Architecture and early evolution of the Oslo Rift

TECTONOPHYSICS ELSEVIER

Tectonophysics 240 (1994) 173- 189

Architecture

and early evolution of the Oslo Rift B.Sundvoll

a, B.T. Larsen

‘*

~’Mineralogical-Geological ’ Statoil A /S,

Museum, Sarsgt. I, N-0562 Oslo, Norway P.O. Box 300, N-4001 Sta~~anger, Nonvuy

Received 14 January 1993; revised version accepted 5 October 1993

Abstract A revised assessment of architecture and pre-rift fabric connections of the Oslo Rift has been undertaken and linked to a new appraisal of observations and data related to the initial phase of the rift evolution. In addition to half-graben segmentation, accommodation zones and transfer faults are readily identified in the linking sectors between the two main grabens and between graben segments. Axial flexures are proposed between facing half-grabens. The accommodation zones were generally sites of volcanism during rifting. Pre-rift tectonic structures played an influential role in the rift location and development. The deviant N-S axis of the Vestfold graben segment is viewed as related to pre-rift structural control through faults and shear zones. This area was probably a site ol Proterozoic/ Palaeozoic crustal and lithospheric attenuation. Field evidence suggests that the rift started as a crustal sag with no apparent surface faulting in a flat and low-lying land at a time about 305-310 Ma. Volcanism, sub-surface sill intrusion and faulting started about simultaneously some time after the initial sag (300-305 Ma). Faulting and basaltic volcanism were initially localized to transfer faults along accommodation zones and a NNW-SSE transtensional zone along the eastern margin of the incipient Vestfold graben segment. This transtensional zone was probably created by right-lateral simple shear tracing pre-rift structures in response to a regional stress field with the tensional axis normal and the maximum compressional axis parallel to the NNE-SSW-trending rift axis.

1. Introduction Studies of the architecture of the East African rift system have furnished several important empirical relations that have improved the understanding of rifting mechanisms and rift development in general and thus may be applicable to other rifts as well (Rosendahl, 1987; Rogers and

* Present address: Norsk Hydro A/S, P.O. Box 200, N-1321 Stabekk, Nonvay. 0040-l 95 l/94/$07.00 0 1994 Elsevier SSDf 0040-1951(94)00103-G

Science

Rosendahl, 1989; Morley et al., 1990). Linked to rift architecture is also the relationship to pre-rift tectonic fabric (Rosendahl, 1987; Daly et al.. 1989). The progressive development from pre-rift conditions into the initial rifting stage is probably the most important and critical phase in the rift evolution for establishing constraints on causal connections and rifting mechanism. This phase of the rifting is in many cases difficult to study, due to extensive syn-rift sedimentation and/ or magmatism or post-rift processes.

B.V. All rights reserved

Fig. 1. Oslo Rift, elements and structures. Rift elements: AGS = Akershus graben segment, BB = Ramble block, HTR = Hedemark-TM region, KB = Kongsberg block, OG = Oslo Graben, SC = Skagerrak Grahen, k’GS = Vestfold graben segment, TQ = Tomquist zone, OB = 0stfold block. Fault zones: A0F = Aremark-0yeren f.z., EF = Engerdal fz., KP = KristiansandPorsgrunn f.z.. MS = Meheia-Sokna f.z., 0 = oslofjord fz.. OF = Gsensje fz., RF = Rendalen fz., Ml= Randstjord-Hunnedal f.z. Lakes: El = Eikeren, MI = Mjosa, RA = Randsfjorden, 7Y = Tyrifjorden, 0Y = @yeren. Cities and districts: A = hendal. BO = Rohusliin (Sweden), D = Drammen, HA = Hamar, HO - Horten, HO = Hgnefoss. K = Kristians&d, L = Langcsund, S = Skien. OFJ = Oslofjord. SB = Sevaldrud explosion pipe.

B. Sundr~oll, B. T. Larsen / Twtonophyws

The Permo-Carboniferous Oslo Rift, situated in the southwestern part of the Fennoscandian shield (Fig. l), may in many respects suit such a study because its partly eroded state of preservation permits a direct exploration of its internal structure while almost no post-rift tectonic or magmatic activity has affected it.

175

240 (19941 173- IN9

A revised descriptive evolution model for the Oslo Rift has been presented in Neumann et al. (1992) with encompasses six progression stages: (1) proto-rift stage; (2) initial rifting stage; (3) main rifting stage; (4) central volcano and composite intrusion stage; and (6) aftermath stage. This paper deals primarily with stages ( 1) and (2)

-

Fig. 2. Oslo Rift, regional setting. Horizontal lines = minimum area affected by syn-rift magmatism and tectonics. B = Bamble area. E = Eastern segment. K = Kongsberg area, M = Median segment, R = Rogaland area, TA = Telemark-Agder area, w = Western segment. DC = Dal Group, TG = Telemark Group. DB c=Dalsland boundary fault zone, G‘ = GZita-alv fault zone. K)’ = Kristiansand-Porsgrunn fault zone, MS = Meheia-Sokna fault zone. My = Mylonite zone. OK = Oslofjord-Koster fault zone. P = Protogine zone, TE = Trans-European fault, 7Q = Tornquist zone. W = Varberg-Viineren fault zone, RFff = Ringk&ing-Fyn High.

of this model, discussing pre-rift setting, rift architecture and influence of pre-rift structures on the structural evolution of the rift.

