Syn-rift thermal structure and post-rift evolution of the Oslo Rift (southeast Norway): New constraints from fission track thermochronology

EPSL ELSEVIER

Earth and Planetary Science Letters 127 (1994) 39-54

Syn-rift thermal structure and post-rift evolution of the Oslo Rift (southeast Norway)" New constraints from fission track thermochronology Max Rohrman, Peter van der Beek, Paul Andriessen Faculty of Earth Sciences, Vrije Unicersiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

Received 12 April 1994; revision accepted 5 August 1994

Abstract

The Permo-Carboniferous (305-240 Ma) Oslo Graben in southeast Norway is a classical magmatic continental rift. We present fission track (FT) data on apatites, zircons and sphenes from the rift and surrounding areas in order to clarify its syn- and post-rift thermal evolution. Zircons and sphenes within the rift record FT ages of between ~ 270 and 180 Ma (i.e., syn-post-rifting). In contrast, the Precambrian basement surrounding the graben yields zircon F T / s p h e n e FT ages of ~ 650-500 Ma, indicating that heating associated with rifting was focused inside the rift as a result of advective heat flow. Syn-rift to early post-rift temperatures reached > 240°C at the present erosion level in the graben. Syn-rift advective heating ( ~ 270-260 Ma) was probably a result of large-scale batholith intrusion; post-rift ( ~ 220-180 Ma) heating seems to have occurred by hydrothermal circulation of high-temperature (100-300°C) fluids. The FT data, organic geochemical indicators and fluid inclusions suggest that hydrothermal circulation followed a highly complex pattern controlled by the tectonic and volcanic structure of the rift. Apatite FT ages and confined track length distributions reveal the post-rift exhumation history of the area. Apatite FT ages are continuous within and outside the graben and decrease from Triassic (200-240 Ma) in the southeast to Jurassic ( ~ 160 Ma) in the northwest, indicating that post-rift exhumation took place on a much larger scale than Permian rifting. Modelling of apatite FT ages and track lengths suggests cooling/denudation events in the Triassic, Jurassic and Neogene, accounting for a total post-Permian erosion of 3-4 km. Triassic-Jurassic denudation coincides with the migration of rifting from the Oslo-Skagerrak area to the North S e a / D a n i s h Basin, as revealed by offshore seismic data, and caused destruction of the hydrothermal regime. Neogene uplift and erosion of the area is also supported by seismic evidence. The inferred timing and amount of regional post-rift erosion suggests that post-rift subsidence and sedimentation within the Oslo Graben was minimal.

I. Introduction

A q u a n t i t a t i v e u n d e r s t a n d i n g of the t h e r m a l evolution o f rifted basins p r o v i d e s an i m p o r t a n t c o n s t r a i n t on t e c t o n i c m o d e l s o f b a s i n f o r m a t i o n a n d has obvious uses in h y d r o c a r b o n e x p l o r a t i o n . G e n e r a l l y , b a s i n m o d e l l i n g studies p r o c e e d f r o m

an analysis of the s e d i m e n t a r y r e c o r d , from which tectonic s u b s i d e n c e p a t t e r n s a n d the t h e r m a l evolution of the l i t h o s p h e r e can b e quantified. However, in m a n y basins the most d e t a i l e d r e c o r d is d e r i v e d from t h e post-rift s e d i m e n t a r y s e q u e n c e s , a n d the syn-rift evolution o f t e n r e m a i n s obscure. T h e highly m a g m a t i c P e r m o - C a r b o n i f e r o u s

0012-821X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0012-821X(94)00166-9

40

M. Rohrman et al. / Earth and Planetary Science Letters 127 (1994) 39-54

(305-240 Ma) Oslo Graben in southeast Norway has not evolved into a deep sedimentary basin; all the post-rift sediments, if ever deposited, were stripped away by erosion and the syn-rift succession preserves mainly volcanics. Whereas conventional basin analysis techniques can thus not be applied here, this situation does provide the unique possibility to directly sample the syn-rift structural, magmatic and sedimentary record of a failed rift system. For this reason, the Oslo Graben has been a classical area for studying the relationships between lithospheric extension and magmatism for more than a century, and a detailed picture has emerged of the riffs syn-rift tectonic and magmatic evolution [1-3]. In contrast, very little is known about the post-rift history of the area and, for instance, the puzzling absence of the post-rift sediments remains unresolved [3]. In the absence of a sedimentary record, fission track (FT) thermochronology yields information on the thermal history of an area which cannot be obtained otherwise. The property that fission tracks have of annealing at relatively low temperatures over geologically relevant time spans (10 6108 y) makes them suitable for studying the thermal evolution of upper crustal rocks. The temperature interval over which annealing rates increase from very slow to almost instantaneous in geological terms (the partial annealing zone, PAZ) [4,5] is well established, at 60-120°C for apatites [6]. The kinetics of annealing in apatite, as recorded by horizontal confined track length distributions [11,12], are now becoming understood in a quantitative manner [13,14]. As a result, apatite FT thermochronology is presently able to record not only the amount and timing of cooling of a sample, but also its cooling trajectory below 120°C [14,15]. For the higher temperature zircon and sphene FT thermochronometers only the effective retention temperatures (i.e., the high-temperature limits of the PAZ) are known with some confidence. These are 240 _+ 25°C for zircon [7-9] and ~ 300°C for sphene [5,10]. In this paper, we present FT data on apatites, zircons and sphenes from the Oslo Rift and surrounding areas in order to obtain a more complete picture of its syn- and post-rift thermal

evolution. Apatite FT thermochronology, quantified by modelling of ages and track length distributions, records the post-rift cooling and denudation history of the area. The zircon and sphene systems provide details on maximum paleotemperatures and the distribution of heat during and after rifting. The data presented here provide a number of new constraints to be incorporated into tectonic models of the evolution of the Oslo Rift and demonstrate how FT thermochronology can be applied to unravel the evolution of rift zones which do not preserve a sedimentary record. In particular, we address questions such as the dynamics (active vs. passive asthenosphere involvement) and kinematics (simple vs. pure shear) of rifting, the roles of advective and conductive heat transport, and the timing and amount of post-rift erosion.

