XXVII.—A Structural History of the Girvan District, S.W. Ayrshire

Abstract

SynopsisAn analysis of the structural style of the Palæozoic rocks exposed around Girvan in S.W. Ayrshire suggests that it was fashioned by at least nine phases of deformation. Five of these phases were active when maximum pressures were horizontal and together constitute the local expression of the Caledonian orogeny. The oldest phase recognized, the Main fold phase, consisted of overfolding due to an over-riding maximum pressure from between 150° and 170° and was penecontemporaneous with thrusting generated by a modal maximum stress from 145° to 325°. Three important thrust belts resulted but there were considerable local swings in the azimuths of maximum stress, a variation which culminated in the propagation of the Ardwell fold and reverse fault phase by a maximum stress from 110°. Later Caledonian movements, presumably influenced by an increase in overburden due to folding and thrusting, were represented by two episodes of wrench faulting: an older Ardwell “cross-fold” phase due to a maximum stress from north-east to south-west and a younger Main wrench system with a maximum stress aligned at 160–340° which generated an intricate pattern of sinistrai and dextral shears together with second-order relatives. The three following phases, the first, second and third “normal” phases represented an episode in stress history when the relief of pressure was horizontal and aligned from approximately north to south, north-north-west to south-south-east and just north of west to south of east respectively. Maximum pressures tended to vacillate between the vertical and near horizontal and as a result much of the faulting was oblique-slip. All three phases were responsible for important faults and some evidence is put forward to suggest that all three are possibly Proto-Armorican or probably not later than Armorican, although the existence of a few mineral veins shows that reactivation certainly took place in Borcovician times. An analysis of the tensional joints recorded for the area indicates that, apart from the dominant Caledonian and less conspicuous Armorican sets, Tertiary joints were formed when there was a relief of pressure from north-east to south-west and this final phase of deformation was accompanied by the intrusion of dykes.The study also includes discussions on the methods and merits of the resolution by stereographic means of the stress fields responsible for oblique-slip faults and on the nature and origin of first- and second-order wrench faulting in the light of the abundant statistical data available.