FACTORS CONTROLLING THE GEOMETRY OF SOUTHERN ARIZONA EXTENSIONAL BASINS

Analysis of gravity and magnetic anomaly data, together with surface geologic and subsurface drilling information, has defined the subsurface basement highs and lows of several basins in southeastern Arizona. The resulting patterns show that these basins are made up of a series of sub-basins separated by buried bedrock highs. When combined with fault and fold patterns defined on the intervening ranges from geologic mapping and potential field data, a polygonal fault pattern of normal, strike-slip, and block rotation motions is obtained. Analysis of these data yields at least three episodes of extension. Northeast-southwest extension began in Oligocene-Miocene together with rhyolitic to andesitic volcanism. More continuous northwest trending faults and trends then developed during mid-Miocene northeast-directed extension associated with detachment faulting and bimodal volcanism. Finally, north-south directed faults and trends, representing Pliocene to Recent basin and range west-directed extension were superimposed. Sub-basin shapes are mainly half-grabens. Extension occurs variably between northeast trending corridors about 35 km wide, so that there are large amounts of strike-slip motion between corridors. Both pre-existing and coeval structures during basin and range formation control the basin geometry, and synforms in the detachment fault surface generally correspond to subbasins and antiforms to bedrock highs. Continued activity on many faults within the basins controls present-day erosional patterns and must be an important factor in basin hydrology. All of the larger ranges are cored by Tertiary intrusives, suggesting that magnmatic inflation is an important factor in creating the total relief between basins and some ranges. More than 4 km of inflation can be demonstrated for the Catalina and Rincon ranges. Potential field data strongly suggests that major amounts of magmatic underplating accompanied basin and range formation.