Geological and Topographical Features of the Zenis Ridge

The Zenis Ridge locates SE part of the Nankai trough, and appears with a NE-SW direction from Izu-Ogasawara Arc (IOA) to Shikoku Basin. R/V Yokosuka bathymetrical survey and Shinkai 6500 dive survey were carried out in order to study the lithology and tectonics around the Zenis ridge. This ridge is composed three continual blocks such as 1) NE block, 2) Central block, and 3) SW block from east to west. The water depth of this ridge increases southwestward, and maximum depth is about 4000m at the southernmost of SW block. NE block develops as a part of IOA. Steep slope developed on the northern flank, but different topographical character such as valleys with gentle slope are observed on the southern flank of this block. On the Central block, steep slope develops on the northern side, but the step terrace with gentle slope expose on the southern side of this block. The northern flank of this block was composing of fresh basaltic to andesitic volcanic rocks. These are typical Island-Arc type volcanic rock, and these rocks show 6.40-5.33Ma (K-Ar age). Well-bedded sand to mudstone exposed along the southern slope of this block. Nannofossil from these sedimentary rocks indicates Late Pliocene (CN 11b to 12a) age. Some NS trend valleys, E-W and NE-SW trend fault blocks are observed around there. Vesicomyidae clams were observed in fractured sediments distributed along this fracture zone. SW block is different from other two blocks on the topographic and geological view. This block tilt to northwestward, and thick sediment are observed by seismic data. MORB type of volcanic rock was sampled from base of this block. Some active fault was estimated around the Zenis ridge based on the seismic survey. However, result of this survey such as topographic features, discovered Vesicomyidae clams and fractured sediment distribution indicate the active fault should exist along the southern foot of this ridge and cold seepage community with active fault formed there.