Neogene erosion along the southwestern flank of the Galiuro Mountains in the Basin and Range province of southeastern Arizona has exposed multiple strands of a mid-Tertiary low-angle normal-fault system that offsets Oligocene-Miocene and older strata. This fault system along the range front is overlain unconformably by onlapping Miocene-Pliocene basin fill but forms the structural boundary between the Galiuro range and the adjacent San Pedro trough, and it served as the breakaway zone for the extensional detachment system associated with the nearby Catalina-Rincon core complex. Younger high-angle normal faults of comparatively modest displacement cut various strands of the low-angle normal-fault system and locally displace basin fill. The antiquity and nature of the Galiuro range front challenge the common assumption that all morphologically prominent blocks of the Basin and Range province are young relief features bounded by high-angle normal faults.

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