Imaging Extensional Versus Strike-Slip Tectonics in the Northern Walker Lane
John Louie, Graham Kent, and Kenneth D. Smith
Nevada Seismological Laboratory, University of Nevada, Reno
Patricia Cashman and James Trexler
Dept. of Geological Sciences and Engineering, University of Nevada, Reno
PROJECT SUMMARY
Intellectual Merit– The Walker Lane Belt of faults, exhibits a division into two types of
tectonic domain: one of east-northeast-striking faults with left-lateral displacement; and a
second of northwest-striking faults with right-lateral displacement. The north-striking Genoa
and Mount Rose fault systems form the Sierra range-front fault between Carson Valley and
Reno, Nevada. These two systems of north-striking fault strands are poorly characterized with
respect to their components of right-lateral versus normal displacement history and future
potential. New paleoseismic trenching and shallow seismic imaging of one fault in the Mount
Rose system shows pure normal displacement, at low angle. These observations point to the
need for seismic-reflection imaging of the two principal Tertiary-to-Quaternary sedimentary
basins associated with the Genoa fault and the Mount Rose fault zone, to characterize the
displacement history of the faults. The proposed reflection surveys will reveal the internal
stratigraphy and deformation of these two basins. This investigation of faulting history will test
hypotheses of the northern Walker Lane Belt developing as a rift in the earliest stages of
initiation. The basins likely record deformation history since the mid-Tertiary, and should show
the shift in style from early east-west extension to the current trans-tension.
A UNR-USGS-nees@UTexas collaboration recently performed minivibe surveys of the
basin below the Reno urban area, achieving clear stratigraphic and fault imaging despite urban
noise. A seismic acquisition firm with an established record of basin and fault imaging in
Nevada will perform 14.8 line-kilometers of innovative 2D surveying. In the first project year
Optim will survey a 7.2-km transect across the south Reno basin to characterize the history of
the Mount Rose fault zone, and an 7.6-km transect across Carson Valley to investigate the
Genoa fault. Both surveys will deploy industry-sized heavy vibrators; and, a new design of
wireless, portable 3-component (3C) geophone recorders. With the dense, 2D-3C data, Pvelocity optimization analyses by the Seismo Lab group will extend to recover S-velocity and
Poisson’s ratio sections on both lines, and use the velocity sections to derive prestack depthmigrated cross-section images of P-P, S-S, as well as P-S and S-P converted-wave reflections.
The multi-component imaging will lead to tests of reflector properties such as amplitude versus
offset analysis, and fault and stratigraphic physical properties. The 3C data will thus allow
separation of fractured from sealed faults, and sedimentary from volcanic basin stratigraphy. To
assess and locate microearthquakes along the faults, the project will deploy about a dozen L43C seismometers over a small region for 18 months, as an EarthScope/USArray Flexible Array
deployment. In combination with the reflection lines, the microearthquake records will
characterize the faults and their tectonic styles affecting the Neogene basins.
Broader Impacts– The nature and activity of the faults along the Sierra Nevada range
front are crucial inputs to an earthquake-hazard assessment of the Reno–Carson City and Lake
Tahoe urban areas of Nevada and California. The possible linking and simultaneous rupture of
the Genoa and Mount Rose fault system provides the longest and most dangerous fault rupture
threatening these urban areas, with event magnitudes of 7.5 a possibility. The proposed
reflection surveys will thus provide critical hazard assessment, helping to protect the urban
1
population and economy. USGS survey efforts will be restricted for the foreseeable future to the
denser urban areas, for the purpose of fault location. Such restrictions will not allow
investigation of fault history through the stratigraphy and structure of the south Reno and
Carson Valley basins, which are not urban. As well, the project will support graduate and
undergraduate students, helping to strengthen Nevada’s scientific and technical workforce.
2