Published Quaternary geochronologic data show the importance of dating geomorphic surfaces with multiple geochronometers

Peter O. Gold, & Whitney M. Behr

Published August 14, 2016, SCEC Contribution #6800, 2016 SCEC Annual Meeting Poster #090

Quaternary fault slip rates rely on geochronologic dating methods for interpreting the ages of displaced geomorphic surfaces such as alluvial fans and fluvial terraces. Most commonly employed among these are cosmogenic exposure dating, uranium-series dating, optically stimulated luminescence, and radiocarbon dating. Because the geologic samples collected may have experienced very different pre-, syn-, and post-formation histories, the dates may be interpreted as max ages, min ages, or something in between, with implications for whether a fault slip rate should be considered an upper or lower limit. For this reason, the practice of using multiple geochronometers to constrain a range of possible surface ages and slip rates is ideal and has become common enough to begin assessing how well dates obtained using different methods agree. To this end, as well as to assess the typical scatter within populations of exposure dates, we have compiled data from >120 published studies in all reporting >1500 dates of different types from >300 alluvial fans and fluvial terraces. Where two dating methods were used, it was most common to compare exposure dates to those obtained using another method. In such cases, the exposure dates were almost without exception older (likely due to inherited nuclides), and for only ~50% of these studies did the dates differ by an amount attributable to different depositional processes. On average, the standard deviation of exposure dates from clasts is 30-40% of the median age, indicating a degree of scatter that is typically addressed by excluding, on average, 25% of dates as outliers. Inheritance in exposure ages from clasts remains challenging estimate, and only in rare cases does subtracting the nuclide concentration found in the modern wash from that measured on surface bring the latter into concordance with independent dates. Cosmogenic depth profiles are useful for estimating inheritance in the clast size sampled, but 2 in 3 depth profiles exhibited distributions too scattered for a date to be calculated. Given uncertainties related to depositional process and internal scatter in individual datasets, a review of published Quaternary geochronologic data shows the importance of dating geomorphic surfaces with more than one method in order to derive a more realistic range of surface ages.

Key Words
Quaternary geochronology, cosmogenic exposure dating, OSL, U-series dating, geologic slip rates

Citation
Gold, P. O., & Behr, W. M. (2016, 08). Published Quaternary geochronologic data show the importance of dating geomorphic surfaces with multiple geochronometers. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology