Abstract
Abstract. We apply cosmogenic-nuclide burial dating using the 36Cl-in-K-feldspar∕10Be-in-quartz pair in fluvially transported granitoid clasts to determine the age of alluvial sediment displaced by the Mission Creek strand of the San Andreas Fault in southern California. Because the half-lives of 36Cl and 10Be are more different than those of the commonly used 26Al∕10Be pair, 36Cl∕10Be burial dating should be applicable to sediments in the range ca. 0.2–0.5 Ma, which is too young to be accurately dated with the 26Al∕10Be pair, and should be more precise for Middle and Late Pleistocene sediments in general. However, using the 36Cl∕10Be pair is more complex because the 36Cl∕10Be production ratio varies with the chemical composition of each sample. We use 36Cl∕10Be measurements in samples of granodiorite exposed at the surface at present to validate calculations of the 36Cl∕10Be production ratio in this lithology, and then we apply this information to determine the burial age of alluvial clasts of the same lithology. This particular field area presents the additional obstacle to burial dating (which is not specific to the 36Cl∕10Be pair, but would apply to any) that most buried alluvial clasts are derived from extremely rapidly eroding parts of the San Bernardino Mountains and have correspondingly extremely low nuclide concentrations, the majority of which most likely derive from nucleogenic (for 36Cl) and post-burial production. Although this precludes accurate burial dating of many clasts, data from surface and subsurface samples with higher nuclide concentrations, originating from lower-erosion-rate source areas, show that the age of upper Cabezon Formation alluvium is 260 ka. This is consistent with stratigraphic age constraints as well as independent estimates of long-term fault slip rates, and it highlights the potential usefulness of the 36Cl∕10Be pair for dating Upper and Middle Pleistocene clastic sediments.
Highlights
36Cl/10Be burial datingIn this paper we apply the method of cosmogenic-nuclide burial dating (“burial dating”) using the 36Cl-inK-feldspar/10Be-in-quartz (36Cl/10Be) nuclide pair to determine the age of buried alluvial sediment exposed in a sedimentary section that has been offset along a strand of the San Andreas Fault in southern California
For the minority of samples in this study that have relatively high nuclide concentrations because they were sourced from relatively low-erosion-rate environments, we use a surface bedrock sample to show that production rate calculations based on sample composition and independently calibrated parameters correctly predict the 36Cl-infeldspar/10Be-in-quartz production ratio, at least for K-rich, Cl-poor feldspars characteristic of typical granitoid rocks
Geomorphic relations and provenance data indicate that upper Qo has been displaced along the Mission Creek strand of the San Andreas Fault relative to its source area by at least 4 km (Fosdick and Blisniuk, 2018)
Summary
In this paper we apply the method of cosmogenic-nuclide burial dating (“burial dating”) using the 36Cl-inK-feldspar/10Be-in-quartz (36Cl/10Be) nuclide pair to determine the age of buried alluvial sediment exposed in a sedimentary section that has been offset along a strand of the San Andreas Fault in southern California. If the half-life of the nuclide ratio is shorter, the ratio in a buried sample diverges from the production ratio faster, and, assuming that other uncertainties in production ratios and concentration measurements are equivalent, a burial age estimate based on the measured ratio is more precise at younger ages We show this mathematically by simplifying the complete equations for burial dating (Granger, 2006) by assuming that a sample experiences a two-stage burial history consisting of (i) a period of surface exposure that is short enough that radioactive decay during the exposure period can be disregarded, followed by (ii) instantaneous burial at a depth large enough that post-burial nuclide production is negligible. It is not possible to measure these properties in sediment source areas for times in the past, and, in addition, even if these properties were known, the uncertainty in estimating 36Cl production rates due to thermal neutron capture can be 25 % or more (Alfimov and Ivy-Ochs, 2009)
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