Abstract

Compilation of new data derived from recent field mapping in the British Isles upholds the earlier conclusion by E.M.Anderson (1942, 1951) that approximately north-south and east-west episodic stress fields have occurred.

In general, the regmatic patterns in the metamorphic terrains of Scotland are dominated by a shear system resulting primarily from a north-south stress system with the Great Glen direction predominating. In Northern Ireland, however, the fault and fold patterns suggest an E-W stress field. Large dislocations are common but second and third order features are notably rare.

In the younger, less competent sedimentary Phanerozoic rocks of the British Isles the horizontal displacements are considerably less but, nevertheless, the two dimensional regmatic patterns and associated drag folding characteristic of both stress systems described above are present.

Three distinctive patterns are most useful for examining Anderson’s hypothesis: a) 60–120° intersections outlining rhomboids whose long axes are nearly north-south for the N-S stress system and nearly east-west for the E-W stress system, b) 90° intersections outlining squares and rectangles whose long axes are oriented according to the azimuth of the dominant fracture, and c) 30–150° intersections outlining rhomboids whose long axes are inclined approximately 45° east and west of north. The 60–120° rhomboid pattern may or may not have occurred at the same time but the 90° and 30–150° intersection pattern implies at least two distinctive stress systems separated in time. Thus, only 60–120° intersections are properly regarded as conjugate. The scale of the square, rectangle and rhomboid patterns in the British Isles ranges from microscopic to at least 200 kilometers on a side.