the oslo rift—its evolution on the basis of geological and geophysical observations

Tecfonophysics, 178 (1990) 11-28

Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

11

The Oslo Rift-its evolution on the basis of geological and geophysical observations

H.E. RO, F.R. LARSSON, J.J. KINCK and E.S. HUSEBYE

Department of Geology, University of Oslo, P. 0. Box 1047, Blindern, 0316 Oslo 3 (Norway)

(Received November 6,1989; revised version accepted December 22,1989)

Abstract

Ro, H.E., Larsson, F.R., Rinck, J.J. and Husebye, ES., 1990. The Oslo Rift-its evolution on the basis of geological

and geophysical observations. In: E.-R. Neumann (Editor), Rift Zones in the Continental Crust of Europe-Geo-

physical, Geological and Geochemical Evidence: Oslo-Horn Graben. Tectonophysics, 178: 11-28.

The formation and evolution of the Oslo Rift have puzzled geoscientists for more than a century. We have

reexamined this problem in view of recent detailed geophysical information bearing on crust and lithosphere structures

in the general rift area. For example, the rift contours closely correlate with a Moho elevation of 3-5 km. The crustal

thinning partly predates the rifting, and apparently the same is the case for lithospheric deformations. Cambro-Silurian

sediments are found mainly within the present day rift and this implies rifting contemporaneous with uplift. The

southern rift segment, the Skagerrak Graben, appears to be somewhat different from the northern segment, the Oslo

Graben, with relatively less magmatism and larger subsidence. Crustal laminae are found inside and especially outside

the rift area including sub-Moho dipping reflectors whose origin is attributed to magmatic intrusions and/or

stretching. The strength of the lithosphere increases northwards with increasing thicknesses of both the brittle crust and

the lithosphere.

A variety of continental rifting mechanisms has been examined in view of the observational data at hand. Basically,

a passive rifting mechanism is presumed with dominant east-west tensional stresses albeit we are unable to be specific

on driving forces. The model presented in this paper reflects concepts introduced by Dunbar and Sawyer (1988) and

Neugebauer (1983) on preweakening of the crust and lithosphere and diapirism as a transport mechanism. The essential

elements of the model are: (1) the area in question was preferential for rifting in view of previous crust and lithosphere

deformations from Late Precambrian to Silurian times; (2) lithosphere stretching is needed for creating favorable

conditions for diapirism; and (3) the shield strength increases rapidly towards the northern part of the rift preventing

further migration.

Introduction

The Oslo Rift has intrigued scientists for nearly two centuries according to a historical overview

given by Dons (1978). The outcome of persistent

and comprehensive studies of the general area is not only detailed maps over the large variety of deformed rocks of Precambrian age, Cambro-

Silurian sediments and Permian volcanos, but also major contributions to the advancement of geo-

logical sciences, in particular in the fields of crys-

tallography and geochemistry. A long-standing problem which remains largely unresolved is the

initiation of the rifting process, its evolution in

terms of crustal deformation, magmatism, and

finally its termination after a relatively brief period of 60 Ma. Obviously, a rifting process would in-

volve not only the crust, but the entire lithosphere and hence a need for geophysical investigations as means for mapping of deep seated structural fea-

tures. Implicit here is that such knowledge would

provide a clue to a better understanding of the

oo40-1951/90/$03.50 0 1990 - Elsevier Science Publishers B.V.

H.E. RO ET AL.

rifting process. A basic problem is that geophysical observations reflect the present day structural environment, whereas a major element in the hypotheses on the rift evolution is the pre-rift structural environment.

The first attempt of crustal mapping of the Oslo Rift was tied to seismic refraction profiling

and detailed gravity measurements (see Husebye

and Ramberg, 1977). The main outcome of these efforts was indications of an elevated Moho beneath the rift (Tryti and Sellevoll, 1977) and that

the bulk of intrusive material should be located in the middle crust. Only a very generalized explanation was given for these observations, namely crustal thinning caused by influx of lithospheric/ asthenospheric material.

Today the situation is radically different regarding the geophysical data bearing on the rift, in particular its southward extension into Skagerrak.

In view of the hydrocarbon potential of the area, extensive marine seismic surveys on commercial

basis have been carried out. In addition, a purely scientific survey for deep crustal mapping of the

entire Skagerrak was undertaken in the winter of 1987. Altogether 1730 km of reflection seismic profiles (16 s TWT) were collected by the research

vessel M/V “Mobil Search” (Husebye et al., 1988). These data give a structural resolution in depth which until now has not been available from this area (e.g. see Kinck et al., 1990; Larsson and

Husebye, 1990; Lie et al., 1990; Pedersen et al., 1990).

The aim of this study is to examine current geological and geophysical data and structural knowledge bearing on the Oslo Rift area, and on this basis forward a model for the evolution of the rift.

Geological framework

Figure 1 shows the main structural elements of Skagerrak and the adjacent areas. The Pre- cambrian rocks adjacent to the Oslo Rift north of the Tornquist-Teisseyre line (TTL) were presuma- bly formed by crustal accretion between 1800 and 1500 Ma, reflected in a successive younging of the rocks southwestwards (Berthelsen, 1987). The last and apparently the most prominent Precambrian

deformation event, the Sveconorwegian-Grenvil- lean orogeny (1200-950 Ma), left the southwestern part of the Baltic Shield with a generally

north-south oriented fracture system (Lindh, 1980). However, the Bamble region exhibits a

mainly NE-SW trend (Ramberg et al., 1977) while in southern Sweden the main structural direction

is NNW-SSE (Kornfalt and Larsson, 1987).

