The Geology of the Shimanto Belt has been examined in the Miyama area, north of Owase, eastern Kii Peninsula, Southwest Japan. The strata in this area can be divided lithologically into three thrust-bounded formations or tectonostratigraphic units : the Mizunomitoge, Ookogawa and Shimakatsuura Formations from north to south. The Mizunomitoge Formation consists mainly of melange rocks with block-in-matrix fabrics. The Ookogawa Formation is a unit of sandstone-dominated coherent beds, and the Shimakatsuura Formation is also a unit of coherent beds mainly composed of alternating beds of sandstone and mudstone. Radiolarian fossils from these formations indicate that they are of Upper Cretaceous age. The mudstones from these formations show nearly the same illite crystallinity value belonging to the level of diagenetic and lower anchimetamorphic zones. The diagenesis and metamorphism occurred probably by the rise of geothermal gradient in a short period during the Early and Middle Miocene. The thermal maturity also suggests that these strata were accreted in a shallower tectonic level of a convergent margin.

抄録全体を表示

A new combination system for paleomagnetic measurement incorporates the following four instruments: a ring-core-type flux-gate spinner magnetometer, an alternating field (AF) demagnetizer, an anhysteretic remanent magnetization (ARM) superimposer, and a magnetic susceptibility anisotropy meter.
The system is composed of the following mechanisms and circuits:
(1) a specially designed ring-core sensor and its driving circuits,
(2) a sensor holder incorporating three sets of coils, which are utilized for AF demagnetization, ARM overprinting, and susceptibility anisotropy measurement, respectively,
(3) a mechanism for sample rotation and resetting for 6-spin measurement, using a pair of non-magnetic ultrasonic motors and motor drivers,
(4) a u-metal magnetic shield case,
(5) a dernaguetizing current generator and power amplifier,
(6) a regulated current source for ARM overprinting and maguetic susceptibility aniso-tropy measurement,
(7) a central controller (notebook-personal computer), digital voltmeter, GP-IB Interface, I/O board, and relay board.
In this system, the whole measurement process is controlled by the central computer, which executes the sample rotation and reset during 6-spin magnetization measurement, the ON/OFF and wave-form control of the demagnetizing current during stepwise AF demagnetization, and the ON/ OFF control of the DC magnetic field during ARM overprinting or susceptibility anisotropy measurement.
The noise level of the magnetometer corresponds to 3 × 10⁻⁸ kA/m (= emu/cc) for 30 cc samples, using 10 replications of stacking measurement. The maximum field intensity of the AF demagnetizer is 45 mT. The measured results are presented by computer graphics on the screen of a computer immediately after each measurement. The total weight of the system is only about 35 kg, which is light enough for field use.

抄録全体を表示

A new method using a “Demagnetization Plane” is proposed to present the results of stepwise demagnetization of remanent magnetization. This method facilitates quantitative analysis of stepwise demagnetization results, allowing distinction between the multi-components of remanent magnetization. An AF demagnetization test on Pleistocene marine siltstone and sandy siltstone samples from the Boso Peninsula, central Japan, shows that most of the stepwise AF demagnetized vectors align on a plane containing their origin. This plane is called the “Demagnetization Plane”. The spectrum of remanent magnetization for demagnetization steps can be read from the plots on the Demagnetization Plane. Quantitative analysis of the variation of remanent magnetization in Boso sediments shows that the remanent magnetization can be separated into soft, inter-mediate and hard components, using normalization with anhysteretic remanent magnetization.

抄録全体を表示

GPS surveys have been carried out at 12 stations near benchmarks in the Akaishi mountains area, central Japan. We compared the ellipsoidal heights obtained through GPS surveys with the orthometric heights obtained through leveling surveys. We also investigated crustal movements between the time of the leveling surveys and the GPS surveys. The geoid undulation differences were determined by subtracting the ellipsoidal heights from the orthometric heights after correcting the crustal movements.
The results show that the geoid undulation obtained is significantly steeper than that of the ORI-89 Geoid Model, which is thought to be the most reliable geoid model for Japan and its vicinity.

抄録全体を表示