Active Faults and Neotectonics

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Active Faults and Neotectonics at Geothermal
Sites in the Western Basin and Range Preliminary
Results John W. Bell Alan R. Ramelli Nevada
Bureau of Mines and Geology University of Nevada,
Reno Funded by DOE grant DE-FG36-021D14311 to
GBCGE
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• Overview of Quaternary fault distribution in
  western BR
• High-temperature (gt100o C) geothermal sites
• Proposed model Young (Holocene) fault movement
  is a criterion for potential high-temperature
  resources
• Methodology for detecting fault occurrence and
  age
• Examples of new fault data found at selected
  high- temperature sites
• Conclusions
• Most high-temperature sites are spatially and
  structurally associated with seismically active
  Holocene faults-- providing a potential
  exploration tool

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• W-NW-oriented crustal extension facilitates
  dilation on N-NE-striking faults (Faulds et al.,
  2005).
• GPS data indicate extensional strain on
  NE-striking faults (Blewitt et al., 2003).
• Other structural models have included stress
  redistribution along fault ruptures (Caskey and
  Wesnousky, 2000 Wesnousky and Temple, 2004)

• Regional tectonic models account for some, but
  not all, high- temperature geothermal
  occurrences.
• Not all favorably oriented faults are
  high-temperature geothermal sites.
• In this study, we propose a simple model in
  which age of faulting (recent crustal strain
  release) is a criterion for geothermal potential.

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• Faults provide conduits for thermal
  water
• Recent fault ruptures provide best potential for
  new or re-opened apertures
• Surface-rupturing earthquakes (gtM 6.5-7.6) open
  pathways along fault planes from crustal depths
  to ground surface
• Conceptual model
• Recent (Holocene) crustal strain release along
  faults may facilitate geothermal potential

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• Methodology
• Identify, determine the spatial association, and
  age of most recent faulting at 37
  high-temperature (gt100o C) sites (Coolbaugh et
  al., 2003)
• Employ low-sun-angle aerial photography to
  detect unmapped Holocene (seismically active)
  faults
• Establish maximum age of faulting with
  Quaternary stratigraphic relationships
• Conduct exploratory trenching and age dating

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USGS Quaternary fault and fold database of the
US http//earthquake.usgs.gov/regional/qfaults
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• Low-sun-angle aerial photography
• Large-scale (112,000) BW
• Early morning and late afternoon sun conditions
• Sun angle 10-25o
• Allows detection of small, subtle fault traces
  not visible on conventional mapping photography

(Camels courtesy of D.B. Slemmons)
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Steamboat geothermal area
Mt Rose Hwy
Low-sun-angle aerial photograph early morning
conditions Steamboat Hills
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Central Nevada seismic belt
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1954 Dixie Valley
Magnitude 6.8-7.6 earthquakes
1915 Pleasant Valley
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Geothermal sites structurally associated with
zones of historical faulting
Leach
Dixie Valley
Dixie Comstock
Stillwater
Eightmile
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Dixie Valley geothermal area
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Stillwater seismic gap
12 ka shoreline scarp
fault scarp
2.5 ka sinter
Most recent event offsets 2.5 ka sinter deposits
(Lutz et al., 2003)
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Eightmile Flat geothermal area

• New 1954 surface ruptures found on Eightmile
  Flat fault
• Pre-1954 fault event on Eightmile Flat fault
  offsets Holocene-age lacustrine deposits

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Identify prehistoric Holocene faults

• Stratigraphic datum provided by late Pleistocene
  Lake Lahontan
• High shoreline at 13 ka
• Multiple recessional shorelines
• Lake sediments provide datable material, e.g.,
  radiocarbon and tephra

Pleistocene Lakes in the Great Basin
(Reheis, 1999)
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Holocene faults and high temperature geothermal
sites
Fly Ranch
Gerlach
Trego
Empire
Kyle
Needle Rock
Geothermal data from Coolbaugh et al. (2003)
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Kyle Hot Springs Holocene faults found to control
hot springs
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Empire geothermal area
San Emidio fault at geothermal site found to be
Holocene age
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New Holocene fault traces found in San Emidio
Desert south of Empire geothermal area Possible
structural connection to Pyramid Lake geothermal
area
trench
trench
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New Holocene faults found at Gerlach geothermal
area
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(No Transcript)
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New Holocene faults found at Fly Ranch geothermal
area
Fly Ranch geothermal area to south
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New Holocene faults found at Trego geothermal area
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New faults and fault ages found near Needle
Rock/North Pyramid Lake geothermal area
trench
Needle Rock geothermal area to west
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2-3 ka beach gravel
30 ka tephra
shear zone
Hydrothermally altered basalt
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New Holocene faults found at northeast margin of
Pyramid Lake May be structurally connected to
faults in Empire/San Emidio Valley area
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Holocene and historical faults associated with 31
of 37 high-temperature geothermal sites
High probability that unmapped faults will be
found at other sites
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Not all Quaternary faults are sites of geothermal
activity including those in favorable
NNE-striking orientations
Spatial-temporal associations suggest that recent
seismic strain release is a factor in occurrence
of geothermal activity
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• Preliminary Conclusions
• Most high temperature geothermal sites are
  spatially and structurally associated with
  seismically active Holocene faults
• Out of a total of 37 high-temperature sites, at
  least 31 sites are located in close proximity to
  Holocene faults. Five sites are associated with
  historical fault ruptures
• Analysis of low-sun-angle aerial photography and
  field investigation indicate that numerous
  unmapped and undated Holocene faults are present
  near geothermal sites
• Results support the conceptual model that
  geothermal potential may be related to recent
  crustal strain release along faults
• Results suggest that active fault identification
  may be useful exploration tool in identifying and
  segregating sites of potential geothermal
  resources