2. Geological setting The Oslo Rift (Fig. 1) consists of an onshore part known as the Oslo Graben and its offshore continuation, the Skagerrak Graben. The Oslo Graben is itself subdivided into two segments: the Akershus Graben segment (AGS) in the north, and the southern Vestfold Graben segment (VGS) (Fig. 1). The present-day width of the Oslo Graben is around 50 km; crustal thinning, reactivated basement faults and evidence of magmatic activity, however, occur over an area about three times as wide [1,2,16]. The rift developed in a high-grade Precambrian basement. Within the Oslo Graben Precambrian basement gneisses, pre-rift Cambro-Silurian sediments [17] and syn-rift Permo-Carboniferous magmatic rocks (with minor sedimentary intercalations) are exposed (Fig. 2). Pre-rift sediments were deformed during the Caledonian orogeny, followed by peneplanation and deposition of the Late Carboniferous Asker Group clastics in a coastal plain to shallow-marine environment [18,191. Rift-related magmatic activity started with sill intrusion and extrusion of plateau lavas at 305295 Ma [3,20]. E N E - W S W to E - W crustal extension, resulting in block faulting and reactivation

41

M. Rohrman et al. / Earth and Planetary Science Letters 127 (1994) 39-54

of Precambrian fault systems, coincided with the main phase of volcanic activity. Vertical motions along the eastern master fault of the rift, the Oslofjorden fault zone, reach 3 km in the south and decrease towards the north to 1-2 km [1]. From 275 Ma onwards the magmatic style changed from extrusion of plateau lavas to caldera formation along (N)NW-(S)SE trends, followed by the intrusion of intermediate to granitic batholiths. Magmatic activity started later and lasted longer in the AGS (295-240 Ma) than in the VGS (305-265 Ma) [20]. At 240 Ma, after about 60 m.y. of rift activity, the geological record

61N

60N

61N 59N

60N

59 N

58 N

9E

IOE

11E

Fig. 2. Geological m a p of the Oslo Graben (after [1]), with sample localities inside the rift zone. Notation as in Fig. 1.

in the Oslo Graben ended. There is no indication of post-rift sediments. The offshore Skagerrak Graben, in contrast, preserves a post-rift sedimentary sequence which thickens towards the south above a Lower Triassic unconformity [21]. Estimates of post-rift erosion of the Oslo Graben lie between 2 and 4 km [16,22]. Stretching factors for the Oslo Graben based on fault offsets are < 1.1; crustal thinning is estimated at 1.3 [16].

3. Fission-track analysis 8E

IOE

12E

Fig. 1. Simplified tectonic map of southern Norway and the Skagerrak area (modified from [2]), with sample locations in the Precambrian basement outside the rift zone. A F T (bold), Z F T (italics and labelled 'Z') and SFT ages (labelled 'S') are indicated for each sample. Inset shows location in Scandinavia. F B Z = F e n n o s c a n d i a n B o r d e r Z o n e ; P K F = Porsgrunn-Kristiansand fault; A G S = Akershus Graben segment; VGS = Vestfold Graben segment.

3.1. Apatite The apatite fission track (AFT) data are summarized in Table 1 (experimental details are in the caption). The A F T ages range from 128 + 16 to 284 + 38 Ma and are continuous between the rift and surrounding areas (Figs. 1 and 2). The

42

M. Rohrman et al. / Earth and Planetary Science Letters 127 (1994) 39-54

Table 1 Apatite fission track ages and horizontal confined track length data

Sample

locality

number

p, (Ns)

p, (IV)

p~ (N~)

of

grains

Age

p(%2)

_+1o

(xl06 cm 2)

(xl06 cm 2)

(xl08 cm 2)

(Ma)

(%)

10 15 20 50P 15 50 P 50 P 50 P 50 P

1.887 (237) 0.471 (229) 0.111 (644) 2.4 (916) 0.121 (780) 2.0 (744) 6.0 (2242) 0.85 (1995) 2.8 (2401)

2.580 (162) 0.578 (154) 0.139 (403) 1.5 (572) 0.185 (598) 0.95 (1421) 3.0 (2557) 0.50 (1179) 1.4 (3305)

0.0256 0.0258 0.0258 0.406 0.033 0.406 0.406 0.406 0.406

0.0255 (1507) 239-'z.22 25 0.0256 (1517) 175:1:15' 2.5 (162_+16) 0.0256 (1517) 171+19 0.5

U Mean track Standard no. length deviation of (ppm) (Bm) (p.m) tracks

Bamble B9 DL129 TN250 BAM85 A' BAM88A BAM91A BAM93 A BAM96 A BAM97 A

Kragem Odengard. V. Tvedestrand Tromoy Arendal Arendal Blakstad Haugland Mykland

(1517) 217-+25 (1517) 235+_27 (1494) 239-~?.0 (2020) 197-+26 (834) 195:1:17 (2020)256+31 (2020)242-+24 (2020) 208-+23 (2020)247:1:24

75 50 75 99.5

16 4 8 23 10 14 46 8 21

13.29-~0.10 12.85~'0.15 13.83:L0.14 12.05:L-0.11

1.03 1.54 1.43 1.06

100 100 100 100

12.08:L-O.14 12.12-1"0.15 10.90"!'O.19 11.99-!-0.14

1.38 1.50 1.94 1.36

100 100 101 101

14 9

12.47-~0.12 12.86!-O.15

1.15 1.48

100 98

20

12.56i-0.18

1.42

128

3 25

13.11-+O.24 1.53 12.60-1-0.16 1.64

40 100

31 18

12.70i-0.16

1.59

100

13.42-+0.11

1.06

100

12.16-'~O.13 1.31

100

13.81-+0.19 12.93-J:0.13 13.57-+0.20 14.19!-0.08

1.17 1.30 1.96 0.83

60 100 100 100

12.73_-H).10 1.04

101

Telemark T3B T8

Nissedal Tinnsje

13 14

1.843 (456) 0.841 (321)

2.280 (282) 1.551 (296)

T14

Hedalsfjella

11

1.716 (640)

3.122 (582)

(164+13)

T15 A T17

Hedalsfjella Bromma riv.

15 18

0.222 (134) 2.00 (649)

0.490 (146) 4.28 (695)

0.0256 (1517) 128+16 0.0261 (1545) 142~.11

10 10

0.0256 (1517) 209-J:18 25 0.0242 (1430) 129-+14 <<1 (110"J:12) 0.0255 (1507) 233-+25 2.5 (196:1:23)

Kongsberg K1 K5

Hoksund Kongsberg

14 13

3.590 (570) 1.123 (229)

5.101 (405) 2.747 (280)

K9

Jonsknuten

15

0.287 (211)

0.433 (159)

K11

Kloftefoss

E~stfold 02 Vesterey isl. 03 O6 07 O10 O12 O14

Nesodden Ski lake ~yeren R~:lenssjeen Askim Moss

23

0.668 (402)

1.229 (370)

21 14 20 19 12

0.167 0.649 0.707 0.819 3.759

0.270 1.097 1.050 0.989 5.825

(124) (194) (187) (313) (915)

(85) (164) (139) (189) (709)

