Latest Caledonian to Present tectonomorphological development of southern Norway

Marine and Petroleum Geology 27 (2010) 709–723

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Latest Caledonian to Present tectonomorphological development of southern Norway Roy H. Gabrielsen a, d, *, Jan Inge Faleide a, Christophe Pascal b, Alvar Braathen c, d, Johan Petter Nystuen a, Bernd Etzelmuller a, Sejal O’Donnell a a

Department of Geoscience, University of Oslo, Norway The Geological Survey of Norway (NGU), Trondheim, Norway The University Centre in Svalbard (UNIS), Longyearbyen, Norway d Center of Integrated Petroleum Research, University of Bergen, Norway b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 December 2008 Received in revised form 15 May 2009 Accepted 3 June 2009 Available online 24 June 2009

The regional resultant stress field of the northeastern North Atlantic has shifted significantly throughout the Phanerozoic. In Fennoscandian parts of the Caledonian orogen, mountain building, which was characterized by NW-SE contraction (reference to present North), was followed by a collapse with transport both parallel and transverse to the mountain chain. The Late Palaeozoic – Mesozoic saw several stages of E-W to NW-SE extension, varying in time and position. Local episodes of inversion are traceable in some cases, particularly in connection with deep-seated and long-lived zones of weakness. The Cenozoic has to a larger degree been affected by compression, including folding and basin inversion. Again some of the more pronounced effects of local inversion are related to pre-existing fault systems. Neogene uplift of the western mountainous area in Scandinavia can be unravelled by potential field study, AFT data and reflection seismic sections. Assuming that the region is close to isostatic equilibrium, the uplifted areas must be supported at depth by substantial volumes of low-density material within the crust or the mantle, close to the crust/ mantle interface or close to the lithosphere/asthenosphere interfaces. A series of NW-SE-oriented cross-sections, representing successive stages of development from the late Caledonian to Present were prepared and used to analyse the tectonomorphological development as related to the shifting stress configurations. Three stages are inferred:

Keywords: Post-Caledonian uplift Tectono-morphology North Atlantı´c margin South Norway

Stage 1; Devonian – late Permian denudation of the Caledonian mountain chain: During the latest stage of closing of the Iapetus Ocean in early to mid Devonian times, the Caledonian mountain chain reached its maximum elevation of perhaps as much as 8–9 km. Including the foreland basins to the east, the average topographic gradient of the eastern flank may have been in the order of 1,3 . The denudation of the Caledonian mountains continued into the Permian and by the end of the Permian, the mountain system was more or less obliterated with a gradient of the eastern flank probably continued to fall to less than 0,1. Stage 2; Triassic – late Cretaceous; Tectono-thermal uplift of central south Norway. Uplift of central south Norway was rejuvenated in Triassic times and continued throughout the Jurassic and early Cretaceous. The gradient of the eastern flank increased to a maximum of approximately 0,15 and master structural elements like the Lærdal-Gjende-Olestøl Fault Complex again played important roles in the development. Stage 3; Tertiary – Present; Renewed tectono-thermal activity and post-glacial rebound. A second phase of post-Caledonian tectono-thermal uplift of south central Norway started in Oligocene time, again increasing the gradient of the eastern flank of south Norway (around 0,2 ). Ó 2009 Elsevier Ltd. All rights reserved.

* Corresponding author at: Department of Geoscience, University of Oslo, Sem Selands vei 1, P.O. Box 1047, 0316 Oslo, Norway. E-mail address: [email protected] (R.H. Gabrielsen). 0264-8172/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2009.06.004

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1. Introduction Recent studies demonstrate that secondary lithospheric response to large-scale tectonic events like rifting and plate collision is more long-lived than previously acknowledged (Van Wees and Beekman, 2000; Ziegler et al., 2002) and that vertical movements in areas that have undergone repeated deformation are particularly complex (Odinsen et al., 2000b; Gabrielsen et al., 2005). However, even areas with a seemingly simple deformation history, like single-phase extension, are surprisingly complex. For example, polyphase compression commonly characterizes the post-rift development of rifts and passive margins (Cloetingh et al., 2008), which in turn may be interrupted by periods of accelerated post-rift subsidence (Cloetingh et al., 1990; Cloetingh and Kooi, 1992; Sales, 1992; Gabrielsen et al., 2001). Such deformation adds to and overprints the effects of syn-rift processes, like rift flank uplift. It is a general characteristic of passive continental margins that the post-breakup uplift is asymmetric, so that it is steeper on the ocean-facing side (Holtedahl, 1953; Peulvast, 1975; Ollier, 1982, 1995; Summerfield, 1985; Boulton et al., 1991; van der Beek et al., 1995; Japsen and Chalmers, 2000; Lidmar-Bergstro¨m et al., 2000). Therefore, in cases where extension is superimposed on older structures, morphology and denudation history of uplifted areas on continental margins commonly are complex and can rarely be explained by one mechanism alone. Furthermore, the different episodes of subsidence and uplift are likely to be characterized by contrasting wavelengths and amplitudes (Gabrielsen et al., 2005; Gallagher et al., 2008; Cloetingh et al., 2008). For the Norwegian eastern North Atlantic (Fig. 1a), the effects of regional uplift of the margins were acknowledged already in the early 20th century (Reusch, 1901a; Nansen, 1922), but has recently received renewed attention, partly because the effects of the uplift influenced hydrocarbon prospectivity (e.g. Nyland et al., 1992; Skagen, 1992; Dore´ and Jensen, 1996; Riis, 1996; Gabrielsen and Kløvjan, 1997; Chalmers and Cloetingh, 2000; Japsen and Chalmers, 2000; Cloetingh et al., 2005). It is also realized that the uplift and its shifting character through time has influenced not only the development of the present relief and morphology, but also the provenance of sediments and sedimentation pattern in the offshore basins (Faleide et al., 2002). A similar interest is recognized for the study of uplift and denudation of east and west Greenland and its continental shelf (Dam et al., 1998; Chalmers, 2000; Mathiesen et al., 2000; Japsen et al., 2006). One key requirement for the quantification of uplift, denudation, sediment transport and deposition along continental passive margins is the firm establishment of the chronology and rates of landscape changes in terms of the geological time scale (Cloetingh et al., 2005). Hence, for the analysis of the processes that contribute to the uplift of passive margins, it is a necessity that all major events are dated and that they can be characterized in the context of maximum uplift (amplitude), rate of uplift, horizontal extent (wavelength) and rock volumes eroded and redeposited. The identification of areas that are under influence of more than one (thermo-tectonic) process is important. Finally, lithospheric heterogeneities, contrasting mechanical properties and zones of weakness need to be taken into consideration (Cloetingh and Burov, 1996; Pascal and Gabrielsen, 2001; Cloetingh et al., 2003). 2. The post-Caledonian uplift history of southern Norway The continental shelf and the mainland of Norway is indeed a classical area for the study of the rise of the land as a result of the glacial rebound (Esmark, 1826; Reusch, 1901a; Nansen, 1922; Torske, 1972). However, studies of the last twenty years have provided new basic insight and have demonstrated that the area

