THE MOUNTAIN-FRONT, NORMAL PIRATE FAULT AT ALAMO CANYON, SANTA CATALINA MOUNTAINS, SOUTHEASTERN ARIZONA

The Pirate fault is a Basin and Range, mountain-front normal fault whose ~30 km-long accessible trace defines the western boundary of the Catalina-Rincon metamorphic core complex in southeastern Arizona. The fault, with an estimated vertical displacement of ~ 4 km, separates the down-dropped Cañada del Oro basin on the west from the uplifted Santa Catalina structural block on the east. Remnants of the fault have been exposed by erosion at a number of locations along its trace, which permit examination of fault morphology and, perhaps, give insight to the fault’s development and evolution. At Alamo Canyon the fault is well displayed as an eastward-trending structural domain consisting, from top to bottom, of (1) a planar fault-surface, (2) a ~ 1 m-thick indurated fault-core cataclasite, (3) a ~ 45 m-thick unit of fault breccia grading progressively with structural depth into unbrecciated but increasingly fractured biotite granite of Alamo Canyon (Kga) footwall, all of which have been invaded pervasively by conspicuous red hematite. The hanging wall at this site, however, poses a problem of interpretation because it appears to consist of an ~80 m-wide block of largely undeformed, hematite-free (Kga?) leucogranite in direct contact with the hematite-rich fault surface.