ACTIVE FAULTING AND STRESS REDISTRIBUTIONS IN THE DIXIE VALLEY, BEOWAWE, AND BRADYS GEOTHERMAL FIELDS: IMPLICATIONS FOR GEOTHERMAL EXPLORATION IN THE BASIN AND RANGE

摘要

Detailed and preliminary field investigations delineating the most recent fault ruptures in vicinity of the Dixie Valley, Beowawe, and Bradys geothermal fields allow an assessment of static stress changes and their possible influences on the geothermal environments. Our models for the Dixie Valley and Beowawe regions show increased failure stress on faults and fractures associated with the geothermal reservoirs with contributions from both increased shear stress and decreased fault-normal stress. These stress changes are especially pronounced for the Dixie Valley geothermal field where large increases in failure stress are concentrated between Holocene rupture endpoints. The portion of the fault with the most enhanced tensile stresses lies at shallow crustal levels between the northern and southern limits of the production field. The sense of stress changes from both Holocene and historic fault ruptures in Dixie Valley are consistent with recent borehole studies of in situ stress and fracture permeability which show the Dixie Valley fault and fault-parallel fractures are critically stressed for failure and hydraulically conductive within the geothermal field. Structural relations in the Bradys geothermal field are analogous to the relations in Dixie Valley, and we suspect that a similar sense of stress change has influenced the Bradys geothermal environment. Our investigation shows that induced stress concentrations at the endpoints of normal fault ruptures may promote favorable conditions for hydrothermal activity. This may be accomplished in two ways: 1) Nearby fault ruptures may induce afteshocks associated with small amounts of slip on macro fractures along the fault and within the damage zone of the fault, thereby producing open fractures without producing large stress drops on the fau and 2) If the geothermal field is adjacent to fault rupture endpoints, then it is conceivable that decreases in fault-normal stress may be large enough to produce significant increases in fracture dilatancy, thereby increasing hydraulic conductivity. Our studies illustrate that a detailed understanding of the neotectonic framework and the mechanics of faulting processes are fundamental to developing conceptual models for controls on the state of stress and fracture permeability in geothermal fields.