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

Abstract

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.

U–Pb zircon geochronology and geodynamic significance of ‘Newer Granite’ plutons in Shetland, northernmost Scottish Caledonides

“…2). Flinn (1977) proposed sinistral strike-slip displacement of at least 200 km, but if it is the continuation of the Great Glen Fault, as commonly supposed (Fig 1; Flinn 1961Flinn , 1977Flinn , 1993Watts et al 2007), displacements may be at least 700 km (Dewey & Strachan 2003).…”

“…Given that formation of the Walls Formation basin and its subsequent deformation are thought to have been related to sinistral displacement along the Walls Boundary Fault (Seranne 1992;Dewey & Strachan 2003), it is tempting to suggest that the fault acted as a steep conduit that facilitated emplacement of the Sandsting Complex. The pluton is an important marker in the geological history of the Walls Boundary Fault as it contains xenoliths of blastomylonites formed during early sinistral displacements, but itself is only affected by cataclasites and gouges associated with dextral, post-Devonian reactivation of the fault (Watts et al 2007).…”

Basement-influenced rifting and basin development: a reappraisal of post-Caledonian faulting patterns from the North Coast Transfer Zone, Scotland

“…Both fault systems consistently cross-cut Caledonian thrusts and associated fabrics (Holdsworth et al 2007). All exposed fault zones are characterized by red-orange haematite staining, brecciation, the localized development of incohesive gouge along larger slip surfaces and calcite mineralization (cements, veining) (Fig.…”

“…It has been proposed that the basin formed initially due to post-orogenic collapse of the Caledonian mountain belt during the Devonian (McClay et al 1986;Norton et al 1987;Coward et al 1989;Seranne 1992). More recently, others have related Devonian extension in northern Scotland to regional sinistral transtension associated with left-lateral displacements along the Great Glen-Walls Boundary Fault system (Dewey & Strachan 2003;Watts et al 2007). A number of studies have cast doubt on basement reactivation models in the West Orkney Basin and along the Scottish north coast (e.g.…”

Extensional tectonics in the North Atlantic Caledonides: a regional view

“…Middle(?) Devonian inversion along these basement-rooted faults has been linked to sinistral strike-slip fault movements (Coward et al 1989;Watts et al 2007), possibly related to Devonian sinistral slip on the Highland Boundary Fault to the south.…”