COMPACTION-CORRECTED INCLINATIONS FROM SOUTHERN CALIFORNIA CRETACEOUSMARINE 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 Ame rica; 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 a nd compaction of marine sediments. To understand the magnitude of buri al compaction effects on the post-Cretaceous motion history of the Pen insular Ranges-Baja Borderlands terrane, a rock magnetic, compaction, and paleomagnetic study of the Ladd Formation and the Point Loma Forma tion from southern California was conducted. The anhysteretic anisotro py of remanence of the characteristic remanence-carrying grains and in dividual magnetic grain anisotropy were used to correct the inclinatio ns of each formation. Individual magnetic grain anisotropy was determi ned by both compaction experiments and redeposition of a magnetic sepa rate in a DC magnetic field. Standard paleomagnetic studies of the uni ts indicated that previous Ladd Formation results could be reproduced, and a correction was made at the sample level. We were unable to adeq uately reproduce earlier results of the Point Loma Formation, so the a verage remanence anisotropy was used to correct the previously reporte d mean direction. The mean inclination of the Ladd Formation was corre cted from 46 degrees (alpha(95)=8 degrees) to 58 degrees (alpha(95)=4 degrees), and the mean inclination of the Point Loma Formation was cor rected from 39.5 degrees+/-5.4 degrees (normal) and -36.4 degrees +/- 16.6 degrees (reversed) to 56.0 degrees+/-5.1 degrees (normal) and -53 .0 degrees+/-16.7 degrees (reversed). These results suggest that the P eninsular Ranges-Baja Borderland terrane has been part of the western North America since the Late Cretaceous and that clay-containing sedim entary rocks may typically experience from 10 degrees to 15 degrees of inclination shallowing due to burial compaction.