NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of the SW Norwegian passive margin based on low-temperature thermochronology 243

Tectonic evolution of the SW Norwegian passive margin based on low-temperature thermochronology from the innermost Hardangerfjord area

Karen C. Johannessen, Fabian Kohlmann, Anna K. Ksienzyk, István Dunkl & Joachim Jacobs

Johannessen, K.C., Kohlmann, F., Ksienzyk, A.K., Dunkl, I. & Jacobs, J.: Tectonic evolution of the SW Norwegian passive margin based on low- temperature thermochronology from the innermost Hardangerfjord area. Norwegian Journal of Geology, Vol 93, pp. 243–260. Trondheim 2013, ISSN 029-196X.

The post-Caledonian structural and morphological evolution of the North Sea rift margin in southwestern Norway is largely unresolved. A compre- hensive understanding of the importance of onshore fault reactivation and the magnitude of crustal uplift has been hindered by the near absence of post-Devonian sediments. This study aims to delineate the tectonic history of the passive margin hinterland from the Permian onwards by means of apatite fission-track (AFT) analysis and (U–Th)/He thermochronology. AFT analysis has been performed on 32 samples from the steep flanks of the innermost segments of the Hardangerfjord. The resulting cooling ages range from Late Triassic to Late Cretaceous and define a general positive age-elevation trend that is locally disturbed by large age offsets, suggestive of post-Mid Jurassic faulting on the order of several hundred metres. Four samples from the Eidfjord and Ulvik districts have been analysed by the (U–Th)/He method, giving primarily Cretaceous single-grain ages. ­Thermal history modelling reveals two distinct episodes of accelerated cooling (2–6°C/Myr-1), confined to the Permo–Triassic and the latest ­Cretaceous to Cenozoic. The high Permo–Triassic cooling rates may be explained by flexural rift–flank uplift and increased tectonic activity onshore as a response to rifting in adjacent offshore areas. Rapid exhumation throughout the Cenozoic is consistent with sustained elevated topography­ and periodic reju- venation of relief as a result of local fault activity. In combination with previously reported AFT data, the results presented in this contri­bution sug- gest that faulting has exerted a major control on the overall morphology of the passive margin. The Mesozoic–Cenozoic ­exhumation history reflects a complex interplay between tectonic activity, flexural uplift and erosion.

Karen C. Johannessen, Department of Earth Science, University of Bergen, P.O. Box 7803, 5020 Bergen, Norway. Fabian Kohlmann, Department of Earth Science, University of Bergen, P.O. Box 7803, 5020 Bergen, Norway. Anna K. Ksienzyk, Department of Earth Science, University of Bergen, P.O. Box 7803, 5020 Bergen, Norway. István Dunkl, Geoscience Center, University of Göttingen, Goldschmidtstrasse 3, 37077 Göttingen, Germany. Joachim Jacobs, Department of Earth Science, University of Bergen, P.O. Box 7803, 5020 Bergen, Norway.

E-mail corresponding author (Karen C. Johannessen): [email protected]

Introduction into the hinterland at constant, low elevations and may potentially reveal essential aspects concerning the The tectonomorphological evolution of the North Sea style of rift-margin evolution over time. The Eidfjord rift margin remains incompletely understood. While area in the innermost part of the Hardangerfjord is the offshore geology has been extensively studied regarded as one of the classical sites for low-temperature in connection with hydrocarbon exploration in the thermochronology in Norway, and a number of studies North Sea, less attention has been devoted to onshore have been undertaken in this particular area (Andriessen areas. Post-Devonian sediments are scarce in southern & Bos, 1986; Rohrman et al., 1995; Leighton, 2007). Norway, making quantification of crustal uplift and Some of the youngest apatite fission track (AFT) ages fault displacements difficult. Low-temperature thermo­ in Norway are found at low elevations in the inner chronological techniques can be employed to constrain regions of the Hardangerfjord. The combination of the timing of vertical movements through the uppermost young AFT ages close to sea level and high-relief few kilometres of the crust and are at present the topography makes the inner Hardangerfjord and its most effective means of obtaining information about tributaries excellent targets for detailed studies on exhumation and topographic evolution prior to the the timing and magnitude of Mesozoic and Cenozoic Quaternary glaciations. exhumation. Previous work in the region has, however, failed to provide a comprehensive representation of The glacially shaped fjords in southern Norway offer a the exhumation history, mainly because the so-called unique opportunity to sample across the passive margin vertical profiles that have been sampled extend over 244 K.C. Johannessen et al. NORWEGIAN JOURNAL OF GEOLOGY significant horizontal distances (i.e., >20 km). Numerous Hordaland–Sogn areas, respectively, suggesting that the brittle structures dissect the basement of the inner tectonic history of southern Norway may be much more Hardangerfjord and major linear features, including complex than that portrayed by Rohrman et al. (1995). fjords and deeply incised glacial valleys, characterise the landscape. Limited structural work has been conducted The Oligocene–Neogene accelerated cooling and the so far and consequently, little is known about the nature domal pattern of AFT ages presented by Rohrman et of these structures, their possible displacements and al. (1995) are widely cited in the literature as supportive their effect on the distribution of low-temperature evidence for large-scale, Cenozoic tectonic uplift of thermochronological data in the area. southern Norway, which is required to explain the presence of a putative Cretaceous–Palaeogene peneplain The pioneering, low-temperature, thermochronological at high elevations (e.g., Japsen & Chalmers, 2000; work in Norway was conducted in the Eidfjord area by Lidmar-Bergström et al., 2000; Gabrielsen et al., 2010). Andriessen & Bos (1986), who documented a general Recently, the origin of the topography in southern increase in AFT age with elevation and suggested a Norway has received renewed attention (Lidmar- decrease in cooling rate from the Permian to the Jurassic. Bergström & Bonow, 2009; Nielsen et al., 2009a, b, 2010a, The sampling density of this reconnaissance study was b; Chalmers et al., 2010; Gabrielsen et al., 2010). The quite low and no vertical profiles were collected. highly controversial ICE (Isostasy–Climate–Erosion) hypothesis by Nielsen et al. (2009b) challenged the A more detailed dataset was presented by Rohrman et concept of Cenozoic topographic rejuvenation by stating al. (1995), who included the Eidfjord area in a regional that the present mountains are remnants of Caledonian AFT study of southern Norway. In their study, a vertical topography, sustained through isostatic rebound of a profile was sampled along the Måbødalen road from sea buoyant crustal root and shaped through the combined level in Eidfjord to the Hardangervidda plateau, covering effects of glacial erosion and periglacial processes. The a lateral distance of ~20 km. Based on the regional age available low-temperature thermochronological data distribution and thermal history modelling, Rohrman from southern Norway are compatible with gradual et al. (1995) suggested two major episodes of rapid denudation of a long-lived orogen (Nielsen et al., 2009b), exhumation, a Triassic–Jurassic phase that started in the showing that the AFT record is not as unambiguous as east and migrated westwards and an Oligocene–Neogene previously assumed. uplift event characterised by domal warping of the AFT isochrons, indicating greater exhumation in the interior Here, we test the viability of the exhumation scenarios than along the coastline. The Neogene cooling–spike depicted by the domal uplift model by applying AFT hypothesis was largely based on the thermal histories analysis in combination with (U–Th)/He thermo­ of the youngest samples from the inner fjord regions, chronology to samples from steep vertical profiles from while an apparent radial pattern of increasing AFT ages the fjord flanks in the Eidfjord–Ulvik district, one of from the interior towards the coast was interpreted as the key localities of Rohrman et al. (1995). The study an indication of domal uplift, with southern Norway is motivated by the need for a higher–resolution AFT behaving as a structurally coherent block since Permian sampling strategy and a better control on the structural times. complexity at the scale of outcrops and vertical profiles. Our specific foci are the timing, rate and driving forces In a later contribution, Redfield et al. (2004, 2005) of exhumation and the extent to which Mesozoic and documented significantly offset AFT ages across Cenozoic faulting has affected the morphological segments of the Møre–Trøndelag Fault Complex in Mid evolution of the North Sea rift margin. Norway, suggesting that the evolution of the North Atlantic passive margin may largely have been governed by reactivation of older structures during the Mesozoic– Geological setting Cenozoic. Normal faulting of the innermost proximal margin has been proposed as the main control on rift– The geology of the inner Hardangerfjord region is escarpment rejuvenation along the coast from Møre dominated by Proterozoic basement rocks, partly to Troms, substantiating that fault activity may have over­lain by strongly sheared Cambro–Silurian meta­ continued into the most recent geological past (Redfield sedimentary rocks and Caledonian nappes, belonging & Osmundsen, 2012). While the Møre–Trøndelag region to the Lower and Middle Allochthon (Roberts & Gee, faces the active spreading system in the North Atlantic, 1985). The tectonostratigraphy is dissected by densely the coastal region of southern Norway borders the spaced brittle structures with unknown origins and aborted rift in the North Sea. The styles of rift-margin records of displacement. N–S- and WNW–ESE- evolution, particularly in relation to the magnitude and trending lineaments are particularly abundant. These timing of faulting, may differ significantly between the populations roughly parallel major Svecofennian and two realms. Recently, Leighton (2007) and Ksienzyk Sveconorwegian shear zones and may belong to a (2012) reported different Mesozoic and Cenozoic cooling Proterozoic fracture pattern that extends across large histories for adjacent structural blocks in the Bergen and parts of Fennoscandia (Gabrielsen et al., 2002). NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of the SW Norwegian passive margin based on low-temperature thermochronology 245

Figure 1. Simplified geological map of southern Norway. Thick black lines illustrate the location of major extensional faults and ductile shear zones developed or reactivated during late- to post-Caledonian orogenic collapse. The study area (B) is located in interior southwestern Norway, directly to the west of the highest topography. The location of the Hardangerfjord Shear Zone (HSZ) is indicated by the thick grey line. Note that the thick- ness of the HSZ is not drawn to scale. After Andersen (1998), Fossen & Hurich (2005), Redfield & Osmundsen (2009) and Sigmond (1998).