Another major structural trend is the TTL and its westward continuation, the Fjerritslev Fault Zone (FF). The Precambrian TTL is considered to be a

boundary or transition zone between the stable Baltic Shield to the northeast and the mobile belts of northwestern Europe to the southwest (Pegrum, 1984; Berthelsen, 1987). The FF was probably a

zone of weakness in the Precambrian and was definitely active in Permian and Triassic times

(Ziegler, 1988). The extent of lateral movements along the TTL/FF system in Skagerrak has been

debated (e.g. Pegrum, 1984) but recent studies suggest relatively small movements in the order of

20 km (Liboriussen et al., 1987). Although details

regarding tectonics and sedimentation within the Sveconorwegian Province in the Late Precambrian

are sparse, it seems clear that a sedimentary basin existed in the northern part of what was later to

become the Oslo Rift (Bjorlykke, 1983). This implies an early stage of crustal thinning, at least in

the northern segment of the Oslo Rift, around 650 Ma ago.

Cambrian to Late Carboniferous

In the Early Cambrian the southern part of the Baltic Shield had a low relief which evolved into a shallow-water continental shelf environment

(Ramberg and Spjeldmes, 1978). During the

Cambrian and Lower Ordovician eustatic sealevel changes lead to a thin, continuous sedimentary

cover over major parts of the Baltic Shield (Ram-

berg and Spjeldnres, 1978; Bjsrlykke, 1983). In

Silurian times, the present-day Oslo Rift area was part of a basin with periodic rapid sediment accu- mulation (Bjerlykke, 1983). This basin evolved mainly in response to Caledonian nappe loading in the northwest. However, it differs from a typical foreland basin in that the rift axis is offset 45” with respect to the nappe front, and having a

THE OSLO RIFT-ITS EVOLUTION ON THE BASIS OF GEOLOGICAL AND GEOPHYSICAL OBSERVATIONS 13

61

5:

5'

5! j-

6 10 14 E

61

59

57

55

6 IO 14 E

Fig. 1. Regional structural map based on Fkxlen (1973), Ramberg et al. (1977) Berthelsen (1987), Liboriussen et al. (1987) Aubert et

al. (1990), Buer (1990) and Ro et al. (1990). Hatching marks the Oslo Rift proper and the Tornquist-Teiseyre line. The location of

the seismic Lines 00-2 and OG-7 are also shown. The following abbreviations are used: CDF= Caledonian Deformation Front,

SNF = Sveconotwegian Front, PZ = Protogine Zone, MZ = Mylonite Zone, OF = Oppland Fault Zone, LF = Loten Fault Zone,

0F = 0ymark Fault, VKF = Vaderiiarna-Koster Fracture, MAF = Meheia-Adal Fault Zone, PKF = Porsgrunn-Kristiansand Fault

Zone, BZ = Bamble Zone, NDE = Norwegian-Danish Basin, FB = Farsund Basin, RFH = Ringkabing-Fyn High, TEF = Trans-

European Fault, FF = Fjerritslev Fault Zone, 7TL = Tomquist-Teiseyre line, AC = Akershus Graben, VG = Vestfold Graben,

SC = Skagerrak Graben, HG = Horn Graben.

north-south direction approximately parallel to from the data at hand an element of stretching/

the axis of the later Oslo Rift. This points to the thinning can neither be excluded nor supported in

influence of an older weakness zone in the a conclusive manner. The southern part of Skager-

crust/lithosphere beneath the basin, controlling rak is more distant from the nappe front and has

its subsequent shape and development. According not been subjected to severe Caledonian deforma-

to N. Spjeldnzes (pers. commun., 1989) this basin tion. Foreland basin features are less clear here. In

must be considered an atypical foreland basin and fact, EUGENO-S Working Group (1988) use the

14

notation “marginal basin” in view of the exten-

sional features present. Most recent sedimentary thickness estimates

are in the order of 1 km in the coastal areas of western Sweden and about 200 m in eastern

Sweden (N. Spjeldnres, pers. commun., 1989). About 2 km of Cambro-Silurian sediments are found inside the Oslo Rift (Bjorlykke, 1983). In- side the Skagerrak Graben and locally along the TTL in Kattegat as much as 4-6 km and 3 km respectively of Cambro-Silurian sediments have

been found (Kornfalt and Larsson, 1987; Ro et al., I990), indicating the existence of major de-

pocenters in Skagerrak and the adjacent areas.

This implies that crustal thinning must have taken

place in this area prior to the rifting in Late

Carboniferous/Early Permian times.

Essentials of the Permian Oslo Rift formation

The Oslo Rift extends from the Caledonian deformation front in the north to the TTL in the south giving it a total length of more than 400 km. The rift proper (hatched in Fig. 1) can be divided

into three distinct grabens. These are, from the north: the Akershus Graben, the Vestfold Graben and the Skagerrak Graben. The first two con- stitutes the onshore Oslo Graben. The width of

the rift is less clearly defined. However, on the western side these faults were active during the

rifting process: the Oppland Fault Zone, the Meheia-Ada1 Fault Zone and the Porsgrunn- Kristiansand Fault Zone. To the east these faults

were active: the Loten Fault Zone, the Oymark Fault (which probably connects with the Vlderoama-Koster Fracture) and a large fault

northeast of the Borglum Fault (Fig. 1; Floden, 1973; Buer, 1990; Ro et al., 1990). The rift terminates against, or overlaps with, the TTL/FF in southwestern Skagerrak where its western border is not well defined. This area is rather complex due to intersection of the Porsgrunn- Kristiansand Fault Zone and Triassic tectonism along the FF causing a restructuring of the fault pattern here.

Observations of Permian volcanism and dyke

intrusions contemporary with rifting, together with Cretaceous and Tertiary volcanism are shown in

60

58

56

H.E. RO ET AL.

6 9

58

56

Fig. 2. Volcanic activity. Legend: 1 = Permian dykes; 2 =

Permian volcanism; 3 = Mesozoic volcanism; 4 = Tertiary

volcanism. (Modified after Kinck et al., 1990.)

