tectonic evolution of the sw norwegian passive margin based on

243

Introduction

The tectonomorphological evolution of the North Sea rift margin remains incompletely understood. While the offshore geology has been extensively studied in connection with hydrocarbon exploration in the North Sea, less attention has been devoted to onshore areas. Post-Devonian sediments are scarce in southern Norway, making quantification of crustal uplift and fault displacements difficult. Low-temperature thermo-chronological techniques can be employed to constrain the timing of vertical movements through the uppermost few kilometres of the crust and are at present the most effective means of obtaining information about exhumation and topographic evolution prior to the Quaternary glaciations.

The glacially shaped fjords in southern Norway offer a unique opportunity to sample across the passive margin

into the hinterland at constant, low elevations and may potentially reveal essential aspects concerning the style of rift-margin evolution over time. The Eidfjord area in the innermost part of the Hardangerfjord is regarded as one of the classical sites for low-temperature thermochronology in Norway, and a number of studies have been undertaken in this particular area (Andriessen & Bos, 1986; Rohrman et al., 1995; Leighton, 2007). Some of the youngest apatite fission track (AFT) ages in Norway are found at low elevations in the inner regions of the Hardangerfjord. The combination of young AFT ages close to sea level and high-relief topography makes the inner Hardangerfjord and its tributaries excellent targets for detailed studies on the timing and magnitude of Mesozoic and Cenozoic exhumation. Previous work in the region has, however, failed to provide a comprehensive representation of the exhumation history, mainly because the so-called vertical profiles that have been sampled extend over

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 contri bution 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]

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

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

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

244 K.C. Johannessen et al. NORWEGIAN JOURNAL OF GEOLOGY

Hordaland–Sogn areas, respectively, suggesting that the tectonic history of southern Norway may be much more complex than that portrayed by Rohrman et al. (1995).

The Oligocene–Neogene accelerated cooling and the domal pattern of AFT ages presented by Rohrman et al. (1995) are widely cited in the literature as supportive evidence for large-scale, Cenozoic tectonic uplift of southern Norway, which is required to explain the presence of a putative Cretaceous–Palaeogene peneplain at high elevations (e.g., Japsen & Chalmers, 2000; Lidmar-Bergström et al., 2000; Gabrielsen et al., 2010). Recently, the origin of the topography in southern Norway has received renewed attention (Lidmar-Bergström & Bonow, 2009; Nielsen et al., 2009a, b, 2010a, b; Chalmers et al., 2010; Gabrielsen et al., 2010). The highly controversial ICE (Isostasy–Climate–Erosion) hypothesis by Nielsen et al. (2009b) challenged the concept of Cenozoic topographic rejuvenation by stating that the present mountains are remnants of Caledonian topography, sustained through isostatic rebound of a buoyant crustal root and shaped through the combined effects of glacial erosion and periglacial processes. The available low-temperature thermochronological data from southern Norway are compatible with gradual denudation of a long-lived orogen (Nielsen et al., 2009b), showing that the AFT record is not as unambiguous as previously assumed.

Here, we test the viability of the exhumation scenarios depicted by the domal uplift model by applying AFT analysis in combination with (U–Th)/He thermo-chronology to samples from steep vertical profiles from the fjord flanks in the Eidfjord–Ulvik district, one of the key localities of Rohrman et al. (1995). The study is motivated by the need for a higher–resolution AFT sampling strategy and a better control on the structural complexity at the scale of outcrops and vertical profiles. Our specific foci are the timing, rate and driving forces of exhumation and the extent to which Mesozoic and Cenozoic faulting has affected the morphological evolution of the North Sea rift margin.

Geological settingThe geology of the inner Hardangerfjord region is dominated by Proterozoic basement rocks, partly over lain by strongly sheared Cambro–Silurian meta-sedimentary rocks and Caledonian nappes, belonging to the Lower and Middle Allochthon (Roberts & Gee, 1985). The tectonostratigraphy is dissected by densely spaced brittle structures with unknown origins and records of displacement. N–S- and WNW–ESE-trending lineaments are particularly abundant. These populations roughly parallel major Svecofennian and Sveconorwegian shear zones and may belong to a Proterozoic fracture pattern that extends across large parts of Fennoscandia (Gabrielsen et al., 2002).

significant horizontal distances (i.e., >20 km). Numerous brittle structures dissect the basement of the inner Hardangerfjord and major linear features, including fjords and deeply incised glacial valleys, characterise the landscape. Limited structural work has been conducted so far and consequently, little is known about the nature of these structures, their possible displacements and their effect on the distribution of low-temperature thermochronological data in the area.

The pioneering, low-temperature, thermochronological work in Norway was conducted in the Eidfjord area by Andriessen & Bos (1986), who documented a general increase in AFT age with elevation and suggested a decrease in cooling rate from the Permian to the Jurassic. The sampling density of this reconnaissance study was quite low and no vertical profiles were collected.

A more detailed dataset was presented by Rohrman et al. (1995), who included the Eidfjord area in a regional AFT study of southern Norway. In their study, a vertical profile was sampled along the Måbødalen road from sea level in Eidfjord to the Hardangervidda plateau, covering a lateral distance of ~20 km. Based on the regional age distribution and thermal history modelling, Rohrman et al. (1995) suggested two major episodes of rapid exhumation, a Triassic–Jurassic phase that started in the east and migrated westwards and an Oligocene–Neogene uplift event characterised by domal warping of the AFT isochrons, indicating greater exhumation in the interior than along the coastline. The Neogene cooling–spike hypothesis was largely based on the thermal histories of the youngest samples from the inner fjord regions, while an apparent radial pattern of increasing AFT ages from the interior towards the coast was interpreted as an indication of domal uplift, with southern Norway behaving as a structurally coherent block since Permian times.

In a later contribution, Redfield et al. (2004, 2005) documented significantly offset AFT ages across segments of the Møre–Trøndelag Fault Complex in Mid Norway, suggesting that the evolution of the North Atlantic passive margin may largely have been governed by reactivation of older structures during the Mesozoic–Cenozoic. Normal faulting of the innermost proximal margin has been proposed as the main control on rift–escarpment rejuvenation along the coast from Møre to Troms, substantiating that fault activity may have continued into the most recent geological past (Redfield & Osmundsen, 2012). While the Møre–Trøndelag region faces the active spreading system in the North Atlantic, the coastal region of southern Norway borders the aborted rift in the North Sea. The styles of rift-margin evolution, particularly in relation to the magnitude and timing of faulting, may differ significantly between the two realms. Recently, Leighton (2007) and Ksienzyk (2012) reported different Mesozoic and Cenozoic cooling histories for adjacent structural blocks in the Bergen and

245NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of the SW Norwegian passive margin based on low-temperature thermochronology

expression of the HSZ is a several kilometre-wide zone of mylonites and the estimated down-to-the-NW displace-ment is on the order of 5 km (Fossen & Hurich, 2005).

