The use of paleomagnetic analysis to assess nonbrittle deformation within the San Andreas Fault Zone

Abstract

A recently developed paleomagnetic extraction technique successfully permits sample collection from unconsolidated sediments. To test the robustness of the extraction technique, as well as the resolution limits of the resultant magnetic vector directions, a total of 238 paleomagnetic samples were taken from unconsolidated strata exposed in an excavation across the San Andreas fault zone in the Carrizo Plain, California. Variable fault offset values for the most recent earthquakes at this locality favored the likelihood of rotational, nonbrittle deformation. Such deformation could possibly be recorded in the DRM (detrital remagnetization) of sediment deposited across the zone. The variability of the magnetic declination directions found for the 29 magnetically stable groups forces us to conclude that paleomagnetic results calculated in this study are not precise enough to discern reasonable (<20°) amounts of rotation. These results question a similar soft-sediment study upon which this study was modeled which claims to have successfully detected less than 20° in rotational deformation using an identical sampling method. -from Authors