has repeatedly been affected by vertical displacement in response to events of plate tectonic scale. Examples are the uplift and denudation following the Caledonian Orogeny, thermo-tectonic and isostatic responses to late Palaeozoic, Mesozoic and Cenozoic extension and basin formation, and the effects of glacial rebound (see Gabrielsen et al., 2005 for summary). It is clear that the separation of and interplay between the effects of these large-scale events are not trivial, and even more so because the events sometimes overlap in time. Only recently, after Apatite Fission Track (AFT) data have become available, has it become possible to start the work to distinguish between the different events as seen in the framework of geological time (Hendriks and Andriessen, 2002; Hendriks and Redfield, 2005). In the following, we make an attempt to summarize the status of knowledge regarding the topographic development and exhumation of the mainland of South Norway and its continental shelf in a WNW-ESE-oriented profile extending from north of Bergen in the west to north of Oslo in the east (Fig. 1b and c). During this compilation, it was demonstrated that to a great extent, highresolution data are still lacking for several periods and areas, an observation that inspired the Norwegian part in preparation for the TopoEurope initiative (Cloetingh et al., 2007). It is evident that an assessment of an elevation profile that extends for more than 500 km and that covers a time span of more than 300 Ma is non-trivial inasmuch as the uncertainties affiliated with the data for the time periods included in the study increases with age. Also the type of data available for the different time spans varies. For example are the reconstructions from the late Caledonian development derived from combining depth data on metamorphic grades with (foreland) basin reconstructions and the spatial distribution of sedimentary facies, estimated water depth and sediment transport direction. In contrast, estimate of the Mesozoic relief to a greater degree rests on a combination of restored offshore gradients derived from reflection seismic data and exhumation gradients based on Apatite Fission Track (AFT) study. On this background it would be meaningless to compare tectono-topographic profiles for each of the time spans included in the study. Therefore, at the present stage, we have concentrated on calculation of the regional topographic gradient in each case, using the point of constructed or inferred maximum elevation in the western part of the profile, and the closest point of zero elevation to the east. 2.1. The latest Caledonian During the final closure of the Iapetus Ocean, the Caledonian Orogeny in Scandinavia was dominated by SE-directed obduction and thrusting over a westerly to northwesterly dipping subduction zone (e.g. Hossack and Cooper, 1986; Dunning and Pedersen, 1988; Pedersen et al., 1988). The closure was complete by 410–405 Ma (early Devonian) and by 400 Ma, extensional collapse, first utilizing the basal thrust system, was evident, spreading to a steeper fault system which became active between 405 and 359 Ma (Fossen et al., 2008). In this process, the over-thickened (>60 km) and unstable crust soon became thinned by an amount of 15–20 km by thermal crustal delamination. This was accompanied by very rapid uplift and exhumation (Krogh, 1980; Andersen and Jamtveit, 1990; Austrheim, 1991; Andersen, 1998; Engvik and Andersen, 2002). The exact present position of the suture in the Scandinavian Caledonides, that is likely to have coincided with the greatest crustal thickness and hence, greatest surface relief, has been long disputed (Dewey, 1969; Smithson and Ramberg, 1970; Mason, 1974; Mykkeltveit et al., 1980; Fichler and Hospers, 1990; Klemperer and Hobbs, 1991), but is by most workers regarded to be positioned inside the present continental shelf and to the west of the Viking

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Fig. 1. a) Areas of pronounced uplift (red rings) and subsidence (blue rings) in the North Atlantic. Based on Japsen and Chalmers (2000). b) Location maps for key profile and seismic sections displayed in Figs. 2 and 4. c) Main tectonic elements of the study area and its surroundings. ESP ¼ East Shetland Platform, HP ¼ Horda Platform, MB ¼ Møre Basin, OG ¼ Oslo Graben, SG ¼ Skagerak Graben, STZ ¼ Sorgenfrei-Thornquist Zone, MTFC ¼ Møre – Trøndelag Fault Complex, ØFC ¼ Øygarden Fault Complex, HFSZ ¼ Hardengerfjorden Shear Zone, NSD ¼ Nordfjord-Sogn Detachment, LGOFC ¼ Lærdal-Gjende-Olestøl Fault Complex.

