Average geologic slip rates along the central Garlock fault, in eastern California, are thought to have been relatively steady at 5-7 mm/yr since at least the Late Pleistocene, yet present-day rates inferred from geodetic velocity fi elds are indistinguishable from zero. We evaluate the possibility of non-steady slip over millennial timescales using displaced Late Holocene alluvium along the central Garlock fault in Pilot Knob Valley. Truncation of a Late Holocene alluvial fan deposit against a shutter ridge requires a minimum of 30-37 m of displacement since deposition of the fan; maximum allowable displacement is 43-50 m. The extent of soil development atop the fan surface and optically stimulated luminescence ages bracket fan deposition between 3.5 and 4.5 ka. Together, these data require that slip rates during the Late Holocene were ~7-14 mm/yr, with a preferred rate of ~11-13 mm/yr. Our results, in conjunction with previous estimates of displacement over the past ~15 ka, require signifi cant temporal variations in strain release along the Garlock fault and confi rm previous suggestions that interactions among fault systems in eastern California give rise to alternating periods of fault activity and quiescence.

Exposed Pliocene–Pleistocene terrestrial strata provide an archive of the spatial and temporal development of a basin astride the sinistral Garlock fault in California. In the southern Slate Range and Pilot Knob Valley, an ~2000-m-thick package of Late Cenozoic strata has been uplifted and tilted to the northeast. We name this succession the formation of Pilot Knob Valley and provide new chronologic, stratigraphic, and provenance data for these rocks. The unit is divided into five members that record different source areas and depositional patterns: (1) the lowest exposed strata are conglomeratic rocks derived from Mio­cene Eagle Crags volcanic field to the south and east across the Garlock fault; (2) the second member consists mostly of fine-grained rocks with coarser material derived from both southern and northern sources; and (3) the upper three mem­bers are primarily coarse-grained conglomerates and sandstones derived from the adjacent Slate Range to the north. Tephrochronologic data from four ash samples bracket deposition of the second member to 3.6–3.3 Ma and the fourth member to between 1.1 and 0.6 Ma. A fifth tephrochro­nologic sample from rocks south of the Garlock fault near Christmas Canyon brackets deposition of a possible equivalent to the second member of the formation of Pilot Knob Valley at ca. 3.1 Ma. Although the age of the base of the lowest mem­ber is not directly dated, regional stratigraphic and tectonic associations suggest that the basin started forming ca. 4–5 Ma. By ca. 3.6 Ma, the northward progradation fanglomerate sourced in the Eagle Crags region waned, and subsequent deposition occurred in shallow lacustrine systems. At ca. 3.3 Ma, southward progradation of conglomerates derived from the Slate Range began. Circa 1.1 Ma, continued southward progradation of fanglomerate with Slate Range sources is characterized by a shift to coarser grain sizes, interpreted to reflect uplift of the Slate Range. Overall, basin architecture and the temporal evolution of different source regions were controlled by activity on three regionally important faults—the Garlock, the Marine Gate, and the Searles Valley faults. The timing and style of motions on these faults appear to be directly linked to patterns of basin development.

The Walker Lane belt and Eastern California shear zone of California, USA, are active, plate boundary–related dextral systems with transtensional and transpressional deformation, respectively. They are separated by the sinistral Garlock fault, creating a complex system without an overall integrated formation and evolution model. We examine the deformation within the eastern segment of the Garlock fault zone over geologic timescales by determining the slip history of faults. We assess the progression of faulting and associated deformation along the WSW-striking Garlock fault zone and how it applies to the overall NNW-directed dextral system. Previous studies found that large synthetic fault strands take up 30% of the slip of the Garlock fault zone and have proposed multiple mechanisms to explore how to accommodate regional NNW-directed shear across the Garlock fault without cutting its trace. We analyze an unstudied section of faulting in one of the more complex areas of regional deformation via compiled and reinterpreted published geologic data for an analysis of total and incremental slip on the main faults of the eastern Garlock fault zone. We identify geologic offset features to interpret total slip, timing, and deformation evolution. We find that 30% of the total slip of the Garlock zone occurs on strands other than the Garlock fault sensu stricto, with the locus of main slip sidestepping during the evolution of accommodation of through-going, regional dextral shear. Our results support ideas of the creation and evolution of the regional dextral system via stress concentration on a sub-Garlock lithospheric anisotropy with a resulting lowering of the plastic yield stress. Our results also show an eastward increase in fault system complexity, which may imply an underappreciated seismic hazard of the eastern Garlock fault zone.

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