GRAVITY-ANOMALIES AND MAGMATISM ALONG THE WESTERN CONTINENTAL-MARGIN OF THE BRITISH-ISLES

Seismic data show that the western margin of the British Isles in the region of the Hatton Bank comprises a thin wedge of sediments, an extr usive volcanic sequence, stretched continental crust, and a thick high -velocity lower-crustal body. The sediments represent a load on the su rface of the stretched crust, which should subside under their weight. The extrusive volcanic rocks defined by a seaward-dipping reflector s equence, were emplaced in the crust during the later stages of rifting . The lower-crustal body has been interpreted as underplated material which re-thickens the crust and causes uplift. Backstripping technique s have been used to evaluate the contribution of sediment loading and underplating to the observed crust and mantle structure and isolate th e initial rift configuration of the margin. The contribution of these processes depends, however, on the elastic thickness, T-e, of stretche d lithosphere. We have constrained I, by computing the combined gravit y anomaly of rifting, sedimentation and underplating and comparing it to the observed free-air gravity anomaly. Underplating gives rise to a distinct pattern of gravity anomaly 'highs' and flanking 'lows': the highs reflect the relatively dense uplifted crust and the lows less de nse underplated material. The best fit between observed and calculated anomalies is for a model in which the T-e of stretched lithosphere is low (<5 km) and the width of the initial rift is narrow (<75 km). Sin ce T-e is low, isostatic anomalies at the margin would be expected to be of small-amplitude and this is indeed the case al the Hatton Bank m argin. Sensitivity studies show that large-amplitude isostatic anomali es would be expected, however, if T-e is high-with negative anomalies of up to 30 mGal over the underplated region and positive anomalies of up to 10 mGal in flanking regions. Such anomaly patterns are seen wes t of Ireland and Scotland suggesting that underplating may be a widesp read feature of the British Isles margin.