NEW GEOLOGIC SLIP RATES ALONG THE BLUE CUT FAULT, EASTERN TRANSVERSE RANGES: IMPLICATIONS FOR SLIP TRANSFER TOWARDS THE EASTERN CALIFORNIA SHEAR ZONE

In complex plate boundary fault systems like that of the San Andreas Fault in southern California, identifying where and how slip is transferred across the network is challenging due to the lack of well-constrained spatial and temporal slip rate and earthquake recurrence information on faults. To better evaluate where and how slip from the San Andreas Fault is transferred to faults of the Eastern California Shear Zone (ECSZ) we are conducting a detailed slip rate study on one of the faults of the Eastern Transverse Ranges (ETR), the province in between the San Andreas Fault and the ECSZ. Faults of the ETR, consisting of five left-lateral east west trending strike slip faults, have been hypothesized to accommodate clockwise microblock rotation allowing slip transfer between the San Andreas Fault to the west and the ECSZ to the northeast. However, the location of slip transfer and the amount of slip is poorly constrained from direct field evidence. Here, we present new slip rate data that support the model of microblock rotation within the ETR and allow for ~1.49 – 2.44 mm/yr of left-lateral motion to be accommodated by the Blue Cut Fault. The two faults within the ETR with current seismic activity and large cumulative offsets are the Pinto Mountain Fault, which bounds the northern edge of the range, and the Blue Cut Fault, which divides the region in the center. Geologic slip rates along the Pinto Mountain Fault have revealed slip from 1.3 - 3.6 mm/yr, yet no firm tectonic geomorphologic-based geologic slip rates exist along the Blue Cut Fault. Our initial ¹⁰Be dates from offsets of three different age alluvial surfaces and dating of a mixture of surface clasts including boulder tops, cobble, and amalgamated pebble samples suggest late Pleistocene slip rates of 1.84 – 2.44 mm/yr and 1.49 – 1.89 mm/yr at two sites along the central Blue Cut Fault, while further surface exposure dating is ongoing. These new rates allow us to better quantify the amount of slip transfer across the ETR towards the ECSZ. Initial slip rate data across three distinct time intervals during the late Pleistocene indicate that slip rate on the Blue Cut Fault may decrease over time, which implies that the rate of block rotation may have slowed, or that slip may have transferred from the Blue Cut Fault onto other faults in the region, such as the Pinto Mountain or Eureka Peak Faults.