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Paleomagnetism of the middle Cretaceous Iritono granite in the Abukuma region, northeast Japan 总被引：1，自引：0，他引：1
Ken-ichi Wakabayashi Hideo Tsunakawa Nobutatsu Mochizuki Yuhji Yamamoto Yutaka Takigami 《Tectonophysics》2006,421(1-2):161-171
We have studied the paleomagnetism of the middle Cretaceous Iritono granite of the Abukuma massif in northeast Japan together with 40Ar–39Ar dating. Paleomagnetic samples were collected from ten sites of the Iritono granite (102 Ma 40Ar–39Ar age) and two sites of its associated gabbroic dikes. The samples were carefully subjected to alternating field and thermal demagnetizations and to rock magnetic analyses. Most of natural remanent magnetizations show mixtures of two components: (1) H component, high coercivity (Bc > 50–90 mT) or high blocking temperature (Tb > 350–560 °C) component and (2) L component, relatively low Bc or low Tb component. H component was obtained from all the 12 sites to give a mean direction of shallow inclination and northwesterly declination (I = 29.9°, D = 311.0°, α95 = 2.7°, N = 12). This direction is different from the geocentric axial dipole field at the present latitude (I = 56.5°) and the typical direction of the Cenozoic remagnetization in northeast Japan. Since rock magnetic properties indicate that the H component of the Iritono granite is carried mainly by magnetite inclusions in plagioclase, this component probably retains a primary one. Thus the shallow inclination indicates that the Abukuma massif was located at a low latitude (16.1 ± 1.6°N) about 100 Ma and then drifted northward by about 20° in latitude. The northwesterly deflection is attributed mostly to the counterclockwise rotation of northeast Japan due to Miocene opening of the Japan Sea. According to this model, the low-pressure and high-temperature (low-P/high-T) metamorphism of the Abukuma massif, which has been well known as a typical location, would have not occurred in the present location. On the other hand, the L component is carried mainly by pyrrhotite and its mean direction shows a moderate inclination and a northwesterly declination (I = 42.8°, D = 311.5°, α95 = 3.3°, N = 9). Since this direction is intermediate between the H component and early Cenozoic remagnetization in northeast Japan, some thermal event would have occurred at lower temperature than pyrrhotite Curie point ( 320 °C) during the middle Cretaceous to early Cenozoic time to have resulted in partial remagnetization. 相似文献
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Hideo Tsunakawa 《Tectonophysics》1983,95(3-4)
The K-Ar dating method was applied to the volcanic rocks, the boninite and related rocks, from the Bonin Islands. Four fresh samples gave K-Ar ages of about 40 Ma. These ages suggest that the main volcanism of the Bonin Islands occurred about 40 Ma ago, as a result of the initiation of subduction of the Pacific plate at its western margin. Results of the K-Ar dating on other rocks suggest that some volcanism or thermal disturbance may have occurred 5 ~ 10 Ma ago, at least, in Chichijima. 相似文献
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Masaki N. Nishino Xiao-Dong Wang Masaki Fujimoto Hideo Tsunakawa Yoshifumi Saito Shoichiro Yokota Wei Bian Chun-Lai Li Masaki Matsushima Hidetoshi Shibuya Hisayoshi Shimizu Futoshi Takahashi Toshio Terasawa 《Planetary and Space Science》2011,59(5-6):378-386
Because the solar wind (SW) flow is usually super-sonic, a fast-mode bow shock (BS) is formed in front of the Earth's magnetosphere, and the Moon crosses the BS at both dusk and dawn flanks. On the other hand, behind of the Moon along the SW flow forms a tenuous region called lunar wake, where the flow can be sub-Alfvénic (and thus sub-sonic) because of its low-density status. Here we report, with joint measurement by Chang’E-1 and SELENE, that the Earth's BS surface is drastically deformed in the lunar wake. Despite the quasi-perpendicular shock configuration encountered at dusk flank under the Parker-spiral magnetic field, no clear shock surface can be found in the lunar wake, while instead gradual transition of the magnetic field from the upstream to downstream value was observed for a several-minute interval. This finding suggests that the ‘magnetic ramp’ is highly broadened in the wake where a fast-mode shock is no longer maintained due to the highly reduced density. On the other hand, observations at the 100 km altitude on the dayside show that the fast-mode shock is maintained even when the width of the downstream region is smaller than a typical scale length of a perpendicular shock. Our results suggest that the Moon is not so large to eliminate the BS at 100 km altitude on the dayside, while the magnetic field associated with the shock structure is drastically affected in the lunar wake. 相似文献
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Nobutatsu Mochizuki Hideo Tsunakawa John Cassidy 《Physics of the Earth and Planetary Interiors》2006,154(2):168-179
We report new palaeointensity results concerning the Auckland geomagnetic excursions using the double heating technique of the Shaw method with low temperature demagnetisation (LTD-DHT Shaw method). The excursional palaeodirections recorded in six volcanoes of the Auckland volcanic field, New Zealand, have been classified into three groups: north-down (ND), west-up (WU) and south-up (SU) directions. In the present study, five to six consistent palaeointensities have been obtained from each of five volcanoes recording the Auckland geomagnetic excursions. The Wiri (27 ka), Crater Hill and Puketutu volcanoes (ND group) yielded mean palaeointensities of 10.6 ± 1.2 (1σ), 11.8 ± 2.8 and 11.1 ± 0.4 μT, respectively. The Hampton Park volcano (55 ka; WU group) gave 9.5 ± 1.2 μT while the McLennan Hills volcano (SU group) gave 2.5 ± 0.5 μT. It is notable that consistent palaeointensities have been obtained from the three different volcanoes which have almost the same palaeodirections (ND group), possibly supporting the reliability of the palaeointensity data. These five palaeointensities for the Auckland geomagnetic excursions correspond to virtual dipole moments (VDMs) of 0.6-2.1 × 1022 A m2, whereas three mean palaeointensities obtained from the Auckland volcanoes having non-excursional palaeodirections are 13.1-40.0 μT giving stronger VDMs of 2.1-6.9 × 1022 A m2. These results suggest that the dipole component of the geomagnetic field reduced to about 2 × 1022 A m2 or less during the Auckland geomagnetic excursions. 相似文献
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Abstract Extensive subduction-related and intraplate volcanism characterize Cenozoic magmatism in the North Is., New Zealand. Volcanics in the central North Is., predominantly intermediate to felsic, form above the dipping seismic zone and show tectonic/geochemical features common to magmatism in most subduction zones. Basaltic volcanism in Northland, the northern part of the North Is., has chemical characteristics typical of intraplate magmatism and may be caused by the upwelling of asthenospheric materials from deeper parts of the mantle. The rifting just behind the present volcanic front (the Taupo-Rotorua Depression), which follows the trench ward migration of the volcanic front and the gradual steepening of the subducted slab, is also a feature of the North Is. A possible mechanism for the back-arc rifting in the area is injection of asthenospheric materials into the mantle wedge; this asthenospheric flow results from the mantle upwelling beneath Northland and pushes both the rigid fore-arc mantle wedge and the subducted slab trenchwards. This mechanism is also consistent with the stress fields in the North Is.: dilatation in Northland, northwest-southeast tension in the Taupo-Rotorua Depression, and the northeast-southwest compression in the fore-arc region. 相似文献
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The direction of propagation of magma-filled cracks was theoretically examined for a two-dimensional model. The analytical result indicates that magma-filled cracks, which have magmatic pressures larger than a critical value, propagate in parallel with the maximum principal direction of far-field stress. This may give one of theoretical grounds to the dike method by which the regional stress field is estimated. 相似文献