Abstract of paper presented at Association of Engineering Geologists 1995 annual meeting, Sacramento, California.

Timing of faulting in the foundation of the once proposed Auburn dam site, California

D. K. McMillan, K-prime Geoscience Associates, Inc.
D. R. Van Alstine, Applied Paleomagnetics, Inc.

Extensive studies in the late 1970s resolved the age of all but 1 meter of net slip on the foundation fault F-1 in the now abandoned Auburn dam site. The unresolved slip is apparently younger than 130 Ma, but its minimum age could not be constrained. To elucidate timing of latest motion on F-1, we conducted a paleomagnetic study of Auburn foundation rocks and shear-zone materials, integrated with a time/temperature/uplift history based on igneous/metamorphic petrology and regional geology. If a consistent “secondary” magnetization associated with mineralization or uplift is present in the shear zone, especially in the geologically youngest “clay gouge,” this would imply that F-1 has been inactive since the magnetization was acquired.

The time/temperature/uplift history for the Auburn dam site is consistent with a nearly constant cooling rate of 5°C/m.y., from 540°C at 190 Ma (amphibolite-grade metamorphism) to surface temperature at 87 Ma (beginning of marine sedimentation in the Chico Formation). The oldest paleomagnetic signal that survived regional metamorphism is ca. 132 Ma, acquired when the Auburn section had cooled to about 250°C. The youngest paleomagnetic signal exhibits exclusively normal polarity, lies in the northwest quadrant, and was probably acquired during the Cretaceous interval of nearly constant normal polarity that persisted from 118 to 83 Ma. The Cretaceous normal-polarity magnetization is undisturbed by intense tectonic deformation immediately adjacent to F-1 and can even be recognized in the 1.5 to 6.5 cm thick “clay gouge” within F-1. Survival of Cretaceous secondary magnetization(s) in F-1 implies inactivity along F-1 during the Cenozoic and demonstrates that paleomagnetism can help constrain the paleoseismic history elsewhere along the Foothills fault system.