A detailed examination of the boundary between Peach and Home's (1899) Northern and Central Belts, in the Lower Palaeozoic rocks of the Southern Uplands of Scotland and their Irish continuation in Longford–Down, demonstrates it to be a major Caledonian sinistral wrench fault. Maps and descriptions of outcrops of the fault at Slieve Glah in County Cavan, at Orlock Bridge on the NE coast of Down, at Cairngarroch on the Rhinns of Galloway and at Garvald, 11 km SW of Dunbar, are presented. A distinctive fault fabric, clearly the result of repeated sinistral slip at the first three localities, characterises these outcrops along the 400 km trace. The fabric elements include: (1) lenticular shearing of arenites in the fault protolith, (2) a fault-associated phyllonitic fabric, with abundant pressure-solution seams, overprinting the regional S1, (3) numerous foliation-parallel quartz segregations, (4) refolding of the regional S1 cleavage, segregation veins and the phyllonitic fabric in at least two generations of steeply plunging, sinistrally-verging folds, (5) a non-penetrative crenulation cleavage, (6) a locally developed sinistral S–C fabric, and (7) typical protomylonite, mylonite and ultramylonite textures in thin sections of the fault rocks. The zone of fault-associated deformation varies from a few metres across at Garvald to over 1 km at Slieve Glah. The fault thus differs from other tract-defining faults in the Southern Uplands, particularly in its clear evidence of ductile, quasi-plastic, deformation at depth, in its lack of associated imbrication, in its significant refolding and overprinting of the regional S1 cleavage and in its uniquely large stratigraphic effects.The main fabric-generating movement postdates the regional, accretion-related, S1 cleavage but predates minor Caledonian (c. 400 Ma) intrusions. For its entire length the fault separates the Northern Belt, with proof only of Upper Ordovician turbidites, and the Central Belt, where turbidite deposition began in the Silurian. The age of the base of the turbidites decreases southward by some six graptolite zones at the fault trace. The fault thus either (a) excises the equivalent of four tracts from a single accretionary-prism terrane, or (b) juxtaposes two distinct terranes, one of late Ordovician and the other of Silurian age. In either event the sinistral slip is probably in excess of 400 km.
In the south-westerly Irish continuation of the Southern Uplands ample evidence of sinistral transpression includes: (a) the clockwise transection of F 1 folds by S 1 cleavage combined with sub-horizontal extension in the cleavage plane, (b) local development of vein arrays deformed by sinistral simple shear and (c) an abundance of sinistral wrench faults. The sinistral movements are dated relative to the local history of Caledonian deformation and the intrusion of two suites of Caledonian lamprophyre dykes. Clockwise transection of folds by cleavage is not a feature of the Northern Belt or of the northern tracts of the Central Belt in County Down. Its geographic distribution suggests that sinistral transpression was first effective during the accretion of Llandovery-age tracts mid-way south in the Central Belt. Sinistral movements continued episodically during the deformation of early pre- and post-S 1 vein sets and was finally expressed in widespread wrench faulting which continued beyond the date of intrusion of the younger dykes, at about 400 Ma.
Coastal exposure in County Down provides an almost complete cross section of the continuation of the Southern Uplands fold belt in Ireland. Intensely folded Ordovician and Silurian turbidite successions are segmented by at least ten major strike parallel faults. In each segment the fold envelope descends northward but the strike faults throw up progressively younger sediments to the south. A system of upright folds, with slaty cleavage axial plane, but locally transecting, becomes progressively complicated southward by the clear development of two further phases of folding.and cleavage with constant geometry and opposing vergence. The chronology of cleavage, folding, later kink-band and brittle deformation is punctuated by successive late Caledonian lamprophyre dyke swarms. The total structure is consistent with a model of diachronous deformation and northward translation and rotation at a destructive plate margin.
In common with many other regions of exposed continental basement, the Late Archaean to Palaeoproterozoic Lewisian Complex, NW Scotland, preserves numerous examples of faults that appear to reactivate pre-existing compositional and structural heterogeneities in the host gneisses. A regionally recognized set of late Laxfordian sinistral strike-slip faults and fractures are spatially associated with pre-existing NW-SE-trending ductile shear zones of Inverian and Laxfordian age. Field observations suggest that most of the sinistral displacements have been accommodated along laterally persistent faults (here termed principal displacement zones (PDZ)) that lie sub-parallel to the pre-existing foliation in the shear zones. Geometric and orientation data collected during structural logging of the PDZ faults have been used to quantitatively test the influence of lithology and pre-existing structural geometry on the spatial patterns of fault development. Stereographic analysis shows a strong geometrical correspondence between the intensity and form of the pre-existing anisotropy and the alignment of the PDZ brittle faults. Spatial clustering of PDZ faults varies depending on lithology (amphibolite v. acid gneiss v. quartz-mica schist). A close correlation exists between the geometry and intensity of the pre-existing foliation and fault spatial clustering. The results demonstrate that reactivation of pre-existing anisotropies in typical continental basement gneisses exert a significant control on brittle fault development and growth in the upper crust.
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