The NE–SW-trending Hardangerfjord Shear Zone (HSZ) expression of the HSZ is a several kilometre-wide zone of is the largest tectonic feature in the Hardanger region and mylonites and the estimated down-to-the-NW displace- follows the Hardangerfjord northeastwards to the Gran- ment is on the order of 5 km (Fossen & Hurich, 2005). vin area, where it is exposed subaerially. Caledonian thrust sheets are mainly preserved in the down-faulted The HSZ belongs to an array of major W–NW-dipping hangingwall west of Granvin and are mono­clinically shear zones in the western part of southern Norway folded in the half-graben that overlies the shear zone (Fig. 1). These structures accommodated significant (Fossen & Hurich, 2005). As a result of large-scale uplift extensional displacement in connection to the collapse and accelerated denudation of the footwall, Cale­donian of the Caledonian orogen in the Devonian (Fossen, 1992; nappes and underlying parautochthonous metasedimen- Andersen, 1998). The bulk displacement along the HSZ tary rocks are merely preserved as outliers to the south- took place in the ductile regime immediately following east of the shear zone (Sigmond, 1998). The sub­aerial the termination of the orogeny (Fossen & Hurich, 2005). 246 K.C. Johannessen et al. NORWEGIAN JOURNAL OF GEOLOGY

The HSZ can be traced along the entire length of 1983) was performed prior to analysis. TrackKey (Dunkl, the Hardangerfjord and its offshore continuation is 2002) was applied to calculate AFT ages. A cursor with a represented by a set of NE–SW-trending lineaments light-emitting diode was used for length measurements. and NW-dipping reflectors that extend through the Ling Only horizontal TINT-type tracks (Lal et al., 1969) in Depression, across the North Sea and possibly link up apatites oriented parallel to the crystallographic c-axis with the Highland Boundary Fault in Scotland (Færseth were measured, following the recommendation of et al., 1995; Fossen & Hurich, 2005). Seismic reflection Barbarand et al. (2003). profiling has revealed an abrupt increase in the crustal thickness across the HSZ, from 28 km in the northwest (U–Th)/He analyses were conducted at the Geoscience to 34 km in the southeast (Hurich & Kristoffersen, Centre at the University of Göttingen, Germany. Three 1988). The shear zone may thus be part of an innermost grains with good crystal morphologies were hand-picked boundary–fault system (Mosar, 2003) that separates the from each sample. All selected crystals appeared to be thinned crust affected by the North Atlantic rifting from free of inclusions and microfractures. The length and the hinterland of the Norwegian passive margin. width of each apatite was measured and the grains were packed in separate platinum capsules. He-degassing was The brittle Lærdal–Gjende Fault (LGF) transects the performed under vacuum in a sealed furnace by heating Caledonian thrust sheets in the area directly to the the sample to c. 870°C for 20 min with an infrared diode northwest of the Hardangerfjord and is interpreted as laser. A hot blank was established prior to every analysis. an upper–crustal expression of the HSZ, formed during Subsequent to degassing, the obtained 4He was spiked late stages of the Devonian orogenic collapse (Fossen with a known amount of 99+% 3He and purified with a & Hurich, 2005). Reactivation of the LGF is believed to SAES Ti–Zr getter at 450°C. The isotopic–ratio analysis have taken place during Permian and Late Jurassic–Early was performed with a Hiden triple-filter quadrupole Cretaceous times (Andersen et al., 1999), concomitantly mass spectrometer, equipped with an ion-counting with major rift phases in the North Sea (e.g., Færseth et detector. Re-extraction of He was executed in order to al., 1995) and associated pulses of fault activity in coast- quantify excess gas derived from mineral inclusions proximal areas (Torsvik et al., 1992; Eide et al., 1997; within the apatite. Following re-extraction, the platinum Ksienzyk, 2012). K–Ar illite dating of fault gouges has capsules were retrieved from the vacuum chamber. Each revealed additional episodes of reactivation during the apatite grain was then dissolved in 2% nitric acid and latest Cretaceous–Palaeogene (Ksienzyk, 2012). spiked with a solution containing known concentrations of 235U and 230Th. The isotopic ratios in the solution were measured by a Perkin Elmer Elan DRC ICP–MS with an Methods APEX micro flow nebuliser, and the 238U, 232Th and Sm contents were calculated from the obtained ratios. AFT analyses were performed on 32 samples from three vertical profiles in the innermost Hardangerfjord area, Thermal history modelling was performed with HeFTy applying the external detector method to determine 1.7.5 (Ketcham, 2005). The annealing model of Ketcham single-grain ages and U-concentrations. Standard et al. (2007a) was applied as a basis for estimation of procedures for heavy-mineral separation, including kinetic behaviour, and Dpar (Donelick et al., 1999) was Wilfley table, magnetic and heavy–liquid separation, used as a kinetic annealing parameter. Confined track were employed in order to isolate apatite for AFT and lengths were corrected by c-axis projection, as proposed (U–Th)/He analysis. For AFT analysis, apatites were by Ketcham et al. (2007b). Zircon fission–track ages of embedded in epoxy resin and internal crystal surfaces 306 ± 22 Ma from Eidfjord (Andriessen & Bos, 1986) were exposed using standard grinding and polishing were implemented as thermal constraints in all models. techniques. Each sample mount was etched in 5 M nitric The zircon fission–track system records cooling of acid (HNO3) for 20 seconds at 20 ± 1°C. A mica plate the inner Hardangerfjord area roughly concomitantly was then attached to each sample to act as an external with the initiation of rifting and volcanism in the detector for induced fission. Thermal neutron irradiation Oslo region (Neumann et al., 1992) and possibly also was conducted at the Garching Forschungsreaktor FRM in the North Sea (Dunlap & Fossen, 1998). Permian II at the Technical University of Munich, using a thermal magmatic activity in western Norway is manifested neutron flux of 1016 neutrons/cm-2. Induced tracks in by ~260 Ma, coast-parallel, dolerite dykes, which crop mica were revealed through etching in 40% hydrofluoric out in the outer Hardangerfjord region (Færseth et acid (HF) for 20 min at room temperature. al., 1976; Løvlie & Mitchell, 1982; Fossen & Dunlap, 1999). There are no indications of magmatism around AFT analyses were executed in the thermochronology the inner Hardangerfjord at this time and the studied laboratory at the University of Bergen with an Olympus area is situated too far from both tectonically active BX51 optical microscope, equipped with a CalComp regions for magmatic heating to have had any effect on Drawing-board III digitising tablet and a computer- the fission–track system. The zircon fission–track ages controlled Kinetek stage, run by the FTstage software from Eidfjord are therefore taken to record denudation by Dumitru (1993). Zeta calibration (Hurford & Green, rather than cooling following a magmatic event and NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of the SW Norwegian passive margin based on low-temperature thermochronology 247

are considered reasonable as a constraint for AFT Customary measurements of etch pit diameter (Dpar) thermal history modelling. A present-day annual mean were performed for all samples, resulting in mean values surface temperature of 7 ± 3°C was applied as a second between 1.18 and 1.69. All samples display similar, constraint. rather low Dpar values, indicative of near end-member fluorapatite compositions.

Results The Osa profile north of the Hardangerfjord comprises 12 samples collected over a relief of 1330 m and displays Apatite fission–track analysis a wide range of AFT ages from 111 ± 9 to 221 ± 36 Ma. A general positive correlation between age and elevation is The AFT ages from the inner Hardangerfjord area range evident (Fig. 3A). from 97 to 221 Ma, with a majority of ages between 120 and 170 Ma. Except for the Late Triassic KJ–24 from The AFT ages from the Simadalen profile in the high elevation at the Hardangervidda plateau and the innermost part of the fjord range from 120 ± 7 to 169 ± Late Cretaceous BG–16 from the hangingwall of the 10 Ma (Fig. 3B). All samples collected from sea level to HSZ, all ages are Jurassic or Early Cretaceous. The results an elevation of 604 m a.s.l. exhibit ages spanning from obtained from AFT analysis of individual samples are 120 ± 7 to 124 ± 7 Ma. A strong correlation between age presented in Table 1, and Fig. 2 displays the age and and elevation is observed, although the age variation location of all analysed samples. that is recorded by this specific portion of the profile is limited. The uppermost samples reveal a strong trend Track lengths were measured for eleven samples, which of progressively increasing AFT ages with elevation and were selected to represent different structural blocks display a greater span of ages over approximately the and elevations within the vertical profiles. The mean same vertical distance as the lowermost portion of the track lengths (MTLs) are confined to a limited range of profile. All samples have similar MTLs in the range of values between 10.41 ± 0.25 μm and 11.66 ± 0.23 μm. 10.93 ± 0.20 to 11.52 ± 0.20 μm.

