Possible kilometer-scale hydrothermal circulation within the Iheya-North field, mid-Okinawa Trough, as inferred from heat flow data

Abstract

We obtained 78 heat flow measurements during 2002-2008 in the area of the Iheya-North hydrothermal field in the middle Okinawa Trough, in order to clarify the spatial extent of the hydrothermal circulation system. Within a small basin surrounded by knolls, three distinct zones are identified with different heat flow values, which we termed the high-, moderate-, and low-heat-flow zones. In the high-heat-flow zone located near the western edge of the basin, extremely high and widely scattered heat flow values (0.01-100 Wm-2) were measured within ∼500 m of the active hydrothermal mounds, venting black smoker fluid of maximum 311 °C. With increasing distance east of the high-heat-flow zone, heat flow gradually decreases from 1.0 to ∼0.1 Wm-2 in a region where surface sediment is dominated by clay and a high-resolution bathymetry indicates a smooth seafloor surface. We term this area the moderate-heat-flow zone. Further to the east (∼2 km from the high-heat-flow zone), the seafloor consists of coarser sediment with a rugged surface, and heat flow is very low (<0.1 Wm-2), as designated the low-heat-flow zone. 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 high-heat-flow zone can drive this kilometer-scale hydrothermal circulation within the Iheya-North knoll complex, if the sediment below the moderate-heat-flow zone is impermeable enough to prevent vertical fluid migration but is permeable enough to encourage horizontal flow. Although some geological data such as piston core samples, surface morphology and seismic data would support this inference, this hypothesis needs to be tested through more direct evidence and by numerical simulation studies.