SUB-BASIN STRUCTURE OF TWO SOUTHERN ARIZONA EXTENSIONAL BASINS

Detailed analysis of gravity and magnetic anomaly data, together with surface geologic and subsurface drilling information, have defined the subsurface basement highs and lows of the San Pedro and Santa Cruz basins in southeastern Arizona. The resulting patterns show that both of these basins are made up of a series of sub-basins separated by buried bedrock highs. When combined with fault 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. Northeast trending faults and trends, representing Oligocene-Miocene northwest-directed extension, are the most disrupted by northwest and north-south trends and faults. The northwest trending faults and trends, representing mid-Miocene northeast-directed extension, are more continuous and mainly only disrupted by the north-south directed faults and trends, representing Pliocene to Recent basin and range west-directed extension. These relationships agree with geological interpretations previously reported. Analyses of the overall pattern of offset of depocenters represented by gravity anomaly minima suggest that extension occurred by block rotation, with significant strike-slip offset across block boundaries parallel to the extension direction, especially for the northeast-southwest directed extension. In some cases, blocks were dragged by extension of adjoining blocks, resulting in local block rotation. In most cases, the sub-basin shape is asymmetric, suggesting that the primary extension mode was by half-graben formation.