Deposits in Magmatic Arc and Trench Systems
Nearly 30,000 km of trench floor fringe the western, northern, and eastern sides of the Pacific Ocean. About 55 percent of this contains less than 400 m of sediment. The remainder comprises trenches with deposits as thick as 2,500 m. Most of the empty or nearly empty trenches border the island arcs of the western Pacific, and include Mesozoic to late Cenozoic pelagic or hemipelagic beds. Although the Panama Trench (?) and the southern part of the Middle America french coniain 400 to 600 m ot pelagic and hemipelagic deposits, the remaining eastern Pacific trenches (i.e., southern part of the Peru-Chile Trench and the Washington-Oregon Trench) and the central and eastern segments of the northern (Aleutian) trench are filled or nearly so with a thick (1,000 to 2,000 m) two-layer sedimentary sequence. Off South America the older unit (200 to 250 m thick) is typically composed of landward dipping pelagic and hemipelagic beds of Tertiary age, whereas in the northwestern Pacific the lower layer is a hemiterrigenous unit composed of pelitic turbidites interlayered with pelagic beds. In both regions the upper unit is a wedge-shaped body of flat-lying terrigenous turbidites that thins seaward over the landward dipping older beds. Only about one million years old, the wedge formed in response to the low sea levels and to increased continental erosion that accompanied episodes of expansive continental glaciation. Similar wedges may have formed only infrequently during the Mesozoic and Tertiary.
In the recent past, many geologists have speculated that the Mesozoic eugeosynclinal rocks flanking much of the Pacific represent raised and deformed trench fillings. However, modern trenches are not of geosynclinal proportions as they are narrow troughs incapable of accumulating a section of terrigenous turbidites thicker than about 3 km. Also, because the terrigenous wedge of existing trenches may be a unique body of Pleistocene age, there is reason to believe that Mesozoic trenches, like most modern ones, may have been chiefly the site of pelagic and hemipelagic sedimentation. In recent years it has become known that trenches are not static downwarps but mark the junction of rapidly converging lithospheric plates. Hence, many engeosynclinal masses are now thought to represent tectonically offscraped and partially subducted Mesozoic trench deposits. This model greatly compounds the problem of the presumed missing pelagic deposits. During the 100 my or so involved in the formation of the typical ensimatic eugeosynclinal assemblage, the volume of pelagic and hemipelagic deposits that would have been tectonically injected into the trench by the incoming oceanic plate is itself nearly of geosynclinal proportion. The pelagic debris should be especially concentrated in the melange units. These units, however, as well as the remainder of the Pacific eugeosynclinal rocks, are composed chiefly of terrigenous detritus.
If the earth is not expanding, we reason that the paucity of oceanic off scrapings in Pacific fold belts means that the greater part of the sediment reaching a trench, whether pelagic or terrigenous, whether deposited there or carried there on the back of an oceanic plate, is subducted beneath the adjacent insular or continental crust. This conclusion also means that the bulk of the terrigenous masses of the Pacific eugeosynclines are not deformed trench deposits.