Journal of the Geological Society, London, Vol. 150, 1993, pp. 473-488, 10 figs. Printed in Northern Ireland

Subsidence analyses from the North Sea 'triple-junction'

N. WHITE & D. LATIN Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK

Abstract: When compared with theoretical subsidence curves calculated from the uniform stretching model, water-loaded subsidence data from the 'triple-junction' region of the North Sea suggest that the Jurassic-Cretaceous rifting event caused lithospheric thinning by a factor of c. 2.0. Although somewhat larger than the stretching factors found elsewhere in the North Sea, this amount of thinning is anticipated from the overall geometry of the three-graben system and is consistent with the observed volume and elemental and isotopic composition of the Jurassic Forties volcanic province. Apart from in the southern North Sea, Permo-Triassic extension is thought to have been relatively minor in comparison to the later Jurassic-Cretaceous phase. The anomalously small amount of Late Jurassic syn-rift subsidence in those wells where local fault-controlled effects are minimal, supports the well-known idea of localized relative uplift or 'doming' in the triple-junction area (c. 104 km 2) prior to and during the early stages of the Jurassic-Cretaceous rift phase. A time-dependent differential stretching model, in which the lithospheric mantle is initially stretched by a greater amount than the crust is stretched might provide an explanation. Such a model would require the total amount of stretching integrated over space and time to be the same for the lithospheric mantle and for the crust in order to avoid space problems. Alternatively, the same data could be explained by invoking a small transient thermal anomaly in the asthenosphere.

In recent years the North Sea has been an important area structure is simple, it could be argued that the discrepancy for testing the lithospheric stretching model of McKenzie remains elsewhere. Nonetheless, the burden of proof lies (McKenzie 1978; Jarvis & McKenzie 1980). So far, with those who argue in favour of more complicated published studies have been confined to the Central Graben stretching models: the simplest model must be shown to fail (Sclater & Christie 1980; Barton & Wood 1984; Hellinger et within the bounds of error for the observations in areas with al. 1989), the Viking graben (Giltner 1987; Badley et al. the best constrained data. 1988; Ziegler & Van Hoorn 1990; Marsden et al. 1990; Aside from this issue, several others need to be White 1990) and the Moray Firth Graben (Sclater & addressed before we present our preliminary results from Christie 1980). The purpose of this paper is to present some the triple junction. The first concerns the importance of preliminary results of subsidence analysis on wells primarily earlier stretching episodes (Carboniferous-Permian and from the Forties and Fisher Bank Basin areas or Triassic). In the central and northern North Sea, Permian 'triple-junction' where the Moray Firth, Viking, and Central and Triassic sediments are only rarely penetrated completely Graben meet (Glennie 1990). This area is significant since by drilling. While the geophysical characterization of their it was the most magmatically active part of the entire rift distribution and thickness is relatively straightforward in the system during the Jurassic-Cretaceous rifting event southern North Sea, it becomes much more conjectural (Woodhall & Knox 1979; Fall et al. 1982; Dixon et al. 1981; north of the Ringk0bing-Fyn high (Glennie 1990). As a Latin et al. 1990; Latin & Waters 1991). result, the Permo-Triassic rift system is poorly understood The early work of Sclater & Christie (1980) and Barton north of about 75 ° . & Wood (1984) concludes that the amount of stretching Obviously, a full understanding of the structural (/3=initial thickness of the crust or lithosphere/final evolution of the North Sea will only be reached when all thickness of the crust or lithosphere) determined from stretching events are taken into account. In the Central and subsidence analyses agrees, within error, with that Viking Graben, attempts to do this have so far tended to determined using crustal thickness data. However, Ziegler exploit our poor understanding of the Triassic event as a (1982), Badley et al. (1988), and Ziegler & Van Hoorn means for explaining various discrepancies (e.g., Giltner (1990) argue that the amount of extension measured from 1987; Hellinger et al. 1989; Marsden et al. 1990). We argue normal faults on seismic reflection profiles is considerably that it is more satisfactory to start off with the later less than that predicted by the other two measurements. better-constrained Jurassic-Cretaceous event and gradually Subsequently, evidence for an 'extension discrepancy' has work backwards in time when better data are available to been put forward for other basins (e.g. Moretti & Pinet constrain previous stretching phases. Where the Permo- 1987; Pinet et al. 1987). More recent work on the East Triassic sediments of the central and northern North Sea are Shetland Basin (c. 61°N), where the major basement- reasonably well-constrained (e.g. in the East Shetland extending normal faults are more clearly observed than Basin: Badley et al. 1988; and in the Egersund Basin: Steel elsewhere in the North Sea, has shown that minimum & Ryseth 1990) the amount of stretching appears to have extension estimates calculated from normal faulting agree, been relatively small (i.e. /3< 1.2). In the Outer Moray within error, with those predicted by the subsidence Firth, Boldy & Brealey (1990) argue that it is difficult to analyses and crustal thinning (White 1990). Since such discern any evidence for active fault control within the thin agreement can only be demonstrated in areas where the Permo-Triassic succession. In contrast, south of the data are excellent and accurately calibrated and where the Rynk0bing-Fyn high (e.g. the Danish Basin: S0rensen 473

