DIRECT DATING OF FAULT SCARPS IN UNCONSOLIDATED SEDIMENT USING COSMO-MORPHOLOGICAL EVOLUTION

 

 

Morphological dating, using fitting of scarp topographic profiles to solutions of the geomorphic diffusion equation, has long been a mainstay of paleoseismology. Unfortunately, large variability in geomorphic diffusivity limits quantification of ages using this approach. The precision of morphological dating can potentially be enhanced by combining it with measurement of cosmogenic nuclide accumulation in materials beneath the scarp. In order to test this approach, we have measured in situ ³⁶Cl in depth profiles beneath two apparent single-event scarps of known age: a Holocene terrace-riser scarp at Cajon Pass in southern California and a Late Pleistocene fault scarp in western Nevada, and a previously undated multiple-rupture scarp in central New Mexico. Comparison of the measurements for the Cajon terrace scarp with preliminary predictions based on the known scarp age and topographic profile show good agreement. The agreement is also good for two cosmogenic profiles at the Nevada scarp, but the (apparent) hanging wall profile shows a deficit of ³⁶Cl, rather than the expected excess. This may have resulted from sampling across a minor splay, rather than the main rupture. At the New Mexico scarp, cosmogenic ³⁶Cl measurements correspond well with stratigraphic indicators of multiple ruptures and has enabled establishment of a rupture chronology. These results are encouraging for routine application of cosmogenic nuclide accumulation as a complement to standard morphological dating.