�@	Active structures and their relation to earthquakes along the eastern margin of the Japan Sea
		K-Ar ages of white micas from pelitic schists of the Bayanhongor area, west Mongolia
		The granitoid series in Bayankhongor area, central Mongolia
		A practical guide to writing geological reports in English

Active structures and their relation to earthquakes along the eastern margin of the Japan Sea

Yukinobu Okamura, Shin-ichi Kuramoto and Mikio Satoh

1998

vol. 49 (1), p. 1-18, 14 figs.

Keywords: Japan Sea, active fault, earthquake, plate boundary

Abstract: The distribution and characteristics of active faults and folds along the eastern margin of the Japan Sea were clarified mainly on the basis of seismic data of the Geological Survey of Japan. The area was divided into the southern, middle and northeastern parts according to their structural style. The southern part is more than 100 km wide and 250 km long extending from Sado Island to the Oga Peninsula, where the major faults tend to be concentrated in two fault zones. In the middle part, a major fault zone forms the Okushiri Ridge which extends from the west of the Nishitsugaru Basin to the west of Rumoi over a distance of 450 km, where the ridge consists of several swells which are about 100 km in length. To the west of the southern part of the Okushiri Ridge, another active fault zone has formed in the Japan Basin. The northeastern part consists of an area of continental slope about 150 km wide, with water depths shallower than 2,000m. Previous data suggests that a couple of small scale active fault zones extend in a NNE direction in this part and that no distinct active structures are observed to the west of the Musashi Bank or in the Tartary Trough, which are located in the northern extension of the Okushiri Ridge. Fault models of the 1964 Niigata earthquake, the 1983 Nihonkai-chubu earthquake and the northern part of the 1993 Hokkaido-oki earthquake roughly agree with the active faults. In contrast, the southern part of the fault models for the 1993 Hokkaido Nansei-oki earthquake does not correlate with the active structures. Many active faults have been formed in the zone ranging in width from several tens of km to more than 100 km along the eastern margin of the Japan Sea, where the dimensions and structure of the faults are comparable with onshore reverse faults in the NE Japan Arc. This fact strongly suggests that the present eastern margin of the Japan Sea can not be defined as a convergent plate boundary.

K-Ar ages of white micas from pelitic schists of the Bayanhongor area, west Mongolia

Chikao Kurimoto, Floragiin Tungalag, Lkhamsuren Bayarmandal and Niidengiin Ichinnorov

1998

vol. 49 (1), p. 19-23, 6 figs., 1 table.

Keywords: Mongolia, Bayanhongor, white mica, K-Ar age, pelitic schist.

Abstract: The Bayanhongor area is about 500 km west of Ulaanbaatar, and is geotectonically divided into the Baidrag, Burdgol, Bayanhongor, Zag and Hangai Zones from south to north. K-Ar age determination was made on white micas from two samples of pelitic schists which were collected at the northern extremity of the Bayanhongor Zone. The white micas gave K-Ar ages of 453.9�}9.1 Ma and 447.4�}9.0 Ma, indicative of the age of metamorphism.

The granitoid series in Bayankhongor area, central Mongolia

Yuhei Takahashi, Sambuu Oyungerel, Kazuki Naito and Baljinnyamiin Delgertsogt

1998

vol. 49 (1), p. 25-32, 7 figs.

Keywords: granitoid, magnetite-series, ilmenite-series, ore deposits, Bayankhongor, Mongolia

Abstract: The granitoid series in Bayankhongor area, central Mongolia, has been investigated using magnetic susceptibility and chemical data. In Riphean, the ilmenite-series granitoids were dominant. In early Paleozoic, the ilmenite-series granitoids were dominant, but the magnetite-series granitoids were also distributed in southwestern part of the study area. In late Paleozoic, the magnetite-series granitoids were predominant. The boundary between magnetite-and ilmenite-series granitoids shifted northeastward in early Paleozoic to late Paleozoic. Shifting of the boundary between the two series may be genetically related to the development of the Khangay Zone, which consists of middle to late Paleozoic strata. Mesozoic granitoids are ilmenite-series granitoids.
�@In late Paleozoic, Cu and Au deposits are distributed within the magnetite-series granitoids area. Sn and W deposits occur around the boundary between the two series and in the ilmenite-series dominant area. These distribution patterns indicate that the redox potential decreases from southwest to northeast in late Paleozoic granitoids of the study area.

A practical guide to writing geological reports in English

Morimasa Yoshii

1998

vol. 49 (1), p. 33-53, 3 tables.

Keywords: guide, English, capitalization, italicization, numeral, abbreviation, punctuation