0.0255 (1507) 1893:10 <<1 (161 _+14) 0.0261 (1545) 221_+33 75 0.0256 (1517) 176-+21 50 0.0261 (1545) 204:!:25 50 0.0242 (1430) 217-+23 10 0.0261 (1545) 196_+14 25

2

7 1 7 6 6 35

Median segment (Hedmark) M1 M2 A M5 M6 M7

EidsvoII Engerfjellet Roverud Kongsvinger Vestmarka

19 20 20 20

0.226 0.350 1.226 1.215

(191) (161) (547) (453)

0.222 (94) 0.474 (109) 2.088 (466) 1.931 (360)

0.0255 0.0261 0.0261 0.0261

(1507) (1545) (1545) (1545)

284:1:38 224+30 178+15 190-!-_17

99 90 50 50

1 2 12 11

12.75~0.19

1.21

40

13.27!-O.19 13.73!'O.15 13.91-+0.14

1.69 1.52 1.42

80 100 100

M. Rohrman et aL / Earth and Planetary Science Letters 127 (1994) 39-54

43

Table 1 (continued) Oslo Rift - Basement and Pa/aeozoic sediments ORll Sundvollen 11 3.741 (386) 5.349 OR15A Jeleya 18 1.112 (428) 1.366 OR22 Holmestrand 16 2.267 (749) 3.668 OR27 A Brandbu 12 0.392 (35) 0.851 Os/o Rift- Permian lavas and sediments OR9 Engelstad 23 1.569 (782) 2.768 OR20 Kjels~s 13 0.761 (236) 1.254 OR32 Sundvollen 18 1.122 (373) 1.721 OR38 Brummundal 23 1.004 (289) 2.272

OR39A OR45 OR46 OR47 OR49 OR52 OR53 OR54 Oslo Rift OR14 OR26 OR28 OR33

Brummundal 10 Gullholmen 18 Sletter isl. 16 Gyrihaugen Skrukkelia 16 Jeleya 20 Horten 20 Hillestad 14 - Permian intrusives Larvik 13 Lunner 23 Skimtesaeter 12 Hurdal 20

(276) (263) (606) (38)

2.551(13584) § 0.0256 (1517) 0.0255(1507) 0.0256 (1517)

217...~5 228+')0 183+14 130"]:31

50 95 10 50

35 8 22 0

13.27!-'0.13

1.09

100

12.192-0.24

1.32

31

(675) (196) (286) (327)

17 7 10 14

13.81_+0.08 0.87 13.72_+0.15 1.09 13,44_+0,12 1.21

110 110 100

13 5 11

13.34i'0.07 13.23¢-0.09 13.48~0.12 13.47_+0.13 13.24:1.'0.12 13.35_+0.12 13.26~-0.15 13.48:L'0.11

1.29 1.56 1.18 1.29 1.25 1.18 1.42 1.10

200 160 100 100 100 100 90 100

12.29-2.-0.14 1.43

100

12.38:1_-0.14 1.37

100

1.269 (432) 0.549 (246) 1.125 (463)

2.133 (363) 0.897 (201) 1.910 (393)

0.0242(1430) 163~12 50 0.0255(1507) 178:1:20 25 0.0242(1430)183+17 50 0.0255 (1507) 16~+8 <<1 (134+1 O) 0.0256(1517) 168+14 10 0.0261 (1545)186¢20 25 0.0261 (1545)179-~_16 10

1.077 0.943 0.488 1.241

(206) (530) (233) (295)

1.798 1.551 0.759 2.002

(172) (436) (181) (241)

0.0261 0.0261 0.0261 0.0261

(1545) (1545) (1545) (1545)

182+91 25 185+16 99 195+92 75 186+19 75

10 9 4 12

2.465(1005) 0.457 (190) 0.657 (222) 0.644 (285)

3.763 0.722 1.147 0.909

(767) (182) (194) (201)

0.0255 0.0255 0.0255 0.0242

(1507) (1507) (1507) (1430)

194+14 5 187+23 50 170-~12 50 199-~21 5

24 4 7 5

= track density ( × 106 tracks c m - 2 ) ; N = number of tracks counted; subscripts 's', 'i' and 'd' denote, respectively, spontaneous and induced tracks and tracks in the fluence monitor glass; P(X 2) = )t'2 probability of the single-grain ages representing one normal distribution. Errors in r are calculated following [23]. Samples were dated by the external detector method using a ~" calibration [24], unless otherwise indicated. Age determinations by R o h r m a n using ~"= 11760 _+ 425 for NBS963, except § where ~ = 124 +_ 7 for CN2 glass. Details of the r calibrations are available on request from the authors. A By Andriessen using ~"= 11134 +_ 344.0 for NBS963 glass. * Samples labelled B A M are from Van H a r e n and R o h r m a n (internal report, CIGO, Amsterdam, 1988). P Age determined using population method. + Mean age is given when samples fail the g 2 test ( P ( x z) < 5%); for a comparison with other ages the pooled age is given in parentheses [23]. Track length determinations by Van der Beek, R o h r m a n and Vogel-Eissens. Durango and Fish Canyon apatite were used as standards for length calibration. Mean lengths are 14.27 _+ 0.14 and 14.68 _+ 0.11 tzm, with standard deviations of 1.0 and 0.72 /zm respectively. A

1000

15

A

800

A

m

=~" 600 _0

400

cm12 200 11

0 i~

I

150

I

200 ~age(Ma)

I

250

,

,

,

,

i

150

,

,

+

,

I

,

,

i

,

I

,

,

,

200

250 Fission-Track age (Ma)

Fig. 3. (a) Plot of apatite fission track ages against elevation. Bars represent age errors ( + ltr). • = Samples from the Oslo Rift; • = Bamble, Ostfold and Median segment; • = Kongsberg and Telemark. (b) Plot of apatite fission track age against m e a n confined track length; symbols as in (a).

44

M. Rohrman et al. / Earth and Planetary Science Letters 127 (1994) 39-54

however, a geographical correlation: The AFT ages young towards the northwest (Fig. 1), which does coincide with the regions of higher elevation

ages show no clear correlation with elevation (Fig. 3a), although there is a slight trend towards low A F T ages at the higher elevations. There is,

Bamble :

2

DL129

I= =~

0

235 + 27 Ma .(z2) = 5o

300 235 Ma

12.y pm

30

I

240

2010 •="

•

-2

180

60

120

T17

Telemark:

• 142 Ma Ii

•

b ~ s t l o l d : (~7

•

240

~

142 +11Ma

[30L 12.60~m

P(~)=10R

1/20f10 ,°'il"i~U ~

60

120 180 240

4

age (Ma)

12 16 (pro)

o" =1.96~

20

10 120 60

OR39

Akershus:

.

