Vector magnetic analysis within the southern Ayu Trough, equatorial western Pacific

Abstract

We present the analysis of magnetic field data of the Ayu Trough, a divergent plate margin located in the southern Philippine Sea plate. The survey, which mapped both axial and off-axis regions of the southern Ayu Trough, recorded the total field using a proton precession magnetometer towed behind the ship and the vector magnetic field using a shipboard three-component fluxgate magnetometer together with multibeam bathymetry. The two magnetic field anomalies show good agreement, indicating that the reduction of vector field data was successful. Our inspection of E–W profiles shows that there is little correlation in magnetic anomalies between adjacent profiles and lack of symmetry with respect to the trough axis. However, a careful examination of gridded magnetic anomaly map reveals that symmetry may be found to the north of 1°N along N120E, which is roughly the direction of spreading at this region as revealed by the fine-scale seafloor fabric by our survey. Vector magnetic anomalies exhibit important differences with strike of magnetic boundaries within the inner region between Tobi and Mapia Ridges trending predominantly N–S, whereas in the outer region they are almost E–W. This finding suggests that the outer region appears to retain the magnetization and crustal structure of the old West Philippine Basin, while the inner region is inherited from the E–W seafloor spreading in the Ayu Trough. The short along-axis segmentation and complex seafloor morphology caused by slow oblique spreading may explain the lack of symmetry in the magnetic anomalies with respect to the trough axis and a wider occurrence of 3-D magnetic source crust roughly to the south of 1°N.