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NORWEGIAN JOURNAL OF Deep , neotectonics and formation in , northern 189

Deep weathering, neotectonics and strandflat formation in Nordland,

Odleiv Olesen, Halfdan Pascal Kierulf, Marco Brönner, Einar Dalsegg, Ola Fredin & Terje Solbakk

Olesen, O., Kierulf, H.P., Brönner, M., Dalsegg, E., Fredin, O. & Solbakk, T.: Deep weathering, neotectonics and strandflat formation in Nordland, northern Norway. Norwegian­ Journal of Geology, Vol 93, pp. 189–213. 2013, ISSN 029-196X.

The strandflat area of Nordland is a region with frequent deep weathering and increased seismic activity. We argue that there is a causal relationship between the phenomena. We have carried out chemical and geophysical studies of the weathering and measured the tectonic strain using the GPS method. A parallel and shallow zone of increased seismicity along the coast largely reflecting extensional stress has previously been mapped. New GPS data and previous datasets such as DInSAR, repeated levelling and focal plane solutions indicate that the outer Ranafjorden area is under E–W extension and subsidence. In 1998–1999, a local seismic network detected earthquake clusters along N–S-trending zones. An irregular relative subsidence pattern on the order of 1 mm yr-1 has previously been recorded on InSAR–PS data. During the period 1999–2008, GPS stations to the west of the earthquakes moved c. 1 mm yr-1 to the NW relative to GPS stations to the east. Deeply weathered bedrock occurs around Vest- fjorden and southwards to Ranafjorden. Electrical resistivity profiling reveals weathered bedrock extending to 20–100 m depth over large areas. Calculations of weathering indices and mass balance from XRF analysis show leaching of the major elements. We argue that the weathering is partly of Early age, since the Late to Early Cretaceous faults in the –Vesterålen area are little affected by the alteration. The weathered­ sites can be found up to an altitude of 500 m a.s.l. and were most likely exhumed during the Plio–. This observation is support­ed by previous marine-geological studies suggesting a Pleistocene of ~500 m along the coast. The unloading of the crust thus most likely caused flexuring and accompanying fracture extension. The Nordland strandflat may consequently represent a deeply weathered and peneplaned surface of ­ to Early Jurassic age that has been exhumed and levelled during Plio–Pleistocene erosion. The deeply weathered and fractured basement rocks could have facilitated effective glacial erosion during the Pleistocene. Glacial erosion combined with freezing/thawing and abrasion by ocean waves during non-glaciated periods would have assisted in the formation of a relatively wide strandflat along the Nordland coast as well as a rim of low land around and peninsulas.

Odleiv Olesen, Marco Brönner, Einar Dalsegg & Ola Fredin, Geological Survey of Norway, P.O. Box 6315 Sluppen, 7491 Trondheim. Halfdan Pascal Kierulf, Norwegian Mapping Authority (Kartverket), 3507 Hønefoss. Terje Solbakk, Norwegian Petroleum Directorate, P.O. Box 600, 4003 .

E-mail corresponding author (Odleiv Olesen): [email protected]

Introduction preglacial marine abrasion (Reusch, 1894) to subaerial denudation at a in the peripheral parts The strandflat as a geomorphological element was of (Ahlmann, 1919). Asklund (1928) and defined by Reusch (1894). The timing and mechanism of Büdel (1978) argued that the strandflat represented an its formation have been discussed at great length over the etch-plain with developed mainly by pre- last century. The strandflat is mainly developed between Cretaceous processes in a tropical climate. Gjessing Stavanger and Magerøya in (Holtedahl, 1998) (1967) suggested that the pre-existence of a mature and its width varies from 50–60 km along parts of the palaeolandscape () approaching sea level Nordland and Møre coasts to more or less zero in the would enhance the effect of different strand-forming Stad area (Fig. 1). The strandflat is an uneven and partly mechanisms. Nansen (1922) emphasised, with the submerged bedrock platform extending seawards from basis from his experience, frost weathering as an the coastal mountains. It consists mostly of numerous important agent. More recent workers have argued that low islands, and shallow sea areas often limited a combination of processes such as frost-shattering, to 20–40 m depth (Vogt, 1900; Holtedahl, 1998). Its abrasion by ocean waves and sea ice, and glacial inner boundary is defined by a sharp change of slope erosion are needed to explain the strandflat formation (knickpoint), but can locally be more diffuse where the (e.g., Møller & Sollid, 1973; Klemsdal, 1982; Larsen & strandflat continues more gradually into the mainland. Holtedahl, 1985; Holtedahl, 1998). There is thus, at the The strandflat commonly occurs as a rim of low land moment, no consensus concerning the mechanism and lying around islands and peninsulas. Opinions with timing of the strandflat formation. Most researchers, regard to origin and formation vary greatly from mainly however, seem to favour a Plio–Pleistocene age. 190 O. Olesen et al. NORWEGIAN JOURNAL OF GEOLOGY

We have, in the present study, acquired new data on deep involving erosion of a pre-existing weathered surface. A weathering and neotectonic deformation in the Nordland Pleistocene exhumation of the basement rocks along the area and aimed at testing if the new observations fit any coast of Nordland could also result in uplift and flexuring of the previously proposed mechanisms. We especially involving local extension. Recorded neotectonic wanted to check the proposed model of strandflat deformation can consequently add to our understanding formation by Asklund (1928) and Büdel (1978) of the strandflat-forming agents.

Geological and geomorphological setting

The bedrock geology (Fig. 2) of mainland Nordland is dominated by the Caledonian Upper and Uppermost that were thrust onto lower allochthons and the subjacent basement during the Scandian continent-continent collision in and Early time (Gee & Sturt, 1985; Roberts & Gee, 1985; Gustavson & Gjelle, 1991). In the Devonian, the were dismembered during a late gravity collapse phase of the Scandian (Rykkelid & Andresen, 1994; Braathen et al., 2002; Eide et al., 2002). The dominating lithologies of the complexes are gneisses, schists and . The tectonic basement windows, , Høgtuva and , consisting of granitic gneisses, are situated to the north of Ranafjorden while the Batholith, comprising granitoids in the Uppermost , is situated to the south. The granitoid rocks within the tectonic windows represent an extension of the Transcandinavian Igneous Belt, TIB (Gaal & Gorbatschev, 1987; Henkel & Eriksson, 1987), that continues northwestwards below the Caledonian nappes from the Fennoscandian of central and southern . The basement of the Lofoten–Vesterålen islands and on Hamarøya in northern Nordland is dominated by Palaeoproterozoic and Archaean, polymetamorphic, migmatitic gneisses and TIB intrusions of mangeritic to charnockitic composition.

The Lofoten–Vesterålen archipelago is a NNE–SSW- to NE–SW-oriented basement high (Åm, 1975; Brekke & Riis, 1987; Løseth & Tveten, 1996; Olesen et al., 1997; Tsikalas et al., 2005), situated between 67°24’N and 69°20’N latitude at the narrowest part of the Norwegian . It is flanked by deep Late Palaeozoic/ Early Mesozoic and half-grabens (Mjelde et al., 1993; Tsikalas et al., 2005; Bergh et al., 2007; Hansen et Figure 1. Earthquakes (1980–2011), deep weathering, strandflat, post- al., 2012) which experienced major phases of extension glacial faults, domes and areas of interpreted Plio–Pleistocene during the Jurassic to Palaeogene. deposition and main erosion along the Norwegian continental margin (modified from Møller & Sollid, 1973; Riis, 1996; Holtedahl, 1998; Dehls et al., 2000; Bungum et al., 2010; Lagerbäck & Sundh, 2008; Peulvast (1986) described the main morphostructural Fredin & Husteli, 2012; Olesen et al., 2012, 2013). The areas of Plio– features of the Lofoten islands as NE–SW-oriented Pleistocene sedimentation and erosion are coinciding with present-­day mountains or ridges arranged en échelon seismicity, indicating that recent loading/unloading is causing flexur­ between longitudinal , sounds and basins, which ing and faulting in the lithosphere. Focal plane solutions (Dehls et are cross-cut by oblique or perpendicular features, al., 2000; Hicks et al., 2000b) indicate the dominating compression­al splitting the islands into smaller blocks. The landscape events in the areas with loading, whereas the regions with recent unloading have predominantly extensional or strike-slip events. The results from a complex tectonic history, characterised locations of grus and clay (Fredin & Husteli, 2012; Olesen mainly by extension, normal faulting and blockwise et al., 2012) occur most commonly within the coastal areas of Plio– vertical movements and erosion (Griffin et al., 1978; Pleisto­cene erosion. The black frame depicts the Nordland study area. Peulvast, 1986; Løseth & Tveten, 1996; Bergh et al., 2007; NORWEGIAN JOURNAL OF GEOLOGY Deep weathering, neotectonics and strandflat formation in Nordland, northern Norway 191

Figure 2. Map of the Nordland margin, including source catchment area of glacial erosion (green dashed line) and area of offshore deposition (iso- pach map of thickness of the Naust Formation in milliseconds of two-way travel time, where 1 ms is ~1 m). The blue line marks the present shelf edge. Adapted from Dowdeswell et al. (2010). Zones of deep weathering more than 100 m thick and more than 1 km wide occur within the eroded area indicating that the present landscape is to a large degree of Mesozoic age. Subcrop units (modified from Sigmond, 2002) underlying the Naust Formation are mainly , Cretaceous and Jurassic sedimentary rocks (hatched pattern). The strandflat is adapted from Vogt (1900), Møller & Sollid (1973) and Holtedahl (1998). We interpret this geomorphological feature as a deeply weathered and eroded basement similar to the sub- cropping units of sedimentary rocks farther west. Earthquakes from the period 1989–2011 are shown in yellow and the size of the circles reflects the magnitude on the Richter scale. The red frame depicts the location of Fig. 5.

