Compaction-corrected inclinations from southern California Cretaceous marine sedimentary rocks indicate no paleolatitudinal offset for the Peninsular Ranges terrane

Paleomagnetic data have been used extensively to delineate the terrane displacement and accretion history of the western margin of North America. However, the anomalously shallow paleomagnetic inclinations used to indicate large-scale northward translation might be alternatively interpreted as due to postmagnetization tilting of batholithic rocks and compation of marine sediments. To understand the magnitude of burial compaction effects on the post-Cretaceous motion history of the Peninsular Ranges-Baja Borderlands terrane, a rock magnetic, compaction, and paleomagnetic study of the Ladd Formation and the Point Loma Formation from southern California was conducted. The anhysteretic anisotropy of remanence of the characteristic remanence-carrying grains and individual magnetic grain anisotropy were used to correct the inclinations of each formation. Individual magnetic grain anisotropy was determined by both compaction experiments and redeposition of a magnetic separate in a DC magnetic field. Standard paleomagnetic studies of the units indicated that previous Ladd Formation results could be reproduced, and a correction was made at the sample level. We were unable to adequately reproduce earlier results of the Point Loma Formation, so the average remanence anisotropy was used to correct the previously reported mean directon. The mean inclination of the Ladd Formation was corrected from 46˚ (alpha-95=8˚) to 58˚ (alpha-95=4˚), and the mean inclination of the Point Loma Formation was corrected from 39.5˚+/- 5.4˚ (normal) and -36.4˚ +/- 16.6˚ (reversed) to 56.0˚ +/- 5.1˚ (normal) and -53.0˚ +/- 16.7˚ (reversed). These results suggest that the Peninsular Ranges-Baja Borderland terrane has been part of the western North America since the Late Cretaceous and that clay-containing sedimentary rocks may typically experience from 10˚ to 15˚ of inclination shallowing due to burial compaction.