Crustal extension, subsidence and inversion in NW Europe

Abstract

Examination of a regional grid of deep and commercial seismic data extending from Cardigan Bay (offshore Wales) to the south of the Paris Basin (Aquitaine–Provence–French Alps) has demonstrated that the deep Mesozoic sedimentary basins along this transect are underlain by low-angle crustal detachments. Such features are interpreted to be Variscan thrusts, reactivated as normal faults during Mesozoic crustal extension; they rise to surface in basement rocks surrounding Mesozoic basins, and can be matched to outcrop structures in S. Wales, S. Ireland, Cornwall, Devon and Brittany. Inversion of the NW European Mesozoic basins occurred in the Cenozoic, but was preceded by minor inversion and uplift of surrounding massifs at various times in the Mesozoic. Such uplift can be demonstrated to have occurred via crustal shortening and re-thrusting along Variscan thrusts at depth. The uplift of basins above such thrusts is concomitant with continued subsidence of basins in the foreland, and can explain the juxtaposition of over-thick sedimentary successions against basins with a predominant unconformity of similar age. In the Celtic Sea area (offshore SE Eire), the North Celtic Sea Basin contains over 2000 m of late Jurassic–early Cretaceous sediments, whilst the South Celtic Sea Basin has virtually no late Jurassic or early Cretaceous preserved: a prominent Aptian–Albian unconformity overlies early–mid-Jurassic sediments. The two basins are separated by the Pembroke Ridge, an ancient ‘high’ where reactivated Variscan thrusts are thought to crop out (Coward and Trudgill 1989). The Carboniferous–Permian–Triassic succession of the Plymouth Bay Basin (Western Approaches Trough) is another example of an ‘overthickened’ sedimentary succession . . .