Hendriks et al., 2007; Hansen et al., 2012). The alpine- and variation in relief (Lidmar-Bergström, 1995; Olesen type mountains rise up from sea level to more than 1200 et al., 2012). The locations of numerous sounds and m above sea level (western Hinnøya and Moskenesøya). islands, not only in the Lofoten–Vesterålen area, but also Paleic surfaces of a smooth hilly landscape (Langøya and on the strandflat along the Norwegian coast, could to a Andøya), wide valleys (Vestvågøya) and large large extent be the result of exhumation and erosion of a at sea level are also developed (Fig. 3). The northward weathered basement. Holtedahl (1998) especially pointed extension of the –Borg (Fig. 3A) on out the horizontality of the strandflat and Vestvågøya in Lofoten coincides, for example, with a local concluded that it was developed during several stages. offshore basin (F. Riis pers. comm., 2013) indicating that The strandflat is consequently polycyclic and remnants the present-day landscape in the Lofoten–Vesterålen– of higher levels are present close to the backwall in many area is reflecting exhumed Mesozoic areas, and also at some distance inside the fjords. basement structures. The 440 m-deep Sortlandsundet basin, representing a half- and defined by a From a correlation between offshore geology and normal in the southeast (Davidsen et al., 2001; Bøe onshore morphological elements, analysis of weathering et al., 2010), and the up to 800 m-deep and fault-bounded surfaces, and apatite fission-track analysis, Riis Mesozoic basins in the Træna–Meløy area (Bøe et al., (1996) suggested that the enveloping summit level of 2008) fit into this tectonic scheme. originated as a in the Jurassic. By the Mid Jurassic, the parts of Mid Norway which were Recent studies, however, have shown that the processes exposed to erosion in the Triassic had been eroded down proposed for the development of the Lofoten–Vesterålen to a peneplain close to sea level, which was exposed to landscape are not sufficient to explain the magnitude weathering in a warm and humid climate (Bøe et al., 192 O. Olesen et al. NORWEGIAN JOURNAL OF GEOLOGY

from www.Norgei3D.no (A) (B) (A) (A) (C) Rødsand Hinnøya

Sortlandsundet Storheia

Fig. 4a Brennvinshaugen

VestvågøyaVestvågøya, Profile, Profile 1 1

View View from from southwest southwest

(C) AndenesAndenes Fig. 4a (B) (B) Hadseløya, Profile 4

View from west

(D) (D)

Ramsåa Ramsåa

Ramsåa Ramsåa Andøya, Profile 3 Andøya, Profile 3

View from south View from south

Fig. 4b

View from east

Figure 3. Aerial photography draped over topographic models (courtesy of www.norgei3d.no) covering the four locations studied with electrical resist­ ivity traversing (ERT). (A) Rise in the Leknes-Borg valley, Vestvågøya, (B) Brennvinshaugen, Hadseløya, (C) Ramsåa, Andøya, (D) Hamarøya. Three of them are located on the strandflat while the fourth on Hadseløya (Brennvinshaugen) is located in an open valley on the hangingwall block of the northeastward extension of the West Lofoten Border Fault of Bergh et al. (2007) cropping out at the base of the alpine range in the background (Osmundsen et al., 2010). The length of the profiles shown in Fig. 8 illustrates the scale of figures. The locations of the figures are shown in Fig. 2.

2008, 2010). These results seem to agree with the fission- kaolinite and smectite which are, to a large extent, the track results (Hendriks et al., 2007). result of chemical weathering caused by percolating acidic groundwater in a humid climate under tropical to subtropical conditions in the Jurassic and Early Deep weathering Cretaceous (Lidmar-Bergström et al., 1997, 1999). The saprolite was partially eroded before the Cretaceous, and Following the studies by Lidmar-Bergström (1995), it is the remnants of this chemical weathering were preserved now generally accepted that the joint-controlled valley below shales and carbonates deposited during the Late landscape of southwestern Scandinavia was formed Jurassic and Cretaceous transgressions (c. 400 m higher during erosion of a thick Triassic/Jurassic saprolite along eustatic sea level). Bodies of saprolite up to 60 m thick regional fault and fracture zones. This system of joint- have been found in joint-controlled valleys in aligned valleys could be traced northwards along the (Lidmar-Bergström et al., 1997). west coast of Sweden all the way to and farther west along the coast of (Fig. 1). Reusch (1903) also suggested, based on his hypothesis The weakness zones in this sub-Cretaceous etch surface of ‘superimposed valleys’, that southeastern Norway are partly due to the presence of clay minerals such as had been covered by Cretaceous sedimentary rocks. NORWEGIAN JOURNAL OF GEOLOGY Deep weathering, neotectonics and strandflat formation in Nordland, northern Norway 193

Reusch (1878) had earlier concluded that the landscape in eastern and southern Norway (e.g., at Lassedalen, in the Oslofjorden area was relatively little affected by Gjerpen, Heskestad, Skreia and Feiring), yielded Mid the glacial erosion. He especially referred to a location Triassic to Early Jurassic ages (Ineson et al., 1975, 1978; at Ødegårdsbukta, 2–3 km to the east of Nevlunghavn Ihlen et al., 1978, 1984). Clay-poor grus weathering in , where a large block of has been from breccias and fracture zones was sampled in a large removed by the inland ice. No weathering surface has number of these locations (P. Ihlen, pers. comm., 2013; formed on this newly exposed bedrock surface during T. Vrålstad, pers. comm., 2013). The Mesozoic ages most the last 10,000 years, indicating that the typical limestone likely represent the same phase of deep weathering as has weathering in the Oslofjord region must be much older recently been reported from two exploration wells on the than Pleistocene. It could, in principle, represent the base High (Fredin et al., 2012a), and not hydrothermal of the original Mesozoic weathering. alteration associated with the formation of the mineral deposits. The magmatic activity in the had Interpretations of aeromagnetic data in the Oslofjord already ceased in the Early Triassic (Sundvoll & Larsen, region indicate the presence of saprolite exceeding 200 1994). Exhumation of the Mesozoic in m in thickness in fracture zones (Olesen et al., 2007). southeastern Norway was initiated during the Early Clay-rich zones with thicknesses of more than 200 m and the uplift and erosion accelerated during have also been observed (Palmstrøm et al., 2003; Olesen the Neogene (Riis, 1996; Lidmar-Bergström et al., 1999). et al., 2007), e.g., in the Lieråsen and Romeriksporten tunnels in the Oslo Region. If the distribution of the There has previously been a widespread view that clay- valleys and mountain ridges, and the sounds and bearing weathering surfaces in Scandinavia formed fjords, are to some extent the visible expression of deep during the Mesozoic whilst the gravelly, so-called ‘grus’- weathering and subsequent erosion during the ice ages, weathering is much younger, perhaps of or the remains of deep weathering onshore Norway can Plio–Pleistocene age (Peulvast, 1985; Sørensen, 1988; possibly be of Mesozoic age. The majority of the K–Ar Lidmar-Bergström et al., 1999). Paasche et al. (2006) dating in the 1970s and 1980s of assumed hydrothermal have suggested a Tertiary age for the coarse-grained clay alteration associated with Permian fluorite and weathering in Vesterålen. It is, however, important to sulphide vein deposits, as well as regional fault zones realise that a generalised tropical weathering profile

(A) (B)

~14 m

~18 m

(C) (D) Figure 4. (A) A c. 14 m-thick layer of saprolite in the quarry at Brennvinshaugen on - øya. A quartz vein and the preserved fabric of the original rock confirm the in situ state of the material. (B) Saprolite with numerous core­stones in an abandoned quarry at Næver- nes on Hamarøya. (C) Example of a very sharp transition between weathered and fresh bedrock in a road-cut on Hamarøya. (D) Sap- rolite in at Amundvatnet to the east of Jektvik in Rødøy. Locations A–C are shown in Figs. 2 & 3 and location D in Fig. 5A. TWIN 194 O. Olesen et al. NORWEGIAN JOURNAL OF GEOLOGY

(A) ›

(B) NORWEGIAN JOURNAL OF GEOLOGY Deep weathering, neotectonics and strandflat formation in Nordland, northern Norway 195

› Figure 5. Results from a detailed neotectonics study in southern Nord- Severe problems occurred during the construction land. The observed 1998–1999 seismicity (Hicks et al., 2000a) in the of the 726 m-long Rørvikskaret road tunnel (E10) on outer Ranafjorden area occurs along NNW–SSE-trending clusters. The shallow earthquakes coincide with mapped fractures and faults with Austvågøya in Lofoten in the early 1970s. The tunnel pronounced escarpments. GPS stations to the west of the earthquake caved in during the construction and the collapse clusters have moved c. 1 mm yr-1 to the W relative to the stations on the extended along a weakness zone with a high swelling- eastern side during the period from 1999 to 2008. (The velocities are clay content all the way to the top of the mountain pass relative to station NE08 which is the southernmost station in the GPS (Palmstrøm et al., 2003). In the end there was an open net). The ellipses represent the 95% confidence region. The uplift data hole from the tunnel to the top of the pass located 100 m in green colours are acquired from height measurements of acorn and above the tunnel. The tunnel project was delayed by three bladder marks (Bakkelid, 1990, 1991, 1992; Olesen et al., 1995). The contoured green isolines reveal a relative subsidence of the seismic­ years and was not completed until 1974 after five years of ally active area. (A) Fault-plane solutions indicate N–S compression construction. We think that the tunnel collapse may have and E–W extensional faulting (Hicks et al., 2000a). The zones of deep occurred along a deeply weathered major fracture zone. weathering have been mapped by Gjelle et al. (1985) and Wilberg (1989). (B) The uplift data in colours are acquired using the DInSAR The localities with the most pervasive weathering in permanent scatterer method (Dehls et al., 2002). The irregular ­relative Nordland (Vesterålen and Hamarøya) are generally subsidence pattern (blue areas) seems to coincide with elongate clusters­ found in bedrock ranging from mafic to felsic of small and shallow earthquakes. composition (from gabbro to (quartz-)monzonite, syenite and granite). Weathering in felsic lithologies has been encountered in the Jektvik and Melfjorden areas (Gjelle et al., 1985; Wilberg, 1989) of the Svartisen and Høgtuva tectonic windows, respectively (Figs. 2, 4D, 5A). Up to 200 m-wide and 15 km-long zones of deeply weathered rocks occur along the foliation of the granitic (Acworth, 1987) is separated into two upper soil gneiss. Only very small amounts of clay minerals were horizons (red sandy soil and laterite) and four lower found in the samples from Nordland, and few definite alteration zones (two clay zones, a granular friable layer signs of clay minerals indicative of advanced weathering (grus weathering) and a lowermost zone of fractures and were detected. fissured rock). Clay-rich and clay-poor weathering can consequently be formed at the same time but at different depths. The total thickness of the profile may reach 100 Neotectonics m in normal bedrock but may be significantly thicker along regional fracture zones. The coastal zone of Nordland and the passive continental margin are seismically relatively active areas compared to Remnants of subtropical weathering occur below the rest of Fennoscandia (Figs. 1, 2). Below the up to 1500 Mesozoic sedimentary rocks on Andøya in Vesterålen m-thick Plio–Pleistocene wedge along the continental (Dalland, 1975, 1981; Sturt et al., 1979) and on the edge to the west there is a seismic zone with relatively continental shelf (Roaldset et al., 1993; Smelror et al., 2001; deep compressional events (Byrkjeland et al., 2000). Mørk et al., 2003; Riis et al., 2013). Weathered bedrock The seismicity in the Træna Basin offshore Nordland with a thickness of up to eight metres has previously been coincides with the thickest unit of the Naust Formation observed up to an altitude of c. 500 m above sea level in (Fig. 2). Another cluster of earthquakes occurs on the the Lofoten–Vesterålen area (Peulvast, 1985, 1986; Paasche Utrøst Ridge and along the adjacent continental slope et al., 2006; Strømsøe & Paasche, 2011). A rather wide area c. 160 km to the west of the Lofoten islands (Fig. 2). This of weathered basement was also discovered and mapped area is characterised by large-scale Neogene erosion (see on Vestvågøya by NGU in the 1980s. The site is located Fig. 3.14a in Norwegian Petroleum Directorate, 2010) due at Rise, an agricultural area situated in a rather wide and to Mid Miocene uplift (Rise et al., 2013), which caused flat, NE–SW-striking valley (Fig. 2A). Small outcrops reduced slope stability and formation of numerous with weathered material are visible today, but photos and deep canyons along the continental slope. Successions reports from NGU staff (P.-R. Neeb, pers. comm., 2010; of submarine fan/slide deposits up to 700 m thick occur Fig. 6.3 in Brönner et al., 2012a) witnessed a significant on the continental rise at the outlets of the canyons. Two amount of saprolite in this area, close to the surface and fault-plane solutions in this area show deep compressional covered by only a thin layer of soil. Mørk et al. (2003) faulting (Dehls et al., 2000; Hicks et al., 2000a). The reported a distinct change in stratigraphic mineralogy of observed earthquakes may consequently also be related to cores from shallow drilling offshore Lofoten–Vesterålen: Pleistocene erosion and sedimentation. from mica/illite + chlorite + feldspar + mixed-layer clay- rich compositions in the Lower Triassic to kaolinite- There is ample evidence for recent movements in dominated and feldspar-poor compositions in the Upper the Ranafjorden area (Olesen et al., 1995). The 1819 Triassic–lowermost Jurassic deposits. This Late Triassic earthquake (magnitude 5.8) occurred in the outer transition coincides with changes from arid continental to Ranafjorden area and is the largest earthquake registered humid continental and paralic environments. on mainland Fennoscandia in the past few centuries 196 O. Olesen et al. NORWEGIAN JOURNAL OF GEOLOGY