The HSZ belongs to an array of major W–NW-dipping shear zones in the western part of southern Norway (Fig. 1). These structures accommodated significant extensional displacement in connection to the collapse of the Caledonian orogen in the Devonian (Fossen, 1992; Andersen, 1998). The bulk displacement along the HSZ took place in the ductile regime immediately following the termination of the orogeny (Fossen & Hurich, 2005).

The NE–SW-trending Hardangerfjord Shear Zone (HSZ) is the largest tectonic feature in the Hardanger region and follows the Hardangerfjord northeastwards to the Gran-vin area, where it is exposed subaerially. Caledonian thrust sheets are mainly preserved in the down-faulted hangingwall west of Granvin and are mono clinically folded in the half-graben that overlies the shear zone (Fossen & Hurich, 2005). As a result of large-scale uplift and accelerated denudation of the footwall, Cale donian nappes and underlying parautochthonous metasedimen-tary rocks are merely preserved as outliers to the south-east of the shear zone (Sigmond, 1998). The sub aerial

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).


1983) was performed prior to analysis. TrackKey (Dunkl, 2002) was applied to calculate AFT ages. A cursor with a light-emitting diode was used for length measurements. Only horizontal TINT-type tracks (Lal et al., 1969) in apatites oriented parallel to the crystallographic c-axis were measured, following the recommendation of Barbarand et al. (2003).

(U–Th)/He analyses were conducted at the Geoscience Centre at the University of Göttingen, Germany. Three grains with good crystal morphologies were hand-picked from each sample. All selected crystals appeared to be free of inclusions and microfractures. The length and width of each apatite was measured and the grains were packed in separate platinum capsules. He-degassing was performed under vacuum in a sealed furnace by heating the sample to c. 870°C for 20 min with an infrared diode laser. A hot blank was established prior to every analysis. Subsequent to degassing, the obtained 4He was spiked with a known amount of 99+% 3He and purified with a SAES Ti–Zr getter at 450°C. The isotopic–ratio analysis was performed with a Hiden triple-filter quadrupole mass spectrometer, equipped with an ion-counting detector. Re-extraction of He was executed in order to quantify excess gas derived from mineral inclusions within the apatite. Following re-extraction, the platinum capsules were retrieved from the vacuum chamber. Each apatite grain was then dissolved in 2% nitric acid and 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 APEX micro flow nebuliser, and the 238U, 232Th and Sm contents were calculated from the obtained ratios.

Thermal history modelling was performed with HeFTy 1.7.5 (Ketcham, 2005). The annealing model of Ketcham et al. (2007a) was applied as a basis for estimation of kinetic behaviour, and Dpar (Donelick et al., 1999) was used as a kinetic annealing parameter. Confined track lengths were corrected by c-axis projection, as proposed by Ketcham et al. (2007b). Zircon fission–track ages of 306 ± 22 Ma from Eidfjord (Andriessen & Bos, 1986) were implemented as thermal constraints in all models. The zircon fission–track system records cooling of the inner Hardangerfjord area roughly concomitantly with the initiation of rifting and volcanism in the Oslo region (Neumann et al., 1992) and possibly also in the North Sea (Dunlap & Fossen, 1998). Permian magmatic activity in western Norway is manifested by ~260 Ma, coast-parallel, dolerite dykes, which crop out in the outer Hardangerfjord region (Færseth et al., 1976; Løvlie & Mitchell, 1982; Fossen & Dunlap, 1999). There are no indications of magmatism around the inner Hardangerfjord at this time and the studied area is situated too far from both tectonically active regions for magmatic heating to have had any effect on the fission–track system. The zircon fission–track ages from Eidfjord are therefore taken to record denudation rather than cooling following a magmatic event and

The HSZ can be traced along the entire length of the Hardangerfjord and its offshore continuation is represented by a set of NE–SW-trending lineaments and NW-dipping reflectors that extend through the Ling Depression, across the North Sea and possibly link up with the Highland Boundary Fault in Scotland (Færseth et al., 1995; Fossen & Hurich, 2005). Seismic reflection profiling has revealed an abrupt increase in the crustal thickness across the HSZ, from 28 km in the northwest to 34 km in the southeast (Hurich & Kristoffersen, 1988). The shear zone may thus be part of an innermost boundary–fault system (Mosar, 2003) that separates the thinned crust affected by the North Atlantic rifting from the hinterland of the Norwegian passive margin.

The brittle Lærdal–Gjende Fault (LGF) transects the Caledonian thrust sheets in the area directly to the northwest of the Hardangerfjord and is interpreted as an upper–crustal expression of the HSZ, formed during late stages of the Devonian orogenic collapse (Fossen & Hurich, 2005). Reactivation of the LGF is believed to have taken place during Permian and Late Jurassic–Early Cretaceous times (Andersen et al., 1999), concomitantly with major rift phases in the North Sea (e.g., Færseth et al., 1995) and associated pulses of fault activity in coast-proximal areas (Torsvik et al., 1992; Eide et al., 1997; Ksienzyk, 2012). K–Ar illite dating of fault gouges has revealed additional episodes of reactivation during the latest Cretaceous–Palaeogene (Ksienzyk, 2012).

MethodsAFT analyses were performed on 32 samples from three vertical profiles in the innermost Hardangerfjord area, applying the external detector method to determine single-grain ages and U-concentrations. Standard procedures for heavy-mineral separation, including Wilfley table, magnetic and heavy–liquid separation, were employed in order to isolate apatite for AFT and (U–Th)/He analysis. For AFT analysis, apatites were embedded in epoxy resin and internal crystal surfaces were exposed using standard grinding and polishing techniques. Each sample mount was etched in 5 M nitric acid (HNO3) for 20 seconds at 20 ± 1°C. A mica plate was then attached to each sample to act as an external detector for induced fission. Thermal neutron irradiation was conducted at the Garching Forschungsreaktor FRM II at the Technical University of Munich, using a thermal neutron flux of 1016 neutrons/cm-2. Induced tracks in mica were revealed through etching in 40% hydrofluoric acid (HF) for 20 min at room temperature.

