Near-seafloor magnetics using the AUV URASHIMA at strongly magnetized active back-arc volcanoes of the Okinawa Trough
Abstract
Near-seafloor magnetic fields near the Irabu knolls were obtained by using the AUV URASHIMA during the R/V Yokosuka cruise YK14-16. The Irabu knolls are located on the axial segment boundary of the back-arc rift zone in the Okinawa Trough and consist of basalt to andesite. Active hydrothermal vents have been confirmed at the top of the conical knoll and along the rim of the caldera. Seafloor rock samples from the same region were used for rock magnetic measurements. The integrated analysis of the magnetic anomaly and rock magnetic properties led to the following conclusions: (i) The Irabu hydrothermal fields (IHFs) are associated with reduced magnetization reflecting the hydrothermal alteration of magnetic minerals present in the crust and the deposits of non-magnetic hydrothermal material. (ii) The basaltic rocks show high natural remanent magnetization (NRM) intensity ranging from 7 to 214 A/m. The extremely strong NRM was caused by less oxidation, abundant single-domain-titanomagnetite grains formed under proper crystal growth rates, and low Ti content for titanomagnetites. These strongly magnetized host rocks produce large variations of magnetic anomalies in the Irabu knolls, resulting in a clear magnetic contrast between the IHFs and their surroundings areas. (iii) The low magnetization zones (LMZs) related to the IHFs are located at the rim of the caldera floor in an elongated direction parallel to the local strike of the caldera, and extend into the caldera wall. These observations suggest that the hydrothermal fluids ascended through the caldera fault and the occurrence of hydrothermally altered zones in both the caldera floor rim and wall. (iv) The LMZ extends across several hundred meters along the caldera rim. Compared with similar hydrothermal fields of the Hakurei and Brothers situated in other arc-back-arc volcanoes with summit calderas, it is clarified that hydrothermal systems controlled by caldera faults have horizontal spatial scale equal to or larger than those of detachment-controlled large hydrothermal fields at slow-spreading ridges such as the TAG. It is implied that the permeability structure and style of hydrothermal circulation may play important roles in the formation of the larger demagnetized hydrothermal fluid pathways at caldera-controlled systems.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMGP31D..02F
- Keywords:
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- 1517 Magnetic anomalies: modeling and interpretation;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 1519 Magnetic mineralogy and petrology;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 1525 Paleomagnetism applied to tectonics: regional;
- global;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 8137 Hotspots;
- large igneous provinces;
- and flood basalt volcanism;
- TECTONOPHYSICS