(Muir , 1989; Bungum & Olesen, 2005). The 1977/1978 and 1992 and contained 10,000 and 200 effects of the earthquake were quite dramatic and it was shocks, respectively, but no main shock was recorded. The followed by an extensive aftershock sequence that lasted magnitudes of the earthquakes were up to 3.2 and 3.6, and for more than 10 years (Heltzen, 1834). The event caused the distance from Ranafjorden to the locations of the two widespread damage to the foundations and chimneys swarms is 60 and 160 km, respectively. The strike trend of wooden buildings as well as very extensive rockfalls, of the main active faults within the Meløy swarm (Fig. a landslide, liquefaction phenomena and a variety of 5) was N20°–35°E and the dip was 60°SE (Bungum et disturbances in fjords and in the sea (Heltzen, 1834; al., 1979; Gabrielsen & Ramberg, 1979; Vaage, 1980). The Muir Wood, 1989; Bungum & Olesen, 2005). faulting had a large component of normal fault motion and a minor component of right-lateral strike-slip. The During the operation of a seismic network in 1997– strike trend of the composite fault-plane solution (NE– 1998, approximately 350 seismic events were located SW) for the events (Atakan et al., 1994) is the in the Ranafjorden area (Hicks et al., 2000a). The main same as that of the Meløy fault plane. Although the dip part of the activity, approximately 250 earthquakes, direction deviates considerably (i.e., oblique-slip fault occurred in the western parts of the network (Fig. 5), with a normal component dipping NW), both sets might the same area in which many of the effects of the 1819 be ascribed to the same stress situation. earthquake were reported. These earthquakes occurred as NNW–SSE-trending swarms. The two largest The anomalous seismic activity in the Nordland area earthquakes had magnitudes of ML 2.7 to 2.8. Fault- is, consequently, evidence of crustal movements along plane solutions indicate E–W extensional faulting (Hicks mainly N–S-trending structures. Most of the events et al., 2000a). The observed seismicity seems to occur suggest normal faulting (Hicks et al., 2000a), and the along N–S- to NNW–SSE-trending fractures and faults coast-perpendicular tensional axes are consistent with with pronounced westward-facing escarpments cutting what would be expected from stress due to differential the glacial morphology in the Handnesøya, Sjona and unloading and post-glacial uplift. The Ranafjorden Austerdalsisen areas (Figs. 6, 7 and Olesen et al., 1995). area in northern Norway is a region of increased These scarps have been attributed mainly to plucking seismicity and anomalous land uplift. There is, for effects by the moving inland ice. Ice-plucking features example, evidence for recent movements in the bedrock may, however, be indirectly related to neotectonics. indicated by an anomalously low uplift of acorn barnacle and bladder wrack marks on the islands of and The spectacular earthquake swarms (Fig. 2) on Meløy in the outer Ranafjorden area (0.0, 0.06 and (Bungum et al., 1979; Vaage, 1980) and Steigen (Atakan 0.07 m from 1894 to 1990 compared to 0.23–0.30 m et al., 1994) are also located within the seismicity zone in the area to the north and to the south (Bakkelid, of the Nordland coast. The two swarms occurred in 1990, 1991, 1992; Olesen et al., 1995). Figs. 2 & 5 show

Figure 6. Several vertical fracture zones on Hand- nesøya (Olesen et al., 1995). The western blocks seem to be downfaulted. The main scarp heights are, however, the effect of sub-glacial plucking from the moving inland ice (Olesen et al., 2004). The two easternmost scarps seem to coincide with linear seismicity clusters shown in Fig. 5. Looking north from the quay in . NORWEGIAN JOURNAL OF GEOLOGY Deep weathering, neotectonics and strandflat formation in Nordland, northern Norway 197

formation of basins several hundred metres deep. The most likely situation, on the other hand, is a migration of the subsiding area that we also observe on a local scale when the seismicity clusters move from one area to another with time.

In order to retrieve further crustal deformation data for the Ranafjorden area, Dehls et al. (2002) applied differential SAR interferometry (DInSAR). As part of a continuing investigation into neotectonic activity in the area, it was decided to investigate the use of differential interferometry to measure surface movements in the area. At Permanent Scatterers, i.e., individual phase- stable, point-wise radar targets, displacement data can be retrieved with millimetric accuracy (Fig. 5B). The PS grid can be seen as a high spatial density, natural geodetic network. An irregular subsidence pattern in the order of c. 1 mm yr-1 was observed on DInSAR permanent scatterer data in the outer Ranafjorden area (Dehls et al., 2002). The relatively low seismicity occurring at a depth of 2–12 km (Hicks et al., 2000a) could therefore create the observed irregular subsidence pattern at the surface. A network of benchmarks to measure the active strain in the Ranafjorden area by use of the Global Positioning System (GPS) was established in 1994 (Olesen et al., 1995). The net was remeasured in 1999 and 2008 and the results are reported in the present paper.

The maximum horizontal in situ stress in Nordland is usually higher than 30 MPa and oriented N–S at moderate depths of 200–800 m, and causes exfoliation in natural and man-made cuttings and spalling in tunnels (Myrvang, 1993). In the Kobbelv/ area, which is situated to the east of Vestfjorden, rock bursting and buckling are observed at the surface (Myrvang, 1993). Figure 7. (A) Aerial photo­graph of NNE–SSW- and NNW–SSE- trending fractures shown by small black arrows to the south of Aust- It is considered likely that the varying stress fields erdalsisen in Rana (Fig. 5). The fractures seem to offset the morpho- and vertical movements were caused mainly by an logy but the apparent offsets were most likely caused by the plucking interaction of the ridge-push force from the Knipovich of the westward-moving inland ice. (B) Erosional features south of Austerdalsisen looking to the north (UTM coordinates 455,500E / and Mohn ridges and other effects such as postglacial 7,376,500N, UTM zone 33). The location is situated close to the small rebound, gravitational effects from mass excess within black arrow in the northeastern corner of the aerial photograph in the Lofoten–Vesterålen metamorphic complex and (A). The foliation of the mica schist is subparallel to the bedrock sur- mountainous areas in Scandinavia (Muir Wood, 2000; face. The moving inland ice has been plucking blocks from the bedrock Olesen, 1991; Fejerskov & Lindholm, 2000; Lindholm along the steeply dipping N–S-trending fractures. The escarpment et al., 2000; Pascal & Cloetingh, 2009), in addition to must hence have been formed by ice plucking beneath the westward- sedimentary loading (Byrkjeland et al., 2000; Hicks et moving inland ice. al., 2000b) and unloading in the coastal and offshore areas (Bungum et al., 2010; Olesen et al., 2013).

The Fennoscandian Shield was affected by a Neogene the average annual uplift in mm. Since a total of five phase of passive doming (approximately 1000 m levelling points constitutes the anomalously low uplift amplitude) in southern Norway and in the Lofoten– area, we regard these observations as fairly reliable even area (Riis, 1996). Hence, the present elevation of though this particular levelling method is not properly Scandinavia is partly the result of Neogene uplift. The tested and documented (Bakkelid, 1992). It is not likely combined effect of tectonic uplift of Fennoscandia and that the observed, high, annual subsidence rates in the the onset of the northern hemisphere glaciation led to outer Ranafjorden area have been constant for several greatly increased erosion and sedimentation. More than hundreds of thousands of years at the same location. 50% of the volume of Cenozoic sediments registered Had this been the case, the result would have been the offshore was actually deposited during the last 2.6 Myr. 198 O. Olesen et al. NORWEGIAN JOURNAL OF GEOLOGY

Fjeldskaar et al. (2000) argued that the long-term Neogene unweathered samples from fresh rock through cutting uplift of western Scandinavia is still active and can explain and sampling well inside of possible superficial approximately 1 mm yr-1 of the present uplift of the southern alteration. All samples were first dried and then and northern (Nordryggen). The examined under a binocular microscope. Grain- size, geodynamic modelling of the uplift within the Lofoten– SEM and XRF analyses were completed at the laboratory Troms area showed positive deviations from the observed of the Geological Survey of Norway in Trondheim. Grain uplift, indicating a long-term tectonic component being sizes were determined on dried samples with a Coulter partly responsible for the present-day uplift. Vestøl (2006) LS Particle Size Analyzer. Scanning electron microscopy subtracted the modelled glacial isostatic uplift of Lambeck analyses were performed on surface bulk fine matrix et al. (1998) from his own postglacial uplift data and samples, and semi-quantitative analyses of grain arrived at a similar conclusion. The anomalous uplift area chemistry and mineralogy were completed according to in northern Norway, however, is larger, revealing a longer energy dispersive spectrometer techniques (Goldstein, et wavelength component including most of Nordland. al. 2003). Samples were prepared according to standard There is also some evidence from uplifted pre-Weichselian techniques for XRF examination at the laboratory of the sediments and along the coast of Geological Survey of Norway. The analytical techniques that the Norwegian coast may have been subject to tectonic are described in the accreditation documentation uplift in the order of 0.1–0.3 mm yr-1 during the Quaternary of the NGU laboratory from 2008. Precision of the (Holtedahl, 1984, 1998; Larsen & Holtedahl, 1985; Sejrup, analysis depends on the element that is analysed but 1987). concentrations are generally determined with error margins on the order of ppm or better.

