Chie Honsho

Three-dimensional magnetic and gravity studies of the Rodriguez Triple Junction in the indian Ocean

The Rodriguez Triple Junction (RTJ) is a ridge-ridge-ridge triple junction connecting three mid-ocean ridges in the Indian Ocean: the Central Indian Ridge (CIR), Southeast Indian Ridge (SEIR), and Southwest Indian Ridge (SWIR). We carried out three-dimensional analyses of Sea Beam bathymetry, magnetic, and gravity anomaly data covering an area of about 200 km × 140 km at the triple junction and obtained the magnetization distribution and the Bouguer anomaly field. In the magnetic inversion result some of the magnetic lineations of the CIR and SEIR are recognized within the SWIR valley, and the Bouguer anomaly field shows little variation over the SWIR valley in spite of its large topographic relief. These results indicate the tectonic rifting at the SWIR by crustal thinning without volcanic activity. A 4-km-wide graben in the western CIR flank was found to have extremely high magnetization and lower Bouguer anomaly than the eastern flank of the CIR. Because the CIR and SEIR have similar spreading directions and rates at the triple junction, we consider the graben represents a fossil propagating rift of the SEIR, which began to propagate toward the CIR at about 1 Ma and then became inactive when the SWIR grew to reach the CIR + SEIR spreading axis at about 0.5 Ma. We suggest the above process repeats several times and composes part of the triple junction evolution on a longer timescale. We also suggest that the whole process is closely related to formation of the CIR segments at the triple junction since 20 Ma.

The magnetic signature of ultramafic-hosted hydrothermal sites

Unlike basalt-hosted hydrothermal sites, characterized by a lack of magnetization, the magnetic signature of ultramafic-hosted hydrothermal sites remains poorly known, despite their wide occurrence at slow-spreading ridges and their strong mineral potential. The first high-resolution magnetic surveys of such ultramafic-hosted sites, achieved by deep-sea submersible on four sites of the Mid-Atlantic Ridge, reveal positive magnetic anomalies, and therefore a strong magnetization at the largest sites. This observation reflects the presence of a wide mineralized zone beneath these sites, the stockwork, where several chemical processes concur to create and preserve strongly magnetized magnetite. Beyond pointing out the importance of subsurface chemical processes in hydrothermal activity, the aging of oceanic lithosphere, and the ocean chemical budget, our results have immediate application for detecting and characterizing economically valuable deep-sea mineral deposits.

Crustal movements on Shikoku, southwestern Japan, inferred from inversion analysis of levelling data using ABIC

Abstract With an inversion technique based on Akaikes Bayesian Information Criterion (ABIC) we analyzed levelling data for 1893–1983 on Shikoku, southwestern Japan, where large interplate earthquakes have periodically occurred at intervals of about 120 years, releasing tectonic stress produced by steady relative motion of the Philippine Sea and the Eurasian plates. Through the inversion analysis we reconstructed the pattern of crustal movements on Shikoku over the last 120 years, including the occurrence of the 1946 Nankaido earthquake (M 8.1). The result clearly shows that the crustal movements on Shikoku include significant secular vertical motion (uplift in the south and subsidence in the north) in addition to cyclic motion related to the periodic occurrence of interplate earthquakes at the Nankai trough. Contrary to the widely accepted theory, we could not find any correlation between the secular vertical motion and the coseismic vertical displacement. The secular uplift motion on southern Shikoku estimated from the levelling data completely agrees with that inferred from the present heights of marine terraces formed by eustatic sea level changes and crustal uplift for the last 105 yr. This suggests that the fundamental causes of the short-term (102 yr) and long-term (105 yr) movements on southern Shikoku are the same, the steady subduction of the Philippine Sea plate at the Nankai trough. On northern Shikoku, on the other hand, the pattern of secular crustal motion estimated from the levelling data is quite different from that of the Quaternary uplift inferred from the present heights of eroded flat surfaces, indicating the existence of some unknown tectonic process controlling the very long-term (106 yr) crustal movements.