Figure 2. Simplified geological map of the study area (after Sigmond, 1998), showing sample locations, AFT ages (circles) and mean (U–Th)/He ages (stars). Areas where vertical profiles have been sampled are marked by dashed lines. 248 K.C. Johannessen et al. NORWEGIAN JOURNAL OF GEOLOGY U 9.4 5.2 8.5 9.4 4.1 4.4 8.7 0.7 7.2 5.5 8.0 4.5 8.6 8.7 9.8 31.9 24.1 10.3 19.3 17.0 10.3 10.8 11.5 14.8 10.5 17.7 10.6 21.4 13.4 18.7 10.6 13.6 (ppm) SD 0.13 0.10 0.12 0.08 0.07 0.08 0.10 0.08 0.09 0.07 0.07 0.10 0.07 0.10 0.12 0.07 0.09 0.13 0.08 0.08 0.09 0.07 0.09 0.12 0.12 0.10 0.09 0.10 0.10 0.10 0.10 0.08 (μm) 1.47 1.57 1.27 1.47 1.46 1.65 1.48 1.40 1.51 1.50 1.53 1.33 1.51 1.59 1.35 1.47 1.38 1.45 1.39 1.40 1.18 1.38 1.31 1.50 1.50 1.55 1.55 1.49 1.34 1.67 1.62 1.59 (μm) Mean Dpar Mean 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 63 100 100 100 100 100 100 100 100 100 100 tracks No. of No. SD 2.37 1.86 2.32 1.76 2.47 2.60 2.04 1.82 1.95 2.00 1.95 N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A (μm) N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A (μm) N/A N/A N/A N/A N/A N/A MTL ± 1 σ MTL 11.43± 0.18 10.90 ± 0.24 11.51 ± 0.19 11.66 ± 0.23 10.55 ± 0.18 10.59 ± 0.25 11.11 ± 0.26 10.93 ± 0.20 11.52 ± 0.20 10.41 ± 0.25 11.22 ± 0.19 iii (χ ) 5.2 6.2 0.1 (%) 24.2 24.3 78.3 32.7 10.7 49.4 99.6 12.2 99.0 99.6 96.8 82.8 98.7 87.5 66.2 58.1 21.2 26.9 98.0 73.7 95.6 56.8 80.1 27.3 36.6 94.0 67.8 33.7 30.3 P ii ) ≥ 5 (Galbraith, 1981), indicating a single age population in the analysed sample. in population age single (Galbraith, 1981), indicating a ≥ 5 ) 2 (Ma) 97 ± 5 137 ± 6 165 ± 7 125 ± 9 120 ± 7 175 ± 8 128 ± 8 116 ± 6 111 ± 9 159 ± 9 163 ± 9 163 ± 9 123 ± 7 124 ± 7 153 ± 8 130 ± 6 105 ± 5 124 ± 8 125 ± 9 123 ± 8 172 ± 13 155 ± 10 138 ± 11 132 ± 10 221 ± 36 169 ± 11 166 ± 11 130 ± 11 169 ± 10 184 ± 12 155 ± 10 131 ± 10 Central age ± 1 σ age Central i Nd 7923 7923 7923 7923 7923 7923 7923 7923 7923 7923 7923 7923 7923 7923 7923 7923 7923 7923 7923 7923 8071 7923 8071 7923 6971 7923 6971 7923 6971 7923 8071 7923 Dosimeter 19.960 20.078 20.196 17.719 20.314 17.837 20.432 17.955 20.550 18.073 20.668 20.786 18.190 20.904 18.308 21.022 18.426 21.140 21.257 18.544 18.993 18.662 19.072 19.370 16.661 19.488 16.895 19.606 16.942 19.724 18.914 19.842 ρ d (10^5) 78 Ni i 830 646 825 512 598 750 414 777 793 823 1464 3304 3420 1592 2487 1013 1853 1114 1190 1131 1108 1542 1140 1691 1255 2007 1140 1806 1380 1624 1154 Induced 6.275 14.28 5.757 6.237 0.913 7.949 5.370 15.93 18.644 44.726 34.128 13.779 14.683 24.791 12.051 21.761 11.800 13.895 12.563 10.763 14.899 12.090 18.513 13.907 23.555 13.769 25.546 21.952 11.173 13.610 11.909 13.155 ρ i (10^5) i 76 Ns 770 569 366 755 510 958 357 780 274 304 837 708 692 829 483 708 906 227 829 801 916 701 743 397 679 523 600 1822 2264 1961 1099 Spontaneous 9.806 9.446 3.555 6.963 6.067 11.25 6.179 8.262 3.081 3.170 9.773 7.864 0.890 6.722 8.010 5.119 7.508 9.919 2.944 9.186 9.674 8.092 5.709 7.568 6.840 24.665 22.592 19.547 12.898 12.957 10.043 11.654 ρ s (10^5) 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 grains No. of No. 3 1 15 15 28 49 45 25 25 442 643 791 982 106 196 332 550 604 749 948 807 128 511 101 189 1310 1047 1210 1345 1025 1226 1057 (m asl) (m Elevation Easting 6704522 6704005 6703732 6703384 6718152 6705845 6702043 6706130 6707545 6718079 6719552 6718446 6718091 6720443 6717643 6720845 6717210 6709345 6717419 6717880 6709857 6691882 6710405 6701607 6711186 6706814 6705475 6719421 6705609 6719094 6705192 6701787 . N = number of counted tracks. of number N = . 5 UTM (zone 32 V) (zone 32 UTM 382646 382410 382239 382092 391372 394397 383333 387723 383168 392399 398231 392936 393337 397460 394223 395733 394777 397618 395211 397022 397021 368998 397235 367631 397243 370234 383695 372345 382736 398209 382574 371904 Northing Apatite fission-track data. fission-track Apatite ) = probability value of the chi-square function at n-1 degrees of freedom, where n = No. of crystals.= where n of freedom, degrees n-1 at function chi-square the of value = probability ) P( χ when The chi-square test is passed 2 no. Apatite fission track analysis was performedwas analysis track fission Apatite the externalby geometry4π/2π using a method detector 7. ± of 251 value zeta a and U with 15 ppm glass standard IRMM-540R an applying and ages were calculated by 0.5, of factor = track density in 10 in density track ρ = Sample KJ-15 KJ-14 KJ-13 KJ-12 KJ-31 KJ-11 KJ-28 KJ-9 KJ-27 KJ-8 KJ-26 KJ-7 KJ-6 KJ-25 KJ-5 KJ-24 KJ-4 KJ-22 KJ-3 KJ-1 KJ-21 BG-53 KJ-20 BG-27 KJ-19 BG-26 KJ-18 BG-16 KJ-17 BG-14 KJ-16 Table 1. Table BG-13 ii) iii) P( χ i) NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of the SW Norwegian passive margin based on low-temperature thermochronology 249

Figure 3. Age-elevation trends and track-length distributions for the three studied vertical profiles. A general trend of increasing age with elevation is apparent for all profiles. All samples have wide track-length distributions and short MTLs that show positive correlation with elevation. Error bars are ± 1σ. (A) Osa profile: ages clearly increase with elevation, but the age-elevation trend is locally inverted. (B) Simadalen profile: the lower- most sample has a significantly older age and a longer MTL than what would be expected from the general trend. (C) Bu profile: the samples can be grouped into two distinct age-elevation trends with similar slopes.

The Bu profile consists of eight samples collected from Leighton (2007) have yielded ages ranging from 98 to 166 sea level to an elevation of 1310 m on the southern Ma and 129 to 178 Ma, respectively, The previous studies margin of the inner Hardangerfjord. The obtained AFT found a weaker correlation between age and elevation ages span from 124 ± 8 to 175 ± 8 Ma (Fig. 3C). A fairly than what is documented in the current contribution. strong positive correlation between age and elevation This is not surprising, considering the greater lateral is evident. A discontinuity in the general age-elevation extent of their vertical profiles. trend is present in the middle of the profile at an elevation of ~700 m, where AFT ages are found to jump Apatite (U–Th)/He analysis abruptly from Early Cretaceous in the lower portion to Middle Jurassic in the upper part of the transect. Four samples were dated by apatite (U–Th)/He analysis. Similar age-elevation gradients are apparent within both Because the alpha-ejection correction proposed by Farley segments. The MTLs of the samples in the Bu profile et al. (1996) requires a homogeneous U–Th-distribution, range from 10.90 ± 0.24 to 11.66 ± 0.23 μm. it may not provide a good representation for apatites from the autochthonous basement in interior Additional samples collected close to sea level generally southwestern Norway. Pronounced concentric zoning, display ages between 116 Ma and 130 Ma and MTLs of characterised by U-rich cores and relatively wide, ~11 μm. The samples collected from the westernmost U-poor rims, appears to be very common in apatites part of the studied area, along the western margin of from this area. Assuming that the analysed grains exhibit the Granvinfjord, have the youngest ages of all analysed decreasing concentrations of parent nuclides from the samples. BG–26 from the outer Granvinfjord records an core towards the rim, the FT-correction will over-estimate age of 105 ± 5 Ma and a short MTL of 10.41 ± 0.25 μm, alpha-ejection, and the corrected (U–Th)/He ages will be while BG–16 from an elevation of ~500 m northwest of too old. There are currently a limited number of models Granvin displays an even younger age of 97 ± 5 Ma. developed to correct for zoning in the calculation of the

FT-factor (e.g., Meesters & Dunai, 2002; Hourigan et al., The ages presented above are generally similar to the 2005). In order for these approaches to be successful, ages obtained in previous AFT studies of the inner the spatial distribution of U–Th in the analysed grains Hardangerfjord area. Vertical profiles and nearby sea- must be known. The exact nature and extent of zoning level samples analysed by Rohrman et al. (1995) and of individual apatites from the inner Hardangerfjord 250 K.C. Johannessen et al. NORWEGIAN JOURNAL OF GEOLOGY 2 2 2 2 0 2 2 2 2 2 2 2 No. term. 54 52 55 56 58 63 61 74 70 65 49 59 Prism rad. (μm) rad. 8 6 5 6 9 6 6 7 6 10 11 14 1 σ (Ma) 93 141 200 227 106 125 204 147 155 150 114 252 -corr. He -corr. t age (Ma) age F

