PACIFIC PLATE MOTION AND UNDULATIONS IN GEOID AND BATHYMETRY

Previous studies have shown that the Pacific geoid and gravity fields exhibit lineated anomalies, trending approximately in the direction of absolute plate motion over the underlying mantle. Because the undulat ions obliquely cross fracture zones they have often been attributed a convective origin. Recently, lithospheric boudinage caused by diffuse extension has been proposed as a possible mechanism. We have examined the undulations in the free-air anomalies, geoid and bathymetry over a portion of the Pacific Plate to determine quantitatively how the undu lations are related to plate motion. We compare the observed data to a n axisymmetric, sinusoidal undulation defined in an arbitrary frame of reference; in particular, we seek the north pole of this reference fr ame that maximizes the correlation between data and model. Poles that are close to the Pacific hotspot pole represent copolar undulations po ssibly related to plate motion. The distance between the best-fitting poles and the hotspot pole is determined as a function of undulation w avelength and reveals several minima (with distance < 10 degrees) for discrete geoid wavebands centered on wavelengths of 160 km, 225 km, 28 7 km, 400 km, 660 km, 850 km, 1000 km and 1400 km. Bathymetry data hav e copolar bathymetric expressions as well, giving an implied admittanc e of 2-3 m/km. The most co-polar geoid/bathymetry undulations (with po les within 2-3 degrees of the average Pacific Euler pole) have wavelen gths of similar to 280 km and similar to 1050 km, respectively. The la tter could have a convective origin or be related to the spacing of ho tspot swells. The former may reflect lithospheric boudinage formed in response to diffuse extension, but could also have a dynamic origin si nce flexural dampening may only have attenuated the bathymetric amplit ude by 50% or less. Radiometric dating of volcanic ridges found in the troughs of prominent gravity lineations gives ages that correlate wel l with documented changes in Pacific and Indo/Australian Plate motion, suggesting the ridges formed in response to intermittent plate bounda ry stresses and not as a direct consequence of small-scale convection or diffuse extension.