Paleo-and rock magnetic experiments were carried out on the Cretaceous Yezo Supergroup in the Urakawa area (southern central Hokkaido) in order to detect tectonic episodes of the ancient eastern Eurasian margin. Characteristic remanent magnetization determined for five sites resides in magnetite, and has primary origin as indicated by positive result of tilting test. Bedding-parallel fabric of anisotropy of magnetic susceptibility and anisotropic acquisition of isothermal remanent magnetization (IRM) indicate significant inclination shallowing after deposition and lock-in of remanence. Shallowing ranges from 4° to 11° for the five sites on the basis of IRM anisotropy. The flattening-corrected mean inclination is still anomalously shallow (31°). Considering the correlation between the obtained inclination data and the geomagnetic polarity Chron C33r−C32r, a block containing the Urakawa area was located at a low latitude (16.7°N) during Campanian and then has experienced at largest 3400 km northward transportation. Transcurrent fault motions along the continental margin may be responsible for the transportation, as inferred from extremely large rotational motion (−161°). The Cretaceous Yezo forearc basin contains an exotic landmass that had evolved along a remote continental margin, then been transported as a result of oblique subduction.
Cretaceous granitic rocks, consisting of Itoshima granodiorite and Sawara granite, are occurred at upper reach of the Nakagawa river, Fukuoka Prefecture. The Sawara granite that is divided lithologicaly into fine-grained facies and the main facies indicates Rb-Sr whole rock isochron ages of 101.2±10.9 Ma and 96.6±5.9 Ma, respectively. Both granites also suggest different initial Sr isotopic ratios and geochemical features, indicating they have been originated from different source magmas. Rb-Sr mineral isochron ages using biotite of the main facies of the Sawara granite and the Itoshima granodiorite are 88.3±0.5 Ma and 88.9±0.2 Ma, respectively. These ages represent the time when the rocks cooled down to closure temperature of biotite. As a result, activity of the Cretaceous granitic rocks in this area is estimated as follows. The magmas of the Sawara granite (fine-grained and main facies) have been intruded into the Itoshima granodiorite during 101 to 97 Ma, and temperature of the granodiorite have been heated to over the closure temperature of biotite Rb-Sr system. After that, the rocks in the area gradually cooled down to the closure temperature of biotite around 88 Ma.
The Neogene formations in the eastern San’in district facing the Sea of Japan contain a large amount of felsic to intermediate volcanic and volcaniclastic materials. Alteration of these materials was investigated by means of X-ray diffractmetry using Cu Ka radiations. Five types of alteration are discriminated: diagenetic, caldera-type alkaline, regional intermediate, hydrothermal, and weathering. Diagenetic alteration zone is characterized by authigenic zeolites and classified into three zones: the fresh, the clinoptilolite-mordenite, and the analcime zones. Three zones are distributed in ascending order according to stratigraphy. Caldera-type alkaline alteration zone in this district (Teragi Caldera) is nearly same mineral assemblage to diagenetic alteration zone. However, zeolites occur at shallower burial depths in response to higher geothermal gradients. Regional intermediate alteration zone is characterized by smectite-chlorite series clay minerals. This zone is classified into three zones: the smectite, the smectite-chlorite mixture, and the chlorite zones, and is the most widespread. Hydrothermal alteration is divided into two types by mode of occurrence of alteration minerals. One is the vein-type in which alteration minerals occur in veins or amygdals. Another is the massive type in which alteration minerals occur as massive in whole rock. Alteration minerals are silicates, carbonates, sulfates, sulphides, oxides, hydroxides and others. Weathering is characterized by halloysite. The diagenetic alteration is variously overlapped by other types of alteration in each area. The caldera-type alteration is restricted in the Teragi Caldera only. The weathering crust covers the surface of post-Pliocene volcanic rocks. The magnetic susceptibility of altered rocks was measured. There are significant differences in the magnetic susceptibility among the altered rocks of each of the alteration type.
This paper describes the lithostratigraphy, microscopic characteristics, and conodont ages of Upper Triassic siliceous micrite from Kutajima Island, Kagoshima, southwest Japan. The siliceous micrite (ca. 34 m thick) comprises calcified radiolarians, thin-shelled bivalves, and calcispheres disseminated in a lime-mud matrix. Biostratigraphic data of conodonts show that the siliceous micrite accumulated in early Norian (Epigondolella quadrata Zone) through middle Norian (Epigondolella spiculata Zone to Epigondolella elongata Zone) with low sedimentation rates of 5.2 to 11.3 mm/kyr. The biotic constituents and lithologic characteristics of the siliceous micrite suggest the accumulation of deep-water oceanic basin. On the basis of lithologic characteristics and conodont ages, the siliceous micrite of Kutajima Island is compared with the siliceous micrite of the Sambosan accretionary complex, defined as a Late Jurassic to Early Cretaceous subduction-generated accretionary complex in southwest Japan.
Faults or shear fractures are sometimes observed in borehole cores. Fault-slip data from those samples can be used to understand (paleo) stress states at depths. This paper describes the mathematical methods of conversion from the fault-slip data measured from a core with an arbitrary attitude in a laboratory to the data with the attitude of the rock before the core was drilled. The data conversion uses a rotation matrix from the laboratory data to the in-situ ones, where the matrix is determined from the orientation of the core axis and attitude of the reference groove of an oriented core or from the orientation of the axis and bedding attitude in the sampling horizon and that in an unoriented core. Worked problems are presented: one using artificial data and the other using actual data from a well in Niigata Prefecture, Japan.
In the middle of the 8th century, a prospecting party under the Provincial Government of Michinoku discovered a gold placer at Nonodake Hill, the Province of Michinoku, Northeast Japan. The placer was worked by corvee labor. It was the earliest gold mining in Japan. K. Kudaranokonikishi, Governor of Michinoku, offered 33.75 kg of gold to the Emperor Shomu in 749. The people of the northern parts of Michinoku were saddled with 9.4 g of gold in poll tax from 752. The gold was used for gilding of the great bronze statue of Buddha at Nara, which was under construction. The statue, 15.8 m in height, was completed in 757. A total of 150 kg or more of gold was gilded the statue and others. In 760, the Japanese Government minted the first gold coin in Japan, which was named Kaikishoho. The working was interrupted because of a rebellion by the natives against the Government in 774, and was reopened after 38 years’ disturbances of war. The gold diggings decreased in production from the early part of the 9th century. The placer gold had been almost exhausted in the 15th century. A very small quantity of gold is still obtained from the remains of the diggings.
Molluscan and pollen fossils in the mangrove swamp are known from the upper Lower-lower Middle Miocene sediments in Southwest Japan. Pollen analysis was performed for the purpose of finding a fossil mangroves in Northeast Japan. Sixteen samples were collected from two sites of the Middle Miocene Myozawa- bashi Formation in the Oguni area, Yamagata Prefecture, Northeast Japan. As a result, evergreen Quercus occurred dominantly in the pollen composition of both sites, which is characteristic for the NP-2 zone of the Neogene pollen zones. In particular, mangrove pollen (Kandelia sp.) fossils were found in an oyster fossil bed. This is the northernmost record of fossil mangroves in Japan. The living mangrove Kandelia candel (L.) has a northern limit at South Kyushu (Kiire, Kagoshima Prefecture), and so the average winter (January) temperature of Oguni town in the early Middle Miocene period is estimated to have been at least 9°C higher than today. Since a mangrove swamp-dwelling bivalve Geloina was recently found in northern Iwate Prefecture, the mangrove pollen may be found in more northern places than the Oguni area.