AFT analyses were executed in the thermochronology laboratory at the University of Bergen with an Olympus BX51 optical microscope, equipped with a CalComp Drawing-board III digitising tablet and a computer-controlled Kinetek stage, run by the FTstage software by Dumitru (1993). Zeta calibration (Hurford & Green,


Customary measurements of etch pit diameter (Dpar) were performed for all samples, resulting in mean values between 1.18 and 1.69. All samples display similar, rather low Dpar values, indicative of near end-member fluorapatite compositions.

The Osa profile north of the Hardangerfjord comprises 12 samples collected over a relief of 1330 m and displays a wide range of AFT ages from 111 ± 9 to 221 ± 36 Ma. A general positive correlation between age and elevation is evident (Fig. 3A).

The AFT ages from the Simadalen profile in the innermost part of the fjord range from 120 ± 7 to 169 ± 10 Ma (Fig. 3B). All samples collected from sea level to an elevation of 604 m a.s.l. exhibit ages spanning from 120 ± 7 to 124 ± 7 Ma. A strong correlation between age and elevation is observed, although the age variation that is recorded by this specific portion of the profile is limited. The uppermost samples reveal a strong trend of progressively increasing AFT ages with elevation and display a greater span of ages over approximately the same vertical distance as the lowermost portion of the profile. All samples have similar MTLs in the range of 10.93 ± 0.20 to 11.52 ± 0.20 μm.

are considered reasonable as a constraint for AFT thermal history modelling. A present-day annual mean surface temperature of 7 ± 3°C was applied as a second constraint.

Results

Apatite fission–track analysis

The AFT ages from the inner Hardangerfjord area range from 97 to 221 Ma, with a majority of ages between 120 and 170 Ma. Except for the Late Triassic KJ–24 from high elevation at the Hardangervidda plateau and the Late Cretaceous BG–16 from the hangingwall of the HSZ, all ages are Jurassic or Early Cretaceous. The results obtained from AFT analysis of individual samples are presented in Table 1, and Fig. 2 displays the age and location of all analysed samples.

Track lengths were measured for eleven samples, which were selected to represent different structural blocks and elevations within the vertical profiles. The mean track lengths (MTLs) are confined to a limited range of values between 10.41 ± 0.25 μm and 11.66 ± 0.23 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.


Tabl

e 1.

Apa

tite f

issio

n-tra

ck d

ata.

Sam

ple

no.

UTM

(zon

e 32

V)

Elev

atio

nN

o. o

fSp

onta

neou

siIn

duce

diD

osim

eter

iC

entra

l age

± 1σ

iiP (χ

)iiiM

TL ±

1σ

SDN

o. o

fM

ean

Dpa

rSD

U

Nor

thin

gEa

sting

(m a

sl)gr

ains

ρs (1

0^5)

Ns

ρi (1

0^5)

Ni

ρd (1

0^5)

Nd

(Ma)

(%)

(μm

)(μ

m)

track

s(μ

m)

(μm

)(p

pm)

BG-1

337

1904

6701

787

2520

6.84

060

013

.155

1154

18.9

1480

7112

3 ±

830

.3N

/AN

/A0

1.62

0.10

9.8

BG-1

439

8209

6719

094

1057

2011

.654

679

13.6

1079

316

.942

6971

184

± 12

67.8

N/A

N/A

01.

340.

1010

.6BG

-16

3723

4567

1942

151

120

10.0

4374

321

.952

1624

16.8

9569

7197

± 5

36.6

N/A

N/A

01.

550.

0918

.7BG

-26

3702

3467

0681

425

2012

.957

916

25.5

4618

0616

.661

6971

105

± 5

80.1

10.4

1 ±

0.25

2.00

100

1.50

0.12

21.4

BG-2

736

7631

6701

607

128

2012

.898

1099

23.5

5520

0719

.072

8071

130

± 6

95.6

11.4

3± 0

.18

1.82

631.

310.

0917

.7BG

-53

3689

9866

9188

23

202.

944

227

5.37

041

418

.993

8071

130

± 11

98.0

N/A

N/A

01.

180.

094.

5K

J-1

3970

2267

1788

094

820

7.50

870

812

.090

1140

21.2

5779

2316

6 ±

1121

.2N

/AN

/A0

1.40

0.08

8.0

KJ-

339

5211

6717

419

749

205.

119

483

7.94

975

021

.140

7923

169

± 11

58.1

N/A

N/A

01.

450.

135.

5K

J-4

3947

7767

1721

055

020

6.72

269

210

.763

1108

21.0

2279

2316

3 ±

987

.5N

/AN

/A0

1.47

0.07

7.2

KJ-

539

4223

6717

643

332

207.

864

708

12.5

6311

3120

.904

7923

163

± 9

82.8

N/A

N/A

01.

590.

108.

7K

J-6

3933

3767

1809

119

620

3.17

030

46.

237

598

20.7

8679

2313

2 ±

1099

.6N

/AN

/A0

1.51

0.07

4.4

KJ-

739

2936

6718

446

106

203.

081

274

5.75

751

220

.668

7923

138

± 11

99.0

N/A

N/A

01.

530.

074.

1K

J-8

3923

9967

1807

945

206.

179

357

14.2

882

520

.550

7923

111

± 9

12.2

10.5

9 ±

0.25

2.47

100

1.51

0.09

10.3

KJ-

938

7723

6706

130

2820

6.06

751

012

.051

1013

20.4

3279

2312

8 ±

849

.4N

/AN

/A0

1.48

0.10

8.5

KJ-

1139

4397

6705

845

1520

6.96

375

514

.683

1592

20.3

1479

2312

0 ±

732

.711

.51

± 0.

191.

8610

01.

460.

0710

.3K

J-12

3820

9267

0338

498

220

9.44

656

913

.779

830

20.1

9679

2317

2 ±

135.

2N

/AN

/A0

1.27

0.12

9.4

KJ-

1338

2239

6703

732

791

2022

.592

2264

34.1

2834

2020

.078

7923

165

± 7

24.3

N/A

N/A

01.

570.

1024

.1K

J-14

3824

1067

0400

564

320

24.6

6518

2244

.726

3304

19.9

6079

2313

7 ±

624

.210

.90

± 0.

242.

3710

01.

470.

1331

.9K

J-15

3826

4667

0452

244

220

9.80

677

018

.644

1464

19.8

4279

2313

1 ±

100.

1N

/AN

/A0

1.59

0.08

13.6

KJ-

1638

2574

6705

192

189

207.

