MORPHOLOGICALLY DUCTILE CRITERIA FOR THE SENSE OF MOVEMENT ON SLICKENSIDES FROM AN EXTENSIONAL DETACHMENT FAULT IN SOUTHERN SPAIN

New criteria for the sense of movement on slickenside surfaces display ing evidence for mesoscopic ductility are described in the Neogene Jal oche extensional detachment fault in southern Spain. This detachment d evelops spectacular slickensides and fault gouges, and four of the cri teria for the sense of movement observed along the highly polished sli p surfaces are new to the literature. They are: (1) synthetic secondar y fractures with hangingwall drag-effects in the form of roll-over mic rosynclines; (2) metre-scale oval-shaped asymmetric culmination and de pression features; (3) microthrusts verging towards the direction of m otion of the opposite block and dragging/overthrusting previous planar elements on the fault surface; and (4) flakes of fault-surface materi al trailed and plastered in the direction of the missing block. The ge ometry and microstructure of these features reveal that they deformed in a morphologically ductile manner by the coupled processes of catacl astic flow and shear localization, probably during aseismic sliding. D uring transient seismic pulses, deformation occurred along shiny/polis hed slickenside fault planes occasionally bearing jigsaw implosion bre ccias. Fluids under high pressure present initially may have promoted distributed cracking, whereas the process of dilatancy hardening may h ave served to facilitate cataclastic flow in the deformation history. (C) 1997 Elsevier Science Ltd.