Methods On Hadseløya, a quarry with deeply weathered basement rocks of c. 14 m thickness was investigated (Figs. 2, Chemical analysis 3B, 4A) on the flank of a wide valley and is covered by only a thin layer of soil (<1 m). The parent rock type During the mapping we investigated publications mangerite can be observed on-site in some corestones. (Peulvast, 1985, 1986; Paasche et al., 2006), and reports The crystalline fabric of the primary basement rocks (Hillestad & Melleby, 1967; Hillestad, 1987), visited is very well preserved and a quartz vein was observed known quarries and former pits, and interviewed local within the saprolite, which indicates the in situ character people. Saprolitic bedrock in the Lofoten–Vesterålen area of the weathering process. The site shows a rather and on Hamarøya is used by local farmers to construct homogeneous, clay-poor, granular material with only a and maintain small gravel roads. Furthermore, recent few corestones (Fig. 4A). air photographs were used to detect likely locations with possible weathered rocks. We found three locations Chemical weathering indices were calculated according onshore Lofoten–Vesterålen, on Vestvågøya, Hadseløya to Price & Velbel (2003), where mainly major element and Andøya, with exposures or reports of remarkably oxides were used. For all indices (CIA, CIW, PIA) we deep weathering of the bedrock. Moreover, an open pit calculate the proportion of certain major elements on Hamarøya was studied. in relation to others. For example, an abundance of

relatively immobile Al2O3 is checked against rather Furthermore, we have followed in Sturt’s and Dalland’s mobile elements such as CaO, Na2O and K2O. The footsteps and excavated a trench on the beach at Ramsåa Chemical Index of Alteration (CIA) is defined by Nesbitt close to the southern basin edge and investigated the & Young (1982) as (100)[Al2O3/(Al2O3 + CaO + Na2O deep weathering and the sediment–basement transition. + K2O)], the Chemical Index of Weathering (CIW) by The primary rock is mainly granite and the weathered Harnois (1988) as (100)[Al2O3/(Al2O3 + CaO + Na2O)] material, which is found at the southern flank of the and the Plagioclase Index of Alteration (PIA) by Fedo et

Ramså Basin at the surface, is of greyish colour and al. (1995) as (100)[(Al2O3 – K2O)/(Al2O3 + CaO + Na2O – relatively clay-rich. It differs greatly from the ones K2O)]. The indices should be interpreted like this: Fresh observed at the three other locations in Lofoten– bedrock <= 50 and Totally weathered bedrock = 100. Vesterålen and Hamarøya and we could no longer recognise the fabric of the parent rock. Sturt et al. (1979) The principle behind the applied mass–balance analysis excavated a trench and studied the weathering profile at is described by Brimhall et al. (1985), Brimhall & Ramsåa. They found at least two different types of altered Dietrich (1987) and Chadwick et al. (1990), where the bedrock. They concluded that a reasonable amount of element composition of the saprolite is compared with weathered basement had been eroded from the nearby the fresh bedrock composition. This comparison allows mountains and deposited on top of the saprolite that a quantification of the degree of weathering that the most probably occurs in an in situ position. bedrock has been subjected to, and which of the major elements that have been leached. A major obstacle for this The selected localities were sampled for both fresh analysis is that saprolite undergoes changes in density bedrock and saprolite. Care was taken to extract and volume during the weathering process, which NORWEGIAN JOURNAL OF GEOLOGY Deep weathering, neotectonics and strandflat formation in Nordland, northern Norway 199 hampers a straight calculation of mass loss from the distances of tens to hundreds of kilometres. We have, fresh bedrock. To overcome this, an immobile element therefore, established a GPS network designed to

(usually Zr or the oxide TiO2) is used as a reference to investigate the possible presence of active strain in the which all other elements are compared (Brimhall et al., Ranafjorden area (Olesen et al., 1995). Three 15–20 1985; Brimhall & Dietrich, 1987; Chadwick et al., 1990). km-long profiles are located across outer, central and As long as the immobile element has a weathering rate inner Ranafjorden. If there was any active neotectonic that is an order of magnitude slower than other elements, deformation in the Ranafjorden area, as indicated by this method is assumed to yield reliable results. previously published land uplift observations, this could be recorded by GPS in less than a decade (Olesen et al., Electrical resistivity traversing (ERT) 1995). Similar networks have been established in Sweden (Talbot, 1992; Pan et al., 2001). Pan et al. (2001) have The 2D electric resistivity traversing (ERT) method reported differential horizontal displacements along the has been applied to study the depth of the border zone between the Fennoscandian Shield and the (Brönner et al., 2012b). Data were collected using a European lowland. cable system developed by the Institute of Technology of Lund University (Dahlin, 1993). The system consists The applied GPS software GAMIT/GLOBK makes of a relay box (Electrode Selector ES10–64 C) and of two use of the so-called Double Difference approach, or four multi-electrode cables. The ABEM Terrameter where a network of GPS stations is analysed in a single SAS 4000 (ABEM, 1999) resistivity instrument contains adjustment. A least-square adjustment is used for an integrated PC for full control of the data acquisition parameter estimation. This implies that parameters process and storage of data. In this survey, four cables which vary with time, for example the troposphere, were used with the gradient electrode configuration have to be estimated as piecewise linear parameters. The and 10 m electrode spacing. A maximum depth atmospheric zenith delay was estimated with a 2-hourly, range of about 120 m may be reached with the latter piecewise, linear model together with a daily troposphere configuration. The resolution decreases with depth and gradient. The analysis strategies included ocean-loading resistivity data deeper than c. 80 m are, by experience, less coefficients from the FES2004-model, and a VMF1 reliable. The entire system can be rolled along the profile mapping function to model trophospheric delay; and an such that the extent of a profile is almost unlimited. absolute antenna phase centre variation model was also used. We have used a cut-off elevation of 7°. All resistivity measurements give an apparent resistivity value that represents a weighted average resistivity which To give GPS velocities a physical meaning they have to be resulted from the resistivity of each heterogeneous volume realised in a reference frame. A reference frame is realised in the surroundings of the measurement points (note: as a list of station coordinates and velocities given rela- heterogeneous volume in terms of resistivity and size for tive to a given origo, with a given orientation and scale. the purpose of this study). To find the specific resistivity ITRF2005 is such a reference frame. To ensure the long- of each part of the heterogeneous investigated volume, the term stability of our results and to have a precise realisa- data are inverted. This is done by dividing the profile into tion of our velocities in the reference frame, the stations blocks, each characterised by specific resistivity values. in our GPS network were analysed together with a net- The resistivity values of the blocks are adjusted following work of permanent GPS stations with known coordinates an iterative procedure until the theoretical model fits the and velocities (Trondheim, Bodø, Andøya and , see measured data. The resistivity of the bedrock, which is Kierulf et al., 2013). Daily solutions for approximately five obtained by ERT, depends on the porosity, water saturation days in 1999 and five days in 2008 are combined to esti- and resistivity of groundwater in cracks and fracture mate velocities to the north and to the east. zones, as well as on the clay content that is produced by weathering and other geological phenomena. Results Resistivity measurements were inverted using the computer program RES2DINV (Loke, 2010). Different Chemical analysis methods of inversion are applied with variations in the inversion parameters, attenuation factors and In many samples from Nordland, the amount of clay filters (Brönner et al., 2012a). This did not result in any minerals was too small to be analysed with the present fundamental changes in the main features of the inverted XRD methodology. However, in some samples, primary profiles. clay minerals such as illite and chlorite were easily identified. Optical microscope inspection of material GPS network with a weathered appearance indicated that weathering- prone minerals such as feldspars were still looking quite By using static GPS (Global Positioning System), it fresh. A brownish tinge, indicating oxidation, in samples is possible to achieve a relative positioning between that had the most weathered appearance appears to two stations that is better than a few millimetres over be superficial. SEM studies of fine grains confirmed 200 O. Olesen et al. NORWEGIAN JOURNAL OF GEOLOGY the impression from optical microscopy that very Hadseløya and Hamarøya (Table 2). The Zr content was little chemical etching has been taking place in silica- used as a reference immobile element in the calculations rich minerals such as quartz and feldspars. However, (Brimhall et al., 1985) and a high mass loss of c. 64% was biotite quite commonly seems to have been altered to observed. The upper range can be considered as indicative vermiculite and smectite. of highly leached bedrock. Since optical microscopy and SEM studies show limited etching of silicate-rich minerals, Grain-size distribution in all investigated soil profiles is it appears likely that phyllosilicates (mainly biotite) and dominated by sand and gravel with very little silt and clay other ‘dark’ minerals such as hornblende and pyroxene (generally <5%). In some cases we also sampled glacial have been chemically altered. In some samples there are overlying the saprolite; these samples commonly signs of reprecipitation of oxides and hydroxides, which is contained more clay and silt, showing an incorporation in agreement with saprolite formation. of finer material from elsewhere. The observed saprolite in Nordland is mostly a clay-poor Weathering indices and mass balance calculations and brownish material. The large amounts of observed (degree of leaching) from XRF analysis, on the other deep-weathered material together with the indicated hand, commonly show a moderate degree of mineral even greater thickness of the existing regolith raise the alteration with weathering indices between 54 and 74, question about a possible maximum thickness of regolith and bulk leaching of the major elements (Si, Al, Na and, consequently, the possible thickness of a transition and K) in the range of 30–65%. The Ramså saprolite zone from soil or the sedimentary succession to reaches a very high degree of weathering whereas the unweathered basement. Moreover, what is the nature of grusy types generally show only a moderate degree the transition from fresh to weathered bedrock? Gerrard (Table 1). The degree of weathering of the samples from (1988) described this transition to be usually quite sharp Brennvinshaugen on Hadseløya is somewhat higher than for crystalline rocks like granite. We did not observe in the other grus–type localities. The mass balance was the actual boundaries at the main locations, but we calculated for both weathered and fresh mangerite on discovered some smaller exposures where the boundary

Table 1. Chemical weathering indices (CIA, CIW, PIA) of mangerites at Tortenåsen and Nævernes on Hamarøya and Brennvinshaugen on

Hadseløya calculated mainly from major element oxides (Price & Velbel, 2003), The Chemical Index of Alteration (CIA): (100)[Al2O3/

(Al2O3 + CaO + Na2O + K2O)], the Chemical Index of Weathering (CIW): (100)[Al2O3/(Al2O3 + CaO + Na2O)] and the Plagioclase

Index of Alteration (PIA):(100)[(Al2O3 – K2O)/(Al2O3 + CaO + Na2O – K2O)]. The indices should be interpreted like this: Fresh <= 50 and Totally weathered = 100.

