Assessing Structural Controls on Geothermal Fluids from a Three-dimensional Geophysical Model of Warner Valley, Oregon USA

Warner Valley in southern Oregon (USA) is the site of a geothermal system that hosts several hotsprings in addition to Crump geyser – a geysering well that soon after being drilled in 1959 underwent frequent eruptions of boiling water. This, and thermochemical studies that estimate reservoir temperatures of 150°C, have prompted ongoing geological and geophysical investigations. Warner Valley is situated in a tectonically complex region in the northwest corner of the Great Basin - a basin and range province characterized by east-west extension. The regional geology consists predominantly of Neogene volcanics that have been faulted by a series of obliquely oriented NW and NNE-trending extensional faults. The valley forms an asymmetric graben, with the NNEtrending range front fault along the west Warner escarpement exposing over 600m of section. Warner Valley seems to be similar to other extensional geothermal systems in the Great Basin, which arise from deep circulation of meteoric water along major normal faults. This is evident in the proximity of Crump Geyser to the principal rangefront fault. Localization of surface hydrothermal features and patterns of borehole temperatures and flow suggest that secondary fault interactions may play a role in controlling geothermal fluids along the rangefront, but their presence within the basin is obscured by the basin fill. The large contrast in properties (density and magnetic susceptibility) between the basin sediments and volcanic rocks render potential field methods (gravity and magnetics) particularly well-suited to mapping and modeling subsurface geologic structures such as faults that juxtapose contrasting rock types and lead to distinct gravity and magnetic anomalies. We have performed geophysical studies in Warner Valley and surrounding regions, collecting high-resolution gravity and ground magnetic data along several detailed transects around Crump Geyser as well as regionally to characterize intra-basin and basinbounding faults, constrain basin geometry, study fault interactions, identify areas favorable to hydrothermal flow, and ultimately to guide exploration of the area’s geothermal system.

[1]  E. E. Larson,et al.  Combined Palaeomagnetic and Petrological Delineation of Faults in Miocene Basalts of Oregon , 1966, Geological Magazine.

[2]  E. E. Larson,et al.  The Steens Mountain (Oregon) geomagnetic polarity transition: 3 , 1987 .

[3]  J. Watt,et al.  Geophysical Studies in the Vicinity of the Warner Mountains and Surprise Valley, Northeast California, Northwest Nevada, and Southern Oregon , 2009 .

[4]  C. Finn,et al.  High-resolution aeromagnetic mapping of volcanic terrain, Yellowstone National Park , 2002 .

[5]  J. Faulds,et al.  The role of accommodation zones and transfer zones in the regional segmentation of extended terranes , 1998 .

[6]  N. V. Peteron.  Lake County's new continuous geyser , 1959 .

[7]  Robert W. Simpson,et al.  Approximating edges of source bodies from magnetic or gravity anomalies , 1986 .

[8]  R. Fisher.  Dispersion on a sphere , 1953, Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences.

[9]  J. H. Stewart,et al.  Accommodation zones and transfer zones : the regional segmentation of the Basin and Range province , 1998 .

[10]  H. Patton.  P-wave fault-plane solutions and the generation of surface waves by earthquakes in the western United States , 1985 .

[11]  J. Kirschvink.  The least-squares line and plane and the analysis of palaeomagnetic data , 1980 .

[12]  L. Muffler,et al.  Classification of public lands valuable for geothermal steam and associated geothermal resources. [Geothermal steam act of 1970] , 1971 .

[13]  F. J. R. Syberg.  A FOURIER METHOD FOR THE REGIONAL-RESIDUAL PROBLEM OF POTENTIAL FIELDS* , 1972 .

[14]  M. B. Dobrin.  Introduction to Geophysical Prospecting , 1976 .

[15]  Lindrith Cordell,et al.  Limitations of determining density or magnetic boundaries from the horizontal gradient of gravity or pseudogravity data , 1987 .

[16]  M. Landisman,et al.  Rapid gravity computations for two‐dimensional bodies with application to the Mendocino submarine fracture zone , 1959 .

[17]  G. Olhoeft,et al.  Density of rocks and minerals , 1984 .

[18]  A. Meigs,et al.  Faulting in a propagating continental rift: Insight from the late Miocene structural development of the Abert Rim fault, southern Oregon, USA , 2010 .

[19]  R. Blakely,et al.  Distribution of buried hydrothermal alteration deduced from high‐resolution magnetic surveys in Yellowstone National Park , 2014 .

[20]  R. W. Craig,et al.  The geothermal hydrology of Warner Valley, Oregon; a reconnaissance study , 1981 .

[21]  Richard S. Smith,et al.  Tilt-depth method: A simple depth estimation method using first-order magnetic derivatives , 2007 .

[22]  S. Klemperer,et al.  ATV magnetometer systems for efficient ground magnetic surveying , 2011 .

[23]  M. Prévôt,et al.  The Steens Mountain (Oregon) geomagnetic polarity transition: 1. Directional history, duration of episodes, and rock magnetism , 1985 .

[24]  J. Phillips.  Designing matched bandpass and azimuthal filters for the separation of potential-field anomalies by source region and source type , 2001 .

[25]  Lindrith Cordell,et al.  A terracing operator for physical property mapping with potential field data , 1989 .

[26]  R. Simpson,et al.  Northward migration of the Cascadia forearc in the northwestern U.S. and implications for subduction deformation , 2001 .

[27]  R. Blakely.  Potential theory in gravity and magnetic applications , 1996 .