Geological Slip Rate Estimate for the Calico Fault at Newberry Springs, California: New Age Constraints from Optically Stimulated Luminescence Dating

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We measured displacements of two distinct alluvial fans inferred to be of late Pleistocene age near Newberry Springs, California, to investigate the long-term slip rate of the Calico Fault in the Eastern California Shear Zone. To estimate their ages, we measured 10Be terrestrial cosmogenic nuclide (TCN) concentrations in depth profiles and boulders. For the older fan surface, a surface age of 346±24 ka was estimated, defining a time-average slip rate of 3.2±0.4 mm/yr, nearly twice that estimated in previous studies. TCN data from the younger alluvial fan exhibit a high degree of scatter, suggesting that older fan material could have been redeposited in the younger fan, and consequently causing inheritance of several 10Be signals and biasing the TCN age estimate. If that is the case, then the age of the younger fan surface should be younger than the TCN estimate. Therefore, to better constrain the age of the younger fan and its corresponding slip rate, we further investigate by collecting samples for optically stimulated luminescence (OSL) dating and conducting a characterization of soil physicochemical properties for comparison with regional soil geochronology. Initial analyzes of soil physicochemical properties, corroborates a younger age than the TCN estimates. Three samples for OSL dating were collected by hammering tubes into sandy layers in the alluvial deposit, in the same pit where samples for TCN were previously collected. Presently, samples are being processed and analyzed in the University of Cincinnati Geochronology Laboratory. The cross-correlation between OSL and TCN data for the same location will help to: 1) test the inheritance hypothesis, 2) interpret surface processes related to alluvial fan genesis and evolution, 3) and in addition evaluate the dating techniques sensitivities to distinct surface processes. In parallel will also validate regional soil geochronology observations. Furthermore and most significant, by improving age constraints, we will refine the average slip rate estimate for the Calico fault over the last few tens of thousands of years, which might be crucial to investigate discrepancies between geodetic and geologic slip rate estimates at the Mojave Desert section of the Eastern California Shear Zone

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Presented at the AGU Fall Meeting on December 14, 2018 in Washington, D.C.