168 Ma • •

Ill

•

.

• d

2,0

, I ~ ~,~=~:-=:'-~ , 4 8 12 16 l e n g t h (~,n)

120 180 240 age (Me)

300

2

168+ 14 Ma

I~r/ ';~.;r~m

.(~)=1o

180

20

120

10

-,

.

60

60

120 180 240

.

4

age (Me)

Vesffold: 0 R 5 4

"" •

12 16 (pro)

180

%

•

8

length

/ p(~2)=50

2114Ma -=='= • • •

60

t

8 length

L2t:25M. I135m]

6O

C

4

120 180 240 a g e (Me)

120

•

.

1

• ,b • •=

.

•

60

_ 300

2

'

I

M

• .=.=. 186•Ma ",,

.

--

8 length

20

.

12 16 (~m)

=°

10

.

e 60

60

120 180 240 age (Ma)

4

8 length

12 16 (pat)

Fig. 4. Representative AFT samples. Single-grain age distributions plotted in a radial plot (left) and as age histograms and probability curves (centre), and histograms of confined track length distributions (right).

M. Rohrman et al. / Earth and Planetary Science Letters 127 (1994) 39-54

in Telemark (Figs. 1 and 2). This pattern suggests that post-Permian cooling and denudation took place on a much larger scale than the present study area, that the area developed as a coherent block after ~ 240 Ma and that there has been more than one Meso-Cenozoic exhumation event. The spread in ages for samples with similar elevations suggests long residence times in the PAZ. The present-day PAZ for western Scandinavia lies between 60 and 105°C, as estimated from the results from the nearby Siljan borehole in central Sweden [25]. This relatively low temperature for total annealing (105°C) implies heating times of at least 100 Ma [6]. Practically all samples have mean lengths of between 12 and 14 p-m (Fig. 3b). The track length distributions are relatively narrow, with standard deviations of < 1.5 p-m for most samples; none have standard deviations > 2 p-m. These narrow distributions result from a conspicuous lack of very short ( < 11 p-m) and long ( > 14 p-m) tracks. The southwestern, low-lying (elevations < 300 m) Bamble sector records high A F T ages (195 + 17-256 + 31 Ma) with mean lengths of between 11.99 and 12.85 p-m (Fig. 4a). One sample (BAM 96) has a markedly lower mean track length (10.90 p.m) and larger standard deviation (1.94 p-m). Samples with relatively high C1/(F + C1) ratios, such as TN250 and DL129 (C1/(F + C1) = 0.6 to 0.8) [26] (see also Nijland, pers. commun., 1993) show ages similar to the other samples with more normal C1/(F + CI) ratios of < 0.2, although their mean track lengths are slightly longer, due to the higher resistance of Cl-rich apatites to annealing [12]. The boundary between the Bamble and the adjacent Telemark sector is formed by the Porsgrunn-Kristiansand fault zone, a Precambrian shear zone which was repeatedly reactivated. Because A F T ages do not vary across this fault zone (e.g., compare BAM96 and BAM93) the latter has not been active since at least the Permian. Samples from the Kongsberg sector, with elevations of between 300 and 1000 m, have overall younger ages (129 + 14-233 + 25 Ma) than samples from Bamble and are similar to the A F T ages in Telemark (128 + 16-239 + 22 Ma) (Fig. 4b). There is again no jump in ages across the fault separating the Kongsberg and Telemark sec-

45

tors. Track lengths vary between 12.47 and 13.42 p-m and standard deviations from 1.06 to 1.64 p-m.

A F T ages on the eastern flank of the Oslo Rift, in the Ostfold and Median sectors, are the oldest encountered in the study area (176 + 21284 + 41 Ma) (Fig. 4c). Elevations here are very low ( < 150 m). Track lengths seem to be slightly longer than in the west (12.16-14.19 p-m), and standard deviations are 0.83-1.69 p-m. These Triassic A F T ages agree with results from further east in Sweden, where ages around 220 Ma were reported, with mean lengths of 12.9-14.0 p-m [27]. Samples from within or near the eastern border fault system of the Oslo Graben ( 0 3 and M2) have relatively old ages (221 + 33-284 __+_38 Ma) and mean lengths of 13.81 p-m (standard deviation = 1.17 p-m), suggesting that relatively low temperatures prevailed within the fault system during and after rifting. The analyzed pre-rift sediments in the Oslo Graben ( O R l l , 15 and 22) all belong to the Upper Silurian Ringerike group [28]. The A F T ages lie between 183 + 14 and 228 + 20 Ma. The age of OR15 (228 + 20 Ma) is older than most of the ages in the Oslo Rift, again suggesting cooler conditions near the Oslofjorden fault. Mean track length distributions for these samples are similar to those from the Precambrian regions (mean l e n g t h = 12.19-13.27 /zm, standard deviation = 1.09-1.32 p-m). Most of the rocks sampled in the Oslo Graben are Permian extrusives from different stratigraphic levels. As a result of block faulting during rifting all the samples are now at approximately the same elevation (0-100 m). Rb-Sr ages for the lavas lie between 295 and 275 Ma [20]. Samples from the southern Vestfold lava plateau (OR9, 53 and 54), the central Krokskogen plateau (OR32 and 47) and from the lavas on Jelcya and surrounding islands in the southeast (OR 45 and 52) show similar results: all the sampled extrusives from the rift have A F T ages in the range 163 + 12-195 + 22 Ma, with mean lengths around 13.4 /.tm (Figs. 4d, e and f). Rapid cooling in volcanic apatites produces characteristic long track lengths ( ~ 15 p-m) with a narrow distribution ( ~ 1 p-m) [11]. Thus, the A F T ages and track lengths of the

46

M. Rohrman et aL / Earth and Planetary Science Letters 127 (1994) 39-54

rhomb porphyry (RP) lavas imply complete annealing following initial burial. This is also indicated by the growth of zeolites irp vesicles of the lower lava flows [B.T. Larsen, pers. commun., 1991]. The youngest sediments of the Oslo Graben stratigraphy are found in the Brummundalen area, north of Lake Mjosa. Here, fluvial-aeolian sandstones of the Brummundalen Formation overlie RP lavas with a Rb-Sr age of 279 _+ 9 Ma [20]. The A F T age of these lavas (OR39) is similar to that of the other RP lavas in the rift (168 _+ 14 Ma). The Brummunddal sandstone itself (OR38) yields an A F T age of 162 + 8 Ma. These ages also support complete annealing since deposition of the syn-rift sediments in the Oslo Graben. Permian intrusives in the Oslo Graben (OR14,

