The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 108, Issue 9
Displaying 1-6 of 6 articles from this issue
  • Hanako Domitsu, Miki Shiihara, Masayuki Torii, Shinji Tsukawaki, Motoy ...
    2002 Volume 108 Issue 9 Pages 545-556
    Published: September 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The piston cored sediment KT96-17 P-2 from the southeastern marginal part of the Oki Trough in the southern Japan Sea contained five volcanic ash layers. We studied the morphologic feature and chemical composition of volcanic glass shards and mineral composition in each ash layer. Between the Aira-Tn Ash (AT) and Kikai-Akahoya Ash (K-Ah) layers in the core, we recognized two ash fall deposits derived from the Ulleung Island. The glass shards from these two ash fall deposits have different chemical composition. This fact suggests that the Ulleung-Oki Ash (U-Oki) which has been regarded as one of important time-marker tephras in the paleoceanographic study of the Japan Sea might not be a unique layer but comprised of these two ash fall deposits from the Ulleung Island. It also implies that the U-Oki Ash might possibly be correlatable not only to U-2 but also to U-3 or U-4, which are three thick pumice layers between the AT and K-Ah layers in the Ulleung Island. We also recognized the Daisen-Kusadanihara Pumice (KsP) layer between the AT and K-Ah Ash layers in the core. AMS 14C datings were conducted on planktic foraminiferal tests picked from the horizons above and below the KsP layer in the core. The AMS 14C ages were 18, 070±50 yrBP (GX-26361) and 18, 470±160 yrBP (GX-25558), respectively. The eruption age of KsP was estimated to be 20, 260-22, 130 cal yrBP (2 sigma range) by calibration to calendar ages using CALIB 4.3 program.
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  • Kiyoshi Kato, Hironori Shimizu, Yukiyasu Saka
    2002 Volume 108 Issue 9 Pages 557-574
    Published: September 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Kurosegawa Terrane intervening in the Chichibu Terrane represents a serpentinite melange zone. It is characterized by various kinds of exotic rocks, including granitoids, metamorphic rocks of different metamorphic degrees and Siluro-Devonian sediments, and is intimately associated with the Cretaceous forearc basin sediments. The Chichibu Terrane of the central Kii Peninsula has long been believed to consist only of its southern belt, with its northern and middle belts missing due to fault-tectonics. We revealed that the terrane comprises the northern and the middle belts as well as the southern belt, separated by longitudinal faults from each other. Each belt is characterized by different lithology, structure and metamorphic grade of degree, and consists mainly of the Jurassic-Early Cretaceous accretionary complex. Peculiar rocks exotic to the accretionary complex and characteristic of the Kurosegawa Terrane are found in the middle belt. They are named the Gobanseki Metagabbro, the Dorokawa Metagabbro and the Kosakatoge Complex. Occurrence of rocks typical of the Kurosegawa Terrane in the middle belt safely attests that the middle belt is represented by the Kurosegawa Terrane and that the Kurosegawa Terrane, without an essential break in between, extends from the eastern end up to the central part of the Kii Peninsula where the deepest erosion level of the Chichibu Terrane is exposed on the ground surface due to the later tectonic uplift. Occurrence of the Kurosegawa Terrane in the central Kii Peninsula cannot be compatible with the nappe model proposed to explain the formation of the Kurosegawa Terrane and would be more satisfactorily explained by a new model that the Kurosegawa Terrane is a product of episodic fault tectonics in the late Early Cretaceous period.
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  • Hiroyoshi Arai
    2002 Volume 108 Issue 9 Pages 575-590
    Published: September 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    En-echelon veins hosted by sandstone and mudstone of the Atokura Formation were examined to clarify the paleostress field in that formation which constitutes the Atokura Nappe in a northern part of the Kanto Mountains. The veins are morphologically classified into two groups:1) a similar fold type with smoothly curved limbs, the vein tips pinch out and are not linked to any other fractures (type A) and, 2) a chevron fold type, the vein tips linked to joints formed before the vein opening (type B). The two types show a different texture and assemblage of infilling minerals. The type A consists mostly of fibrous to polygonal quartz and chlorite with lesser amounts of calcite and albite, whereas the type B of polygonal to prismatic calcite and quartz with lesser amounts of plagioclase and chlorite. Regardless of the vein type, morphology of the en-echelon sigmoidal veins is represented by a similar fold, but that of bridges is close to a parallel fold. The presence of undulatory extinction and subgrains in the fibrous quartz and pressure-solution cleavage in the bridges, which are considered to have simultaneously formed, indicates that the en-echelon veins were formed in a brittle-ductile shear zone. According to the paleostress analysis with conjugate sets of type-A vein arrays, the maximum (σ1) principal stress axis is inferred to have been oriented in the NW-SE to NNE-SSW direction and horizontal, intermediate (σ2) principal stress axis vertical, and minimum (σ3) principal stress axis in the NE-SW to WNW-ESE direction and horizontal. The veining could have occurred during the formation of E-W trending open upright folds of the Upper Cretaceous Atokura Formation.
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  • Ken-ichi Kano
    2002 Volume 108 Issue 9 Pages 591-605
    Published: September 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Precise mapping and analysis of bedding attitudes of the very-weakly metamorphosed Mesozoic accretionary complex in the Mino-Tanba Belt around the NNW-SSE to N-S trending Yanagase Fault, Inner Zone of Southwest Japan, have identified N-S trending map-scale folds with steeply-plunging axes. Horizontal buckling about subvertically plunging rotation axes after the regional steepening of bedding sufraces produced these folds. The Yanagase Fault occurs just on the hinge surface of the largest fold. The fold continues several kilometers or more to the north of the fault tip. The spatial relationship suggests that the Yanagase Fault has its origin as a kind of "fold-related fault" that has developed in association with the growth of the map-scale chevron folds. The fault displaces the basement accretionary rocks several kilometers left-laterally, whereas the confirmed maximum topographic offset along the fault is only about 170 m. This suggests that the Late Quaternary faulting has occurred on the weak surface in the pre-existing fault zone.
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  • Tetsuro Hirono, Tadashi Yokoyama, Manabu Takahashi, Satoru Nakashima, ...
    2002 Volume 108 Issue 9 Pages 606-609
    Published: September 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
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  • Tetsuji Onoue, Hitoshi Tanaka
    2002 Volume 108 Issue 9 Pages 610-613
    Published: September 15, 2002
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Upper Triassic bivalves were discovered from the Sambosan Subterrane of a Late Jurassic to earliest Cretaceous accretionary complex along the uppermost reaches of Motoidani Gully, Itsuki-mura, Kumamoto Prefecture. The examined succession entirely comprises an accreted oceanic-rock assemblage of the basaltic volcaniclastic rocks (ca. 60 m thick) conformably succeeded by bedded black limestone (ca. 10 m thick), which is, in turn, over-lain by partly dolomitic, massive limestone (ca. 50 m thick). Two Late Triassic bivalvian assemblages were recognized in the examined succession. One is the Carnian St. Cassian-type Assemblage from the upper part of the volcaniclastic rocks and the lowermost part of the bedded black limestone. The other is the Norian Megalodontid Assemblage from the lower to middle part of the massive limestone, with an emphasis upon the successive relation of the basaltic volcaniclastic rocks to the overlying limestone. Occurrence of Gruenewaldia decussata and G. woehrmanni of the St. Cassian-type Assemblage, and the Megalodontid Assemblage implies a Tethyan affinity of the Sambosan oceanic rocks.
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