The onset and timing of Caledonian sinistral shear in County Down

Abstract

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.

The nature and regional significance of structures in the Gala Group of the Southern Uplands terrane, Berwickshire coast, southeastern Scotland

“…This is supported by existing biostratigraphic and radiometric data which suggest that the onset of thrusting ranges from the Ashgill in the northwest through to the Wenlock in the southeast (Barnes, Lintern & Stone, 1989). In addition to the contractional deformation, a regional component of sinistral shear becomes important during deformation from the late Llandovery onwards (Anderson & Oliver, 1986;Anderson, 1987;Barnes, Lintern & Stone, 1989).…”

“…In addition to the contractional deformation recognized throughout the Southern Uplands terrane, a regional component of sinistral shear becomes important during deformation from the late Llandovery onwards (Anderson & Oliver, 1986;Anderson, 1987;Barnes, Lintern & Stone, 1989). In the northwestern parts of the thrust wedge, sinistral fault-and shearzones, often with associated, steeply plunging late folds ('F3') are superimposed upon pre-existing contractional structures ('F1-F2') including tract-bo unding early thrusts such as the Orlock Bridge Fault ( Fig.…”

“…Numerous studies have documented the structures present in this terrane in some detail, notably in Northern Ireland (e.g. Anderson & Cameron, 1979;Anderson & Oliver, 1986;Anderson, 1987), southwestern Scotland (e.g. Stringer & Treagus, 1980, 1981Anderson & Oliver, 1986;Kemp, 1986Kemp, , 1987Barnes, Anderson & McCurry, 1987;Stone, 1995Stone, , 1996Lintern & Floyd, 2000) and inland regions of the Southern Uplands (e.g.…”

The Southern Uplands Terrane in Scotland – a notional controversy revisited

“…In general, the Hawick Group tracts are more folded and are affected by a stronger cleavage than is apparent in the older tracts to the north, with a tendency for the cleavage to be clockwise transecting, up to 20° locally (Stringer & Treagus 1980;Anderson 1987). The plunge of the fold hinges may be highly variable and folds are downward-facing locally.…”

“…The widespread evidence of fold-transecting cleavage patterns and vein arrays (e.g. Anderson 1987) demonstrates a degree of sinistral shear within the accretionary complex, as do the steeply-plunging D3 folds. But these are relatively small-scale structures, and evidence for sinistral shear on a larger scale has proved generally elusive.…”