Fig. 2. The bulk of the igneous activity is located in the central part of the rift. In addition,

volcanism has also been observed in the northern part of the Ringkobing-Fyn High, in the

Norwegian-Danish Basin and in Skagerrak (Holmsen, 1959; Am, 1973; Rasmussen, 1974). Dykes are found along the Swedish west coast, in the Bamble region and along the TTL in Scania

(Klingspor, 1976; Bergstrom et al., 1982). Accord- ing to Kinck et al. (1990), magmatism is confined

to areas with crustal thicknesses less than ap-

proximately 35 km. On land volcanic activity started in the south

around 300 Ma ago and gradually spread northwards with a rate of l-2 cm aa1 (Sundvoll et al.,

1990). The bulk of igneous rocks were emplaced within 20 Ma, but magmatic activity persisted for

another 40 Ma (Ramberg and Larsen, 1978; Ram- berg and Spjeldnses, 1978; Neumann et al., 1986).

Geophysical data: analysis and results

Approximately half of the Oslo Rift is situated on land with the other half extending into Skager- rak (Fig. 1). This land/marine segmentation is reflected in the geophysical data. For example, seismic profiling on land is modest and confined to refraction surveys. The structural resolution of

THE OSLO RIFI-ITS EVOLUTION ON THE BASIS OF GEOLOGICAL AND GEOPHYSICAL OBSERVATIONS 15

these data is relatively poor. Southern Skagerrak is

credited with a certain hydrocarbon potential

which has motivated extensive seismic profiling

surveys in recent years. In the northern Skagerrak, only a few shallow seismic surveys have been carried out @loden, 1973; Solheim and Gronlie,

1983). The notable exception here is the Mobil

Search survey in the winter of 1987 where ap-

proximately half of the 1730 km of deep (16 s TWT) seismic reflection profiles was collected in northern Skagerrak (Husebye et al., 1988). These

data constitute, together with approximately 5900 km of commercial data (Ro et al., 1990), the main data base for this investigation. The results of the seismic observations/interpretations are, in terms of Skagerrak sedimentation history, basin subsi-

dence analysis, lower crust laminae analysis and

crustal thicknesses, presented by Ro et al. (1990),

Pedersen et al. (1990), Larsson and Husebye (1990) and Kinck et al. (1990) respectively.

Gravity and aeromagnetic mapping are rela-

tively uniform over the entire area. Variations in

water depths and sediment overburden in Skager- rak may mask and/or weaken such anomalies. Other geophysical observations considered are the spatial earthquake occurrence and the few heat flow observations available (for an overview, see Husebye and Ramberg, 1977).

Below we will give an integrated land/marine

presentation of the geophysical observations

bearing on the Oslo Rift. The presentation is focused on results considered essential for the

problems discussed and with references to the data sources and analysis procedures used.

Seismic crustal studies-h4oho depth mapping

A comprehensive overview of the crustal mapping efforts in the Skagerrak area has recently

been presented by Kinck et al. (1990). They used available seismic data together with 3-component

seismological recordings from the large aperture NORSAR array (Berteussen, 1977), “overlying” the northern end of the rift, to produce a detailed 2D Moho depth map over the region under investigation (Fig. 3). In addition, they made a map over the thickness of the crystalline crust (Fig. 4). The main feature of these two maps is a marked

60

56

56 6 9 12

Fig. 3. Moho depth below sea level (contour interval 2 km) and

earthquakes as reported for the period 1951-1988. Open

squares: M, = 2.4-3.5; filled squares: ML = 3.6-5.5. (Mod-

ified after Kinck et al., 1990.)

60

56

-60

-58

-56 -- I

6

Fig. 4. Thickness of crystalline crust with 2 km contour inter-

val. (Modified after Kinck et al., 1990.)

H.E. RO ET AL.

crustal thinning from the shield towards the basin

areas offshore. The Oslo Rift is marked by a

pronounced Moho elevation (up to 5 km) along its entire length. The thinning is shifted slightly eastward relative to the rift in the south. The Moho trends in Skagerrak seem to follow mainly the

Skagerrak Graben. The crustal thinning beneath the FF seems to be less pronounced, whereas a

rapid change in thickness appears to be associated

with the ‘ITL in Kattegat (Fig. 3).

An essential element associated with the Oslo Rift is the influx of magmatic material to the rift (Neumann et al., 1986). Such bodies are inferred

from gravity measurements (Ramberg, 1976; Wes- se1 and Husebye, 1987). The question is if they can be detected by seismic means. With this in mind we examined velocity-depth relations from the refraction profiles intersecting the rift, but found no clear evidence for anomalous velocity variations across the rift. After all, the minimum den-

sity contrast at mid-crustal levels needed for ex-

plaining the gravity anomaly associated with the rift is in the order of 0.06-0.10 g crnm3 which

implies a relatively weak velocity contrast. Fur-

thermore, Aki et al. (1977) used the P-wave travel time residuals observed across the NORSAR for a lithospheric tomographic study of the array area. They detected a moderate velocity anomaly in the

lithosphere (at approximately 90-130 km depth) beneath the northern rift segment. In a tomo-

graphic study of the lithosphere and upper mantle (O-600 km), based on seismograph network data from southeastern Norway, southern Sweden and Denmark, Husebye et al. (1986) obtained the fol-

lowing results: southern Sweden, bounded by TTL to northern Jutland in the south and 11.3”E to the

west, exhibits a clear high-velocity anomaly down to approximately 300 km depth, including the whole lithosphere and upper part of the astheno-

sphere. Likewise, southern Norway and Denmark exhibit relatively low velocities with Skagerrak

and the Oslo Rift as a sort of lithospheric transition zone. In a recent P,/S, tomographic investigation, Bannister et al. (1990) reported anoma- lously low P, and S,, velocities for the TTL, the Oslo Rift and the central parts of the Norwegian Caledonides, the latter being most pronounced. We take the two latter studies to imply that the

lithosphere underlying the Skagerrak-Oslo Rift is slightly different from that of adjacent areas.

