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Neo-tectonics research at the western edge of North America being conducted by Jon Lewis:
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| California's Coso Geothermal Field | |
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The area
of the Coso Geothermal Field of eastern California's Mojave
Desert is the site of large-scale "transtension" in response
to motion of the Sierran block with respect to the southern
Basin and Range of North America (ongoing work by Lewis
and Pluhar at UC Santa Cruz). This area provides the
opportunity to examine how the crust accommodates such boundary
conditions by using both seismicity data and the time-integrated
record of shearing deformation in rocks dominantly less than 3.4
Ma. Results from the immediate vicinity of the
geothermal field are shown in the block diagram below.
The view is toward the SSE approximately parallel to the
seismicity at Wild Horse Mesa shown on the map below.
The seismogenic strain results in the block diagram are
from Unruh et al. (2002, GSA memoir 195). The
results from brittle faults are from ongoing work. Seismic
events in the Indian Wells Valley (nearly 3500 events total, 964
events from SW Indian Wells Valley) have been analyzed and
the results suggest fine spatial-scale partitioning of strain
into two plane strains.
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| Cascadia | |
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Active deformation at the Cascadia convergent margin remains a fundamental problem for geologists and geophysicists. Using background seismicity I have explored seisomgenic strain in the shallow crust surrounding the Oregon Coast block, a kinematically distinct block underlain by anomalously thick mafic basement. My analyses take advantage of micropolar continuum theory as developed by Rob Twiss and coworkers at UC Davis. This approach entails numerically inverting earthquake focal mechanism data for best-fitting partial strain-rate tensors. The results summarized below will be published in the coming months (Lewis, Unruh & Twiss, in press, Geology). The Euler poles and associated small circles are as follow: m = McCaffrey et al. (2001, GRL), s = Savage et al. (2000, JGR) and w = Wells et al. (1998, Geology). The Euler pole of Wells et al. (1998) was modified after acceptance of the 1998 paper been and has been further updated (Wells and Simpson, 2001, Earth Space Planets). Both modified poles are closer to the Euler pole of McCaffrey et al. (2001) shown below. Our seismogenic strain results are most consistent with Eulor poles located along the eastern Oregon-Washington border. |