Graben (Fig. 1c). The bulk exhumation following the delamination has been calculated to between 7 km (based on metamorphic gradients; Milnes and Koestler, 1985; Se´ranne and Se´guret, 1987; Milnes et al., 1997) and 13 km (AFT-data; Andriessen and Bos, 1986)

in western Norway. A greater part of the crust is supposed to have been removed after mid Devonian times by erosion that affected the hangingwall of crustal-scale detachments. This amount of exhumation has been taken as an indication that the crest line of

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the Scandinavian Caledonian mountain chain was raging more than 5000 m above sea-level (Gabrielsen et al., 2005), but even elevations close to the maximum of what the continental crust could sustain (8–9 km) have been proposed (Dewey et al., 1993; Andersen, 1998). In the present slope calculation a figure of 8000 m has been used. The domination of west-facing faults associated with high gravitational instability and the development of intramountaneous basins infilled by coarse clastic sediments in late Devonian times (Bryhni and Skjerlie, 1975; Bryhni, 1978; Norton, 1986, 1987) show that the geometry of the mountain chain soon became asymmetric with a steep western flank characterized by a great topographic relief. A strike-slip component, either primary (Roberts, 1983) or secondary (Steel, 1976, 1978) has been suggested for the configuration of these basins, but more recent works suggest that strikeslip is of less importance (Se´ranne and Se´guret, 1985; Se´guret et al., 1989; Norton, 1986; Osmundsen and Andersen, 2001). On the eastern flank of the orogen, stacked foreland basins are well documented from eastern Norway (Ringerike sandstone Group; e.g. Nystuen, 1981; Bjørlykke, 1983). It has been suggested that these basins stretched well into the Baltic (Plink-Bjo¨rklund et al., 2004), indicating a very wide and shallowly dipping eastern flank of the orogen. Recent AFT-data from Finland and southern Sweden suggest that only a thin foreland-related sedimentary cover can have been present in these areas (Hendriks and Redfield, 2005) so that its eastern extent and thickness is not yet established. It should be recognized, however, that thick, Silurian foreland basin sediments are preserved in down-faulted, Permian basins in the Oslo rift, in the Skagerrak Graben and along the Sorgenfrei-Thornquist Zone. Unaffected by this dispute, it is well established that the western margin of the foreland basin system was situated to the west of the present Oslo area. It is clear that deposition in these basins shifted from marine to terrestric in the Wenlock to Ludlow (late Silurian; e.g. Bjørlykke, 1983), and that the foreland basin system remained at or close to sea-level for an extended time. Hence, half-wavelength can be estimated to approximately 450 km in the late Caledonian stage (Fig. 2a). 2.2. Late Devonian–Carboniferous Collapse of the Caledonian mountain chain most probably started before the orogenic contraction was complete, i.e. prior to 405 Ma (Fossen et al., 2008). The process involved crustal-scale transverse, top-to-west and northwest-sense (Se´ranne and Se´guret, 1987; Norton, 1986; Se´ranne, 1992; Dunlap and Fossen, 1998; Krabbendam and Dewey, 1998; Fossen, 1998; Andersen et al., 1999; Eide et al., 2002, 2005) and ENE-WSW-directed orogenparallel (Braathen et al., 2000, 2002; Osmundsen et al., 2003, 2005) extensional faulting. The collapse was extremely fast and a total relative uplift in the order of 30–60 km occurred over a time span of only 5–10 Ma (Andersen and Jamtveit, 1990; Terry et al., 2000; Engvik and Andersen, 2002), bringing the middle and lower parts of the crust to be exposed on the surface in late Devonian time (Eide et al., 2003; Fonneland and Pedersen, 2003). In the following stages of the collapse, fault activity strongly contributed to the modification of the topography of the mountains and the system of intramountaneous basins continued to develop along its western topographic margin. During this process, extensional fault activity became centred along the Nordfjord-Sogn Detachment Zone (Norton, 1986; Se´ranne et al., 1989; Andersen and Jamtveit, 1990; Andersen, 1998), thus moving the crest line of the mountain chain eastward and away from the extinct suture zone. Still, however, the relief was strongly asymmetric with rapid reduction of the relief of the western flank due to faulting and sedimentary infilling of tectonically formed accommodation space. We suggest that the half

wave-length of the uplift in South Norway, as measured to the western margin of the foreland basins was still in the order of 400 km and that the amplitude had been cut to maximum 4 km by the latest Devonian earliest Carboniferous (c. 360 Ma) (Fig. 2b). About this time the tectonic component in the topographic development of the mountain chain had seized so that the relief became controlled by surface processes alone. Thus, the mountain chain should be regarded non-orogenic after this point in time and is, in order to separate later tectonotopographic uplift of western Norway from the effects of the Caledonian mountain building, termed the Scandinavian mountain chain in the following. Thus, by the late (mid?) Carboniferous the elevation and relief of the Scandinavian mountain chain had been dramatically reduced and fault activity in the west central part of the mountains had seized. Faulting associated with relaxation of the Variscan Orogeny occurred in the southern North Sea (Glennie, 1998; Ziegler, 1990), but early Carboniferous faulting is not documented in the northern North Sea. Even farther north, observations from the Barents Sea and Svalbard of shallow basins being filled with mature sandstone, indicate that peneplanation was well underway and that only very modest faulting occurred (Steel and Worsley, 1984). It is therefore reasonable to assume that the Late Carboniferous was characterized by a very moderate relief, and with amplitude of half a kilometre or even less. The topographic crest line of southern Norway was moved farther into the now relaxed hangingwall of the central west-facing master faults in western Norway (Fig. 2c). 2.3. Late Carboniferous–Permian The latest Carboniferous–Permian saw a new regional stress situation with development of a rift system in northwest Europe, associated with reorganization of the Laurentian-Baltic plate configuration (Ziegler, 1990; Ziegler and Cloetingh, 2003). Southwestern and central Norway now consisted of a thick, stable and cold lithosphere, whereas the substratum of the North Sea still may have been relatively warm due to a heritage from the Caledonian mountain building (Neumann et al., 2004; Wilson et al., 2004). The Carboniferous-Triassic period of crustal stretching in northwest Europe was heralded by widespread volcanism and was succeeded by thermal subsidence, resulting in the development of large and wide depocenters such as Southern and Northern Permian Basins of the central and southern North Sea (Ziegler, 1990; Glennie, 1998; Heeremans and Faleide, 2004; Neumann et al., 2004). In the northern North Sea little evidence exists for Carboniferous extension and basin formation, so that most workers assume that rifting started in Permian times there (Beach et al., 1987; Gabrielsen et al., 1990; Færseth et al., 1995; Færseth, 1996). The extension persisted throughout the Permian, causing the development of a system of large half-grabens. This was followed by thermal relaxation lasting from Schytian to Bajocian-Bathonian (Badley et al., 1985; Gabrielsen et al., 1990; Steel, 1993; Færseth et al., 1995; Odinsen et al., 2000a,b). Hence, Permian N-S-striking rotated fault blocks are found on both the western flank (Giltner, 1987; Ziegler, 1990; Roberts et al., 1995) and particularly below the Horda Platform at the eastern margin of the (Jurassic-Cretaceous) Viking Graben. Here, rift units with varying polarities along strike are seen in reflection seismic sections (Gabrielsen et al., 1990; Færseth et al., 1995; Heeremans and Faleide, 2004). Structural analysis and numerical modeling (Christiansson et al., 2000; Odinsen et al., 2000a,b) confirm that the Permo-Triassic event affected a wider crustal transect than that which was affected by extension in the later Jurassic-Cretaceous rifting. The amount of extension also generally was greater in the Permo-Triassic event, with the exception for the very centre of the Viking Graben proper. Taking into consideration the extremely wide fault blocks that characterize

