The Importance of Magnetostratigraphy for Studies of Tectonic Rotations: Examples from the Mio-Pliocene of California

Abstract

Magnetostratigraphy provides a means of identifying paleomagnetic samples with a strong secondary overprint as well as a means of determining whether the time-averaged field can be assumed to be a geocentric axial dipole. Two Neogene sedimentary sequences in California provide examples of the application of magnetostratigraphy to studies of tectonic rotations. In the case of the Ridge Basin sediments of southern California, the magnetostratigraphic analysis leads to a new interpretation of previously published data. In the case of the Purisima Formation of coastal central California, it provides evidence that an observed rotation of 50° in less than 3.4 million years is due to tectonic rather than geomagnetic or paleomagnetic factors. The rate of rotation of the Purisima Formation is among the highest rates ever documented and demonstrates that for older rocks it may be impossible to distinguish clockwise rotations from counterclockwise rotations.