A three-dimensional gravity study of the Rodrigues Triple Junction and Southeast Indian Ridge

Abstract Three-dimensional analysis of gravity and bathymetry data is used to investigate the density structure of the Rodrigues Triple Junction and the first segment of the Southeast Indian Ridge south of the triple junction. The distribution of mantle Bouguer and residual gravity anomalies suggests that focused upwelling of mantle is occurring along the nearly co-linear Central Indian and Southeast Indian Ridge limbs of the triple junction. In contrast, the mantle Bouguer anomaly over the Southwest Indian limb of the triple junction shows little variation despite nearly 4 km of topographic relief within this segment of the Southwest Indian Ridge. The absence of a significant mantle Bouguer anomaly over the Southwest Indian Ridge near the triple junction suggests that the rift valley observed on this segment is not completely compensated by thinning oceanic crust; only ∼ 1 km of crustal thinning within the axial valley is expected, based on our gravity analysis. Further, no clear long-wavelength thermal signal associated with lithospheric growth can be associated with the Southwest Indian Ridge in this region. These observations imply that focused upwelling is not occurring on this segment of the Southwest Indian Ridge and require a primarily dynamic compensation mechanism for the extreme axial topography. Our analysis indicates that the Rodrigues Triple Junction system is dominated by mantle upwelling associated with the Southeast and Central Indian Ridges.

High‐resolution acoustic mapping to understand the ore deposit in the Bayonnaise knoll caldera, Izu‐Ogasawara arc

We collected deep-sea multibeam, side scan, and subbottom profiler data using an autonomous underwater vehicle at the Bayonnaise knoll, a submarine caldera located in the rift zone of the Izu-Ogasawara arc. We aimed to reveal topographic and geological features and the origin of a hydrothermal field called the Hakurei site in the caldera. We performed seafloor classification by textural analysis using calibrated side-scan sonar data, which provided an effective means to understand the geology and to highlight potential areas of hydrothermal constructions. The high-resolution bathymetric map illustrates that the Hakurei hydrothermal field is distributed over a landslide landform in the caldera wall. The distribution of hydrothermal vents indicates that the slip surface has served as a major route of hydrothermal fluids. The radial alignment of chimneys and mounds indicates radial routes of hydrothermal fluid and/or belching along fragile lines in the landslide landform. Various postcaldera activities are inferred including the formation of a lava dome, a pyroclastic cone, and subsequent phreatic explosions. A general volcano-tectonic structure extending across the caldera in a NW-SE direction is interpreted as an inferred boundary fault of the North Myojin Rift. Analogous to the Hokuroku basin and land kuroko deposits, it is suggested that the main contributing factor in the formation of kuroko deposits was volcano-tectonic activity that dominated the margin of the back-arc rift basin. The intersections between the margin of a rift basin and the surrounding knolls have a high potential for ore-forming areas.

High‐resolution magnetic signature of active hydrothermal systems in the back‐arc spreading region of the southern Mariana Trough

High-resolution vector magnetic measurements were performed on five hydrothermal vent fields of the back-arc spreading region of the southern Mariana Trough using Shinkai 6500, a deep-sea manned submersible. A new 3-D forward scheme was applied that exploits the surrounding bathymetry and varying altitudes of the submersible to estimate absolute crustal magnetization. The results revealed that magnetic-anomaly-derived absolute magnetizations show a reasonable correlation with natural remanent magnetizations of rock samples collected from the seafloor of the same region. The distribution of magnetic-anomaly-derived absolute magnetization suggests that all five andesite-hosted hydrothermal fields are associated with a lack of magnetization, as is generally observed at basalt-hosted hydrothermal sites. Furthermore, both the Pika and Urashima sites were found to have their own distinct low-magnetization zones, which could not be distinguished in magnetic anomaly data collected at higher altitudes by autonomous underwater vehicle due to their limited extension. The spatial extent of the resulting low magnetization is approximately 10 times wider at off-axis sites than at on-axis sites, possibly reflecting larger accumulations of nonmagnetic sulfides, stockwork zones, and/or alteration zones at the off-axis sites.

Absolute magnetization of the seafloor at a basalt-hosted hydrothermal site: Insights from a deep-sea submersible survey

The analysis of high-resolution vector magnetic data acquired by deep-sea submersibles (DSSs) requires the development of specific approaches adapted to their uneven tracks. We present a method that takes advantage of (1) the varying altitude of the DSS above the seafloor and (2) high-resolution multibeam bathymetric data acquired separately, at higher altitude, by an Autonomous Underwater Vehicle, to estimate the absolute magnetization intensity and the magnetic polarity of the shallow subseafloor along the DSS path. We apply this method to data collected by DSS Nautile on a small active basalt-hosted hydrothermal site. The site is associated with a lack of magnetization, in agreement with previous findings at the same kind of sites: the contrast between nonmagnetic sulfide deposits/stockwork zone and strongly magnetized basalt is sufficient to explain the magnetic signal observed at such a low altitude. Both normal and reversed polarities are observed in the lava flows surrounding the site, suggesting complex history of accumulating volcanic flows.