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1986) there is good evidence for larger amounts of 6 km (Marsden et al. 1990; Kusznir et al. 1991). If the elastic Permo-Triassic stretching. thickness of the lithosphere is very small then the question The distribution of pre-Jurassic rift-related igneous rocks arises as to whether or not errors in other measurements (in in the North Sea (Dixon et al. 1981; Latin et al. 1990; particular palaeobathymetry; Wood 1982) and model Glennie 1990), also suggests that Permo-Triassic stretching parameters (e.g. lithospheric thickness and density) will be was more important in the southern North Sea. Extensive of more significance. If so, the standard approach, which Early Permian magmatism has been recorded in the determines the stretching factor from one-dimensional southern North Sea whereas none occurs to the north (other subsidence analysis assuming Airy isostasy, is justified. than in the Midland Valley and in the Oslo Graben). No Meanwhile, it is important to note that the flexural Triassic volcanic rocks have yet been drilled to our cantilever model requires the observed fault geometry and knowledge. The age of the magmatism suggests that the the degree of extension accommodated as input before basin earlier episode of rifting may well have been predominantly subsidence can be forward modelled. To date, modelling has Early Permian in age. Even though an Early Permian (?) been carried out on two-dimensional sections. Although this stretching episode may have been large (fl<2.0?) in the approach may be useful in areas where the fault geometry is southern North Sea, it is clear that the interval of time that simple, clearly observed, and, above all, two-dimensional passed before Jurassic stretching started (c. 100 Ma) is (e.g. the Viking Graben?), it cannot be applied to areas longer than the thermal time constant for the lithosphere (c. such as the triple-junction where normal faulting is complex 60Ma). Hence the thermal perturbation induced by the and difficult to resolve and where there are likely to have earlier event will have largely decayed away by Mid-Jurassic been large rotations about vertical axes. Here, we argue that times. a simpler approach based on fitting theoretical subsidence The second issue concerns the existence, volume, and curves to backstripped data is more fruitful at this elemental and isotopic composition of the Jurassic preliminary stage since it does not require detailed magmatism which occurs in the triple-junction. The most knowledge of the normal faulting and is independent of the important question is whether or not the amount of extensional slip-vector across the zone of interest. Jurassic-Cretaceous stretching is large enough to generate the melt without resorting to elevated temperatures in the Subsidence and stretching asthenosphere at the time of rifting (i.e. a hotspot or plume). Recent modelling Latin & Waters (1991) suggests Figure 1 illustrates the overall structure of the triple- that providing the stretching factor in the Forties region is junction area and shows the location of the 41 wells >1.5 and preferably c. 2.0 in order to raise the base of the presented here (see Table 1 for key). About 20 wells occur lithosphere to 80km depth, there is no difficulty in within the triple-junction itself. To avoid local syn-rift uplift producing sufficient melt. The presence of a convective effects which may occur at the crests of large (>10 km wide) plume in the asthenosphere (e.g. Watson & McKenzie 1991; rotating fault blocks (Barr 1987a, b; Jackson et al. 1988), a Latin et al. 1992) would give rise to melt volumes several significant number of the wells chosen occur within the orders of magnitude greater than is observed. A transient hanging walls of major fault-bounded blocks of the region. thermal anomaly within the asthenosphere cannot, however, The position of each well with respect to the underlying be ruled out. structure is indicated in Table 1. Some of these 'hanging Thirdly, there is the question of whether or not a pre-rift wall' wells are actually sited on the crests of smaller (<5 km and perhaps an early syn-rift (Bajocian/Bathonian) 'dome' wide) fault-bounded blocks (A. Roberts, pers. comm. existed within the area of the triple-junction and central 1991). However, the arguments of Barr (1987a, b), Jackson North Sea (Eynon 1981; Leeder 1983; Ziegler 1982). There et al. (1988), and White (1990) suggest that blocks of this are two concerns. (1) What is the evidence for either width and smaller do not significantly perturb the average syn-rift subsidence. doming or reduced syn-rifl subsidence? (2) If there was a dome then how big was it and what caused it (e.g. was it Throughout, the standard backstripping approach is used related to a transient thermal anomaly in the astheno- to determine water-loaded basement subsidence as a sphere)? Erosion of pre-rift sediments and the absence of function of time. A more detailed discussion of the method syn-rift sediments have been used as evidence for doming and its application is given by Steckler & Watts (1978), (Ziegler 1982; Underhill & Partington 1993). However, it is Sclater & Christie (1980), Wood (1982) and Barton & Wood often difficult, using well data alone, to discriminate (1984). Theoretical subsidence curves (McKenzie 1978; between regional erosion and erosion due to local footwall Jarvis & McKenzie 1980) are fitted to determine r, the uplift and rift-flank uplift (Barr 1987a, b; Jackson et al. stretching factor. 1988). Complexity of fault geometry and large thicknesses of post-rift subsidence in the triple-junction region make it The importance of previous stretching episodes difficult to demonstrate large-scale erosion of pre-rift Our first concern is the importance of Permo-Triassic sediments on seismic reflection data. stretching and how, if ignored, the compaction and thermal Finally, a more recent debate has arisen concerning the implications of earlier subsidence affect the value of fl importance of allowing for lithospheric flexure when determined for the Jurassic-Cretaceous. As discussed analysing subsidence in extensional sedimentary basins. above, there is a limited amount of evidence suggesting the Admittance studies of the free-air gravity field and load Permo-Triassic stretching was much less significant than topography indicate that the elastic thickness of the crust Jurassic stretching north of the Ringk0bing-Fyn high. In the beneath the North Sea is less than 5 km (Wood 1982; Barton past, Ziegler (1982) has proposed that there may be large & Wood 1984). More recent work using a flexural cantilever thicknesses of Triassic sediments in the Central and Viking model to elucidate extensional basin development produces Graben. This hypothesis has not yet been tested. Here, we best results when the elastic thickness is between 1.5 and ignore the thermal and compactional effects of the

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SOUTH VIKING GRABEN i

=GERSUND Mo,:,AY,:,,:, ]- GRABEN \ 10 _ .

Fig. 1. General structural map of central North Sea where Moray Firth, Central, J L and Viking Graben meet to form so-called 'triple-junction' (redrawn from tectonic map of North Sea published by GECO Exploration Services & Alastair Beach Associates 1989). Quadrants and their numbers indicated; normal faults are shown as solid lines with ticks on down-thrown sides; wells are numbered in solid circles (see Table 1). Each quadrant is about 60 km wide and 110 km long.

Permo-Triassic event for reasons outlined below. However, Figure 3 illustrates what the effect of large thicknesses of if, as Roberts et al. (1990) argue, the earlier phase of Triassic sediment would have on the thermal subsidence stretching cannot be so lightly dismissed, what will be the gradient and hence the calculated Jurassic stretching factor effect on the conclusions we present? for well 37 (see Table 1). the stratigraphy of this well is also Figure 2a shows the water-loaded subsidence for well 18 shown in Fig. 4 where only the observed data are used in (see Table 1) on the edge of the Egersund sub-basin where backstripping, suggesting a fl value of 1.5-1.6. When 3 km large thicknesses of Triassic and Lower Jurassic sediments of Triassic sediment are arbitrarily added to the section and are observed. The data have been fitted assuming a Triassic backstripping then carried out (Fig. 3a), it is clear that the stretching event from 225-190Ma and a Jurassic- gradient of the post-rift or thermal subsidence has decreased Cretaceous event from 150-100 Ma. In this particular area slightly. However, the decrease in gradient is insufficient to both stretching events are small (/7 < 1.15). Figure 2b shows alter significantly the previously chosen value of/7. Note the the fl values which would have been determined for the poor fit between the observed and predicted syn-rift Jurassic-Cretaceous stretching event if sediments older than subsidence as in Fig. 4. Figure 3b shows the result when 150 Ma were assumed to be fully compacted basement and if only 1 km of Triassic sediment is added. Therefore we feel the theoretical curves only include the Jurassic-Cretaceous confident that the gradient of post-rift subsidence is not event. The values are higher (i.e. fl = 1.2-1.25) for two greatly changed by ignoring the effects of possibly large reasons. First the theoretical curves in this case have not thicknesses (c. 3 km) of Triassic and earlier sediment. Since been 'pulled down' by an earlier thermal event which is still the gradient of the post-rift subsidence is the most sensitive decaying at 150 Ma. The second reason is because the effects measure of /7, the values we choose will be largely of decompacting the pre-150 Ma sediment are ignored and unchanged. We reiterate that a far more important source of therefore the 150Ma to present-day subsidence appears error exists, namely the uncertainty in lithospheric thickness larger than it should be. Either effect will, on its own, lead and density. to an estimate of/7 for the later event which is slightly larger than it should be. The effect of excluding a Triassic event in Palaeobathymetry the theoretical curves is better illustrated by comparing Figs 2b and c. It is a slightly more important effect than the effect The most significant source of error in determining the of including the decompaction of pre-150Ma sediments, water-loaded subsidence is the palaeobathymetry during shown in Fig. 2d. basin subsidence. Although there is a large degree of

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Table 1. Well location references, maximum and minimum fl values, penetration of Triassic rocks, location of wells with respect to underlying major normal faulting (H, on footwall crest; L, within hanging wall), and numbering system used in text and Fig. 1

Number Well reference Stretching factor Triassic rocks Drilled on penetrated ( T) high or low

1. Oxy 14/19-4 1.40-1.50 T H/L 2. Amerada-Hess 15/21a-8 1.05-1.10 H 3. Phillips 15/27-2 1.05-1.10 H 4. BP 21/2-4 1.45-1.55 H 5. BP 21/6-1 1.50-1.60 T L 6. Be 20/10-1 1.20-1.25 T L 7. BP 20/10-2 1.25-1.35 T L 8. Shell 22/6-1 1.80-2.00 H 9. Oxy 22/la-3 1.70-1.85 H 10. Sun 22/1-2a 1.35-1.45 H 11. Conoco 15/30-1 1.60-1.70 H/L 12. BP 15/24-1 1.45-1.55 H/L 13. BP 15/20-2 1.05-1.10 H/L 14. BP 16/13a-2z 1.80-2.00 T H/L 15. Trans 16/26-2 1.80-2.00 H/L 16. Gulf 16/27b-4z 1.80-2.00 H/L 17." BP 16/28-3 1.20-1.25 L 18. Esso 17/9-1 1.15-1.20 L 19. Superior 22/5b-2 1.15-1.20 L 20. Britoil 22/3a-1 1.65-1.75 H/L 21. Burmah 22/2-2 1.40-1.50 H/L 22. Total 22/7-1 1.80-2.00 T L 23. Sun 22/8-1 1.80-2.00 H/L 24. Sun 22/8a-2 1.80-2.00 H/L 25. Amerada-Hess 22/14a-2 1.40-1.50 T H 26. Phillips 22/14-1 1.80-2.00 H/L 27. Total 22/15-1 1.35-1.45 T L 28. Conoco 7/9-1 1.20-1.30 T H 29. Conoco 8/21-1 1.20-1.25 T H/L 30. BP 7/12-3a 1.20-1.25 L 31. Oxy 22/19-1 1.35-1.45 T L 32. BP 22/24a-1 1.15-1.20 T H 33. Shell 22/23b-1 1.30-1.40 T H 34. Shell 22/21-2 1.15-1.20 L 35. Shell 21/30-1 1.15-1.20 T H 36. Amoco 27/10-1 1.10-1.15 H/L 37. Ranger 27/27a-2 1.50-1.60 T H 38. Shell 29/3-1 1.15-1.20 T H 39. Shell 29/9a-1 1.80-2.00 T L 40. Britoil 29/14b-lb 1.50-1.60 T L 41. Britoil 30/2-1 1.70-1.90 H/L

