The sheared porphyrite dykes of South Galloway

Abstract

Summary A group of more than 100 dykes, mainly porphyrites with some microdiorites, quartz-diorites, and lamprophyres, trends east-north-east from the Solway coast between Creetown and Gatehouse-of-Fleet; the dykes are intruded along the strike of the closely folded Lower Palaeozoic sediments, the belt in which they are emplaced being some eight miles in length and four miles wide. Most of the dykes possess directional textures, formerly described as foliation; these textures are partly of primary origin but mainly due to shearing. The shearing has taken place on vertical surfaces which lie either parallel to the length of the dykes or make small acute angles with their walls. A feature shown by most of the dyke-rocks is the development of numerous flakes of white mica within their fabrics; in the sheared dykes the micas are oriented along the surfaces of shear. The mica-forming materials may have been derived from an internal source in the zone of shearing, or as emanations from subjacent granite masses. The relationships between the shear directions, transverse joints, and quartz veins which cross the dykes are analysed, and experimental work due to Reidel is cited in support of the suggested origin of the structures in a belt of shearing. The attitude of the shear surfaces in relation to the ellipsoids of stress and strain is also examined. The dykes are considered to fill tear (or transcurrent) fractures, on which lateral movement recurred after the emplacement of the igneous bodies. The implications of this mechanism in respect of a wider area are indicated.