Abstract

The gravity and total magnetic field over Jersey, Guernsey, Herm and Jethou, and the gravity field over Alderney and Sark, have been surveyed in detail at 470 gravity stations and 847 magnetic locations as the basis of Bouguer and magnetic anomaly maps, which can be integrated with the marine gravity and aeromagnetic maps of the region. There is a striking correlation in location and relative amplitude of gravity and magnetic anomalies on the islands. The St Peter Port Gabbro of Guernsey can be modelled as a laccolith no more than ∼0.8 km thick and extending several km NE subsurface beyond its present outcrop limit. The aeromagnetic and gravity high over SE Jersey can be modelled as a circular laccolithic lens c. 12 km in diameter and c. 2 km thick with about 1 km of cover; thus the dioritic and gabbroic outcrop in that area does not reflect a great volume of such material at <l km depth. Smaller gravity and magnetic highs are centred over Alderney and Sark and may be due to similar but smaller buried intrusions, and other small anomalies near Le Doit in SW Guernsey and in the sea ∼10km S of Guernsey may also have the same origin. There are positive magnetic anomalies E of Sark and N of Guernsey which have zero or negative gravity expression. The granites of western Jersey have low gravity and magnetic expression; modelling indicates that the NW and SE granites are linked at shallow depths and also underlie central and much of eastern Jersey; the existence of negative magnetic anomalies to the NE and S of Jersey suggest that the granites may also extend offshore in those directions subsurface. The Pentevrian outcrop area of Guernsey is characterized by intermediate values of the Bouguer anomaly and positive magnetic anomaly fields, and the geophysical maps suggest that Guernsey lies at the N end of a promontory of Pentevrian outcrop beneath shallow sedimentary cover.

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