Locality CIA CIW PIA Degree of weathering

Tortenåsen, E6 Hamarøya 59.8 73.5 65.6 Moderate Nævernes, Hamarøya 59.2 69.3 63.0 Moderate Nævernes, Hamarøya 54.3 59.7 55.2 Low Brennvinshaugen, Hadseløya 62.5 72.2 67.0 Moderate Ramsåa, Andøya 96.4 97.3 97.3 High

Table 2. Brimhall mass–balance calculations for weathered and fresh mangerites at Tortenåsen and Nævernes on Hamarøya and Brennvins­ haugen on Hadseløya. The loss of mass through chemical weathering is estimated at 43 to 67%. The Zr content has been used as a reference immobile element in the calculations (Brimhall et al., 1985). Sample SiO Al O Fe O TiO MgO CaO Na O K O MnO Zr Si+Al+Na+K SUM Locality 2 2 3 2 3 2 2 2 No (%) (%) (%) (%) (%) (%) (%) (%) (%) (ppm) (%) (%) Tortenåsen, E6 Hamarøya, 58959 -46.2 -9.2 -1.4 -0.2 -0.1 -0.5 -2.8 -3.9 0.0 819 -62.2 -64.3 saprolite Tortenåsen, E6 Hamarøya, 58990 0 0 0 0 0 0 0 0 0 310 0 0 fresh bedrock Nævernes, Hamarøya, 58967 -28.0 -7.0 -1.2 0.2 -0.3 -1.6 -2.2 -2.8 -0.1 1040 -40.0 -43.1 saprolite­ Nævernes, Hamarøya, 58993 0 0 0 0 0 0 0 0 0 653 0 0 fresh bedrock Brennvinshaugen, Hadseløya, 58970 -43.7 -12.8 -0.2 -0.1 -0.2 -1.0 -3.9 -5.0 0.0 235 -65.4 -67.0 saprolite Brennvinshaugen, Hadseløya, 58995 0 0 0 0 0 0 0 0 0 89 0 0 fresh bedrock NORWEGIAN JOURNAL OF GEOLOGY Deep weathering, neotectonics and strandflat formation in Nordland, northern Norway 201 was sharp (Fig. 4C), but most likely did not represent basement. The corestones of fresh basement became the actual base of the weathering. On the other hand, larger with depth, but they were still truncated by large from the resistivity profiles there was none that showed fracture zones with a significant low resistivity. a transition front above a continuous unweathered

(A) Boundary fresh/weathered rock NW Rise, Vestvågøya, Lofoten Fracture zone SE (A)Elev. 0 Boundary fresh/weathered rock 100NW Rise, Vestvågøya, Lofoten 160 Fracture zone SE Elev. 0 340 (A)100 50 500 Boundary fresh/weathered rock 160 660 820 980 1140 1300 1520 m (A)NW Rise, Vestvågøya, Lofoten Boundary fresh/weathered rock 0 340 Fracture zone SE Elev.50 500 NW0 Rise, Vestvågøya, Lofoten 660 820 980 1140 1300 1520 m 100 Fracture zone SE Elev. 0 160 100-500 160 340 50 500 660 820 980 1140 1300 1520 m -100-50 340 50 500 0 660 820 980 1140 1300 1520 m -100-1500 -50m Vertical exaggeration: 1.5 1 5 15 65 250 1000 4100 16000 ohm·m -150-50 (B)-100 m Vertical exaggeration: 1.5 1 5 15 65 250 1000 4100 16000 ohm·m -100 Ramsåa, Southern edge of the Ramså (B)-150 S Andøya, Vesterålen Basin from geological map N m -150 Ramsåa, Southern edge of the Ramså Vertical exaggeration: 1.5 Elev. 0 1 260 5 15 580 65 910 250 1000 1230 4100 160001550 ohm·m 1880 2360 m mS Andøya, Vesterålen Basin from geological map Vertical exaggeration: 1.5N (B) 0 1 5 15 65 250 1000 4100 16000 ohm·m Elev. 0 260 580 (B)-40 Ramsåa, Southern edge of910 the Ramså 1230 1550 1880 2360 m 0 Basin from geological map -80S RamsåaAndøya,, Vesterålen Southern edge of the Ramså N Basin from geological map Elev.-40S 0 Andøya 260, Vesterålen 580 N -120 910 1230 1550 1880 2360 m Elev.-800 0 260 580 910 1230 1550 1880 2360 m -160 -120-40m0 Vertical exaggeration: 2 1 5 15 65 250 1000 4100 16000 ohm·m -160-80-40 (C)m -80 Transition zone containing weathered/ Vertical exaggeration: 2 -120SW 1 5 15 65 fractured 250 basement 1000 and sediments4100 16000on ohm·m Elev.. NE (C)-120 Brennvinshaugen, top -160350SW Transition zone containing weathered/ m Hadseløya , Vesterålen fractured basement and sediments on Elev..-160300 Vertical exaggeration:2040 m 2 NE m Brennvinshaugen1 5 15, 65 250 1000top 4100 16000 ohm·m (C)350 1620 Vertical exaggeration: 2 250 Hadseløya1 5 , Vesterålen 15 65 Transition 250 zone 1000 containing 4100 weathered/ 16000 ohm·m (C)300SW 2040 m fractured basement and sediments1300 on NE Elev..200SW Transition zone containing weathered/ 1620 350250 Brennvinshaugen , fractured basement980 top and sediments on Elev.. NE 150 HadseløyaBrennvinshaugen, Vesterålen, 660 top 1300 300200350 2040 m 340 100 0 Hadseløya, Vesterålen 980 300 1620 2040 m 250150 660 1620 50 340 1300 200100250 0 0 980 1300 200 15050 660 980 -50 150 340 660 100m0 0 Vertical exaggeration: 1.5 0 340 100-5050 100 225 500 1100 2500 5800 13000 29200 ohm·m m Vertical exaggeration: 1.5 (D)500 100 225 500 1100 2500 5800 13000 29200 ohm·m NW0 Nævernes, Hamarøya SE (D)Elev.-50 m 100 310 470 630 790 960 1120 Vertical 1280 exaggeration: 1480 m 1.5 -5080NW Nævernes, Hamarøya m 100 225 500 1100 2500 5800 13000 29200 ohm·m Vertical exaggeration: 1.5SE Elev.40 (D)80 100 100 225 310 500 470 1100 630 2500 790 5800 13000 960 29200 1120 ohm·m 1280 1480 m 0 (D)40NW Nævernes, Hamarøya SE Elev.NW Nævernes, Hamarøya -4080 100 310 470 630 790 960 1120 1280 1480 m SE Elev.0 80 100 310 470 630 790 960 1120 1280 1480 m -8040 -40 40m 0 Vertical exaggeration: 1.5 -80 m0 -40 100 225 500 1100 2500 5800 13000 29200 ohm·m Vertical exaggeration: 1.5 -40 -80 100 225 500 1100 2500 5800 13000 29200 ohm·m -80m m Vertical exaggeration: 1.5 100 225 500 1100 2500 5800 13000 29200 ohm·m Vertical exaggeration: 1.5 100 225 500 1100 2500 5800 13000 29200 ohm·m

Figure 8. Resistivity profiles at four different locations in Nordland: (A) Rise at Vestvågøya in Lofoten, (B) southern margin of the Ramså Basin at Andøya in Vesterålen. (C) Brennvinshaugen at Hadseløya in Vesterålen and (D) Nævernes on Hamarøya. The vertical lines indicate possible faults, represented by vertical low-resistivity zones. The horizontal line shows a top of unweathered bedrock interpretation (often referred to as rockhead) The hatched area in (B) marks a transition zone with possibly saprolite and/or sedimentary rocks. The deeply weathered bedrock extends typically to a depth of more than 50 m and is continuous for a distance of more than 1 km along the profiles. The weathering is more than 100 m deep along numerous steeply dipping fracture zones. Deeply weathered bedrock has been observed at all four locations. The locations of the profiles are shown in Figs. 2 & 3. 202 O. Olesen et al. NORWEGIAN JOURNAL OF GEOLOGY

Electrical resistivity traversing quarry in weathered mangerite. The layer is underlain by blocks of high resistivity with a rather irregular surface We have acquired resistivity profiles and samples for truncated by at least five steep zones with low resistivity. geochemical and mineralogical analysis at four sites in The quarry is situated at c. 450 m and coincides with one the Lofoten–Vesterålen and Hamarøya areas (Brönner et of these vertical low-resistivity zones. The high-resistivity al., 2012a; Olesen et al., 2012). The interpreted rockhead values most likely represent bedrock that is unaltered or is marked as dashed lines in the profiles in Fig. 8. The little affected by weathering. The irregular surface of the resistivity data show a 20–100 m-thick layer of mostly high-resistivity bodies along Profile 3 (Fig. 8C) probably low resistivity in all four areas. Different vertical low- reflects the transition of saprolite/saprorock to barely resistivity zones were marked as possible faults where the fractured and unweathered basement. The zones of saprolite is interpreted to extend to a depth of more than low resistivity in between represent larger fractures and 100 m (Fig. 8). Seismic-refraction data at Vestvågøya weakness zones probably filled with deeply weathered and Andøya support the conclusion of deep weathering rock. Except for some features where the resistivity on the strandflat as well as on the pala eosurfaces at contrast is large and obviously related to fracturing and an altitude of 300–500 m (Hillestad & Melleby, 1967; advanced weathering, the variations could simply be Hillestad, 1987; Brönner et al., 2012a; Olesen et al., 2012). caused by variations in fluid saturation, but – and this seems to be more likely regarding the observed large Resistivity and seismic profiling were also carried out at amount of saprolite – it could also indicate fractured Rise on Vestvågøya and indicate a thickness of weathered bedrock. Previous measurements of seismic velocity and fractured basement of more than 100 m (Fig. 8A). on fresh rock samples in the Lofoten–Vesterålen area The primary bedrock type is mangerite. The observed (Chroston & Brooks, 1989) are significantly higher than weathered basement shows both physical and chemical seismic velocities deduced from both shallow and deep weathering and is of a clay-poor sandy to gritty nature. refraction experiments (Sellevoll, 1983; Brönner et al., The colours are brown to black with a metallic glint. 2012a), revealing that the crust in the Lofoten–Vesterålen The saprolite is very similar to the one observed on is highly fractured. A high fracture density could help to Hadseløya and Hamarøya. A locality 4 km to the north of accelerate weathering downwards. Rise (between Saup and Tangstad) was sampled and the porosity and permeability of the sample were measured The thickness of observed weathered basement in the to 24.3% and c. 300 mD (Table 3), respectively (at 20 bar Ramså Basin is up to 30 m (Midbøe, 2011) but resistivity NCP/20°C) (Brönner et al., 2012a). data along two of the six Ramså profiles (Brönner et al., 2012a) indicate a much thicker regolith. The The Andøya location is in one of the best developed boundary of the basin can be observed on three of the strandflats in Norway (Fig. 3C). The studied location is six resistivity profiles as a rather sharp and significant situated in northeast Andøya in the vicinity of the Ramså resistivity contrast. The ERT data show that the Ramså Basin, the only known Mesozoic basin onshore Norway. Basin extends farther to the southeast than shown on Deeply weathered material can be found exposed on the bedrock geological map by Henningsen & Tveten the beach and also from well cores which were drilled (1998). Potential Mesozoic sedimentary rocks are through the sedimentary succession of the Ramså Basin characterised by rather low resistivity. The Ramså profile into the basement (Midbøe, 2011). The weathered layer (Fig. 8B) shows a transition zone, where the resistivity beneath the basin is overlain by Jurassic and Cretaceous seems to change gradually and rather irregularly, which sedimentary rocks. The burial of the weathered basement might indicate thick packages of deeply weathered below Mesozoic sediments proves the weathering to basement on top of fractured basement, possibly with be rather old with a minimum Mesozoic age (Dalland, layers of Mesozoic sediments on top. A noticeable 1975), but K–Ar age dating indicated an older, Late structural change at c. 800 m correlates with the basin Palaeozoic age (Sturt et al., 1979). boundary on the geological map. Southwards from here, a high-resistivity layer with a rather irregular surface The ERT profile on Hadseløya (Fig. 8C) is oriented NE– is observed, overlain by an up to c. 70 m-thick low- SW along the road to Storheia that runs parallel to a resistivity sequence.