Bold indicates wells on hanging wall that encounter Triassic rocks.

uncertainty and considerable problems in making accurate uncertainty in palaeowater depth. The most important estimates, some workers think that sediments from Triassic difference between the Wood (1982) and the Bertram & to present-day within the North Sea were deposited Milton (1989) approaches concerns palaeowater depth predominantly in shallow (< c. 300m depth) water e.g. during the Early Cretaceous. According to Wood, inner Sclater & Christie 1980; Wood 1982 and references therein). shelf conditions dominated during the Early Cretaceous, However, others argue that large variations in palaeobathy- followed by a deepening to maximum depths during the metry (>lkm) should be anticipated and that such Mid-Cretaceous, with a gradual shallowing throughout the variations will have an important impact on the stratigraphi- Chalk. From Berriasian to Barremian times, the water cal development of the basin (Bertram & Milton 1989; depth is thought to have been no deeper than about 80 m. Milton et al. 1990; Barr 1991; Marsden et al. 1990). This conclusion is based on the presence of Haig's (1979) The approach taken here is that of Wood (1982) who Ammobaculites association of benthic Formaminifera. The assigns different levels of uncertainty to palaeowater depth main aggulutinating genera are Trochammina, Vernu- at different times based on sedimentological and micro- linoides, Haplophragmoides and Ammobaculites, together palaeontological arguments. Where we show water-loaded with the calcareous forms Lenticulina, Epistomina, subsidence curves, the length of the error bar indicates the Gavelinella and Conorboides. The presence of Epistomina,

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ESSO 17#9-1 ...... I ...... i ...... i ...... ~.o.o I: ;".,'~, !:.:.I ::i 37 ~,2~,A-2 0.0 ...... I ..... '.'''1 ...... I ...... "''1 ...... - ...... i:"; ...... , ...... '...... i".~.. ",,.:... 161.01:- ...... "...... --1 '3 1.0 ::.. ,.0 I ...... "..... ii ...... 11 ...... !i1' 1 1 1 15 .~.. '.i' .. I ...... Ill ...... ' -..?. 2n F ...... I ...... l ...... I ...... i ...... 'l 1.15, 1.15 ..... "~50.0 200.0 150.0 100.0 50.0 0.0 , + -.-... r~ (ma) 2.0 s s% " ""'" "...... '.i (a) '" P'"'"I 1.5 ESSO 17/9-1 1.6 0.0 - ...... -- 4 .n50.0j~ i i ..... I ...... I ...... I ...... I ...... t-':. 200.0 150.0 100,0 500 00 "'1~-... i" "::::::"7;.:: Time (Ma) '0 :::::::c!t II 1.25 . ,,,,,I , I''', ,'''' I''' ''' '' '1''' '''''''1 RAN 2~27^-2 "~Io' lm.o ,oo.o m.o o.o 0.0 ~...... I ...... ~.1 ...... I ~ ...... I ...... - Tmne (Ma) (b) "::-7.:::i:j.. ~. 1.o . i -.'.. E,~SO 17#9-1 0"0 I-' ...... I ...... I ...... I ...... l ...... ':] '", ":i..:.:!:7.::il li~.. il." II "" "".. "w" i. I ,, ;'..,;:. ~ 2.0 1.5 'E" I "" "':":ii:: .... • " ...... !:}l:lL::: J o i "° ~" .... ::::..... 7 1.6 "250.0 200.0 150.0 100.0 50.0 O.O I ...... ::: : :: :::::::::::::::::::::::::::::: : "rime (Ua) '£:o ...... ~.:o ...... ;~:o ...... ;~:o ...... ~ ~ ...... 0° 1.25 (Ma) (b) @) Fig. 3. Subsidence analyses for well 37 (22/27a-2, see Table 1 and

...... +...... ~ 17/9-1 Fig. 1). (a) Water-loaded subsidence with maximum and minimum o'(i::~- ..... , ...... , ...... , ...... ,..i theoretical subsidence curves assuming three extra km of Triassic sediment (see Fig. 2 for details and compare with Fig. 4). (b) as for ~ ...... i...... (a) assuming one extra km of Triassic sediment. ~10 ...... "...... I t !.! ...... I I , 1.1,1.15 Bertram & Milton's main concern is with those who 2-1", I .I...... I ...... "1 "~:o' ...... ;~o ...... loo.o m.o o.o completely ignore palaeobathymetry when carrying out r~ (i~ basin analysis. In general, they agree with the estimates of Wood (1982) and Barton & Wood (1984) although they (d) argue that palaeowater depths increased rapidly during rifting resulting in a maximum of 1 km by about Hauterivian Fig. 2. Subsidence analyses for Egersund Basin well (number 18, times. Water depths are then assumed to decrease linearly Fig. 1 and Table 1): solid vertical bars, water-loaded subsidence to the present day. In other words, much of the observed including uncertainty in palaeowater depth; dotted lines, best-fitting subsidence in the North Sea is inferred to have been due to maximum and minimum theoretical subsidence curves (numbers at the infilling of a deep hole. It is ironic that the rejection of right-hand-side are fl values). (a) Total observed subsidence fitted this particular explanation for post-rift subsidence and the with theoretical curves which allow for Triassic and Jurassic realisation that such sediments were deposited in shallow stretching: (b) observed post-Jurassic subsidence fitted with curves water originally led to the development of the stretching which only allow for Jurassic stretching; (¢) total observed model (McKenzie 1978). subsidence fitted with curves which only allow for Jurassic The problems in assuming such a dramatic variation in stretching; (d) observed post-Jurassic subsidence fitted with curves water depth can be examined by backstripping well data which allow for Triassic and Jurassic stretching as in (a). using Bertram & Milton's palaeobathymetry. The resulting water-loaded subsidence for 3 wells (30, 37, and 14) is shown in Fig. 5. In each case the thermal subsidence an aragonitic foraminiferid, in areas like the Egersund Basin gradient implies extremely small stretching factors (fl suggests shallow water, inner shelf conditions (20-80 m). 1.05), given that the flexural rigidity of the lithosphere is Rare specimens of Lenticulina and Textularia foeda suggest less than 5km. These very low values are a direct even shallower conditions (0-30 m). By early Aptian times, consequence of the proposed palaeobathymetry which Epistomina is no longer present in well cuttings, although causes the thermal subsidence gradient to be reduced almost this absence may be due to poor preservation. The to zero (compare same wells in Fig. 4 and Table 1). For a Aptian-Albian period was viewed as a period of rapidly variety of reasons (overall crustal thinning, organic increasing water depth: 0-50 m at the end of the Barremian maturation, etc.), very low values of end Jurassic stretching increasing to 200-600 m at the end of the Albian. are unlikely to be realistic.

