GEOPHYSICAL MODELLING OF MAGNETIC ANOMALIES OVER THE LIZARD COMPLEX OPHIOLITE, SW ENGLAND

Abstract

The Lizard ophiolite of SW England is a key geological terrane in the United Kingdom that represents a fragment of oceanic lithosphere emplaced northwards upon continental crust by thrust faulting during the evolution of the Variscan orogeny. The ophiolite consists of a sequence of serpentinized peridotites and overlying oceanic crustal rocks (mainly gabbros and dolerite dykes) that are well-exposed around the coastline of the Lizard peninsula but are poorly exposed inland. Aeromagnetic surveying (most recently during the Tellus SW campaign in 2013) shows a band of major magnetic anomalies that trends in a WNW-ESE direction over the peridotite outcropping at Coverack. However, in detail these anomalies do not correspond directly to the mapped outcrop patterns in the ophiolite. Hence, this project aims to improve our understanding of the structure and subsurface geometry of ophiolite via 2D and 3D magnetic modelling of the source of these anomalies. 3D modelling of the Tellus SW aeromagnetic anomaly data shows that the main anomaly over the ophiolite is caused by a wedge-shaped region of high magnetic susceptibility that extends down to depths of c. 1.5 km along the eastern coast of the ophiolite. The thickness and lateral N-S extent of this body decreases to the west. Further to the WNW, the modelled region of anomalously high susceptibilities is offset to the north and becomes shallower and narrower. This thinner unit appears to correlate well with the mapped outcrop of the Traboe Cumulate Complex, that consists of metamorphosed mafic/ultramafic lower crustal rocks.

Magnetic profiles extracted from the Tellus SW database, extending ~9 km SSW from the northern margins of the main anomaly to beyond Goonhilly Downs, also cross additional thinner and laterally discontinuous aeromagnetic anomalies that lie within a major western mapped unit of peridotites. 2D forward modelling of these profiles suggests that they are best explained by a series of bodies with contrasting magnetic susceptibilities that dip broadly to the NNE and are terminated to the N by S-dipping faulted contacts. The main anomaly is attributed to a thrust slice of high susceptibility rocks of the Traboe Cumulate Complex. This overlies a series of NNE-dipping sheet-like zones of lower and more variable susceptibilities within the underlying peridotites. These could represent different thrust slices, but are perhaps more simply interpreted as zones of variable degrees of serpentinization within the peridotites, with the extent of alteration controlled by fluid pathways that parallel the major thrust structures. The more discontinuous WNW-ESE-trending anomalies over the western peridotite are shown to in part spatially correspond to mapped outcrops of the Kennack Gneiss, a banded felsicmafic intrusive complex intruded into the peridotites along thrust-related contacts during intraoceanic thrusting. The 2D models suggest that these rocks form discrete lens/wedgeshaped bodies of high magnetic susceptibility that again dip to the NNW. Finally, modelling of a ~9 km long profile running NNE from offshore through the coastal section at Coverack and beyond suggests that the high susceptibility serpentinized peridotites exposed at Coverack extend to depths of at least 2.5 km. They are inferred to be underlain to the SSW by peridotites with lower susceptibilities that appear magnetically similar to the western peridotites traversed by the other modelled profiles.