2. Terminolugy and structures The regional terminology used in this paper follows that of Dons (1978) with some additions. The Oslo region (Fig. 2, inset) is a non-genetic geographical term describing an area in southern Norway containing Palaeozoic rocks in contrast to the surrounding Precambrian high-grade metamorphic terrain. The Oslo Graben is the down-faulted crustal block confining the rocks of the Oslo region and the adjacent Precambrian Kong&erg block (Fig. 1). The Oslo Graben consists of two graben segments: the southern Vestfold segment, and the northern Akerhus segment (Ramberg and Larsen, 1978). An additional graben structure, the Skagerrak Graben (Fig. 11, is located in the off-shore Skagerrak area (Ro et al., 199Oa). The Oslo Rifr (Fig. 1) is defined as the magmatic and tectonic entity formed by the PermoCarboniferous rifting event in southeastern Norway (and adjoining parts of southwestern Sweden) and in the Skagerrak Sea, covering a minimum area of about 510 km X 120 km (from SHrna in western Sweden to the southern part of the Skagerrak Sea, Fig. 2). The Oslo Rift is terminated to the south by the Tornquist zone, and there seems to exist a structural link between this zone and the rift (Ro et al., 199Ob). The rift-affected Precambrian area partly covered by Caledonian nappes northeast of the Oslo Graben (including the Stirna alkaline complex) is here tentatively called the Hedernark-Trysil region (Fig. 1). The overall rift axis as defined by the fault pattern and geophysical features like gravity and magnetic anomalies, is roughly oriented NNESSW (15-20”E of N). However, the Vestfold graben segment deviates from this direction with an axis about 7”W of N. The Bamble block has an apparent structural trend about NE-SW and the 0stfold block and the Hedemark-Trysil region

about NNW-SSE (Ramberg. 1976; Kamberg et al.. 1977). The Oslo Graben is primarily characterized by its extensive magmatic activity, expressed at the present erosion level mainly by plutonic intrusions. The Skagerrak Graben is covered by postrift sediments and no direct evidence of any magmatic activity has so far been ascertained. However, a belt of rift-related ore-deposits along the southern coast of the Bamble area and a large Permian monzonitic dyke continuing off-shore in the same sector, indicate plutonic intrusion in the southern Skagerrak area (Fig. 1). A magnetic anomaly in this off-shore area similar to the anomaly covering a composite monzonitic pluton in the Vestfold segment, suggests the presence of an analogous pluton in the southwestern part of the Skagerrak Graben (Am, 1973).

3. The pre-rift setting The pre-rift geology of the Oslo region has been reviewed by Ramberg (1976), Ramberg and Spjeldnzs (19781, and Bockelie and Nystuen (1985). The tectonic lineament and fracture pattern have also been addressed by Ramberg et al. (1977). 3.1. Precambrian The Oslo Rift is situated almost entirely within the Sveconorwegian domain of the Fennoscandian (Baltic) shield with crustal ages of < 1800 Ma (Fig. 2). To the east the Sveconozwegian domain is separated from the generally older Svecofennian craton by the Protogine zone (Ga&l and Gorbatschev, 1987). To the north and northwest it is confined by the Caledonian orogen (Fig. 2). Earlier (16OO-1200 Ma) tectonothermal events (GaM and Gorbatschev, 1987; Starmer, 1991) are partly obliterated by the Sve.conoxwegian (Grenvillean) orogeny, occurring between 12OOand 900 Ma, and in the central parts also by postorogenic @OO-850 Ma) granites &rthelsen, 1980; Gaal and Gorbatschev, 1987). The eastern margin of the area affected by Sveconorwegian post-oro-

B. Sundtall,

B. T.

Larserl/ Trcronophysics

Fig. 3. Proto-rift (stage It development. Precambrian fractures and mylonite zones. I= presumed extent of shallow depression with Asker Group sediments, 2 = additionai areas where early sill intrusions in Lower Palaeozoic sediments took place, 3 = post-erogenic (Sveconorwegian) granites, 4 = Precambrian gabbros. A = Asker area, B = Bohus-Iddefjord granite, F = Fib granite, R = Ringerike area. S = Skien area. (Partly after Ramberg and Larsen. lt)78.)

genie granites is made up of the Iddefjord-Bohus granite and the Fl?i granite, forming a NNWSSE-trending belt transecting the Oslo region (Fig. 3). In addition to the Protogine zone, several other major fracture or fault zones originating in Precambrian times are cutting the Sveconorwegian domain and dividing it into distinct tectonic regions (Fig. 2). West of the Oslo region the Kris-

