Application of uranium-series dating of pedogenic carbonate and beryllium-10 cosmogenic exposure dating to neotectonic and geochronologic problems.

摘要

Determining reliable ages for alluvial fan surfaces in arid to semi arid environments is important for many earth science applications. Herein, 230Th/U (U-series) dating of pedogenic carbonate is applied to several abandoned alluvial fans in southern California with the goal of improving our understanding of both the age of deposits, and the history of their surfaces. U-series dating of carefully selected milligram size samples of visibly pure dense pedogenic carbonate by thermal ionization mass spectrometry is found to yield reliable ages for surfaces ranging from ∼6 ka to over 100 ka. Combining U-series minimum ages with 10Be surface exposure and depth profile models can provide more accurate and precise ages than either technique could yield alone. Results suggest that surface cobbles dated by 10Be cosmogenic exposure are commonly anomalously young most likely due to partial shielding as cobbles are exhumed from depth, and that boulders are a more favorable target for surface exposure dating. Combining U-series ages from pedogenic carbonate with modeling of 10Be depth profiles is a powerful technique that can constrain both the age and erosion rate of a surface, however it is most useful when soil and geomorphic interpretations can be used to constrain model results.;The surfaces studied are alluvial fans that are offset across major strands of the San Andreas fault system in southern California, and slip rates are determined for two fault strands. The Coachella Valley segment of the San Andreas fault has a slip rate of between 12--22 mm/yr (averaged over 50 +/- 5 ka) with a preferred interpretation of 15.6 mm/yr. The southern end of the Elsinore fault has a slip rate of 1.5 +/- 0 4 mm/yr (averaged over ∼42 ka).