Remagnetization of Cretaceous Forearc Strata on Santa Margarita and Magdalena Islands, Baja California Sur: Implications for Northward Transport Along the California Margin
Hagstrum, J T (1), Sedlock, R L (2)
(1)U.S. Geological Survey 345 Middlefield Road, Menlo Park, CA 94025 United States; (2) Department of Geology San Jose State University, San Jose, CA 95192-0102 United States
Abstract:
Paleomagnetic data for two sections of Cretaceous forearc strata with differentstructural attitudes on Santa Margarita and Magdalena Islands in Baja California Sur, Mexico, indicate that these rockshave been remagnetized, probably during the late Cenozoic. The in situ paleomagnetic directions, however, are similarto data from other Cretaceous rocks on peninsular California with unexpectedly shallow inclinations and easterlydeclinations. These data have been interpreted as indicating either northward tectonic transport ($10\deg$ to $15\deg$ oflatitude) and clockwise rotation ($\geq20\deg$), or compaction shallowing of magnetic inclinations in sedimentary rocksand southwestward tilting of plutonic rocks. The available paleomagnetic data for Cretaceous forearc strata in southernand Baja California can be divided into three groups: (1) sections with normal- polarity magnetizations that fail fold testsand are remagnetized, (2) sections with normal-polarity magnetizations with no or inconclusive fold tests that may ormay not be remagnetized, and (3) sections with both normal and reversed polarity intervals where pervasiveremagnetization has not occurred. Other rocks of the Mesozoic Great Valley Group, Coast Range ophiolite, andFranciscan Complex in California also have secondary magnetizations with directions similar to younger geomagneticfield directions. Although these widespread remagnetizations could have variable local causes, we propose regionalburial and uplift, related to changes in subduction parameters, as a possible explanation. Two episodes ofremagnetization are apparent: one in the Late Cretaceous and a second in the late Cenozoic. On the other hand, theunremagnetized and apparently reliable data from sedimentary and plutonic rocks on the Baja peninsula consistentlyindicate northward translation ($14\deg\pm 3\deg$) and clockwise rotation ($29\deg\pm 8\deg$) with respect to NorthAmerica since the Late Cretaceous.
