We studied low prairie (Mima) mounds and ridges with sorted stone borders separated by broad rubbly soil intermounds in areas near Mount Shasta, northern California. An earlier study ascribed a purely physical origin for these soil features based on a four-stage conceptual model. Mounds were interpreted as periglacially produced clay domes formed in polygonal ground and stone perimeters as loose gravity accumulations in unexplained shallow trenches at dome peripheries. The model was widely cited to account for similar stone-bordered prairie mounds and rubbly soil intermounds in the Pacific Northwest. Our observations and measurements indicate, however, that these mounded landscapes are more complex, and that a polygenetic origin best explains them. We suggest that combined bioturbation, seasonal frost action, and erosion processes, with occasional eolian inputs, best account for the mounds, their well sorted stone borders, and the poorly sorted rubbly soil intermound pavements. We propose a transitional, eight-stage conceptual model to explain this complex landscape. The model may generally explain the origin of other similar strongly bioturbated, cold winter-impacted, erosion-prone mounded tracts in the Pacific Northwest.

Neogene sediments in a structural and geomorphic high in the southwestern Honey Lake basin represent lacustrine deposition from 3.7 to 2.9 Ma, interrupted once by a significant lowstand. Tephras in the upper section are 3.26 Ma and 3.06 Ma. A thick debris-flow bed, truncated by an erosional surface and overlain concordantly by a thin interval of subaerial sediments, is evidence for lake-level fall at ca. 3.4 Ma. The dominant structure is a broad east-southeast–plunging anticline cut by several sets of faults. These include northwest-striking dextral and northeast-striking sinistral strike-slip faults and a conjugate set of west-northwest–striking thrust faults; all are consistent with north-south shortening. Mutually crosscutting relationships between faults, and tilt fanning of the dextral faults, indicate that tightening of the anticline was synchronous with faulting. A Quaternary strand of the dextral Honey Lake fault crops out near the northern end of the exposure, suggesting that the cause of the local shortening and uplift was a contractional stepover between two strands of the Honey Lake fault. The Neogene section limits this faulting to some time after 2.9 Ma. The Honey Lake basin lies at the intersection of the Walker Lane with the Sierran frontal fault system. Although the timing of tectonic disruption was roughly consistent with passage of the triple junction to the west and with uplift and exhumation of several nearby basins, the described deformation seems to be directly related to dextral faulting, dating the propagation of a strand of the Honey Lake fault through the southwestern Honey Lake basin.

The Honey Lake fault zone is one of four major, northwest-striking dextral faults that constitute the northern Walker Lane in northwestern Nevada and northeastern California. Global positioning system (GPS) geodetic data indicate that the northern Walker Lane accommodates ~10%–20% of the dextral motion between the North American and Pacific plates. Regional relations suggest that dextral movement in the Honey Lake area began ca. 6–3 Ma. Five 31.3–25.3 Ma ash-flow tuffs, totaling ~250 m in thickness, were distinguished in a paleovalley in the Black Mountain area of the Diamond Mountains, southwest of the Honey Lake fault. Four of these tuffs, totaling ~200 m in thickness, also occupy a paleovalley in the Fort Sage Mountains northeast of the fault. On the basis of the similar tuff sequences, we infer that the Diamond and Fort Sage Mountains contain offset segments of a once-continuous, westerly trending late Oligocene paleovalley. Paleomagnetic data from the 25.3 Ma Nine Hill Tuff indicate negligible vertical-axis rotation in the Diamond and Fort Sage Mountains. Correlation of the paleovalley segments in the Diamond and Fort Sage Mountains suggests 10–17 km of dextral displacement across the Honey Lake fault. About 10 km of offset is favored on the basis of constraints near the southeast end of the fault. The spread of possible offset values implies long-term slip rates of ~1.7–2.8 mm/yr for a 6 Ma initiation, and ~3.3–5.7 mm/yr for a 3 Ma initiation. These rates are comparable to slip rates inferred from Quaternary fault studies and GPS geodesy.

Abstract Within the western Great Basin, a system of dextral strike-slip faults accommodates a significant fraction of the North American–Pacific plate motion. The northern Walker Lane in northwest Nevada and northeast California occupies the northern terminus of this fault system and is one of the youngest and least developed parts of the North American–Pacific transform plate boundary. Accordingly, the northern Walker Lane affords an opportunity to analyze the incipient development of a major strike-slip fault system. In northwest Nevada, the northern Walker Lane consists of a discrete ~50-km-wide belt of overlapping, curiously left-stepping dextral faults, whereas a much broader zone of disconnected, widely-spaced northwest-striking faults characterizes northeast California. The left steps accommodate little shortening and are not typical restraining bends. The left-stepping dextral faults may represent macroscopic Riedel shears developing above a nascent lithospheric-scale transform fault. Strands of the northern Walker Lane terminate in arrays of northerly striking normal faults in the northwestern Great Basin and along the eastern front of the Sierra Nevada. These relations suggest that dextral shear in the northern Walker Lane is transferred to ~NW-SE extension in the Great Basin. Offset segments of a west-trending Oligocene paleovalley suggest ~20–30 km of cumulative dextral slip across the northern Walker Lane. Strike-slip faulting began between 3 and 9 Ma, indicating a long-term slip rate of ~2–10 mm/yr, which is compatible with GPS geodetic observations of the current strain field .