240 11994) I73- IN

171

tiansand-Porsgrunn and Meheia-Sokna fault zones separate the Kongsberg-Bamble area from the Rogaland-Telemark area (Bugge, 1928). East of the Oslo region the Svecono~egian domain is divided into three units: the Western, the Median and the Eastern segments, roughly delimited by two fracture zones: the Dalsland boundary fault and Gota-alv fault-zone: and the Mylonitc zone and V~neren-Varberg fault zone (Park et al.. 1990). In SE Norway the Dalsland boundary fault merges with the Q)yeren-Aremark fault zone, and the Protogine zone is split into the Osen-Rendal and Engerdal fault zones. The dips of these fault zones are generally SE to E west of the Oslo region, and W in those to the east. They have been interpreted by some workers as Sveconorwegian thrust belts (Berthelsen, 1980). The occurrence of mafic dyke swarms and a spatially associated post-intrusive (left-lateral) shear zone along the coast of Bohuslan and in the Oslofjord area (Fig. 1) have been taken to indicate the existence of a major Precambrian tectonic zone under the southeastern part of the Oslo Graben (Berthelsen, 1980; Hageskov, 1985). The fault zones in the east and in the west apparently converge upon the area covered by the Oslo Graben. The Mylonite zone extends on to the western shore of lake Mjosa (Sigmond et al., 1984). Further on to the west to lake Randsfjorden, the exposed Precambrian displays affinity to the Median segment rocks (H~~ltedahl, 1939). The rocks in most of the Precambrian fracture zones are blastomylonites with apparent marks of repeated fragmentation (Swensson, 1990; Buer. 199Ob). The youngest events are characterized by narrow sectors of steep faults with slickensides and associated quartz-carbonate matrix (friction breccias) that in many cases can be determined to be post-Caledonian. They are generally considered to be of Permo-Carboniferous age.

To the north of the Oslo region (Fig. 2), sedimentary rocks (Hedmark Group) of Late Precambrian (Riphean-Vendian) age occur (Nystuen,

1987). 3-4 km of sediments consisting mainly of coarse feldspathic sandstone or arkoses (sparagmites), were deposited in faulted basins or aulacogens during the split-up of a Late Proterozoic supercontinent. Some basaltic volcanism is also associated with the sediments (Nystuen, 1987). Tectonostratigraphic considerations have suggested that the rift basins were not formed in situ, but originally were developed some 150-250 km to the north-northwest of their present position and subsequently thrust into their current location during the Caledonian orogeny as one gigantic nappe complex (Nystuen, 1987).

3.3. Lower Palaeozoic Lower Palaeozoic sediments were deposited on a peneplained shelf area to the southeast of the main Fennoscandian margin of the Iapetus ocean during Cambrian-Silurian time. In the Oslo region a marine sequence of about 6OO-1400 m (Lower Cambrian-Lower Silurian), is overlain by a continental sequence (Upper Silurian) of about 500-1200 m (Henningsmoen, 1978). In the Skagerrak Graben Palaeozoic sediments in excess of 5 km, most of which may be of Early Palaeozoic age, have been inferred from seismic profiles (Ro et al., 1990a). Observations also indicate the previous existence of Lower Palaeozoic sediments (and syn-rift magmatic rocks) in areas adjacent to the Oslo and Skagerrak grabens. Examples are the Sevaldrud explosion pipe located west of lake Randsfjorden (Fig. 1) and the small half-graben on the western shore of lake (dyeren (Ramberg, 1976). Hydrocarbon minerals (coal blend) are found in breccias and faults in the Precambrian areas adjacent to the Oslo region (Dons, 1956). Such hydrocarbons are believed to have originated from Cambrian alum shales which are known to be an oil-prone source, and thus indicate the former presence of Cambrian sediments in areas considerably larger than the Oslo and Skagerrak grabens. Stratigraphic sections have been interpreted to show an increase in thickness of the deposits along the axis of the Oslo region as compared to

the flanks. This depth zonation also roughly corresponds to inferred lithofacies belts of the postCambrian deposits (Ramberg and Spjcldmes. 1978). The inferred basin-like development. with an axis about N-S to NNE-SSW, seems to have started during the Middle Ordovician and was amplified during the Silurian (Rambcrg and Spjeldnzs, 1978). However, the above interpretations of thickness variation and lithofacies belts have been questioned on the basis of the strong Caledonian shortening of the sediments (Bockelie and Nystuen, 1985). Possibly a basinal development was accentuated by Late Precambrian faulting (Ramberg, 1976). Evidences of post-Sveconorwegian, prc-rift magmatism are sparse in the Oslo region and its adjacent areas. However, the Fen carbonatitc, signifying an intraplate magmatic event at about 540 Ma (Andersen and Taylor, 1988), is situated within the rift zone only 12 km west of the Oslo region. 3.4. Devonian to Late Carboniferous Almost no trace of the geological record from this period is preserved in the area considered. At places like Brumunddal, Jeloya and Langesund, the Lower Palaeozoic sediments show distinct signs of erosion and weathering prior to rift depositions, often expressed as a brecciation and a deep red coloring of the top-most layers of the Lower Palaeozoic beds (Henningsmoen, 1978). Both the youngest sediments of the Lower Palaeozoic sequence and the proto-rift beds immediately above are indicative of non-marine conditions. This suggests that the Oslo region was part of a land mass (Old Red Continent?) that was exposed to erosion during most of the time represented by the hiatus between the Lower and Upper Palaeozoic sediments. Presumably the area was again levelled into a sub-‘Permian’ (actually sub-Moscovian) peneplain (Henningsmoen, 1978).

