Abstract:
The most recent dating of Midway Island (Dalrymple and others, 1977) eliminates the need for previously proposed major Pacific plate rotation rate changes between 25 and 42 MYBP. However, the resulting constant rotation rate of .83 degrees /m.y. about a fixed Hawaiian hotspot will not place the Pacific paleo-equatorial sedimentation rate maximum on the equator for times older than 25 MYBP (Van Andel and others, 1975). This discrepancy can be eliminated if the Hawaiian hotspot and therefore the rotation rate frame of reference relative to the earth's equator moved south by 6 degrees between 42 and 25 MYBP. Using the constant rotation rate of .83 degrees /m.y., the equatorial sedimentation rate maxima indicate that 3.5 degrees of the latitudinal shift occurred abruptly upon reorientation of plate motions at 42 MYBP, followed by a further more gradual shift of 2.5 degrees between 42 and 25 MYBP. The 2.5 degrees shift shows up as an offset in the present Hawaiian island chain trend (the last 25 m.y.) relative to the interval between 42 and 25 MYBP. Independent evidence for motion of the hotspot reference frame relative to the earth's poles is provided by paleomagnetic inclination data. Recent drilling of Suiko seamount (IPOD Leg 55) suggests a southward shift of the Hawaiian hotspot of 6 degrees + or -4 degrees since 58 MYBP. A 6 degrees paleolatitude discrepancy is also seen in 65 m.y. old sediments from a giant piston core (LL44-GPC3) north of Hawaii. Published Pacific paleomagnetic data of sufficient precision and of the right ages are limited to 3 DSDP sites (167, 315, 317). Given a constant rotation rate of .83 degrees /m.y. about both the Hawaiian and Emperor poles, the 3 sites have minimum latitude shifts of 12 degrees , 6 degrees and 4 degrees and are all consistent with southward motion of the Hawaiian hotspot.