Chaotic breccia along the Dent Fault, NW England: implosion or collapse of a fault void?

Abstract

A body of chaotic breccia along the reverse-oblique Dent Fault zone is ascribed to hanging-wall collapse into persistent voids created by geometric mismatch of fault walls, although some implosion into transient voids is a possibility. The breccia comprises a 20 m wide body of hanging-wall lithologies, with a chaotic clast-supported fabric that contrasts with the fitted-fabric breccias typical of the Dent Fault damage zone. The breccia body has crude bedding defined by clast shape and size contrasts. The void fill is cut by Variscan fault strands, which, together with its ferroan calcite and barite cement, prove its late Carboniferous rather than recent age. It is shown that any fault void, transient or persistent, had a smaller aperture than the final width of the breccia body, and no more than 5 m; a span that can be supported to depths of 2 or 3 km. However, cement zonation in the breccia fill suggests that the void opened in multiple increments, each of an aperture compatible with the maximum displacement in any one event along the Dent Fault. The Dent Fault example highlights the possible general importance of fault-void collapse but also the problems in distinguishing it from implosion processes.