Abstract
Surface geophysical, airgun-OBS (ocean bottom seismograph), and deep-towed surveys by R/V Kairei KR01-15 cruise, and OBS array observations were conducted to characterize geophysical features of a hydrothermal system in Suiyo Seamount as a part of Archaean Park Project. Geomagnetic results from surface and deep towed magnetometers suggest 1) the seamount was formed during Brunhes Epoch (after 0.78 My), and 2) a low magnetization area exists on its summit and is limited only on the eastern side with a diameter of 800 meters. 2-D ray tracing results, using four OBS data with a GI gun across the seamount in NE-SW direction, indicate that a P wave velocity of 2.2∿4.2 km s-1 (2 km thickness) is located all over the observation line. The OBS array observations reveal the seismic activity and the hypocenter determination indicates that the locations of these earthquakes are focused in a chimney-like shape within its diameter of 3 km just beneath the Suiyo volcanic cone at a depth of 3∿7km. We propose a model to explain these geophysical observations; the former main conduit of Suiyo Seamount is in the middle of cooling after the last eruption, and its heat drives the hydrothermal system in Suiyo Seamount. We perform a numerical simulation of a magma injection in the conduit with its diameter of 1 km and its cooling process has two different mechanisms; 1) a major hydrothermal circulation exists in the uppermost crust with P wave velocity of 2.2∿4.2 km s-1 (2 km thickness), suggesting a higher porosity region, and 2) thermal conduction is dominated in the deeper part. In the thermal conduction region, the existence of a cooling area inside and a heating area outside near the former main conduit leads to thermal differential stress, which is the cause of earthquakes below a depth of 3 km. This is a reasonable explanation of chimney-like shape hypocenter locations. The magma injection in the conduit with its diameter of 1 km has enough heat to drive hydrothermal circulation at least for a few hundreds years and the hydrothermal circulation is maintained for long times enough to make the low magnetization area with its diameter of 800 meters through the high alteration of the rock. This stable existence of the hydrothermal system in Suiyo Seamount would be an important factor to discuss regarding the origin and evolution of subsurface-vent biospheres, especially when the hydrothermal system has been isolated.
