The thermal impact of Paleocene magmatic underplating in the Faeroe-Shetland-Rockall region

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Voluminous magmatism on the northwest European shelf during Palaeogene continental breakup is believed to result in crustal thickening by gabbroic accretion to the base of the crust totalling 4-5 km west of Scotland, 5-8 km around the Faeroe Islands, but only 1-2 km beneath much of the Faeroe-Shetland Basin. Simple models of heat conduction predict high heat-flow (>120mWm -2) following underplating of 5 km of gabbro. This is, however, not substantiated by thermal modelling derived from vitrinite reflectance and apatite fission track data from wells in the Rockall Trough, which show peak heat-flow of <70mWm-2 in the central Rockall Trough and 55-78 mWm-2 in the Faeroe-Shetland Basin. Present day values are 45 mWm-2 and 55-63 mWm-2, respectively. The additional heat-flow is about 60% of that predicted by conduction models assuming instantaneous underplating of a single body. Underplating appears to have taken place progressively over 11 Ma (51-62 Ma) in the form of small plutons, thus reducing the thermal impact compared to an instantaneous event. Nonetheless, variability in underplating is the dominant influence on regional heat-flow and may have had a major impact on source-rock maturation. The oil generating window is predicted to have shallowed by 1.4 km in the northern Rockall Trough in the early Eocene. Present day heat-flow readings indicate that the background heat-flow against which the underplating peak is superimposed increases from 45 mWm-2 in the southern Rockall region, to 50mWm-2 in the northern Rockall, and reaches 55-60mWm-2 in the Faeroe-Shetland Basin. This pattern must reflect changes in the radiogenic content of the underlying crystalline basement and/or its thickness, as most of the underplating-derived heat has now dissipated. © Petroleum Geology '86 Ltd. Published by the Geological Society, London.

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Petroleum Geology Conference Proceedings

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