As mentioned, seismic results reflect present day structural conditions but in a rift context a separation of pre- and post-rifting states is im- portant, We consider this to be feasible regarding

the pronounced Moho elevation characterizing the rift. In a “back projection” analysis of crustal

thicknesses to pre-basin times (Precambrian),

Pedersen et al. (1990) estimated the northern

Skagerrak crust to be relatively thin (35 km) while

the crust beneath southern Skagerrak must have been thicker, i.e. 38-40 km. Kinck et al. (1990)

have found that geological ages and crustal thicknesses are closely correlated in Fennoscandia. For the Sveconorwegian rocks, in which the rift is

embedded, the crustal thickness should be expected to be about 40 km whereas it is presently around 30 km. Derived stretching coefficients f/3-

values) are in the order of 1.05-1.20 (Wessel and

Husebye, 1987; Pedersen et al., 1990) and thus incompatible with a pre-rift thickness of 40 km.

These estimates, apparently contradictory, are

easily reconciled given a pre-rift thinning of ap-

proximately 5 km. This is in general agreement with the geological evidence for the existence of Precambrian and Cambro-Silurian basins in the Oslo Rift area.

Seismic reflection profile.9

The seismic reflection profiles OG-2 and OG-7 (Figs. 5 and 6 ) show a clear tectonic subsidence

inside the Skagerrak Graben and uplift and erosion especially to the east (in the order of 4 km).

The following thermal subsidence is uniform over the entire profile (Fig. 6). Seismic mapping of

outcrops (Fig. 7) reveals that all Paleozoic sediments to the east of the Skagerrak Graben were

removed prior to Mesozoic sedimentation. The Paleozoic strata of the fault-blocks inside the Skagerrak Graben increase in thickness towards

the center of the graben (Fig. 6). This indicates a pre-rift basin controlled primarily by flexure. The doming of the rift basement and erosion of the Paleozoic rift sediments may have been caused by local intrusion of magmatic material. A magmatic body seems to be present in the northern end of


SW DISTANCE (km) NE 0.0 20.0 40.0 60.0 60.0 100.0 120.0 140.0 160.0 160.0 200.0 220.0 240.0

q ~~o~~r;d;~n~~ C&m?-Silurian sediments, q Permian deposits, . agmatrc rocks

Fig. 5. Depth-converted line drawing of seismic profile OG-2 (Fig. 1) running in a SW-NE direction along the axis of Skagerrak

Graben. Note the fairly uniform thickness of Cambro-Silurian sediments except within the graben. South of the Fjerritslev Fault

Zone (FF) Rotliegendes and Zechstein deposits overlay the Cambro-Silurian sediments. North of the FF Zechstein deposits are

removed and the Permian deposits are post-rift sediments.

the Skagerrak Graben (Fig. 5) (Flodtn, 1973; Solheim and Grsnlie, 1983). The presumed out-

crop of this body coincides with a gravity anomaly (Fig. 12). Within the Skagerrak Graben, syn-rift sediments are only occasionally found at the graben border faults (Figs. 5 and 6). The thickness of Permian (post-rift) sediments increases towards

the FF (Fig. 5). Post-tectonic erosion during the Permian turned the northern part of Skagerrak

into a smooth base Triassic peneplain which has

been only slightly disturbed by later tectonic events (Figs. 5 and 6). South of the FF Rotliegendes

NW DISTANCE (km) SE 0.0 20.0 40.0 60.0 60.0 100.0

a. Skagerrak Graben w OG-7 ~15.0 ,“I,‘, , ,I,” ,I I ’

e Crystalline crust, 0 Cambro-Silurian sediments, Mesaoie sediments, aPermian deposits.

Fig. 6. Depth-converted line drawing of OG-7 (Fig. 1) crossing

the Skagerrak Graben. Note the approximately 4 km of

Cambro-Silurian sediments towards the graben flanks (faults A

and B) and the almost complete lack of Paleozoic sediments

southeast of the graben. Syn-tectonic sediments seem to be

present close to the faults A and B. In the center of the graben

a clear doming and erosion of the Cambro-Silurian sediments

can be seen. Post-rift subsidence appears to be uniform over

the entire profile.

sediments and thick Zechstein evaporites were de- posited, the latter resulting in salt tectonics in Jurassic times. Triassic sediment thicknesses in-

crease from the Skagerrak Graben towards the FF. Influence of inversion during the Alpine (Zie-

gler, 1988) orogeny is manifested by uplift and erosion of Chalk deposits directly above the FF. Finally, the north- and eastward extent of the

various sediments was altered by Pleistocene gla-

cial erosion which was also responsible for the Norwegian Channel-a prominent bathymetric

feature with a maximum depth of approximately

700 m (Figs. 5, 6 and 7). For details on the

i 9 10 1;

Fig. 7. Map showing outcrop of sediments. Figures on the map

give typical velocities for each layer. Modified after Ro et al.

(1990).

18

sedimentation history of Skagerrak we refer to

Skjerven et al. (1983) and Liboriussen et al. (1987).

Seismic reflectivity-spatial laminae distribution

From deep seismic profiling surveys it is well established that the crust is not transparent in the sense that numerous reflectors are observed particularly in the lower crust in extensional areas

(Matthews and Smith, 1987). Skagerrak is no exception in this respect as demonstrated by Larsson and Husebye (1990). The thickness and intensity

of the reflective zone in the lower crust is strongly variable; being more clearly defined in the northeast of the surveyed area than in the southwest.

The advantage with the Mobil Search survey was

the profiling grid used enabling a direct compari-

son of lamination features at intersecting profiles. For example, profile OG-7, perpendicular to the rift, is almost void of laminae within the rift (Fig.

8) whereas on OG-2 (Fig. 9) running along the rift axis, laminae are relatively abundant. We have considered two potential explanations for this ob- servation:

(1) Faults and tilted blocks may scatter and/or attenuate the seismic signals. Attenuation effects are also obvious in areas with a thick sedimentary

column as in southwestern Skagerrak.

(2) Alternatively, reflectivity is strongly depen- dent on structural trends and/or laminae geome-

try (e.g. Reston, 1988). Larsson and Husebye

(1990) examined all their laminae observations and special attention was given to sub-Moho dipping reflections. These are observed both inside

and outside the rift, but are especially abundant on the eastern side. However, no systematic pat-

H.E. RO ET AL.