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Distance Fig. 2. The development of a ENE-WSW-oriented elevation profile across south Norway as obtained from compilation, analysis and comparison of several independent datasets (see Fig. 1 for location and main text for description of data). a) Caledonian - post collision, b) late Devonian, c) late Carboniferous, d) late Permian, e) late Triassic, f) late Jurassic, g) late Cretaceous, h) early Tertiary, i) late Tertiary, j) Present.

the Permo-Triassic event, and the slightly larger bulk extension (b ¼ 1,27 for the Permo-Triassic and 1,15 for the Jurassic-Cretaceous; Christiansson et al., 2000; Odinsen et al., 2000b) elastic responses must have been significant at the edges of the master fault blocks and even greater along the master faults that

delineated the graben structures, resulting in strong erosion of the hanging walls of the larger fault blocks (Gabrielsen et al., 1990; Færseth et al., 1995). Large-scale faults and fault zones onshore southern Norway give clear evidence for Permian activity (Gabrielsen et al., 1990; Færseth

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et al., 1995). These faults worked in harmony with fault activity which was resumed along the Nordfjord-Sogn Detachment Zone, Lærdal-Gjende-Olestøl Fault System and the Hardangerfjord and Ryfylke Shear Zones (Norton, 1986, 1987; Torsvik et al., 1992, 1997; Andersen et al., 1999; Eide et al., 1997, 1999; Mosar, 2003). These were large-scale faults with vertical down-to-the west displacements of several kilometres, and elastic and isostatic uplift of the footwall probably was in the order of thousands of meters, thus continuing the transfer of the zone of maximum elevation even

farther eastwards and into the wider area occupied by these faults. In the Oslo Graben of eastern Norway, subsidence was heralded by transgression in Westphalian times (312–300 Ma; Olaussen, 1981; Olaussen et al., 1994). Here, activity along NNW-SSE-trending normal faults accelerated between 305 and 299 Ma, and was accompanied by onset of volcanism (Neumann et al., 2004). The topography associated with the Carboniferous–Permian is not well constrained by direct observations or models. Although it is known that sediment deposition associated with the North Sea

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fault blocks took place in a terrestrial environment, the thickness of the sedimentary deposits and the mere sizes of the fault blocks suggest that the area was subdivided into elongated large-scale depocenters separated by N-S-trending elongate ridges defined by the shoulders of uplifted fault blocks. Erosion of the fault block crests shows that these were exposed. Because of the fault activity, mainland Norway most probably had an enhanced relief compared to that of the Early Carboniferous. Furthermore, subsidence going on in the Oslo Region (Olaussen et al., 1994) (Fig. 2d) added to the total relief. The axis of maximum altitude is likely to have been situated in the footwalls of the large, west-facing fault systems of south Norway, namely the Lærdal-Gjende-Olestøl Fault Complex and associated faults (Mosar, 2003). This implies that the half-wavelength of the system shrunk to 350–400 km and that the amplitude increased, but still remained within the same order of magnitude as that of the Carboniferous (more than 500 m). 2.4. The Triassic–Late Jurassic In the North Sea the Triassic and earliest Jurassic were characterized by thermal subsidence following the Permian extension (Gabrielsen et al., 1990; Odinsen et al., 2000a,b). At the triple junction between the Central Graben, the Moray Firth and the Viking Graben of the central southern North Sea, pronounced thermal doming occurred (Underhill and Partington, 1993, 1994), whereas subsidence increased in the northern North Sea from Bajocian – Bathonian times due to renewed, E-W-oriented extension. The principal axis of extension in the northern North Sea changed to NW-SE during its Jurassic development (Dore´ and Gage, 1987; Færseth et al., 1997; Gabrielsen et al., 1999; Pascal et al., 2002). By the latest Jurassic, subsidence had not yet reached its maximum, and the relief in the North Sea was still moderate. The topography was dominated by up-rotated crests of fault blocks with their long axes oriented parallel to the graben axis (Kyrkjebø et al., 2001; Kjennerud et al., 2001), was enhanced by elastic response to faulting on the one hand (Roberts and Yielding, 1991; Yielding and Roberts, 1992), and smoothed by gravitational destabilization and erosion and infilling of accommodation space on the other (Alhilali and Damuth, 1987; Gabrielsen et al., 2001). Maturation study of organic material from late Jurassic sediments found along Jurassic fault lines between islands of the archipelago of western Norway indicate that these sediments have not been buried below a few hundred meters since their deposition (Fossen et al., 1997), defining an approximate position for the maximum flexure of the Jurassic and younger uplift profile. In their AFT-study, Rohrman et al. (1995), investigating an NWSE-profile across southern Norway, and comparing data from samples taken at sea-level along the western coast with samples collected inland, found that the coastal samples yielded cooling ages of 180–200 Ma years, compared to the inland samples that gave ages of 100–150 Ma. It was concluded that south Norway was uplifted with a domal geometry, with the most uplifted areas to be found in central south Norway. According to Rohrman et al. (1995), uplift had commenced in the Triassic and strong erosion persisted throughout the Jurassic, corresponding to a high cooling rate of 2– 2.5  C/Ma and the exhumation of a rock column between 1.3 and 3.5 km. Later works have revealed that abrupt changes in AFT-ages occur across faults (Redfield et al., 2004, 2005a,b), suggesting that, rather than defining a regular, dome-shaped feature, the south Norwegian uplift was asymmetric with a steeper and strongly faulted (north)-western flank. When considering the reliance of the AFT-data for southwestern Norway, it is of importance to note that in contrast to what is the case for central and eastern Scandinavia, AFT-data from the study area reflect real geological events

