Chaotic breccia zones on the Pembroke Peninsula, south Wales: Evidence for collapse into voids along dilational faults

Abstract

Chaotic breccias and megabreccias – locally called gash breccias – hosted within the Pembroke Limestone Group (Visean, Mississippian, lower Carboniferous) of southwest Wales are re-mapped along with spatially-related crackle and mosaic breccias. Of thirteen studied megabreccia bodies, seven lie along steep, NNW- or NNE-striking strike-slip faults originating during north–south Variscan (late Carboniferous) shortening, though reactivated during later extension. Four bodies are conformable with E–W striking, steeply-dipping bedding, and two have irregular or indeterminate margins. The bedding-parallel zones are interpreted as the dilational tips of listric normal faults, and the cross-strike faults as transtensional transfer zones. Sub-horizontal clast fabrics suggest brecciation by gravitational collapse into opening fissures rather than by cataclasis along the faults. Most fissures have geometrically matched margins produced by this dilational faulting, and only locally have the indented margins indicating solutional processes. The most likely age for the main fissure extension and fill is late Triassic, based on analogous dated fills at the eastern end of the Bristol Channel Basin. The Pembroke megabreccias blur the distinction between fault rocks formed by deformation and those formed by redeposition along fault zones.