Coupling biomarker thermal maturity with K-Ar dating to assess the earthquake slip history of the central San Andreas fault

The San Andreas fault consists of a stable creeping section sandwiched between two earthquake-producing locked sections. Historical earthquakes have ruptured the northern and southern sections, but it is unclear whether large earthquakes can propagate into the stable, creeping central section. Making this distinction has significant implications for seismic hazard in California, as the maximum potential earthquake is markedly greater if the whole San Andreas fault can rupture in a single event. Here, we search for evidence of large earthquake slip along the central San Andreas fault by measuring biomarker thermal maturity on samples collected from the San Andreas Fault Observatory at Depth (SAFOD).

Biomarkers are organic molecules preserved in sedimentary rocks. Because biomarker structure and abundance are altered when heated, they are an effective tool in identifying earthquake slip in the rock record. We measured 40 samples along the entire SAFOD core and found evidence of a ~3.5 m wide zone (located at 3193 - 3196.5 m within the core) of high thermal maturity representing temperatures of over 700°C at coseismic timescales. These high thermal maturities fall within an extremely deformed ultracataclasite adjacent to the Southern Deforming Zone. Given the width of this heating signal, it likely represents the propagation of multiple earthquakes within this volume. No other section of the core, including the actively creeping Central and Southern Deforming Zones, show any evidence of coseismic heating through biomarker maturity, implying that seismic slip is localized within this section of the fault.

We couple our biomarker thermal maturity results with new preliminary K-Ar ages on SAFOD samples to date the resetting or partial resetting of fault rock ages from coseismic heating. Analyses were completed on bulk samples of the same material analyzed for thermal maturity. Two distinct populations of ages occur along the core. The younger set is restricted to the high thermal maturity region with a mean age of 3.5 Ma (2.9 - 4.0 Ma). While the older group, consist of ages that cluster around 12.2 Ma and 62.3 Ma. These younger ages likely result from thermal resetting of illite grains during coseismic heating. Therefore, we consider these as maximum ages for the large earthquakes detected within the SAFOD core.