The Pirate fault is a basin-range normal fault that separates footwall bedrock of the Santa Catalina Mountains on the east from downfaulted Neogene basin fill underlying Oro Valley, which forms a sedimented corridor to the west between the Santa Catalina and Tortolita Mountains (Pearthree and others, 1988). According to both McCullough (1963) and Budden (1975), this basin-margin, range-front structure was first termed the Pirate fault by Wallace (1954), although his own thesis makes no explicit claim for originating the nomenclature. The trend of the fault is northeasterly at the foot of Pusch Ridge near Tucson, but swings to nearly due north along the range front north of Catalina State Park. In detail, the structure is a complex fault zone, with brecciation and complex fracturing of varying intensity present for 100-200 m into footwall bedrock (Davis and others, 1994). By contrast, offset basin fill forming the hanging wall is little deformed, at least at exposed horizons. The contact between faulted basin fill and variably deformed bedrock is a thin gouge seam, only 1-5 cm thick, that dips consistently at angles of 50-55 degrees beneath Oro Valley. This contact is here inferred to be the main displacement surface of the Pirate fault zone, and its trace is the feature plotted along the trend of the fault. Adjacent to the gouge seam in the footwall is a cohesive sheet of compact fault breccia, composed of crushed bedrock and typically 4 m thick (Davis and others, 1994).