568

523

11.9

0982

319

.724

7923

155

± 10

33.7

N/A

N/A

01.

670.

108.

7K

J-17

3827

3667

0560

910

120

5.70

939

711

.173

777

19.6

0679

2312

5 ±

994

.0N

/AN

/A0

1.49

0.10

8.6

KJ-

1838

3695

6705

475

120

8.09

270

115

.93

1380

19.4

8879

2312

4 ±

827

.311

.22

± 0.

191.

9510

01.

550.

1013

.4K

J-19

3972

4367

1118

612

2620

9.67

480

113

.769

1140

19.3

7079

2316

9 ±

1056

.811

.52

± 0.

201.

9510

01.

500.

1210

.6K

J-20

3972

3567

1040

510

2520

9.18

682

913

.907

1255

18.6

6279

2315

3 ±

873

.7N

/AN

/A0

1.38

0.07

10.5

KJ-

2139

7021

6709

857

807

209.

919

906

18.5

1316

9118

.544

7923

124

± 7

26.9

10.9

3 ±

0.20

2.04

100

1.39

0.08

14.8

KJ-

2239

7618

6709

345

604

208.

010

829

14.8

9915

4218

.426

7923

123

± 7

66.2

N/A

N/A

01.

380.

0911

.5K

J-24

3957

3367

2084

513

4520

0.89

076

0.91

378

18.3

0879

2322

1 ±

3698

.7N

/AN

/A0

1.35

0.12

0.7

KJ-

2539

7460

6720

443

1210

209.

773

837

13.8

9511

9018

.190

7923

159

± 9

96.8

11.1

1 ±

0.26

2.60

100

1.33

0.10

10.8

KJ-

2639

8231

6719

552

1047

208.

262

780

11.8

0011

1418

.073

7923

155

± 10

6.2

N/A

N/A

01.

500.

079.

4K

J-27

3831

6867

0754

549

2011

.25

958

21.7

6118

5317

.955

7923

116

± 6

99.6

10.5

5 ±

0.18

1.76

100

1.40

0.08

17.0

KJ-

2838

3333

6702

043

1310

2019

.547

1961

24.7

9124

8717

.837

7923

175

± 8

10.7

11.6

6 ±

0.23

2.32

100

1.65

0.08

19.3

KJ-

3139

1372

6718

152

1520

3.55

536

66.

275

646

17.7

1979

2312

5 ±

978

.3N

/AN

/A0

1.47

0.08

5.2

i) ρ

= tra

ck d

ensi

ty in

105 .

N =

num

ber o

f cou

nted

trac

ks.

ii) A

patit

e fis

sion

trac

k an

alys

is w

as p

erfo

rmed

by

the

exte

rnal

det

ecto

r met

hod

usin

g a

4π/2

π ge

omet

ry fa

ctor

of 0

.5, a

nd a

ges

wer

e ca

lcul

ated

by

appl

ying

an

IRM

M-5

40R

stand

ard

glas

s w

ith 1

5 pp

m U

and

a z

eta

valu

e of

251

± 7

. iii

) P(χ

2 ) =

prob

abili

ty v

alue

of t

he c

hi-s

quar

e fu

nctio

n at

n-1

deg

rees

of f

reed

om, w

here

n =

No.

of c

rysta

ls. T

he c

hi-s

quar

e te

st is

pas

sed

whe

n P(χ2

) ≥ 5

(Gal

brai

th, 1

981)

, ind

icat

ing

a si

ngle

age

pop

ulat

ion

in th

e an

alys

ed s

ampl

e.


Leighton (2007) have yielded ages ranging from 98 to 166 Ma and 129 to 178 Ma, respectively, The previous studies found a weaker correlation between age and elevation than what is documented in the current contribution. This is not surprising, considering the greater lateral extent of their vertical profiles.

Apatite (U–Th)/He analysis

Four samples were dated by apatite (U–Th)/He analysis. Because the alpha-ejection correction proposed by Farley et al. (1996) requires a homogeneous U–Th-distribution, it may not provide a good representation for apatites from the autochthonous basement in interior southwestern Norway. Pronounced concentric zoning, characterised by U-rich cores and relatively wide, U-poor rims, appears to be very common in apatites from this area. Assuming that the analysed grains exhibit decreasing concentrations of parent nuclides from the core towards the rim, the FT-correction will over-estimate alpha-ejection, and the corrected (U–Th)/He ages will be too old. There are currently a limited number of models developed to correct for zoning in the calculation of the FT-factor (e.g., Meesters & Dunai, 2002; Hourigan et al., 2005). In order for these approaches to be successful, the spatial distribution of U–Th in the analysed grains must be known. The exact nature and extent of zoning of individual apatites from the inner Hardangerfjord

The Bu profile consists of eight samples collected from sea level to an elevation of 1310 m on the southern margin of the inner Hardangerfjord. The obtained AFT ages span from 124 ± 8 to 175 ± 8 Ma (Fig. 3C). A fairly strong positive correlation between age and elevation is evident. A discontinuity in the general age-elevation trend is present in the middle of the profile at an elevation of ~700 m, where AFT ages are found to jump abruptly from Early Cretaceous in the lower portion to Middle Jurassic in the upper part of the transect. Similar age-elevation gradients are apparent within both segments. The MTLs of the samples in the Bu profile range from 10.90 ± 0.24 to 11.66 ± 0.23 μm.

Additional samples collected close to sea level generally display ages between 116 Ma and 130 Ma and MTLs of ~11 μm. The samples collected from the westernmost part of the studied area, along the western margin of the Granvinfjord, have the youngest ages of all analysed samples. BG–26 from the outer Granvinfjord records an age of 105 ± 5 Ma and a short MTL of 10.41 ± 0.25 μm, while BG–16 from an elevation of ~500 m northwest of Granvin displays an even younger age of 97 ± 5 Ma.

The ages presented above are generally similar to the ages obtained in previous AFT studies of the inner Hardangerfjord area. Vertical profiles and nearby sea-level samples analysed by Rohrman et al. (1995) and

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.


area have not been determined. Due to the persistent, yet not precisely quantified excess of actinides in apatite 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 reported in the text. FT-corrected ages are included in Table 2 for comparison.

The obtained single-grain (U–Th)/He ages range 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 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 (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 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 samples, and all except for one of the selected crystals were whole. The age dispersion must thus reflect other factors.