Table 3. Analysis of permeability and porosity (at 20 bar NCP/20°C) of a sample from a locality between Saup and Tangstad at Vestvågøya. The measurements were carried out at Weatherfords laboratory in Stavanger.

Klinkenberg corr. Pore volume Porosity gas permea­ Grain volume Grain density Length Diameter 20 bar 2 bar Sample id. bility 20 bar atm/20°C atm/200°C NCP/20°C NCP/20°C (cm) (cm) NCP/20°C (ml) (g/ml-1) (ml) (g/ml-1) (mD) Saup-2 2.20 3.60 291 4.71 14.71 2.67 0.243 NORWEGIAN JOURNAL OF GEOLOGY Deep weathering, neotectonics and strandflat formation in Nordland, northern Norway 203

The different characteristics of the two exposed deep- by a number of different geophysical phenomena weathering profiles, with a rather homogeneous saprolite causing the Earth’s crust to deform. The most dominant and only a few corestones on Hadseløya and a rather processes are plate tectonic and glacial isostatic dense distribution of corestones with granular weathered adjustments. To study a local phenomenon such as material in between on Hamarøya, could be interpreted neotectonic deformation in the Ranafjorden area, we are as two insights into different stages or depths of the only interested in the local relative movement, not the regolith (Brönner et al., 2010). Whilst Hadseløya shows a regional or large-scale movements due to plate tectonic sequence within the saprolite, the exposure on Hamarøya and glacial isostatic adjustments. Hence, we look at represents a deep section, and is within or close to the velocities relative to the southernmost station in the saprorock and consequently closer to the unweathered network (NE08). To do this we subtract the velocity for basement rock. It remains unclear whether or not the rate this station (14.18 mm yr-1, 15.24 mm yr-1 and 7.37 mm of erosion on Hamarøya was higher or the weathering yr-1 in the east, north and upward direction, respectively). process was slower and/or initiated at a later time. The relative horizontal velocities are included in Table 4 and plotted in Fig. 5. Based on our observations, we conclude that there are still thick piles of weathered basement rocks along the To estimate reliable uncertainties for GPS velocities based Nordland coast and especially on the Lofoten–Vesterålen on campaign measurements is not a straightforward task. islands, but also on the mainland such as at Hamarøya, The uncertainties included in Table 4 are based on the Misvær and Jektvik (Figs. 2, 4, 8). Deep weathering of assumption that the measurements have no temporal pyroxenite is reported from Misvær (located 30 km correlation. Earlier studies have shown a significant southeast of Bodø) by Ihlen (2008). All observed, major temporal correlation in GPS time series (e.g., Mao deep-weathering locations occur in rather flat, low-lying et al., 1999). Williams et al. (2004) studied the noise areas where erosion is considered to be low to moderate. characteristics in detail and found that a combination of Considering a generally much higher amount of erosion white noise and flicker noise is appropriate for most GPS in the mountainous areas where glacial erosion occurs time series. However, it is not possible to find the noise to a large degree as plucking, deep weathering is likely to characteristics with only a few days of campaign data. still have considerable impact on the topography of the Norwegian landscape. We have instead examined how the temporal correlation increases the uncertainty estimates by simulating GPS campaigns with permanent GPS stations in the area. The RMSs for the these permanent stations series are 1.3– The velocities estimated in this study are realised in 1.8 mm in the east component, 1.3–1.4 mm in the north ITRF2005. ITRF2005 is a global reference frame affected component and 3.1–3.3 mm in the height component.

Table 4. Results from GPS measurements in local coordinates. The velocities are relative to the station NE08. The velocity of NE08 in the global reference frame ITRF2005 is 14.18 mm yr-1 in the east and 15.24 mm yr-1 in the north component. Five of the original benchmarks in the inner Ranafjorden area had been destroyed during the 1994–2008 period. The uncertainties are the one sigma values. E-velocity N-velocity ID Longitude Latitude mm yr-1 mm yr-1 ne03 14.1147536 66.3306682 0.26 ± 0.10 -0.19 ± 0.10 ne05 14.0519408 66.3587042 0.20 ± 0.15 0.23 ± 0.20 ne08 13.6829771 66.1762583 0.00 ± 0.09 0.00 ± 0.10 ne09 13.6007343 66.2251341 0.14 ± 0.07 0.75 ± 0.09 ne10 13.5810583 66.2662831 0.12 ± 0.11 0.10 ± 0.13 ne11 13.6278273 66.2877472 0.23 ± 0.10 0.41 ± 0.12 ne12 13.6575577 66.3273056 -0.42 ± 0.07 0.67 ± 0.08 ne13 13.0789199 66.1529632 -0.18 ± 0.08 0.38 ± 0.09 ne14 13.0137884 66.1893526 -0.34 ± 0.08 0.18 ± 0.09 ne15 12.9660966 66.2001071 -0.62 ± 0.08 0.04 ± 0.09 ne16 12.9561028 66.2279716 -0.42 ± 0.09 0.18 ± 0.11 ne17 12.8933904 66.2716375 -0.60 ± 0.07 0.86 ± 0.08 ne18 12.9989946 66.3385106 -0.27 ± 0.09 0.55 ± 0.11 ne19 13.2135926 66.5046635 -0.53 ± 0.09 0.62 ± 0.11 ne20 13.1445155 66.4001871 -0.84 ± 0.09 0.53 ± 0.10 204 O. Olesen et al. NORWEGIAN JOURNAL OF GEOLOGY

