The middle to upper Miocene diatom biostratigraphy has been established for the Nomura Formation exposed along the Otonashigawa River in the Tsugawa area, Niigata Prefecture, Japan. Neogene North Pacific diatom zones NPD5B through NPD6A are recognized in the Nomura Formation. Sediment accumulation curve contructed by diatom biochronology indicates that the sedimentation rate remarkably decreased to 0.3-0.15 meters per 1000 years around the base of the Nomura Formation. The last occurrence of Denticulopsis dimorpha var. areolata was recognized between the diatom biohorizons D59 (93 Ma) and D60 (9.2 Ma), and this event can potentially serve as a stratigraphic marker to improve the resolution of late Miocene diatom biostratigraphy. A mass occurrence of Goniothecium rogersii has been found in the lower part of zone NPD 5D between the horizons D56 and D58 where the species composes over 20 % of the total diatom assemblages. This event consists of four peaks of the abundance and persisted from 9.75Ma to 9.45Ma. The event has been recognized widely in several stratigraphic sections of the northern part of the Neogene Niigata sedimentary basin.
Eruption ages of the ejecta from Kutcharo and Mashu calderas were systematically determined by 14C dating. 16 charred samples were newly obtained from the Mashu and Nakashumbetsu Tephra Formations around the calderas and dated by AMS and β-counting methods. Examined units are Ma-d, Ma-e, Ma-f, Ma-j, Ma-k, Ma-l and Ml-a in the Mashu ejecta and 6 Nakashumbetsu tephra layers including Kutcharo Pumice Flow Deposit I (KpI), which is the youngest caldera-forming product from Kutcharo caldera. Results of the 14C dating range from 3,660 ±40 yBP to 36,080±1,300 yBP, and are consistent with the tephrostratigraphy. Calendar age for KpI was newly calculated at almost 40 ka and this age shows there was about 70,000 years recurrence interval between KpI and KpIV caldera-forming eruptions. Mashu caldera has appeared on the eastern part of Kutcharo caldera immediately after the KpI eruption, and calendar age for its main caldera-forming eruption were determined at ca. BC 5,600.
In northern Kitakami area, early Cretaceous plutonic masses are intruded into the Mesozoic sedimentary rocks. Those masses are aligned linearly almost on parallel three lines, trending NNW-SSE. In the ”Rikuchuseki”district of the 1/50,000 scale geological map series, six early Cretaceous plutonic masses occur and the five masses of them are on the alignments mentioned above. The Hiraniwa mass is on the western alignment, the Tenjinmori, Oguni and Numabukuro masses on the central alignment, and the Kawaguchi mass on the eastern alignment. The masses on the western alignment consist mainly of gabbro, quartz diorite and granodiorite. They construct the zoned pluton of which central part is more felsic. Tha gabbroic rocks contain some amounts of K-feldspar and mafic rock-facies of the masses are rather monzonitic. The masses on the central alignment consist mainly of tonalite, associating with small amount of gabbro or diorite. The masses on the eastern alignment consist mainly of gabbro with small amount of granodiorite and characterized by their high magnetic susceptibility. In the northeastern margin of the district, the Takinosawa mass occurs. It is a part of the early Cretaceous large plutonic masses occurring along the sea shore of the Pasific Ocean. In the western and eastern parts of the district, several small gabbroic dikes and masses occur. They were weakly metamorphosed and considered to be intruded before the intrusion of the early Cretaceous plutonic masses. In the northwestern part of present district, intrusions of andesite dike and tuff breccia occur, though small in amount. K-Ar age of the hornblende in andesite dike is 15.2±0.9 Ma.
