Abstract
Abstract Data on the crustal structure, bulk composition, and eruption ages of an arc seamount were obtained in an investigation aimed at studying the spatial and temporal variations in the magma composition of an intra-oceanic arc. We conducted gravity and magnetic surveys of the Horeki seamount and evaluated the density and magnetization structure using inversion and forward modeling. The seamount is located on the back-arc side of the Izu-Ogasawara arc. This seamount has an elliptical shape, a flat summit, and satellite ridges and cones on its northern and southern flanks. The flanks are consistent with a higher density anomaly, with an estimated mean density corresponding to dense basaltic rocks. A low-density anomaly is distributed in the seamount top, indicating that the top likely consists of porous basalts or differentiated rocks. The prominent circular low Bouguer gravity anomaly, which appeared in the northern part of the flat-topped summit, indicates that a light-density material fills the summit. The main body of seamount is normally magnetized. Combined with the age of the rocks, the volcanism constructing the main body may be most robust in the Gauss chron. The deeper part of seamount may consist of intrusive rocks, with induced magnetization over remanence. The eastern part of the northern ridges is reversely magnetized, while the western part is normally magnetized. These features and the ages of the sampled rocks suggest that these ridges were constructed in the Matuyama and Brunhes chrons, respectively. The southern part of the seamount flanks shows weak normal magnetization, probably caused by the small cones with different polarities of remanent magnetization.
Highlights
The Izu-Ogasawara arc is an active, approximately1200-km-long intra-oceanic arc associated with PacificPhilippine Sea plate convergence in the western Pacific (Fig. 1)
The result shows that the calculated Bouguer gravity anomaly with the assumed density of 2510 kg/m3 has the lowest correlation with variations in water depth in the area around the seamount (30 × 30 km)
The flanks of the seamount are consistent with a higher density anomaly, with a range from +300 to +400 kg/m3
Summary
The Izu-Ogasawara arc is an active, approximately1200-km-long intra-oceanic arc associated with PacificPhilippine Sea plate convergence in the western Pacific (Fig. 1). We present gravity and magnetic data on the Horeki seamount that was collected in a 2004 survey aboard the R/V Kairei (KR04-04 cruise). The result shows that the calculated Bouguer gravity anomaly with the assumed density of 2510 kg/m3 has the lowest correlation with variations in water depth (topography) in the area around the seamount (30 × 30 km).
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