4. The proto-riit stage The earliest phase of the Oslo Rift is marked by the deposition of a thin cover of sediments,

the Asker Group, upon the rather even surface cutting through the various strata of the underlying, folded Lower Palaeozoic sequence. In the central Oslo-Asker area the Asker Group has been subdivided into three formations: the Kol&s (lower), the Tanum (middle) and the Skaugum (upper) formations (Dons and Gyory, 1967). The sediments of the Asker Group occur throughout most of the central and southern part of the Oslo region, but are missing in the areas north of Nordmarka (Fig. 3). The thickness of the Asker Group varies from < 90 m in the southwest (Skien area) to about 15-30 m in the northernmost (Ringerike) area. However, stratigraphic correlations between the central area and the southern districts are not unequivocal (Olausson et al., 1994). Lithologically the Kolsis Formation has been interpreted to represent fluvial sediments deposited in an arid climate and a flood plain environment (Olaussen et al., 1994). Anhydrite has been identified locally, but no volcanic material (Henningsmoen, 1978). The sedimcntology of the Tanum Formation suggests deposition in a marine deltaic to continental environment (Olaussen et al., 1994). In the Asker area a 5-m-thick layer of fossiliferous shales occurs in the uppermost part of the formation displaying an ample flora and freshwater fauna of Late Carboniferous to Early Permian age (Ramberg and SpjeldnEs, 1978). A thin (OS-2 m), and in many places partly eroded, bed of limestone can be traced from the Asker-Oslo area southward to the Skien area below a distinct horizon of quartz-conglomerate in the upper part of the suggested Tanum Formation equivalents. In the Oslo area Foraminifera of Moscovian (Westphalian C/D) age have been documented, indicating that portions of this limestone represent a marine incursion (Olaussen, 1981). The notable change in lithology of the Tanum Formation from the underlying Kolsis Formation and the occurrence of volcanic fragments in the sediments have been interpreted as reflecting topographic modifications caused by fault movements (Dons and Gyory, 1967). Faults intersecting the Asker Group, older than the overlying stage 2 lavas, are documented in the Ringerike area (Larsen, 1978).

The Skaugum Formation, primarily developed in Asker and the adjacent district to the west, is composed predominantly of volcanic material (agglomerates and tuffs) reflecting strong volcanic activity (Henningsmoen, 1978). The volcanic vents were probably located to the southwest of the Oslo area (Drammen). The sediments of the Skaugum Formation suggest fluvial deposition in an environment changing between flood plain and braided stream (Henningsmoen, 1978; Olausson et al., 1994). In the Skien area a 2-4-m-thick sequence of pyroclastic sediments at the top of the Asker Group resembles the Skaugum Formation (Olausen, 198 1‘I. A complex of basaltic to ~mainly~ syenitic sills and dykes occurs in most of the Oslo Graben area (Langesund-southwestern shore of lake Mjosa), intruding the lower part of the pre-rift Cambrian-Silurian sediments (Fig. 3). The rocks have been dated to about 300 h 5 Ma ~Sundvoll et al., 1992). Both field evidence and radiometric age data establish this complex as the earliest intrusive magmatic activity in the Oslo Graben. On the basis of available time-scale calibrations. geodynamic considerations and stress-field implications, the complex has been associated with the proto-rift stage, probably coeval with the deposition of the upper part of the Asker Group. The parental magma of this complex was presumably of alkali-basaltic composition, enriched in H,O and CO,, and isotopically similar to that of the main stock of Oslo Rift lavas (Sundvoll et al.. 1992). 5. The initial rifting stage The initial rifting stage (Fig. 4) is defined as the period covered by the first volcanic sequence of basaltic lavas succeeding the Asker Group sediments and underlying the rhombporphyry (RP) plateau lavas of the subsequent (main rifting) stage 3 (Ramberg and Larsen, 19710. This basaltic sequence is named the B, unit in the Permo-Carboniferous stratigraphy of the Oslo Rift. The area1 extent of the B, unit is roughly congruent with that of the proto-rift sediments. Radiometric age determinations of the RP lavas of the succeeding stage indicate that the B, unit

B. Sundl~oll, B. T Larsen / Tectonophysics 240 (1994) 173-180

1x0

‘.\

I

. . .

.

1’

I

\

Fig. 4. Early rift (stage 2) development. Dotted areas = presumed extent of B, basal& dotted lines = dolerite and ultramafic dykes, continuous lines = active faults, stars = presumed volcanic centres, D = Drammen, E = Eikeren, H = Horten-Holmestrand, J = Jeleya, K = Krokskogen. N = Nevlunghavn, S = Skien, SK = Skrim.

is older than about 294 f 5 Ma. The B, Kolds basalt at Krokskogen (see below) yielded a Rb-Sr age of 291 f 8 Ma (SundvoIl and Larsen, 1990). The thickness, volcanological pattern and the petrology of the B, unit deviate considerably from area to area. In the Skien area (Fig. 4) it is made up of a > 1500 m sequence of more than 150 individual, thin (l-5 m) flows (Segalstad, 1979). The component of tuffs and ag&omeratic beds increases to the south; they are especially abun-

