Yozo Hamano

Geomagnetic poles over the past 10,000 years

Locations of the geomagnetic pole over the past 10,000 years have been calculated by averaging the VGP positions obtained from paleomagnetic data. The distribution of the geomagnetic pole was elongated to the direction parallel to the meridian of 45° and 225° longitude, and westward movement of the pole was predominant throughout this period. The time sequence of the polar motion can be divided into three intervals, the intervals between ca. 10,000 B.P. and ca. 7000 B.P., between ca. 7000 B.P. and ca. 3700 B.P., and between ca. 3700 B.P. and the present. During the period between ca. 7000 and ca. 3700 B.P., the range of the movement of the geomagnetic pole was limited within 5 degrees around the geographical pole. Before and after this period, the movement was very active, fluctuating over 10 degrees. The results of the last 2000 years show good consistency with the geomagnetic pole calculated from archaeomagnetic data by Merrill and McElhinny [1983].

An experiment on the post-depositional remanent magnetization in artificial and natural sediments

Artificial and natural sediments were consolidated at constant loading rate in a known magnetic field, and the resultant remanent magnetization was observed. The experiment revealed that both the artificial and natural sediments can acquire remanent magnetization during the consolidation process. The acquisition process was determined as a function of the void ratio, which is the volume ratio of water to solid powder in the sample. The artificial sediment acquired remanence with the void ratio varying from 1.4 to 0.9. On the other hand, the range of the void ratio in the natural sediments is from 6 to 3. These void ratios roughly correspond to a depth of 15 cm and 2.5 m, respectively. The gradual acquisition of remanence during the consolidation shows that the remanence indicates some average of the ambient field over a period of time which depends on the sedimentation rate. A simple model based on relaxation qualitatively explains the above result. Results of alternating field demagnetization indicate that the stability of the remanence is not determined by bodily rotations of magnetic grains, but is controlled by coercivity of the magnetic grains. The recorded field direction in the deep-sea sediments did not show a systematic deviation from the applied field direction.

Oblique and near collision subduction, Sagami and Suruga Troughs —preliminary results of the French-Japanese 1984 Kaiko cruise, Leg 2

Abstract Leg 2 of the French-Japanese 1984 Kaiko cruise has surveyed the Suruga and the Sagami Troughs, which lie on both sides of the northwestward moving and colliding Izu-Bonin Ridge, the northernmost part of the Philippine Sea plate. The transition from the Nankai Trough to the Suruga Trough is characterized by northward decrease in width of the accretionary prism, in good agreement with the increasing obliquity between the through axis and the direction of the convergence, as the strike of the convergent boundary changes from ENE-NNE to south-north. South of the area, the southern margin of the Zenisu Ridge shows contractional deformations. This supports the interpretation made by the team of Leg 1 who studied the western extension of the area we studied, that it is an intra-oceanic thrusting of the ridge over the Shikoku Basin. In the Sagami Trough, where the relative motion is highly oblique to the plate boundary, active subduction is mostly confined in the east-west trending portions of the trough located south of the Boso Peninsula and along the lower Boso Canyon, near the TTT triple junction. In between, the present motion is mainly right-lateral along the northwest trending Boso escarpment. However, an inactive but recent (Pliocene to lower Pleistocene) accretionary prism exists south of the Boso escarpment, which suggests that the relative motion was more northerly than at present before about 1 Ma ago.

Paleomagnetic study of the Central Andes: counterclockwise rotation of the Peruvian block

Kono, M., Heki, K. and Hamano, Y., 1985. Paleomagnetic study of the Central Andes: counterclockwise rotation of the Peruvian Block. Journal of Geodynamics, 2: 193-209. Paleomagnetic study was performed on Mesozoic and Tertiary rocks from Peru and northernmost Chile. Comparisons of these results as well as other data from the Central Andes with paleomagnetic poles from South American craton strongly support the orocline hypothesis of Carey for the formation of the Arica (Santa Cruz) deflection. Paleomagnetic declinations of Jurassic and Cretaeous rocks are quite similar to the direction of the present-day structural trend in the Central Andes, which suggests that the mountain belt has rotated in a coherent fashion (i.e., rigid body rotation) in sections of the Central Andes. The occurence of this deformation is certainly post-Cretaceous, with some suggestion that rotation still continued as recently as Neogene. The mechanism of this deformation is not well known, but a differential stretching of the Amazon Basin behind the Peruvian Andes is a possibility.

Experimental studies of He and Ar degassing during rock fracturing

Abstract We studied degassing of He and Ar from granite, basalt and volcanic tuff samples which were subjected to uniaxial compression. From the samples which were fractured in the dilatant region He was always degassed, while degassing of Ar dependent not only on the dilatancy, but also on the type of rock and the compressional conditions such as wet, room dry or vacuum conditions. It is concluded that degassing of rare gases from the compressed samples depends primarily on the generation of new surface area by dilatancy.

