Contrasting Paleo-Seismically Induced Lateral Spreading Mechanisms and Geomorphologies in Dixie Valley, NV - Geotechnical Analysis

Gigantic paleo-lateral spreads have been recognized in Dixie Valley, Nevada, covering more than 20 km² and previously interpreted to be a result of paleo-liquefaction. Dated at 2.0--2.5 ka, a 1.5 km² lateral spread with a lateral displacement of at least 43 m, and a 20 km² lateral spread with a lateral displacement of approximately 500 m were studied, identified as the Dixie Comstock site and the Dixie Meadows site respectively. Zhang and Zhao (2005) used the lateral spread database of Bartlett and Youd (1992) to estimate a greatest lateral displacement of just over 10 m, worldwide. The geothermal industry has substantial subsurface thermal and chemical proprietary data. However, prior to Watts (2008) subsurface investigations had not been carried out in order to obtain geotechnical data for these lateral spreading features. In addition to the interpretation of a liquefaction failure, previous investigators also interpreted a compressed toe at both lateral spreads, and noted the lack of a graben at the Dixie Meadows site.;Field observations, laboratory data, and analyses show that the nature of these lateral spreads is more complex than previously realized. In addition to the gravels, sands, and silts anticipated in a closed basin where alluvial fans gently interfinger with lacustrine deposits, the subsurface investigation revealed that the Dixie Comstock site is underlain by meters of clay, a material not considered to liquefy. The discovery of clay indicates that in addition to liquefaction, cyclic softening and translational/rotational failure are possible failure mechanisms.;An earthquake referred to as the Bend event has been identified along the Dixie Valley range front fault, dated at 2.0--2.5 ka and estimated magnitude of MW 7.1--7.3, is interpreted to have provided the seismic source for these failures (Caskey et al., 1996; Bell et al. 2004). Results herein indicate that given a large earthquake (> MW 5.0) and peak ground accelerations (>0.22), liquefaction, cyclic softening, and translational/rotational failure are all possible failure mechanisms at the Dixie Comstock site, likely acting in combination. Cyclic softening and liquefaction were found to be unlikely failure mechanisms at the Dixie Meadows site, where liquefaction is the preferred failure mechanism. Observations also indicate that compression at the toe of the Dixie Comstock site likely results from failed buttressing in the passive toe composed of clay. Buttressed compression in the toe at the Dixie Meadows site is not supported by data, due to a lack of deformed subsurface horizons. It is postulated that the toe at the Dixie Meadows site represents runout of liquefied material over undisturbed material, followed by differential settlement during dewatering and preserved by subsequent preferential erosion in the topographic lows. Finally, it is interpreted that the Dixie Meadows site has a graben approximately 500 m wide, partially filled and overprinted by a combination of subsequent alluvial fan action and the modern Dixie Meadows.