A Geophysical Study on Spreading Ridges and Hydrothermal Deposits in the North Fiji Basin Using Sea-Surface Magnetic and Bathymetry Data

We performed the sea-surface magnetic and bathymetry data using R/V Onnuri in the North Fiji Basin (KF-1, KF-2 and KF-3 areas) in order to investigate the submarine hydrothermal deposits. The North Fiji Basin is belong to one of back-arc basins and the active basin that has generated back-arc movement through several spreading ridges from 12 Ma. The bathymetry of KF-1 shows narrow horst between two NW-SE trending grabens. The magnetic anomaly of KF-1 exhibit high values in the northern part and low values in the southern and northeastern parts of the survey area. The RTP (Reduction to the Pole) anomaly of KF-1 displays the detachment of central spreading axis consistently. The bathymetry of KF-2 represents high topography due to the triple junction and the rift valley connecting to N-S segment from the triple junction. The magnetic anomaly of KF-2 has high values in N-S segment and North Fiji Fracture Zone, shown as T-shape anomalies over the triple junction. The bathymetry of KF-3 presents a dome structure and several seamounts scattered around the spreading axis. The magnetic anomaly of KF-3 shows high values from south to center and low values in the northern part of the area. KF-1, KF-2, and KF-3 consistently display high magnetic anomalies at spreading axes. The RTP results of the study areas are similar to the result of the previous studies for the structure of the North Fiji Basin. As for magnetic data inversion, the spreading ridges of the study areas show Central Anomaly Magnetization High (CAMH) and low magnetization anomalies areas are mainly identified at the flanks along the axes, indicating possible hydrothermal deposits. The results of the 2D forward modeling along in A-A' line in KF-2, with RMS (Root Mean Square) of 4.212 nT, propose the good correlation of 2D structure model for the spreading axis.