Sadao Sasajima

Paleomagnetic studies combined with fission-track datings on the western arc of Sulawesi, east Indonesia

Abstract Paleomagnetic studies in conjunction with fission-track dating on the western arc of Sulawesi yield important evidence bearing on the tectonic history of the area. During the Paleogene to Early Miocene time interval the paleomagnetic pole for southwestern Sulawesi was situated at 36.5°E 44.8°N. This pole position is significantly different from that in the time interval Middle Miocene to Recent, which is consistent with the north pole of the axial geocentric dipole. This fact suggests that subsequent to the Paleogene to Early Miocene period, possibly 19–13 m.y. B.P., a major tectonic event occurred which caused about 40 degrees of anticlockwise rotation of the area. It is suggested by the present work that the postulated collision followed by welding of eastern Sulawesi with western Sulawesi during the Pliocene (Katili, 1978) may be the tectonic event mentioned above. In addition, our data does not support the hypothesis that western Sulawesi has been derived from the dispersal of Gondwanaland.

Paleomagnetic evidence for clockwise rotation of the northern arm of Sulawesi, Indonesia

Abstract Paleomagnetic results from the northern arm of Sulawesi show that the arm has been subjected to a clockwise rotation of more than 90° and that its rotational motion began no later than the middle Miocene. The mean direction showing a normal polarity at the Eocene to the early Miocene is D = 98.0° and I = 6.9° . A declination value of D = 50.1° obtained from Miocene rocks indicates a transition stage of the rotational motion. The datum from Plio-Pleistocene volcanics is D = −4.6° and I = −9.3° . This suggests that the rotational motion terminated before the initiation of volcanic activity during the Plio-Pleistocene.

Paleomagnetic evidence for the paleoposition of Sumba island, Indonesia

Abstract Jurassic paleomagnetic directions have been obtained for the sedimentary rocks in Sumba. The mean direction is 59.2° in declination and −44.2° in inclination. The reliability of the direction is ascertained through thermal demagnetization and the presence of normal and reversed polarities. Comparison with the Permian paleomagnetic direction of Timor indicates that Sumba was subjected to a clockwise rotation through 79.4° relative to Timor since the Jurassic. When Sumba is restored by a counter-clockwise rotation, the Sumba pole is gradually approaching the Jurassic pole of Australia as well as the Timor Permian pole. This implies that, at least until the Jurassic, Sumba and Timor were situated at the Australian continental margin and that Sumba rotated clockwise during or after the Jurassic.

Long-term creep experiment on some rocks observed over three years

Abstract Since August 1974 the authors have been carrying out an experiment on creep by bending three small granite beams and three gabbro beams. While making measurements, an optical flat is set to produce interference fringes of Na-D light upon the upper polished surface of the beam, which was previously bent convex upward. By analyzing the fringes the profile of the surface is determined with in an accuracy of one-tenth of a wavelength. The routine determination of profiles gives a change in the amount of bending with time. A similar experiment on two large beams of granite was carried out for 21 years by Kumagai and Ito. It took 10 years in order to find out the secondary creep of granite. However, in this three-year experiment it has been found that the secondary creep of the granite specimens gives a viscosity of about 1 · 10 20 poise, which is comparable to that obtained over 20 years in the previous experiment (Kumagai and Ito). As for the gabbro specimens, the present authors cannot yet definitely recognize the secondary creep, because gabbro creeps much more slowly than granite. Both this and the previous experiments have revealed that the creep of rocks does not show a steady and monotonous progress, but a repeated “turn back” with various periods, some of which are longer than one year. In order to explain the creep accompanied by the “turn back” phenomenon, the authors venture to hypothesize that the elastic constants of a test-piece slightly vary with time during the advance of creep.

Paleomagnetic evidence of a drift of the Japanese main island during the paleogene period

So far no paleomagnetic study of Paleogene rocks in Japan has been reported. In this paper paleomagnetic results are presented for Paleogene volcanic rocks which were taken from various localities in Southwest Japan. Thermal and alternating field demagnetization experiments have been carefully undertaken to clean the secondary unstable components in the natural remanent magnetization. From the result obtained it is shown that the Paleogene paleomagnetic pole deduced from Southwest Japan is appreciably far from that from the Eurasian continent. This fact might be well accepted as an additional proof supporting a hypothetical drift of the Japanese main island relative to the continent, since a certain time before the Cretaceous period, which was previously reported by the two authors. Such a drift of the Island should still continue during the Paleogene period until the Miocene epoch when the present geographic location of the Island was almost established.

Long-term creep experiment of rock with small deviator of stress under high confining pressure and temperature

Abstract Long-term creep tests of gabbro which have been performed with a maximum bending stress (20 bar) under a high confining pressure (1 kbar) and various temperatures, are described. Methods and techniques used in the experiment are mainly similar to those reported previously by the same authors (Ito and Sasajima, 1980) except for the application of high pressure and temperature. The techniques include the bending system, size and preparation of the sample, and the determination of its deformation by use of interference fringes of Na-D light. In order to measure a very small deformation of creep, intermittent breaks of the application of loading, confining pressure and temperature are necessary, and the creep curve is constructed from the intermittent advance of permanent deformation. The experiment has revealed two strange phenomena : one is a sinuous progress of the creep curve, and the other is that the deformation recovery shows strange behavior after the unloading. These results are discussed in close connection with the mechanism of the “turn back of creep” denoted by Ito and Sasajima (1980). The mean creep curves, at 25°C. 95°C and 150°C, obtained so far lead to viscosities of 1.6 · 10 20 , 1.9 · 10 19 and 4.8 · 10 18 poise, respectively and the maximum strain rates employed in the samples were 4.2 · 10 −14 , 3.6 · 10 −13 and 1.4 · 10 −12 /sec, respectively, which cover the geological strain rate. Although we have only three data points, the logarithm of viscosity is linearly related to the reciprocal of absolute temperature (see Fig. 7), and an activation energy for creep of gabbro is found to be 7.6 kcal/mol. It should be noted that viscosities obtained are considerably smaller than those estimated for the crust and mantle, and that the activation energy is surprisingly smaller than those obtained by high-pressure experiments of rock deformation, which have been carried out under a strain rate larger than 10 −8 /sec.