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Structural Interpretation of the Dixie Meadows Geothermal Prospect using Joint Modeling of Gravity and Magnetic Data

Schwering, Paul C. and Karlin, Robert E.
[email protected]

The Dixie Meadows geothermal prospect, located ~150 km east of Reno, Nevada, lies within an active, NNE‐trending fault zone between the Stillwater Range to the east and Dixie Valley to the west. Geothermal surface expressions consist of advanced argillic alteration, fumaroles, and hot springs. Joint modeling of gravity and magnetic datasets indicates that intersecting faults and splaying normal faults control geothermal fluid flow. Some faults appear to be blind faults with little or no surface expression.

This study incorporates data from 80 aeromagnetic transects flown by the USGS in 2002 and 516 gravity stations acquired in 2010 by Zonge Geosciences, Inc. for Ormat Technologies Inc., covering ~150 square kilometers. Forward modeling of the gravity data, supported by available well data, indicates a basin thickness of ~2 km in the basin center, abruptly decreasing to within 500 m in the intrabasin – a 1‐2 km wide zone on the basin margin, adjacent to the rangefront. A reduced‐to‐pole magnetic map reveals normal fault‐bounded magnetic anomalies. The horizontal derivative of the gravity data delineates the strike of normal faults. Joint forward modeling of the gravity and magnetic data, supported by other geothermal exploration methods, indicate the presence of three generations of normal faulting; the oldest, steeply‐dipping NW‐striking fault is intersected by modern, moderate‐ to steeply‐dipping, NNEstriking faults that are superimposed upon older, steeply‐dipping, N‐striking, right‐stepping faults.

Normal faults primarily strike NNE and dip ESE, orthogonal to regional WNW extension of the northwestern Basin and Range province. Joint gravity and magnetic modeling reveals tens of meters of total displacement at the rangefront and >1 km of displacement in a blind, sub‐parallel piedmont fault, which indicates that the modern rangefront fault developed relatively recently. The NNE‐trending rangefront and piedmont faults have N‐striking, right‐stepping segments inherited from Tertiary faults created by E‐W extension. Joint modeling indicates two blind, splaying normal fault segments between the rangefront and piedmont faults, or intrabasin, occur near the hot springs and fumaroles. In addition, a steep, NW‐striking normal fault cuts through the Stillwater Range and into the basin. It is concealed by alluvial cover, but is geophysically delineated and spatially correlates with an abrupt lateral change in shallow (1 m) temperatures and with measured changes of total dissolved solids and pH in spring fluids. The NW‐striking fault exhibits stratigraphic offset in the Stillwater Range but does not cut the Quaternary fan surface, and is interpreted to be a remnant of an earlier Tertiary extensional episode.

The interpreted structural model is defined by three generations of extensional faulting. The oldest, NW‐striking fault appears to be a hydrologic barrier confining the intrabasin geothermal system. The youngest, NNE‐trending rangefront fault and piedmont fault are superimposed upon older, right-stepping, N‐striking fault segments. A pair of splaying normal fault segments in the intrabasin spatially correlates with surficial thermal features, suggesting that the splaying faults are hydrothermal conduits. The geophysical approach applied in this investigation delineates blind faults that control hydrothermal fluid flow and provides a testable structural model for further exploration and potential development.


AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013