The Sambagawa and Chichibu pelitic rocks are in direct contact in Kohtaki outcrop, which has been found out in field studies carried out in the Odai area, eastern Kii Peninsula. In the Kohtaki outcrop, it is suggested that difference in degree of metamorphic recrystallization between the two lithologies can be recognized by naked eyes, i.e. the Sambagawa pelitic phyllite is more highly recrystallized than the Chichibu pelitic mixed rock. In addition to this observation, overall difference in their constituent protolith was used to determine the boundary between the Sambagawa and Chichibu belts. The result suggests that the pelitic mixed rock in the Chichibu belt overlies pelitic phyllite in the Sambagawa belt with a moderately south dipping tectonic boundary, in contradiction with a previous study. It is also discussed that the juxtaposition of the two belts already completed by the end of the Sambagawa main deformation stage, Ds, which is associated with regional-scale E-W stretching. The Kohtaki outcrop likely preserves original structural relationships between the Sambagawa and Chichibu belts.
To determine the eruption age of the felsic lava flows distributed in the northeast of Izu Peninsula, we have performed fission-track dating of zircon from three volcanics named Izuyama, Higaneyama and Tenshozan. The resulting ages are 0.2±0.1, 0.26±0.04 and 0.1±0.1 Ma for Izuyama, Higaneyama and Tenshozan, respectively.
Sodic-calcic amphiboles (winchite and richterite) occur in dolerite in the Nedamo Terrane, Early Carboniferous accretionary complex. Sodic-calcic amphiboles occur generally in high-P/T metamorphic terranes such as the Sambagawa Terrane, the Suo Terrane etc., whereas they also occur in manganese ore deposits and alkali igneous rocks. Sodic-calcic amphiboles of a high-P/T metamorphic type tend to show the feature of high-Al2O3 and low-TiO2. The sodic-calcic amphiboles of the dolerite in the Nedamo Terrane are difficult to be recognized as the high-P/T metamorphic type because they do not show the feature of high-Al2O3 and low-TiO2, that is they contain quite low glaucophane component. However, the sodic-calcic amphiboles could be formed as a metamorphic mineral in a subduction zone considering that some sodic-calcic amphiboles which show low glaucophane component occur certainly in high-P/T metamorphic rocks, bulk chemistry of the dolerite shows the feature of within-plate tholeiite, occurrence of the sodic-calcic amphiboles is different from that of sodic-calcic amphiboles in alkali igneous rock or contact aureole, and the dolerite does not undergo alteration or metasomatism.
A stick-slip experiment was conducted using dry cylindrical samples saw-cut at an angle of 30° under confining pressures ranging from 30 MPa to 120 MPa. The samples used in the experiment were quartz-free basalt, gabbro, and peridotite. Loading on the rock samples was applied by a manual micro-discharge pump at a controlled rate of ～ 0.003 mm/s. Solenoids aligned circumferentially with the sample were used as sensors for detecting the magnetic field signal, while two pairs of electrodes, each consisting of two copper plates (30×30 mm), were used as sensors for detecting the electric field signal. The electrode pairs were aligned circumferentially with the cylindrical sample, one perpendicular to and the other parallel with the saw-cut surface. Variations in both the electric and magnetic fields were detected at the point of slippage during stick-slip motion in all samples, even in the quartz-free samples. However, no such signals were detected during stable sliding. In almost all of the gabbro samples, the amplitude of the electric field variation measured by the electrode pair aligned parallel to the fault was longer than thet obtained by the electrode pair orientated normal to the fault. This indicates that the gabbro samples are anisotropic. Moreover, the separation of electric charges between the faults and electric discharge is considered to occur simultaneously because the signals detected by the coils and the signals observed on the electrodes occurred almost simultaneously. Furthermore, the amplitude of the electric field variation measured by the electrode pairs was found to increase with the stress drop during stick-slip. Next, signals induced in each of the four copper electrodes were measured individually to confirm the anisotropy of the electric field variation. Results showed that the signals detected by the electrodes positioned parallel sides of the fault of the gabbro sample exhibited opposite polarities in the early stages, confirming anisotropy parallel and normal to the fault. For the basalt samples, although the electric and magnetic fields were too low to measure differences in the two directions for some samples, similar tendencies as those for the gabbro samples were observed.