NW DISTANCE (km) SE 0.0 20.0 40.0 60.0 60.0 100.0

0.0

‘;‘ 10.0

t 20.0

I JO.0 I-

“, 4O.O - OG-9 _ __ ;__ ___

D 50.0 , 3 , 7 * 3 I 7 7 ’ I I 3 1 ’ I

Fig. 8. Depth-migrated line drawing of OG-7. Reflectivity is

weaker inside and northwest of the graben than to the south-

east. Note the elevated Moho beneath the graben and that

sub-Moho reflections are observable.

tern in these observations was found. This does

not necessarily preclude magmatic underplating

but may imply that the lateral extent of such bodies is in the order of lo-30 km, i.e. less than

the profile spacing. To summarize, laminae observations can not be uniquely related to rift fea-

tures since no specific pattern/fabric has been recognized so far. High-quality reprocessing of the original data may change this.

Larsson and Husebye (1990) converted laminae observations into a “reflectivity measure” tied to the count of reflectors within rectangles of 1 km vertical extent and 10 km horizontal extent. These

rectangles were added up over intervals of 30 km horizontally and then normalized before plots such

as shown in Fig. 10 were produced. Most of these

reflectivity plots exhibit two conspicuous peaks;

one at or near the Moho and a second at depths of

lo-20 km, averaging 15-16 km for the surveyed area. As the “Moho peak” clearly marks the transition between the lower crust and the upper man-

tle, the secondary reflectivity peak is taken to reflect the transition from the brittle upper crust

SW DISTANCE (km) NE 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 160.0 200.0 220.0 240.0

0 ._-- - . _- 6 50.0 , a,, , , , , ,, , ,, , , ,, , , ,,, , ,, , ,~,,, ,, , , ,, ,, ,, , , , , , ,

Fig. 9. Depth-migrated line drawing of OG-2. Reflectivity inside the Skagerrak Graben is stronger than on OG-7. Note the elevation

of the reflective zone and also of the Moho beneath the graben.

THE OSLO RII=-ITS EVOLUTION ON THE BASIS OF GEOLOGICAL AND GEOPHYSICAL OBSERVATIONS 19

REFLECTIVITY (refl./unit box)

05 10 15 20

?----

Fig. 10. Reflectivity-depth plot for profile OG-7 between 80

and 110 km horizontal distance. The uppermost peak is taken

to mark the transition between the brittle upper and the more

ductile lower crust. The lowermost peak coincides with the

Moho as found by Kinck et al. (1990).

to the ductile lower crust. This is to a large extent supported by the coinciding first or second order

velocity discontinuity in the crust as derived from refraction profiles (Lund et al., 1987; Egilson and

Husebye, 1989). On the basis of these reflectivity peaks Larsson

and Husebye (1990) constructed a map of the

ratio between the transparent upper crust and total crust thicknesses (Fig. 11). This map indicates a general increase in the thickness of the

brittle part of the crust and hence the strength of the crust towards the north of Skagerrak. How-

59 N

; 8 9 10 11 12 E

Fig. 11. Map of the brittle crust in percent of total crustal

thickness in Skagerrak, defined by depths of reflectivity peaks

as shown in Fig. 10 (from Larsson and Husebye, 1990).

ever, for a constant lithosphere thickness this en-

tails a weakening of the lithosphere per se.

Gravity and magnetic measurements

The Bouguer anomaly map for the study area is shown in Fig. 12. The data used are those made

available by national geodetic agencies in Norway,

Sweden and Denmark, and supplemented with

Ramberg’s (1976) measurements in the Oslo Graben. The perhaps most essential feature is a gravity high coinciding with the Oslo Graben and continuing into Skagerrak where it disappears.

This strong anomaly, partly overlapping the southern part of the Vestfold Graben, implies that this graben should have a seaward extension. If so,

the Skagerrak Graben is taken to start where thick

sedimentary strata emerge (Figs. 1 and 5). Exten-

sive analyses of this onshore anomaly, using for-

ward (Ramberg, 1976) as well as inverse modelling

(Husebye et al., 1978; Wessel and Husebye, 1987), imply that its origin is in the middle crust and that

it extends eastward lo-30 km out of the graben.

The density contrast needed to explain the Oslo Graben gravity high is modest and in the order of 0.06-0.10 g cmP3 (Wessel and Husebye, 1987).

We have also examined available aeromagnetic data for evidence of magmatic activity in the

Skagerrak Graben (Fig. 13). As with gravity, the

anomalies were found to disappear southward. This may partly be due to a weakening of poten-

tial anomalies caused by large water depth and thick sedimentary columns. The magnetic field

strength is inversely proportional (3rd power) to the distance of the anomalous body. Such over-

burdens are non-existent in the northern parts of

the rift where the strong magnetic anomalies neatly coincide with gravity and outcrops of volcanics.

The lack of pronounced magnetic anomalies in the area east of the Skagerrak Graben, i.e. north of Jutland and along the west coast of Sweden, with thin sedimentary cover and shallow waters, pre-

clude intense volcanic activity in these areas. The notable exception is the mentioned dykes along the Swedish coast (Fig. 2).

We have given special attention to the free-air gravity data collected along profile OG-7 (Fig. 14) since this profile crosses the Skagerrak Graben

H.E. RO ET AL.

