ABSTRACT: Volcanic Reactivation of Central Pacific Seamounts

Barbara H. Keating

Geological studies of the Line Islands seamount chain, in the central equatorial Pacific, have suggested that the chain has a very complex history. The chain consists of three provinces: a northern province consisting of long ridges (250-1200 km in length) and large seamounts (50-70 km in width), a central province of massive ridges, and a southern province dominated by small seamounts (10-15 km in width). This variation in morphology is very different from that of other seamount chains in the Pacific. SeaMARC II side-scan sonar surveys conducted over several ridges and seamounts show volcanoes intruding the carbonate caps of these structures. On one unnamed seamount in the northern part of the chain, 120 volcanic centers have been mapped, and large lava flows can be seen radiating from cones and covering the low reflectivity substrates, interpreted as carbonate bank deposits. Similar volcanic structures are present on the seafloor adjacent to seamounts.

Volcanic breccias or limestone breccias containing volcanic fragments have been obtained from every dredge recovered from this seamount group. Radiometric dating of rocks from these dredges suggests that the seamounts were initially formed in the Late Cretaceous. Eocene ages (roughly 45 my) are also documented from radiometric dating and fossil dating along the length of the seamount chain. The Eocene reactivation of seamounts coincides with the major change in plate motion recorded in the bend of the Hawaiian-Emperor seamount chain and initiation of subduction and volcanism along the Mariana and Bonin Island Arcs.

The Eocene volcanic reactivation has for the most part brecciated and destroyed the carbonate caps on these seamounts. Most of the seamounts and ridges display extremely rough topography, particularly when compared to seamounts of the Hawaiian chain. If the volcanic reactivation event is a result of stresses on the plate associated with a change in plate motion, then reactivated volcanism should be seen in other parts of the Pacific, particularly in the Gilbert-Ellice Seamount chains. Preliminary radiometric dating of rocks from some isolated dredges from that group have produced Cenozoic ages.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990