DEVELOPMENT OF LUMINESCENCE DATING METHODS IN TECTONICALLY ACTIVE AND ARID REGIONS: DATING FANGLOMERATES FROM ALLUVIAL FANS, COACHELLA VALLEY, SOUTHERN CALIFORNIA

In this study, we expand the available alluvial fan chronology and refine the slip rate of the Mission Creek fault, one of the main strands of the San Andreas Fault in the Coachella Valley. We test a new luminescence dating methodology that utilizes silicate minerals extracted from the coarse-grained fanglomerate abundant in alluvial fans. We measured luminescence ages of K-rich feldspars utilizing post-infrared high-temperature infrared (pIRIR₂₂₅) protocols for both small aliquots and single grains. Our results demonstrate that single grain analyses, finite mixture model approaches, and grain-size dependent environmental radioactivity measurements are essential for age assessment of fanglomerate deposits. OSL data from twelve samples indicate that alluvial fan formation in the Coachella Valley happened during the glacial period MIS 4 (65 ± 10 ka); interglacial periods MIS 3 (51.04 ± 5.36 ka), MIS 5 (75.83 ± 8.68 ka and 100.24 ± 11.23 ka), and MIS 9 (320.17 ± 33.82 ka); and the Holocene. The calculated minimum slip rate along the Mission Creek fault in Indio Hills area at Hidden Palms is 7.0 ± 1.3 to 8.4 ± 1.4 mm/yr which is smaller than the measured slip rate along this fault at Biskra Palms. Since our OSL age provides a maximum age constraint (i.e. sediment deposition in the alluvial fan prior to offset) and hence a minimum slip rate, this rate could suggest that the slip rate along the Mission Creek fault is constant at least between Biskra Palms and Indio Hills. On the other hand, there is a possibility that the slip rate along the Mission Creek fault has slowed from Biskra Palms towards the northwest and the slip might then have been transferred to the Banning fault.