SCEC Award Number 14217 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Collaborative Research: Extending the record of incremental slip rate variation on the central Garlock fault throughout the Holocene using newly developed luminescence sediment dating
Investigator(s)
Name Organization
James Dolan University of Southern California Edward Rhodes University of California, Los Angeles Sally McGill California State University, San Bernardino
Other Participants One USC graduate student as lead author on anticipated paper. Another USC graduate student to help with fieldwork. One UCLA graduate student as lead author on anticipated 2nd paper. An additional UCLA grad student to help with fieldwork. One CSUSB undergraduate student to conduct senior thesis mapping project and to assist with fieldwork.
SCEC Priorities 1a, 2a, 4c SCEC Groups Geology, SoSAFE, SDOT
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 Our prior SCEC-funded work at the Christmas Canyon West site on the central Garlock fault developed a new single-grain feldspar IRSL (Infra-Red Stimulated Luminescence) technique (Rhodes, 2015). Application of this technique to 21 samples from alluvial deposits offset 23.5 ± 2.5 m yields a minimum slip rate for the past c. 1900 years that is nearly twice as fast as longer-term rates (Dolan et al., in prep, to be submitted May 2015). The rapid slip rate during the late Holocene corresponds to a cluster of four surface ruptures, which was preceded by a paleo-earthquake lull lasting ~ 3300 years. These observations suggest that the Garlock fault experiences strain “super-cycles” comprising multiple earthquakes and periods of strain accumulation that are considerably longer (or faster) than a simple, single-event strain accumulation–release earthquake cycle. In addition to preparation of a manuscript documenting these results, our efforts in 2014 included: (1) initial field work and sampling for IRSL dating in Pilot Knob Valley, inside China Lake Naval Air Weapons Station (results pending); (2) continued work at the Christmas Canyon site to determine mid- to late Holocene incremental slip rates which constitutes a Chapter of USC Ph.D. student Lee McAuliffe’s 2014 docctoral dissertation; and(3) completion of Thomas Crane’s CSUSB M.S. thesis which suggests a pronounced eastward decrease in the Garlock fault slip rate, with the south of the Quail Mountains about 60% and the rate in the Avawatz Mountains about 25% of the slip rate south of the Slate Range.
Intellectual Merit The degree to which fault loading and strain release rates are constant in time and space is one the most fundamental, unresolved issues in modern tectonics. Yet our current attempts to understand the mechanics of fault-system behavior remain severely data-limited. There are far too few data sets in which the timing of earthquakes can be compared directly with incremental fault slip rates, and the Christmas Canyon West site is one of them. Our results (Dolan et al., in prep.) yield a minimum slip rate of 12.8 ± 2.4 mm/yr for the central Garlock fault for the past c. 1900 years, significantly faster than longer-term (8-13 ka) rates of 6.5±1.5 mm/yr. The rapid slip rate during the late Holocene corresponds to a cluster of four surface ruptures documented at a nearby trench, which was preceded by a paleo-earthquake lull lasting ~ 3300 years that presumably corresponded to a 0 mm/yr “slip rate”. These observations suggest that the Garlock fault experiences strain “super-cycles” comprising multiple earthquakes and periods of strain accumulation that are considerably longer (or faster) than the simple, single-event strain accumulation–release earthquake cycles envisaged by classical elastic rebound theory. Moreover, the extreme variations in Garlock fault slip rate suggest that fault strength varies significantly over millennial time scales, with the fault acting stronger during lulls, and weaker during the clusters. These results have important implications for understanding of how faults store and release energy and for determining the system-level controls on earthquake occurrence in time and space.

Rhodes’s work has also led to the development of a new Infra-red stimulated luminescence (IRSL) dating technique that can be used for dating fault rupture events (Rhodes, 2015, Quaternary International; SCEC publication #2100). This K-feldspar based method is preferred in southern California over optically stimulated luminescence (OSL) of quartz, as quartz grains in tectonically active areas tend to have poor characteristics for dating. Rhodes and his students have investigated the effect of major composition, trace elements, aluminum-silicon order-disorder, and microstructures such as exsolution lamellae on the IRSL behavior of a wide range of feldspars. Measurement methods such as post-IR IRSL (pIR-IRSL), grain selection techniques such as the Super-K method and a new visual representation of acceptable contamination (the “Grid of Concern”) have been developed. The pIR-IRSL method has proven effective in a wide variety of fluvial and alluvial environments due to improvements in laboratory procedures and single-grain data analysis. Super-K (SuK) mineral separation by heavy liquid (2.565 g.cm-3) increases single-grain yield by isolating the target mineral, K-feldspar, from other feldspars and denser minerals. A two-step pIR-IRSL measurement protocol has been adopted, with IR stimulation at 50 C followed by IR stimulation at 225 C. The higher temperature signal (pIR-IRSL_225) is more stable than lower temperature IRSL measurements and results in minor (~10-15%) or, in some cases, no age correction for anomalous fading. Single-grain data analysis techniques provide higher precision dates by isolation of the well-bleached single-grain population (i.e., the grains for which the IRSL signal has been reset at time of deposition) in conjunction with a stratigraphic Bayesian age model. Procedural and analytic improvements for the single-grain pIR-IRSL technique broaden opportunities to date active faulting using sand from alluvial and fluvial deposits in trenched faults and from fluvial terraces.
Broader Impacts Our work has contributed to the professional development of numerous graduate and undergraduate students. It has been one of the projects in Lee McAuliffe’s Ph.D. dissertation (USC, 2014) and formed the basis for Thomas Crane’s M.S. thesis (CSUSB, 2014). Several UCLA graduate students (Nathan Brown, Mike Lawson and Chris McGuire) and undergaduates (Tomas Capaldi and Juliet Olsen) have also contributed to the project this year.
Exemplary Figure Figure 4. Scatter plot of qualitatively estimated ages and offset from the Slate Range West and East sites (PKV), from the Quail Mountains site (QMTNS), and from the Avawatz Moutains sections 2 and 3 (AVA). Linear regression lines were fit to each series of points, showing that the Garlock fault slip rate dramatically decreases eastward, with the rate south of the Quail Mountains being 60% and the rate in the Avawatz Mountains being 25% of the rate south of the Slate Range. (Figure from Thomas Cranes M.S. thesis, CSUSB, 2014).
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