The geologic history of the Mid-Pacific Mountains in the central North Pacific Ocean; A synthesis of deep-sea drilling studies

Abstract

The Mid-Pacific Mountains constitute one of the largest aseismic rises in the central North Pacific Ocean. They have been generated by mid-plate volcanic events prior to Barremian time, but their volcanic activity continued through the remainder of the Cretaceous. Evidence of the latest stages of this volcanism are the trachytic ashes included in mid- and Late Cretaceous sediments and the presence of guyots atop the main volcanic pedestal. The thermal and volcanic history of the oldest part of the Pacific Plate and its plate tectonic movements since Cretaceous time have led to considerable horizontal and vertical movements of the Mid-Pacific Mountains. Reconstruction of their subsidence and evidence from the sediments from Site 463 suggest that they once, in Cretaceous time, constituted large, tropical volcanic islands which were covered by vegetation and which shed their erosional debris over the adjacent island slopes. Neritic fossils in Maastrichtian sediments document the presence of shoal areas until the end of the Mesozoic. Since then they have subsided to their present water depth. The horizontal movements of the Pacific Plate have carried the Mid-Pacific Mountains from a position well south of the Cretaceous equator to their present position under the unproductive surface waters of the subtropical central North Pacific Ocean. Site 463, on the western Mid-Pacific Mountains, probably crossed the equator in Maastrichtian time. Shallow-water-derived calcareous fossils are incorporated into the pelagic sediments covering the Mid-Pacific Mountains. They have been displaced from their source areas along the flanks of seamounts over the adjacent regions during times of low sea-level stands. Debris of land plants in Aptian sediments documents the presence of emergent volcanoes during that time. The pelagic sediments penetrated at Site 463 consist largely of a sequence of Cretaceous chalks, limestones, and cherts which accumulated fast and which document the presence of highly productive surface water masses around the former volcanic islands and above the shoals. The development of oxygen-deficient depositional environments and the lack of evidence for intensive reworking suggest at the same time very sluggish water movements in the meso- and bathypelagic environment during Early and mid-Cretaceous times. The Cenozoic calcareous oozes, on the other hand, are very condensed. They are interrupted in several places by hiatuses, and despite their position well above the CCD they show effects of dissolution and poor preservation of the calcareous faunas and floras. The frequency of reworked pelagic material together with the hiatuses indicate episodes of intensive renewal of the meso- and bathypelagic water masses which generated intensive sea-floor erosion and which were probably triggered by the climatic deterioration in the polar regions