26 and 28) vary in A F T age between 170 + 12 and 194 + 14 Ma, and their mean track lengths lie between 12.29 and 12.38 and have comparable track length distributions (standard deviation = 1.37-1.43/xm). Permian intrusive rocks thus have A F T ages and mean track lengths that are similar to those of the extrusives. 3.2. Zircon

Zircon fission track (ZFT) ages were determined for eight samples. The results are summarized in Table 2. Only two samples from Precambrian basement gneisses outside the rift zone could be dated, the other samples appearing metamict after etching. These samples (T18 and M2) give very old ages of 611 + 55 Ma and 634 +

Table 2 Zircon and sphene fission track ages

Sample

locality

lithostratigraphic unit

number Ps (N,) of grains (xl 08 cm 2)

p~(NJ

p~ (N~)

(xl 06 cm2)

(xl 08 cm 2)

Age _+1a (Ma)

p(z2) Uranium (%)

(ppm)

Zircon T18 M2 OR6 ORll A

Bruflat Engerfjellet Porsgrunn Sundvollen

Precambrian gneiss 10 Precambrian gneiss 10 Ordovician sandstone 14 Upper Silurian sandstone 15

25.4 (782) 24.7 (666) 12.0 (335) 24.3 (2281)

12.2 11.5 15.9 4.41

(188) (154) (222) (207)

2.84 (1682) 2.84 (1682) 2.84 (1682) 0.212(4277) 2.84 (1682) 0.212(4277) 0.212(4277) 0.212(4277)

611_+55 50 634!-62 90 228:!:22 75 475-~_491' <<1 (351-+34) 523_+53 95 181+18 25 264_+30 75 20~0 95

OR16 OR20A OR22~ OR26 A

Asker Kjels6s Holmestrand Lunner

Ordovician sandstone Permian rhyolitictuff Upper Silurian sandstone Permian gabbro

12 10 10 11

22.0 (510) 14.7 (1090) 18.6 (1033) 16.8 (1240)

12.4 (144) 5.17(191) 4.44(123) 5.30(196)

BAM81 A" Arendal BAM84 ATromey BAM91A Arendal BAM94 A Blakstad BAM96 A Haugland T3A Nissedal OR28 SkJmtesaeter

Precambrianskarn Precambriangranulite Precambrian skam Precambrian amphibelite Precambrian amphibelite Precambrian ampllibolite Permian syenite

6 6 6 6 6 5 10

20.5 21.9 10.4 32.5 43.9 34.2 1.26

8.68(411) 9.38(308) 4.67 (221) 16.1 (542) 26.2 (517) 18.0 (87) 1.54(220)

75 70 88 127 69 152 131 156

0.027(1632) 0.027(1632) 0.027(1632) 0.027(1632) 0.027(1632) 2.84 (1682) 0.027(1574)

665+45 657+49 627-+64 574+36 468_+30 542!-69 243-+23

51 55 27 95 155 100 8

Sphene (1942) (1436) (981) (2193) (1692) (330) (359)

10 10 95 50 10 50 90

Notation as in Table I. Age determinations labelled A by Andriessen using ( = 308 _+ 18 for zircon (NBS962) and 11134 _+ 344.0 for sphene (NBS963); others by R o h r m a n using ~ values of 307 _+ 9 (NBS962) and 108 _+ 4 (CN-1) for zircon and 105 +_ 3 (CN-1) for sphene.

M. Rohrman et al. / Earth and Planetary Science Letters 127 (1994) 39-54

complete annealing and temperatures in excess of 240°C around that time. Two samples ( O R l l and 22) of the Late Silurian Ringerike sandstones (Wenlockian-Ludlowian stratigraphic age of 425-410 Ma) [28] also show discordant Z F T ages. O R l l fails the X 2 test; the single-grain ages show a large spread (Fig. 5d) and suggest partial annealing, as single-grain ages both older and younger than the stratigraphic age are observed. The maximum temperature of this sample probably never significantly exceeded 200°C. OR22 shows a more uniform grain age distribution of 264 _+ 30 Ma. No single-grain ages older than depositional ages were encountered (Fig. 5e), implying near or total overprinting and representing consistency with the maximum temperatures near the base of the zircon PAZ. This Z F T age indicates cooling since a Permian (syn-rift) heating event. Two Permian samples were analyzed, a rhyo-

62 Ma respectively (Figs. 5b and c), indicating that they have not been affected by a Permian thermal overprint. Within the Oslo Graben four samples were taken from pre-rift sediments. Two of them (OR6 and 16) belong to the U p p e r Ordovician Lang0yene Formation, which is of Ashgillian (440-435 Ma) stratigraphic age [29]. OR16 has a Z F T age of 523 + 53 Ma. Most of the single-grain ages are higher than the stratigraphic age, suggesting that the sample has experienced minor partial annealing since deposition. If we adopt a zircon PAZ between 170 and 240°C for this area [7-9], the paleotemperature for OR16 has probably always been below 200°C. As the Early Palaeozoic sediments have also been buried during Silurian foreland basin development, it is difficult to estimate the temperature disturbance as a result of Permian rifting. OR6, however, has a post-rift Triassic age of 228 + 22 Ma, suggesting

0 R l l (zircon) 750 . 600

BAM91 (sphene)

627_+ 64 Ma [ 627 Ida •

.

II

1°

•

o

a

\

-. t.50

N

3OO

1~

T18 (zircon)

![

611 Ha

-

b

• -m

% \

t:

150

I~0 4~ ~ age (Ma)

611+ ~ Ma

\ 5o

150 300 4S0 600 age (Ma)

M2 (zircon)

150 300 450 600 age (Ma)

45o

0R22 (zircon)

-145o

-~

634+ 62 Ma P(~?) = 90

100

150 300 450 600

OR20 ([zircon) 400 2

age (Ma)

300 i-

.~. ~--u~"u~"

181+ 18 Ma

21111

..-

634 Ma

~

47

o

C

\

-2 I f

,,

150 300 450 600 age (Ma)

~,

• 5O

IO0

150 300 450 600 age (Ma)

Fig. 5. R e p r e s e n t a t i v e Z F T and SFT samples. Single-grain age d i s t r i b u t i o n s p l o t t e d in a r a d i a l plot (left) and as age h i s t o g r a m s a n d p r o b a b i l i ~ curves (right). D a s h e d lines in (d), (e) and (f) d e n o t e d e p o s i t i o n a l age of the samples.

48

M. Rohrman et aL / Earth and Planetary Science Letters 127 (1994) 39-54

lite tuff from the Nittedal caldera (OR20) and a gabbro plug from Hadeland (OR26). The emplacement (Rb-Sr) ages for these rocks are ~ 265 Ma [20]. They show post-rift Z F T ages of 181 _+ 18 Ma and 200_+ 20 Ma respectively. These ages indicate almost total annealing and temperatures near the base of the Z F T prevailing into the Triassic, i.e. after rifting activity in the Oslo Graben had ceased.