Quasi-periodic wind signal as a possible excitation of Chandler wobble

The atmospheric contribution to the excitation of the Chandler wobble (CW) is studied for a period of about 11 years period beginning in September 1983, using the wobble data set (SPACE93). Two atmospheric angular momentum (AAM) functions are employed; one is based on Japan Meteorological Agency (JMA) data, and the other is based on U.S. National Meteorological Center (NMC) data. In the vicinity of the Chandler frequency, the excitation power for both AAM functions is comparable with that inferred from the observed wobble. If we separate the AAM excitation into wind and pressure effects, the wind contribution exceeds the pressure contribution around the Chandler frequency. In addition, the JMA wind AAM function reveals a spectral peak around the Chandler frequency. Significant coherence with the inferred excitation exceeding the 99% confidence level can be recognized for both JMA and NMC wind excitation. We propose that the wind contribution to the excitation of polar motion may be somewhat larger than at other frequencies because of the existence of quasi-periodic atmospheric variations that have been found in other investigations. In a narrow-band analysis of the excitation using least squares fit sinusoids, we found that both the amplitude and phase of the inferred excitation near the Chandler band indicate their strong dependence on the assumed Chandler period and that an assumed CW period of 431 days caused atmospheric and observed excitations to agree most closely.

Paleomagnetic study of Cretaceous rocks of Peru, South America: Evidence for rotation of the Andes

Abstract Paleomagnetic data for the Cretaceous volcanic and sedimentary rocks in the Andean region of Peru are given. Reliable paleomagnetic field directions were obtained for three Cretaceous (Albian to Cenomanian) formations from calcareous sediments in northern Peru. Stable remanent magnetization directions were also derived from twelve Cretaceous lava flows and dikes in coastal Peru. Paleomagnetic data of the same age from the stable areas of South America such as Brazil demonstrate that the paleomagnetic poles are nearly coincident with the present pole, but Peruvian paleomagnetic directions studied here showed several tens of degrees of counterclockwise declination shifts. This suggests counterclockwise tectonic rotation of an extensive block which includes the whole of Andean Peru.

Paleomagnetism of late Archean rocks of Hamersley basin, Western Australia and the paleointensity at early Proterozoic

We report the results of paleomagnetic and rock magnetic measurements of late Archean rocks from Hamersley basin, Western Australia, and the paleointensity determination for early Proterozoic. Basalts and banded iron formations have two to four components of remanent magnetization, and have consistent directions for different localities, indicative of geomagnetic origin. Rock magnetic measurements of basalts reveal that the main magnetic mineral is a fine-grained magnetite present in ∼10 ppm in mass, which do not alter when heated up to its blocking temperature. We interpret the stable component up to ∼390 ◦ C as the post-tilting thermal overprint from uplift at � 2.0 Ga, and the higher temperature component as the pre-tilting thermoviscous remanence during burial metamorphism. From the Thellier type paleointensity experiments using the thermal overprint component, we obtain a mean virtual dipole moment (VDM) estimate of (1.8–3.6) × 10 22 Am 2 . This suggests that early Proterozoic was characterized by a 1 weak geomagnetic field of less than one-half of the present.

Long-term seafloor geomagnetic station in the northwest Pacific: A possible candidate for a seafloor geomagnetic observatory

For two years, geomagnetic variations have been measured at the seafloor in the northwest Pacific. The seafloor data consist of the geomagnetic vector field measured by a three-component fluxgate magnetometer and the absolute scalar total force measured by an Overhauser (1953) magnetometer with attitude measurements for both orientation and tilt. Using the attitude data, the geomagnetic data at a site in the northwest Pacific (41o06′08″N, 159°57′47″E, -5580 m), hereafter referred to as NWP, were converted into the same reference frame as land and satellite measurements. Short-period variations of the converted vector data were examined by Hamano’s (2002) global time domain analysis method, which showed compatibility of the seafloor geomagnetic observatory data with the existing land observatory network. The smooth and gradual change of the Earth’s main field (i.e., the geomagnetic secular variation) was also found consistent with those predicted by the latest International Geomagnetic Reference Field (IGRF-10; IAGA, 2005) and by Ørsted Satellite (Olsen, 2002) for not only the scalar field but also the vector field. This means that observation of the geomagnetic vector secular variation is now feasible on the seafloor.

Trench triple junction off Central Japan—preliminary results of French-Japanese 1984 Kaiko cruise, Leg 2

Abstract Leg 2 of the French-Japanese 1984 Kaiko cruise has surveyed the trench triple junction off central Japan, where the Japan, Izu-Bonin and Sagami Trenches intersect. The Izu-Bonin Trench is deeper than the Japan Trench and filled by a thick turbiditic series. Its anomalous depth is explained by the westward retreat of the edge of the northwestward moving Philippine Sea plate. On the contrary to what happens in the Japan Trench, horst and graben structures of the Pacific plate obliquely enters the Izu-Bonin Trench, suggesting that the actual boundary between these two trenches is located to the north of the triple junction. The inner wall of the Izu-Bonin Trench is characterized in the triple junction area by a series of slope basins whose occurrence is related to the dynamics of this area. The northernmost basin is overthrust by the edge of the fore-arc area of the Northeast Japan plate. The plate boundary is hardly discernible further east, which makes it impossible to locate precisely the triple junction itself. These features suggest that large intra-plate deformation occurs there due to the interaction of the plates involved in the triple junction and the weak mechanical strength of the wedge-shaped margin of the overriding plates.