Abstract
The Devonian Orcadian Basin in northern Scotland belongs to a regionally linked system of post-Caledonian continental basins extending northwards to western Norway and eastern Greenland. Extensional fault systems that cut the Orcadian Basin sequences are commonly assumed to be Devonian, with some limited inversion and reactivation proposed during the Carboniferous and later times. We present a detailed structural study of the regionally recognized fault systems exposed in the Dounreay area of Caithness, which host significant amounts of authigenic mineralization (carbonate, base metal sulphides, bitumen). Structural and microstructural analyses combined with Re–Os geochronology have been used to date syndeformational fault infills (pyrite) suggesting that faulting, brecciation and fluid flow events are likely to have occurred during the Permian (267.5 ± 3.4 [3.5] Ma). Stress inversion of fault slickenline data associated with mineralization suggest NW–SE regional rifting, an episode also recognized farther west in Sutherland. Thus a dominant set of Permian age brittle faults is now recognized along the entire north coast of Scotland, forming part of the regional-scale North Coast Transfer Zone located on the southern margin of the offshore West Orkney Basin. Supplementary material: Onshore and offshore fault and fracture lineament data are available at https://doi.org/10.6084/m9.figshare.c.2182433 .
Highlights
It has long been recognized that individual regional faulting episodes are often associated with characteristic fault rocks and/or mineral vein fillings (e.g. Sibson 1977; Passchier & Trouw 2005)
We present a detailed structural study of the regionally recognized fault systems exposed in the Dounreay area of Caithness which host significant amounts of authigenic mineralization
Provided there is field and microstructural evidence to show that dated fault rocks or vein fills are syntectonic with respect to faulting, this allows dating of the mineral fills to be used to constrain the absolute timing of brittle deformation events (e.g. Vernon et al 2014; Holdsworth et al 2015)
Summary
It has long been recognized that individual regional faulting episodes are often associated with characteristic fault rocks and/or mineral vein fillings (e.g. Sibson 1977; Passchier & Trouw 2005). The absolute dating of fault movements based on geochronological dating of fault rocks or newly-formed syn-tectonic minerals has proved to be rather more problematical This is due to both a lack of sufficiently robust isotopic systems and suitable geological materials, especially in upper crustal settings (e.g. see van der Pluijm et al 2001 and references therein). Absolute ages of both base metal sulphide mineralization (e.g., molybdenite, pyrite, chalcopyrite) and hydrocarbon maturation (oil, bitumen) can be determined by Re-. Provided there is field and microstructural evidence to show that dated fault rocks or vein fills are syntectonic with respect to faulting, this allows dating of the mineral fills to be used to constrain the absolute timing of brittle deformation events (e.g. Vernon et al 2014; Holdsworth et al 2015)
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