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Description
The Helendale fault extends from the Mojave River area southeastward through Lucerne Valley into the San Bernardino Mountains. The fault is the westernmost of a series of sub-parallel, northwest-striking, right-lateral faults in the central Mojave Desert which have been zoned by the State of California as requiring further study under the Alquist-Priolo Earthquake Fault Zoning Act. I characterize the fault in the Lucerne Valley region using seismicity, aerial photography, and by exposing the fault in several exploratory trenches. The surface trace of the fault is expressed as a zone of discontinuous scarps, tonal and vegetation lineaments, right-laterally-offset drainages and ridges, linear drainages, sidehill benches, breaks-in-slope, and shutter ridges. The fault displaces alluvium of Pleistocene and Holocene age. Several seasonally active artesian springs delineate portions of the fault in this area. I studied the subsurface character of the fault in detail at two locations. A trench was excavated near Rabbit Springs Ranch across a 0. 5 m high scarp and exposed lacustrian and alluvial sediments which were plastically deformed. Two zones of open fractures associated with the formation of the surface scarp are present in the lacustrian units directly below the scarp. A charcoal sample collected within deformed sediments above the fractures yielded a calibrated age of 2330+28/-189 yrs BP, indicating rupture during the late Holocene. Evidence for several surface rupturing events is apparent in trenches excavated near Waverly Road. At this site the fault is observed to be nearly vertical and offsets or truncates alluvial sediments in a brittle fashion. Fissures lined with carbonate and filled with sediment are resheared to within 0.2 m of the surface, higher than the level of carbonate cementation. This observation suggests two events with sufficient time to allow for the accumulation of the carbonate lining. I interpret the reshearing to be the same event dated at Rabbit Springs, as well as in an adjacent trench (T-3) at Waverly Road. The faulting in T-3 ruptures nearly to the surface in a brittle manner; charcoal collected from 1.5 m depth is dated at 4415+445/-494 yrs BP, again indicating late Holocene rupture. One sample from the innermost lining of one fissure fill yielded a 14C date of 9230 ± 100 yrs BP, suggesting an event at about this time assuming the carbonate was in equilibrium with the atmosphere. Alternatively, the actual age of the event could be 1-2 ka younger, depending on the amount of inherited age for the carbonate. In any case, these data suggest two Holocene surface ruptures in the Lucerne Valley area with evidence of both plastic and brittle deformation in the near surface.