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1 ox~ 14/19-4 7 BP 2o/10-2 o.o ~...... "" ..... '.'" " "":!'.' .... ' ...... :1 o.o ...... , ...... 1' ''-.''..""' ...... ' ...... :1 " '"' ...... L,:, ...... ~ ...... t. L):i ,; t l o ' ' ...... !" ...... --1 ~o t ...... '.l... :"-.....= ...... I 1.25

...... , ...... , ...... , ...... :::':L::.: , , , , :t .35 "" o ~.o ,~o ,oo.o ~o.o .... o.o 1.5 2"6.o ...... ~:o ...... ;~:o ...... ;oo:o ...... ~.'o ...... o o

AMERADA-HESS 15/21A-8 8 SHELL 22/6-1 o.o 1: ...... l ~ ...... L.'. ' "'..~.!..'.. ', ...... "',--.:/"i ...... •...... I

...... ': ,.o ;71 ...... ool i i L 1.8 2.0 ,o.~.o ...... ,soo' ...... ,ooo'...... ~o o o 1.1 2 o ...... ,'~,:o ...... ;':o ...... '~.;, ......

PHILLIPS 15/27-2 9 0~, 22/1A-3 o.o "'~ .... ,~',';"!"'I ...... I ...... • .. ""-F ...... i q ......

1.0 ":')' ...... [~:i: h ' , l ,.o ...... Ii.-., I ...... t...I ...... ,,:~ 1.o5

~ ...... , ...... , ...... ~ ...... ~ 1.1 2.O ~oolo L"-I::::" 1.7 ",...... BP 21/24 1.85 , ...... I £;.' ...... i ...... i ...... 30 ...... ,,,I,,,,,,,,,I,,,,,,,,,I .... i i i i i" . '..:... "200.0 150.0 100.0 50.0 0.0 "(%

1 O SUN 2211-2A 0.0 , ,, -' ' ' ','i' I ;'''~ ...... I ...... I ...... - ...... ::i:i.1i111!i! . I"'."..

1.45 "'"-...... 1.0 ...... l -. "'ii...... '. 1.55

,,,,,,1,,,,,,,,,I,,,,,,,,,1,,,,,,,,, 1 35 3~.o'"'" ' ,r, o o ,ooo 5o.o o o 2.0 1.45

BP 21/6-1 30 '`''''''''1'''',,,1'1`,,,,,`',1,,,, I I I O.O ~ ...... I ...... o,' ' "!:..,...... , ...... 200.0 150.0 100.0 50.0 0.0 ".o%. .

]_ "1 CONOCO 15/30-1 '°F II ...... ",: 1 0.0., ...... i ' "'-~%'. .... i ...... i ...... -I F 1::::l'.,...,.,~ 1.5 ,.F,,:o ...... ~'o ...... ,'~:o ...... ;#o:o ...... ~.'o"" ..... :;,, ,0-:- ...... 1 1.6 • i-. ""?...... !.I| .....L.. L!,,.... "

4~ BP 20/10-1 '"'r o.o ~ ...... l ...... "~.l-:'..'' ', ..... l ...... ! ...... 1.6 "°i "ii :l 1.7

1.0 ...... " ]]]::]i:i: ""| ...... 3 "~().0r,,, ,,,,,,I,,,150.0 ,,,,,,I 100.0...... 50.0I ...... ~"10.0 =~: L , .o., :':o', :, I,, 11"2.25 Fig. 4. Subsidence analyses for wells located in and on the peripheries of 'triple-junction' region. In many cases, only thermal part of subsidence curve shown (see Fig. 1 and Table 1 for location, etc.). See Fig. 2 for explanation.

It is important to emphasize that there is no independent evidence that water-depths were as great as 1 km by the end Stretching in the triple-junction region of the Jurassic stretching phase. What f6w data are available Figure 4 shows the water-loaded subsidence data fitted with suggests that water depths were considerably shallower theoretical curves for all 41 wells from the triple-junction (Glennie 1990). Moreover, we feel that it is essential to region. Note in Table 1 that 18 of these wells penetrate assess palaeobathymetry separately as Wood (1982) has down to Triassic or older sediments and 13 occur within the done rather than to vary it on a largely ad hoc basis in order hanging walls of major normal faults. A total of seven wells to explain possible anomalies in the stretching history. For occur in hanging walls and encounter Triassic (highlighted in want of compelling evidence to the contrary, we use the bold in table 1). We have ignored the effects of salt approach taken by Wood (1982). diapirism since it is thought to have been relatively modest

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2 or 15r24..1 /_q SUPERIOR 22/58-2 o.o~ ...... , .... i'.,'.' ' ', ...... , ...... 0n .vI I I I ,'-'~....,.,, = ...... , ...... r....'," .... " ...:% "i:.'?.... "* .....::::::::::::::::::::::!"t 1~oo!I '""iLiilL]il]]i]!i]iiiil.i ...... ill 1.45 ....::L!:q:::::: 1.15 1.2

13 ~* ,~2 2.0n, i o.o ~ ::,:: :,~. , , :y:, . , , :, . , , , , ...... ,,,,,,,,, ,i,,,,,,,,,i,,,,,,,,,I,, ,,,,,, 3"~.0 150.0 100.0 ,'50.0 0.0

~0 BNOC 22t3a-1 "~ i'"f:~""l ...... , ...... :1

~?,,o ...... ,'~.', ...... ~ ...... 0o i ...... i ...... ,.o- ...... ~II[Il F 14 B~,~,~z ! ""1t' ", ,;1 0.0 -''''''~''1 ...... I ' '.'*-"... ' ..... "'[ .... 't'''l'''''''''- 1.65 .~.o- ~me (Ma) I ...... ', ii 1.75 ! i i". '", 1.0 "'|" "'"""

'•~=,:o ...... ,~o ...... ~o ...... 'ol,o l:'i;' ' !!!i;!il] ...... ~8 2.0 ...... 1 BURMAH 22/2-2 o.o ...... ,, .:.y ...... , ...... , ...... 2.0 • . i f..... 30 ...... 1 ...... I .... = .... I .... , .... I , t * i iii I "250.0 200.0 150.0 100.0 50.0 0.0 ki..."1.1 ...... 1.0 ..... '"H ' ' • .. OPwl II 1!!:': !iii...... o.o~ ...... ',"'i" ..... ': ...... ' ...... R ~o tilL[Ill,Ill? 1.4 ,.ol ...... 1;iill, ...... i;,j.. 1 1.5

3Ol~tllllltli fill, ,II II 11 111 l, ,llllll ll, , 1.8 ~o ,~oo ,ooo 50o oo

2.0 22 To~,~ ~-~

0.0., .... ~ ,e, , I ...... "I'| ' ' ' ;'.'"" '' I ...... I ......

"'".i'"'".. 0.0 ,0 1t~! ...... ,,~ ,o ...... q:II!::LL!, • "'"'.-....,, "'.[,...., ,..... 1.8 2.0 ,.o ili,il ill 18 'o ~l"l"J"i 2.0,.~.0 ...... ~.'o ...... L~.'o ...... 4o~:o' ...... :..L.h...... o.,

3.0 ii III II I I II I II II I I I , I , , I I , * I I I I I I l I I I II 200.0 150.0 100.0 50.0 0.0 2 3 SUN 22/8-1

17 ..,6n~-~ 0.0 , , , ,, , ,, i ,i, , r.,.4.,"", "'". ,, i ,,, , ,, , , ,i, ,, , , ,,, ,~ o.o I: ...... ~ I ~ ...... ;"~:: ...... -I o%.',., '°I ....:::::::::::::::::::::::::::::::::::::::::::::::: ,o ....::::i:1! L'LI ~ ...... ] L ' 1 1.8 1.2 20 ...... J ...... I ...... i .... J, [; i't" 2.0 20 I" .... lllll I .... lllllll,,,, .... I .... ,,,,,I,,,,.,,,,'t 1.25 2oo.o 15o.o 1oo.o so.o o.o "250.0 200.0 150.0 100.0 50.0 0.0

Fig. 4--(Continued). Secondly, the best-fitting theoretical curves indicate a this far north (Glennie 1990). Nevertheless, we acknowledge signficant misfit between the observed and predicted syn-rift that specific conclusions drawn from any single well must be subsidence. The usual way to account for a striking misfit qualified by the possiblity of local diapirism. between observed and predicted syn-rift subsidence is to These data taken as a whole show several important appeal to local footwall uplift effects (Barr 1987a, b). This features. Firstly, the thermal subsidence (c. 100Ma to explanation is probably valid for those wells which are present) often has a very steep gradient within the clearly sited on the crests of large tilted blocks (e.g. wells 8 triple-junction, even allowing for considerable uncertainty in and 9). However, the results from the seven wells which the palaeowater depth estimates. According to the analyses, occur in hanging walls and which appear to have a complete observed subsidence gradients suggest fl values of 1.8-2.0 stratigraphy, cannot be so easily explained. for nine wells most of which are sited in Quadrant 22. The Perhaps the best way to account for this striking misfit is other wells give fl values ranging from 1.15 up to 1.6. to allow the lithospheric mantle to stretch more than the

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2 4 SUN 22JSA-2 30 BP 7r12"3A °'°1- .... I1-I ' ' I ...... I ...... -I o.o~ ..... :,~+q..,,.: ...... , ...... , ...... E '"( ......

