The Geologic History of the Mid-Pacific Mountains in the Central North Pacific OceanA Synthesis of Deep-Sea Drilling Studies

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 midand 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 mesoand 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 mesoand bathypelagic water masses which generated intensive sea-floor erosion and which were probably triggered by the climatic deterioration in the polar regions. INTRODUCTION Morphology, Age, Plate-Tectonic Setting, and Previous Studies The Mid-Pacific Mountains, located south of the Hawaiian Ridge, constitute a prominent structural high on top of Mesozoic, probably Jurassic and Lower Cretaceous crust (Larson, 1976) of the central north Pacific Ocean. It is one of a series of submarine oceanic plateaus of volcanic origin that is typical of oceanic crust of Mesozoic age (Schlanger et al., 1981). These features rise as many as several kilometers above the surrounding deep ocean floor, and are capped by very thick, mostly calcareous sedimentary sequences. Many oceanic plateaus have been investigated by the Deep Sea Drilling Project, because of their peculiar structural configurations, and their volcanic rocks and sedimentary sequences, which generally are different from those Initial Reports of the Deep Sea Drilling Project, Volume 62. Department of Geology, University of Oslo, Norway. U.S. Geological Survey, Denver, Colorado. Department of Atmospheric and Oceanic Science, University of Michigan, Ann Arbor, Michigan. 5 U.S. Geological Survey, Menlo Park, California. 6 Deep Sea Drilling Project, Scripps Institution of Oceanography, La Jolla, California. of the surrounding ocean floor; Magellan Rise, Shatsky Rise, Hess Rise, Manihiki Rise, Tuamotu Plateau, and Ontong-Java Plateau are the best-known examples of oceanic plateaus in the Pacific Ocean. Most of these western Pacific oceanic plateaus are known to have a crust and early sedimentary sequence of late Mesozoic age, and to have undergone considerable subsidence from early subaerial positions to their present water depths. The sediments which accumulated after cessation of the pedestal-building volcanic activity record not only the effects of horizontal plate-tectonic movements (Lancelot, 1978), but also the effects of vertical subsidence caused by cooling of the underlying oceanic crust. The subsidence histories might be much more complicated than that of normal oceanic crust, however, because of the thermal effects of volcanic events associated with these large rises (Schlanger et al., 1981). In general, the depositional environments on top of these rises have been above the CCD since their early origins in Mesozoic time. The depositional environments at any one locale change with time and, during subsidence, the sediments in general document crossing through the shallow photic zone, with neritic faunas, into mesoand bathypelagic water depths. Therefore, the sediments consist largely of calcareous sequences, in