REVISION OF PALEOGENE PLATE MOTIONS IN THE PACIFIC AND IMPLICATIONS FOR THE HAWAII-EMPEROR BEND

Understanding the relative motion of the Pacific, Farallon, and Vancouver plates in the Paleogene has important consequences for deciphering the relationship between absolute and relative plate motions in the Pacific Ocean basin, the history of circum-Pacific subduction and the cause of the Hawaii-Emperor Bend (HEB). We quantitatively model the Farallon-Pacific, Vancouver-Pacific and Pacific-Antarctic seafloor spreading history during the Middle Paleocene to Eocene (i.e. 62 Ma – 33 Ma). We use Hellinger’s best-fitting criteria, extended by Chang and co-authors to include uncertainties, to compute half-stage rotations based on an extensive dataset of magnetic and fracture zone identifications on the Pacific and Antarctic plates. Finite rotations cannot be directly computed in this case due to the subduction of the conjugate Farallon ridge flank.

We find a well-constrained increase in Farallon-Pacific full-spreading rates between 57.5 and ~56 Ma from 76±10 to 106±4 mm/yr, followed by a stepwise increase to 176±16 mm/yr from 49.7 to 40.1 Ma. No statistically significant change in spreading direction accompanies the increases in Farallon-Pacific spreading rates. We propose the initial acceleration in Farallon-Pacific rates (i.e. at ~57.5-56 Ma) reflects an eastward acceleration in Farallon Plate motion, as it precedes west Pacific subduction and is not associated with any significant change in Pacific-Antarctic spreading.

Our results support a major plate reorganisation around ~57.5-56 Ma, rather than at HEB formation time (~47.5 Ma). Our model suggests the HEB cannot be related to relative or absolute changes in Pacific Plate motion, based on the lack of statistically significant change in Farallon-Pacific spreading direction. This allows us to reject a recent suggestion (Koivisto et al., JGR 2014) that a combination of true polar wander and absolute plate motion changes are responsible for the HEB and the associated mismatch in paleomagnetically and hotspot track-derived paleolatitudes for the Emperor chain dated from ~82 to ~50 Ma. We attribute the formation of the HEB to the previously suggested slowdown of the southward motion of the Hawaii hotspot and plume dynamics.