Figure 4. Uncorrected (U–Th)/He ages displayed with their corre- sponding AFT ages. The majority of the (U–Th)/He ages are younger 99 75 69 93 83 140 162 151 115 120 115 170 than the ages obtained from AFT analysis. The sensitivity ranges for age (Ma) age Uncorr. He Uncorr. the AFT and AHe systems are ~120–60°C and ~70–40°C, respectively. Error bars are ± 2σ. iii t F 0.70 0.70 0.71 0.71 0.73 0.74 0.74 0.78 0.78 0.76 0.68 0.73 1 σ 5.6 5.5 9.2 5.6 5.5 5.4 5.4 5.4 5.4 5.5 5.5 9.4 (%) area have not been determined. Due to the persistent, yet not precisely quantified excess of actinides in apatite Sm (ng) 0.169 0.243 0.298 0.255 0.262 0.649 0.871 0.629 0.930 0.451 0.251 0.212 cores, the true He age is expected to lie closer to the

uncorrected age than the FT-corrected age. Uncorrected ages are therefore considered more reliable and are ii 5.3 6.2 eU 11.7 32.4 13.7 13.1 18.8 20.6 23.1 14.4 12.3 17.1 reported in the text. F -corrected ages are included in

[ppm] T Table 2 for comparison. 0.4 0.5 0.4 1.5 1.6 1.6 1.1 1.1 0.9 0.4 0.7 0.8 The obtained single-grain (U–Th)/He ages range Th/U from 69 to 170 Ma, with a majority of Late Cretaceous ages close to sea level and Early Cretaceous ages on the Hardangervidda plateau. Fig. 4 shows the obtained ages 1 σ 2.6 2.4 2.5 2.4 2.4 2.4 2.4 2.5 2.4 2.5 2.5 2.5 (%) together with the corresponding fission track ages. Both samples from the Osa profile, KJ–8 and KJ–25, show poor reproducibility of single-grain ages. Large dispersion in Th (ng) 0.015 0.045 0.022 0.051 0.061 0.140 0.113 0.042 0.088 0.029 0.032 0.022 (U–Th)/He ages may result from variations in physical parameters between analysed grains, as the diffusion of He out of the apatite crystal is strongly dependent on 1 σ 2.4 1.9 2.1 2.4 2.3 1.9 1.9 2.3 1.9 2.0 2.2 2.7 (%) the grain dimensions and the extent of fragmentation (Brown et al., 2013). There is no clear relationship between age and grain size for the inner Hardangerfjord U

(ng) samples, and all except for one of the selected crystals 0.037 0.094 0.051 0.035 0.039 0.090 0.098 0.039 0.098 0.067 0.047 0.029 were whole. The age dispersion must thus reflect other factors. 1 σ 2.5 1.9 2.0 2.4 2.6 2.0 1.9 2.3 2.0 2.1 2.0 2.8 (%) Considerable dispersion may also arise from differential radiation damage between the grains in an aliquot. Alpha i recoil damage has been found to impede He diffusion, He (ncc) 0.499 1.813 1.162 0.444 0.461 1.444 2.425 0.757 1.859 1.091 1.176 0.358 shifting the sensitivity range of the (U–Th)/He system towards higher temperatures (Shuster et al., 2006). This effect is most pronounced for old samples that have (m asl) (m

Elevation been subjected to extensive radiation. A general positive 45 15 1226 1210

Apatite (U–Th/He) data. (U–Th/He) Apatite relationship between effective U-concentration (eU) and (U–Th)/He age is found for two of the analysed samples, no.

Sample suggesting that much of the dispersion may be explained Amount of 4He is given in nano-cubic-cm at standard temperature and pressure. temperature at standard 4He is given in nano-cubic-cm Amount of + 0.235 [Th]). in ppm (eU = [U] U-concentration eU is the effective et al. (1996). according to Farley FT-correction Table 2. Table KJ-8 KJ-11 KJ-19 KJ-25

i) ii) iii) by radiation effects. NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of the SW Norwegian passive margin based on low-temperature thermochronology 251

Thermal history modelling 5. Some general trends are shared by the majority of the thermal history models, i.e., rapid Permo–Triassic Representative thermal history models from the Bu cooling into the partial annealing zone (PAZ) followed by profile south of the inner Hardangerfjord and the slow cooling through the temperature interval between Simadalen profile north of the fjord are shown in Fig. 80°C and 60°C and, finally, accelerated cooling starting

Figure 5. Thermal history models based on AFT data from two of the ver- tical profiles, with tempe- rature constraints shown as boxes. Acceptable t–T paths are marked in green, while 100 good paths are indicated by purple lines. The dark blue cooling path in each model represents the weighted mean path. The sensitivity range for the AFT method (i.e., the partial annealing zone, PAZ) is marked in pale blue. The samples are arranged according to their elevations. (A–C) Modelled cooling paths for samples from the Bu pro- file. Note the progressively earlier exit from the PAZ with increasing altitude. The proposed thermal history model for KJ–28 indicates cooling out of the PAZ already in the latest Early Cretaceous. (D–F) Thermal history models for the Simadalen samples. All samples record rapid cooling into the PAZ wit- hin the time interval from the Permian to the Early Jurassic. The latest exit from the PAZ is indica- ted for the middle sample, KJ–21. 252 K.C. Johannessen et al. NORWEGIAN JOURNAL OF GEOLOGY in the Late Cretaceous–Palaeogene. The samples from history of the inner Hardangerfjord region is characterised the north flank of the Hardangerfjord consistently record by prolonged time intervals of slow cooling. The shallow a later onset of accelerated cooling, and hence a later age-elevation gradient from the three vertical profiles exit from the PAZ, than samples from corresponding suggests the presence of an exhumed PAZ. All analysed elevations on the south flank. samples have experienced similar, protracted cooling histories, but AFT ages are locally offset across short horizontal distances. The vertical profiles are dissected by Summary numerous lineaments. Offset ages may thus be explained by displacement of samples from the fossil PAZ along When all the analytical results from AFT and (U–Th)/He local, small-scale faults. analyses are considered together, some distinct trends are evident: Early Cretaceous AFT ages predominate close to For the Osa profile, the general increase in AFT age sea level, while Jurassic ages are recorded by most samples with elevation (Fig. 3A) is in good agreement with the from high elevations on the Hardangervidda plateau. notion of an exhumed PAZ. The disordered and locally Consistent ages, generally within the range 111–130 Ma, inverted age-elevation gradient may reflect offset ages are evident at low elevations throughout the study area. or simply natural variability. A number of N–S- to NW– However, the samples collected close to sea level, north of SE-trending faults transect the profile, but it cannot be the inner Hardangerfjord, systematically display younger determined which of the individual structures might AFT ages, shorter MTLs and thus longer residence times have accommodated displacement. in the PAZ than samples at corresponding elevations from the southern margin of the fjord. This pattern is The Simadalen profile comprises five samples, of which not as pronounced at higher elevations. The (U–Th)/ four have been collected from the northern margin of the He system records cooling in the Early Cretaceous on inner Hardangerfjord, while the sea-level sample comes the Hardangervidda plateau and in the Late Cretaceous from the southern margin. An apparent break in slope is at low elevations, i.e., roughly 40 Myr later than the ages evident in the age elevation plot from the Simadalen profile obtained from AFT analysis. at around 124 Ma (cf., Fig. 3B). Considering the nearly identical, short MTLs and broad track-length distributions displayed by the uppermost and lowermost sample, this Discussion feature is unlikely to reflect an Early Cretaceous episode of accelerated cooling. Faulting can produce unrealistically Interpretation of vertical profiles steep, age-elevation gradients and the age distribution in the Simadalen profile more likely results from post-Mid Vertical profiles provide detailed information about the Jurassic fault activity. The most obvious linear feature cooling style within a geographically restricted area. that transects the profile is the Simadalsfjord and the age A positive correlation between apatite fission–track distribution in the vertical profile is consistent with down- age and elevation reflects the movement of the rock to-the-SE displacement across a fault that underlies this column through the temperature gradient associated fjord. Many Norwegian fjords follow the trends of old with increased fission–track stability (Wagner & Reimer, fracture zones (e.g., Nesje et al., 1992; Nesje & Whillans, 1972). Implicit in the concept of vertical profiles is the 1994), as brittle fault rocks are easier for glaciers and river assumption that all samples have experienced similar systems to remove. A small number of NE–SW- to NNE– thermal histories (Gallagher et al., 2005). At any time in SSW-trending lineaments dissect the upper part of the the geological past, samples derived from low portions sampled profile, and movement along these structures may of the profile were exposed to higher temperatures than potentially have affected the age-distribution pattern. The samples that presently reside at higher elevations. Fossil close spacing of lineaments makes it difficult to isolate the partial annealing zones (PAZ) may be identified when effect of individual faults. A throw of ~650 m is inferred fission–track age data from vertical profiles are plotted from the age of the sea-level sample and the age-elevation against sample elevation (Gleadow & Fitzgerald, 1987; gradient in the upper part of the profile, assuming that the Fitzgerald & Gleadow, 1990; Fitzgerald et al., 1995). A bulk displacement was accommodated by a single fault. shallow age-elevation gradient and a large proportion of extensively annealed tracks are indications of a fossil The mean (U–Th)/He ages of KJ–11 and KJ–19 are PAZ. The base of the exhumed PAZ is typically perceived younger than the obtained AFT ages by ~40 Ma and as a break in slope associated with a change in age- indicate cooling through the He partial retention zone elevation gradient. Samples below the break in slope in the Cretaceous. The nearly similar difference in AFT display similar fission–track ages and predominantly and (U–Th)/He ages for the lowermost and uppermost long tracks, indicative of rapid cooling through the PAZ samples points towards a common Jurassic–Late (e.g., Gleadow et al., 1986). Cretaceous thermal history for the entire profile. This implies that the apparent differential exhumation of From the wide track-length distributions and short MTLs the upper and lower portions of the profile may best be of the analysed samples, it is apparent that the thermal explained by post-Late Cretaceous fault activity. NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of the SW Norwegian passive margin based on low-temperature thermochronology 253

