Abstract:
The Okinawa trough is a backarc basin, located between the Ryukyu arc-trench system and the Asian continent. It is considered to be in a rifting stage of the continental lithosphere. The trough contains both hemipelagic and volcanic sediments, and numerous hydrothermal sites have been discovered inside the trough. Iheya-North hydrothermal field is surrounded by the Iheya-North knolls in the middle Okinawa trough. Active chimneys as well as diffuse venting area has been located and studied in detail through various geological and geophysical surveys. To clarify the spatial scale of the hydrothermal circulation system, intensive heat flow measurements were carried out and 78 heat flow data were obtained from 2002 to 2008 in and around the knolls. In 2010, drilling study was carried out during the IODP Expedition 331, and new subbottom temperature data were obtained around the hydrothermal site. Three distinct zones are identified with different heat flow values which we termed the high-heat-flow zone (>1 W/m (super 2) ; HHZ), moderate-heat-flow zone (1-0.1 W/m (super 2) ; MHZ), and low-heat-flow zone (<0.1 W/m (super 2) ) within 3 km from the active hydrothermal field. In the HHZ located near the western edge of the basin, extremely high and widely scattered heat flow values were measured within approximately 500 m of the active hydrothermal mounds, venting black smoker fluid of maximum 311 degrees C. With increasing distance east of the HHZ, heat flow gradually decreases towards MHZ and LHZ. We suggest that such anomalously low heat flow can be explained by the recharge of seawater into the formation, and that hydrothermal vents or diffuse flow in the HHZ can drive this kilometer-scale hydrothermal circulation. During IODP Expedition 331, we carried out coring and in-situ temperature measurements in the HHZ and LHZ. We could not obtain enough core (less than 1% core recovery). In the HHZ, the temperature data showed over 55 degrees C only few meters below the seafloor. After drilling, the temperature in the bore hole at HHZ recorded around 300 degrees C, which is consistent with temperatures of black smokers. Hemipelagic sediment including pumiceous layer were found in the LHZ. The temperature at 37 m below seafloor (mbsf), was 6 degrees C. Between 70 and 90 mbsf, the coarser sediments were cored, and temperature increases from 25 degrees C to 40 degrees C. At the maximum drilled depth of 151 mbsf, the temperature was 90+ or -5 degrees C, which was measured with thermoseal strips. Low thermal gradient in the upper 40 m supports the downward fluid flow. Seismic reflection data indicate a couple of horizontal layers continuously below MHZ. these layers could inhibit vertical fluid flux. Thus we assume that vertically-impermeable surface layers serve to generate kilometer scale hydrothermal circulation. In order to estimate permeability structure below HHZ through LHZ, we conducted a 2-dimensional numerical simulation, and the results will be shown.