SLIP RATE OF THE WESTERN GARLOCK FAULT NEAR LONE TREE CANYON, MOJAVE DESERT, CALIFORNIA

A channel incised into a Late Pleistocene (19.1 to 11.5 C-14-ka) alluvial fan that drains southeastward from the southern Sierra Nevada has been left-laterally offset at least 66 ± 3 m across the western Garlock fault. Vertical offset of the fan into which the channel incised is negligible. The site is located in the NW1/4 of section 26, T.31S., R.36E., about 3 km west of the western end of the 3.5-km-wide step-over that separates the western segment of the Garlock fault from the central and eastern segments. Eleven trenches and pits were excavated to determine the geologic history of the site. Radiocarbon dating of detrital charcoal indicates that the channel incised into the fan deposits sometime between 11.5 C-14-ka and 7.2 C-14-ka. These constraints result in a left-lateral slip rate of at least 7.2 ± 2.1 mm/radiocarbon-year (2-sigma), and the calibrated rate is at least 6.3 ± 2.0 mm/yr (2-sigma). Additional left-lateral slip may have occurred on another fault strand, a few hundred meters to the northwest, at the range front of the southern Sierra Nevada. However, the calibrated left-lateral slip rate on the range-front fault is no more than 3.6 ± 1.1 mm/yr and could be zero. Aggradation within the offset channel continued at least until 5.5 C-14-ka, burying several Native American hearths. Another stream incision occurred in the late Holocene, and this younger channel was partially filled by 2550 ± 200 years BP (calibrated age). The projected left-lateral offset of this buried channel is 7.5 ± 5.5 m. This estimate is based on projecting the channel wall to the fault; a three-dimensional excavation was not done. At least one faulting event occurred during the filling of this channel (at 2550 ± 200 cal years BP, if the detrital charcoal samples had no inherited age at the time of deposition). At least one other faulting event occurred after the channel had filled. This suggests an average recurrence interval of 1150 to 2700 years and an average slip per event of 3.75 ± 2.75 m. The recurrence interval and slip per event could be smaller if additional faulting events occurred that were not detected in the trench. The fact that the average slip per earthquake must equal the slip rate multiplied by the average recurrence interval further constrains the average recurrence interval to be less than 1500 years, the average slip per earthquake to be at least 3.5 m, and the slip rate to be less than 8 mm/yr.