The Bu profile on the southern flank of the inner significantly lower Permo–Triassic exhumation rates Hardangerfjord displays two distinct, nearly linear age- for the hangingwall of the LGF (20–40 m Myr-1) elevation trends (Fig. 3C). This distribution may reflect relative to the footwall (40–90 m Myr-1), which clearly movement along roughly N–S-trending faults that dissect demonstrate significant displacement and probably the profile. The younger age-elevation trend is evident repeated reactivation of the brittle segment of the HSZ for samples that were collected farther east, suggesting during the development of the North Sea rift. The area later exhumation of the eastern block relative to the studied in the current work is located in the transition western block and thus down-to-the-W displacement, zone where the ductile shear zone disappears beneath the with a total throw of >1000 m. Caledonian thrust sheets and the first clear indications of brittle overprint are evident. Although Permo–Triassic The vertical profiles presented in this contribution have reactivation of the ductile southwestern portion of the been selected with the purpose of obtaining closely spaced HSZ has been proposed (Færseth et al., 1995) there samples and minimising the lateral sampling distance is no conclusive evidence for post-Devonian brittle within each profile. Where the distance between samples is displacement (Fossen & Hurich, 2005). The Permo– large enough to cross several faults, important information Triassic cooling rate inferred from the thermal history regarding differential exhumation, and thereby aspects models for the inner Hardangerfjord samples suggests of the tectonic evolution, may be overlooked. The inner an exhumation rate of >100 m Myr-1. in the footwall Hardangerfjord region is strongly affected by brittle of the shear zone, assuming a geothermal gradient of deformation. This feature is readily observed from the 20°C, which is similar to the present gradient (Pascal close spacing of lineaments evident on aerial photographs et al., 2010). Taking this comparably high exhumation (Fig. 6). The complexity of the analysed vertical profiles rate into account, it is conceivable that Permo–Triassic stresses the necessity for closely spaced data points and brittle reactivation of the inner Hardangerfjord segment demonstrates the advantages of sampling the steepest may have taken place. The onshore expression of the profile, rather than the profile with the easiest accessibility, Permo–Triassic rift phase appears to have been related as has largely been done in the past. to both regional rift-flank uplift and reactivation of post-Caledonian structures resulting in differential Exhumation history of the inner Hardangerfjord area denudation of adjacent fault-bounded blocks.

The obtained thermochronological data provide The thermal history models reveal low Jurassic– information about the temperature history and clues Cretaceous cooling rates (<1°C Myr-1). This implies that for the tectonic evolution from the Permian until the the Late Jurassic North Sea rift did not significantly affect Palaeogene (Fig. 7). Thermal history modelling indicates onshore areas inland of the coastal region. Previous relatively fast cooling (2–6°C Myr) during the Permo– studies have suggested a minor effect of the Jurassic rift Triassic, consistent with the timing of the earliest, well- phase even in coast-proximal areas (e.g., Færseth et al., documented, North Sea rift phase (e.g., Ziegler, 1990; 1995), which is corroborated by the lack of evidence Færseth et al., 1995) and with previously published for post-Triassic igneous activity onshore (Fossen & zircon fission–track and 40Ar/39Ar data from southern Dunlap, 1999). Although the North Sea rift phase had a Norway (Andriessen & Bos, 1986; Dunlap & Fossen, limited effect on the regional exhumation rate, extensive 1998). Significant unroofing of the inner Hardangerfjord fault reactivation may have exerted a major control on region appears to have taken place at this time. Permo– local exhumation patterns. The distribution of AFT Triassic crustal extension in the North Sea may have ages reveals significant age differences over short lateral been a dominant driving force for the rapid exhumation distances, suggesting episodes of accelerated cooling, of the margin. Assuming an early rift margin comparable possibly associated with Mid Jurassic–Early Cretaceous, to a large-scale flexural cantilever, in the sense described fault-related uplift and denudation. Such local cooling by Kusznir et al. (1991), extensional displacement along events of limited magnitude cannot be resolved major basin-bounding faults would lead to considerable by thermal history models based on AFT analysis. isostatic uplift of the rift flank. Increased relief would Palaeomagnetic and radiometric data from fault rocks enhance the efficiency of surface processes, creating a obtained from Hordaland and adjacent regions indicate regional effect on exhumation rates. This effect would reactivation of structures during Late Jurassic–Early be most pronounced in coastal areas. The exact degree Cretaceous times (Torsvik et al., 1992; Eide et al., 1997; to which rift-flank uplift has contributed to the cooling Fossen et al., 1997; Andersen et al., 1999; Ksienzyk, 2012). of the inner Hardangerfjord area is uncertain, as the hinterland may already have been topographically high An episode of accelerated cooling appears to have in the Permo–Triassic. commenced in the latest Cretaceous–Palaeogene. The tectonic activity offshore Norway had shifted to the Down-to-the-NW extensional reactivation of the North Atlantic by this time (Doré et al., 1999). Accelerated Lærdal–Gjende Fault and affiliated structures (Andersen Cenozoic exhumation is difficult to explain through et al., 1999) may have exerted the main control on the rift-related regional uplift, as the study area is situated exhumation of the hinterland. Leighton (2007) estimated far from the locus of active extension. Erosion rate is 254 K.C. Johannessen et al. NORWEGIAN JOURNAL OF GEOLOGY

Figure 6. (A) Aerial photo­graph of Simadalen and surrounding areas. The most pronounced lineaments are marked by red lines. N–S-trending structures are particularly abundant. The majority of the N–S-trending frac- tures in the area appear to be subvertical or steeply dipping (see inset stereo- graphic plot). Additional fracture systems include a NE–SW- to ENE–WSW- trending set, with indivi- dual structures that paral- lel the Simadalsfjord, and a second set consisting of structures that vary in orientation between E–W and ESE–WNW. (B) Stee- ply dipping to subvertical, roughly N–S-trending fractures in the southern flank of the Simadalsfjord (blue and yellow arrows). An array of moderately to steeply SE-dipping structu- res is evident in the depres- sion associated with the major fracture zone (blue arrow). View towards the S from the Hardanger- vidda plateau north of Kjeåsen on the north flank of the fjord.

mainly controlled by relief (e.g., Summerfield & Hulton, factors, such as climate (Nielsen et al., 2009b). AFT 1994) and the Palaeogene episode of rapid cooling likely analysis cannot distinguish between processes that might reflects elevated topography in Norway at this time. have led to cooling. However, some information on the The high relief may have resulted from topographic recent topographic evolution can be extracted. While rejuvenation of a subdued Cretaceous–Palaeogene thermal histories derived from AFT analysis may be used landscape (e.g., Gabrielsen et al., 2010). Alternatively, the to infer patterns of denudation, cooling is not a direct topography has remained high ever since Caledonian proxy for uplift (see England & Molnar, 1990; Fitzgerald times, and the rate of cooling has been affected by other et al., 1995; Gunnel, 2000). To produce a Cenozoic NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of the SW Norwegian passive margin based on low-temperature thermochronology 255

Figure 7. Proposed structu- ral evolution of the inner Hardangerfjord region. The north and south flanks of the innermost Hardan- gerfjord are shown in light red and blue, respectively. The general thermal evolu- tion of the vertical profiles northwest and southeast of the inner Hardanger- fjord is displayed in the cartoon to the right. (A) The Permo–Triassic was characterised by rapid regional exhumation rela- ted to the rifting in the North Sea, and localised accelerated exhumation associated with footwall uplift along the HSZ. At this time the inner Har- dangerfjord samples cooled into the PAZ. (B) Exten- sive small-scale reactiva- tion of structures affected the area during the Mid/ Late Jurassic–Early Creta- ceous. By the latest Early Cretaceous, rocks derived from the Hardangervidda plateau south of the inner Hardangerfjord had cooled out of the PAZ. (C) Ceno- zoic reactivation across the inner segments of the Hardangerfjord resulted in accelerated exhumation of the north flank. The samples from the northern margin, which had resided at deeper crustal levels throughout the Meso- zoic, were then juxtapo- sed against their southern counterparts. The palaeo­ topography of the area remains largely unknown and is not indicated here.

cooling signal of the magnitude seen in the modelled greater depths than the coastline. Consequently, AFT thermal histories, the Palaeogene–earliest Neogene data from the hinterland are not in favour of Cenozoic tectonic uplift inferred for interior southern Norway tectonic uplift with subsequent preservation of low- must have been accompanied by overburden removal relief geomorphological features. The exhumation on the order of ~2 km. This estimate is inconsistent scenario suggested for the inner Hardangerfjord area is with the preservation of an uplifted peneplain, unless compatible with the ICE hypothesis, although Nielsen the planation surface was buried by a thick sediment et al. (2009b) do not consider faulting an essential cover. Vitrinite reflectance data from the Late Jurassic landscape-forming process. Both the distribution of AFT Bjorøy Formation on Sotra (Fossen et al., 1997) and AFT ages across the passive margin (Leighton, 2007; Ksienzyk thermal history models from the basement in the same et al., 2014) and the cooling rates obtained from thermal area (Ksienzyk, 2012) indicate a maximum burial depth history modelling support repeated topographic of 1–1.5 km for the coastal region. It is highly unlikely rejuvenation resulting from fault-controlled uplift. that the Hardangervidda plateau has been buried to 256 K.C. Johannessen et al. NORWEGIAN JOURNAL OF GEOLOGY

