Abstract
Geological and geophysical studies along the entire length of the Line Islands were undertaken in order to test the hot spot model for the origin of this major linear island chain. Volcanic rocks were recovered in 21 dredge hauls and fossiliferous sedimentary rocks were recovered in 19 dredge hauls. Volcanic rocks from the Line Islands are alkalic basalts and hawaiites. In addition, a tholeiitic basalt and a phonolite have been recovered from the central part of the Line chain. Microprobe analyses of groundmass augite in the alkalic basalts indicate that they contain high TiO (sub 2) (1.0-4.0 wt %) and Al (sub 2) O (sub 3) (3.4-9.1 wt %) and are of alkaline to peralkaline affinities. Major element compositions of the Line Islands volcanic rocks are very comparable to Hawaiian volcanic rocks. Trace element and rare earth element analyses also indicate that the rocks are typical of oceanic island alkalic lavas; the Line Islands lavas are very much unlike typical mid-ocean ridge or fracture zone basalts. Dating of these rocks by (super 40) Ar- (super 39) Ar, K-Ar, and paleontological methods, combined with Deep Sea Drilling Project data from sites 165, 315, and 316 and previously dated dredged rocks, provide ages of volcanic events at 20 localities along the chain from 18 degrees N to 9 degrees S, a distance of almost 4000 km. All of these dates define mid-Cretaceous to late Eocene edifice or ridge-building volcanic events. Eocene volcanic events took place from 15 degrees N to 9 degrees S, and Late Cretaceous events took place from 18 degrees N to 9 degrees S. In the southern Line Islands both Cretaceous and Eocene events took place on the same edifice or ridge, indicating recurrent volcanism at a single locality. The irregular distribution of atolls in the chain, the fact that Late Cretaceous reefs flourished along a distance of approximately 2500 km in the central and southern Line Islands, and the observation that spatially closely related seamounts exhibit different subsidence histories are interpreted as indicating that large segments of the chain have not followed a t (super 1/2) related subsidence path. Magnetic surveys of 11 seamounts show that four of the seamounts, from the central Line chain, give virtual geomagnetic poles which fall well to the north of virtual geomagnetic poles of Cretaceous seamounts. These four poles agree with other paleomagnetic data of middle-late Eocene-early Oligocene age from the Pacific. One of these four seamounts yielded a (super 40) Ar- (super 39) Ar total fusion age of 39 Ma. Because the poles of all four seamounts fall into a tight group we infer that they are probably of middle-late Eocene age to early Oligocene age. The other three seamounts as well as one seamont from the Line Islands analyzed previously all give virtual geomagnetic poles which agree with Late Cretaceous paleomagnetic data from the Pacific. Of these four seamounts, three give Late Cretaceous (super 40) Ar- (super 39) Ar ages ranging from 71 to 85 Ma; another, in the southern Line Islands, is interpreted, on paleontological evidence, to be of Late Cretaceous age. The origin of the Line Islands has been ascribed by previous workers to the effects of a single hot spot and to the action of four hot spots. The single hot spot model cannot account for all of the volcanic edifices in the Line Islands, although it does explain a general age progression of 9.6 + or - 0.4 cm/yr from north to south along the chain derived from a number of dated edifices. The four hot spot model accounts for more of the dated volcanic edifices but still does not explain all of the available data. The petrologic data argue against a mid-ocean ridge or transform fault origin proposed by earlier workers. The complex volcanic province represented by the Line Islands remains a challenge to existing models for the origin of midplate volcanism in the Pacific. An atoll drilling program which could determine edifice building histories, paleolatitudes of seamount formation, and subsidence rates and patterns in the Line Islands is needed.