"°I ....t L'i il)ii'i~ " 1 ,.o ...... [::i:::i:i

' i',i~ .8 20 I',,, ...... I ...... 1 , , ,';'1',-,.+ ,;I. ,,m.o ,®.o .~.o o.o 2.0 do...... +o ...... +o ...... !! l!: ,!o 2 5 ,AMERADA-HESS 22114,~2 / O.O 31 o~ 22/19-1 o..~ ...... , ...... ,,.,,.,:.+,..,..: ,...... , ...... + -' '' ''' '' ' l ' ' ' '' '' ' ' i ' "."-.'..... ' ' ' ' ' i '',,, ,,,, i,,,,,, ,, , ....-iii ......

1.o ...... '"...... ~Dii~

,o I ...... i,:: 1.35 ...... i!:::l:li:!:i• '... " , , ...... ~t 1.5 t ,-m,~:o...... +g.'o...... ,'a,.'o...... ;='o' ...... ;o~ ...... o o , ,.o 1.45

26 PHILUPS 22/14-1 ,,,,,1 .... I,,,,,,,,,I,,,,,,,,,l,,,,,,,,,I,,,,,,ij o.o t"" "" 'i*"' ...... ' ...... -I $'~54:).0 200.0 150.0 100.0 50.0 O.O

32 .... '~',-'

°I ...... , l oo I"".'"'...... IIi: l' 1.8 z?.g:o...... ,":o...... g.;"'"' ";.o 2.0 ,.o .15 27 TOTN_22/15-1 ~ .2 0.0 ''''''+'' i '''''' ',' ; i ''v1'~ ...... '' '' I''''''''' I'''''''''.~ ~I.0

'... ~ ...... ,.

1.0 ::o" fi ...... ~.'o ...... ,'.~;o...... ;0020' ...... ia.;,1...... o.o ii]iii[ii!' 1.35 33 smz,,mmm-, 2.0 - 1.4500 ...... , ...... ,~..+...... :, ...... , ...... ,,,,.. • ...... , ...... ; ...... i: ...... %,

S.~!~I.O''''*'''''I'''''''''I'''''''''I'''''''''I'''''''''200.O150.0 I00.0 50.0 0.0 ,0 IJ ...... 28 ~,~, 1.3 0.0]...... +"'I .... i 'l' ' i' i ' L~ I"t.'.' ' ' I ...... I ...... '-I 2.0 ' '.. ,i...... :! 1.4 I ...... I I "'i"ii"... 1 • .... ,,I,,,,,,,,,i,,,, ..... I,,,,,,,,,I,,,,,llji ,.o I ...... "i] ...... r~ 1.2 ' ...... !~l.a "~o.0'--" 200.0 154).0 1oo.0 5o.0 o.o

4 SHELL 22/21-2 ,m;:o ...... ~,:o ...... ;~,~...... ;~:0 ...... ~a ...... 'go 0.0~ ...... ,,,.(.., ...... , ...... , ...... 29 comm~+-, 0"0 I- ...... +''1 ..... ~' ' "*' I~v:~. ' ...... I ...... I ...... 2[ 1.o "...... L!il:"i:::::~::'il"l , 1.15 ...... i ...... ~...... i. i ...... 1|'. ~;:o . ,5o.o, .... mo.o, ..... ~o.o ::rS:+o.o 1.2 '°I ::::::::::::::::::::l4 1.2 ,_° ...... , ...... , ...... , ...... , ...... 21 1.25 2S0.0 200.0 150.0 100.0 50.0 0.0

Fig. 4--(Continued).

crust within this area, at least during the early stages of can melt at depths of 80 km or less (Latin & Waters 1991). rifting (Latin 1990). This suggestion is not a new one (e.g., Finally, a significant increase in the rate of subsidence Royden & Keen 1980; Hellinger & Sclater 1983) although between 65 and 50 Ma is evident in many wells (e.g. wells 2, we do emphasize the necessity of conserving the total 3, 9, and 13). For the purposes of this study, we have amount of extension accommodated by the crust and interpreted this increase as a minor stretching event. As lithospheric mantle at any one time in order to avoid space discussed below, the evidence that the increase is due to problems. If heterogeneous stretching curves had been fitted stretching is poor. The important point, however, is that by to the data in Fig. 6, the total amount of lithospheric treating it as a stretching event, the magnitude of thinning (crust + lithospheric mantle) predicted would have Jurassic-Cretaceous stretching is minimised. been slightly greater than the values given here (Latin 1990). These preliminary results are in good agreement with Stretching on the periphery of the triple-junction the amount of stretching required to produce the observed It is clear that post-Triassic subsidence away from the volumes of alkali basalt, provided that dry asthenosphere triple-junction fits the uniform stretching model reasonably

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3 5 SHELL 21/30-1 39 s+, 2919A-1 °'° I~, ...... l ...... l :'t,.-..,.,l..... 1 ...... , ...... '~l o.o .,,,,~,,~,,I,,,,,,~l,l,,+.,+,,,,vl,,,,,,,,,i,,,,,,,, i '."...... IIIII~IIIIIZIII..... ,o I ....'+ ....tl +i +::i...... i, +- 11.2ol5 ,o ";;";""..I;;;;i;i:.-.,...... !...!ll.. ,-. , , , , ,-, 1...I,.t.I...... 1.8 z~.o ...... ffo+'o...... ;~o:o...... ;g£i ...... ~ ...... o.o "+,.o 2.0 AMOCO 27/10-1 , * l I . I , I . l l I l , I . l I I . l I I I l l I I . . l I I i i i IIi i i i i i i i i 36 ~:o' 200.0 150.0 100,0 50.0 0.0 0.0 ....,,...... I''" ' I" ! ...... 1: .i~:.i.:;.ii!...... t ...... 101: t...... +' ...... , ...... ++ "/':";'~'~','.,...... :i 1.1 •I=.0 ~®.0 =.o 0+ 1.15

3 7 R/U~GER22/27A-2 40 BRITOIL 29/14B-1A o.o +...... , ...... , ,,..?...... , ...... , ...... , O~ I,...... "'I ...... ' ' ' I ' ' "~'I. .... I'''''''''I' ...... -I

ii o ';i", i ++ ",...... i; ...... i ' % i ...... 1 ,o ' "::;t;]i.~,;...i~,~..i!q' 1.5 1.6 " ...... i1111:iii 1.5 1.6

. , I I I I s'~o ...... ~o:o ...... ;~'o ...... ;oo:t; ...... ~.~ ...... o.o 4 1 BRrrOIL 30/2-1 SHELL 29/3-1 o.o ~...... , ,.,..y~ .... , ...... , ......

