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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/3/473/4892332/gsjgs.150.3.0473.pdf by guest on 28 September 2021 474 N. WHITE & D. LATIN 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.