CORRELATION OF MARINE TERRACES USING LUMINESCENCE DATING TECHNIQUES TO TEST QUATERNARY SLIP RATE ESTIMATES OF THE NORTHERN SAN ANDREAS FAULT

Dating of offset marine terraces on the Northern San Andreas Fault has been hindered by technical challenges in Quaternary geochronology. U-series analyses of coral from two exposures of sediment mantling the lowest extensive terrace along the Northern California coast give an approximate age of 80,000 yr B.P., which is consistent with marine oxygen isotope substage 5a chronology. Earlier work investigated the area using optically stimulated luminescence (OSL) from quartz minerals and a single-aliquot regenerative-dose (SAR) approach to date similar localities. These data suggest that either the quartz OSL may have reached dose-saturation levels, or was poorly bleached prior to deposition, and give underestimated results compared to U-series ages. The post-infrared high-temperature infrared stimulated luminescence (pIRIR₂₉₀) technique for feldspars has been shown to produce accurate ages up to 600 ka and in suitable low-dose environments, up to ~1 Ma. This project is investigating alternative luminescence dating approaches, including pIRIR₂₉₀ dating of feldspar minerals using SAR protocols, to correlate the 5a marine terrace on both sides of the San Andreas. Under low-intensity safe-lighting conditions, samples were prepared using physical and chemical methods to isolate K-feldspar grains. Prepared samples are currently being analyzed using pIRIR₂₉₀ protocols. Ages calculated will correlate the Point Arena Terrace across the fault; and, in doing so, allow determination of Quaternary slip rates. Preliminary results suggest the Point Arena Terrace is older than calculated from previous quartz OSL analysis, indicating an older upper limit for feldspars, and success of pIRIR₂₉₀ protocols.