3.3. Sphene Sphene fission track (SFT) ages for six Precambrian and one Permian sample are listed in Table 2. Four of the SFT ages are from the Bamble sector, two are from Telemark (Fig. 1). The ages decrease from 657 + 49 Ma (BAM84) on the Bamble coast near Arendal to 574_+ 36 Ma (BAM94) towards the Porsgrunn-Kristiansand fault (PKF). On crossing this fault into the Telemark region we see ages of 468_+ 30 Ma (BAM96) close to the fault and 551_+ 70 Ma (T3A) further to the north; the SFT ages seem to decrease to the northwest. As for the A F T ages, neither do the SFT data show a jump across the PKF. Samples from near the Skagerrak coast (BAM81 and 84) have SFT ages similar to those reported at Lake V~nern in Sweden (Fig. 1) (i.e., ~ 650 Ma) [27]. The old SFT ages indicate, in accordance with the Z F T results, that the flanks of the Oslo Rift have not experienced thermal resetting associated with Permo-Carboniferous rifting. Sample OR28 was taken from a Late Permian syenite in the AGS (Fig. 2). Its Rb-Sr age is 248 _+ 4 Ma [20], making it one of the youngest intrusions within the rift zone. The SFT age is 243 + 23 Ma, indicating that the pluton cooled to temperatures of < 300°C immediately after intrusion.

4. Interpretation of the fission track results

The syn-rift tectonic and magmatic evolution of the Oslo Graben is well understood from a large number of geological and geochronological studies [3,20]. The FT data presented above now

allow a detailed reconstruction of the thermal evolution of the rift from Permian to recent times.

4.1. Syn-rift thermal evolution The Z F T and SFT data indicate that rocks within the Oslo Graben have experienced temperatures up to the zircon PAZ ( ~ 170-240°C). The maximum temperature and the timing of heating, however, vary strongly between different localities. The presence of laumontite within vesicles in the lower lava flows of the VGS [B.T. Larsen, pers. commun., 1991] and the absence of a regional greenschist facies metamorphism [30] limits the absolute temperature maximum for the presently exposed rift to < 300°C. Peak temperatures prevailed during the syn-rift and early postrift phases ( ~ 270-180 Ma). An older ( ~ 400 Ma) event, possibly caused by Silurian burial in a foreland basin setting [17], remains obscure as a result of the Permian overprint. However, the inferred Silurian foreland basin would have had a much wider extent than the present-day rift; the fact that the flanks of the Oslo Graben record Late Proterozoic-Cambrian Z F T and SFT ages ( ~ 650-500 Ma) suggests that any Silurian burial heating would have been moderate. The Z F T / SFT ages from the Precambrian basement possibly reflect exhumation related to the formation of the regional sub-Cambrian peneplain [27]. The fact that Z F T resetting was restricted to the presently exposed rift makes a simple burial scenario implausible. Moreover, Z F T annealing by burial and conductive heating alone would imply either a post-Permian cover exceeding 5 km or extremely high ( > 60°C/km) regional geothermal gradients. Both of these conditions seem highly unlikely. Therefore, advective heat transport must have played a major role in establishing the Permo-Triassic peak temperatures. Thermal resetting around 270-240 Ma is logically explained by heating associated with the emplacement of syn-rift plutons. The Triassic-Early Jurassic Z F T ages (230-180 Ma) recorded in some samples are, however, more difficult to explain. Considering the large variation in Z F T age over relatively small distances, and the fact that Z F T age patterns seem to be controlled by

M. Rohrman et al. / Earth and Planetary Science Letters 127 (1994) 39-54

structural trends (Fig. 6), we suggest that this late resetting is controlled by hydrothermal fluid circulation. Several lines of evidence support this interpretation. Conodont colouration studies [31] have demonstrated highly variable conodont alteration indices (CAI) ranging from 3 to 5 in the Silurian sediments of the Oslo and Ringerike areas, indicating peak temperatures varying locally between 110 and as much as ~ 400°C. The reflectance (R 0) of vitrinite-like macerals [32] from the Cambrian-Ordovician alum shale lies between 2 and 6 and confirms the pattern indicated by the CAI values. However, temperatures are difficult to estimate from vitrinite reflectance at R 0 values above ~ 2. K-Ar dating of illites associated with hydrothermal ore deposits in the northern AGS yields ages of 191 +_ 2-244 + 4 Ma [33]; similar ages are found near Skien [34]. These ages are concordant with the reset Z F T ages and there seems to be a geographical correlation between the localities of reset Z F T ages and the occurrence of ore deposits associated with hydrotherreal veining (Fig. 6). Although some of these veins have a Permian age, they were probably reactivated by late-stage fluids, as is evident from the record of fluid inclusions in various parts of the rift [35,36]. Estimated temperatures are of the order of 230-300°C [36], which is enough to reset Z F T and SFT thermochronometers. The only SFT sample within the rift, however, is not reset, which is again evidence of an irregular, probably vein-dependent hydrothermal circulation pattern. The complex circulation pattern seems to be controlled by the volcanic structure within the rift: both Z F T age lows and hydrothermal ore deposits are concentrated within collapsed calderas or their feeder plugs (Fig. 6). This is in contrast with FT results from the Late TriassicEarly Jurassic Newark Basin in the eastern USA [37], which show a more regular and large-scale pattern of hydrothermal circulation. This discrepancy is probably the result of the great structural complexity attained by the Oslo Graben at the end of rifting. Old A F T ages near the eastern master fault of the Oslo Graben suggest that this was a cooler area of infiltration, similar to the master fault of the Newark Basin.

49

4.2. Post-rift thermal evolution AFT ages generally decrease from Triassic ( ~ 200-240 Ma) in the southeast (Bamble and Ostfold/Median segment samples) to Jurassic ( ~ 150-200 Ma) in the northwest (Telemark and AGS samples). Fig. 7 shows thermal histories for a number of representative samples. Maximum temperatures are inferred from the zircon and sphene data. The temperature history below 120°C is modelled from the A F T ages and length distributions [15] (and Van der Beek, internal report, Vrije Universiteit Amsterdam, 1992). The lack of short tracks (length < 10 ~m) in most samples indicates that cooling from 120 to ~ 70°C

61N

BON

59N

9E

10E

11E

Fig. 6. Map showing geographical correlation between volcanic structure, hydrothermal ore deposits, K-Ar ages of hydrothermally grown clay minerals (italic numbers) and ZFT ages (bold numbers with errors) within the Oslo Rift. Modified after [19] and [36]; K-Ar ages from [33,34]. Paleo-heat flow highs ( + ) and lows ( - ) , from organic geochemical indicators [31,32] and the AFT data presented here are also indicated.

