NEW LONG AND INTERMEDIATE RANGE SLIP RATES FOR THE CENTRAL CALICO FAULT OF THE EASTERN CALIFORNIA SHEAR ZONE

The Eastern California Shear Zone (ECSZ) accommodates 10 to 20 percent of the North America-Pacific plate boundary shear. Several recent geodetic and geologic studies of the Mojave section of the ECSZ, south of the Garlock Fault, have resulted in the identification of an apparent discrepancy between the rates from the two types of data sets. This, in turn, has led to numerous studies seeking to quantify alternative mechanisms to accommodate shear not accounted for by fault offsets. In this study we are conducting additional geologic rate determinations over different time scales and on multiple faults within the Mojave section of the ECSZ. Herein, we report the preliminary results of a study of the central Calico Fault near Newberry Springs. This study included the mapping and sampling of two alluvial fan surfaces with widely varying apparent ages and offsets. The Autumn Leaf Road alluvial fan (AF) is the older of the two surfaces exhibiting a strongly developed desert pavement, common but spaced boulders of gneiss, granite, rhyolite, and basalt (all are characterized by well-developed rock varnish), and 1120 m of right lateral offset along the Calico Fault. The Poniente Drive alluvial fan (PF) is characterized by well-preserved channels and boulder trains (with little to no apparent rock varnish), and 85 m of dextral offset along the Calico Fault. Three pits, one on both sides of the AF and one on the SW side of the PF, were dug and sampled for depth profiles and Cl-36 age determinations. Twenty-five surface samples, eight from the PF surface, were collected from boulders and cobble-sized clasts of quartzite for Be-10 age dating. Preliminary ages from boulders suggest that the AF has an age of ~290 ka, yielding a rate of 3.9 mm/yr , while the PF has an age of 39–55 ka, yielding a rate of 1.5 to 2.3 mm/yr. While the PF rate is consistent with previous studies of this fault, the rate from the AF suggests a much faster earli er rate than previously suggested.