Divergence between paleomagnetic and hotspot-model–predicted polar wander for the Pacific plate with implications for hotspot fixity

Abstract

If mantle plumes (hotspots) are fixed in the mantle and the mantle reference frame does not move relative to the spin axis (i.e., true polar wander), a model of plate motion relative to the hotspots should predict the positions of past paleomagnetic poles. Discrepancies between modeled and observed poles thus may indicate problems with these assumptions, for example, that the hotspots or spin axis have shifted. In this study, I compare paleomagnetic and hotspot-model–predicted apparent polar wander paths (APWP) for the Pacific plate. Overall, the two types of APWP have similar shapes, indicating general agreement. Both suggest ~40° total northward drift of the Pacific plate since ca. 123 Ma. Offset between paleomagnetic and hotspot-predicted poles is small for the past ca. 49 Ma, consistent with fixed hotspots during that time, but the offsets are large (6–15°) for earlier times. These differences appear significant for the Late Cretaceous and early Cenozoic. During the period 94–49 Ma, the hotspot model implies the paleomagnetic pole should have drifted ~20° north without great changes in rate. Measured paleomagnetic poles, however, indicate rapid polar motion between 94 and 80 Ma and a stillstand from 80 to 49 Ma. Comparison with global synthetic APWP suggests that the 94to 80-Ma polar motion may be related to true polar wander. The stillstand indicates negligible northward motion of the Pacific plate during the formation of the Emperor seamounts. This observation is drastically different from most accepted Pacific plate motion models and requires rethinking of western Pacific tectonics. If the Emperor seamounts show relative motion of the plate relative to the Hawaiian hotspot, the implied southward hotspot motion is ~19°. Lack of a diagnostic coeval phase of polar wandering in global APWP and consideration of the significance of the Hawaiian-Emperor bend imply that true polar wander is probably not the cause. Likewise, mantle-flow models do not readily explain the large southward drift of the hotspot or its inferred large westward velocity component. Thus, current models for the formation of the Emperor seamounts appear inadequate, and new ideas and further study are needed. Comparison of the Pacific APWP with a global APWP, both rotated into the Antarctic reference frame, shows an offset of ~10°, 335 *E-mail: wsager@ocean.tamu.edu. Sager, W.W., 2007, Divergence between paleomagnetic and hotspot-model–predicted polar wander for the Pacific plate with implications for hotspot fixity, in Foulger, G.R., and Jurdy, D.M., eds., Plates, plumes, and planetary processes: Geological Society of America Special Paper 430, p. 335–357, doi: 10.1130/2007.2430(17). For permission to copy, contact editing@geosociety.org. ©2007 The Geological Society of America. All rights reserved. on May 29, 2012 specialpapers.gsapubs.org Downloaded from