Controlling Structures

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Controlling Structures

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Controlling Structures:
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Structure Description Associated Topography Tectonic Setting
Major Normal Fault Normal faults are structures in which the hanging wall is down dropped along the fault plane relative to the foot wall. They are the predominant type of structure in extensional tectonic environments, but are commonly encountered in a number of geologic settings. Major range front faults account for 3% of structural controls in the Great Basin. Mountainous, Horst and Graben Extensional Tectonics, Rift Zone
Termination of a Major Normal Fault Major normal faults sometimes terminate into multiple horsetailing splays that result in increased hydrothermal fluid flux. As of 2011, 22% of all known Basin & Range structures hosting geothermal systems occur at normal fault terminations or tip-lines with multiple closely-spaced faults that enhance permeability. Mountainous, Horst and Graben Extensional Tectonics, Rift Zone
Stepover or Relay Ramp in Normal Fault Zones Stepover or relay ramps between overlapping fault strands are one of the most favorable structural settings for geothermal activity. As of 2011, ~32% of the more than 200 document geothermal fields in the Basin & Range were associated with step-overs or relay ramps in normal fault zones. Increased fracture density generated by intersections between multiple overlapping fault strands results in enhanced permeability that facilitates hydrothermal fluid flow. Mountainous, Horst and Graben Extensional Tectonics
Apex or Salient of Normal Fault Normal faults may intersect in the subsurface to form a fault apex or salient. Apices or salients of normal faults account for 3% of structural controls in the Great Basin. Mountainous, Horst and Graben Extensional Tectonics
Fault Intersection Fault intersections between normal faults and transversely oriented strike-slip or oblique-slip faults account for 22% of the Basin & Range structures known to host geothermal systems as of 2011. Multiple minor faults in these areas create connections between major structures, allowing fluids to flow through highly fractured dilational quadrants. Mountainous, Horst and Graben, Flat Extensional Tectonics, Strike-Slip
Accommodation Zone Accommodation zones between overlapping, oppositely dipping normal fault systems account for 8% of all Basin & Range structures hosting geothermal systems. Multiple fault intersections in the subsurface in these zones enhances permeability and hydrothermal fluid flow. Mountainous, Horst and Graben Extensional Tectonics
Displacement Transfer Zone Displacement transfer zones represent a major subset of fault intersection types, hosting 5% of the known geothermal systems in the Basin & Range as of 2011. Intersections between strike-slip faults in the Walker Lane and N- to NNE-striking normal faults commonly host geothermal systems, focused along the normal faults proximal to their dilational intersections with nearby dextral faults. Mountainous, Horst and Graben, Flat Extensional Tectonics, Strike-Slip
Pull-Apart in Strike-Slip Fault Zone Pull-aparts are dilatational features that form along accommodation bends in strike-slip fault zones. 4% of all known Great Basin geothermal systems are located in pull-aparts, but are more abundant within or along the margins of the Walker Lane. Mountainous, Horst and Graben Strike-Slip
Intrusion Margins and Associated Fractures Hydrothermal circulation often occurs at the lithologic boundaries of igneous intrusions and along the fractures they induce in the country rock. Shield Volcano, Lava Dome, Stratovolcano, Cinder Cone, Caldera Depression Extensional Tectonics, Subduction Zone, Rift Zone, Hot Spot
Stratigraphic Boundaries Hydrothermal circulation may occur at the contacts between different lithologies. All All
Fissure Swarms Fissure swarms typically result from tensional forces in volcanic regions and are the predominant host to geothermal systems in Iceland. Shield Volcano, Lava Dome, Stratovolcano Extensional Tectonics, Hot Spot
Caldera Rim Margins Caldera rims often act as adequate hydrothermal conduits. They are particularly productive when intersected by other faults. Shield Volcano, Caldera Depression Extensional Tectonics, Subduction Zone, Rift Zone, Hot Spot
Lithologically Controlled Hydrothermal fluids may be lithologically controlled as is the case in the Salton Trough region where sandstones often lack cement (due to certain properties of the local hydrothermal fluid) allowing for increased permeability, capped by less porous shales. All Non-Tectonic, Strike-Slip

These values are part of Category: Geothermal Controlling Structures, and are used for the Property:ControlStructure.[1]

References

  1. James E. Faulds,Nicholas H. Hinz,Mark F. Coolbaugh,Patricia H. Cashman,Christopher Kratt,Gregory Dering,Joel Edwards,Brett Mayhew,Holly McLachlan. 2011. Assessment of Favorable Structural Settings of Geothermal Systems in the Great Basin, Western USA. In: Transactions. GRC Anual Meeting; 2011/10/23; San Diego, CA. Davis, CA: Geothermal Resources Council; p. 777–783