Permo-Carboniferous mylonite formation in the Walls Boundary Fault system, Shetland

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Abstract
SUMMARY: The Walls Boundary Fault system of Shetland is a major zone of transcurrent faulting which has been active on more than one occasion. It may include a northward extension of the Great Glen Fault. Evidence suggests that the latest major episode of movement, which resulted in the formation of a gouge filled fracture, is of Lower Jurassic age. Earlier movements suggested by lithological offsets across the fault are supported by the presence of bands and slices of mylonite within the fault zone. A mineralogical investigation of the mylonites shows that growth and recrystallisation of K-feldspar and white mica occurred during mylonitisation, and is responsible for partial overprinting of the potassium-argon age of the host rock. 40 Ar/ 39 Ar stepheating spectra from two samples suggest that the phase of movement associated with the development of the observed ductile mylonites occurred during the latest Carboniferous or early Permian, although earlier movements are not ruled out.