Morphologically ductile criteria for the sense of movement on slickensides from an extensional detachment fault in southern Spain

Abstract

New criteria for the sense of movement on slickenside surfaces displaying evidence for mesoscopic ductility are described in the Neogene Jaloche extensional detachment fault in southern Spain. This detachment develops spectacular slickensides and fault gouges, and four of the criteria for the sense of movement observed along the highly polished slip surfaces are new to the literature. They are: (1) synthetic secondary fractures with hangingwall drag-effects in the form of roll-over microsynclines; (2) metre-scale oval-shaped asymmetric culmination and depression features; (3) microthrusts verging towards the direction of motion of the opposite block and dragging/overthrusting previous planar elements on the fault surface; and (4) flakes of fault-surface material trailed and plastered in the direction of the missing block. The geometry and microstructure of these features reveal that they deformed in a morphologically ductile manner by the coupled processes of cataclastic flow and shear localization, probably during aseismic sliding. During transient seismic pulses, deformation occurred along shiny/polished slickenside fault planes occasionally bearing jigsaw implosion breccias. Fluids under high pressure present initially may have promoted distributed cracking, whereas the process of dilatancy hardening may have served to facilitate cataclastic flow in the deformation history.