Magnetostratigraphy of Neogene Sediments in the San Jacinto Fault Zone, Southern California, and Paleomagnetic Evidence for Block Rotation

SCHEUING, D. F., and L. SEEBER, Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY

A suite of oriented paleomagnetic samples from a continuous sedimentary secton in the right-lateral San Jacinto fault zone allows the interpretation of magnetic stratigraphy, documentation of the clockwise rotation of crustal blocks, and identification of several potentially adverse effects on the precision of paleomagnetic directions. Accounting for these effects has led to a clearer understanding of the structural evolution and depositional patterns within the fault zone.

The 500-m sampled interval includes all of the upper Pleistocene Ocotillo Formation (alluvial sands and interbedded shales) and the upper 25 m of the underlying Borrego Formation (lacustrine deposits). Most samples contain two magnetic components: a low-temperature component that fails the fold test (suggesting its recent acquisition), and a high-temperature component that passes the fold test. The magnetostratigraphy of the sampled interval, calibrated by vertebrate biostratigraphy, implies the base of the Ocotillo Formation is 1.25 million years old. Compared to published ages for the base of this formation towards the southeast, these new data suggest the onset of alluvial deposition was diachronous within the fault zone.

Scatter in paleomagnetic directions results from two depositional and two structural effects. Depositional effects include paleocurrent-induced grain orientation and inclination shallowing due to compaction. Structural overprints include shear associated with both faulting and folding that induces rotation of magnetic grains. Removal of samples affected by these factors suggests an average clockwise block rotation of 21 degrees plus or minus 7 degrees over the past 1.3 m.y. and further that rotation has been progressive over this interval.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)