38 fl ".8"'. 0.0 ,,,,, d+~, i ~.,,**,,+i,,,,,,,,,i,,,.,,,,,I... • "':','., i,,,..,,,, ",.If...... '-.. I

1.0 "" ""I "'"" ...... ! (. 1.0 ""=::;"~. I

.....~+;iii;:ii;i ..... iii ij 2.O 1.7 II ...... 1.9 =.o. " ...... Ir i+ 1.15 + l lt ...... m0 ~n ¥I11 Ill 11 l|lll l. Ill 1| 1. III ll II 111 l 11 200.0 150.0 100.0 GO.O O.O '-~;, ...... ,d,.', ...... ;'=', ...... ;~:,,' ...... ='.~ ...... ~;.,

Fig. 4--(Continued).

well. For example, in the Egersund Basin (Fig. 1), wells 18, increase can be. Its magnitude decreases away from this 29, and 30 show little or no evidence for diminished syn-rift area such that near the western edge of the Egersund Basin subsidence. This observation is true of several other wells: (Figs 1 and 2), the increase in subsidence is absent (e.g. 2, 3, 4, 6, 19, 34, and 35. The stretching values summarized wells 18, 39, 40, and 41). in Table 1 are in good agreement, where there is overlap, Most workers agree that this increase in subsidence with those published previously: Sclater & Christie (1980), cannot be completely accounted for either by an increase in Barton & Wood (1984) and White (1990) all argued that clastic sedimentation rate as Scotland underwent rapid uplift within the deepest parts of the Moray Firth, Viking, and or by a rapid decrease in palaeowater depth (see, e.g. Central Graben fl reaches a maximum of c. 1.6 for the Rochow 1981; Thorne & Watts 1989; Milton et al. 1990). Jurassic stretching event. Knox et al. (1981) emphasize that Palaeocene sedimentation was largely restricted to parts of the older Mesozoic graben zones while the remainder of the North Sea underwent Palaeocene subsidence slight or intermittent subsidence. In other words, the The rapid increase in subsidence that lasted from Late increase in subsidence was not just due to the building out Cretaceous to Early Eocene times has been widely of thick sequences of proximal sedimentary wedges from the documented (Parker 1975; Knox et al. 1981; Mudge & Bliss Scottish Mainland with coeval deposition of distal, deeper 1983; Johnson, 1987). As pointed out above, we have equivalents further out (Parker 1975). interpreted this increase in subsidence rate as a stretching A rapid change in palaeobathymetry at the end of the event in order to ensure that Jurassic-Cretaceous stretching Cretaceous is also an unlikely explanation for the increase in values are minimized. The purpose of this section is to subsidence. Water-loaded subsidence for six wells which review the various published explanations. show large subsidence 'kicks' at about 60 Ma are shown in The increase in subsidence at the end of the Cretaceous Fig. 7. No corrections for palaeobathymetry have been is best seen in the Outer Moray Firth Graben, in the made. In general, the water-loaded subsidence increases by northern part of the Central Graben, and in the southern 0.5-1 km over a period of 10 Ma. Removing this kick would end of the Viking Graben (Nielsen et al. 1986). Wells 2, 3, require a rapid decrease in palaeowater depth of about the 5, 9, 11, 13, 23, and 29 (Fig. 4) illustrate how large this same amount. In order to avoid introducing a similar kick

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0 BP 7/12-3a 4 BP 16/13A+2Z

°°I ...... ' ..... :'i'" ...... '1 1.0 L. , : __] ~ -.+.. -...... ~ ...... " 'z !:. :1 .: I :: !i:: 1.05

...... , .... '.',..,, ...... ";::::::::-,: 1.1 2.0 2"~10.0 150.0 100.0 50.0 o.O 1 Time (Ma)

3.0 +! ',, I , '''''''''1'''''''''. *°i "'1 1.0 ,~:o ...... ~,:o ...... ,'~.'o ...... ,o~:o ...... ".o ...... % ...... -.::: ...... 8 ...... :iiii"";+,,.+.~.. '; ...... i ]~rne(Ma) ...... ,., ,,% ...... 1.05 ~ 2.0 Fig. 6. Subsidence analyses for well 14. Thin dotted vertical lines 1.1 represent water-loaded subsidence determined by assuming Airy isostasy; thick vertical bars represent water-loaded subsidence for 11 IIII,t,,I,*ItjIIII|I,,,I,,IIIIIIIII'''I''I'I''IL 1~so.o 2o0.0 lm.o 100.0 50.0 0.0 no loading correction (i.e. assuming infinitely strong lithosphere;

"13me (Ma) see also Fig. 4).

(b) that it is difficult to discern any evidence for active fault control at this time. 14 e, ~, ~. It is likely that this increase in subsidence is much more 0.0 ,, ,,,,-, ,i , ,, ,, ,,, , I '' ''' '' '' I'' '' ''' '' I' ''''''''- widespread than is documented in the literature. It certainly i occurs as far west as the Porcupine Basin (west of Ireland)

1.0 .... :...... PHILLIPS 15/27-2 8 ...... ,...... + "-.i i ' ri ...... 0"01- ..... ~~~'I ...... I ...... + ..... 4

2.0 1.05 m ...... "+''' '11155!L i 1.1 ++i~ 1.0 '"...... " 1 3011,,,,,,,,i,,1,,,1,, ,,,,,,11,1,,,,,,, ,,I,1,,,,,,, 1.05 "R50.O 200.0 150.0 1(30.0 50.0 u.O

"rime (Ma) 2 ,l ill,Ill I,i Illl,II II I ' • .0 150.0 100.0 L ~ ~ ~ 50.0 0.0

(c) lime (Ma)

Fig. g. Subsidence analyses for three wells having applied .(~ OXY 22/1A-3 palaeobathymetry proposed by Bertram & Milton (1989) (1 km .,T, 9, ~g I l,e, 4~ ,.4.., , i , , , , , , , ,, I , , , , , , , , water depth at beginning of Cretaceous decreasing linearly to ' "" " " ~" "++I, present-day). Maximum and minimum theoretical subsidence curves • are fitted to thermal subsidence gradient. 1.0 °~ 8 .. "'O ,...... +.... within the Cretaceous, water depths of about 1 km would ~ 2.0 1.7 have to be invoked for most of the post-rift period (>120 Ma). 3.~i o...... 150.0, ...... 100.0, ...... 50.0, ...... 0 Knox et al. (1981) and Mudge & Bliss (1983) suggest that a period of widespread uplift and erosion followed ]1me (Ma)

deposition of Danian chalk and marl. Danian sediments 24 SUN 22/8A-2 were removed from all but the deepest parts of graben. 0.0 , , '#;"-.' ' ' I ''~' ..... I ...... - Resedimented chalk deposits, slumping, and debris flows "*" +*4.+o ~...... :~ have been tentatively interpreted as evidence for earthquake 1.0 ~-.+ activity (Morton 1982). While most workers agree that there °• '-+ ...... : was a period of uplift and tilting at the end of the "D 1.8 ~ - Cretaceous (Milton et al. 1990), there is disagreement about ~'1 2.0 how the subsequent period of rapid subsidence can be explained. Parker (1975) and subsequently Rochow (1981),

Morton (1982) and Conort (1986) favour reactivation of 3.?&~o ...... 100.0, ...... 50.0, ...... 0.0 major Mesozoic normal faults, in other words, a small "lime (Ma) degree of stretching. However, as Milton et al. (1990) point Fig. 7. Subsidence analyses for three wells which show pronounced out, the small degree of Palaeocene displacement on basin increase in rate of subsidence during Palaeocene. No palaeowater margin faults could equally have been caused by differential depth correction has been applied but the same theoretical curves as compaction. Boldy & Brealey (1990) go further and argue in Fig. 4 have been used.