. \ , I / . .3u-

6’ 8’ loo 12O 111O

Fig. 12. Bouguer anomaly map compiled from Ramberg’s (1976) and the Swedish (1971) and Danish (1978) Geodetic Institutes’

Bouguer anomaly maps. The Oslo Rift appears as a positive anomaly along its entire length, with the largest anomalies between the

southern part of the Vestfold Graben and northernmost Skagerrak Graben. The Tomquist-Teisseyre tine (TTL) appears as a clear

low-anomaly feature, probably reflecting thick sediment cover. Between the TTL and the Swedish coast a weak positive anomaly

seems to continue into the Oslo Rift. The presumed Skagerrak volcano offshore southern Norway exhibits a clear positive anomaly of

more than 60 mGal.

approximately perpendicularly. The data were cor- mantle. The initial model was chosen on the basis rected according to the reference geoid formula. of the seismic interpretation of the sedimentary Airy isostasy was presumed, densities for the sedi- column and Moho depths (Figs. 6 and 8). The mentary column were obtained from nearby wells, final model, shown in Fig. 14, gives a reasonable and a density contrast of 0.41 g cmW3 was taken fit to the observations. Noteworthy is the indica- between the crystalline crust and the underlying tion of relatively heavy material in the upper and


Fig. 13. Magnetic map from Am (1973) showing clear positive anomalies in the southern and northern Oslo Graben. The southern

anomaly correlates neatly with the outcrop of a larvikite body. In Skagerrak the magnetic field is much weaker except for a positive anomaly associated with the Skagerrak volcano.

lower part of the crust southeast of the graben. The upper anomaly reflects the short gravity gradient across the southeastern flank of the graben (similar to Wessel and Husebye’s results). The lower crust anomaly is less well resolved, but may indicate an existence of heavier material in the same depth as where the laminae are most dense.

Within the graben no anomalous body is needed to explain the gravity observations. This is somewhat surprising as the seismic data imply local doming in the central part of the graben (Fig. 6). The explanation for this may be that the density contrast between an intrusive body and the coun- try rocks is not sufficiently high, or that the pres- ence of a magmatic body is masked by overlying

22 H.E. RO ET AL.

FREE.AIR 00-7 NW se

: : ID IO 80

Len& (km)

Fig. 14. Profile OG-7 with observed (crosses) and calculated

(solid) free-air gravity anomaly. The densities used for the

sediments are: Quatemary-Tertiary = 2.15 g cm-‘; Chalk-mid

Jurassic = 2.35 g cme3; mid Jurassic-base Triassic = 2.5 g

cme3 and Paleozoic sediments = 2.7 g cme3.

sediments whose densities have not been correctly estimated.

To summarize, the gravity results imply that magmatic material is rather uniformly distributed in the Vestfold and Akershus Grabens in a depth range of lo-20 km. There is evidence of magmatic intrusions within the crust east of the rift also for the Skagerrak Graben where the magmatic activity

appears to have been relatively modest.

Earthquake activity-heat flow observations

The instrumental data of the Nordic earth-

quake (EQ) database (Seismological Observatory, Helsinki) from 1951 to present has been used. We

have plotted epicenter locations together with Moho depth (Fig. 3). As shown by Kinck et al. (1990), there is a striking correlation between EQ locations and crustal thicknesses. This is even more pronounced for large magnitude events. Dominant tectonic features like the Oslo Rift and

the ‘ITL/FF are at present not particularly seismically active with a possible exception where the two structural zones intersect. A long after- shock sequence following the 5.5 earthquake in outer Oslo Fjord in 1904 had mainly epicenters east of the rift. It should also be mentioned that

seven small earthquakes in the northern part of

the Oslo Graben beneath the NORSAR array

have well resolved focal depths of 20 and 32 km, coinciding with the depth of crustal intrusives as

found by Wessel and Iiusebye (1987), and with the lowermost part of the crust respectively. At present, however, the Oslo Rift is tectonically dead. The few heat flow observations available from the Oslo Rift area (Grsnlie et al., 1977) show no anomalous high values as expected, implying a

thermally dead rift as well.

Summary of geological and geophysical characteris- tics

Based on the observations presented above we can summarize the dominant features associated

with the Oslo Rift during its short, active period

as follows:

- At least two stages of crustal thinning/stretching prior to rifting. This is based on both geological and geophysical evidence. - Moderate uplift associated with rifting, reflected in the lack of Paleozoic sediments outside

the rift. - Start of rifting is prior to or nearly contemporaneous with volcanism. - Dyke volcanism took place also outside the rift during its initial stage.

- Volcanism possibly migrated from the Vest-

fold Graben and northwards.

- Main volcanic activity is short-lived with magmatic intrusions primarily in the northern rift

segment.

- Bulk of magma located at lo-20 km depths inside, and extending lo-30 km eastward of the

rift in the north, possibly more in the south. - Southern end of the rift overlaps with/ terminates against the ‘M’L.

- Contempor~eous movements along the TTL and formation of the Alborg Trough. - Laminae data imply a progressively thicker

brittle part and hence a stronger crust northwards. - Rift surface contours coincide with a Moho elevation of 3-5 km. - Oslo Rift activity ceased in Late Permian times. The tectonic activity shifted westward, thus

activating the FF.


Discussion

One of the most striking surface features of

geodynamical processes in the Earth are continen-

tal rifts and as such among the most extensively studied. Their origins are still subjects of debate

and the Oslo Rift is no exception in this respect. Before discussing specific models for the Oslo Rift’s origin, we will give a brief outline of major rifting hypotheses (for a detailed review, see Zie-

gler, 1988, chapter 10).

Active and passive rifting mechanisms

Active mechanisms for rift formation associate the surface rifting with mantle convection. Nor-

mally, this hypothesis involves a time/space sta-

tionary mantle plume which channels mantle/

asthenospheric material into the lithosphere and thereby creating a large swell such as observed

beneath Iceland today (diameter approximately 1000 km) (White and McKenzie, 1989). The ten-

sional loading stresses caused by such a lithospheric doming fracture the lithosphere and thus initiate the rifting. This mechanism is termed “ac-

tive” because the upwelling material provides the

driving force. It is not considered to be relevant for the Oslo Rift due to lack of evidence for large-scale doming, a short life span and magmas originating in the lithosphere (Neumann et al., 1986).

The passive rifting mechanisms are related to stretching of the lithosphere due to preexisting tensional stresses with accompanying passive up-

welling of the underlying mantle (Turcotte and

Emerman, 1983). The driving forces may be gener- ated at plate margins or alternatively by thermal

and/or loading stresses, or by asthenospheric drag on the lithosphere due to mantle convection.