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(Hendriks and Redfield, 2005) and are considered reliable for correlation purposes. From radiometric age determination, it is known that activity along the Lærdal-Gjende-Olestøl Fault Complex and affiliated structures occurred mainly in the Permo-Triassic, and continued throughout the Mesozoic (Andersen, 1998; Braathen, 1999; Eide et al., 1997, 1999; Torsvik et al., 1997), again demonstrating that the zone of maximum altitude was located in the footwall of the active master fault systems. This also seems to be in general agreement with the observations of Redfield et al. (2005a,b) for the MøreTrøndelag Fault Complex. Based on this, and taking the high erosion and cooling rates into consideration an amplitude in excess of 500 m and a half-wavelength of 350–400 km seem the most likely for the Triassic (Fig. 2e). Into the Jurassic the axis of maximum altitude migrated eastwards, shortening the half-wavelength to approximately 250 km. The relief was still moderate and a maximum of a few hundred meters of altitude was maintained. Data from the Norwegian continental shelf indicate input of large volumes of siliciclastic sediments in the Triassic (Steel and Ryseth, 1990), but reflection seismic data give little information about the source area for these sediments. It is, however, generally assumed that the Norwegian mainland was the most likely source (Mearns, 1992; Morton, 1992). In the late Jurassic, however, subsidence patterns and sedimentary wedges with internal clinoforms demonstrate propagation of clastic sedimentary units (?sands). These sediments were derived from the Norwegian mainland and transported into the North Sea through the major feeder systems, which are synonymous with the present major fjords, namely the Nordfjord, Sognefjord and Hardangerfjord (Fig. 3a–c). This indicates a considerable uplift of west southern Norway and is supported by observations in the Skagerrak area (between Norway and Denmark) where the entire Triassic–late Jurassic sequence below the base Chalk (mid Cretaceous) reflection is rotated and eroded (Fig. 3d). An extrapolation of the gradients in this region suggests a maximum elevation in the order of 1 km in South Norway (Fig. 2f). 2.5. Late Cretaceous In the North Sea, crustal extension attained its climax at the transition from the Jurassic to the Cretaceous and thermal cooling caused rapid subsidence, with water depths of more than 500 m in the early Cretaceous (Kyrkjebø et al., 2001). Crests of (Jurassic) rotated fault blocks were subareally exposed and contributed to rugged topography before they became submerged in early late Cretaceous. The deep post-rift basin, which had its regional axis oriented parallel to the western shore of southern Norwegian mainland and locally reached water-depth of 1000 m (Kjennerud et al., 2001), contributed significantly to the total topographical gradient between the deepest part of the basin and the summit of the rising mainland. This was further exaggerated by the isostatic response accompanying fast sediment infill of the basin. The period of post-rift subsidence induced by thermal cooling and sediment loading lasted over a period of 70 Ma, and was fulfilled by Maastrictian times implying that by the latest Cretaceous the northern North Sea was a wide, low-relief shallow basin (Gabrielsen et al., 2001; Faleide et al., 2002). Unfortunately, no calculation of the corresponding isostatic response of mainland Norway to the sedimentary loading is available. Onshore south Norway, a shallow sea is believed to have transgressed the southwesternmost tip of the Norwegian mainland towards the end of the Cretaceous, although no sediments of Cretaceous age are preserved (Sigmond et al., 1984; Ziegler, 1990; Encl. 23 and 24). Indeed, Dore´ (1992) ascribed the development of the so-called ‘‘paleic surface’’ (Reusch, 1901b; Gjessing, 1967) to

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Fig. 3. Reflection seismic lines (see Fig. 1b for location) displaying effects of uplift of the south Norwegian mainland. a) E-W-line outboard Nordfjord where Oligocene – Miocene sequences (yellow) are rotated and eroded, demonstrating the effect of post Miocene uplift. The prograding unit is the Upper Pliocene. Note truncation of the unit by the base Pleistocene. b) Rotated and faulted wedge of late Jurassic sediments (yellow) outboard Sognefjord. c) Late Jurassic outbuilding sequence outboard Hardangerfjorden, sourced from the emerging Norwegian mainland to the east. Note basinward prograding clinoforms near the base of the unit. d) Uplifted, rotated and eroded Triassic – early Cretaceous sequence in the Skagerrak (between Norway and Denmark). The reflection base Chalk can be correlated across the Pleistocene Norwegian Channel as a flat surface not affected by the uplift.

