Geomorphic evidence for tear faults accommodating lateral propagation of an active fault-bend fold, Wheeler Ridge, California

Abstract

Wheeler Ridge is the topographic expression of an actively growing fault-bend fold developed in the hanging walls of a system of mostly north-vergent blind thrusts. The eastern tip of the fold system has migrated laterally by approximately 3 km during the last 120 ka perpendicular to the regional shortening direction. Comparison of surface topography and structural cross-sections indicates that eastern Wheeler Ridge comprises three fold segments, whose topographic expression steps southward towards the eastern end of the structure. Lateral propagation of the fold is associated with numerous tear faults, expressed as fault scarps that face towards the eastern end of the fold. We suggest that these tear faults accommodate earthquake-related processes that collectively build plunge of the fold and lateral propagation on blind thrusts. We envisage that the fold will grow primarily by thrust faulting events with similar displacement(s) along strike that are terminated abruptly at tear faults, build displacement over multiple earthquake cycles and then step eastward to form a new tear fault. Formation of especially large tear faults is inferred to be in response to an increase in rock strength governed by lithology. The high fault displacement vs length ( D L ) for Wheeler Ridge may be related to high strain rates across a restraining bend in the nearby San Andreas strike-slip fault system.