The movement history and fault rock evolution of a reactivated crustal-scale strike-slip fault: the Walls Boundary Fault Zone, Shetland

The Walls Boundary Fault Zone is a crustal-scale strike-slip fault that cuts Precambrian–Caledonian basement terranes in Shetland and has been interpreted as the northern continuation of the Great Glen Fault Zone in Scotland. This paper presents the first detailed account of the kinematic history and fault rock assemblages associated with the onshore evolution of the Walls Boundary Fault Zone. These observations suggest that it initiated as a major late Caledonian (Silurian–Devonian) sinistral strike-slip fault associated with the successive development of mylonites and cataclasites. These fault rocks are preserved only locally, and elsewhere are obscured by the effects of later brittle overprinting and dismemberment of the fault zone during dextral strike-slip reactivation, probably during late Carboniferous inversion of the Orcadian Basin. This led to the development of extensive cataclasite and fault gouge assemblages, which are widely preserved along the Walls Boundary Fault. Narrow zones of post-Triassic dip-slip, and finally sinistral strike-slip, reactivation are localized within earlier-formed gouge-filled fault cores. There are some similarities to the kinematic history of the Great Glen Fault Zone, most notably the recognition of late Caledonian sinistral shear and post-Devonian dextral reactivation, but the post-Triassic reactivation histories appear to differ significantly.