The inflection point of the cooling paths in the thermal (Sibson, 1977) and are thus not expected to record the history models is generally found in the uppermost most recent episodes of fault activity. part of the PAZ. Thus, most of the thermal evolution following the Late Cretaceous–Palaeogene shift in Present-day seismicity around the Hardangerfjord cooling rate occurs outside the sensitivity range of the implies ongoing periodic reactivation of brittle structures AFT method and cannot be interpreted directly. The in the area. Faulting related to differential glacio-isostatic majority of the presented models display accelerated compensation has been suggested as the dominant cooling in the most recent geological past. Rohrman et cause of the recorded seismic activity (Gudmundsson, al. (1995) put considerable emphasis on the latest portion 1999). Other studies (Bungum et al., 2010; Hicks et al., on the cooling history, i.e., from ~30 Ma onwards, and 2000) have considered far-field stresses induced by sea- argued for rapid Oligocene–Neogene cooling resulting floor spreading in the North Atlantic to be of greater from 1–1.5 km of domal tectonic uplift. The proposal of importance. Redfield & Osmundsen (2012) explain the a Neogene uplift event was largely based on the generally brittle deformation of the post-rift margin partly in terms high proportions of short tracks measured in the samples of lithospheric flexure generated by changing vertical from the Eidfjord area. Neogene cooling is a well-known loads. Erosion, transport and deposition constantly modelling artefact in thermal history models from AFT change the load on the lithosphere, thereby forcing the data (Redfield, 2010) and has previously been observed crust to rearrange. The redistribution of mass across in models from areas where recent cooling is not the passive margin may have been an essential driving supported by independent geological observations (e.g., force for onshore tectonic activity after the abortion of Danišík et al., 2012). The thermal sensitivity range of the North Sea rift in the Early Cretaceous. Both the fault the AFT system does not extend beyond the boundaries activity and the accelerated Cenozoic cooling obtained of the PAZ. Consequently, modelled cooling paths at from AFT thermal history modelling are readily temperatures below 60°C should merely be regarded as explained by rapid erosion of a topographically high vague suggestions. However, the short MTLs and young margin, accompanied by a shift of mass to offshore areas. ages obtained from some of the samples from the inner Hardangerfjord area (e.g., BG–26 and KJ–27) imply Combined results from AFT analysis, radiometric prolonged residence within the PAZ. Rapid, recent dating of fault rocks and seismology suggest that cooling is thus required to account for the data. For faulting might have been an ongoing process along samples derived from sea level and to a lesser degree the North Sea rift margin since Permian times to the intermediate elevations, Pliocene–Pleistocene glacial present. In the inner Hardangerfjord region and on the erosion may account for a substantial part of the most westernmost Hardangervidda plateau, there is generally recent cooling. Assuming that ~1 km of overburden no topographic expression of this extensive fault activity. was removed from the inner fjord arms during the last Whether fault displacements on the order of hundreds to 2.5 Myr, the particularly high cooling rate (>3°C Myr-1) thousands of metres would be preserved as prominent inferred for temperatures below ~35°C reflects fjord landscape features, depends on the time frame of faulting incision rather than regional uplift and denudation. This and the rate of erosion. Considering that the history of effect explains why samples collected at low elevations faulting recorded by the AFT data might date back as far require more rapid Neogene cooling than samples from as the Middle Jurassic, sufficient time has been available the Hardangervidda plateau. to allow levelling of any topographic expression from small-scale faults. Palaeogene and Neogene fault activity AFT ages, MTLs and thermal history models generally in the interior of southwestern Norway is difficult indicate later exhumation of the northern flank of the to reconcile with the notion of a peneplain that has innermost Hardangerfjord relative to the southern flank. essentially been preserved in a pristine state since its Samples obtained from similar elevations on opposite formation. Some authors have advocated the existence of margins of the fjord display slightly offset cooling paths a number of peneplains at different elevations (Riis, 1996; in the upper PAZ, suggesting that juxtaposition may Lidmar-Bergström et al., 2000), but no direct evidence have taken place subsequent to the time at which the for fault displacement of surfaces has been put forward. sea-level samples cooled out of the sensitivity range The absence of a clear topographic expression of faulting of the AFT method. It is plausible that the observed is easily explained in a setting dominated by a constantly differential cooling signatures are in part attributed to eroding, high topography. Erosive glaciers of the kind Cenozoic episodes of fault reactivation. K–Ar illite data described by Nielsen et al. (2009b) would efficiently indicate reactivation of the Lærdal–Gjende Fault at ~60 obliterate any remaining surface manifestations of fault Ma (Ksienzyk, 2012). No further record of Palaeogene displacement. fault reactivation is available from southwestern Norway. In this context, it is essential to note that most previous Regional perspective studies aiming to date fault activity (e.g., Andersen et al., 1999; Larsen et al., 2003) have focused on cohesive The general pattern of fission–track ages from the fault rocks. Cohesive breccias and cataclasites originate Bergen–Hardangerfjord region reveals progressively from brittle processes operating at greater crustal depths later exhumation from the coast towards the interior. NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of the SW Norwegian passive margin based on low-temperature thermochronology 257

Fission– track data from the Bergen and Sunnhordaland rifting, culminating with opening of the Atlantic Ocean areas are generally consistent with relatively early in the Eocene (Lundin & Doré, 1997). Changes in exhumation to shallow crustal levels (Rohrman et al., the orientation of the stress field have taken place in 1995; Leighton, 2007; Ksienzyk et al., 2014). Samples between individual rift phases (Færseth et al., 1995; from coast-proximal regions are typically characterised Færseth, 1996). The traditional landscape–development by Permian–Jurassic AFT ages and intermediate MTLs models furthermore fail to account for pre-rift structural on the order of 12–14 μm. In comparison, the data inheritance. This is critical, considering that the from the inner Hardangerfjord area reveal relatively Precambrian structural grain has exerted a major control late exhumation of the hinterland, evident by mainly on the development of the North Sea rift (Færseth et al., Jurassic–Cretaceous fission–track ages and short MTLs. 1995; Gabrielsen et al., 2002).

The AFT age distribution in southern Norway is largely Rohrman et al. (1995) set the stage for future fission– structurally controlled (Leighton, 2007; Ksienzyk et al., track studies in southern Norway by providing a 2014). Landscape development appears to have been comprehensive regional AFT record, but overlooked governed by down-to-the-W displacement along large- local differences in cooling signatures in their effort to scale extensional structures in association with the cover a geographically large area. The domal uplift model Permian and Mesozoic rift phases. Local exceptions suffers from the implicit presumption that southern to the general trends are abundant both in coastal and Norway behaved as a coherent block since Permian in inland areas and may reflect differential vertical times and does not fully describe the pattern of AFT movement across small-scale faults, as is suggested ages presented in the current contribution and other here for the inner Hardangerfjord region. Larger- recent studies (Leighton, 2007; Ksienzyk et al., 2014). We scale structures with easterly dip directions (e.g., the propose that the AFT age distribution rather results from Hjeltefjord Fault Zone; Fossen, 1998) produce locally a combination of fault-related uplift and denudation reversed age–distribution patterns (Ksienzyk, 2012). and regional differences in exhumation rate during both North Sea rift phases. The extensional tectonic activity The AFT data from southwestern Norway clearly in the North Sea has exerted a first-order control on the demonstrate that the evolution of the North Sea margin regional AFT age distribution onshore. Zircon fission– is much more complex than the scenarios portrayed track ages increase from the coast towards the interior, by classic passive-margin, landscape–development indicating more pronounced erosion along the coastline models (Fig. 8; see Gilchrist & Summerfield, 1990; Kooi during Permo–Triassic times (Ksienzyk et al., 2014). This & Beaumont, 1994; Ollier & Pain, 1997; Gallagher et al., pattern is consistent with large-scale, flexural rift-flank 1998). Although simplified, these models are well suited uplift, causing coastal samples to reach shallow crustal to a range of passive margins worldwide (see compilation depths at an earlier stage than inland samples. The AFT by Gallagher & Brown, 1997). The complexity of the age distribution in interior southern Norway largely Norwegian margin reflects a history of polyphase reflects exhumation of individual structural blocks

