Hiromi Fujimoto

The interaction of viscous heating with grain‐size dependent rheology in the formation of localized slip zones

The formation of localized shear zones is important for understanding many local and global processes in geodynamics. We have developed a self-consistent thermal-mechanical model together with a rheology which depends on temperature, strain-rate and grain-size distribution. The grain-size distribution has contributions from both dynamic recrystallization and grain-growth processes, and is governed locally by a nonlinear ordinary differential equation. A one-dimensional model with 104 points is employed to resolve all of the scales involving grain-size and temperature. We found that grain-growth inhibits the development of shear zones, and that there is a delicate interplay between viscous heating and grain-growth process in determining whether narrow fault zones are developed quickly. For realistic parameters of rheology and grain-boundary processes for wet olivine, the magnitude of the rate of grain-growth is crucial to determine whether shear zones are stable or unstable at temperature T ≃ 1000 K or shear stress σ ≃ 100 MPa.

The eastern and western ends of Nankai Trough: results of box 5 and box 7 Kaiko survey

Abstract Seabeam mapping and detailed geophysical surveying have been conducted over the eastern and western ends of the Nankai Trough. The eastern survey covers the transition between the large Izu-Bonin (Izu-Ogasawara) Ridge collision with Honshu and the Nankai Trough subduction. It includes a northeast trending basement ridge, the Zenisu Ridge, to the southeast of Nankai Trough as well as two large channel systems, one following the trench, the Nankai channel and the other coming from the north, the Tenryu Canyon. The Zenisu Ridge is a zone of recent intra-plate shortening consisting of three distinct segments with an increasing deformation from southwest to northeast. We interpret this gradient of deformation as a way to absorb the kinematic discontinuity between the diffuse shortening prevailing over the Izu-Bonin Ridge and the concentrated shortening along the Nankai Trough. The shortening axis appears to change from north-south along the Zenisu Ridge to northwest-southeast along the Nankai Trough and finally to east-west within Honshu. The western survey covers the junction between the Nankai and Ryukyu Trenches and the Kyushu-Palau Ridge which is subducted without major internal deformation. The ridge appears to act as an indenter upon the margin. The tip of the indenter is presently situated under the upper accretionary prism.

Relict hydrothermal events along the super-slow Southwest Indian spreading ridge near 63°56'E-mineralogy, chemistry and chronology of sulfide samples

Abstract During the Indoyo cruise in 1998 with Shinkai 6500 a massive sulfide field was located in a water depth of about 2940 m, near 27°51′S/63°56′E, east of the Melville fracture zone. The newly discovered hydrothermal field is situated at the summit of an axial volcanic ridge, informally called, Mount Jourdanne. Within the field, inactive chimneys as well as hydrothermal mounds were observed on the ocean floor but no indications such as shimmering water, chemical anomalies in the water column or faunal relicts, which might point to an active vent site, were observed. Our discovery proves, for the first time, that high-temperature massive sulfide chimneys can also develop at a super-slow spreading ridge such as the Southwest Indian Ridge. This discovery of massive sulfides will possibly open new aspects in the understanding of hydrothermal processes along super-slow spreading ridges, since the sulfide samples from Mt. Jourdanne show interesting differences in their mineralogical, as well as chemical compositions, when compared to sulfide samples from faster opening ridge environments. To reconstruct the history of the evolution of this field, age dating studies were carried out on the recovered sulfide samples. These indicate activity between 70xa0000 and 13xa0000 years ago.

Nankai Trough and the fossil Shikoku Ridge: results of Box 6 Kaiko survey

Abstract Seabeam mapping and detailed geophysical surveying have been conducted over the Nankai Trough where the fossil Shikoku Ridge is subducted below southwest Japan. The geometry of the oceanic lithosphere bending under the margin as well as the three-dimensional structure of the accretionary prism have thus been determined in detail. Three 350° trending, probably transform faults have been identified in the area of the survey. They do not extend further south and appear to be limited to the last phase of spreading within the Shikoku Basin, probably between 15 and 12 Ma; this last phase of spreading would then have been accompanied by a sharp change in spreading direction from east-west to N 350°. The two eastern transform faults limit a zone of reduced Nankai trench fill of turbidites opposite to the Tosa Bae Embayment. This observation suggests that the Tosa Bae Embayment actually results from this reduced supply of trench fill to the imbricate thrusting process. The accretionary prism can be divided into three different tectonic provinces separated by continuous mappable thrusts, the Lower and Upper Main Thrusts. Surface shortening is limited to the lower accretionary prism south of the Upper Main Thrust (UMT) whereas uplift with possible extension characterizes the prism above the UMT. Deformation, due to the relative plate motion, mostly affects the lower accretionary prism south of the UMT.

