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Basin Analysis |
1 School of Construction and Earth Sciences, Oxford Brookes University, Gipsy Lane, Oxford OX3 0BP, UK
2 British Geological Survey, Nicker Hill, Keyworth, Nottinghamshire NG12 5GG, UK
Gravity, aeromagnetic, seismic, borehole and outcrop data from the Isle of Man area have been used to study the structural framework and tectonic evolution of the central part of the Irish Sea since the Ordovician. The area has been affected by two major compressional events (Caledonian and Variscan), three rifting events (early Carboniferous, early Permian and ?late Jurassic) and a period of thermal uplift (early Tertiary). A large part of the Isle of Man consists of early Ordovician turbidites (‘basement’) interpreted to have been deposited in a back arc or fore arc basin that was inverted during the Caledonian orogeny. Major NE-SW and subsidiary NW-SE strike-slip faults were initiated at this time, some of which were later reactivated to form important basin controlling faults in the Irish Sea. Post-Caledonian basin subsidence was initiated in the early Carboniferous with a period of NW-SE directed extension followed by thermal sag which ceased at the end of the Carboniferous as a result of Variscan compression. E-W extension in the early Permian led to the development of new extensional faults which have influenced the thickness of the Collyhurst Sandstone in the offshore. Post-rift thermal sag continued throughout the late Permian and the Triassic. A late Jurassic period of extension has been proposed in order to explain post-Triassic offset across major normal faults east of the Isle of Man although direct evidence of syn-rift or post-rift strata has been removed by erosion during uplift in the early Tertiary. This uplift was due to thermal doming of the crust, an event which was associated with the intrusion of a large number of dolerite dykes. The structural evolution of the Isle of Man has also been influenced by a major NW-SE trending basement fracture known as the ‘Central Valley Lineament’ which extends into the offshore to the SE and NW of the island. The lineament appears to be important in defining areas of different hydrocarbon prospectivity in the Irish Sea.
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