintaining nearly their original orientation. Deformation is stronger on the opposite flank, a,~ shown bv the lar~er fault offsets, the

-4--(D Oblique V. D. 60*

Fig. 8. Apparatus used for oblique extension.

Fault pattern at the end of the 1 st phase before sedimentation

a

Fault pattern at the end of the 2 st phase with sedimentation

b /

?ig. 9. Top views of an oblique extension model: (a) at the end

)f the strongly coupled 1st phase and (b) at the end of the

~eakly coupled 2nd phase. The arrows indicate the sense of

displacement of the mobile basal plate. Scale is given by 5 cm

spacing of white marker lines at the model surface.

faults being rotated to become nearly perpendicular to the stretching direction.

The progressive sedimentation modelled during phase 2 (Fig. 10) did not produce the distinct asymmetry as previously obtained in orthogonal extension experiments (Fig. 7b). However, a slight axial bulging resulted from displacement on border faults. As in previous experiments, thinning curves (Fig. 10) indicate that the asymmetric thinning of the upper brittle layer is controlled by the localized and symmetric thinning of the lower ductile layer. Finally, the faulted blocks situated above the basal mobile plate show a tilt which is

46 .I.~P. B R U N A N D V. T R O N

C~ 0 0 , - 0 0 0 z Z z ,,r, i--

I I i

. . I

/

t I J

I I l I l

0 d o

~ d 0 0

I I I

O. ~ ~ . o o 0 C5

E 0

0

- t

9 0.0 o

©

¢o .E

~o II

0

o"

. o 0

.o

D E V E L O P M E N T OF T H E N O R T H VIKING GRABEN: INFERENCES FROM LABORATORY MODELLING 47

displacement

mobile wall

Oblique V.D. Fig. 1 l. Structure of models used to study the effects of inherited zones of weakness in the brittle layer.

more pronounced than in orthogonal extension experiments and which increases up to 10 ° outside the basin.

Effects of inherited zones of weakness in the brittle layer

A third type of experiment was carried out using the same procedure as the type 2 experiments. A thin layer (2 ram) of silicone putty was placed in the upper brittle layer, parallel to and dipping toward the basal velocity discontinuity (Fig. 11). This was done to simulate a zone of weakness inherited from an earlier geological event.

Fault pattern at the end of the 1 st phase

Most of the features observed in previous experiments were reproduced in type-3 experiments: en-echelon fault patterns, block rotation above the basal mobile plate (Fig. 12) and only slightly asymmetric basin fill (Fig. 13). The most significant differences are the development of a linear graben parallel not only to the zone of weakness but also to the pattern of thinning (Figs. 12 and 13). The thinning curves of both brittle and ductile layers show strong fluctuations adjacent to the zone of weakness. Although the oscillations in adjacent layers are out of phase, they are mutually compensated, as demonstrated by the low residual amplitudes of variation in the total thinning curve. The smoothed oscillations, drawn with dashed lines on the thinning curves, show that, although the asymmetry of thinning in the brittle layer is smaller than in the previous types of experiment, the symmetry and concen- tration of thinning in the ductile layer are similar. Note that normal-sense shearing of the zone of weakness in the brittle layer induces a local uplift of the ductile layer (1//3 > 1.0). It should be noted that normal faults directly above the zone of weakness are parallel to the zone despite its obliquity to the stretching direction.

Discussion

Fig. 12. Top view of an oblique extension model with a weak

zone of silicone in the sand layer. The photograph was taken

at the end of the strongly coupled 1st phase before the start of

sedimentation. Spacing of white marker lines = 5 cm.

Crustal-scale structure of the Viking Graben

Figure 14 shows an analysis of thinning in the North Viking Graben along the NSDP84-01 seis-

48 J.-P. BRUN AND V. TRON

mic line (Fig. lb) in the light of the present experimental results.

Despite differences in the interpretation of the seismic data, the layered lower crust is clearly separated from the transparent upper crust by a continuous line. The BCU is taken as a reference for the top of the crust. Reference thicknesses for the upper and lower crust are measured at the eastern end of the section (Horda Platform). Two interpretations of Moho geometry at the graben axis, those of Pinet (1989) (1 in Fig. 14) and Klemperer (2 in Fig. 14), have been taken into account.

The thinning curve in the upper crust shows an asymmetry directly comparable to that observed in the experimental models (Figs. 7, 10 and 11). Solution 2 (Fig. 14) for the Moho (Klemperer, 1988) gives a value for thinning that is similar to that obtained from the experimental models, being fairly symmetrical and more localized than thinning in the upper crust. The fact that the maximum value of thinning is only slightly higher

in the lower crust than in the upper crust suggests that the entire layered lower crust is not ductile during extension. If the thickness of the ductile crust is effectively lower than that of the layered lower crust, then the measurements made on the seismic section probably underestimate the real thinning of the lower ductile crust. Despite these uncertainties, the thinning pattern for the North Viking Graben obtained assuming the Moho geometry of Klemperer (1988) is fairly similar to those obtained from laboratory experiments. By contrast, the flat Moho of Pinet (1989) gives a thinning curve for the layered lower crust without significant variation from the Horda Platform to Tampen Spur. This corresponds to a fairly constant value of stretching of 1.3-1.4, a result which is not easily reconciled with the gradients of thinning observed in the upper crust or subsidence history (Klemperer, 1988), and which has never been observed in any of the experiments.

A comparison with experimental results leads us to interpret the Moho geometry beneath the

0.5

0.0

TOTAL f

THINNII~G

0.75

't j SA.O J

0.5

0.0

toi

0.5 t 0.0

0.75 0.5 0.25

V.D. 60 ° 0 3 cm

13 =1.12

Fig. 13. Cross - sec t ion of a n ob l ique ex tens ion m o d e l wi th a w e a k zone ot sd l cone m the b a n d layer a n d p rog res s ive s e d i m e n t a t i o n

d u r i n g the weak ly c o u p l e d 2 n d phase .

