Orthogonal fracture systems at the limits of thrusting: an example from southwestern Wales

A well-developed orthogonal vein system is located in the autochthonous foreland just beyond the limits of Variscan thrusting in southwestern Wales. The two vein sets trend 200° and 290°. perpendicular and parallel, respectively, to Variscan structures. The veins contain stretched quartz fibers indicating horizontal extensions of between 0.3 and 2.5%. The 200° set has greater extension with a distinctive morphology, including shorter more curvilinear fractures that have components of normal offset. Vein intersections are simple, indicating episodic growth by alternating regional propagation of each set. During propagation, maximum compression was subvertical and the horizontal principal stresses switched direction with the alternation of set growth. The system formed during waning Variscan compression when the directions of horizontal principal stresses were still controlled by the tectonic compression. The differential stress may have been greater during propagation of the 200° Set. This example indicates that orthogonal fracture systems in forelands beyond thrusting: (1) are initiated by fracturing parallel to tectonic compression; (2) form in a few stages that are controlled by changes in regional stress; and (3) form during waning tectonic compression that controls fracture orientations