Structural history of the Silurian rocks of the coast west of Gatehouse, Kirkcudbrightshire

Abstract

Synopsis The rocks exposed in the coastal sections between Gatehouse and Creetown, Kirkcudbrightshire, are considered to belong to the Hawick Rocks (Upper Valentian), and have been affected by five main episodes of deformation, three of which are of Caledonian age. The first phase produced E.N.E.-W.S.W. similar folds and associated tensional strike faults, forming the structural basis of the area and determining the outcrop distribution. During the second phase these folds were modified, and a new set of concentric folds, comparable in strike but with axial surfaces dipping at around 45° S.S.E., was generated, along with a conjugate set of wrench faults. Folds of the third phase have E.-W. axes and steep eastward plunges. The three phases are equated respectively with F 1 F 2 and F 3 of the Whithorn area ( Rust 1965a ). The fourth phase gave rise to folds with near-horizontal axial planes, and frequently developed as minor thrusts, and the final deformation gave rise to dextral kink-bands trending N.-S. The two last are correlated with F 4 and F 5 of Whithorn, to which Hercynian and Alpine dates are provisionally allocated. A swarm of dykes was emplaced between the first and second phases, and further dykes were intruded after the second phase.

Asymmetrical folding in the Hawick Rocks of the Galloway area, Southern Uplands

“…Our investigation also indicates that F t folds predominate in the Hawick Rocks, and that locally developed post-Fj folds can be assigned to a single (F 2 ) phase. The two-phase fold history that we propose conflicts with the more complex polyphase sequences of folding postulated for parts of the Galloway area by Rust (1965a) and Weir (1968).…”

“…Two principal phases of folding in the Ordovician and Silurian rocks of southwest Scotland have recently been proposed by Weir (1979) who clearly envisages a reappraisal of his earlier structural chronology similar to our interpretation. The detailed polyphase structural chronology postulated for the Hawick Rocks in the Whithorn (Rust 1965a) and Gatehouse (Weir 1968) areas, however, has not been revised; the main differences between our interpretation and those proposed by Rust and Weir are outlined below. Rust (1965a, p. 108) and Weir (1968, pp.…”

“…We interpret the steeply plunging folds as part of a continuous spectrum of variation in plunge and intensity of the F t folding, although subsequent rotation of 'packets' of folds between strike dislocations may account for restricted areas of steep and downward-facing F t folds (Stringer and Treagus, in press). F 2 and F 4 folds (Rust 1965a) and^" and^ folds (Weir 1968) have been distinguished in the Whithorn and Gatehouse areas, largely on the basis of their relationships to supposed F 3 structures. Deformation of F 2 folds by F 4 was not recorded by Rust, and the overprinting evidence cited by Weir (e.g.,/ 4 folds deforming s, a and s 2 , Weir 1968, fig.…”

“…a. Kirkcudbright area (after Craig and Walton 1959, fig. 3) and Gatehouse area (after Weir 1968, fig. 2).…”

Late Caledonian dyke‐swarms in Southern Scotland: New field, petrological and geochemical data for the Wigtown Peninsula, Galloway

“…Weir 1968;Anderson and Cameron 1979) precedes D2 folding. This deformation, as argued earlier, must have been closely related to D, and to thrusting, and therefore dyke emplacement was contemporaneous with the later stages of thrusting.…”