dant in outcrops of the B, unit at Nevlunghavn and adjacent skerries to the south. However, this latter area may constitute a larger and separate volcanic province as indicated by magnetic and seismic data from the off-shore area north of the Skagerrak Graben (Floden, 1973; Solheim and Gronlie, 1983). The Skien basalts are in general distinctly silica-undersaturated and alkaline (Segalstad, 1979). Their isotope chemistry suggests that they were drawn from a different mantle source than the B, basalts elsewhere in the Oslo Graben (Neumann et al., 1992). In the northeastern part of the Skrim area (Fig. 4) a sequence of about 300-400 m of B, basaltic lavas occur (Rohr-Torp, 1973). These lavas are possibly associated with the Skien basalts. The occurrence of basaltic boulders in the syn-rift deposits along the eastern margin of the outer Oslofjord area suggests that B, basalts are also present in the southeastern part of the Vestfold segment (Olaussen et al., 1994). In the northeastern margin of the Vestfold lava plateau the B, unit can be traced from the Jeloya-Horten-Holmestrand area in central Oslofjord, to the southwest shore of lake Eikern (Fig. 4). The integral thickness and number of flows decrease from east to west: about 800-1500 m (> 60 flows) at Jeloya, 200 m ( < 40 flows) at Horten and 150 m (g 20 flows) at Holmestrand. The basalt chemistry indicates two to three different series probably originating from separate eruption centres and possibly from two distinct secondary sources (Schou-Jensen and Neumann, 1988; Neumann et al., 1990): (1) a group of mafic, silica-undersaturated, TiO,-rich basalts occupying the lower part of the stratigraphy; and (2) a group of evolved, potassic trachybasalts in the middle and upper part. At Jelaya also a third group of silica-undersaturated basalts occurs, accompanied by numerous pyroclastic flows. This last unit most likely erupted from a volcanic centre close by. Debris flows containing portions of Upper Silurian and Asker Group sediments are interpreted as syn-volcanic land-slides in response to normal faulting along the adjacent Oslo-fiord fault zone (Schou-Jensen and Neumann, 1988). In the Horten-Holmestrand area two to three conglomerate horizons with well rounded boulders of

5. Sundr~oll, B. T. Lurwn / Tectonophysics 240 0944)

basaltic material, indicate episodic eruption activity in a lacustrine environment (Bragger, 1933). Southwest of lake Eikern, the B, unit is observed to be about 150-200 m thick. Northeast of Holmestrand numerous sills of TiO,-rich magma, conceivably belonging to the first basalt sequence, intrude the Upper Silurian sediments (K&r, 1908; Bragger. 1933). Within three of the calderas in the area north and south of Drammen the B, unit is also partly preserved, but a detailed stratigraphy is not available (Oftedahl, 1953). In the Krokskogen lava area (Fig. 4) the B, unit is made up of a single, relatively thick (10-30 m) flow of tholeiitic composition (Weigand, 1975). This flow (the Koisis basalt) is missing in the westernmost part of the Krokskogen area. This is conceivably caused by a N-S-trending, pre-B, fault escarpment as the basalt is relatively thick in areas close to where it disappears. Eastward it probably originally covered a major part of the Nordmarka area as it is documented from within the Nittedal caldera northeast of Oslo. The occurrence of volcanic material of alkali basaltic composition in the proto-rift Skaugum Formation implies that volcanism started earlier in areas to the south and southwest of Oslo (Jelsya-Holmestrand-Drammen), and that this early volcanism was of a mildly alkaline character (Weigand. 1975). This suggests that the B, unit in those latter areas is, at least partially, older than the Kolsis basalt.

6. Rift architecture Ramberg (1976) and Ramberg and Larsen (197X) have emphasized the (right-lateral) enechelon pattern of the graben segments in the Oslo Graben. However, the grouping of graben segments along the entire rift describes a slender S- or sigmoidal shape, and all segments are generally ~~symmetric and composed of half-grabens. To describe the architecture it may thus be awarding to apply the linked half-graben concept developed in the East African rift system (Rosendahl, 1987).

173-f NO

IX1

Fig. 5 shows an attempt to adapt this idea to the Oslo Rift structural pattern. A number of interesting features appear. The successive (south to north) half-grabens have different polarity progression in the Skagerrak and Oslo grabens. In the Skagerrak Graben it is right-left (Roscndahl family la type), in both the Vestfold and Akershus segments it is left-right (Rosendahl family lb type). The geometry is complicated by the fact that several of the half-grabens in the central and southern part of the rift interact with similar polarity half-grabens or blocks (Roscndahl family 3i type). Examples are: Bamhlc block and the western half-graben of the Skagerrak Grabcn and the western half-grabens of the Akcrshus and the Vestfold segments. The Bstfold block has been suggested to constitute a horst block by Ramberg and Larsen (19781, but evidence for this is inconclusive. The Bohuslan area (Fig. 1) is also affected by Permo-~arb~~nifcrous rn~l&rn~~tisrn and faulting, and the existence of a m~~nz~)niticpluton offshore in the southwestern part of this area is suggested by the pattern of the RP dykc intrusions (Sundvoll, in prep.). Structural elements similar to those identified as acc~)mmodation or transfer zones in the East African rifts are recognized in the B;erumOyangen area and the Skien area. They can also be inferred in the Langesund-N Skagerrak area. The Oslo Rift accommodation zones are all characterized by the presence of early basaltic volcanism, partly of atypical compositi(~ns c~~mpared to the main stock of Oslo Rift basalts (alkali-olivinc basalts). The Baerum-0yangen and the Skicn areas are also distinguished by later, polygcnctic central volcanoes that subsequently collapsed into calderas. The triangular joint between the Skagerrak Graben. the Tornquist zone and the southwestern part of the Bamble block (Fig. 1) may possibly also be recognized as an accommodation zone; Geophysical investigations (Halvorsen, 1970; Am, 1973) suggest that basaltic magmatism belonging to the earliest phase of the rift evolution, also occur in this area. The central uplifted area between the two overlapping, opposing half-grabens in the Skagerrak Graben (Fig. Sb) has been suggested to constitute an accommodation zone (Ro et al., 1990).

