INFLUENCE OF MECHANICAL STRATIGRAPHY ON FAULT SCARP DEVELOPMENT IN EXTENSIONAL TERRANES (Invited Presentation)

Surface expression of normal fault zones in active extensional terranes is widely variable, from nearly imperceptible surface traces with no discernible topographic expression to major topographic expression with pronounced fault scarps. This geomorphic expression is partially related to rate of fault slip and associated accommodation space development, and infilling rate. In sediment starved systems where fault scarps are not masked by sediment or volcanic infilling, dramatic fault scarp morphologic differences occur. In these cases, fundamental controls on fault scarp morphology are exerted by the mechanical stratigraphic character of the faulting sequence. Mechanically competent layers are prone to brittle failure whereas incompetent or ductile strata may tend to arrest fault propagation. In the absence of a weak or ductile layer, normal faults propagating upward may reach the ground surface and cause shear offset or dilational shear offset, producing a fault scarp without or with a dilational fissure developing at the surface. In contrast, a ductile layer at depth can impede upward fault propagation and cause a fault to tip or terminate below the surface. Above such a tip, continued fault displacement at depth is accommodated above the fault tip by folding and distributed deformation instead of discrete faulting. Folding can in turn result in widely variable deformation styles at the surface, including monocline formation with no development of surface fissures, versus dilational fissure development accommodating bending strains if thick competent strata are present above the incompetent fault-terminating layer. Vertical offset (throw), fissure development, and tilting of the hanging wall are key aspects of normal fault expression at the surface that vary as a function of mechanical stratigraphy. Whereas the fault scarp morphology may be predicted from knowledge of the mechanical stratigraphy, the fault scarp surface morphology can also be used to infer the subsurface mechanical stratigraphy. These observations and processes are illustrated using natural examples of active normal fault systems in the Mid-Atlantic spreading center of Iceland and the western Basin and Range province in California (USA).