Creeping Closer: Clay Separation and X-ray Diffraction for Refined K/Ar
Dating of Fault Motion on the Creeping Central Section of the San
Andreas Fault
Abstract
Creeping faults are typically not associated with large earthquakes.
However, new K/Ar dating and biomarker maturity data on the San Andreas
Fault Observatory at Depth (SAFOD) present evidence that large
paleoearthquakes have occurred in the creeping section of the San
Andreas Fault, California. K/Ar ages of bulk samples with evidence of
coseismic heating range from 3.3 to 15.8 Ma, and argon diffusion
experiments suggest that these ages are only partially reset and the
actual event ages may be even younger. Thus, questions remain as to how
we can refine such dates to reveal the precise age and location of these
earthquakes. To refine the ages and more accurately assess seismic
hazard, we date size separates of eight samples from different sections
of the SAFOD core. Following Stokes’ Law, we split each sample into five
size fractions using hydrodynamic settling: <0.2, 0.2-0.5,
0.5-0.8, 0.8-1.4, and 1.4-2 micrometers. The finest size fractions
contain the most authigenic illite, which form during fault slip. We
determined chemical composition and separated illite polytypes using
x-ray diffraction, and also measured K/Ar ages on each sample.
Preliminary results from two scaly black fault rock samples, previously
shown to have hosted earthquakes, (3,193.69 m and 3,193.96 m along the
core) support that the finest size fractions contain the greatest ratio
of authigenic illite. With a York regression between age and detrital
illite abundance, we place the authigenic illite ages at 1.08 ± 2.40 Ma
and 0.88 ± 5.08 Ma for these two samples, and observe that the detrital
illite matches the late Cretaceous age for the country rock. This new
age estimate for the authigenic illite means that large earthquakes must
have propagated into the creeping section within the last million years.
Not only is it significantly younger than the bulk sample age, it is
recent enough that translation of faulted material from the locked
southern San Andreas fault into the creeping section cannot explain the
record. Moving forward, we will expand our procedure to include isotope
dilution for measuring K concentration and analyze the other samples
previously measured for biomarker maturity and bulk K/Ar age. Resulting
insights into the fault rock composition and the timing of past
earthquakes will be crucial in assessing the region’s seismic hazard.