Considerable dispersion may also arise from differential radiation damage between the grains in an aliquot. Alpha recoil damage has been found to impede He diffusion, 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 been subjected to extensive radiation. A general positive relationship between effective U-concentration (eU) and (U–Th)/He age is found for two of the analysed samples, suggesting that much of the dispersion may be explained by radiation effects.

Figure 4. Uncorrected (U–Th)/He ages displayed with their corre-sponding AFT ages. The majority of the (U–Th)/He ages are younger than the ages obtained from AFT analysis. The sensitivity ranges for the AFT and AHe systems are ~120–60°C and ~70–40°C, respectively. Error bars are ± 2σ.

Tabl

e 2.

Apa

tite (

U–T

h/H

e) d

ata.

Sam

ple

Elev

atio

nH

ei1σ

U1σ

Th1σ

Th/U

eUii

Sm1σ

F tiiiU

ncor

r. H

eF t-c

orr.

He

1σPr

ism

No.

no.

(m a

sl)(n

cc)

(%)

(ng)

(%)

(ng)

(%)

[ppm

](n

g)(%

)ag

e (M

a)ag

e (M

a)(M

a)ra

d. (μ

m)

term

.

KJ-

845

0.49

92.

50.

037

2.4

0.01

52.

60.

411

.70.

169

5.6

0.70

9914

18

542

1.81

31.

90.

094

1.9

0.04

52.

40.

532

.40.

243

5.5

0.70

140

200

1052

21.

162

2.0

0.05

12.

10.

022

2.5

0.4

13.7

0.29

89.

20.

7116

222

711

552

KJ-

1115

0.44

42.

40.

035

2.4

0.05

12.

41.

513

.10.

255

5.6

0.71

7510

66

562

0.46

12.

60.

039

2.3

0.06

12.

41.

618

.80.

262

5.5

0.73

6993

558

01.

444

2.0

0.09

01.

90.

140

2.4

1.6

20.6

0.64

95.

40.

7493

125

663

2K

J-19

1226

2.42

51.

90.

098

1.9

0.11

32.

41.

123

.10.

871

5.4

0.74

151

204

961

20.

757

2.3

0.03

92.

30.

042

2.5

1.1

5.3

0.62

95.

40.

7811

514

76

742

1.85

92.

00.

098

1.9

0.08

82.

40.

914

.40.

930

5.4

0.78

120

155

670

2K

J-25

1210

1.09

12.

10.

067

2.0

0.02

92.

50.

412

.30.

451

5.5

0.76

115

150

765

21.

176

2.0

0.04

72.

20.

032

2.5

0.7

17.1

0.25

15.

50.

6817

025

214

492

0.35

82.

80.

029

2.7

0.02

22.

50.

86.

20.

212

9.4

0.73

8311

46

592

i) A

mou

nt o

f 4H

e is

giv

en in

nan

o-cu

bic-

cm a

t sta

ndar

d te

mpe

ratu

re a

nd p

ress

ure.

ii)

eU

is th

e ef

fect

ive

U-c

once

ntra

tion

in p

pm (e

U =

[U] +

0.2

35 [T

h]).

iii) F

T-cor

rect

ion

acco

rdin

g to

Far

ley

et a

l. (1

996)

.


5. Some general trends are shared by the majority of the thermal history models, i.e., rapid Permo–Triassic cooling into the partial annealing zone (PAZ) followed by slow cooling through the temperature interval between 80°C and 60°C and, finally, accelerated cooling starting

Thermal history modelling

Representative thermal history models from the Bu profile south of the inner Hardangerfjord and the Simadalen profile north of the fjord are shown in Fig.

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.


history of the inner Hardangerfjord region is characterised by prolonged time intervals of slow cooling. The shallow age-elevation gradient from the three vertical profiles suggests the presence of an exhumed PAZ. All analysed samples have experienced similar, protracted cooling histories, but AFT ages are locally offset across short horizontal distances. The vertical profiles are dissected by numerous lineaments. Offset ages may thus be explained by displacement of samples from the fossil PAZ along local, small-scale faults.

For the Osa profile, the general increase in AFT age with elevation (Fig. 3A) is in good agreement with the notion of an exhumed PAZ. The disordered and locally inverted age-elevation gradient may reflect offset ages or simply natural variability. A number of N–S- to NW–SE-trending faults transect the profile, but it cannot be determined which of the individual structures might have accommodated displacement.

The Simadalen profile comprises five samples, of which four have been collected from the northern margin of the inner Hardangerfjord, while the sea-level sample comes from the southern margin. An apparent break in slope is evident in the age elevation plot from the Simadalen profile 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 feature is unlikely to reflect an Early Cretaceous episode of accelerated cooling. Faulting can produce unrealistically steep, age-elevation gradients and the age distribution in the Simadalen profile more likely results from post-Mid Jurassic fault activity. The most obvious linear feature that transects the profile is the Simadalsfjord and the age distribution in the vertical profile is consistent with down-to-the-SE displacement across a fault that underlies this fjord. Many Norwegian fjords follow the trends of old fracture zones (e.g., Nesje et al., 1992; Nesje & Whillans, 1994), as brittle fault rocks are easier for glaciers and river systems to remove. A small number of NE–SW- to NNE–SSW-trending lineaments dissect the upper part of the sampled profile, and movement along these structures may potentially have affected the age-distribution pattern. The close spacing of lineaments makes it difficult to isolate the effect of individual faults. A throw of ~650 m is inferred from the age of the sea-level sample and the age-elevation gradient in the upper part of the profile, assuming that the bulk displacement was accommodated by a single fault.

The mean (U–Th)/He ages of KJ–11 and KJ–19 are younger than the obtained AFT ages by ~40 Ma and indicate cooling through the He partial retention zone in the Cretaceous. The nearly similar difference in AFT and (U–Th)/He ages for the lowermost and uppermost samples points towards a common Jurassic–Late Cretaceous thermal history for the entire profile. This implies that the apparent differential exhumation of the upper and lower portions of the profile may best be explained by post-Late Cretaceous fault activity.

in the Late Cretaceous–Palaeogene. The samples from the north flank of the Hardangerfjord consistently record a later onset of accelerated cooling, and hence a later exit from the PAZ, than samples from corresponding elevations on the south flank.

