Deformation in Moffat Shale detachment zones in the western part of the Scottish Southern Uplands

Abstract

-A study of the décollement zones in the Moffat Shale Group in the Ordovician Northern Belt of the Southern Uplands of Scotland reveals a progressive sequence of deformation and increased channelization of fluid flow. The study concentrates on exposures of imbricated Moffat Shale on the western coast of the Rhins of Galloway. Initial deformation occurred in partially lithified sediments and involved stratal disruption and shearing of the shales. Deformation then became more localized in narrower fault zones characterized by polyphase hydrothermal fluid flow/veining events. Deformation continued after vein formation, resulting in the development of low-temperature crystal plastic microstructures and further veining. Late-stage deformation is recorded as a pressure solution event possibly reflecting the cessation of slip on these faults as the slice became accreted. Most deformation can be ascribed to SE-directed thrusting and incorporation of the individual sheets into the Southern Uplands thrust stack. Later sinistral shear deformation, not observed in overlying turbidites, is also localized in these fault zones. The study reveals the likely structures formed at levels of an accretionary prism deforming under diagenetic to low-grade metamorphic conditions.

Imbricated ocean-plate stratigraphy and U-Pb zircon ages from tuff beds in cherts in the Ballantrae complex, SW Scotland

“…1A and 1C). Structures in mélanges and décollement zones in the Ordovician (northern) belt of the Southern Uplands accretionary prism resulted from SE-directed thrusting (Armstrong et al, 1996;Needham, 1993Needham, , 2004. Leggett et al (1983) and Needham (1995) pointed out that the accretionary prism of the Southern Uplands is comparable to that of the Shimanto belt of Japan, a comparison with which we agree.…”

Zircon age constraints on the provenance of Llandovery to Wenlock sandstones from the Midland Valley terrane of the Scottish Caledonides

“…The evolution of this terrane has been interpreted in terms of a fore-arc, supra-subduction zone prism (McKerrow et al 1977;Leggett et al 1979;Leggett et al 1982;Leggett 1987;Needham 2004), although the northern (Ordovician) part has also been interpreted as a subsiding, fore-arc shelf sequence rather than trench deposits (Armstrong et al 1996;Armstrong & Owen 2001). In an alternative model, the terrane is envisaged as having developed from an Ordovician back-arc setting into a mid-Silurian foreland basin that migrated onto the Avalonian continent following closure of the Iapetus Ocean (Morris 1987;Stone et al 1987;Kneller 1991;Kneller et al 1993).…”

The record of coal exploration in the Ordovician-Silurian in the Southern Uplands of Scotland and the north of Ireland

“…Almost all of the shale localities explored for coal are in the Moffat Shale, except those in the southernmost part of the outcrop (e.g., Borrow Head, Slane Coalpits), which may be of slightly younger Silurian age. Repetition of the carbonaceous rocks by thrusting means that outcrops of the Moffat Shale occur at intervals across the structural strike of the SUDLT, separated by successions of turbiditic sandstone (McCurry & Anderson 1989;Merriman & Roberts 2000;Needham 2004;Beamish et al 2010). The fault-repeated stack of alternating sandstone and carbonaceous shale is oriented with highly inclined dip so that the shale beds occur at intervals in a N-S transect through southern Scotland and the north of Ireland.…”

“…Data from such sites, including the evidence of solid carbonaceous residues (the 'coal'), contributes to an understanding of carbon migration in an accretionary prism (Parnell et al 2023). Over the whole SUDLT outcrop, the horizons of carbonaceous shale played a fundamental role in building the prism geometry (McCurry & Anderson 1989;Needham 2004). The thrust-related slices in the prism are numbered as tracts 1 to 10 from north to south (Leggett et al 1979;Leggett 1980;Morris et al 2003).…”