Main deformation models of passive rifting are pure and simple shear. In the first case uniform and symmetrical thinning of the lithosphere is assumed with down-faulting of blocks in the brittle upper crust and ductile extension in the rest of the lithosphere. Simple shear (Wemicke, 1985) involves displacements on a large-scale shear zone dipping through the lithosphere. This shear zone transfers the extension laterally from the upper to

the lower part of the crust and remaining lithosphere, resulting in an asymmetric rift. In both

cases crustal doming and possibly volcanism may

take place albeit on a modest scale. These are both

secondary effects and hence the notation “passive rifting”.

The “diapir” model of Neugebauer (1983) may be considered to be a hybrid of active and passive rifting mechanisms. The essential element of this model is a thermal instability in the lower lithosphere and hence a localized density anomaly. The

ensuing diapiric uprise of light and hot material may in extreme cases continue up to the surface

but in most cases it will terminate at barriers such as the brittle crust or the sub-Moho brittle zone.

Numerical modelling results imply “mushroom- ing” or lateral material flow beneath the men-

tioned barriers. Magmatic intrusions in the lower

crust have been suggested to be associated with

the widespread observations of laminae in basin areas (Meissner, 1986). The diapir model is an

effective way of transporting material through the lithosphere but it is hardly sufficient for providing

the driving force, at least in the initial stage of the rifting.

Dunbar and Sawyer (1988) have considered passive continental rifting conditioned on preexisting weaknesses in the crust and mantle lithosphere

separately. In the first case, the rift would be dry (no magmatism) until a very late stage in the rifting process, whereas in the latter case

widespread volcanism may start 15-20 Ma after the initiation. If the crust and mantle lithosphere weaknesses coincide spatially, then doming, rifting

and volcanism should be essentially symmetric relative to the rift axis.

Rifting of the lithosphere is a complicated process. The area subjected to deformation may have

had different crust/lithosphere thicknesses and hence mechanical strengths. Explicit driving forces

are not well known and besides, the differences between passive and active rifting mechanisms become blurred after the rifting has commenced, as demonstrated by Neugebauer et al. (1983) and Dunbar and Sawyer (1988). With these reserva- tions in mind we have constructed a schematic model for the Oslo Rift (Fig. 15) in order to explain the principle observations listed in the

24 H.E. RO ET AL.

zoo tm

I x

B R -I I

Fig. 15. Schematic cross-sections of the Oslo Rift in Early

Permian times. Profile A represents the northern part of the

rift (the Oslo area) and profile B the southern part (north of

Jutland). In profile A the thinning of the lithosphere is sym-

metric to the rift axis and magma reaches the surface. Magma

bodies within the crust will contribute to local doming within

the graben and give symmetric uplift of the graben flanks.

Lateral magma flow within the crust and out of the rift, due to

to have been subjected to tensional stresses having

an E-W or NW-SE orientation. Dextral move-

ments along the TTL, in response to the

Hercynian/Variscan orogeny, have been sug-

gested to constitute the primary “driving force”

for the Oslo Rift formation (Pegrum, 1984: Zie-

gler, 1988).

Within the North Sea and adjacent basin areas

a large number of graben and rift systems have

been identified (for details, see Ziegler, 1988,

chapters 5, 10). Graben/rift systems in this re-

gion, such as the Oslo Rift, Midland Valley Graben

(U.K.) and the Norwegian-Greenland Sea (NGS)

rift, are of Devonian to Permian ages. The NGS

megarift, upon progressing southward and “enter-

ing” the North Sea, appears to have triggered the

formation of the many grabens here. Another con-

temporaneous megarift was that tied to the Neo-

Tethys sea-floor spreading separating Africa from

Laurasia. According to Ziegler (1988) the North

Sea rift and graben formations coincided in time

with the initial phase of the post-Her~ynian brea-

kup of Pangea (Gondwanaland, Laurasia and

Africa) along the mentioned megarifts. In this

context we consider deformations in Skagerrak

and adjacent areas to be associated with complex

small-scale movements in a fragmented part of the

Laurasian plate. During this period the litho-

sphere here was retained largely intact so the

associated rift driving forces must have been mod-

erate.

The northern part of the rift is embedded in

Precambrian rocks of the Baltic Shield. These

rocks have survived over a time span of at least a

biliion years due to the lithospheric strength of the

area. The presumed deformation forces coming

into being in Carboniferous and Permian times

would not be sufficient for causing rifting unless

the area had been weakened beforehand. The ex-

istence of basins in the rift area from Late Pre-

cambrian to mid-Silurian times (Bjorlykke, 1983;

EUGENO-S Working Group, 1988) indicates that

such a weakening took place. We regard this litho-

spheric weakening and implied thinning to have

been confined mainly to the lower part of the

lithosphere due to the small width of the northern

rift segment. It apparently must have been mod-

crate and note that the present-day thickness of

sloping and differences in crustal strength, can locally produce

gravity anomalies (Wessel and Husebye, 1987). In profile B the

lithospheric thinning is shifted slightly eastward relative to the

rift. The ascent of magma will be obstructed and the bulk of

magma will be emplaced beneath and witbin the crust resulting

in asymmetric uplift relative to the rift surface. The magma

bodies trapped at mid-crust levels and at the Moho may be the

origin of the observed laminae (Fig. 8).

previous section. Basic assumptions are passive

rifting initially, primarily lower lithospheric thin-

ning essentially symmetric vis-8-vis the rift in the

north and slightly asymmetric in the south, and a

form of brittle crust failure with diapiric upwelling

of lithospheric material. A discussion of the Oslo

Rift evolution in view of available geophysical and

geological observations follows below.

