An exploration of hydrothermal deposits by using a deep towed sensor suite in and around the Iheya small ridge

Topographical, geomagnetic and geochemical survey in and around the Iheya small ridge, which is located on the mid Okinawa trough, are investigated by a surface ship, and a deep towed sensor suite. The active plumes are confirmed by WCIs of MBES from the surface ship in the vicinity of the southern foot of the Iheya small ridge. And a lot of whisker-like signals are also recognized on the water column part of SSS record towed at the depth of 1400m. Since it is cleared that these are signals from whole seawater, they are judged as signals caused by plumes from the sea bottom. Some plumes observed by SSS are located in the plume groups by WCIs of MBES, but others exist outside the group. The plume distributions observed by MBES and SSS are compared with the topography, the geological features estimated from the topography, magnetization structure derived from the 3D geomagnetic inversion and a score diagram of geochemistry which is an indicator of hydrothermal activity (low pH, low ORP, high Turbidity). It is found out that the plume distribution is limited to the distribution area of the latest lava field and also corresponds to the low magnetization area. Distributions of scores of 2 and 3 of geochemistry by which all three or two indicate anomalies, are also limited to the area of the latest lava field and the low magnetization area. The scores 2 and 3 do not necessarily coincide with the plume groups by MBES, but the plume distribution by SSS closely corresponds to both score. Chemical components of the plume by MBES are unclear from this survey, but it is estimated to be gas plume associated with submarine volcanic activities because it corresponds to the location of the latest lava field. On the other hand, the plume by SSS indicates the possibility to be hydrothermal plume because they can well correspond to the score of geochemistry. In the area estimated to be covered with the latest or second latest lava, the magnetization in Layer 2(shallow layer) is slightly high, and Layer 3 and deeper (deep layer) have low magnetizations. From above discussions, there is a possibility that hydrothermal alternation has been progressing.