The structure of the Llangranog Lineament, West Wales: A Caledonian transpression zone

The Llangranog Lineament is an important NE to NNE-trending zone of oblique-slip deformation in west Wales. Inland from Aberystwyth the lineament is represented by major NNE-striking faults on the west flank of the Machynlleth inlier and by an important NNE-trending vergence divide, the Glandyfi tract, which separates E-verging structures in most of mid Wales and W-verging structures on the coast of Cardigan Bay. A similar vergence divide, the Sarnau divide, marks the southward continuation of the lineament towards Cardigan. In southwest Ceredigion the lineament coincides with a belt of major NE-striking, oblique-slip faults which dip inwards beneath the Sarnau vergence divide. The lineament forms a distinctive structural zone in which the folds verge outwards from the vergence divide and are transected by the regional cleavage. The geometry, orientation, and relationships of the structures in this zone are indicative of deformation involving both compression and transcurrent shear. Restored sections across the Llangranog Lineament show a positive flower structure which has been disrupted by post-Caledonian extension. The structures developed along the Llangranog Lineament suggest that it is the surface expression of a deep basement fault with a significant strike-slip displacement.