50

M. Rohrman et al. / Earth and Planetary Science Letters 127 (1994) 39-54

took place relatively rapidly, whereas the general scarcity of long tracks ( > 14 ~m) suggests that final cooling to surface conditions took place fairly recently ( a f t e r ~ 30 Ma). A TriassicJurassic onset of cooling is also suggested by the (albeit not very distinctive) mean length maximum at these ages in the age-length plot (Fig. 3b). The distinct staircase-like pattern of the thermal histories from A F T inversion could, to some extent, be an artifact of the annealing model [13] employed, as this model seems to underestimate low-temperature and overestimate hightemperature annealing [38]. For this reason, we have rerun some of the inversions, adopting an annealing model for the fluorapatites [39], which

should exhibit an opposite type of behaviour [38]. Although the early Mesozoic and Neogene cooling events become less pronounced with this model, they are still easily discernable, and therefore we believe them to be real. We suggest that the A F T thermal histories record mainly cooling induced by denudation and that the drop in temperature from closure of the Z F T to A F T systems (Fig. 7) is caused by the destruction of the fluid flow system around ~ 200 Ma. The inferred Triassic exhumation of the southeastern part of the study area correlates well with the offshore sedimentary record• Seismic reflection studies reveal a major base-Triassic unconformity in the northern Skagerrak Graben,

Telemark: T17 i

Oslo rift lavas: OR39 i

i

i

i

50

50

100

100

o~150

i

i

i

i

I

I

150 ,o

200

200

25O

250 f

I

400 300 Bamble: DL129

I

I

I

I

200

100

400

300

200 100 Pre-rift sediments: OR11/OR22 !

50

50

100

100

o¢~150'

i

%1

150

250

250 I

I

I

I

400

300

200

100

400 ,

50

50

100

100

oQ~150 riflin~ 400

300

200 time (Mabp)

rT 100

I

;,-?

t

I

300 200 Oslo rift lavas: OR20

OstfoId: (~7

o I l,o J o,I "[Trt, I

i

,g

I

2OO

i

,

200

2OO

250

I -

,

150

! !

200

i

250

o I l,o I o,J. 400

100

300

f ,rT 200

100

time (Mabp)

Fig. 7. Thermal histories of representative samples from the Oslo Rift and surrounding areas. Shaded areas represent modelled thermal history below 120°C from inversion of A F T data ([15] and Van der Beek, internal report, Vrije Universiteit Amsterdam, 1992); thermal histories within the shaded bands fit the observed A F T ages within lo- error and pass the Kolmogorov-Smirnov test for track length distributions at the 95% confidence limit. Light shading denotes inversion using the D u r a n g o apatite annealing model [13]; darker shading denotes the fluoroapatite model [39] (see text for discussion). Dashed lines indicate interpreted thermal histories using regional stratigraphic data as well as Z F T and SFT ages (shown as crossing error bars where appropriate).

M. Rohrman et aL /Earth and Planetary Science Letters 127 (1994) 39-54

while thick Triassic sequences are encountered toward the south [19,21]. Triassic denudation of the Oslo Rift region is probably the result of a shift in rifting activity from north to south of the Fennoscandian border zone at the Permo-Triassic boundary [40,41]. Jurassic cooling ages do not correspond to a distinctive event in the Skagerrak where the Jurassic sediments are thin. However, rifting activity in the North Sea culminated during this time [42] and we suspect that this strongly influenced drainage and erosion patterns in southwestern Scandinavia. Finally, Neogene uplift and erosion of southwestern Norway, as suggested by the thermal history inversions, is also indicated by the pattern of Mesozoic strata within the Skagerrak, which are truncated by the Quaternary, with subsequently older sequences subcropping landward [43]. A more regional study of the Meso-Cenozoic uplift and erosion of southwestern Norway and its relationship to offshore basin evolution will be published elsewhere [Rohrman et al., in prep.]. If we adopt a normal post-rift geothermal gradient ( ~ 30°C/km), we estimate the total amount of post-Permian erosion for the rift area to be of the order of 3 - 4 km. The present geothermal gradient in the area is ~ 20°C/km [44], and AFT results from the Siljan borehole [25] suggest that this geotherm has been more or less stable since about 100 Ma. However, gradients were probably higher in the past due to the blanketing effect of the eroded Cambro-Silurian and Permian sediments, and locally high (advective) heat fluxes during rifting. Adopting a 20°C thermal gradient would increase our erosion estimate by a factor 1.5, which seems rather high. An estimate of 3 - 4 km of erosion agrees well with the estimated intrusion depth of the presently exposed Permian granitic and granodioritic plutons that has been ascertained from petrological [22] and fluid inclusion [35] studies. The flanks of the rift must also have experienced syn-rift uplift and erosion, as is evident from large conglomerate fans along the eastern border fault of the rift [16,19]. The absence of basement pebbles in these fans limits syn-rift uplift and erosion of the flank to 1-2 km. The total erosion of the areas bordering the rift is thus of the order of 4-5 km, which is not enough

51

to exhume rocks from the zircon PAZ; this would accord with the old Z F F ages encountered. The total amount of erosion within the rift zone corresponds well to the estimated maximum thickness of the Permian lava pile [1,19], suggesting that no significant amount (more than a few hundred metres) of post-rift sediment was ever deposited.

5. Implications for rift models The results of the present study place a number of constraints on tectonic models for the evolution of Oslo Rift. Below, we specifically address the dynamics and kinematics of rifting, the mechanisms of heat transport during rifting and the post-rift evolution of the area.