Figure 8. Models for passive-margin landscape evolution. (A) Downwarp model (Ollier & Pain, 1997): margin development as a denudational response to lithospheric flexure. (B) Scarp retreat model (Gilchrist & Summerfield, 1990): initial exten- sional faulting followed by landward retreat of the escarpment. (C) Pinned divide model (Kooi & Beaumont, 1994): vertical denudation of the initial rift flank from fluvial incision. (D) Footwall uplift model based on AFT ages from the Norwegian margin. Denudation patterns are controlled by differential ver- tical movement of fault-bound blocks. (E) Predicted AFT ages with distance from the coast for each of the presented models. The lowermost dashed line is taken to correspond to the North Atlantic rift phase in the Palaeogene. The rifting of the Nor- wegian margin was initiated in the North Sea in the Permian and culminated with the North Atlantic breakup in the Eocene. In order to be comparable to the passive-margin evolution models, which do not incorporate fjord incision, the portrayed age pattern inland of the escarpment is representative of the ages obtained from the Hardangervidda plateau. (A–C) and (E) are modified from Gallagher et al. (1998). 258 K.C. Johannessen et al. NORWEGIAN JOURNAL OF GEOLOGY resulting from mainly down-to-the-W extensional References faulting and associated footwall uplift. Fault-related exhumation efficiently explains the large age difference Andersen, T.B. 1998: Extensional tectonics in the Caledonides of southern Norway, an overview. Tectonophysics 285, 333–351. between the coast and the interior and the offset of ages Andersen, T.B., Torsvik, T.H., Eide, E.A., Osmundsen, P.T. & Faleide, across major structures. Small-scale faults have affected J.I. 1999: Permian and Mesozoic extensional faulting within the the local AFT record by offsetting ages across short Caledonides of central south Norway. Journal of the Geological lateral distances and even within vertical profiles. Society of London 156, 1073–1080. Andriessen, P.A.M. & Bos, A. 1986: Post-Caledonian thermal evolu- tion and crustal uplift in the Eidfjord area, Western Norway. Nor- wegian Journal of Geology 66, 243–250. Conclusion Barbarand, J., Hurford, A.J. & Carter, A. 2003: Variation in apatite fission-track length measurement: implications for thermal history Regional, low-temperature, thermochronological studies modelling. Chemical Geology 198, 77–106. have provided a general overview of the exhumation Brown, R.W., Beucher, R., Roper, S., Persano, C., Stuart, F. & Fitzge- history of the SW Norwegian passive margin. The rald, P.G. 2013: Natural age dispersion arising from the analysis of wide spacing of data points has, however, entailed a broken crystals. Part I: Theoretical basis and implications for the lack of control of the effect of faulting on the AFT apatite (U–Th)/He thermochronometer. Geochimica et Cosmochi- age distribution, leading to difficulties in recognising mica Acta 122, 478–497. Bungum, H., Olesen, O., Pascal, C., Gibbons, S., Lindholm, C. & temporal and spatial differences in cooling style. Apatite Vestøl, O. 2010: To what extent is the present seismicity of Norway fission–track and (U–Th)/He analyses of vertical profiles driven by post-glacial rebound? Journal of the Geological Society of from the hinterland of the passive margin in the inner London 167, 373–384. Hardangerfjord area reveal offset ages across small- Chalmers, J.A., Green, P., Japsen, P. & Rasmussen, E.S. 2010: The Scan- scale lineaments and fjords, indicating widespread fault dinavian mountains have not persisted since the Caledonian oro- activity from the Middle Jurassic onwards. Thermal geny. A comment on Nielsen et al. (2009a). Journal of Geodynamics history modelling suggests that the exhumation of 50, 94–101. Danišík, M., Kuhlemann, J., Dunkl, I., Evans, N.J., Székely, B. & Frisch, the margin has been closely linked to tectonic activity W. 2012: Survival of ancient landforms in a collisional setting as operating within adjacent rift basins. Relatively rapid revealed by combined fission track and (U–Th)/He thermochrono- -1 cooling is evident for the Permo–Triassic (2–6°C Myr­ ), metry: a case study from Corsica (France). The Journal of Geology corresponding to a significant unroofing of interior 120, 155–173. southern Norway as a consequence of onshore tectonic Donelick, R.A., Ketcham, R.A. & Carlson, W.D. 1999: Variabilities of activity and rift flank uplift during the formation of the apatite fission track annealing kinetics II: Crystallographic orienta- North Sea rift. A second episode of accelerated cooling tion effects. American Mineralogist 84,1224–1234. -1 Doré, A.G., Lundin, E.R., Jensen, L.N., Birkeland, Ø., Eliassen, P.E. & (>3°C ­Myr ) is obtained for the latest Cretaceous Fichler, C. 1999: Principal tectonic events in the evolution of the onwards and may be ascribed to ongoing topographic northwest European Atlantic margin. In Fleet, A.J. & Boldy, S.A.R. rejuvenation, involving fault-related footwall uplift (eds.): Petroleum Geology of Northwest Europe: Proceedings of the and enhanced erosion rates. The stepwise increase 5th conference, The Geological Society of London, pp. 41–61. in AFT ages from the hinterland to the outer margin Dumitru, T.A. 1993: A new computer-automated microscope stage demonstrates that faulting has been a common process system for fission-track analysis. Nuclear Tracks and Radiation onshore throughout the Mesozoic and probably also the Measurements 21, 557–580. Dunkl, I. 2002: TRACKKEY: a Windows program for calculating and Cenozoic. Hence, the apparent domal warping of AFT graphical presentation of fission track data. Computers & Geo­ isochrons that has been emphasised in previous studies sciences 28, 3–12. is merely a result of extrapolation between widely spaced Dunlap, W.J. & Fossen, H. 1998: Early Paleozoic orogenic collapse, samples. High Neogene cooling rates at low elevations tectonic stability, and late Paleozoic continental rifting revealed in the inner fjord arms are not necessarily indicative through thermochronology of K-feldspars, southern Norway. of rapid tectonic uplift, but may rather be attributed to ­Tectonics 17, 604–620. Plio–Pleistocene fjord incision. The low-temperature Eide, E.A., Torsvik, T.H. & Andersen, T.B. 1997: Absolute dating of brittle fault movements: Late Permian and Late Jurassic fault brec- thermochronological record from the SW Norwegian cias in western Norway. Terra Nova 9, 135–139. margin is consistent with sustained elevated topography, England, P. & Molnar, P. 1990: Surface uplift, uplift of rocks, and transformed through regional rift-related uplift, periodic exhum­ation of rocks. Geology 18, 1173–1177. extensional faulting and erosion. Farley, K.A., Wolf, R.A. & Silver, L.T. 1996: The effects of long alpha- stopping distances on (U–Th)/He ages. Geochimica et Cosmochi- mica Acta 60, 4223–4229. Acknowledgements. This study has received funding from Statoil through Fitzgerald, P.G. & Gleadow, A.J.W. 1990: New approaches in fission the Earth System Modelling project at the University of Bergen. We track geochronology as a tectonic tool: Examples from the Trans- thank Arne Johannessen for field assistance and Vibeke Gellein Hanssen antarctic Mountains. Nuclear Tracks Radiation Measurements 17, for help with sample preparation. Members of the Bergen Tectonics and 351–357. Thermochronology research group are thanked for fruitful discussions Fitzgerald, P.G., Sorkhabi, R.B., Redfield, T.F. & Stump, E. 1995: Uplift and feedback. We gratefully acknowledge helpful and constructive and denudation of the central Alaska Range: A case study in the use reviews by K. Gallagher and T.F. Redfield. of apatite fission track thermochronology to determine absolute uplift parameters. Journal of Geophysical Research 100, 175–191. Fossen, H. 1992: The role of extensional tectonics in the Caledonides of south Norway. Journal of Structural Geology 14, 1033–1046. NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of the SW Norwegian passive margin based on low-temperature thermochronology 259