Driving forces-rifting criierim

It is difficult to pin-point one obvious driving

force for the Oslo Rift. However, the area appears

THE OSLO RIFT--ITS EVOLUTION ON THE BASK.3 OF GEOLOGICAL AND GEOPHYSICAL OBSERViTIONS 25

the lithosphere is about 110-130 km increasing to

150-200 km in the central parts of the shield

(Calcagnile, 1982; Lie et al., 1990). Consequently,

the Skagerrak-Oslo Rift area was a strong candi- date for rifting given the appropriate defo~ation forces.

Rifting in progress

The thinning of the lithosphere in Late

Carboniferous/Early Permian had two effects: uplift due to thermal expansion and initiation of

widespread diapirism contributing to local doming. The lateral extent of the uplift is uncertain as

the lateral extent of Silurian sedimentation is not well known. Rifting must have progressed ap-

proximately jointly with uplift in order to preserve the Cambro-Silurian sediments in the rift. Then

surface magmatism started; volcanos and dykes, primarily within the Vestfold and Akershus

Grabens, and dykes outside the grabens were emplaced (Figs. 2 and 15). The extent of volcanism

within the rift is rather exceptional compared to that of the other graben/rift systems within the general North Sea area. It appears to be unevenly distributed with con~ntrations in the Vestfold

Graben and the Akershus Graben while the grav-

ity observations indicate a more uniform distribu-

tion of igneous rocks in the middle crust along the

entire land segment of the rift according to Wessel

and Husebye (1987). The laminae observations in Skagerrak may also be taken as indications of widespread magmatic intrusions. We take this to indicate lateral magmatic flow with the brittle upper crust acting as a barrier (Neugebauer, 1983).

The dykes outside the rift are c~ntemporan~us with the initial rifting stage. The influx of bulk material within the rift is believed to have exerted compressional stresses closing the initial fractures.

The Skagerrak Graben and adjacent areas

The southern part of the rift appears to be somewhat different from the northern part in terms of larger subsidence and less volcanic activity. Both the OG-7 seismic and gravity results and the laminae ~st~bution imply that the magmatic activity is relatively more confined to the lower crust

in the south than in the north. The down-faulting both perpendicular and parallel to the rift axis is taken to signify increasing subsidence southward.

The same phenomenon is seen on profiles parallel with the graben axis and is ma~fested by subsidence analysis (e.g. Pedersen et al., 1990). The general rift area in Skagerrak is wide, in the order

of the thickness of the lithosphere (100-130 km),

indicating a more uniform stretching in the initial

rifting stage. The Skagerrak Graben and Alborg Trough ap-

pear to have been causally connected from Late Carboniferous and up to Triassic times in view of

sedimentation and Permian faulting. The extent of this causality is of interest in a Oslo Rift driving

force context. An E-W stretching in the northern

Skagerrak-Oslo Rift would entail transcurrent movements along the TTL in a NW-SE direction

and extension in a general E-W direction.

Northward migration

The hypothesized northward rift migration, as derived from volcanic age differences between the Vestfold and Akershus Grabens, may either be

genuine or reflect fortuitous circumstances. Part of this migration may reflect magma ascent time as

the crust appears to be more “crushed” in the

Vestfold Graben where the dominant N-S trend-

ing faults of the rift intersect the NE-SW trending

Bamble fault system. The rift termination north-

wards coincides with the Caledonian deformation front (Fig. 1). Evidence for deformations farther

north is not clear and such deformations should not be expected either. The reason for this is the rapid increase in c~st/~thosphere thicknesses and hence a much stronger lithosphere. Whether there was a clear northward rift migration or not, is of less importance for our rift evolution model, espe-

cially since the initial driving force remains uncertain.

Oslo Rift termination

The total life span of some 60 Ma of the Oslo Rift is short compared to other North Sea area grabens and rifts (Ziegler, 1988). We tie this to di~~s~ng stretching with the effect that the rate

26 H.E. RO AL.

of crust/lithosphere cooling became faster than

the net heat influx to the lower lithosphere, thus

terminating the rifting process. In Late Permian to

Early Triassic times the tectonic activity apparently shifted to southern Skagerrak with (re)activation of the FF and rapid subsidence in the Norwegian-Danish Basin and the Farsund Basin (Skjerven et al., 1983; Pedersen et al., 1990;

Ro et al., 1990).

Conclusion-final remarks

Our model for the Oslo Rift evolution is presented in cartoon style (Fig. 15) since it is not

backed by extensive numerical modelling experi-

ments. We have been unable to identify an obvi-

ous driving force for the rift initiation. This model accounts qualitatively for the principal geological

and geophysical features characterizing the Oslo Rift area. These are: - Prerift crustal/lithospheric deformation and thinning. - Anomalous lithosphere beneath the rift seems to partly predate the rifting. - Cambro-Silurian sediments mainly within the rift implies rifting contemporaneous with uplift.

- Gravity and reflectivity (laminae) results imply widespread lower crust magmatic intrusions

also outside the grabens.

- The crust/lithosphere strength increases

northwards. - The southern rift segment, the Skagerrak

Graben, appears less magmatic and is char- acterized by larger subsidence than the northern segments.

- Formation of the Alborg Trough and TTL movements were contemporaneous with rifting.

Implicit in our rift evolution model is that diapirism as a transport mechanism is given a prominent role albeit conditioned on a pre-

weakened crust/lithosphere. The essential ele-

ments here are that (1) the area in question was preferential for rifting in view of past deformation cycles in Late Precambrian and Silurian times; (2) lithosphere stretching is needed for creating favorable conditions for diapirism; and (3) the

shield strength increases rapidly towards the

northern part of the rift preventing further migra-

tion.

Acknowledgements

We acknowledge Professors A. Bjorlykke, K. Bjorlykke, H. Neugebauer, E.-R. Neumann and N. Spjeldnses for many stimulating discussions.

The Oslo Core Group (NLP) is supported by grants from the Norwegian Council for Science and the Humanities (RNF/NAVF: D 48.50.10) and VISTA (V:6217), jointly sponsored by Statoil

and the Norwegian Academy of Science.

Norwegian ILP Contribution No. 105.

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