GEOLOGIC STRUCTURE OF THE ISLAND-SIDE SLOPE OF THE VOLCANO TRENCH IN THE PACIFIC

Abstract

An expedition conducted in 1984 by our Institute on the R/V Akademik Aleksandr Nesmeyanov dredged the central part of the Volcano Trench. It lies between the Izu and the Mariana Trenches and is separated from the Philippine Sea by the Kazan (Volcano) island arc. This study of its island-side slope revealed two tuffaceous-sedimentary and siliceous-carbonate units. The tuffaceous sedimentary unit is over 500 m thick. It consists of greenish-gray, silty, vitric tuffs and clayey tuffites. The siliceous carbonate unit forms the lower slope and is at least 1000 m thick. The rocks here are crushed to the state of mylonite, composed of white, brownish and yellow attrition clay with chert and limestone fragments that range in size from 0.1 mm to 10 cm. The siliceous rocks consist of cryptograined silica, equivalent in composition to quartz and chalcedony and containing sporadic radiolaria and foraminifera with different degrees of preservation, as well as admixed clay and carbonate. They usually exhibit lamination due to the alternation of lenticular pure chert laminae, 1 to 2.5 cm thick, with clearly defined chert-clay, chert-carbonate and carbonate laminae. Additional aspects of the subject are discussed.