Synopsis

The Bouguer anomaly gravity map of the islands of Unst and Fetlar in northeast Shetland shows a high amplitude positive gravity anomaly overlying the metagabbro outcrop (average density 3.0 Mg/m3) of the Shetland ophiolite in Unst. The aeromagnetic map of the same area shows a high amplitude positive magnetic anomaly overlying the serpentinised ultramafic layers (average density of 2.62 Mg/m3) of the ophiolite lying west of the metagabbro. Geologically-predicted gravity-adjusted profiles across the ophiolite confirm the geological interpretation of the ophiolite as a nappe of ultramafic and basic vertical layers overlying the schist-gneiss basement. The base of the nappe reaches a maximum depth of about 3 km (assuming an average density for the basement of 2.75 Mg/m3). The magnetic properties of the serpentinites are too variable to be used in modelling. However, the aeromagnetic profiles along the gravity traverses are closely matched by calculated magnetic profiles based on selected notional susceptibilities.

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