Outcrop-scale manifestations of reactivation during multiple superimposed rifting and basin inversion events: the Devonian Orcadian Basin, northern Scotland

Abstract

The Devonian Orcadian Basin in Scotland hosts extensional fault systems assumed to be related to the initial formation of the basin, with only limited post-Devonian inversion and reactivation. However, a recent detailed structural study across Caithness, underpinned by published Re–Os geochronology, shows that three phases of deformation are present. North–south- and NW–SE-trending Group 1 faults are related to Devonian ENE–WSW transtension associated with sinistral shear along the Great Glen Fault during the formation of the Orcadian Basin. Metre- to kilometre-scale north–south-trending Group 2 folds and thrusts are developed close to earlier sub-basin-bounding faults and reflect late Carboniferous–early Permian east–west inversion associated with dextral reactivation of the Great Glen Fault. The dominant Group 3 structures are dextral oblique NE–SW-trending and sinistral east–west-trending faults with widespread syndeformational carbonate mineralization (± pyrite and bitumen) and are dated using Re–Os geochronology as Permian (c. 267 Ma). Regional Permian NW–SE extension related to the development of the offshore West Orkney Basin was superimposed over pre-existing fault networks, leading to local oblique reactivation of Group 1 faults in complex localized zones of transtensional folding, faulting and inversion. The structural complexity in surface outcrops onshore therefore reflects both the local reactivation of pre-existing faults and the superimposition of obliquely oriented rifting episodes during basin development in the adjacent offshore areas.Supplementary material: Stereographic projections of compiled structural data from individual fieldwork localities are available at https://doi.org/10.6084/m9.figshare.c.5115228

Fracture attribute scaling and connectivity in the Devonian Orcadian Basin with implications for geologically equivalent sub-surface fractured reservoirs

“…1; Allan and Mange-Rajetzki, 1992; Duncan and Buxton, 1995). It should be noted that, strictly speaking, the Clair Group formed in an adjacent basin, in a somewhat different tectonic setting (Dichiarante, 2017;Dichiarante et al, 2020a).…”

“…Deformation bands, gouges and breccias associated with these faults display little or no mineralization or veining. It is suggested that these structures are related to Devonian ENE-WSW transtension associated with sinistral shear along the Great Glen Fault during formation of the Orcadian and proto-West Orkney basins (Wilson et al, 2010;Dichiarante et al, 2020a). Wilson et al (2010) and the trace of the regional-scale transects (WTR1 and WTR2) and the location of the sub-regional transects (DO and SJ).…”

“…Once again, fault rocks associated with these structures display little or no mineralization or veining. Group 2 features are likely due to late Carboniferous-early Permian E-W shortening related to dextral reactivation of the Great Glen Fault (Coward et al, 1989;Seranne, 1992;Dichiarante et al, 2020a).…”

“…Barr et al (2007) noted the presence of dispersed joints, in outcrops, which they attributed to exhumation features being much rarer in cores. In this study, we carried out fracture attribute analyses in areas where Group 3 structures predominate, or at locations where there is good field evidence that pre-existing Group 1 faults have undergone significant later reactivation synchronous with Group 3 age deformation (Dichiarante et al, 2020a). We did not include obvious early (Group 1 and 2) or late jointing in our fracture datasets.…”

A revised age, structural model and origin for the North Pennine Orefield in the Alston Block, northern England: intrusion (Whin Sill)-related base metal (Cu–Pb–Zn–F) mineralization

“…Fault-slip slickenline data associated with each of the mineralized fault groups identified in the field were used to carry out a conventional palaeostress inversion analysis (Angelier 1979(Angelier , 1984Michael 1984) implemented using MyFault® software following the protocols set out by Holdsworth et al (2015) and Dichiarante et al (2020). Importantly, any analysis of this kind is viable only in regions where finite strains are modest and the presence of major fault block rotations can be ruled out.…”