The northwestern Great Basin of the western USA includes one of the youngest and
least developed parts of the Pacific – North American transform boundary. Here, a
system of right-lateral strike-slip faults known as the Walker Lane accommodates
~20% of the dextral motion between the Pacific and North American plates. The rest
of the motion is taken up along the San Andreas fault. Similar to the San Andreas,
the Walker Lane terminates northwestward, but unlike the San Andreas, the terminus
of the intra-continental Walker Lane is not accommodated in a triple junction.
Dextral shear from the Walker Lane is instead transferred to NW-directed extension
within the northwestern Great Basin.
Some of the most prolific geothermal activity in North America occurs within the
northwestern Great Basin. Magmatism is not the cause of this activity, because it
generally ceased 3-10 Ma. The abundant activity probably results from enhanced
dilation on N- to NNE-striking normal faults induced by the transfer of NW-directed
dextral shear from the Walker Lane to NW-directed extension in the Great Basin.
Although faults are known to control most geothermal activity in the Great Basin,
few detailed investigations have been conducted on specific structural controls of
individual fields. Because knowledge of such structures would facilitate
exploration models, we have embarked upon a comprehensive study of the controls on
several geothermal systems. This work includes detailed geologic mapping,
structural analysis, and geophysical investigations.
Our findings from geothermal fields in the Black Rock Desert, Pyramid Lake, and
Carson Sink regions suggest that many systems occupy discrete steps in fault zones
or lie in belts of intersecting, overlapping, and/or terminating faults. In
addition, most fields are associated with steeply dipping faults and, in many
cases, with Quaternary faults. The structural settings favoring geothermal
activity all involve subvertical conduits of highly fractured rock within fault
zones oriented approximately perpendicular to the least principal stress. Features
indicative of these settings that may be helpful in guiding exploration include: 1)
major steps in range-fronts, 2) interbasinal highs, 3) mountain ranges consisting
of relatively low, discontinuous ridges, and 4) lateral terminations of mountain