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late Cretaceous peneplanation. This interpretation is supported by AFT-data (Rohrman et al., 1995), because samples from the ‘‘paleic surface’’ near the present sea level has a comparable signature to samples from the same surface taken in the present high mountains (1000–1500 m of elevation), altogether suggesting a Cretaceous or Palaeogene age for the ‘‘paleic surface’’. Rohrman et al. (1995) also inferred a period of slow cooling for central Norway in the late Cretaceous – Paleogene, and the geothermal gradient sank from approximately 30  C/km to 20  C/km. Accordingly, a stable and very moderate relief is inferred for the late Cretaceous, with a half-wavelength in the order of 200 km and an amplitude of a couple of hundred meters or less (Fig. 2g). Near the southern Norwegian coast, this event is mirrored by the flat geometry and erosive character of the reflection base Chalk (Fig. 4d). This unconformity is of regional significance, and can be traced across later (Pliocene) surfaces like the Norwegian Channel. 2.6. Cenozoic Two pronounced episodes of uplift have lately been identified in the Cenozoic of Norway and its continental margin. The earliest of these (Palaeogene) have been linked to the development and passage of the Icelandic plume in the North Atlantic between 60 and 40 Ma (Nadin et al., 1997; Saunders et al., 1997; Clift et al., 1998) or other tectonic effects like flank uplift associated with continental break-up (Holtedahl, 1967; Torske, 1972; Nesje and Whilans, 1994), whereas the latter (Neogene) vertical displacements commonly are explained by a combination of tectonic uplift, lithostatic response to glacial erosion and redposition and isostatic, post-glacial adjustments (e.g. Riis and Fjeldskaar, 1992; Blythe and Kleinspehn, 1998; Olesen et al., 2002; Gabrielsen et al., 2005). On the continental shelf, the infilling of the shallow low-relief basin established in the late Cretaceous continued, sourced by the emergent East Shetland Platform and Scottish Highlands in the west, and the Norwegian mainland in the east (Faleide et al., 2002). The Cenozoic northern North Sea can loosely be defined as a wide, saucer-shaped basin containing up to 2500 m of sediments in the depocenters. The basin started to subside quickly in the late Palaeocene, perhaps reaching water depths up to 900 metes

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(Gradstein et al., 1994; Kyrkjebø et al., 2001). Sedimentation rates and distribution of the basin infill succession suggest that the basin deepening was accompanied by rise of the Norwegian mainland east of the Øygarden Fault Complex. Seismic stratigraphic data indicate reduced sediment input from most of the south Norwegian mainland in the earliest Eocene suggesting that levelling had taken place. Still, active point sources, like outside the Sognefjord (Jordt et al., 1995, 2000; Faleide et al., 2002) may be taken as indications of renewed, local uplift. Late Eocene to Late Miocene was characterized by shallowing, due to a combination of global sea-level fall (Haq et al., 1987) and tectono-thermal effects (Kyrkjebø et al., 2001). Uplift of the basin flanks in general seems to have occurred in the Oligocene. This development is also mirrored by the initiation of a system of extensional, intraformational faults in the Miocene sequence due to tilting of the Horda Platform, when the uplift was at its maximum. Restoration suggests an SW-directed slope angle of 1.8 (Clausen et al., 1999) (Fig. 2h). AFT-data support the interpretation of the offshore analysis in that rapid exhumation started at approximately 30 Ma (Oligocene) and continued into the Neogene. The uplift was accordingly initiated well before the Pliocene – Pleistocene glaciation and was in the order of 1–1.5 km (Rohrman et al., 1995) (Fig. 2i). Seismic sequence stratigraphic analysis from the continental shelf yields important information for deciphering the uplift of the mainland Norway. Of particular importance are the following observations: The Upper Pliocene and older sequences are tilted away from the mainland. Pleistocene sediments are found to be flatlying above the unconformity. The angular unconformity separating Pliocene and Pleistocene sediments is less pronounced in the central North Sea. Accelerated subsidence is recorded in basins adjacent to the uplifted landmasses (Faleide et al., 2002). This confirms pronounced uplift of south Norway in the Pliocene – Pleistocene. The present topography of southern Norway has kept its asymetric dome-shape with its maximum culmination at 1850 m in the south (Gausta) and 2500 m in the north (Jotunheimen massif; Fig. 4). Particularly its western flank is truncated by deep valleys that probably have long histories as sediment transport fairways (Mearns, 1992; Morton, 1992). However, they were strongly eroded

Fig. 4. Classification of topographic regions and terranes of South Norway (present). Note distribution of mountain terrains with steep and moderate slopes. After Etzelmuller et al. (2007).

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and over-deepened during the Pleistocene glaciations (e.g. Gjessing, 1967; Holtedahl, 1967; Nesje and Whilans, 1994). A detailed analysis of the southwestern part of the uplift, which was performed by analysing 12 profiles radiating out from the present topographical maximum in Bykleheiene (Figs. 4 and 5a) displays several topographic elements that can be correlated from one sector to the next (Fig. 5b). An envelope surface of these demonstrates the strong influence of the strandflat on the topography, but also that landward extension of the strandflat varies greatly from the south and northwards along the coast (Fig. 5c). Also, the surface of the remnant regional dome is broken by a combination of fracture systems (Gabrielsen et al., 2002; Mosar, 2003; Redfield et al., 2005a,b) and erosional valleys that seem to have been stable over an extended time period (?Cretaceous – Present) and that define the present and former regional sediment transport system. The eastern flank of the dome defines a wide slope from the central high plateau and is characterized by wide, shallow valleys (Fig. 2j), distinct from the steeper valleys in the west (LidmarBergstro¨m et al., 2000). During the Pleistocene, denudation seems to have been limited, whilst linear erosion dominated, preserving the Tertiary overall relief configuration. However, the drainage pattern had changed as the main drainage divide became transferred eastwards, mainly due to over-deepening of main valleys and associated river capture. The interaction between erosion and topographic development as a consequence of glacial erosion and post-glacial isostatic rebound is still debated. Still, there is indisputable evidence that the present Norwegian coast is undergoing uplift at a rate that is 0.1–0.3 mm/yr or higher than what can be explained by post-glacial uplift alone (Mangerud et al., 1981; Sejrup, 1987).