SummaryWhen all the analytical results from AFT and (U–Th)/He analyses are considered together, some distinct trends are evident: Early Cretaceous AFT ages predominate close to sea level, while Jurassic ages are recorded by most samples from high elevations on the Hardangervidda plateau. Consistent ages, generally within the range 111–130 Ma, are evident at low elevations throughout the study area. However, the samples collected close to sea level, north of the inner Hardangerfjord, systematically display younger AFT ages, shorter MTLs and thus longer residence times in the PAZ than samples at corresponding elevations from the southern margin of the fjord. This pattern is not as pronounced at higher elevations. The (U–Th)/He system records cooling in the Early Cretaceous on the Hardangervidda plateau and in the Late Cretaceous at low elevations, i.e., roughly 40 Myr later than the ages obtained from AFT analysis.

Discussion

Interpretation of vertical profiles

Vertical profiles provide detailed information about the cooling style within a geographically restricted area. A positive correlation between apatite fission–track age and elevation reflects the movement of the rock column through the temperature gradient associated with increased fission–track stability (Wagner & Reimer, 1972). Implicit in the concept of vertical profiles is the assumption that all samples have experienced similar thermal histories (Gallagher et al., 2005). At any time in the geological past, samples derived from low portions of the profile were exposed to higher temperatures than samples that presently reside at higher elevations. Fossil partial annealing zones (PAZ) may be identified when fission–track age data from vertical profiles are plotted against sample elevation (Gleadow & Fitzgerald, 1987; Fitzgerald & Gleadow, 1990; Fitzgerald et al., 1995). A shallow age-elevation gradient and a large proportion of extensively annealed tracks are indications of a fossil PAZ. The base of the exhumed PAZ is typically perceived as a break in slope associated with a change in age-elevation gradient. Samples below the break in slope display similar fission–track ages and predominantly long tracks, indicative of rapid cooling through the PAZ (e.g., Gleadow et al., 1986).

From the wide track-length distributions and short MTLs of the analysed samples, it is apparent that the thermal


significantly lower Permo–Triassic exhumation rates for the hangingwall of the LGF (20–40 m Myr-1) relative to the footwall (40–90 m Myr-1), which clearly demonstrate significant displacement and probably repeated reactivation of the brittle segment of the HSZ during the development of the North Sea rift. The area studied in the current work is located in the transition zone where the ductile shear zone disappears beneath the Caledonian thrust sheets and the first clear indications of brittle overprint are evident. Although Permo–Triassic reactivation of the ductile southwestern portion of the HSZ has been proposed (Færseth et al., 1995) there is no conclusive evidence for post-Devonian brittle displacement (Fossen & Hurich, 2005). The Permo–Triassic cooling rate inferred from the thermal history models for the inner Hardangerfjord samples suggests an exhumation rate of >100 m Myr-1. in the footwall of the shear zone, assuming a geothermal gradient of 20°C, which is similar to the present gradient (Pascal et al., 2010). Taking this comparably high exhumation rate into account, it is conceivable that Permo–Triassic brittle reactivation of the inner Hardangerfjord segment may have taken place. The onshore expression of the Permo–Triassic rift phase appears to have been related to both regional rift-flank uplift and reactivation of post-Caledonian structures resulting in differential denudation of adjacent fault-bounded blocks.

The thermal history models reveal low Jurassic–Cretaceous cooling rates (<1°C Myr-1). This implies that the Late Jurassic North Sea rift did not significantly affect onshore areas inland of the coastal region. Previous studies have suggested a minor effect of the Jurassic rift phase even in coast-proximal areas (e.g., Færseth et al., 1995), which is corroborated by the lack of evidence for post-Triassic igneous activity onshore (Fossen & Dunlap, 1999). Although the North Sea rift phase had a limited effect on the regional exhumation rate, extensive fault reactivation may have exerted a major control on local exhumation patterns. The distribution of AFT ages reveals significant age differences over short lateral distances, suggesting episodes of accelerated cooling, possibly associated with Mid Jurassic–Early Cretaceous, fault-related uplift and denudation. Such local cooling events of limited magnitude cannot be resolved by thermal history models based on AFT analysis. Palaeomagnetic and radiometric data from fault rocks obtained from Hordaland and adjacent regions indicate reactivation of structures during Late Jurassic–Early Cretaceous times (Torsvik et al., 1992; Eide et al., 1997; Fossen et al., 1997; Andersen et al., 1999; Ksienzyk, 2012).

An episode of accelerated cooling appears to have commenced in the latest Cretaceous–Palaeogene. The tectonic activity offshore Norway had shifted to the North Atlantic by this time (Doré et al., 1999). Accelerated Cenozoic exhumation is difficult to explain through rift-related regional uplift, as the study area is situated far from the locus of active extension. Erosion rate is

The Bu profile on the southern flank of the inner Hardangerfjord displays two distinct, nearly linear age-elevation trends (Fig. 3C). This distribution may reflect movement along roughly N–S-trending faults that dissect the profile. The younger age-elevation trend is evident for samples that were collected farther east, suggesting later exhumation of the eastern block relative to the western block and thus down-to-the-W displacement, with a total throw of >1000 m.

The vertical profiles presented in this contribution have been selected with the purpose of obtaining closely spaced samples and minimising the lateral sampling distance within each profile. Where the distance between samples is large enough to cross several faults, important information regarding differential exhumation, and thereby aspects of the tectonic evolution, may be overlooked. The inner Hardangerfjord region is strongly affected by brittle deformation. This feature is readily observed from the close spacing of lineaments evident on aerial photographs (Fig. 6). The complexity of the analysed vertical profiles stresses the necessity for closely spaced data points and demonstrates the advantages of sampling the steepest profile, rather than the profile with the easiest accessibility, as has largely been done in the past.

Exhumation history of the inner Hardangerfjord area

The obtained thermochronological data provide information about the temperature history and clues for the tectonic evolution from the Permian until the Palaeogene (Fig. 7). Thermal history modelling indicates relatively fast cooling (2–6°C Myr) during the Permo–Triassic, consistent with the timing of the earliest, well-documented, North Sea rift phase (e.g., Ziegler, 1990; Færseth et al., 1995) and with previously published zircon fission–track and 40Ar/39Ar data from southern Norway (Andriessen & Bos, 1986; Dunlap & Fossen, 1998). Significant unroofing of the inner Hardangerfjord region appears to have taken place at this time. Permo–Triassic crustal extension in the North Sea may have been a dominant driving force for the rapid exhumation of the margin. Assuming an early rift margin comparable to a large-scale flexural cantilever, in the sense described by Kusznir et al. (1991), extensional displacement along major basin-bounding faults would lead to considerable isostatic uplift of the rift flank. Increased relief would enhance the efficiency of surface processes, creating a regional effect on exhumation rates. This effect would be most pronounced in coastal areas. The exact degree to which rift-flank uplift has contributed to the cooling of the inner Hardangerfjord area is uncertain, as the hinterland may already have been topographically high in the Permo–Triassic.

