NEW 10BE SURFACE EXPOSURE AGES AND FAULT SLIP RATES, CUCAMONGA FAULT, SOUTHERN CALIFORNIA

Several generations of alluvial fans at Day Canyon host some the best preserved scarps of the Cucamonga Fault. This fault is part of the Sierra Madre fault system on the southern border of the Transverse Ranges, which has generated damaging historical earthquakes in the Los Angeles, California region. In addition, the Cucamonga Fault is located at the apparent junction of the Sierra Madre fault system, San Andreas Fault, and San Jacinto Fault. Here, we report two new ¹⁰Be exposure ages from the surfaces of alluvial fans mapped as Qyf2 and Qyf3. The degree of soil development and clast weathering patterns indicate that these units are younger than the fan mapped as Qyf1, where previous exposure ages yield an average value of 38 ka. A clean and consistent depth profile in the Qyf2 deposit yields an age of 27±2 ka. The depth profile in the Qyf3 deposit shows a jump in ¹⁰Be concentration at a depth of ~1.2 m, which is coincident with our observations of a change in the degree of soil development. We interpret this contact to be the boundary between the inset Qyf3 deposit and an older alluvial fan surface. The three ¹⁰Be concentrations located in the pit above this horizon, as well as a single boulder-top measurement nearby, yield exposure ages of ~15 ka. As for the scarps preserved in the Qyf2 and Qyf3 fan surfaces, previous work has demonstrated considerable dispersion in the vertical separation along the strike of the Cucamonga Fault at Day Canyon. Despite this dispersion, preliminary time-averaged slip rate estimates yield values in the range of 1 to 2 mm/yr for both Qyf2 and Qyf3, which are consistent with estimates derived from the older Qyf1 alluvial fan. These data add to the growing literature of Sierra Madre fault system behavior through time and help to better quantify the regional soil chronostratigraphy.