Subsections

New Terrestrial LIDAR and Cosmogenic Radionuclide Constraints on the Little Lake Fault, Eastern California Shear Zone

Colin Amos, Roland Bürgmann, G. Burch Fisher (UCSB), Dylan Rood (LLNL), Angela Jayko (USGS)

Introduction

An ever-expanding inventory of fault slip-rates spanning various time intervals reveals contrasting spatial and temporal patterns of strain for active faults within the eastern California shear zone (ECSZ). Comparison of these data, derived from geologic, geomorphic, and paleoseismic records, with geodetic estimates of fault loading affords unparalleled opportunity to investigate the dynamics of earthquake processes and the evolution of an intracontinental plate boundary fault system. This work focuses on the Little Lake fault, which occurs along the western margin of the ECSZ between the Sierra Nevada and the Coso Range in east-central California. The fault accommodates 10-20% of the total dextral motion within the ECSZ at this latitude and a smaller fraction of the relative motion between the Pacific and North American plates. Relatively high rates of decadal fault loading described for the Little Lake fault zone from GPS measurements and InSAR data (7 $\pm$ 3 mm$/$yr; Peltzer et al., 2001) suggest potential discrepancies with longer, late Quaternary records of geologic strain. To address this inconsistency, we targeted a series of previously unrecognized fluvial terraces related to overtopping and outflow from pluvial Owens Lake that cross the fault and record dextral offset since Late-Pleistocene time.

Methods

Geologic constraints on displaced Quaternary geomorphic features along the Little Lake fault zone come from geologic and geomorphic mapping and high-resolution digital topography collected using a Riegl LMS-Z420i terrestrial laser scanner (TLS).

Figure 2.3: A) 50-cm DEM of lower Little Lake wash overlain on a hillshade image, both computed from high-resolution terrestrial laser scanning (TLS) surveys. B) Geologic mapping and 50-cm elevation contours of the same area as (A), highlighting right-lateral displacement of the Qt1$/$Qt2 terrace riser by the Little Lake fault.
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Constraints on the age of offset terrace surfaces and risers come from cosmogenic $^{10}$Be samples (currently being processed) from intact, meter-scale outwash boulders preserved on terrace treads bounding each riser. Slip rates were computed using the methods and code outlined by Zechar and Frankel (2009) from probability distributions of displacement and correlated surface age.

Findings

Initial results from TLS measurements of Late-Pleistocene geomorphic surfaces on the Little Lake fault zone suggest between 33 and 38 m of reconstructed right-lateral displacement over this time period. Preliminary correlation of terrace surfaces with exposure dating of fluvially scoured basalt upstream at Fossil Falls ($\sim$16 ka; Cerling, 1990) suggests a right-lateral displacement-rate on the order of $\sim$2 mm$/$yr at 95% confidence.

Figure 2.4: Probability density functions of fault slip rates computed from offset terrace risers and cosmogenic age constraints at Fossil Falls (Cerling, 1990).
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Although more than double the previously reported rate estimate ($\geq$ 1 mm$/$yr; Roquemore, 1988) based on older, poorly constrained basalt flows, this result tentatively suggests that decadal rates of loading ($\leq$ $\sim$ 5 mm$/$yr) on the Little Lake fault zone are not sustained geologically over the $\sim$10,000 year timescale. Future work will explore the implications of this rate variation for the ECSZ and fault networks in general.

Acknowledgements

This work is supported by NSF award EAR 0847990 to Amos.

References

Cerling, T.E., Dating geomorphologic surfaces using cosmogenic He-3: Quaternary Research, 33(2), 148-156, 1990.

Peltzer, G., Crampe, F., Hensley, S., and Rosen, P., Transient strain accumulation and fault interaction in the Eastern California shear zone: Geology 29(11), 975-978, 2001.

Roquemore, G.R., Revised estimates of the slip rate on the Little Lake fault, California: Geological Society of America Abstracts with Programs, 20(3), 225, 1988.

Zechar, J.D., and Frankel, K.L., 2009, Incorporating and reporting uncertainties in fault slip rates: Journal of Geophysical Research-Solid Earth, 114, DOI 10.1029/2009JB006325.

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