Down-to-the-NW extensional reactivation of the Lærdal–Gjende Fault and affiliated structures (Andersen et al., 1999) may have exerted the main control on the exhumation of the hinterland. Leighton (2007) estimated


factors, such as climate (Nielsen et al., 2009b). AFT analysis cannot distinguish between processes that might have led to cooling. However, some information on the recent topographic evolution can be extracted. While thermal histories derived from AFT analysis may be used to infer patterns of denudation, cooling is not a direct proxy for uplift (see England & Molnar, 1990; Fitzgerald et al., 1995; Gunnel, 2000). To produce a Cenozoic

mainly controlled by relief (e.g., Summerfield & Hulton, 1994) and the Palaeogene episode of rapid cooling likely reflects elevated topography in Norway at this time. The high relief may have resulted from topographic rejuvenation of a subdued Cretaceous–Palaeogene landscape (e.g., Gabrielsen et al., 2010). Alternatively, the topography has remained high ever since Caledonian times, and the rate of cooling has been affected by other

Figure 6. (A) Aerial photo graph 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.


greater depths than the coastline. Consequently, AFT data from the hinterland are not in favour of Cenozoic tectonic uplift with subsequent preservation of low-relief geomorphological features. The exhumation scenario suggested for the inner Hardangerfjord area is compatible with the ICE hypothesis, although Nielsen et al. (2009b) do not consider faulting an essential landscape-forming process. Both the distribution of AFT ages across the passive margin (Leighton, 2007; Ksienzyk et al., 2014) and the cooling rates obtained from thermal history modelling support repeated topographic rejuvenation resulting from fault-controlled uplift.

cooling signal of the magnitude seen in the modelled thermal histories, the Palaeogene–earliest Neogene tectonic uplift inferred for interior southern Norway must have been accompanied by overburden removal on the order of ~2 km. This estimate is inconsistent with the preservation of an uplifted peneplain, unless the planation surface was buried by a thick sediment cover. Vitrinite reflectance data from the Late Jurassic Bjorøy Formation on Sotra (Fossen et al., 1997) and AFT thermal history models from the basement in the same area (Ksienzyk, 2012) indicate a maximum burial depth of 1–1.5 km for the coastal region. It is highly unlikely that the Hardangervidda plateau has been buried to

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.


(Sibson, 1977) and are thus not expected to record the most recent episodes of fault activity.

Present-day seismicity around the Hardangerfjord implies ongoing periodic reactivation of brittle structures in the area. Faulting related to differential glacio-isostatic compensation has been suggested as the dominant cause of the recorded seismic activity (Gudmundsson, 1999). Other studies (Bungum et al., 2010; Hicks et al., 2000) have considered far-field stresses induced by sea-floor spreading in the North Atlantic to be of greater importance. Redfield & Osmundsen (2012) explain the brittle deformation of the post-rift margin partly in terms of lithospheric flexure generated by changing vertical loads. Erosion, transport and deposition constantly change the load on the lithosphere, thereby forcing the crust to rearrange. The redistribution of mass across the passive margin may have been an essential driving force for onshore tectonic activity after the abortion of the North Sea rift in the Early Cretaceous. Both the fault activity and the accelerated Cenozoic cooling obtained from AFT thermal history modelling are readily explained by rapid erosion of a topographically high margin, accompanied by a shift of mass to offshore areas.

Combined results from AFT analysis, radiometric dating of fault rocks and seismology suggest that faulting might have been an ongoing process along the North Sea rift margin since Permian times to the present. In the inner Hardangerfjord region and on the westernmost Hardangervidda plateau, there is generally no topographic expression of this extensive fault activity. Whether fault displacements on the order of hundreds to thousands of metres would be preserved as prominent landscape features, depends on the time frame of faulting and the rate of erosion. Considering that the history of faulting recorded by the AFT data might date back as far as the Middle Jurassic, sufficient time has been available to allow levelling of any topographic expression from small-scale faults. Palaeogene and Neogene fault activity in the interior of southwestern Norway is difficult to reconcile with the notion of a peneplain that has essentially been preserved in a pristine state since its formation. Some authors have advocated the existence of a number of peneplains at different elevations (Riis, 1996; Lidmar-Bergström et al., 2000), but no direct evidence for fault displacement of surfaces has been put forward. The absence of a clear topographic expression of faulting is easily explained in a setting dominated by a constantly eroding, high topography. Erosive glaciers of the kind described by Nielsen et al. (2009b) would efficiently obliterate any remaining surface manifestations of fault displacement.

Regional perspective

The general pattern of fission–track ages from the Bergen–Hardangerfjord region reveals progressively later exhumation from the coast towards the interior.

The inflection point of the cooling paths in the thermal history models is generally found in the uppermost part of the PAZ. Thus, most of the thermal evolution following the Late Cretaceous–Palaeogene shift in cooling rate occurs outside the sensitivity range of the AFT method and cannot be interpreted directly. The majority of the presented models display accelerated cooling in the most recent geological past. Rohrman et al. (1995) put considerable emphasis on the latest portion on the cooling history, i.e., from ~30 Ma onwards, and argued for rapid Oligocene–Neogene cooling resulting from 1–1.5 km of domal tectonic uplift. The proposal of a Neogene uplift event was largely based on the generally high proportions of short tracks measured in the samples from the Eidfjord area. Neogene cooling is a well-known modelling artefact in thermal history models from AFT data (Redfield, 2010) and has previously been observed in models from areas where recent cooling is not supported by independent geological observations (e.g., Danišík et al., 2012). The thermal sensitivity range of the AFT system does not extend beyond the boundaries of the PAZ. Consequently, modelled cooling paths at temperatures below 60°C should merely be regarded as vague suggestions. However, the short MTLs and young ages obtained from some of the samples from the inner Hardangerfjord area (e.g., BG–26 and KJ–27) imply prolonged residence within the PAZ. Rapid, recent cooling is thus required to account for the data. For samples derived from sea level and to a lesser degree intermediate elevations, Pliocene–Pleistocene glacial erosion may account for a substantial part of the most recent cooling. Assuming that ~1 km of overburden was removed from the inner fjord arms during the last 2.5 Myr, the particularly high cooling rate (>3°C Myr-1) inferred for temperatures below ~35°C reflects fjord incision rather than regional uplift and denudation. This effect explains why samples collected at low elevations require more rapid Neogene cooling than samples from the Hardangervidda plateau.

