Abstract

This study of the controversial structures of the Rhoscolyn Anticline suggests a different result of two-phase coaxial deformation from Ramsay's Type 3 interference fold patterns. From detailed field observations of the sequence of bedded quartzites, psammites, pelites and oblique quartz veins, with their strong competence contrasts, we conclude that the Rhoscolyn Anticline was an original tight, upright F₁ anticline that has undergone modification and distortion in a second deformation (D₂). This second deformation is an oblique, but near-vertical, pure shear, with a quantifiable strain ratio (R=3) that altered the Rhoscolyn Anticline and its minor structures into a more open, SE-overturned antiform, with c. 260 m hinge migration. Refolded folds are rare, but hybrid F₁ + F₂ minor folds and their fabrics, especially in the region between old and new hinges, provide clues to the two-stage history. Oblique distortion of originally NW-verging F₁ minor folds has resulted in their apparent neutral vergence in the present-day hinge of the Rhoscolyn Anticline. We regard the structures and fabrics in quartzites and psammites as more reliable indicators of the region's deformation history than those in pelites or quartz veins, and this may prove true for other regions of polyphase deformation.