R. Sundrxdl. A. T Larsen ,/ Tectonophysics 240 11994) 173-189

Fig. S. (a) Oslo Rift, architecture: AGS = Akershus graben segment, BB = Bamble block, HTR = Hedemark-Ttysil region, OG = Oslo Graben, SC; = Skagerrak Graben, t’GS = Vestfold graben segment, 0Lt = Bstfold block, B = Brumunddal, B0 = Baerum0yangen area, K = Kristiansand, L = Langesund. NO = Nordmarka area, S = Skien. TQ = Tornquist zone. Numbered lines concur to cross-sections in (bf. (b) Oslo Rift, schematic cross-sections. Cross-section numbers correspond to those in (a). F-site of initial transcurrent fault zone. (Based on Ramberg. 1976, and Ro et al.. 199Oa.)

B. Sundwil, B. T. Lumn / Tecromphysics 240 (1994) 17.3-189

Relation

0 1

100 J

50 km

Fig. 5 continued.

and In the revised nomenclature of Rogers Rosendahl (1989) it may be termed an axial flexure. In the Vestfold segment the NNE-SSWtrending belt of intrusions from the Skrim area to lake Tyrifjord denotes an area with very high topographic relief and indications of uplift of pre-rift rocks (Bragger, 1898). This area may also be associated with an axial flexure. An analogous interpretation may be given to the NE-trending belt of intrusions in the northern part of the Akerhus segment, where high topographic relief and evidence of uplift are also conspicuous. The Nordmarka area, however, may emerge as an accommodation zone between the Akershus segment and the Bstfold block, possibly reflected by the younger (Nittedal) caldera east of Oslo, and the N-S-trending chain of gabbroic necks in the area (Hadeland) to the north (Sundvoll et al., 1990).

IX3

to pre-rift fabric

Some influence of the pre-rift geology on the Oslo Rift evolution has been asserted to in most papers addressing this aspect (Ramberg, 1976; Ramberg et al., 1977; Ramberg and Larsen, 197X; Ramberg and Spjeldmes, 1978; Schanwandt and Petersen, 1983; Neumann et al., 1992). However, the substance of these assessments does not go beyond the statement that some of the major rift faults trail older fracture zones developed in the Precambrian, and that the grabens are grossly superimposed on the supposed Lower Palaeozoic sedimentary basin. Studies of dykes and sills in and outside the graben areas indicate strong influence of pre-rift structures on the strike and offset of these intrusions (Sundvoll and Larsen, 1993; Sundvoll, in prep.). lnvcstigations of the major fault zones in the Oslo Graben (Fig. 1) indicate that these all trace Precambrian. probably Late Proterozoic fault or breccia zones (Swensson, 1990; Buer. 199Ob). Analogously, the prominent tectonic zones in the Hedemark-Trysil region (Engerdal and Oscn-Rendal fault zones, Figs. 1 and Sb) denote belts of repeated crustal movements both in Proterozoic and Palaeozoic times (Nystuen, 1987). However, no general parallelism exists between the Precambrian mctamorphic fabric such as foliation, fold axial traces. etc., and the rift-related structures (Ramberg. 1976). The conspicuous directions of the normal faults and central caldera zone of the Vestfold segment (7”W of N) appear superimposed when compared to the predominant old orthogonal directions ol the adjacent Precambrian areas (Ramherg ct al.. 1977). However, the Kongsbcrg block (Fig. 1) shows a marked N-S fracture pattern as do other Precambrian “windows” inside the graben (examples are the area between Horten and Oslo and the area between lake Randsfjorden and lake Mjosa). As suggested by Starmer ( 19X5), the Kongsberg and Bamble blocks may be continuous through an arcuate sector now covered by the Permian magmatic rocks in the southwestern part of the Vestfold segment. Thus the N-S (and NNE-SSW) structures in this segment may grossly be inherited from the Precambrian terrain.

As outlined above the area between the Kongsberg-Bamble belt and the Western and Median segments of the Precambrian area east of the Oslo Graben (Fig. 2), can be interpreted as a NNW-SSE to N-S zone of pre-rift structural junction. Although earlier concepts of a suture zone along this junction have been negated (Gail and Gorbatschev, 19871, the fact remains that structural geometry (converging thrust or detachment faults) suggests some Proterozoic crustal shortening in this area and a corresponding crustal thickening. Turner et al. (1992) have suggested that posterogenic granites may reflect lithospheric thinning in a declining compressional regime due to an elevation of the asthenospheric-lithospheric boundary after erogenic thickening of the lithosphere. Applying this concept to the late Sveconorwegian granites in southern Norway, together with the proposed crustal shortening, would suggest that the junctional zone suffered pre-rift crustal thickening and lithospheric thinning, which could indicate that this zone developed into a belt of significant lithospheric weakening (Kusznir and Park, 1987). The spatial association of the large syn-rift. A-type granites north and south of Drammen with this zone, is also considered to reflect lithospheric attenuation (Tronnes and Brandon, 1992). Late Precambrian faulting, the Fen carbonatite magmatism and the possible basinal development of the Oslo region during Ordovician and Silurian times may also indicate this area as a zone of crustai/ lithospheric weakness.