3. Stages of post-Caledonian uplift The topographic development of southern Norway and its adjacent continental shelf is more complex than hitherto acknowledged and is affected of several thermotectonic events, several of which have been overlapping in time and space. To analyze the tectono-topographic development in more detail and particularly to understand the mechanisms responsible for its development, it is necessary to acknowledge that the present recorded uplift at any topographic point will be the result of several mechanisms acting at different places and at different times. A starting point for solving such a problem is by recording the amplitude and wavelength associated with these sources. The mechanisms behind the uplift can, in many cases, only be verified by numerical modelling (Cloetingh et al., 2007). The paleo-topographic profiles presented in Fig. 2 is a first attempt to establish a basis for further analysis of a more detailed uplift and denudation history that eventually can provide a background for modelling. It is obvious that the precision in these data is still limited and highly variable. Thus, uncertainties are considerable and increasing with age of the periods analyzed, but the uncertainty affiliated with the profiles of late Caledonian topography are estimated to be in the order of up to 1 km when elevation of the central part is concerned and 50–100 km for the lateral extension. For the profiles for the Cenozoic, the uncertainty is reduced to some hundred meters and tens of kilometres, respectively. Still, based on the change in the ENE-WSW-slope three distinct stages seem to be distinguishable within these regimes of uncertainty (Fig. 6).

Fig. 5. a) Digital topographic map of southwestern Norway (500m DEM) with profile lines. b) Example of profile line (profile 1 in a, oriented E-W) with indication of topographic regimes with elevation gradients. c) Comparison of profile lines displayed in (5a). Note that the profiles are placed side by side in sequence from north (left) to south (right) for comparison, without regard to orientation. Thus, profile 1 is oriented E-W and profile 10 N–S (5a). Three distinct topographic profile regimes are identified, namely those defined by profiles 1–3, 4–7 and 8–10.

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Topographic gradients, Late Caledonian - Present, Eastern slope Elevation (km)

1,8 1,6

Gradient (deg)

1,4 1,2

Stage 1

1

Topographic maximum South Norway

9 8 7 6 5 4 3 2 1 0

CalMax LDev LCarb LPerm LTri

Stage 3

0,8

719

LJur

LCret

ETer

LTer

Pres

Time

0,6

Stage 2 Fig. 7. Development of the relief in the context of inferred maximum elevation in southern Norway from late Caledonian to Present.

0,4 0,2 0 CalMax LDev LCarb LPerm LTri

LJur

LCret ETert LTert

Pres

Time Fig. 6. Development of dip gradient of the eastern slope of the south Norwegian uplift from latest Caledonian to Present. Three stages are inferred. The first stage is related to construction of the Caledonian mountain chain and its denudation, whereas the two latter are related to tectonothermal events, the mechanisms of which are not yet fully understood. The gradients were obtained by calculation the dip angle drawn from the assumed highest topographic point (Fig. 2) to the nearest point to the east assumed to be close to sea level along the key profile as displayed in Fig. 1b.

Stage 1; Devonian – late Permian denudation of the Caledonian mountain chain: During the latest stage of closing of the Iapetus Ocean in early to mid Devonian times, the Caledonian mountain chain reached its maximum elevation of perhaps as much as 8– 9 km. Based on the distance measured from the zone of maximum altitude to the western margin of the nearest foreland basins in the east, the average topographic gradient of the eastern flank may have been in the order of 1.3 . Collapse of its central parts started before the collision was fulfilled, reducing the elevation and moving the topographic maximum eastward. The westerly facing extensional master fault systems in western Norway were important in the development of the overall morphology of the mountain chain and the footwall of these faults most likely defined the highest topographic elevation. Due to reduced erosion, or entrapment of sediments in basins within the mountain chain, the foreland basins to the east became starved. Hence, the active sedimentary system was transferred westwards, changing the eastern topographic gradient to a little less than 1.2 . The most dramatic erosion of the mountains occurred from late Devonian to late Carboniferous. The position of the topographic peak most likely remained stationary, but the maximum topographic elevation was dramatically reduced, and the gradient of the eastern flank was reduced to around 0.1. The denudation of the Caledonian mountains continued into the Permian and by the end of the Permian, the mountain system was more or less obliterated with a gradient of the eastern flank probably continued to fall to less than 0.1. In the meantime, however, the effects of Permian activity in the North Sea and the Oslo Graben had started with slight subsidence in the latest Carboniferous (Ramberg and Larsen, 1978; Olaussen et al., 1994; Larsen et al., 2008), perhaps followed by uplift in the order of 1 km or more, before thermal cooling again brought the basin floor back to about sea level in late Permian (B.T. Larsen pers. com, 2007). Assuming Airy isostacy, the rate of denudation of a mountain relief is broadly determined by the time scale for mountain topography decay and the crust and mantle densities, respectively (Ahnert, 1970; Pinet and Souriau, 1988; Pelletier, 2004). Thus, a the typical time scale for the decay of a mountain chain generated by thick-skin tectonics and with an elevation in the order of 8000 m would be 70 Ma (Allen, 2008). This is in good agreement with the assumption that the relief of the Caledonian mountain chain was

