Geophysical investigation of basalt- to rhyolite-hosted hydrothermal fields in arc and back-arc regions of the Okinawa Trough

Abstract:

In order to understand the geological and geophysical backgrounds of arc-backarc seafloor hydrothermal systems, we performed high-resolution bathymetric and magnetic surveys on arc and back-arc hydrothermal vent fields in the southern Okinawa Trough by using the AUV URASHIMA. The Irabu hydrothermal fields (IHFs) are developed on the axial area of back-arc rift and consist of basaltic lavas. The Tarama hydrothermal field (THF) is developed between back-arc rift and arc along with dacite to rhyolite. Active hydrothermal venting was reported in both the IHF and THF. The distribution of crustal magnetization deduced from the magnetic anomaly revealed that both the IHF and THF are associated with reduced magnetization, as widely observed at lava-hosted hydrothermal fields of mid-ocean ridges (MORs). The spatial scale of low magnetization reflecting the extent of discharge zone is large compared with that found at MORs. Comparisons with detailed bathymetry and magnetization distribution of the Irabu knolls reveal that the magnetization lows are located along the rim of the cauldron structure, indicating that hydrothermal fluids rise along the caldera fault from the deep regions. These results demonstrate that reduced magnetization related to hydrothermal activity occurs in rhyolite-hosted systems as well as basalt-hosted systems, and that the caldera fault is an important pathway in the formation of large discharge zones of hydrothermal systems.