The corresponding RMSs for the mean of five time. The large percentage of leached minerals on consecutive days are 0.9–1.3 mm, 0.9–1.0 mm and 1.9– Hamarøya and Hadseløya could also suggest a fairly old 2.2 mm, respectively. The RMSs are reduced by a factor age for this type of weathering. This is intriguing since of approximately 1.4, 1.4 and 1.6, respectively. If the time this kind of clay-poor weathering has commonly been series contained only white noise, the improvement for considered to be rather young, e.g., Cenozoic (Migon, a five-day campaign relative to daily measurements 1997). Grusy saprolites can also be found in the Småland will have a theoretical factor of 2.2 (square root of 5). area in southern Sweden. These are associated with little In other words, the temporal correlation reduces the mass loss, mechanical disintegration, and an almost improvements from using data from five consecutive complete absence of clay (Lidmar-Bergström et al., 1997). days by a factor of 1.6, 1.6 and 1.4, respectively. If we These saprolites are found in a landscape correlated with assume the same influence from temporal correlation etching and erosion of the sub- peneplain on our campaign measurements as for the permanent during the Tertiary (Lidmar-Bergström 1995), and the stations, our uncertainty estimates for the velocities are weathering occurred most likely during the Tertiary in too optimistic by a factor between 1.4 and 1.6. a cooler climate. Similar grus weathering along the coast of southern Norway (Lista) and on the Utsira High has, An additional problem occurs for campaign networks however, been dated to Late Triassic (Ineson et al., 1978 with only two campaigns. We have no possibility to detect and Fredin et al., 2012a, respectively) implying that this small changes in the campaign setup, neither differences interpretation has to be reconsidered. in the equipment nor in the local electromagnetic environment, that might cause a coordinate offset. The XRF data show that leaching processes have However, the GPS antennas are mounted directly on occurred at all four localities in Nordland. The XRD the ground with permanent screw bolts and the stations analysis, however, is ambiguous, with analytical were equipped with the same type of antennas both problems involved with the clay-poor samples. There in 1999 and in 2008. These factors reduce the risk of are definite signs of alteration of biotite into vermiculite artificial coordinate changes to a minimum and are and smectite, and at some localities small amounts of negligible for the horizontal components. We have opted feldspar have been altered into kaolinite. This process to use only the horizontal components in the rest of this causes expansion and exerts pressure on the surrounding study because of the larger uncertainties in the vertical mineral grains. The process can be likened to physical component. weathering causing disintegration of the rock into a gravelly-sandy saprolite (Nesbitt & Young, 1989). This The GPS campaign measurements in 1999 and 2008 disintegrated rock (gravelly-sandy saprolite), in turn, is indicate that the benchmarks along the western profile then much more porous and rainwater can penetrate far have moved c. 1 mm yr-1 to the NW and W relative to the down into the profile causing chemical alteration mainly stations along the two eastern profiles (Fig. 5 and Table 4). of the mafic minerals. In principle, the characteristics of our samples could be the result of recent weathering under the present-day climatic conditions. It is thus Discussion tempting to assign a Plio–Pleistocene age to most of these saprolites, with the exception of the sub-Jurassic Deep weathering saprolite at Ramsåa. An argument against a recent age (i.e., Quaternary?) could be the actual penetration of Weathering of rocks during different climatic conditions weathering down into the crystalline basement, which is rather complex and is poorly understood. The initial in all four ERT locations in Nordland is in the order trigger for the weathering, the actual type and finally the of several tens of metres. It may consequently appear weathering rate are commonly considered to depend difficult to assign a definitive age to the weathering in on several external factors such as climate, temperature, Norway based on weathering characteristics, position humidity and CO2 content in the atmosphere (Riebe in the or bedrock type. The most compelling et al., 2004; West et al., 2005; Anderson et al., 2007; regional correlation that indicates that the saprolites Brantley et al., 2007), but also on geotectonic processes might have been formed in Mesozoic time is correlation such as uplift and erosion (Raymo & Ruddiman, 1992; of ‘envelope surfaces’ of topography and the fact that Amundson et al., 2007; Rasmussen et al., 2011). The clay these dip down beneath Mesozoic strata offshore mineral content of the samples from the studied locations Norway (e.g., Riis 1996; Lidmar-Bergström et al., 2007). in Nordland was too low to provide a hint of the chemical At several locations in South Sweden and , processes involved in the possible climatic conditions, this relationship can also be observed onshore (Lidmar- which could otherwise have pointed to a specific epoch. Bergström et al., 1999). Most of these localities exhibit Paasche et al. (2006) and Strømsøe & Paasche (2011) extensive saprolite profiles with clay-rich kaolinitic have, however, reported gibbsite and kaolinite from deep weathering with, in places, pure kaolinite and quartz weathering in the Vesterålen area, indicating formation sand (Lidmar-Bergström et al., 1997), indicative of during a warmer and more humid climate as that intense leaching of feldspars in a hot and humid climate reported in Scandinavia during most of Plio–Pleistocene with acidic meteoric water. These saprolites are found in NORWEGIAN JOURNAL OF GEOLOGY Deep weathering, neotectonics and strandflat formation in Nordland, northern Norway 205 landscapes termed as ‘Sub-Cretaceous undulating hilly The volume of sediments deposited along the continental relief’ or ‘Sub-Cretaceous joint valley landscape’. It is margin in the Plio–Pleistocene Naust Formation has worth noting that grus weathering may also constitute a been well mapped during the last decade (Fig. 2; Rise deep section in a generalised tropical weathering profile et al., 2005; Dowdeswell et al., 2010) and can be used to (Acworth, 1987). The total thickness of such profiles may constrain the amount and timing of onshore erosion. reach 100 m in normal bedrock but may be significantly Average sedimentation rates during the last are thicker along regional fracture zones. The uppermost estimated to have been ~0.24 m kyr-1 with 2–3 times clay-rich horizons may have been removed by erosion higher rates for the most recent 600 kyr (Eidvin et al., from the studied locations in Nordland. The Quaternary 2000; Dowdeswell et al., 2010). The mean sediment overburden in the northern Nordland area may to a delivery was 2–3 times higher for the most recent 600 ky large degree represent a glacially reworked saprolite. The than for the period 0.6–2.7 Ma. The weathered bedrock observed anomalous high concentrations of REEs and in Lofoten–Vesterålen can be found up to an altitude of heavy metals such as Mo, Pb and Zn in till in northern c. 500 m above sea level (Peulvast, 1985, 1986; Paasche et Nordland (Finne & Eggen, 2013) can partly be the result al., 2006; Strømsøe & Paasche, 2011). This observation of a weathering process where major elements such as K, is compatible with the conclusions by Dowdeswell et al. Na and Ca have been partly removed by leaching. (2010) of a mainly Pleistocene erosion of ~500 m along the coast of Nordland. The observation contradicts, on Another argument for an Early Mesozoic age of the grus the other hand, the bimodal, Plio–Pleistocene glacial weathering in the Lofoten–Vesterålen is the low degree erosion model proposed by Steer et al. (2012), who of weathering of the and Cretaceous faults suggested that the glacial erosion occurred on the high- in the area. The faults at Rødsanden that constitute parts altitude low-relief surfaces in Scandinavia and in the of the border fault of the Sortlandsundet Basin show, for fjords. These authors did not include any major glacial instance, a low degree of alteration (Osmundsen et al., erosion along the coast of Norway in their model. Their 2010) whereas the hangingwall block of the same fault model is thus incompatible with other geomorphological on Hadseløya has been weathered to a depth of 20–100 observations in Norway (Lidmar-Bergström & Bonow, m over several square kilometres. 2009).

One possible explanation for the ambiguous Neotectonics characteristics of the investigated saprolites is that they have a complex genesis. It appears likely that extensive The present-day seismicity provides a means to estimate weathering of the Scandinavian landmass commenced the timing of Neogene erosion and sedimentation and, in Mesozoic time, possibly as clay-rich, kaolinitic, deep- consequently, the exhumation of a pre-existing deep weathering zones. By latest Cretaceous time the saprolites weathering profile. Fig. 1 shows areas of interpreted Plio– were covered by sedimentary strata and were thus Pleistocene deposition and erosion and Neogene domes protected. Later on, possibly as late as during the Plio– along the Norwegian continental margin, together Pleistocene uplift and erosion events, most of the cover with earthquakes and post-glacial faults (Olesen et al., rocks and saprolites were stripped from the bedrock and 2013). The major glaciations in Fennoscandia during a rejuvenation of bedrock weathering could take place. the last 600,000 years extended all the way to the The occurrence of gibbsite and kaolinite in the grus-type continental margin. One dominant feature is the late weathering on Hadseløya and Langøya in the Vesterålen Plio–Pleistocene Bjørnøya Fan, which is 400 km wide area reported by Paasche et al. (2006) and Strømsøe & and up to 3.5 km thick and is, moreover, co-located Paasche (2011) supports this interpretation. The potential, with seismic activity that is higher than anywhere else younger new weathering episode could have taken place in Fennoscandia (Byrkjeland et al., 2000; Fejerskov & in 1) a cooler climate and 2) in bedrock already weakened Lindholm, 2000), even though it is located in oceanic by the previous weathering episode. Both of these factors crust which normally is aseismic. The stress is expected favour plain oxidation of biotite (cool climate and matrix to be compressional under the load and tensional features permeability in vadose zones and formation of grusy are noted on the continental flanks (Stein et al., 1989), saprolites). During the Quaternary, most of these newly as also modelled by Engen & Watts (2008). Along the formed saprolites were once more stripped, except for coast of northern Norway there is also a parallel, shallow pockets shielded by topographic effects or non-erosive, seismicity delineation which largely reflects extensional cold-based ice sheets. The Nordland landscape, wherein stress conditions (Fjeldskaar et al., 2000; Hicks et al., the saprolites remain, could thus have a Mesozoic origin 2000a, b; Bungum et al., 2010). There are, however, also but with strong imprint of erosive processes in the Late some strike-slip earthquakes here, with coast-parallel Cenozoic. Most of the investigated saprolites appear compressions. young but their formation was preconditioned by processes in the Mesozoic. This also suggests that it will be Because very recent erosion has taken place in the very difficult to obtain an absolute age for the weathering Vestfjorden area (Riis, 1996), the crust in the greater events, since they are interlocked with each other but of Helgeland–Lofoten region may be flexured due different character. to isostatic rebound, which would again result in 206 O. Olesen et al. NORWEGIAN JOURNAL OF GEOLOGY coast-perpendicular extension as shown by Atakan et column, indicating open fractures (Chand et al., 2012). al. (1994), Bungum et al. (1979) and Hicks et al. (2000a) These open fractures resemble the vertical fractures in the Steigen, Meløy and Rana districts, respectively. observed on mainland Nordland (Figs. 6, 7) which are Shallow seismicity with extensional (flexural) most likely also related to Pleistocene unloading (Olesen mechanisms in the coastal regions of Nordland has earlier et al., 2004, 2013). been associated with glacioisostatic adjustments (Hicks et al., 2000a; Bungum et al., 2005). Considering that the Tectonic uplift inferred from marine sediments Plio–Pleistocene loading of the relatively stiff oceanic along the coast of western Norway may be explained by crust causes relatively high seismicity in the Norwegian unloading of the coastal area of Norway. Loading of the Sea, it is also likely that a comparable unloading of the outer shelf and deep water areas in the is coastal areas in western and northern Norway may induce also a likely contributor to this tectonic uplift along the earthquake activity (Olesen et al., 2013). This scenario Norwegian coast. is likely because continental crust is generally weaker than oceanic crust. Figs. 1 & 2 show that the seismicity Strandflat formation in North Norway is to a large extent located in areas of either Plio–Pleistocene erosion or sedimentation. One Resistivity measurements and gravel pits indicate major exception is the Barents Sea region, which is almost that weathering is extending to much greater depths devoid of earthquakes. This phenomenon may be related (commonly 20–100 m) than previous studies (Peulvast, to a low density of seismographs in the greater Barents 1985, 1986; Paasche et al., 2006; Strømsøe & Paasche, Sea region. An alternative explanation is the relatively 2011) have revealed from trenching (2–8 m). The wide area of erosion and uniform unloading, thus not strandflat can represent an old weathering surface that creating sufficient differential stress in the crust (Bungum has been exhumed during Plio–Pleistocene erosion. et al., 2010; Olesen et al., 2013). The numerous, shallow, Mesozoic basins ( et al., 1997; Bøe et al., 2005, 2008, 2010) occurring on the N–S-trending, steeply dipping fractures and faults occur strandflat support this conclusion. The large-scale in the Ranafjorden area. There seem to be vertical offsets present-day topography can consequently be considered of the bedrock surface across these structures (Figs. quite old; outcrops of deeply weathered basement 6, 7). The foliation of the mica schist is subparallel to rocks in the Vestfjorden and Ranafjorden areas, for the bedrock surface. Field inspection revealed that the example, may therefore represent a primary inheritance features are probably of erosional origin since the scarps from the Mesozoic (Olesen et al., 2012). We suggest seem to have been sculptured by the moving inland ice. that the deep weathering in the Hamarøya, Lofoten No offsets of marine sediments have been observed on and Vesterålen areas is preserved because of the Late reflection-seismic profiles in the Sjona or Ranafjorden Pleistocene uplift and erosion of this area. Most of the (Longva et al., 1998; Lyså et al., 2004). Monitoring of ice was transported in ice streams through Vestfjorden the seismicity in the area, however, has shown that more and (Ottesen et al., 2005; Dowdeswell et than 20 earthquakes occurred along one of these fracture al., 2010) leaving the interior of the mainland and the zones in 1998, indicating that the fault is active at depth Lofoten–Vesterålen archipelago relatively unaffected. (Hicks et al., 2000a). Some of the earthquake clusters The inland ice in this area was most likely thin and in the Handnesøya and Sjona areas are located along the erosion was concentrated to local . Some NNW–SSE-trending fracture zones with escarpments of the palaeosurfaces on the uplifted and rotated fault facing to the west. The relatively low seismicity occurring blocks were therefore relatively undisturbed. The little– at a depth of 2–12 km could, therefore, have created the consolidated Mesozoic rocks were easily removed by the observed irregular subsidence pattern at the surface, large amounts of meltwater following each glacial cycle. which was most likely formed by glacial plucking of This interpretation is also consistent with the observed the bedrock along the fractures by the moving inland deep weathering in the mountainous areas of Hadseløya, ice. Ice-plucking features may be indirectly related to Langøya and Andøya. An older Mid Miocene to Early neotectonics. Moving in the Canadian Cordillera uplift phase may have occurred earlier during tend to pluck bedrock along extensional fractures the deposition of the Utsira and Molo formations (Riis, parallel to the direction of maximum horizontal stress 1996; Eidvin et al., 2007; Rise et al., 2013). (Bell & Eisbacher, 1995). The Pleistocene glaciers could, in a similar way, have had a higher degree of bedrock A similar model is also applicable for the Oslofjord– plucking by basal shear along favourably oriented Sørlandet region where there is an abundance of reported fractures in areas with highly anisotropic rock stress. locations of deep weathering (Lidmar-Bergström et al., 1999; Olesen et al., 2007, 2012; Fredin et al., 2012b). Chand et al. (2012) showed that unloading due to erosion The erosion of the Mesozoic cover in this region was and in the SW Barents Sea resulted in the also dependent on the means of transportation. The ice opening of pre-existing faults and creation of new faults, stream in the Norwegian Channel that was established in facilitating fluid migration and eventual escape into the the Late Pleistocene provided both the erosion agent and water from the subsurface. This is, e.g., expressed as the transportation mechanism for the eroded Mesozoic pockmarks, gas hydrates, and even gas flares in the water NORWEGIAN JOURNAL OF GEOLOGY Deep weathering, neotectonics and strandflat formation in Nordland, northern Norway 207 rocks which accumulated as sedimentary wedges nappes, are more resistant to weathering and constitute along the Møre margin (the Fan). The main the mountainous areas along the Nordland coast. Slates difference between the Oslofjord–Sørlandet and the and that are more susceptible to weathering are Vestfjorden–Andfjorden regions is the location relative mostly found on the strandflats, along the fjords and in to the active rifting in the North Sea and the Norwegian the inland valleys (S. Gjelle, pers. comm., 2013). The Sea. The greater Vestfjorden region constitutes a part of formation of the strandflats, mountains and fjords due the Mesozoic rift system with an abundance of uplifted to an exhumation of a deeply weathered bedrock in the and rotated fault blocks (Brekke & Riis, 1987; Løseth & Pleistocene is shown in Fig. 9. Tveten, 1996; Olesen et al., 1997, 2002; Bergh et al., 2007; Hansen et al., 2012), whilst the Oslofjord–Sørlandet Mapping of the deeply weathered bedrock on the region was located more remotely to the southeast. strandflat along the coast of Nordland indicates that Consequently, we find the remnants of deep weathering this mostly horizontal surface close to sea level was on rotated fault blocks in Lofoten–Vesterålen whereas originally an exhumed pre-Quaternary surface. The similar regoliths occur in the relatively gentle landscape deeply weathered and fractured basement rocks could along the coast of southern Norway. have facilitated effective glacial erosion during the c. 40 glaciations in the Pleistocene, as illustrated in Fig. 9. A Triassic or Jurassic age for the exhumed surface in Freezing and thawing combined with abrasion by ocean Nordland is consequently possible. The occurrence of waves during fluctuating sea levels in non-glaciated downfaulted Jurassic rocks to the east of the Vestfjorden periods would have assisted in the formation of a Basin in the Træna–Meløy area (Bøe et al., 2008) suggests relatively wide strandflat along the coast from the presence of a boundary between basement rocks to western Finnmark. and overlying Mesozoic sediments which was situated not far above the present surface. Similar rocks located Lidmar-Bergström (1995) and Riis (1996) concluded immediately to the east of the Øygarden Fault Zone that the southern Norwegian plateau was partly uplifted on Bjorøy south of (Fossen et al., 1997), in the in the Neogene, whereas the northern Scandinavian Griptarane area to the west of Smøla (Bøe & Skilbrei, mountains (northern Nordryggen) originated mainly as 1998) and in the Frøya–Frohavet area (Bøe et al., 2005, a rift-shoulder in Late Cretaceous to Early Tertiary times. 2010) support this conclusion, which was also pointed out In this regard, Hendriks & Andriessen (2002) reported by Holtedahl (1998). Most of the Mid–Late Jurassic basins that apatite fission-track analytical data along a profile are interpreted to represent the remains of a much more from Lofoten into northern Sweden fit best with those extensive Jurassic–Cretaceous sedimentary succession expected from a retreating scarp model. The results are in that covered large parts of coastal Norway (Bøe et al., agreement with the conclusions from isostatic modelling 2010). These sedimentary rocks were down-faulted by Olesen et al. (2002) and Ebbing & Olesen (2005), during tectonic activity in Late Jurassic–Early Cretaceous indicating that the mountains in northern Norway are times, thus escaping later erosion. Bøe et al. (2005) supported by low-density masses at a relatively shallow observed extensive weathering of the fault rocks and the depth in the upper crust. The gravity field in the southern Early Palaeozoic igneous rocks within the Frøyfjorden Nordryggen mountains seems, on the other hand, to be fault zone between and Frøya. Lauritzen et al. (2011) compensated by low-density rocks at typical Moho and have further reported that deep weathering has facilitated sub-Moho depths. the formation of coastal caves along the Norwegian coast (e.g., the Torghatten cave in Nordland). These reports support the conclusion that the Norwegian strandflat is Conclusions an exhumed, weathered and peneplaned surface of Late Triassic to Early Jurassic age that has been modified and 1) Geophysical studies (2D resistivity and refraction levelled during Plio–Pleistocene erosion. The surface seismic) reveal continuous saprolite layers with a was preserved beneath Late Jurassic and Cretaceous thickness of more than 100 m in Lofoten–Vesterålen sedimentary rocks until the Neogene, as illustrated in Fig. and Hamarøya. This conclusion is supported by 9. This conclusion is also supported by the occurrence of observations of clay-rich fracture zones in a 100 a relatively flat and gently westward-dipping basement m-deep road tunnel in Lofoten. surface beneath the Jurassic sedimentary rocks close to the Norwegian coast (Fossen et al., 1997; Fossen, 1998). 2) XRF analysis and mass balance calculations show a The relatively high mountains on the Nordland standflat moderate to high degree of mineral alteration with (e.g., Trollvasstinden [Vega] 799 m, Lovundfjellet weathering indices ranging from 54 to 74 and a [Lovund] 623 m, Trænstaven [Træna] 338 m, and bulk leaching of the major elements (Si, Al, Na and Rypdalstiden [Landegode] 802 m) most likely represented K) in the range of 30–65%. The upper range can be inselbergs on the Mesozoic peneplain. The granitoid rocks considered indicative of intense weathering. within the tectonic basement windows (Svartisen, Sjona and Høgtuva) and in the Bindal Batholith, in addition 3) New GPS data in the Ranafjorden region are to mica gneisses and quartzites within the Caledonian consistent with previous datasets such as DInSAR, 208 O. Olesen et al. NORWEGIAN JOURNAL OF GEOLOGY

