WHY IS NEVADA IN HOT WATER? RELATIONS BETWEEN PLATE BOUNDARY MOTIONS, THE WALKER LANE, AND GEOTHERMAL ACTIVITY IN THE NORTHERN GREAT BASIN

The Walker Lane (WL) accommodates dextral motion of the Sierra Nevada block (SNB) relative to the Great Basin and currently accounts for 15-25% of the Pacific-North American plate motion. Cumulative slip across the WL appears to decrease from 60-80 km to 30-40 km between W-central and NW Nevada concomitant with a decline in slip rates from ~12 mm/yr to 4-8 mm/yr. The WL terminates NW-ward in northern California near the south end of the Cascade arc and latitude of the Mendocino triple junction (MTJ), suggesting a genetic link with the San Andreas fault system (SAF). The northern Walker Lane (NWL) is therefore the youngest, least developed part of the transform boundary and permits analysis of the incipient development of a major strike-slip fault system. In NW Nevada, the NWL consists of a discrete belt of curiously left-stepping, NW-striking dextral faults, which gives way in NE California to a diffuse zone of discontinuous, widely-spaced, NW-trending faults and lineaments. This geometry may reflect progressive development of Riedel shears above an incipient lithospheric-scale transform that is propagating NW in step with N-ward migration of the MTJ, growth of the SAF, and retreat of the Cascade arc. These relations further suggest that the SNB and NW-most Great Basin are essentially one micro-plate and that the SNB has been progressively decoupling from the Great Basin as the WL has evolved.

In the northern Great Basin (NGB), relatively high rates of recent (<10 Ma) WNW-directed extension on NNE-striking normal faults appear to absorb the NW-ward declining dextral motion of the NWL, diffusing that motion into the Basin-Range. Abundant geothermal fields cluster in NNE-trending belts in the NGB (e.g. Humboldt structural zone and Black Rock Desert region) and collectively define a broad, NE-trending zone of high heat flow. Individual fields are largely controlled by NNE-striking normal faults. The WL begins losing displacement to the NW in W-central Nevada near the SE margin of the region of high heat flow. The abundant geothermal fields may therefore result from a transfer of NW-trending dextral shear in the WL to WNW extension in the NGB. Enhanced extension favors dilation and deep circulation of thermal aquifers on the NNE-striking faults. The NNE-trending belts of geothermal fields may reflect loci of strain transfer.