reduced to less that 500 m by early Carboniferous (end Visean) times, corresponding to a time span of 77 Ma. Stage 2; Triassic – Late Cretaceous; tectono-thermal uplift of central south Norway. Uplift of central south Norway was rejuvenated in Triassic times and continued throughout the Jurassic and Cretaceous. The gradient of the eastern flank increased to a maximum of approximately 0,15 and master structural elements like the Lærdal-Gjende-Olestøl Fault Complex again played important roles in the development. The point of maximum uplift again seems to have drifted eastward and the maximum elevation was reduced (Fig. 6). It also seems that the uplift increased throughout the Triassic – Jurassic period when between 1.3 and 3.5 km of overburden was removed (Rohrman et al., 1995, 1996). The mechanism behind the stage 2 uplift is not well understood. It has been suggested that faulting associated with the Jurassic rifting in the North Sea affected a large part of the southwestern Norwegian mainland. Hence, the uplift of southern Norway might be regarded as flank uplift of the North Sea rift system (Rohrman et al., 1995; Mosar, 2003). It remains to be demonstrated, however, that the shape of the uplifted area fits this model inasmuch as recent studies have demonstrated that post-Caledonian, most likely Mesozoic faulting took place much farther to the east than that previously anticipated. Nevertheless, by latest Cretaceous times the relief was again smoothed and marine transgression and deposition may have taken place over large parts of southern Norway and the topographic gradient was reduced to zero (e.g. Dore´, 1992). AFTdata suggest that the geothermal gradient decreased during the Cretaceous, which is in accordance with this period being tectonically stable and that denudation dominated. Stage 3; Cenozoic; renewed tectono-thermal activity and postglacial rebound. The Cenozoic rejuvenation of the uplift of the southern Norwegian mainland encompasses two distinct events. The earliest (Palaeocene – Eocene) has been linked to either primary or secondary effects of the passing of the Islandic plume in the North Atlantic (Rohrman and van der Beek, 1996; Nadin et al., 1997; Dore´ et al., 2008). Correlation to offshore reflection seismic data, extrapolation and restoration of sequence boundaries and mapping of sedimentary units suggest that uplift of south central Norway continued throughout Oligocene times (e.g. Clausen et al., 1999; Faleide et al., 2002), again increasing the topographic gradients of the eastern flank of the south Norwegian uplift to to around 0.2 (Fig. 7). During the Neogene the gradient continued to increase, but the zone of maximum elevation remained stationary, although the water divide migrated eastward due to headward erosion of steep westward-running rivers. 4. Conclusions By combining topographical analysis with geological data from the mainland and geophysical data from the continental shelf of

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southern Norway, it is possible to track the development of the crest line of south central Norway through a period of 400 Ma and also to identify the most important events of uplift and subsequent denudation (Figs. 6 and 7). It seems substantiated that the Caledonian Orogeny resulted in a Himalaya-scale mountain chain, which started to collapse before the contraction had seized. The Caledonian mountain chain was probably levelled out approximately 70 Ma after it had reached its maximum elevation. This was followed by a tectonically quiet interlude in late Carboniferous times, before renewed uplift occurred in association with largescale faulting, reaching a topographic elevation in the order of 1 km in the Permian. Uplift and erosion continued throughout the Mesozoic, but is not well understood and only more detailed mapping of the regional extension and trends of the uplifted areas can reveal whether this event can be ascribed to rift-flank uplift alone. The relief was levelled out by Cretaceous times, before uplift again took place in the Palaeogene. Hence, the present analysis has confirmed knowledge acquired the last two decades considering the complex post-Caledonian uplift-pattern of South Norway. Although these conclusions are based on one ENE-WSWstriking profile across the southern Norway, it needs to be acknowledged that the topographic data have been supplied by information from seismic data from the neighbouring continental shelf and a recently started detailed analysis of the topography of coastal southwestern Norway. Still, it is clear that such data lack much in resolution when palaeo-topographic detail is concerned, but even more so concerning age determination of erosion surfaces. To mend this, a detailed program for field studies and correlation between offshore unconformities and onshore erosion surfaces is required. Acknowledgements The authors would like to thank Torgeir B. Andersen, Bjørn T. Larsen, Odleiv Olesen, Per Terje Osmundsen and Tim F. Redfield for inspiring discussions and Filippos Tsikalas for technical assistance with the map material. Four anonymous referees provided us with valuable comments on an earlier version of the article. References Ahnert, F., 1970. Functional relationships between denudation, relief and uplift in large and mid-latitude drainage basins. American Journal of Science 208, 243–263. Alhilali, K.A., Damuth, J.E., 1987. Slide block (?) of Jurassic sandstone and submarine channels in the basal Upper Cretaceous of the Viking Graben, Norwegian North Sea. Marine and Petroleum Geology 4, 35–48. Allen, P.A., 2008. Time scales of tectonic landscapes and their sediment routing system. In: Gallagher, K., Jones, S.J., Wainwright, J. (Eds.), Landscape Evolution: Denudation, Climate and Tectonis over Different Time and Space Scales. Geological Society of London Special Publication, vol. 296, pp. 7–28. Andersen, T.B., 1998. Extensional tectonics in the Caledonides of southern Norway, an overview. Tectonophysics 285, 333–351. Andersen, T.B., Torsvik, T.H., Eide, E.A., Osmundsen, P.T., Faleide, J.I., 1999. Permian and Mesozoic extensional faulting within the Caledonides of central south Norway. Journal of the Geological Society of London 156, 1073–1080. Andersen, T.B., Jamtveit, B., 1990. Uplift of deep crust during orogenic extensional collapse: a model based on field studies in the Sogn-Sunnfjord Region of Western Norway. Tectonics 9, 1097–1111. Andriessen, P.A.M., Bos, A., 1986. Post-Caledonian thermal evolution and crsuatl uplift of the Eidfjord area, western Norway. Norsk Geologisk Tidsskrift, 243–250. Austrheim, H., 1991. Eclogite formation and dynamics of crustal roots under continental collision zones. Terra Nova 3, 492–499. Badley, M.E., Egeberg, T., Nipen, O., 1985. Discussion on development of rift basins, illustrated by the structural evolution of the Oseberg structure, Block 30/6, offshore Norway. Journal of the Geological Society of London 142, 933–934. Beach, A., Bird, T., Gibbs, A., 1987. Extensional tectonics and crustal structure: deep seismic reflection data from the northern North Sea Viking Graben. In: Coward, M.P., Dewey, J.F., Hancock, P.L. (Eds.), Continental Extensional Tectonics. Geological Society of London Special Publication, 28, pp. 467–476.

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