The nature and significance of the Faroe-Shetland Terrane: Linking Archaean basement blocks across the North Atlantic

Abstract

Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details.Abstract: Core samples of the continental basement rocks that underlie the eastern Faroe-Shetland Basin (FSB) and its inshore margins west of Shetland reveal a suite of predominantly granodioritic to granitic orthogneisses (including TTG), together with lesser volumes of foliated granitoids and subordinate dioritic to mafic gneisses and amphibolites. A small area of lithologically similar gneisses also crops out onshore at North Roe/Uyea west of the Caledonian front in Shetland. In the core samples, coarse grained gneissose textures and mineralogies consistent with upper amphibolite facies metamorphism are overprinted by a weak, but ubiquitous, static greenschist facies retrogression. Later structures include widespread epidote-quartz veining, and local developments of mylonite, cataclasite, pseudotachylite and phyllonite. Regions associated with the Rona Ridge oil fields (e.g. Clair, Lancaster) also preserve extensive brittle fracturing and associated low temperature hydrothermal mineralization (quartz, adularia, calcite, pyrite/chalcopyrite) with significant fracture-hosted hydrocarbons. New and published U-Pb zircon analyses from the gneisses offshore give a relatively narrow range of Neoarchaean protolith ages (ca 2.7-2.8 Ga) spread over a geographic area of 60,000 km 2 west of Shetland. Detrital zircon data from overlying Triassic-Cretaceous sedimentary sequences in the FSB suggest an equivalent limited range of Neoarchaean source materials. Hf isotopic data indicate involvement of Palaeo-to Mesoarchaean crustal sources. Overall, our findings suggest that a major phase of Neoarchaean crustal formation and associated high grade metamorphism dominates basement rocks in the region. They are similar in age and lithology to the protoliths of the nearest onshore Lewisian Complex of NW Scotland (Rhiconich Terrane). However, they lack geochronological or textural evidence for the widespread Proterozoic Laxfordian events (ca 1.7-1.8 Ga) which are widespread in Scotland. This suggests that the Precambrian rocks west of Shetland -the Faroe-Shetland Terrane -can be correlated with the Archaean rocks of the Central Greenland-Rae Craton and that a northern limit of Proterozoic reworking lies just offshore from the north coast of Scotland. Sample HM11705 comes from 13.00 m depth in BGS borehole 88/02. The succession encountered in this borehole was described by Chambers et al. (2005) as comprising quartzo-feldspathic schist cut by cataclasite bands. The analysed sa...

Fractured basement play development on the UK and Norwegian rifted margins

“…This trend continues as recent insights into the West of Shetland basement reservoirs (e.g. Holdsworth et al 2018Holdsworth et al , 2019Trice et al 2018) have provided evidence of common geological process being shared by the Clair Ridge and south Rona basement. Of particular relevance here is the use of outcrop fault and fracture statistics to constrain fracture frequency and fracture aperture distributions (e.g.…”

“…The Rona play is located on the Atlantic margin, West of Shetland, and comprises a narrow (,15 km wide) upfaulted elongated ridge c. 200 km long of naturally fractured Neoarchean granitic-tonalitic orthogneisses, which are likely to be related to the Lewisian Gneiss Complex in NW Scotland (Ritchie et al 2011;Holdsworth et al 2018Holdsworth et al , 2019. The basement ridge is overlain by a diverse Paleozoic-Cenozoic sedimentary succession (Brooks et al 2001, Ritchie et al 2011, as summarized in Figure 4a.…”

“…The Rona Ridge is the best known part of the Neoarchean Faroe-Shetland Terrane located to the north of Scotland and West of Shetland ( Fig. 1) (Ritchie et al 2011;Holdsworth et al 2018). Offshore cores and limited onshore outcrops in Shetland reveal a suite of predominantly granodioritic-granitic orthogneisses, including tonalite-trondhjemitegranodiorite (TTG), together with lesser volumes of foliated granitoids and subordinate dioriticmafic gneisses and amphibolites.…”

“…Offshore cores and limited onshore outcrops in Shetland reveal a suite of predominantly granodioritic-granitic orthogneisses, including tonalite-trondhjemitegranodiorite (TTG), together with lesser volumes of foliated granitoids and subordinate dioriticmafic gneisses and amphibolites. One of the longest and most complete set of cores comes from the Clair Ridge well 206/7a-2 (for a description see Holdsworth et al 2018). These rocks appear to correlate with the Lewisian Gneiss Complex along strike in NW Scotland, having experienced intense amphibolite-facies metamorphism soon after their intrusion at c. 2.8-2.7 Ga.…”

Modeling petroleum expulsion in sedimentary basins: The importance of igneous intrusion timing and basement composition

“…The basement underlying the FSB is composed Neoarchean (ca. 2.7-2.8 Ga) orthogneisses (Holdsworth et al, 2018) that typically contain low concentrations of heat-producing radionuclides 40 K, 232 Th, 235 U, and 238 U (Pollack and Chapman, 1977), which, together with the prolonged time for radiogenic decay, are expected to result in cold (reduced RHP) basement at the time of petroleum generation.…”