Placing the Paleo-Pacific Plate in Paleolatitude and Paleo-Declination

Traditional paleomagnetic methods must overcome many challenges to establish an appropriate paleolatitude framework for oceanic plates, in particular the Pacific plate, the largest oceanic plate on the planet. Drill cores are generally azimuthally unoriented and thus constrain only one of two degrees of freedom. Paleolatitudes from sediments suffer from inclination shallowing, although new methods may allow such a bias to be partly or fully corrected. Drill cores of volcanic rock give a small number of independent samples of geomagnetic secular variation that only modestly average secular variation. Because of secular variation, even the best data from drill cores of volcanic rocks give highly uncertain estimates of paleolatitude and tell us little or nothing about paleo-declination. For these reasons, various research groups, including ours, have sought methods for positioning the Pacific plate relative to the spin axis that overcome the limits of traditional paleomagnetic methods. Analysis of the skewness of marine magnetic anomalies, proposed and originally applied in the 1970s, has allowed determination of 9 paleomagnetic poles from the Cenozoic and Late Cretaceous sequence of magnetic anomalies due to seafloor spreading. The geometry of Pacific-Farallon spreading over this time results in confidence limits that are more compact than those for continental paleomagnetic poles. Moreover, because broad swaths of seafloor, in stark contrast to seamounts, are approximately planar, magnetic overprints cause no bias in the poles that are determined. A second approach for estimating paleo-spin axis locations come from the determination of location and orientation of paleo-equators from sediment accumulation rates, as pioneered by T. Moore and colleagues. Such analysis has several advantages over paleomagnetism. First, negligible time-averaging is required to create a precise record of the location of the paleo-equator when compared with the time scales of secular variation. Second, unlike the paleomagnetic dipole assumption, there are no known biases that affect the estimated location of the paleo-equator. We will further elaborate on these points, compare the results of these and other methods, and explore the implications for the Paleogene global paleolatitude framework and plate circuit.