Interaction of the upwelling plume with the phase and chemical boundary at the 670 km discontinuity: effects of temperature-dependent viscosity

Abstract Numerical simulations have been performed to investigate the interaction of upwelling plumes with the 670 km discontinuity. Temperature-dependent rheology is employed to introduce the low viscosity and strength of the hot plume. The 670 km discontinuity is taken as a phase and/or chemical boundary. The condition for penetration of the plume into the upper mantle is examined by varying the value of the Clapeyron slope of the phase transition and the compositional density difference of the chemical boundary. Possible styles of the mantle convection are classified into six types depending on the nature of the 670 km discontinuity. The plume in the lower mantle can penetrate into the upper mantle and reach the bottom of the lithosphere when the Clapeyron slope is greater than −3 to −4 MPa/K for the pure phase boundary, and when the compositional density difference is smaller than 0.5–1.5% for the pure chemical boundary. The plume can barely penetrate into the upper mantle when the 670 km discontinuity possesses even a small amount of the compositional density contrast (less than 0.5–1.0%) together with the phase transition of the experimentally determined Clapeyron slope of −3 ± 1 MPa/K. This result suggests that the mantle may be of uniform composition or at most has a weak compositional layering if the origin of the plume is in the deep mantle.

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.

Morphological studies of the Ayu Trough, Philippine Sea – Caroline Plate Boundary

The Ayu Trough lies on the southern boundary between the Philippine Sea Plate and Caroline Plate. Although this trough may be the best place to study the evolution and kinematics of the Philippine Sea Plate, the origin and evolution of this trough are poorly understood. Our geophysical and morphological surveys in the northern part of the Ayu Trough revealed that the trough shares morphological similarities with slow-spreading lid-ocean ridges. The seafloor ages and an average spreading rate of the trough were inferred from the avcrage length of the ridge segments, distribution of sediment thickness, and basement subsidence. Based on the seismic section at 3°30N, the opening of the trough started about 25 Ma with an average half-spreading rate 4.1 mm/yr. This spreading rate is much slower than some previous estimates. The present magma production rate and activity of the trough is estimated to be even lower than the 25 m.y. average. The seafloor depth of the trough axis and relief of the axial valley are approximately twice as deep as that of active lid-ocean ridges. The trough axis valley is covered with sediment in contrast with the axial valleys of active lid-ocean ridges. Sedimentation at the trough axis shows that the spreading rate has been quite slow for the last 2.5 Ma.

Development of instruments for seafloor geodesy

We have developed systems for measuring differential displacements across a fault zone, and examined their resolutions through seafloor experiments at relatively short baselines. A system for a seafloor extensometer makes use of precise acoustic ranging with a linear pulse compression technique. The system has a resolution better than 1 cm in acoustic ranging over a baseline of at least 1 km. The most critical problem is correction for temperature variations, and we estimate that the effect can be corrected with cm-order accuracy in the case of a deep-sea experiment. We have also examined a leveling system on the seafloor using an array of ocean bottom pressure gauges and an ocean bottom gravimeter to detect differential vertical motion. The system is estimated to have a resolution of several centimeters in vertical displacement. These system will be useful for triangulation and leveling on the seafloor, but we need further studies over a longer baseline and to achieve better long-term stability.

Deep-tow survey in the KAIKO-Nankai cold seepage areas

Deep-sea biological communities associated with cold venting were located within the eastern portion of the Nankai Trough using a deep-towed TV system. Five continuous lines (approximately 200 km long) were observed. Bivalve communities were found both in the Tenryu Canyon outlet region and at the outcrop of the frontal thrusts of the accretionary wedge off Cape Omaezaki. The former is a site at which biological communities had been found during the KAIKO dives in 1985. However, the second site appears to be associated with larger scale venting and was chosen for the 1989 KAIKO-Nankai submersible cruise. Three small mud volcanoes with diameters of several hundred meters and heights of several tens of meters are found in the Seabeam topography map, but the present survey showed neither evidence of recent venting nor biological communities.

Gravity anomalies of the Mid-Atlantic Ridge north of the Kane Fracture Zone

High-resolution gravity and multibeam bathymetric data were newly obtained from the Mid-Atlantic Ridge (MAR) between latitudes 23°30′N and 28°N. Mantle Bouguer anomalies (MBA) show magmatic accretion focused at discrete centers along the spreading axis except near the oblique portion of the MAR between 24°30′–24°50′N. The MBA also indicates that crustal production near the TAG hydrothermal field has been relatively low during the last 2–3 My. The MBA show 120°-striking off-axis traces, indicating northward migration of the oblique portion and northward extension of the segments south of the oblique zone despite a long-wavelength southward deepening of the axial topography. Observed gravity anomalies support a tectonic interpretation proposed from morphology and geomagnetic anomalies that the segment south of the oblique zone has extended northward by repeatedly adding short segments which migrated westward through asymmetric spreading in the oblique zone.