ABSTRACT:

Distribution patterns, compositional variation of ferromanganese deposits, and their implications to geological setting have been characterized in the Northwestern Pacific Ocean according to shipboard survey and shore-based analysis. Recent submersible observations with Shinkai"s, short-core drilling with Benthic Multi-coring System, and surface ship operations provided us with more details of ferromanganese deposits together with related geological data. We have carried out ICPIES chemical analysis, XRD mineralogical analysis, and cross-checking of age determination on hydrogenetic ferromanganese crusts collected from wide variety of marine geological environments from deep-sea basins to topographic highs of the area. The patterns of regional- to small-scale variability of the deposits are basically controlled by geological/topographic settings. The age determination indicated a close relationship of the deposits to the geological provinces controlled with tectonic evolution as well as local non-tectonic phenomena.

INTRODUCTION

In earlier literatures, the regional patterns of occurrences of marine ferromanganese deposits have been documented in the Northwestern Pacific areas in relation to geological setting of volcanic arcs. For example, Usui et al. (1994; 1997) and Usui and Someya, (1997) described a wide variety of iron-manganese oxide deposits of hydrogenetic, hydrothermal and earlydiagenetic origins which well related with topographic highs over submarine island-arc systems. This paper describes more details of seabed occurrences, growth history, geochemical characteristics of hydrogenetic crust and nodule deposits from three model sites;

  1. a seamount on the Pacific plate,

  2. a subsided submarine volcano in the back are,

  3. outcropped deep-sea floor at the trench.

GEOLOGICAL SETTING

The sea floor morphology of the Northwestern Pacific is characterized by several major island-arc(trench) systems. The western margin of the Pacific plate forms deep-sea basins at water depths greater than 5000 m on which several in active seamounts and ridges are located. The Izu-Bonin and Mariana Trenches lie to the east.

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