A Preliminary Structural Model for the Blue Mountain Geothermal Field, Nevada

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A Preliminary Structural Model for the Blue
Mountain Geothermal Field, Nevada
James Faulds (UNR) and Glenn Melosh (NGP)

• Tectonic Setting
• Regional Setting
• Blue Mountain
• Implications

Supported by Nevada Geothermal Power Company and
DOE
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Geothermal Fields Why Clustered in Northern
Great Basin?
Blackwell and Richards, 2004

• High heat flow through much of western U.S.
• Not higher in northern Great Basin

• Concentrated in NW Great Basin
• NE of northern Walker Lane
• Several NE-trending belts

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Why is Nevada in Hot Water?

• Dextral shear in Walker Lane transferred to WNW
  extension in NW Great Basin
• Accentuates Basin-Range extension, enhancing deep
  circulation of fluids
• Generates major transtensional domain in northern
  Great Basin
• Geothermal belts Loci of strain transfer

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Exploration Challenges

• Hot dry wells
• Overturn in down-hole temperature gradients
• Wet cool wells
• Better characterization and conceptual models are
  critical

Blue Mt., Nevada, USA
Germancek, Turkey
Desert Peak, Nevada, USA
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Methods

• Detailed mapping and reconnaissance
• Structural analysis
• Gravity surveys
• Studies of surficial geothermal features
• Shallow temperature surveys
• Geochemical analyses
• GIS compilation

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Structural Controls
Gerlach
Desert Peak

• Most not on major range-front faults
• Majority on less conspicuous normal faults
• Most common occurrences
• Discrete steps in fault zones
• Intersecting faults
• Overlapping fault zones
• Terminating, horse-tailing faults (some
  range-front faults)
• Small pull aparts within or at ends of
  strike-slip faults

Astor Pass
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Blue Mountain Geothermal Field

• Blind system West flank of Blue Mountain
• No surface hot springs
• Discovered due to mineral exploration in
  hydrothermally altered area
• Exploration by Nevada Geothermal Power
• 40 megawatt plant under construction

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Geologic Setting

• Miocene volcanic strata overlying Mesozoic
  metamorphic-plutonic basement
• NE of Walker Lane in actively extending region
• Gently to moderately east-tilted fault blocks
  (10-40o)
• Mainly systems of normal faults
• Major E-W to WNW extension begins 12 to 8
  MaContinues to present

N
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Blue Mountain Fault Block

• Small isolated block 8 km long
• Tilted 20-40o east
• Bounded by
• NNE fault
• WNW fault
• Merge at geothermal field
• Quaternary offset
• 1 km of relief
• Questions?
• Fault kinematics
• Controls on geothermal fluids

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Productive Wells

• Productive wells all along western apex of Blue
  Mountain
• Productive depths (inner wells) from 600 to
  1,070 m
• High flow rates
• Temperatures to 184-213oC
• Step out well to west
• gt 210oC at 1,730 m
• Intersecting moderate to steep W-dipping fault
  zone (55-60o)

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Fault Kinematics

• SW side dextral-normal
• Offset drainages (Szybinski, 2007)
• Small right steps
• Quaternary offset
• Normal component dominant?
• NW side sinistral-normal
• Small left steps
• Quaternary offset
• Normal component dominant
• Geothermal activity at intersection
• Multiple young scarps
• Complex intersection

SW flank - View NW
SW flank - View NE
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Fault Kinematics

• SW side right-lateral-normal
• Offset drainages
• Small right steps
• Quaternary offset
• Normal component dominant?
• NW side left-lateral-normal
• Small left steps
• Quaternary offset
• Normal component dominant
• Geothermal activity at intersection
• Multiple young scarps
• Complex intersection

Western Apex
NW flank - View NE
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Basement Fabric

• Triassic metasedimentary rocks
• NE- to ENE-striking, NW dipping fabric
• N-striking fabric to south gives way to ENE
  fabric to north
• As orientation of basement fabric changes, so
  does strike of major faults

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Conceptual Model

• Basement fabric affecting normal fault
  orientation
• Induces adjustments or steps
• Produces complex fault intersections and focused
  dilational zone

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Dilatant Zone Closely-Spaced Faults

• Entire NNE fault zone dilational
• Fault intersection at apex
• Dilational
• Dense fracturing
• Natural pathway for deep circulation
• Moderately to steeply W-plunging reservoir

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Implications

• Blue Mountain more complex than many other
  geothermal fields
• Not a simple step over or termination in a
  normal fault zone
• Slightly different setting than previously
  described
• Most common occurrences
• Discrete steps in fault zones
• Intersecting faults
• Overlapping fault zones
• Terminating, horse-tailing faults (some
  range-front faults)
• Small pull aparts within or at ends of
  strike-slip faults
• Complex dilatant fault intersection

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Conclusions

• Geothermal system controlled by a complex fault
  intersection
• Multiple strands connect two oblique-slip normal
  faults
• Dilatant zone with high fracture density
• Induced by strong basement fabric
• Major NNE normal faults skewed to ENE trends
• Adjustments produce fault intersections

• Exploration strategies
• Steps in range fronts and less conspicuous normal
  faults
• Interbasinal highs
• Ranges of low, discontinuous ridges
• Lateral terminations of ranges
• Ends of Walker Lane strike-slip faults
• Apex of small isolated fault blocks