5.1. Dynamics and kinematics of rifting The timing of regional uplift with respect to rifting constitutes a strong argument in discriminating between 'active' and 'passive' rifts (i.e., for inferring the relative importance of mantle plume activity versus in-plane lithospheric forces in generating extension). The A F T data from the Oslo Rift suggest that regional uplift and erosion post-dated rifting by ca. 40-80 m.y. Pre-rift uplift cannot be excluded using the A F T data alone, especially since there has been such a large amount of post-rift erosion. The preservation of Cambro-Silurian (pre-rift) sediments exclusively within the down-faulted rift blocks, however, excludes any major pre-rift uplift, while the sedimentology of the Asker Group indicates that the area was at sea level immediately prior to rifting. Collectively these observations strongly support a passive origin for the Oslo rift [3,19,42]. This conclusion accords with petrological, geochemical and isotopic data [3] and modelling of melt production within the rift [45], which suggest that Oslo Graben magmatism was more strongly controlled by the presence of fluids than by a major temperature anomaly within the lithosphere beneath the rift. The kinematics of rifting (i.e., whether rift formation is mainly a result of symmetric (pure shear) or asymmetric (simple shear) lithospheric

52

M. Rohrman et al. / Earth and Planetary Science Letters 127 (1994) 39-54

extension) is another topic of discussion. It has recently been argued that extension of at least the southern part of the Oslo Rift, the Skagerrak Graben, was controlled by lithospheric simple shear [46,47], with the Porsgrunn-Kristiansand fault (PKF) acting as a main detachment fault in the system. Such a detachment, associated with large-scale vertical movements (of the order of a few kilometres), should show a strong contrast in thermal history between its footwall and hangingwall blocks, with the footwall block experiencing rapid cooling of several hundred degrees Celcius. Neither the SFT nor the A F T data from the Bamble and Telemark regions show such a contrast, indicating that a simple shear model for the Oslo Rift with the PKF as a master fault is not very likely. The Oslo Rift developed in response to lithospheric in-plane forces within a relatively strong Precambrian shield area. Such a pre-rift rheology would favour the development of a narrow pure shear rift [48]. Vertical movements along reactivated Precambrian faults outside the presently exposed rift zone were probably minor (i.e., < 1 km), only accommodating the tensile stress pattern associated with rift formation.

5.2. Heat distribution during rifting The Z F T and SFT ages in the Precambrian areas bordering the rift show that syn-rift heating was focused within the Oslo Graben, although crustal thinning and reactivation of faults took place over a much wider area. The present-day heat flow in southern Norway is relatively low (50-60 mW m -2 [44]), as would be expected such a long time after rifting. Modern rifts (e.g., the Rio Grande, East Africa, and the Rhinegraben) show relatively high and highly variable heat flow values, with means of 70-125 mW m -2. High heat flow values appear to be restricted to the grabens and to be controlled by volcanic structures [49]. The Z F T ages of the Oslo Rift show a similar pattern, suggesting highly variable heat flow during rifting with thermal activity located essentially inside the graben. Assuming 3 - 4 km of post-rift erosion leads to an inferred syn-rift geothermal gradient varying locally from < 50 to 70°C/km. The syn-rift thermal structure thus

appears compatible with that of modern magmatic rifts.

5.3. Post-rift evolution of the Oslo Rift The youngest Z F T ages are Triassic-Early Jurassic (180-200 Ma), suggesting that hydrothermal activity continued to operate long after the main rifting period ended. This late fluid flow regime was probably terminated as a result of erosion around the Triassic-Jurassic boundary, as indicated by the A F T ages. The estimates of erosion from the A F T data correspond well to the inferred thickness of the syn-rift lava pile, suggesting that very little postrift sediment was ever deposited. A number of mechanisms have been proposed to explain the absence of post-rift basin subsidence [50]. Firstly, subcrustal stretching could have been absent below the rift zone, as a result of lithospheric simple shear. This is, however, not very likely in the Oslo Rift because of it's association with major volcanic activity. In addition, there is no basin which records post-rift without syn-rift subsidence and can, as such, be 'linked' to Oslo Rift development. Secondly, the long duration of the syn-rift phase (60 m.y. in Oslo) would allow most of the heat to dissipate during rifting, so that the lithosphere would retain only a very minor temperature anomaly at the end of the rifting phase. In cases when subcrustal thinning is substantially greater than crustal thinning, as has been suggested to account for the volcanism within the Oslo Rift [45], regional syn-rift uplift takes place and rapid subsidence is delayed until the post-rift phase. However, it is possible that Triassic subsidence was not recorded by sedimentation because, by this time, the major depocentres lay towards the south and sediments would thus have bypassed the Oslo Graben [19]. Moreover, there is evidence for a ~ 10 km thick underplate beneath the Oslo Graben [3,22,45], the emplacement of which would counteract post-rift subsidence [51]. Finally, preservation of lithospheric strength during rifting suppresses the relatively small thermal forces originating from non-uniform extension. Although lithospheric strength was probably reduced directly beneath the Oslo

M. Rohrman et al. / Earth and Planetary Science Letters 127 (1994) 39-54

Graben itself, the evidence for rift flank uplift associated with rifting supports a finite strength of the lithosphere during extension. Acknowledgements We thank Bjcrn T. Larsen (Norsk Hydro, Oslo) and Else-Ragnhild Neumann (Mineralog&k-Geologisk Museum, Oslo) for their enthousiastic support of this project and many discussions and suggestions. Constructive reviews by Tony Hurford, Tom Pedersen and an anonymous reviewer are also much appreciated. Lodewijk IJlst and Tineke Vogel-Eissens assisted in sample preparation and processing. Irradiations were carried out at the ECN, Petten, The Netherlands. This study was supported by The Netherlands Organization for Scientific Research ( N W O / G O A grant 750.530.25 to M.R.) [UC]

References [1] I.B. Ramberg and B.T. Larsen, Tectonomagmatic evolution, in: The Oslo Paleorift: a review and guide to excursions, J.A. Dons, and B.T. Larsen, eds., Nor. Geol. Unders. 337, 55-73, 1978. [2] H.E. Ro, F.R. Larsson, J.J. Kinck and E.S. Husebye, The Oslo Rift--its evolution on the basis of geological and geophysical observations, Tectonophysics 178, 11-28, 1990. [3] E.-R. Neumann, K.H. Olsen, W.S. Baldridge and B. Sundvoll, The Oslo rift: A review, Tectonophysics 208, 1-18, 1992. [4] G.A. Wagner, Correction and interpretation of fission track ages, in: Lectures in Isotope Geology, E. J~iger and J.C. Hunziker, eds., pp. 170-177, Springer, Berlin, 1979. [5] P.G. Fitzgerald and A.J.W. Gleadow, Fission track geochronology, tectonics and structure of the Transantarctic Mountains in northern Victoria Land, Antarctica, Chem. Geol. (Isot. Geosci. Sect.) 73, 169-198, 1988. [6] C.W. Naeser, Thermal history of sedimentary basins: Fission-track dating of subsurface rocks, in: Aspects of diagenesis, P.A. Scholle and P.R. Schluger, eds., Soc. Econ. Paleontol. Mineral. Spec. Publ. 26, 109-112, 1979. [7] P. Zaun and G.A. Wagner, Fission track stability in zircons under geological conditions, Nucl. Tracks Radiat. Meas. 10, 303-307, 1985. [8] A.J. Hurford, Cooling and uplift patterns in the Leponfine Alps, South Central Switzerland and an age of vertical movement on the Insubric fault line, Contrib. Mineral. Petrol. 92, 413-427, 1986.

53

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