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Characteristics and Origin of the Cracks Observed by Research Submersible "Shinkai 6500" in the Oceanward Slope of the Northern Japan Trench.

1994; Taylor & Francis; Volume: 103; Issue: 6 Linguagem: Inglês

10.5026/jgeography.103.6_706

1884-0884

Yujiro Ogawa,

Geology and Paleoclimatology Research

The cracks on the sea bottom in the middle of the oceanward slope of the northern Japan Trench off Miyako, Iwate Prefecture, in the depth of 6, 280-6, 210 m, were revisited during Research Dive 134 of “Shinkai 6500”, in July 1992. Mapping and observation including previous dives, 65, 66, 67 and 130 revealed the distribution, features and characteristics of the cracks.The cracks developed at the flank just above the steep cliffs, paralleling the main N-S trending trough of a horst-and-graben structure in the descending Pacific plate. The cracks have several meters depth and height, and are more or less straight, and continue for several tens of metres to several hundreds of metres. The predominant directions are either N-S or N20°E. Cracks are open, and not dislocated but stretched horizontally. Often they are branched into a Y-shape before merging again. Some are arranged in an en echelon fashion. A pressure ridge which was formed by compression of the previously open crack was found.Sediments in and around the cracks are soft, poorly indurated siliceous biogenic clay or claystone, and contain considerable amount of volcanic ash. Pebbles with manganese coating are abundant particularly in and around the cracks, suggesting the presence of oxidized bottom currents. The bottom current eroded the superficial part of the sediments exposing fracture cleaved mud below. Sediments that are rich in clay minerals indicate cohesion. However, if the slope suffers induced differential stress, the sediments may open up horizontally forming cracks or extensional fracture cleavages.This differential stress may be attributed to horizontal shaking during an earthquake, associated with the normal faulting of a horst-and-graben structure. Such an earthquake occurred underneath the study area on March 3, 1933, Magnitude 8.4.