7. Discussion 7.1. Tensional stress and initial fault pattern

The tensional palaeostress in the Oslo Rift has generally been taken as E-W (Ramberg and Larsen, 1978; Ro et al., 1990b; Ro and Faleide. 1992). Alternatively, Buer (199Oa) has suggested a stress field that changed from an initial WSWENE to a WNW-ESE direction of the tensional axis. The apparent symmetry of the Vestfold segment suggests that the general tensional stress

field was about normal to the symmetry axis ot this segment as do the interpretation of large. mainly N-S-trending dykes of RP magma as primary palaeostress indicators of the main rifting stage. However, Rb-Sr ages obtained on the KP dykes (275-270 Ma; Sundvoll and Larsen, 1993) show that they post-date the main rifting stage (295-275 Ma; Neumann et al., 1992). The deviating axis (NNE-SSW) of the Skagerrak Graben has been suggested to retlect left-lateral NNESSW shear (Ro and Faleide, 1992). However, left-lateral movements arc not evident in rift areas outside the Vestfold segment and the Bstfold block. Accordingly, we propose an alternative model depending more on the pre-rift setting and make the syn-rift mantle lithosphere/ lower crust interaction and rift propagation reflect the stress-field direction. The main axes of the post-rift magnetic and gravimetric anomalies, and the crustal thickness and Moho depth patterns (Ro et al., 1990b), suggest that the general tensional stress-field was WNW-ESE, complying to the NNE-SSW t152O”E of N) trend of the rift axis, and semi-parallel to the Tornquist zone. The deviant axis of the Vestfold segment can be explained as dictated by the pre-rift. NNW-SSE to N-S-trending structures. Tectonic response to such a small angular deviation ( < 25”) between the regional stress and the inherited structures, could enforce transtensional NNW-SSE (right-lateral) and NE-SW (left-lateral) components added to the normal faults (Ramberg and Spjeldnaes. 1978; Rosendahl, 1987). Indeed, evidence of transtensional movements, including horizontal or oblique slickenside striations, are observed in several faults in the Vestfold segment and the Oslo area (Stormer, 1935; Ramberg, 1976; Ramberg and Spjeldmes, 1978; Gravcrsen. 1984). Although no detailed analysis of the fault systems has been undertaken, and the timing of the faulting is in most cases unconstrained, it is notable that the right-lateral directions are mainly associated with the major faults like the Oslofjord fault zone (Stormer, 1935; Ramberg, 19761, where as minor faults mostly show left-lateral displacements (Ramberg, 1976). NNE-facing reverse faults are also observed locally (Kiter, 1908; Ramberg and Spjeldrues, 1978).

Satellite image analysis suggests that the earliest lineament trend in the rift zone had a NNWSSE direction and that this lineament pre-dates the formation of the graben structure (Buer, 1990a). The deeper troughs of the lakes Tyrifjord and Eilkeren, the Drammensfjord and the outer Oslofjord all exhibit nearly parallel, NW-SE (4O”W of N)-trending axes (Fig. if. These troughs arc obviously the result of Quaternary glacial erosion, but the disparity between trough axes and glacier movements indicate a pre-existing right-stepping fault system. In the outer Oslofjord area these faults are associated with N-S- and NE-SW-trending. left-stepping faults (Stormer, 1935). Combined these fault systems conspicuousiy resemble a NNW-SSE-trending, rightlateral simple shear fault pattern (Christie-Blick and Riddle, 19X5) with acquired R (N-S), P (4o”W of N). and R’ (NE-SW) shears (Fig. 4). The maximum offset of the strike-slip faults would be about l- 1.5 km based on available maps (Solheim and Grsnlie, 1983; Graversen, 1984). As this fault system is partly obliterated by the Drammen and Finnemarka granites and the Drammen caidera, it must be older than these structures, that is > 270 Ma (Sundvoll and Larsen, 1990). Wether it is also older than the plateau lavas can not he fully ascertained as many of the faults have been reactivated repeatedly during the main rifting event. However, the prominent N-S block faults of the Vestfold segment that developed in the later part of the main rifting stage, appear superimposed on the NW-SE and NE-SW fault systems (Buer. 199Oa). Thus the latter systems are better c(~mprehended if considered older ( > 395 Ma) tlhan the main rifting stage and constituting an early rift transtensional zone. A swarm of large tholeiitic dykes dated at 297 + and Larsen, 19931, with - 9 Ma (Sundvoll strikes (15”W of N to N-S), is situated in the Precambrian terrain north of the Vestfold segment along strike of the proposed transtensional fault zone (Fig. 4). In the Bohuslan area lamprophyric and ultramafic dykes related to the Oslo Rift magmatism are considered to be associated with a NNW-SSE-trending fissure system with a noticeable strike-slip component and which is

older than the tensional fissure with later RP dykes (Ljungner,

system associated 1927).

7.2. Initial rift der~elopment The sedimentological and structural characteristics of the proto-rift stage as outlined above, make any pre- to early rift uplift or doming inconceivable. Also the intrusion of a regional sill complex during the later part the proto-rift stage, indicating at least local compressional stresses inside the sedimentary depression. confute an early doming concept (Sundv~~ll et al., 1992). An appraisal of the pre- to proto-rift observa tions makes it likely that the Oslo Rift started as a crustal sag, creating a shallow basinal depression, probably with a NNE-SSW trend. The first sequence of proto-rift sediments (Kols% Formation) does not indicate surface volcanism, However, sub-surface magmatism in the crust or mantle lithosphere cannot be excluded. Due to the lack of a fossil record an exact timing is impossible, but the sedimentary record strongly suggest a time around or shortly before the Moscovian stage (Westphalian C/D) in the Upper Carboniferous (Olaussen, 1981). According to a recent time-scale calibration (Cowie et al., 1989) this would indicate a date between 300 and 315 Ma. possibly between 305 and 310 Ma. At a date that can be fixed to about 305-300 Ma, widespread sub-surface magmatic activity (sill intrusions) started within the c~)ns(~lid~~ted pre-rift sediment sequence. This magmatism was alkaline in character, possibly episodic. and probably lasted no longer than S-IO m.y. (Sundvoll et al.. 1992). The near-surface magmatic activity certainly indicates that transfer of magmas between mantle and crust had started. However, as a large portion of the intrusions at this stage were of ;I syenitic c{)mposition, this could indicate some intra-crustal residence time. The magmatic style (predominantly sill intrusions) denotes a non-tensional stress regime within the pre-rift sediment sequence t < 2 km of depth). which signifies a transient episode of the general stress field evolution, or a differential stress field with lithospheric depth, or both. Earlier field evidence of coinciding sub-aerial