Postcaldera volcanism and hydrothermal activity revealed by autonomous underwater vehicle surveys in Myojin Knoll caldera, Izu‐Ogasawara arc

Myojin Knoll caldera, one of the submarine silicic calderas lying on the volcanic front of the northern Izu-Ogasawara arc, has attracted increasing attention since the discovery of a large hydrothermal field called the Sunrise deposit. Although numerous submersible surveys have been conducted in Myojin Knoll caldera, they have not sufficiently explored areas to produce a complete picture of the caldera and understand the origin of the Sunrise deposit. We conducted comprehensive deep-sea surveys using an autonomous underwater vehicle and obtained high-resolution bathymetric and magnetic data and sonar images from ~70% of the caldera. The detailed bathymetric map revealed that faulting and magma eruptions, possibly associated with an inflation-deflation cycle of the magma reservoir during postcaldera volcanism, had generally occurred in the caldera wall. The main dome of the central cone was covered with lava flows and exhibits exogenous growth, which is unusual for rhyolitic domes. The magnetization distribution in the central cone indicates preferential magma intrusion along a NW-SE direction. It is presumed that magma migrated along this direction and formed a rhyolite dome at the foot of the southeastern caldera wall, where the Sunrise deposit occurs. The Sunrise deposit is composed mainly of three ridges extending in slope directions and covers ~400 × ~400 m. Magnetization reduction in the deposit area is small, indicating that the alteration zone beneath the Sunrise deposit is slanting rather than vertical. It is presumed that several slanting and near-vertical volcanic vents serve as pathways of hydrothermal fluid in Myojin Knoll caldera.

Magnetic anomalies associated with abundant production of pyrrhotite in a sulfide deposit in the Okinawa Trough, Japan

We report here results from a deep-sea magnetic survey using an autonomous underwater vehicle over the Hakurei hydrothermal site, in the middle Okinawa Trough. Magnetic inversion revealed that the Hakurei site is associated with well-defined high-magnetization zones distributed within a broad low-magnetization zone. Results from rock magnetic measurements, performed on sulfide ore samples obtained by drilling, showed that some samples possessed extremely high natural remanent magnetization (NRM) (as much as 6.8–953.0 A/m), although most of the measured samples had much lower NRM. These high-NRM samples were characterized by high Konigsberger ratios (101−103), indicating much larger NRM than induced magnetization, and contained pyrrhotite as the only magnetic mineral. This suggests that NRM carried by pyrrhotite is the source of the observed magnetic anomalies. The wide range of NRM intensity was considered to be due to a highly heterogeneous distribution of pyrrhotite, because pyrrhotite was commonly identified in both the high- and low-NRM samples. Pyrrhotite production may have been occasionally drastically increased, with highly magnetic ores formed as a result. Rapid burial of active vents may result in the creation of an extensive reducing environment under the seafloor, which is favorable to pyrrhotite production, and may also prevent oxidation of pyrrhotite by isolating it from seawater. Because the magnetization intensity of sulfide ores was highly variable, it would not be straightforward to estimate the quantity of ore deposits from the magnetic anomalies. Nevertheless, this study demonstrates the usefulness of magnetic surveys in detecting hydrothermal deposits. This article is protected by copyright. All rights reserved.

Comprehensive analysis of traveltime data collected through GPS-acoustic observation of seafloor crustal movements†

We have developed a comprehensive inversion scheme to analyze traveltime data collected through GPS-acoustic observations on a campaign basis. Our method uses a quantity called the “Nadir Total Delay” (NTD), which is analogous to the Zenith Total Delay (ZTD) in GNSS analyses, to represent the time variation of the sound speed. The observation equation using the NTD is an approximated formulation. We examine its applicable scope using numerical experiments, and demonstrate that the approximation holds well enough in practice. Traveltime data from all our observation campaigns were utilized together for inversion calculation to determine: (1) the positions of individual precision acoustic transponders (PXPs) at the time of a particular reference campaign, (2) the displacement from these positions of a rigid array at the time of each campaign, and (3) the time variation of the NTD during each campaign. We applied this method to actual data to fix the array geometry more precisely. Also, we have made a first step in detecting vertical motions as well as horizontal motions, though further enhancement of the accuracy will be required. In performing linear regression of array displacements obtained from the inversion, their error covariance has a significant effect on the resulting velocities. When the covariance is taken into account, nearly the same velocities are obtained no matter what campaign is taken as reference in the inversion.