New insights about active detachment faulting in Death Valley, California
Darrel Cowan, University of Washington
Paul Bodin, University of Washington
Mark Brandon, Yale University
To test our hypothesis that detachment faults in Death Valley are active, we deployed 10 portable seismographs, which continuously recorded 3-channel short-period data at 100 samples per second, for 18 months, from July 2012 through January 2014. We relocated a sub-set of 313 earthquakes, which lie within the footprint of the portable network, using a revised 1D velocity model with individual station corrections. The largest earthquake in our dataset was M ~2.5. Using a total least squares (TLS) solution, we determine that the best-fit plane to the earthquake hypocenters dips 7.8° to the NW (azimuth 326°) beneath central Death Valley and the eastern Panamint Mountains. This azimuth is approximately parallel to the strike of the northern Death Valley fault zone. Our result is entirely compatible with: (1) GPS velocities, from sparse stations in and near Death Valley, of ca. 2 mm/yr NW; and (2) earlier studies, based on stratigraphic evidence and regional structural restorations, hypothesizing ten's of kilometers of late Cenozoic dextral transport of tectonic elements. We infer that the Death Valley pull-apart basin is currently opening by oblique slip on (1) an active, but blind, detachment fault, and (2) the system of normal faults bounding the western front of the Black Mountains.
A long-standing question concerns the state of stress attending slip on gently dipping normal faults. At Mormon Point, the Quaternary sediments in the hanging wall above the Mormon Point detachment, which dips NW about 25°, contains dominantly synthetic, and a few antithetic, steeply dipping normal faults with offsets of a few cms to perhaps tens of cms. Groups of students in a senior-level class at the University of Washington have measured the attitudes of the steep faults and stereographically analyzed them to test the hypothesis that they formed in an Andersonian stress state. The analysis involves simply plotting by hand the poles to synthetic and antithetic faults, fitting great circles to the poles, and determining the bisector of the dihedral angle between the great circles. An Andersonian state would predict that the bisector, sigma-1, is perpendicular to the earth's surface. Sigma-1 at Mormon Point plunges ca. 83-88° northwest. We conclude that slip on the Mormon Point detachment is occurring as Anderson would predict.