Geothermal Resource Exploration, Naf El Centro – Superstition Mountain Area, Imperial Valley, California

The geothermal potential of West Mesa has historically been assumed to be small because it is thought to be dominated by a left-stepping transpressional regime that is generally tight and not conducive to open fluid pathways. This may not be the complete picture, however. The 85 MWe Heber geothermal project appears to be located in a right stepover from the northwest trending seismogenic extension of the Cerro Prieto fault, placing it outside the Imperial-San Andreas fault system and on a separate, sub-parallel, extensional trend to the west. A similar right stepover appears to be present at Superstition Mountain where shallow drilling in the 1980’s found temperature gradients exceeding 300oC/km (17.5oF/100 ft.). Through the use of detailed surface mapping, electrical and potential fields geophysical methods and analysis of a relocated earthquake catalog for the region, the Navy Geothermal Program Office is beginning to define the hydrothermal history and the structural and tectonic framework of this thermal anomaly and to delineate active transtensional areas and critically stressed fractures which may serve as conduits for upwelling geothermal fluids. Introduction and Geological Background The Superstition Mountain geothermal prospect is located in the West Mesa area of northwestern Imperial Valley, California (Figure 1) and occurs within the Shade Tree bombing and parachute range of the Naval Air Facility, El Centro. The geothermal potential of West Mesa has historically been assumed to be small because, while the central Salton Sea trough is dominated by the Imperial fault-San Andreas fault trend, a right-stepping transtensional regime that allows for the upward circulation of heat and geothermal fluids, West Mesa has been thought to be dominated by a left-stepping transpressional regime (San Jacinto, Laguna Salida and Elsinore fault zones) that is generally tight and not conducive to open fluid pathways. Recent seismotectonic studies indicate this may not be the complete picture. Figure 1. NAS El Centro project location and the regional structural framework. Edmunds (1977) noted the hypothesis that current tectonic activity on the west side of Imperial Valley may be opening new heat sources there. The 85 MWe Heber geothermal project successfully operates using fluids fed from intersecting northwest trending right lateral strikeslip and northeasterly-trending normal faults (James, et al, 1987) and appears to be located in a right stepover from the northwest trending seismogenic extension of the Cerro Prieto fault (Magistrale, 2002) (Figure 2). This evidence appears to place Heber on a structural trend outside the Imperial-San Andreas fault system and on a separate, sub-parallel, extensional trend to the west. Such a right step would be releasing, creating the critically stressed fractures found at Heber which are