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Voluminous hydrothermal systems are corresponding to the heat flow anomaly discovered from previous geophysical surveys in the Southern Okinawa Trough (SOT), which is an area with a high sedimentation rate supplied by the East China Sea shelf and the island of Taiwan. Our latest fifteen field measurements around the Geolin Mound (GLM) (24° 36.6 ‘N, 122° 53’E) are located at a sedimentary basin west of the Yonaguni Rift, which is nearby the southwestern tip of a NE-SW trending volcano trail in the SOT. The heat flow results illustrate a concentric decrease from 31,477 to 180 mW/m2 in a 1-km radius around the GLM, evidently proving the focus fluid migration along the major fluid conduit. The ultrahigh temperature observations revealed the existence of magmatic components in shallow sediments. Combining the subsurface profiles with the temperature fields around the GLM, a 0.16-km2 acoustic blanking area of the G1 fluid conduit was recognized from the chirp sonar image and numerous interpreted igneous reflections with fluids related features from seismic profiles. As a result, we proposed a conceptual model, which simultaneously illustrates the hydrothermal circulation pattern from the previous general surveys and the enormous thermal anomaly from current detailed measurements. The sill-to-sill feeding sedimentary basin of the GLM is capable of demonstrating the enormous thermal anomaly around the spreading center of the SOT and appropriately represents the potential hydrothermal discharge and recharge by heat flow distribution around the GLM.

Yonaguni Knoll IV is an active hydrothermal venting site located on the southern tip of a cross back-arc volcanic trail in the South Okinawa trough. We have compiled 22 heat-flow measurements on the seafloor around the Yonaguni Knoll IV site that reveal drastic heat-flow spatial variation. To obtain the appropriate heat-flow measurements from our recently rebuilt Lister-type marine heat probe, we have updated the processing algorithm in this study to avoid utilizing the sometimes-unstable temperature evolving data from the frictional heating stage. We have carefully calibrated and processed the observed data to yield robust estimates of the seafloor heat-flow emphasizing in particular that the previous implementation of the data reduction scheme by downsizing model parameters into two subsets to be pursued in two separate steps is not theoretically and practically essential. We also examine the trade-offs of sensitivity on model parameters constrained by the thermal decay data and reveal previously unclear knowledge concerning the intertwined mutual dependence that is critical to the performance the data reduction. A contrast of more than 3000 mW/m2 within short distances implies active local hydrothermal circulation. The observed low heat-flow area could be a cold seawater-charging site whereas Yonaguni Knoll IV might stand for a close by discharging output. Although it demands for detailed mapping to establish the 3D geometry of the local hydrothermal circulation, preliminary evidence suggests that the lateral dimension of the circulation cell is around a few kilometres.