Wednesday, June 08, 2011

Structural controls on geothermal systems in the Great Basin

Why do geothermal systems exist where they do in the Great Basin? Are there common factors that might help us find more of them there and in other areas such as the Basin and Range province of Arizona?

Jim Faulds with the Nevada Bureau of Mines & Geology in Reno, is tackling these questions with a grant from the US DOE Geothermal Technologies Program. Jim presented his preliminary findings this morning at the 3rd day of the week long DOE Peer Review of funded projects. [AZGS presented on our State Geothermal Data project on Tuesday] Jim is focusing on the structural geology controls on geothermal reservoirs. Over dinner last night, he noted that a large number of the producing hydrothermal fields in the Great Basin are ‘blind’ systems, meaning they have no obvious surface signature. Many were found serendipitously, as a result of other types of exploration programs or through grid drilling of geothermal gradient holes.

Jim reported his preliminary analysis of 160 of the 250+ geothermal systems in the Great Basin with the percentage of systems occurring in each of 8 structural settings:

  • Stepovers and relay ramps 32%
  • Intersecting normal faults and transverse slip or obverse slip faults 24%
  • Normal fault termination or tip lines 19%
  • Accommodation zones 8%
  • Displacement transfer zones 5%
  • Pull-aparts 4%
  • Salients or apices of major normal faults 3%
  • Major range front normal faults 2%

He made two other observations – Quaternary faults are generally in or near these systems, and it’s rare to find geothermal systems in areas of maximum displacement or the middles of major normal faults.

This is all pointing towards a model in which geothermal systems form in areas where faulting and fracturing are more distributed through a volume of rock rather than in areas where they are concentrated in single large displacements.

If it proves out, it could offer an exploration approach applicable to blind systems in extensional regimes elsewhere including southern Arizona.

[right, geothermal systems in the Great Basin. Credit, Great Basin Center for Geothermal Energy, UNR]

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