AFT ages, MTLs and thermal history models generally indicate later exhumation of the northern flank of the innermost Hardangerfjord relative to the southern flank. Samples obtained from similar elevations on opposite margins of the fjord display slightly offset cooling paths in the upper PAZ, suggesting that juxtaposition may have taken place subsequent to the time at which the sea-level samples cooled out of the sensitivity range of the AFT method. It is plausible that the observed differential cooling signatures are in part attributed to Cenozoic episodes of fault reactivation. K–Ar illite data indicate reactivation of the Lærdal–Gjende Fault at ~60 Ma (Ksienzyk, 2012). No further record of Palaeogene fault reactivation is available from southwestern Norway. In this context, it is essential to note that most previous studies aiming to date fault activity (e.g., Andersen et al., 1999; Larsen et al., 2003) have focused on cohesive fault rocks. Cohesive breccias and cataclasites originate from brittle processes operating at greater crustal depths


rifting, culminating with opening of the Atlantic Ocean in the Eocene (Lundin & Doré, 1997). Changes in the orientation of the stress field have taken place in between individual rift phases (Færseth et al., 1995; Færseth, 1996). The traditional landscape–development models furthermore fail to account for pre-rift structural inheritance. This is critical, considering that the Precambrian structural grain has exerted a major control on the development of the North Sea rift (Færseth et al., 1995; Gabrielsen et al., 2002).

Rohrman et al. (1995) set the stage for future fission–track studies in southern Norway by providing a comprehensive regional AFT record, but overlooked local differences in cooling signatures in their effort to cover a geographically large area. The domal uplift model suffers from the implicit presumption that southern Norway behaved as a coherent block since Permian times and does not fully describe the pattern of AFT ages presented in the current contribution and other recent studies (Leighton, 2007; Ksienzyk et al., 2014). We propose that the AFT age distribution rather results from a combination of fault-related uplift and denudation and regional differences in exhumation rate during both North Sea rift phases. The extensional tectonic activity in the North Sea has exerted a first-order control on the regional AFT age distribution onshore. Zircon fission–track ages increase from the coast towards the interior, indicating more pronounced erosion along the coastline during Permo–Triassic times (Ksienzyk et al., 2014). This pattern is consistent with large-scale, flexural rift-flank uplift, causing coastal samples to reach shallow crustal depths at an earlier stage than inland samples. The AFT age distribution in interior southern Norway largely reflects exhumation of individual structural blocks

Fission– track data from the Bergen and Sunnhordaland areas are generally consistent with relatively early exhumation to shallow crustal levels (Rohrman et al., 1995; Leighton, 2007; Ksienzyk et al., 2014). Samples from coast-proximal regions are typically characterised by Permian–Jurassic AFT ages and intermediate MTLs on the order of 12–14 μm. In comparison, the data from the inner Hardangerfjord area reveal relatively late exhumation of the hinterland, evident by mainly Jurassic–Cretaceous fission–track ages and short MTLs.

The AFT age distribution in southern Norway is largely structurally controlled (Leighton, 2007; Ksienzyk et al., 2014). Landscape development appears to have been governed by down-to-the-W displacement along large-scale extensional structures in association with the Permian and Mesozoic rift phases. Local exceptions to the general trends are abundant both in coastal and in inland areas and may reflect differential vertical movement across small-scale faults, as is suggested here for the inner Hardangerfjord region. Larger-scale structures with easterly dip directions (e.g., the Hjeltefjord Fault Zone; Fossen, 1998) produce locally reversed age–distribution patterns (Ksienzyk, 2012).

The AFT data from southwestern Norway clearly demonstrate that the evolution of the North Sea margin is much more complex than the scenarios portrayed by classic passive-margin, landscape–development models (Fig. 8; see Gilchrist & Summerfield, 1990; Kooi & Beaumont, 1994; Ollier & Pain, 1997; Gallagher et al., 1998). Although simplified, these models are well suited to a range of passive margins worldwide (see compilation by Gallagher & Brown, 1997). The complexity of the Norwegian margin reflects a history of polyphase

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).


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resulting from mainly down-to-the-W extensional faulting and associated footwall uplift. Fault-related exhumation efficiently explains the large age difference between the coast and the interior and the offset of ages across major structures. Small-scale faults have affected the local AFT record by offsetting ages across short lateral distances and even within vertical profiles.

ConclusionRegional, low-temperature, thermochronological studies have provided a general overview of the exhumation history of the SW Norwegian passive margin. The wide spacing of data points has, however, entailed a lack of control of the effect of faulting on the AFT age distribution, leading to difficulties in recognising temporal and spatial differences in cooling style. Apatite fission–track and (U–Th)/He analyses of vertical profiles from the hinterland of the passive margin in the inner Hardangerfjord area reveal offset ages across small-scale lineaments and fjords, indicating widespread fault activity from the Middle Jurassic onwards. Thermal history modelling suggests that the exhumation of the margin has been closely linked to tectonic activity operating within adjacent rift basins. Relatively rapid cooling is evident for the Permo–Triassic (2–6°C Myr-1), corresponding to a significant unroofing of interior southern Norway as a consequence of onshore tectonic activity and rift flank uplift during the formation of the North Sea rift. A second episode of accelerated cooling (>3°C Myr-1) is obtained for the latest Cretaceous onwards and may be ascribed to ongoing topographic rejuvenation, involving fault-related footwall uplift and enhanced erosion rates. The stepwise increase in AFT ages from the hinterland to the outer margin demonstrates that faulting has been a common process onshore throughout the Mesozoic and probably also the Cenozoic. Hence, the apparent domal warping of AFT isochrons that has been emphasised in previous studies is merely a result of extrapolation between widely spaced samples. High Neogene cooling rates at low elevations in the inner fjord arms are not necessarily indicative of rapid tectonic uplift, but may rather be attributed to Plio–Pleistocene fjord incision. The low-temperature thermochronological record from the SW Norwegian margin is consistent with sustained elevated topography, transformed through regional rift-related uplift, periodic extensional faulting and erosion.

Acknowledgements. This study has received funding from Statoil through the Earth System Modelling project at the University of Bergen. We thank Arne Johannessen for field assistance and Vibeke Gellein Hanssen for help with sample preparation. Members of the Bergen Tectonics and Thermochronology research group are thanked for fruitful discussions and feedback. We gratefully acknowledge helpful and constructive reviews by K. Gallagher and T.F. Redfield.


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