Luminescence dating used to reconstruct fault slip rates in the Mojave Desert, California

The Mojave Desert, California, is crossed by a number of major active fault systems that have played an important role in this region's landscape development and represent a significant seismic hazard to humans. Defining rates of fault displacement and the timing of paleoseismic events is critical to test earthquake recurrence models for seismic hazard mitigation. The Garlock fault is a major active strike-slip fault in southern California, extending 250 km in a broad east-west arc from its intersection with the San Andreas fault to the southern end of Death Valley (Figure 1). In modern tectonic studies several parameters are generally unknown: 1) the degree to which slip of faults, such as the Garlock fault, is clustered into episodes of rapid movement, followed by periods of reduced activity, and 2) whether slip is accommodated by different sub-parallel faults within a single system. These issues are important for understanding fault dynamics and improving earthquake risk assessment.

The key to examining such issues is to document slip rates at a variety of timescales. However, the nature of the geomorphic features in desert regions where faulted landforms are well-preserved often makes them difficult to date reliably. Organic matter for radiocarbon dating is often rare or altogether absent in alluvial fan environments where laterally-offset geomorphic features needed for slip rate studies are found. Terrestrial cosmogenic nuclide dating of alluvial fans in arid zones works well in many cases, but can also be problematic (i.e. van der Woerd et al., 2006; Behr et al., 2010) and often associated with large uncertainties. Luminescence dating complements these other dating methods, and while it has great potential because of its reliance on commonly occurring minerals such as quartz and feldspar, it can also be problematic.

Luminescence dating relies on the fact that naturally occurring minerals such as quartz and feldspar act as dosimeters, recording a proxy for the amount of radiation that they have been exposed to during burial. As the minerals are exposed to radioactive decay from the surrounding sediment, they are able to store within their crystal structure a portion of the energy delivered by that radiation (Rhodes, 2011). This energy is released when the mineral grains are heated or exposed to sunlight. The analogy of a rechargeable battery is useful for understanding how luminescence dating works (Figure 2; Rhodes, 2011). When these mineral grains are exposed to light or heat, trapped electrons that have accumulated during burial are released, similar to a battery being depleted. Once the minerals are re-buried, exposure to natural radiation begins to recharge the mineral grains, similar to recharging a battery.

Luminescence dating is the way we detect how "charged" the battery is in order to know how long it has been "charging". After careful sample collection, being sure not to expose the grains to any light, the minerals are optically stimulated in the laboratory, which releases the stored energy in the form of light. This is the luminescence signal that is observed, and the brightness of this signal is related to the amount of radiation that the sample was exposed to during burial. If this is divided by the amount of radiation that the sample receives each year, the dose rate, then this will give the amount time that the sample has been receiving radiation. This method essentially dates the time that has passed since the sediments were buried, or the timing of deposition.

Luminescence dating can be used to date sediments on timescales of 1 to 250,000 years, with young ages being both precise and accurate (Rhodes, 2011), making it ideally suited for fault slip rate studies. However, in tectonically active areas the luminescence characteristics of quartz and feldspar grains are often not well suited for precise age estimates because they emit a very dim luminescence signal. At the El Paso Peaks paleoseismic site, a trench was re-opened to a depth of approximately five meters and a sequence of 23 samples was collected for luminescence dating, bracketed by units that have been previously dated. This site provides the opportunity to compare luminescence age estimates with a well-established radiocarbon chronology. We are aiming to refine and optimize our luminescence dating technique in southern California by testing a number of different dating protocols and dating the parameters of the luminescence measurements. The El Paso Peaks paleoseismic site, located on the central Garlock fault, was initially trenched by McGill and Rockwell (1998) to investigate paleoseismic evidence for Holocene rupture events (Figure 1). It was later re-opened by Dawson et al. (2003), resulting in a series of 31 radiocarbon dates that have identified six well-constrained earthquakes on this portion of the Garlock fault. This site is located at a small playa that is bounded to the north by the El Paso Mountains and to the southeast by a shutter ridge, and is blocked from freely draining by small, Holocene alluvial fan (Figure 3). The playa has had semi-continuous deposition throughout the Holocene, resulting in well-stratified, distinctive units of graded sand and silt interpreted to represent individual flood events or pulses of a single flood event. By studying the relationship between the different bedding layers that have been disturbed by earthquakes it is possible to put an age constraint on the timing of the faulting events that caused the earthquakes.

The characteristics of the quartz and feldspar grains from our sites in the Mojave Desert are suboptimal and difficult to work with because of dim luminescence signals. However, using the isothermal luminescence signal (Murray and Wintle, 2000), which measures the luminescence that results from heating the sample to 250°C and holding it at this temperature for 60 seconds, preliminary feldspar age estimates are in good agreement with the radiocarbon control ages (Figure 4). Further research is needed to determine the reliability of this signal, but our preliminary results are encouraging. Once we have developed the optimal dating procedure for quartz and feldspar from southern California we will apply that procedure to determine slip rates on numerous active faults in the region. This will allow us to work in areas where previous studies could not be carried out because of lack of suitable material for radiocarbon or U-series dating, as well as date beyond the age range of radiocarbon dating.

• Behr, W.M., Rood, D.H., Fletcher, K.E., Guzman, N., Finkel, R., Hanks, T.C., Hudnut, K.W., Kendrick, K.J., Platt, J.P., Sharp, W.D., Weldon, R.J., Yule, J.D., 2010. Uncertainties in slip-rate estimates for the Mission Creek strand of the southern San Andreas fault at Biskra Palms Oasis, southern California. Geological Society of America Bulletin 122, 1360-1377.
• Dawson, T.E., McGill, S.F., Rockwell, T.K., 2003. Irregular recurrence of paleoearthquakes along the central Garlock Fault near El Paso Peaks, California. Journal of Geophysical Research 108, No. B7, 2356, doi: 10. 1029/2001JB001744.
• McGill, S.F., Rockwell, T.K., 1998. Ages of late Holocene earthquakes on the central Garlock Fault near El Paso Peaks, California. Journal of Geophysical Research 103, 7265-7279.
• Murray, A.S., Wintle, A.G., 2000. Application of the single-aliquot regenerative-dose protocol to the 375C quartz TL signal. Radiation Measurements 32, 579-583.
  
• Rhodes, E.J., 2011. Optically stimulated luminescence dating of sediments over the past 200,000 years. Annual Review of Earth and Planetary Sciences 39, 461-488.
• Van der Woerd, J., Klinger, Y., Sieh, K., Tapponnier, P., Ryerson, F.J., Meriaux, A-S., 2006. Long-term slip rate of the southern San Andreas fault from 10Be-26Al surface exposure dating of an offset alluvial fan. Journal of Geophysical Research 111(B4), 17 pp.