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JOHNSON, HOWARD, DEREK RITCHIE, ROBERT GATLIFF, JOANNE CAVILL, JOE BULAT, AND PAUL WILLIAMSON, British Geological Survey, Murchison House,West Mains Road, Edinburgh, United Kingdom, EH9 3LA

Abstract: Aspects of the structure and evolution of the frontier deep water Porcupine Seabight Basin

The Porcupine Seabight Basin (PSB) includes the Main Porcupine Basin (MPB) and the Seabight Basin (SB) to the south (Fig. 1).The deep crustal structure of the PSB has been the focus of a limited number of deep seismic experiments (e.g.,Whitmarsh et al., 1974; Croker and Klemperer 1989; Makris et al., 1988). Results from the COOLE refraction / wide-angle reflection profiles (Makris et al. 1988) indicate that the crust is 25-28 km thick around the eastern flank of the MPB, and thins gradually to the southwest. Potential field modelling by Conroy and Brock (1989), on a line that bridges the gap between the COOLE profiles, suggests that the continental crystalline crust thins to around only 8 km or so within the centre of the SB; i.e., beta = 3.5. A major discrepancy between large stretching factors (beta >6), calculated from amounts of post-rift subsidence within the PSB, and the much smaller values derived from the variation in crystalline crustal thickness determined by the COOLE profiles has been attributed to the masking effects of underplating (Tate et al. 1993).

Two east-trending, basinwide transects have been constructed from commercial seismic and satellite gravity data. Transect 1 crosses the MPB, and transect 2 crosses the SB.The transects indicate a crustal stretching factor of approximately four. The notable feature of the observed gravity data along transect 1 is the large free air anomaly (+55 mGal) over the centre of the basin. In contrast, transect 2 displays a free-air gravity anomaly profile characterized by edge anomalies flanking an essentially zero anomaly over basin, suggesting that the SB is isostatically compensated. The difference in the degree of isostatic compensation displayed by the two transects is believed to reflect different stretching and subsidence histories in the two sub-basins. Speculatively, this may be associated with different basement provinces either side of the Clare Lineament.

The tectono-stratigraphic development of the PSB has been the subject of considerable debate (e.g., Croker and Shannon 1987; Croker and Klemperer 1989; Shannon 1991;Tate 1993). The fill of the PSB can be divided into pre-rift (?Carboniferous to ?mid Jurassic), syn-rift (?mid to Upper Jurassic) and post-rift (Lower Cretaceous and Cenozoic) seismic packages, that are broadly characterized by tilt-block, wedge-shaped and onlapping geometries. A high amplitude seismic reflector marking the top of acoustic basement has been modelled on the transects as top crystalline basement. On the basin flanks, the basement reflector is commonly poorly developed, but in the central parts of the MPB it is characterized by a strong and up-bowed event that delineates a prominent intra-graben high.

Pre-rift successions are more thickly developed in the north PSB, where sections are up to about 6 km thick. In the south PSB, pre-rift rocks are around 3 to 4 km thick. Syn-rift successions reach up to about 4 km thick on the flanks of the PSB. In the MPB, syn-rift rocks appear to be absent from a central graben high. The post-rift succession is thickest in the centre of the south SB, where it reaches about 8 km. On transect 1, the post-rift succession comprises around 3 km of Cenozoic and 3 km of Cretaceous rocks. On transect 2, however, the Cenozoic strata are only up to about 2 km thick and the Cretaceous succession is expanded to 6 km or so.

The pre-rift succession displays a complex seismic character and includes a local package of continuous and high amplitude reflectors, a number of significant unconformities, and structures that may be due to halokinetic effects.

The syn-rift succession can be separated into two sub-divisions at a prominent intra-Kimmeridgian downlap surface. The lower syn-rift sub-division commonly rests on pre-rift strata that apparently display 'normal flexural drag' in the hangingwall blocks of major normal faults. However, at least some of this configuration may be due to the effects of halokinesis. Commonly, the lower syn-rift succession is severely truncated in the footwall blocks of major normal faults; this may be due to significant uplift and erosion during the second phase of late Jurassic rifting.

The post-rift succession displays a saucer-shaped geometry, with seismic onlap over both the syn-rift and pre-rift successions. The Porcupine Median Volcanic Ridge (PMVR) forms a NNW trending, deeply buried mound, approximately 150 km long, 25 km wide and 2.5 km high within the post-rift succession (Tate 1993).The Ridge is onlapped and buried by Lower Cretaceous sediments and its well-preserved morphology suggests that it may be comprised of both lavas and volcaniclastics. Strongly reflective and parallel layered Lower Cretaceous sediments within the basal part of the post-rift succession appear to interdigitate with volcanic rocks of the PMVR that display a more chaotic seismic facies. Though the age of the Ridge is poorly constrained, the seismic evidence suggests that it post dates the Late Jurassic main phase of lithospheric stretching.

Figure 1. Location map of Porcupine Seabright Basin, inclusive of the Main Porcupine Basin (MPB) and Seabright Basin (SB), and seismic transects.

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