The Pirate fault forms the western boundary of the Santa Catalina Mountains near Tucson and separates the mountains from the Cañada del Oro basin to the west. The fault occurs just east of the communities of Saddlebrooke, Catalina, and Oro Valley. Remnants of this fault, exposed for about 15 miles along the mountain front, are described in a paper from the Arizona Geological Survey (see reference below). The paper describes geological features of 10 sites along the fault trace.
The AZGS says that this fault represents an expression of Basin & Range faulting which was active between 12 million and 6 million years ago. Vertical displacement on the fault is estimated to be about 2.5 miles with the west side down relative to the Santa Catalina Mountains uplift on the east. The fault dips from 50° to 55° west along its entire trace. The Basin & Range era was a time of crustal extension which formed much of the topography in Southern Arizona.
According to AZGS: “ Following cessation of active uplift, the fault was buried under detritus eroded from the uplifted Santa Catalina block and, currently, is being exhumed by the down-cutting Cañada del Oro and its tributaries. This field examination reveals the fault to have left a sparse but diverse collection of remains implying a varied history of fault development and evolution.”
“Deposition of basin-fill material in the Cañada del Oro basin culminated in Pleistocene time (1-2 Ma) following cessation of active uplift on the Pirate fault. Alluvium deposited during this latter time forms the high-stand surface of coalescent alluvial fans composed mostly of detritus eroded from the Santa Catalina Mountains.” That material contains placer gold deposits. The gold was derived from gold-bearing quartz veins in the Santa Catalina Mountains.
The Pirate fault disappears beneath alluvium to both the south and north. If one projects the northern trace, the Pirate fault could intersect the southeast-to-northwest trending Mogul fault. Indeed, near the projected intersection is a decorative stone quarry whose source rock is highly fractured, deformed, and altered bedrock that may be evidence of the projected fault intersection.
Parts of the exposed Pirate fault are stained red by hematite, an iron oxide, suggesting that mineralizing hydrothermal solutions were present during the development of the fault. The exact nature of this mineralization is enigmatic and according to the AZGS, “would seem to defy ready explanation.” “The picture that emerges is that of the Pirate fault as a geologic entity whose tenure as an active participant in the extensional Basin-Range tectonic event has left behind a somewhat sparse and locally enigmatic set of remains from which to infer, caveat emptor, its past.”
Hoxie, D.T., Exhuming the Remains of the Inactive Mountain-Front Pirate Fault, Santa Catalina Mountains, Southeastern Arizona. Arizona Geological Survey, Contributed Report CR-12-F, 18p.
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