Synopsis

Current geological maps of the Ardnamurchan Central Complex show the Fascadale Fault as a short (1.2 km) NW-trending tectonic junction between a sliver of the superstructure of the former Cenozoic volcano and the western margin of its outermost quartz-gabbroic intrusion. To the south, the mapped fault appears to die out in an area of coalescing basic igneous intrusions, whereas to the north, it is buried by alluvium before passing offshore into Fascadale Bay. Detailed bathymetric data obtained from a marine multibeam echo-sounder survey north of the Ardnamurchan peninsula reveal that the onshore fault is in structural continuity with a similarly-trending linear bathymetric feature, which extends almost to the island of Rum. Sea-bed geological structures offset by this major lineament suggest that the segment of the Fascadale Fault mapped onshore marks the southern end of a Cenozoic dextral strike-slip fault (>22 km long), which extends between Ardnamurchan and Rum. Based on these observations, a tectonic model is outlined that suggests that the sites of volcanic centres in the British part of the North Atlantic Igneous Province are controlled by the interaction of NW-trending structures, like the Fascadale Fault, with NNE-trending (Caledonoid) structures in a Cenozoic strike-slip regime.

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