Fossen, H. 1998: Advances in understanding the post-Caledonian Japsen, P. & Chalmers, J.A. 2000: Neogene uplift and tectonics around structural evolution of the Bergen area, West Norway. Norwegian the North Atlantic: overview. Global and Planetary Change 24, Journal of Geology 78, 33–46. 165–173. Fossen, H. & Dunlap, W.J. 1999: On the age and tectonic significance Ketcham, R.A. 2005: Forward and inverse modeling of low-tempera- of Permo–Triassic dikes in the Bergen–Sunnhordaland region, ture thermochronometry data. Reviews in Mineralogy & Geoche- southwestern Norway. Norwegian Journal of Geology 79, 169–178. mistry 58, 275–314. Fossen, H. & Hurich, C.A. 2005: The Hardangerfjord Shear Zone in Ketcham, R.A., Carter, A., Donelick, R.A., Barbarand, J. & Hurford, SW Norway and the North Sea: a large scale low-angle shear zone A.J. 2007a: Improved modeling of fission-track annealing in apa- in the Caledonian crust. Journal of the Geological Society of London tite. American Mineralogist 92, 799–810. 162, 675–687. Ketcham, R.A., Carter, A., Donelick, R.A., Barbarand, J. & Hurford, Fossen, H., Mangerud, G., Hesthammer, J., Bugge, T. & Gabrielsen, R.H. A.J. 2007b: Improved measurement of fission-track annealing in 1997: The Bjorøy Formation: a newly discovered occurrence of apatite using c-axis projection. American Mineralogist 92, 789–798. Jurassic sediments in the Bergen Arc System. Norwegian Journal of Kooi, H. & Beaumont, C. 1994: Escarpment evolution on high- Geology 77, 269–287. elevation rifted margins: insights derived from a surface process Færseth, R.B. 1996: Interaction of Permo–Triassic and Jurassic exten- model that combines diffusion, advection, and reaction. Journal of sional fault blocks during the development of the northern North Geophysical Research 99, 191–209. Sea. Journal of the Geological Society of London 153, 931–944. Ksienzyk, A.K. 2012: From mountains to basins: geochronological case Færseth, R.B., Macintyre, R.M. & Naterstad, J. 1976: Mesozoic alkaline studies from southwestern Norway, Western Australia and East dykes in the Sunnhordaland region, western Norway: ages, geoche- Antarctica. PhD thesis, University of Bergen, 162 pp. mistry and regional significance. Lithos 9, 331–345. Ksienzyk, A.K., Dunkl, I., Jacobs, J., Fossen, H. & Kohlmann, F. 2014: Færseth, R.B., Gabrielsen, R.H. & Hurich, C.A. 1995: Influence of From orogen to passive margin: constraints from fission track and basement in structuring of the North Sea basin, offshore southwest (U–Th)/He analyses on Mesozoic uplift and fault reactivation in Norway. Norwegian Journal of Geology 75, 105–119. SW Norway. In Corfu, F., Gasser, D. & Chew, D.M. (eds.): New Per- Gabrielsen, R.H., Braathen, A., Dehls, J.F. & Roberts, D. 2002: Tectonic spectives on the Caledonides of Scandinavia and Related Areas, Geo- lineaments of Norway. Norwegian Journal of Geology 82, 153–174. logical Society of London, Special Publications 390, doi 10.1144/ Gabrielsen, R.H., Faleide, J.I., Pascal, C., Braathen, A., Nystuen, J.P., SP390.27. Etzelmuller, B. & O’Donnel, S. 2010: Latest Caledonian to Present Kusznir, N.J., Marsden, G. & Egan, S.S. 1991: A flexural-cantilever tectonomorphological development of southern Norway. Marine simple-shear/pure-shear model of continental lithosphere exten- and Petroleum Geology 27, 709–723. sion: applications to the Jeanne d’Arc Basin, Grand Banks and Galbraith, R.F. (1981). On statistical models for fission track counts. Vikin Graben, North Sea. In Roberts, A.M., Yielding, G. & Free- Mathematical Geology 13, 471–478. man, B. (eds): The Geometry of Normal Faults, Geological Society Gallagher, K. & Brown, R. 1997: The onshore record of passive margin of London, Special Publication 56, pp. 41–60. evolution. Journal of the Geological Society of London 154, 451–457. Lal, D., Rajan, R.S. & Tamhane, A.S. 1969: Chemical composition of Gallagher, K., Brown, R. & Johnson, C. 1998: Fission track analysis and nuclei Z>22 in cosmic rays using meteoritic minerals as detectors. its applications to geological problems. Annual Reviews in Earth Nature 221, 33–37. and Planetary Sciences 26, 519–572. Larsen, Ø., Fossen, H., Langeland, K. & Pedersen, R.B. 2003: Kinema- Gallagher, K., Stephenson, J., Brown, R., Holmes, C. & Fitzgerald, P.G. tics and timing of polyphase post-Caledonian deformation in the 2005: Low temperature thermochronology and modelling strate- Bergen area, SW Norway. Norwegian Journal of Geology 83, 149– gies for multiple samples 1: Vertical profiles. Earth and Planetary 165. Science Letters 237, 193–208. Leighton, C.A. 2007: The thermotectonic development of southern Gilchrist, A.R. & Summerfield, M.A. 1990: Differential denudation Norway: constraints from low-temperature thermochronology. PhD and flexural isostasy in the formation of rifted margin upwarps. thesis, Imperial College London, 296 pp. Nature 346, 739–742. Lidmar-Bergström, K. & Bonow, J.M. 2009: Hypotheses and observa- Gleadow, A.J.W. & Fitzgerald, P.G. 1987: Uplift history and structure tions on the origin of the landscape of southern Norway – A com- of the Transantarctic mountains: new evidence from fission track ment regarding the isostacy-climate-erosion hypothesis by Nielsen dating of basement apatites in the Dry Valley area, southern Victo- et al. 2008. Journal of Geodynamics 48, 95–100. ria Land. Earth and Planetary Science Letters 82, 1–14. Lidmar-Bergström, K., Ollier, C.D. & Sulebak, J.R. 2000: Landforms Gleadow, A.J.W., Duddy, I.R., Green, P.F. & Lovering, J.F. 1986: Con- and uplift history of southern Norway. Global and Planetary fined fission tracks in apatite: a diagnostic tool for thermal history Change 24, 211–231. analysis. Contributions to Mineralogy and Petrology 94, 405–415. Lundin, E.R. & Doré, A.G. 1997: A tectonic model for the Norwegian Gudmundsson, A. 1999: Postglacial crustal doming, stresses and fracture passive margin with implications for the NE Atlantic: Early Creta- deformation with application to Norway. Tectonophysics 307, 407–419. ceous to breakup. Journal of the Geological Society of London 154, Gunnel, Y. 2000: Apatite fission track thermochronology: an overview 545–550. of its potential and limitations in geomorphology. Basin Research Løvlie, R. & Mitchell, J.G. 1982: Complete remagnetization of some 12, 115–132. Permian dykes from western Norway induced during burial/uplift. Hicks, E.C., Bungum, H. & Lindholm, C.D. 2000: Stress inversion of Physics of the Earth and Planetary Interiors 30, 415–421. earthquake focal mechanism solutions from onshore and offshore Meesters, A.G.C.A. & Dunai, T.J. 2002: Solving the production-diffu- Norway. Norwegian Journal of Geology 80, 235–250. sion equation for finite diffusion domains of various shapes. Part Hourigan, J.K., Reiners, P.W. & Brandon, M.T. 2005: U/Th zonation II. Application to cases with α-ejection and nonhomogeneous dis- dependent alpha ejection in (U–Th)/He thermochronometry. tribution of the source. Chemical Geology 186, 347–363. Geochimica et Cosmochimica Acta 96, 3349–3365. Mosar, J. 2003: Scandinavia’s North Atlantic passive margin. Journal of Hurford, A.J. & Green, P.F. 1983: The zeta age calibration of fission- Geophysical Research 108, 1–18. track dating. Isotope Geoscience 1, 285–317. Nesje, A., Dahl, S.O., Valen, V. & Øvstedal, J. 1992: Quarternary Hurich, C.A. & Kristoffersen, Y. 1988: Deep structure of the Caledo- erosion in the Sognefjord drainage basin, western Norway. Geo­ nide orogen in southern Norway: new evidence from marine seis- morphology 5, 511–520. mic reflection profiling. In Kristoffersen, Y. (ed.): Progress in studies Nesje, A. & Whillans, I.M. 1994: Erosion of Sognefjord, Norway. Geo- of the lithosphere in Norway, Geological Survey of Norway Special morphology 9, 33–45. Publication 3, pp. 96–101 260 K.C. Johannessen et al. NORWEGIAN JOURNAL OF GEOLOGY

Neumann, E.-R., Olsen, K.H., Baldridge, W.S. & Sundvoll, B. 1992: Geophysical Research 99, 13871–13883. The Oslo Rift: a review. Tectonophysics 208, 1–18. Torsvik, T.H., Sturt, B.A., Swensson, E., Andersen, T.B. & Dewey, J.F. Nielsen, S.B., Gallagher, K., Egholm, D.L., Clausen, O.R. & Summer- 1992: Palaeomagnetic dating of fault rocks: evidence for Permian field, M. 2009a: Reply to comment regarding the ICE hypothesis. and Mesozoic movements and brittle deformation along the exten- Journal of Geodynamics 48, 101–106. sional Dalsfjord fault, western Norway. Geophysical Journal Inter- Nielsen, S.B., Gallagher, K., Leighton, C., Balling, N., Svenningsen, L., national 109, 565–580. Jacobsen, B.H., Thomsen, E., Nielsen, O.B., Heilmann-Clausen, C., Wagner, G. & Reimer, G.M. 1972: Fission track tectonics: The tectonic Egholm, D.L., Summerfield, M.A., Clausen, O.R., Piotrowski, J.A., interpretation of fission track apatite ages. Earth and Planetary Sci- Thorsen, M.R., Huuse, M., Abrahamsen, N., King, C. & Lykke- ence Letters 14, 263–268. Andersen, H. 2009b: The evolution of western Scandinavian topo- Ziegler, P.A. 1990: Tectonic and palaeogeographic development of the graphy: A review of Neogene uplift versus the ICE (isostacy-cli- North Sea rift system. In Blundell, D.J. & Gibbs, A.D. (eds.): Tecto- mate-erosion) hypothesis. Journal of Geodynamics 47, 72–95. nic Evolution of the North Sea Rifts. Clarendon, Oxford, pp. 1–36. Nielsen, S.B., Clausen, O.R., Jacobsen, B.H., Thomsen, E., Huuse, M., Gallagher, K., Balling, N. & Egholm, D. 2010a: The ICE hypothesis stands: how the dogma of late Cenozoic tectonic uplift can no long­er be sustained in the light of new data and physical laws. Jour- nal of Geodynamics 50, 102–111. Nielsen, S.B., Clausen, O.R., Pedersen, V.K., Leseman, J.-E., Gole- dowski, B., Huuse, M., Gallagher, K. & Summerfield, M. 2010b: Discussion of Gabrielsen et al. (2010): Latest Caledonian to present tectonomorphological development of southern Norway. Marine and Petroleum Geology 27, 1285–1289. Ollier, C.D. & Pain, C.F. 1997: Equating the basal unconformity with the palaeoplain: a model for passive margins. Geomorphology 19, 1–15. Pascal, C., Elvebakk, H. & Olesen, O. 2010: An Assessment of Deep Geothermal Resources in Norway. Abstracts and Proceedings, World Geothermal Congress, 25–29 April, Bali, Indonesia. Redfield, T.F. 2010: On apatite fission track dating and the Tertiary evolution of West Greenland topography. Journal of the Geological Society of London 167, 261–271. Redfield, T.F. & Osmundsen, P.T. 2009: The Tjellefonna fault system of Western Norway: Linking late-Caledonian extension, post- Caledonian normal faulting, and Tertiary rock column uplift with the landslide-generated tsunami event of 1756. Tectonophysics 474, 106–123. Redfield, T.F. & Osmundsen, P.T. 2013: The long-term topographic response of a continent adjacent to a hyperextended margin: A case study from Scandinavia. Geological Society of America Bulletin 125, 184–200. Redfield, T.F., Torsvik, T.H., Andriessen, P.A.M. & Gabrielsen, R.H. 2004: Mesozoic and Cenozoic tectonics of the Møre Trøndelag Fault Complex, central Norway: constraints from new apatite fis- sion track data. Physics and Chemistry of the Earth 29, 673–682. Redfield, T.F., Braathen, A., Gabrielsen, R.H., Osmundsen, P.T., Tor- svik, T.H. & Andriessen, P.A.M. 2005: Late Mesozoic to Early Cenozoic components of vertical separation across the Møre– Trøndelag fault complex, Norway. Tectonophysics 395, 233–249. Riis, F. 1996: Quantification of Cenozoic vertical movements of Scan- dinavia by correlation of morphological surfaces with offshore data. Global and Planetary Change 12, 331–357. Roberts, D. & Gee, D.G. 1985: An introduction to the structure of the Scandinavian Caledonides. In Gee, D.G. & Sturt, B.A. (eds.): The Caledonide Orogen – Scandinavia and Related Areas. John Wiley and Sons, Chichester, pp. 55–68. Rohrman, M., van der Beek, P., Andriessen, P.A.M. & Cloetingh, S. 1995: Meso–Cenozoic morphotectonic evolution of southern Nor- way: Neogene domal uplift inferred from apatite fission track ther- mochronology. Tectonics 14, 704–718. Shuster, D.L., Flowers, R.M. & Farley, K.A. 2006: The influence of natural radiation damage on helium diffusion kinetics in apatite. Earth and Planetary Science Letters 249, 148–161. Sibson, R.H. 1977: Fault rocks and fault mechanisms. Journal of the Geological Society of London 133, 191–213. Sigmond, E.M.O. 1998: Odda, bedrock geology map, scale 1:250.000, Geological Survey of Norway. Summerfield, M.A. & Hulton, N.J. 1994: Natural controls of flu- vial denudation rates in major world drainage basins. Journal of