DEFORMATION HISTORY IN NEOGENE SEDIMENTS OF HONEY LAKE BASIN, NORTHERN WALKER LANE, LASSEN COUNTY, CALIFORNIA

Neogene sediments exposed in a broad, WNW-trending faulted anticline exposed at the southern end of Honey Lake basin, record the deformation history in this part of the northern Walker Lane. The sedimentary rocks are exposed along a N-S trending beach cliff 1 km long; this area is in the overlap zone between the southern Honey Lake fault to the west, and the northern Warm Springs Valley fault to the east. They consist of diatomite, conglomerate, sandstone, siltstone, and tephra, and are 435 m thick. Tephras in the upper third of the section are 3.06 Ma and 3.26 Ma, although projection of sedimentation rate indicates the top of the section is as young as 2.5 Ma. Dramatic soft sediment slumping in a 5 m-thick interval low in the section may represent co-seismic deformation. Stratigraphic separation is 60 – 100 m along one of the major strike-slip faults, indicating significant total slip. Faulting orientation, sense and amount of slip, and kinematic indicators were analyzed for over 200 faults to determine timing and evolution of the deformation.

There are four major faulting orientations. (1) NW-striking faults exhibit apparent dextral slip; dips are steep and appear to fan slightly across the anticline. (2) ENE-striking faults have apparent sinistral slip, and faults in several other orientations have been reactivated as sinistral faults. (3) WNW-striking conjugate thrust faults at the core of the anticline are consistent with the shortening direction documented by the anticline itself. (4) N-striking, moderately to steeply E-dipping faults appear to have originated as normal faults. Most have been reactivated as sinistral faults, especially in the southern limb of the anticline.

Field relations document the faulting history in the Honey Lake area since 2.5 Ma. Strike-slip faults are generally younger than fold-related thrust faults, although fanning of dextral faults records continued tightening of the anticline. Sinistral faults are generally younger than dextral faults as indicated by cross cutting relations. However, some thrust faults cut both sinistral and dextral faults -- consistent with tightening of the anticline -- despite a regional tectonic scheme dominated by Walker Lane tectonics. This shortening is probably related to the location of the study area in a right step of an active dextral fault system.