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where it appears to be associated with a phase of minor Lyon-Caen & Molnar 1983). In the East Africa Rift Zone, normal faulting and volcanism (Tate & Dobson 1989; White both Bechtel et al. (1987) and Ebinger et al. (1991) have et al. 1992). A similar increase in rate of subsidence is shown using admittance and coherence techniques that re documented by Keen et al. (1987) on the conjugate may reach values of 17-38 km. Newfoundland margin. We agree with Knox et al. (1981) A difference between the elastic behaviour of different that increased Palaeocene subsidence is a general phenome- regions of continental lithosphere is not unexpected. The non that is linked with the tectonic development of the rheological properties of a material are principally North Atlantic: the initiation of the Iceland hotspot at c. 60 controlled by T', the ratio of the temperature of the Ma cannot be coincidental (White 1989). Small amounts of material to that of its melting point, both measured in Late Cretaceous uplift in the western North Sea could be Kelvin (Ashby & Verrall 1977; Weertman 1978). Materials part of the regional uplift which resulted from the can only maintain stresses over geological time if T' is less development of the plume while the subsequent rapid than about 0.4. For oceanic lithosphere this approximately increase in subsidence could ensue as the European plate corresponds to the 450°C isotherm, in agreement with drifted eastwards relative to the plume. Unfortunately, Watts (1978). If the rheology of the upper continental crust simple calculations show that this mechanism cannot corresponds to that of wet granite, then T' =0.4 generate an increase in subsidence which is sufficiently large corresponds to a temperature of about 100 °C. Although this and rapid (B. Hall, pers. comm. 1992). estimate is not very accurate, it does suggest that the Cathodoluminescence microscopy and fuid inclusion strength of the continental lithosphere is generally small. studies of sandstones in the Witchground Graben (Burley et Significant elastic thicknesses will only occur when the al. 1989) suggest that hot fluids passed through during the temperature gradient is small, when the crust is thick, and Late Cretaceous and Early Tertiary, and Glasmann et al. when loads are imposed very rapidly (as in areas of (1989) argue that illite diagenesis in the northern Sea compression). occurred very rapidly, affecting a thick sequence of sediment In the North Sea, the significance of small values of re is between 66 and 38 Ma. Quartz fluid-inclusion studies in the still debated. Marsden et al. (1990), Kusznir et al. (1991), same area also document the presence of unusually warm and Roberts & Yielding (1991) maintain that elastic saline pore fluids as diagenesis proceeded. thicknesses of 5 km or less have a significant effect on the Much work remains to be done in determining the extent calculated value of ft. They attempt to predict the and magnitude of Palaeogene subsidence on both sides of subsidence history of the northern North Sea using a model the Atlantic. Elemental and isotopic analysis and modelling which incorporates the rheological, thermal, and flexural of the small amounts of melt, which occur within Mesozoic isostatic consequences of lithospheric stretching. Results basins around Britain and Ireland at the same time, will suggest that the Jurassic stretching event is very small (fl probably play an important role in discriminating between reaching a maximum of 1.24 in the deeper parts of the the stretching and thermal uplift hypotheses. Viking Graben). In order to account for the observed crustal thinning on deep refection and refraction profiles The importance of iithospheric strength (Klemperer 1988; Holliger & Klemperer, 1989), the proposed values of Jurassic stretching require Permo- There has been considerable argument over the last 10 years Triassic stretching to be significantly greater than has concerning the flexural rigidity of continental lithosphere previously been estimated (e.g. Barton & Wood 1984). and whether or not flexural rigidity should be allowed for These flexural studies also imply that simple one- when analysing subsidence data. Flexural rigidity, D, is dimensional backstripping, which assumes Airy isostasy, given by considerably overestimates the degree of Jurassic stretching. For the purposes of this paper, it is important that fl D - 12(1 - a 2) (1) values for the Jurassic event are minima rather than maxima: if high values of fl (c. 2) are required (in the where E is Young's Modulus, o is Poisson's Ratio, and z% is absence of a convective plume) to account for the existence the elastic thickness of the lithosphere, z'e is often used in of Jurassic volcanism then it is better to err on the low side. preference to D as an indicator of lithospheric strength. The easiest way to check how our results will be affected by Studies of seamounts and ocean islands show that ~'e of a finite 1% is to compare two extremes of lithospheric oceanic lithosphere depends on its age at the time of loading strength: zero re (i.e. Airy isostasy applies) and large Z'e. (Watts 1978). The elastic thickness of 100 Ma oceanic crust The strength of the lithosphere affects subsidence is 30-40 km. Subsequently, Watts et al. (1982) argued that analysis in two important ways. The first concerns the stretched continental lithosphere should respond in a similar backstripping process where a heterogeneous sediment load fasion: as it cools down after stretching, the continental is converted into a uniform water (or air) load for each lithosphere should become increasingly rigid in its reponse increment of subsidence. When Airy isostasy is assumed, to sediment loading. However, studies of free-air gravity the lithosphere is compensated locally and a substantial anomalies and load topography using admittance techniques amount of sediment-loaded subsidence is due to loading as a by Barton & Wood (1984) in the North Sea, and by Fowler result of the difference in mass between water and sediment & McKenzie (1989) on passive continental margins columns (Steckler & Watts 1978). In contrast, if the suggested that the continental lithosphere has a 1% of less lithosphere is strong then any load regardless of its size or than 5 km. Such values are significantly smaller than those mass and regardless of how rapidly it is imposed can be of coeval oceanic lithosphere where ~e is approximately the supported by lithospheric strength. Hence, the difference depth of the 450°C isotherm. Studies of other regions between sediment- and water-loaded subsidence would be suggest that parts of the continental lithosphere may have much smaller than in the Airy case. much larger elastic thickness (Ahern& Mrkvicka 1984; Figure 6 shows the difference between calculated

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100 km (b)

T~ = 5 k~~ %=0 km~ 7 Fig. 8. Synthetic two-dimensional sedi- mentary basins. In each case heatflow is assumed to be vertical only and basin infill has density of zero. Rifting period is 180-120 Ma, maximum fl = 2, isochrons drawn every 20 Ma. Young's Modulus = 70 MPa, Poisson's ratio = (c) (d) 0.25. Wide basin (times 40 vertical exaggeration) shown for (a) Te = 0 km and (b) re = 5 km; Narrow basin (times %=Okm %--5km 20 vertical exaggeration) shown for (c) lr~ = 0 km and (d) re = 5 km.

water-loaded subsidence for re = 0 km and for an infinitely model is assessed qualitatively. We are wary of using re in large value, assuming finite load shape. The most important this way and prefer to make either maximum or minimum difference is that when the lithosphere is strong, the estimates of the principal variable, ft. Fortunately, because water-loaded subsidence has a steeper gradient than when we choose to fit the gradient of the thermal subsidence Airy isostasy is applied. In reality, some degree of load which contains little short wavelength information of compensation will occur and the water-loaded subsidence sufficient amplitude, the difference between Airy isostasy curve will lie somewhere between these two end-members. and re = 5 km is negligible. It is important to note that regardless of the value of re and If the above arguments concerning the effect that fexure whether it decreases or increases as a function of time, the has on both the backstripping process and on the shape of resultant subsident gradient will always be steeper than for theoretical subsidence curves are correct, then why does the Airy isostasy case. Wood (1982) showed that if re was application of the flexural cantilever model of Kusznir et al. 5 km or less, the true water-loaded subsidence would lie (1991) and Marsden et al. (1990) imply that the Jurassic close to the Airy isostasy case, for load wavelengths of stretching values in the northern North Sea are smaller than greater than 20 km. For that reason, Barton & Wood (1984) if Airy isostasy is assumed? There are probably three and subsequent workers ignored the affects of flexural reasons for this contradiction. Firstly, Marsden et al. are rigidity. Since we determine fl from the gradient of the principally concerned with matching the total subsidence post-rift, thermal subsidence, these arguments alone suggest rather than the thermal subsidence gradient. Secondly, the that if re is finite the fl values given in Table 1 are minima. flexural cantilever model initially relies on identifying the Consideration of the effect of small wavelength variation in correct amount of stretching from observed normal faulting. loading shows that by ignoring re, fl will be underestimated The considerable difficulties in measuring the amount of if a well is situated within a small-wavelength depression and stretching taken up by normal faulting with any degree of overestimated if the well is located on a high. accuracy and the likelihood that any such estimate is The strength of the lithosphere also affects the shape of considerably less than the true extension has been pointed theoretical subsidence curves. For finite loads, thermal out many times (e.g. Sclater & Christie 1980; Le Pichon & subsidence approaches zero, while initial subsidence Sibuet 1981; Jackson & White 1989). Thirdly, undue increases, as larger values of flexural rigidity are used. In emphasis on an arbitrarily chosen re has deflected attention other words, the gradient of the thermal subsidence will from more important sources of error, namely uncertainty in always be smaller for a given value of fl even though the the values of parameters such as thicknesses and densities of total subsidence will generally be greater than if Airy the crust and lithospheric mantle, the temperature of the isostasy applied. This result also implies that if re is asthenosphere, and strain rate. significant, our chosen values of fl are minima since the observed thermal subsidence gradient can only be fitted by increasing ft. When addressing the difference between Airy Discussion isostasy and small but finite re, the spectral content of the To date, few subsidence analyses have been published for basin shape is obviously important, as shown in Fig. 8. For the triple junction area. Wood (1982) and Barton & Wood load wavelengths of >20km, there is no significant (1984) obtained small values for some of the wells in this difference between re = 0 km and re = 5 km (compare Fig. area because they attempted to fit both the syn-rift and the 8a and b). For very large amplitude (c. 3 km) but short post-rift subsidence data with theoretical curves based on wavelength (<20 km) loads, the difference is large (compare uniform stretching. As a result, the theoretical curves Fig. 8c and d). The critical wavelength and amplitude significantly failed to match the steep gradient of post-rift decrease rapidly as re approaches zero, a situation which subsidence. However, Sclater & Christie (1980) only forces us to pose a more important question: what value of attempted to fit the thermal part of the subsidence data and, re should be used: 5, 0.5, or 0.05 km? The flexural cantilever like us, obtained large values for fl (up to 2.0). model itself places no independent physical constraints on Why should we expect large values of fl in this area? the value of re (Kusznir et al. 1991). Instead, re is treated as Simple geometrical arguments suggest that if the stretching an 'engineering-type' variable whose effect on the final factor in each of the three arms of a rift system is equal to fl

