Regional geologic mapping using pictures from the first Earth Resources Technology Satellite (ERTS-1) has led to the recognition of two parallel northeast-trending systems of normal faults, each of which can be traced more than 100 km. Many eruptive centers appear to be localized along these fault systems or along their extensions. The faults are chiefly observed in Phanerozoic rocks and have minor displacement but are interpreted by us to reflect fault zones of major displacement in the crystalline Precambrian basement. The Bright Angel fault system extends as a continuous zone of normal faults from Cataract Creek on the southwest to the Echo Cliffs on the northeast. Beyond the Echo Cliffs, the system continues northeastward to the vicinity of Monument Valley as a more diffuse, discontinuous zone of normal faults. The Bright Angel fault, Vishnu fault, and Eminence Break graben are among the larger individual members of the total system. The Navajo mountain intrusive center lies along the discontinuous part of the system. Three major eruptive centers of the Mount Floyd volcanic field lie on the southwestern projection of the Bright Angel fault system. If the eruptive centers are included as part of the recognizable structural system, the Bright Angel system has a total known length of slightly more than 300 km. The Mesa Butte fault system, as now recognized, extends from Chino Valley on the southwest to Shadow Mountain on the northeast. Bill Williams Mountain, Sitgreaves Peak, and Kendrick Peak are principal silicic to intermediate eruptive centers of the San Francisco volcanic field that appear to be localized along the fault system. Red Mountain, Mesa Butte, and Shadow Mountain are prominent basaltic eruptive centers along the system; monchiquite diatremes at Tuba Butte and Wildcat Peak lie on the northeast projection of the fault system. The total distance from Chino Valley to Wildcat Peak is more than 200 km. Comparison of the Bright Angel and Mesa Butte fault systems with a residual aeromagnetic map of Arizona reveals a close correspondence between the positions of the observed relatively minor normal faults and the margins of a series of large northeast-trending magnetic anomalies. Perhaps the most noteworthy feature of the aeromagnetic map is a 400-km-long northeast-trending belt of large positive aeromagnetic anomalies that extends from the vicinity of Congress to the northern border of Arizona. The Mesa Butte fault system lies along the southeast margin of this anomaly belt. Another large positive anomaly, bounded on the southeast by the Bright Angel fault, corresponds in the Grand Canyon to a belt of Precambrian amphibolite and schist. Most of the large positive aeromagnetic anomalies along the Bright Angel and Mesa Butte fault systems may correspond to similar bodies of mafic metavolcanic rocks, which have been offset along two major and perhaps several minor faults of Precambrian age. The normal faults that displace the overlying Phanerozoic rocks have been formed by renewed movement along these ancient fault zones, in response to dilation of the crust from late Tertiary time to the present. The ancient fault zones inferred to be present along the Bright Angel and Mesa Butte fault systems may be related in origin to the Shylock and Chaparral fault zones in central Arizona described by Anderson (1967). Both the Shylock fault zone and the Chaparral fault have right-lateral transcurrent displacement. As shown by Anderson, the Shylock zone has a probable minimum horizontal displacement of 8 km. A large contrast in the magnetic properties of the rocks on opposite sides of the fault zone, indicated by the aeromagnetic map, suggests that the displacement may be several tens of kilometres or more. Comparably large right-lateral displacements may have occurred along the ancestral Bright Angel and Mesa Butte fault zones. The location of epicenters of recent earthquakes and reports of earthquakes by residents in the region indicate that the Bright Angel and Mesa Butte fault systems are currently active.