Huge deep-sea landslide in the Southern Mariana Trench landward slope: a preliminary report of the R/V Yokosuka & Shinkai 6500 YK15-11 Cruise -

The R/V Yokosuka YK15-11 Cruise was done in 2015 for biological and geological survey using the Shinkai 6500 (=6K) along the southern Mariana Trench landward slope including the Shinkai Seep Field (Ohara et al, 2012, Proc. Nat. Aca. Sci. USA). We report here 6K1429 dive (observer; Ishii) on a huge submarine landslide terrain found in YK14-13 Cruise by mapping of a multi-beam sounder EM122. A small trench-parallel ridge with summit of 2300 m deep and 100 km long exists next to land side of the southwestern most Mariana Trench in collision zone of the Caroline Ridge against the Philippine Sea plate. The landslide terrain exists in the northern slope of the edifice, which is dipping toward a small basin with bottom of 5750 m deep and 70 km long. The terrain consists of two area. One is the shallow concave depression terrain of oval shape with 5 km wide, 6 km long and 600 m deep, ranging from 2700 m to 4500 m deep. The other is convex deposit terrain of fan shape with 5 7 km wide, 7 km long and 300 m high, ranging from 4500 m to 5650 m deep.The former may be induced by the deep landslide of bed rocks, while the latter may be deep sea fan or depositional mound of debris derived from the former. On the bases of EM122 data, the amount of the landslide debris was estimated as about 5 10 km3. A small tsunami might be induced by the landslide.6K1429 dive: 6K landed on the seafloor at 5645 m in the lowermost end of the mound, where she recovered serpentinized mantle peridotite and island arc type volcanic rock (hornblende andesite). Additional 4 serpentinites were collected from shallower 3 stops including final stop at 4108 m in the lower end of concave terrain. The edifice is composed of fore-arc peridotite, like serpentine seamount (Ishii, 1985, Terra Pub.) in Ogasawara forearc. The edifice was made by uplifting of forearc mantle with collision of the Caroline Ridge. During uplifting, a huge landslide occurred.