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o rifling. We feel that the observations presented above are inconsistent with the plume model for rift formation for the -o °° following reasons (see also Eynon 1981; Leeder 1983). (1) If the lithosphere had been thinned by a factor of c. 2.0 over a convecting plume some 100-200°C hotter than the ambient asthenosphere (McKenzie & Bickle 1988) then an average thickness of at least 3-10 km of basalt may be anticipated (White & McKenzie 1980). The observed o °°° thickness and inferred volume of melt (c. i km and c. oo 104 km 3) are more than one order of magnitude less than that observed, in, for example, the Kenya Rift (Latin et al. 1992). (2) The lack of evidence for a plume trail (e.g. migrating ,,.~4 o o~jO o unconformities before and after Mid-Jurassic times; Latin 1990), suggests that whatever doming there was appears to have been short-lived and relatively localized. Hence, the plume model appears to require special pleading. It is, however, worth pointing out that a small (low temperature and lateral extent) transient thermal anomaly would not be inconsistent with observation (e.g. Ziegler 1982; Underhill & Partington 1993). We are fully aware that it is still extremely difficult to distinguish between local erosional effects and regional O- wells with nO volcenlcl ~ '~) '~t/~ doming. Although some of the wells used in this preliminary l- wells containing volcenlcs study were selected so as to minimize local fault-controlled effects, more work is needed before the notion of a transient Fig. 9. Very simplified structural map of central North Sea (see thermal anomaly can be taken more seriously. Figure 1). Thin solid lines represent outline of three graben; thick numbered lines represent contoured stretching factors using smaller of the two values given for each well in Table 1; open circles are Conclusions and further work wells which reached Triassic sediments without encountering The Jurassic-Cretaceous stretching factor in the triple- igneous rocks; solid circles are wells in which Jurassic igneous rocks junction region of the North Sea appears to have been as were cored. high as 2.0. These values are consistent with current theory and observations regarding melt generation (Latin & Waters 1991) and are predicted by geometrical considerations then the stretching factor within this central zone equals f12 (Appendix). Although Permo-Triassic extension undoub- (Appendix). In the North Sea, the maximum stretching tedly occurred, we argue on the basis of the known factor for the Jurassic-Cretaceous event in each rift arm, at thickness and complete lack of magmatic activity during the a considerable distance from the triple-junction, is c. 1.5 Triassic that the amount of stretching was small within the (e.g. Sclater & Christie 1980; Barton & Wood 1984; central and northern North Sea (fl < 1.2, north of 57 °N) Marsden et al. 1990; White 1990). Hence we might expect fl when compared to the Jurassic-Cretaceous event. There is in the triple-junction to be c. 2.25 (Appendix). This some evidence for doming in the area of the triple-junction prediction appears to be supported by our observations (Fig. (C. 10 4 km 2) during the Jurassic-Cretaceous rifting episode, 6; Table 1). but this appears to have been a relatively short-lived and Figure 9 summarizes our results. The largest degree of localized phenomenon, inconsistent with the presence of a stretching (/3 -~ 2.0) occurs in Quadrants 21 and 22, covering large, long-lived, mantle plume. roughly 1.2 x 103 km 2. To the north and to the south in the We realise that there are many errors associated with the deeper parts of the Central and Viking Graben, fl is application of the stretching model and would go no further generally 1.5 although locally it may be greater. Stretching than to say that our results are generally consistent with the factors in the Buchan and Witchground Graben of the data in the North Sea. A detailed three-dimensional analysis Moray Firth are less than 1.5. Figure 9 also shows the of the triple junction region is really required. What we are location of (1) wells which reached Triassic sediments suggesting is not new, Leeder (1983) came to similar without encountering igneous rocks and (2) wells in which conclusions in a very perspicacious paper. Clearly, a more Jurassic volcanic rocks were cored. It appears that the sophisticated model will have to be constructed to account location of the Forties Volcanic Province occurs slightly to for the pre-rift uplift and relative paucity of syn-rift the north of the region which has stretched the most. The subsidence observed within the triple-junction area. As signficance of this offset is unclear: perhaps it reflects the many other workers have shown, a differential stretching affect of local structural control on the emplacement of the model where the lithospheric mantle is initially stretched by igneous rocks. a slightly greater amount than the crust could produce the Whiteman et al. (1975) were the first to suggest that observed uplift. To avoid space problems, it would be North Sea triple-junction was generated by a mantle plume necessary for the time- and space-integrated distribution of (Burke & Dewey 1973). Subsequently, Ziegler (1982) strain within the crust and mantle to be the same. modified this idea suggesting that the doming was caused by Alternatively, a transient thermal anomally could be the emplacement of a large asthenolith immediately prior to invoked. Although such models are ad hoc from an

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observational point of view, some constraints may come 60 ° (see Fig. All. Assume that each arm stretches by a factor /3. from numerical experiments (e.g. Houseman & England Now consider the volume of lithosphere within the 'toblerone' 1986). shaped region before and after stretching:

We would like to thank P. A. Ziegler for organizing an entertaining volume before stretching = 1 x 1 × ~- x a and thought-provoking conference in Glion-sur-Montreux and for v5 the enthusiastic help he has given us over the last seven years. L. volume after stretching = 1 ×/3 x fl9-2- × a/o: Wakefield and M. Giles (Shell); and R. Johns, A. Fernie, and W. Beauchamp (Sun Oil) gave us considerable help. Well data were where a is the initial thickness of the lithosphere and cr is the generously provided by Amerada-Hess, Amoco, British Petroleum, amount that the lithosphere has stretched within the triple junction Burmah, Conoco, Esso, Occidental, Phillips, Ranger, Shell, Sun area. Oil, Superior, and Total. C. A. Calfe, S. Capon, N. J. Kusznir, A. Volume is conserved, so M. Roberts, T. O. Spring, J. Turner, and J. R. Underhill helped in various ways. P. A. Allen, N. J. Kusznir, and R. A. Scrutton a,=fl 2. provided very helpful reviews. Part of this work was funded by Shell Expro. D. 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-- & McKENZIE, D. P. 1989. Magmatism at rift zones: The generation of

Receivcd 14 August 1991; revised typescript accepted 16 October 1992.

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