The San Andreas fault system forms the boundary between the Pacific and North American tectonic plates in California (United States).
Researchers agree that this plate boundary developed about 27 million years ago and that about 315 km of horizontal offset has occurred in the Neogene period (23 million years ago to the present day).
Researchers also think that the San Andreas fault was inactive between about 23 and 50 million years ago, based on the correlation of two sandstone formations that were deposited in a deep-ocean basin about 40 million years ago.
Glenn R. Sharman and colleagues tested this correlation by "fingerprinting" these sandstone formations based on the radiometric age of individual sand grains of the mineral zircon.
They found that the ages of zircon grains were different on either side of the San Andreas fault, suggesting these sandstones were derived from different regions and were never correlative. Sharman and colleagues propose an alternative tectonic reconstruction that implies that at least 50 to 75 km of the previously unrecognized offset occurred between about 23 and 38 million years ago along the San Andreas fault or a predecessor fault.
This finding has important implications for how geoscientists understand the tectonic evolution of the western margin of North America.
A reappraisal of the early slip history of the San Andreas fault, central California, USA, Glenn R. Sharman et al., DOI:10.1130/G34214.1