volcanic activity from about the Moscovian stage is strengthened by the occurrence of beds of clay-like material, interpreted as bentonites, in the uppermost part of the Tanum Formation. The approximate temporal and spatial correlation of extrusive magmatic activity and faulting with the sill intrusions indicates that upper crustal magmatism and tectonism started at about the same time, about 300-310 Ma. In the Oslo Graben area signs of early faulting (Jeloya: Schou-Jensen and Neumann, 1987; Ringerike: Larsen, 1978) are apparently limited to a NNW-SSE- to N-S-trending zone comprising the eastern margin of the incipient Vestfold segment (Oslofjord-Ringerike). This zone (corrcsponding to the axial zone of Schgnwandt and Petersen, 1983) is also the site of the majority of observations of syn-rift, transtensional fault movements and a belt of pre-rift crustal attenuation. The initial faulting that resulted from the first response of the brittle upper crust with its inherited N-S structures, to the WNW-ESE tensional stress field, may be explained in two ways: (1) creating right-lateral shears in the NW-SE to N-S direction and left-lateral shear in the NESW direction from an overall pure shear tectonic response as suggested by Ramberg and Spjeldnaes ( 1978); (2) by a NNW-SSE-trending right-lateral simple shear wrench zone along the OslofjordRingerike belt of pre-rift attenuated crust/ lithosphere creating stepped fault patterns (Fig. 4). The second explanation is preferred because: (a) pure shear creates volume problems because of converging elements (Sylvester, 1988); (b) a major portion of the earliest volcanic activity (Jeloya-Holmestrand, Drammen) is located along this “Oslofjord-Ringerike shear zone”, as can be inferred from the spatial distribution and volcanological assessment of the Skaugum Formation and the B, lavas of the initial rifting stage (Fig. 4). As some of the faults in this zone trace older fractures with apparently opposite strikeslip kinematic signature (Hageskov, 1985; Starmer, t985), only small displacement may have been necessary to re-open fractures to considerable depth due to the “harpoon-effect” envisaged by Black et al. (1985), and facilitate magma ascent.

The deviant spatial position of the NW-SE fault along the lake Eikeren compared to the main fault zone, may signify influence from the Porsgrunn-Kristiansand fault zone. ‘This fault zone probably had a continuation indicated by the NNE-SSW-trending (Ligen) river valley south of lake Eikeren. The left-stepped offset of the lake Eikeren fault compared to the main fault zones, suggests a restraining overstep or fault junction in the area between them (Christie-Blick and Biddle, 1985). The occurrence of sill intrusions of the earliest basaltic sequence and NNEfacing reverse faults support this suggestion. Initial-stage basaltic volcanism is also associated with areas that later developed into accommodation zones like the Bzrum-Oyangen arca, the Skien and the Nevlunghavn-N Skagerrak area (Fig. 4). Most probably the accommodation zones were launched as transfer faults during the initial rifting stage. In the Skien area such transfer faults are inferred between the PorsgrunnKristiansand and the Meheia-Sokna fault zones. Here strongly alkaline magmas of a different mantle source than other B, basalts were erupted, probably reflecting the flanking position of this arca with respect to the rift axis. On the western margin of the Nevlunghavn-N Skagerrak a supposed NNW-SSE-trending fault may have acted as a transfer fault between the Vestfold segment and the Skagerrak Graben. Also in this zone basaltic volcanism took place. The early development of the Barum-0yangen accommodation zone is manifested by NNW-SSE and N-S faults (Isidalen and Langlia fault zones) and the eruption of the voluminous Kolsis basalt. The tholeiitic character of this basalt may have been caused by a deep shear combined with a sudden pressure release (Sundvoll et al., in prep). The above concept of the initial rifting stage would be characterised by a transcurrent stress field with s, trending along the rift axis (NNESSW) and the sg axis WNW-ESE. Compared to earlier concepts, this model more directly takes into account the evident observations of stike-slip components of the major fault systems (horizontal/oblique slickenside striation, reverse faults) and the influence of the pre-rift structures on the early development of the Oslo Rift. The model

B. Sundtdl,

B. T Lmsen / Tectmophysics

can possibly be tested by palaeostress analysis and detailed reflection seismic profiles in the outer Oslofjord as it would imply the existence of flower structures.

Bockelie,

J.F. and Nystuen,

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The rapid increase of extensional stress during the initial rifting stage and the first part of the main rift stage (stage 3) probably extended the rift zone laterally in both axial and transversal directions and initiated the development of the half-graben structures that acquired their final pattern at the end of this stage (Ramb~rg and Larsen, 1978; Neumann et ai., 1992). During the main rifting stage the stress field was tensional, as indicated by the master-fault system and block faulting. In the accommodation zones and axially in the Vestfold segment, polygenetic central volcanoes developed that subsequently collapsed into caldera structures (stage 4).

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