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The chain of islands that forms the Japan arc represents a segment of the orogenic belt that marks the subduction of the Pacific Ocean plates beneath the Eurasian continental plate. The islands are separated from the mainland of Asia by the Japan Sea.Profile P 849 crosses the Japan trench to the east of northern Honshu between North latitudes 39° and 40°. To provide a more coherent picture of trench morphology and trench tectonics, tracings of three additional profiles are included.

The seaward wall of the trench appears on seismic as a faulted flexure in the oceanic basement, which in this region is of Early Cretaceous age. The sediment cover, less than 500-m (1,640-ft) thick, consists of pelagic mudstones and siliceous shales. The most conspicuous features on this flexure are normal faults in a stepwise horst and graben arrangement. The spacing of the faults is between 5 and 10 km (3 and 6 mi). Individual fault throws are of the order of several hundred meters, rarely reaching 1,000 m (3,281 ft). Along this outer wall of the trench, the oceanic basement plunges over a distance of about 50 km (31 mi) from an average depth of 6 km (3.7 mi) in the West Pacific basin to a depth of nearly 8 km (5 mi) in the trench itself. This sudden plunge has been interpreted as the result of tectonic loading as observed in foredeep basins.

The landward wall of the trench is formed by an imbricated and folded wedge of sediments. Below it, the top of the oceanic basement can be traced westward to a distance of around 50 km (31 mi). The shape of the toe and the sporadic development of reflectors suggest that listric faults and submarine slides are present. This observation is in agreement with the findings of the JOIDES coreholes, which encountered only land-derived turbiditic shales in this position. Tectonic instability of the slope appears to have produced a continuous mass transport by submarine slides and turbidity flows into the trench. The pelagic sediments of the Pacific were buried below this heterogenous material derived from the imbricated wedge and the overlying Neogene and were subsequently folded and imbricated with it. This process repeated over and over again is thought to form a "melange."

Japan Sea. There, undisturbed, flat and level Neogene strata overlie oceanic basement. Folding and overthrusting of the thick Neogene sediments along the island arc indicate a renewal of compression and uplift since Pliocene time.

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