Paleomagnetic Study of Neogene Tectonic History of Baja California, Mexico: ABSTRACT

ABSTRACT Paleomagnetic study of 20 Miocene (19 to 5 m.y.B.P.) volcanic flows and one dike in central and southern Baja California has resulted in determination of inclinations too shallow for the present latitude at all sites, suggesting a greater amount of northward movement than can be accounted for by previous studies. Natural remnant directions from 21 units at five sites (24 to 29° north lat.) suggest an average northward translation of roughly 10° since late Miocene time, and a probable 45° clockwise rotation (post 6 m.y.B.P.) of the San Ignacio flows. The Paleomagnetically indicated rate of absolute motion of the peninsula is 11 cm per year since 5 m.y.B.P. and 3.5 cm per year prior to 5 m.y.B.P., assuming an offset axial dipole. Absolute northward motion, assuming a geocentric axial dipole, is 18 cm per year from 0 to 5 m.y.B.P., and 3.5 cm per year from 5 to 19 m.y.B.P. The rates of northward motion described by Atwater and Molnar, and Dickinson for the same time spans are 3.5 cm per year and 1.5 cm per year, respectively. Possible solutions to this discrepancy are: (1) Baja California is part of a broad shear zone of the plate margin, and has had more movement along faults within the proto-gulf and the present margin of western Mexico than previously deduced, (2) the North American plate has no northward motion, or (3) the North American plate has had northward motion since the Miocene, with the amount of motion of the plate margin being equal to that described by T. Atwater and P. Molnar; thus, the paleomagnetic data show both motions. Studies by M. J. Kamerling and B. P. Luyendyk, and continued paleomagnetic studies at San Diego State University, show a comparable amount of northward motion for southern California and northwestern Mexico during the Miocene. Paleomagnetic results would imply that the present palinspastic reconstructions have not completely resolved the tectonic framework of the Pacific-North American plate boundary.