Abstract:
The results from DSDP Leg 55 are a significant advance in a decade-long experiment to test the hot spot hypothesis and several of its more important corollaries for the origin of the Hawaiian-Emperor volcanic chain. Subaerial basalt flows overlain by shallow-water carbonate sediments were recovered at three sites on Ojin, Nintoku, and Suiko seamounts, 600 km, 900 km, and 1300 km, respectively, north of the Hawaiian-Emperor bend. One hundred sixty five meters of sediments and more than 100 lava flows were cored in the multiple re-entry hole on Suiko, which penetrated 550 m into the seamount. The data from Leg 55, when combined with other data, confirm the hot spot hypothesis for the origin of the Hawaiian-Emperor chain and lead to the following conclusions: 1.) The Hawaiian Islands, the seamounts on the Hawaiian Ridge, and the Emperor Seamounts are a single volcanic chain that was generated by continuous movement of the Pacific Plate relative to a single source of hot spot. 2.) Chemical, petrologic, and lithostratigraphic data indicate that the volcanoes in the chain are all very similar to those that form the main Hawaiian Islands. The volcanoes appear to be tholeiitic shields that are commonly capped by lavas of the Hawaiian alkalic suite. 3.) The existence of shallow water carbonate sediments, soils, and subaerial lava flows on Ojin, Nintoku, and Suiko seamounts, and marine seismic data from other seamounts, indicate that many of the volcanoes of the Emperor chain were once islands and have become guyots through processes associated with Darwinian subsidence. 4.) K-Ar age measurements on 27 volcanoes show that the volcanoes increase in age as a function of distance from Kilauea and indicate a velocity of volcanic propagation of about 8 cm/yr. The increase in age is approximately linear through Midway (27 m.y.) to the Hawaiian-Emperor bend (43 m.y.) and Suiko Seamount (65 m.y.). There is no indication of a change in volcanic propagation velocity at the Hawaiian-Emperor bend. 5.) Paleomagnetic data indicate that Suiko Seamount formed at a paleolatitude of 26.9 + or - 3.5 degrees N, well south of its present latitude of 44.8 degrees N but significantly north of the present latitude (19.5 degrees N) of Kilauea volcano. These data show that while the Hawaiian hot spot appears to have remained in southerly latitudes for the last 65 m.y., it has moved southward some 5 to 10 degrees. relative to the earth's axis since the formation of Suiko. 6.) Paleogene, shallow-water carbonate sediments on Suiko, Nintoku, and Ojin seamounts are of the bryozoan-algal facies and indicate formation in temperate, not tropical, waters, i.e., a mean annual surface water temperature of less than 22 degrees C. On the basis of the present knowledge of Pacific paleotemperatures, this suggest a paleolatitude of formation for Suiko of about 25 degrees N of more, which is consistent with the paleolatitude of Suiko determined from paleomagnetic data. Although the kinematic hot spot hypothesis has now been verified for the Hawaiian-Emperor chain and the question of Hawaiian hot spot fixity partly resolved, there is very little information concerning the exact mechanism responsible for the Hawaiian hot spot. We also do not know 1) whether the mechanism that creates and localizes the Hawaiian hot spot also provides the energy, the lava, or both; 2) the cause and effect relationship between the hot spot mechanism and plate motion; or 3) whether the lithosphere or asthenosphere are active or passive in the hot spot mechanism.