W-NW E-SE DEPTH TAMPEN SPUR NORTH VIKING GRABEN HORDA PLATFORM

o1o11 km

10

D E V E L O P M E N T OF T H E NORTH VIKING GRABEN: INFERENCES FROM LABORATORY MODELLING

20

30

40

49

NSDP84-01

THINNING

o...o.os, _It 0.75 0.75

0.25 0.25

Fig. 14. Interpretation of thinning variations of the NSDP84-01 line (see location in Fig. 1). Line drawing of the depth-converted

section after Pinet (1989). Curves show separately, from top to base, the thinning variation for the whole crust, the upper crust and

the layered lower crust. Curves 1 and 2 refer to the two solutions for Moho geometry beneath the graben axis according to Pinet

(1989) and Klemperer (1988), respectively.

graben as a displacement zone in the underlying mantle (Fig. 15) that is continuous with dipping mantle reflectors observed beneath the eastern flank of the graben (3 in Fig. 14). The minimum displacement implied by this interpretation is

around 40 km. Such a zone of movement in the upper part of the lithospheric mantle could be directly compared to the relative movement between the fixed and mobile basal plates of our experiments (Fig. 6). The deformation, which is

Upper \\ . ' \ , ~ ;"

Lower - - ~ ~

Lithosphere

Asthenosphere

Fig. 15. Model of the North Viking Graben extension on a lithospheric scale. The upper part of the lithospheric mantle is affected

by a normal-sense shear zone. Displacement is transferred to the upper brittle crust through diffuse deformation of the lower

ductile crust combining pure shear and simple shear parallel to the Moho. The lithospheric mantle part of the model is based on

experimental results from Beslier and Brun (1991). The dashed line in the mantle indicates a passive marker.

51) , I . -I ' . B R U N A N D V. T R O N

localized in a narrow zone in the mantle, be- comes distributed over a significantly wider part of the upper crust through ductile flow in the lower crust (Fig. 15).

It should be noted that, in contrast to previous "simple shear" models (Beach, 1985, 1986; Beach et al. 1987; or Gibbs, 1984, 1987), the present model does not require any single flat-lying normal fault or fault zones running continuously from the mantle up into the upper crust. Nor does it require a lateral shift between zones of maximum thinning in crust and mantle. Experi- mental models show that the thinning of ductile units controls the asymmetry of the brittle units. This interpretation based here on two-layer models, is also applicable to four-layer lithospheric- type models (Beslier and Brun, 1991; Brun and Beslier, submitted). The existence of the ductile layer allows the britt le-ductile multilayer to thin almost symmetrically, even when asymmetrical structures are developed internally. From this point of view, the controversy between the "pure shear" and "simple shear" interpretations of the Viking Graben does not seem justified.

Relationship between sedimentation and deformation

As mentioned in the introduction, the relationship between sedimentation and deformation in the Viking Graben is a matter of debate. Inter- pretations of well and seismic data (e.g. Fig. 4) fall into two broad categories: some authors maintain that the entire extension occurred during the Upper Jurassic (Beach et al., 1987; Badley et al., 1988; Gabrielsen et al., 1990) while others claim that extension continued into the Lower Cretaceous (Jarvis and McKenzie, 1980; Giltner, 1987; White, 1990; Ziegler, 1990). For the first group, the principal arguments for pre-Creta- ceous extension are the Lower Cretaceous unconformity, which caps most of the faults, and the nearly horizontal stratification of the Lower Cre- taceous. For the second group of authors, whose interpretation we prefer, the main argument is the localization of deformation to the west of the graben in the Tampen Spur area.

Support for the second interpretation comes

from the present experiments, which indicate a decrease of coupling between upper brittle and lower ductile crust. Such a decrease could arise during the Viking Graben extension through two mechanisms, a decrease of strain rate or a decrease of lower crust viscosity. A decrease of strain rate could simply stem from a progressive attenuation of extension from Jurassic through Cretaceous times. A decrease of the lower crust viscosity could have resulted from the local uplift of hot ductile mantle at the graben axis, due to the presence of a mantle detachment zone (Fig. 15). Both phenomena could have produced the same result by lowering the strength of the lower ductile crust.

We interpret the BCU not as marking the end of extension but rather as due to a decrease in strain rate, augmented possibly by transient heat- ing leading to rheological softening of the lower crust. The nearly horizontal stratification of the Lower Cretaceous can be explained, as in the laboratory experiments, as follows: (1) the asymmetrical thinning of the upper brittle crust is controlled by localized and nearly symmetrical thinning of the lower ductile crust; and (2) the stretching direction is at an angle of 60 ° to the graben t r end - - the fan-like sedimentary wedge obtained with perpendicular extension is almost completely absent when the extension is oblique.

Acknowledgements

This work was financially supported by ELF Aquitaine Production. Thanks are due to the Exploration Department of ELF Aquitaine Pro- duction for permission to publish this work and use the seismic line shown in Fig. 4. We are indebted to Alain Allary, Michel Coulon, Pierre Masse and Jean-Paul Richert (ELF Aquitaine Production), and also to Olivier Dauteuil, Philippe Davy and Thierry Souriot (Geosciences, Rennes) for help and comments at various stages. Special thanks are due to Jean-Jacques Kermar- rec for technical assistance in the Tectonic modelling laboratory, to C6cile Dalibard and Arlette Falaise for typing, as well as to Nick Arndt and Michael Carpenter for improvement of the En- glish text.

D E V E L O P M E N T OF T H E NORTH VIKING GRABEN: INFERENCES FROM LABORATORY MODELLING 51

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