Figure 9. Conceptual model for strandflat develop- ment along the Nordland coast from the Triassic to the present day. (A) Triassic–Jurassic subtropical weathering (etching) across the entire palaeosurface with deeper penetration into pre-existing fracture zones. Subsequent block-faulting in the Late Jurassic. The palaeolandscape is partly adapted from the Eas- tern Highlands and western slopes of Australia (Tay- + + + + + + + lor, 2008). (B) Preservation of chemical weathering + + + + + + + below shales and carbonate rocks deposited during the + + + + + + + Late Jurassic to Cretaceous transgression. (C) Uplift + + + + + + + + + and erosion during the Neogene. The bulk of the che- + + + + mical weathering was removed leaving an immature etch-surface landscape with joints and faults as vall- eys, sounds and fjords. Some of the valleys have been ((A)A) excavated by the glaciers to form fjords. Remnants of the old weathering surface are left as strandflats, often Triassic-Jurassic Triassic- occurring as rims of low land around islands and Jurassic peninsulas. The cirques in the mountainous areas are also formed by glacial erosion. The clay zones contai- ning smectite and kaolinite were preserved at depth along the fracture zones (modified after Lidmar- Bergström, 1995; Olesen et al., 2007; Taylor, 2008).

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

(B)(B) LaLatete Jurassic Jurassic - - CrCretaceusetaceous

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

(C) Today (C) Today

Legend + + Granitoid bedrock Marble and mica schist Karst cave Mica gneiss Joints Triassic-Jurassic sediments Base of regolith Cretaceous sediments Channel deposits and buried soils Saprolite Saprolite remnant NORWEGIAN JOURNAL OF GEOLOGY Deep weathering, neotectonics and strandflat formation in Nordland, northern Norway 209

repeated levelling and focal plane solutions indicating periods would have assisted in the formation of a that the area is under WNW–ESE present-day relatively wide strandflat along the Nordland coast extension and subsidence. Such detailed data are, as well as the rim of flat land around islands and however, not available for the remaining parts of the peninsulas. The present-day large-scale topography Nordland coast. along the coast of Nordland may consequently be an inheritance from the Mesozoic, as suggested by 4) A parallel and shallow zone of increased seismicity Asklund (1928) and Büdel (1978). The exhumation largely reflecting extensional stress conditions process may be an important mountain-forming has previously been observed along the coast of mechanism in Nordland. Nordland. Below the up to 1500 m-thick Plio– Pleistocene wedge along the continental edge to the west there is another seismic zone with deeper Acknowledgements. The present paper contains some of the results from compressional events. The seismicity in the Træna the TWIN Project (Tropical Weathering in Norway) that was funded by Basin offshore Nordland coincides with the thickest the Norwegian Petroleum Directorate (NPD) and the Geological Survey unit of the Naust Formation (1500 ms TWT). The of Norway (NGU). Karl Fabian, Bart Hendriks, Peter Ihlen, Jan Steinar shallow seismicity with extensional mechanisms in Rønning and Jan Fredrik Tønnesen at NGU and Dag Bering, Christian Magnus and Jon Arne Øverland at NPD contributed to this study the coastal regions of Nordland is possibly associated with support, advice and discussions. Tormod Henningsen and Peter with flexuring due to the Pleistocene erosion and Midbøe, Statoil, introduced us to the geology of the Ramså Basin. Lars unloading. Bockmann and Knut Gjerde from the Norwegian Mapping Authority performed the two GPS campaigns in the Ranafjorden area in 1999 and 5) N–S-trending, steeply dipping fractures and faults 2008, respectively. The GPS net was originally established in 1994 by Terje Skogseth at the Norwegian University of Science and Technology occur in the Ranafjorden area. The foliation of the (NTNU). Ann Karlsen og Bjørn Nilsen carried out the XRF analyses mica schist is subparallel to the bedrock surface. at the NGU laboratory. Adrian Hall and John-Are Hansen reviewed Observed westward facing scarps along the fractures the manuscript and made suggestions towards its improvement. David are most likely of erosional origin. 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