SCEC Award Number 17187 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Additional isochron burial dating of late Pleistocene alluvial surfaces to constrain long-term San Andreas fault slip rates
Investigator(s)
Name Organization
Nathaniel Lifton Purdue University
Other Participants
SCEC Priorities 1a, 5b, 5c SCEC Groups SAFS, Geology
Report Due Date 06/15/2018 Date Report Submitted 07/17/2018
Project Abstract
This project builds on results from SCEC award #15135 and aims to better constrain the age of up-lifted and deformed Quaternary alluvial surfaces between the Banning and Garnet Hill strands of the San Andreas Fault in the northern Coachella Valley, CA. Specifically, this research focuses on tightening age constraints on the Fw2 surface capping Whitewater Hill (Owen et al., 2014), using cosmogenic nuclide (26Al/10Be) isochron burial dating of clasts from a depth of ca. 10 m below the surface. Previous mapping of the of the Whitewater Hill area (Huerta, 2017) revealed a paleosol buried by ca. 80 m of sediment – burial of this former surface (Fw1 of Owen et al., 2014) was dated for the first time with this technique to 420±90 ka. However, cosmogenic nuclide estimates for the overlying Fw2 surface age remain contro-versial (Owen et al., 2014). Tighter age constraints may enable these soils to be used as piercing points to develop long-term (>100,000 year) slip rates along this section of the San Andreas fault system. Eleven cobble samples were collected in September 2016 from a horizon approximately 10 m below the Fw2 surface. Be and Al are currently being extracted from the samples at PRIME Lab and will be analyzed by Accelerator Mass Spectrometry (AMS). Results will be interpreted in terms of burial age using isochron techniques, which will then be used to better constrain Fw2 surface age. We plan to present results at the SCEC 2018 annual meeting.
Intellectual Merit This project applies a recent technique for dating mid-late Quaternary (1,000,000 – 100,000 y.b.p.) surfaces in southern California. These surfaces are common along the southern San Andreas Fault, but our inability to accurately date them has hindered interpretation of long-term slip rates along the San An-dreas and other faults. Isochron burial dating with in situ cosmogenic 10Be and 26Al has been used for the last several years, but recent advances in AMS at PRIME Lab enable dramatic improvement in detection limits, potentially pushing the lower age limits of the method to well below 100 ka. Our new, 26Al/10Be burial ages from buried paleosols and other subsurface horizons will provide invaluable geochronology from uplifted markers along the fault, and provide new, long-term slip rates that can be used for geophys-ical models and improve our understanding of the rupture history of these fault zones.
Broader Impacts This project has involved 1 MS student from Purdue University. Support for student Sarah Sams is enabling her to learn 10Be and 26Al extraction techniques while processing the samples. Moreover, this project continues a collaboration between CSU Northridge, Purdue University, and the USGS.
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