Paleomagnetism of Cretaceous Red beds from Eastern Part of South China Block

INTRODUCTIONCollisionofIndiawithAsiaappearstohavebroughtaboutlargetectonicdeformationofAsia,especiallyEastAsia(Tapponnieretal.,1986).Post-Cretaceousdeformation(translationand/orrotation)ofSimaoandIndochinablocks,whicharelocatedatthesouthernsideofRedRiverfault(RRF),hasbeenreportedfromCretaceouspaleomagneticin-vestigations(Yangetal.,l995;HuangandOpdyke,l993IYangandBesse,1993).AlsoinSouthChinablock(SCB)locatedatnorthsideoftheRRF,localdeformationofsomeregionsneartheRRFhasbeendocum…     

Direct measurements of mid-depth circulation in the Shikoku basin by tracking SOFAR floats

Mid-depth circulation of the Shikoku Basin was measured by tracking four SOFAR floats drifting at the 1,500 m layer. Two floats were released on 17 April 1988 at 30°N, 135°59E and tracked for 433 days. Another two were released on 3 November 1988 at 29°52N and 133°25E, and tracked for 234 days. Two floats flowed clockwise around the Shikoku Warm Water Mass with a diameter of 400 km centered at 31°N and 136°E and a mean drift speed of 4.5 cm sec^–1. One of the floats showed about ten counterclockwise rotations with a period of about 8 days and a maximum speed of 80 cm sec^–1 in the sea area west to the Izu Ridge. In the east to Kyushu, a southward flow was observed under the northward flowing Kuroshio. The southward flow of 4 cm sec^–1 drift speed was considered to be a part of the counterclockwise circulation at deep layers along the perimeter of the Shikoku Basin. One float remained for 234 days in a limited area of 100 km by 150 km in the western part of the basin.     

Petrology of peridotite xenoliths in lamprophyre from Shingu,Southwestern Japan: Implications for origin of Fe-rich mantle peridotites

Summary Xenoliths of harzburgite, lherzolite, dunite and wehrlite (= Group I rocks) in lamprophyre dikes from Shingu are accompanied by large amounts of ultramafic-mafic xeno liths with Al- and Ti-rich clinopyroxene and/or kaersuite (websterite, clinopyroxenite, kaersutite rock, gabbro and anorthosite) (= Group II rocks). The latter rocks often crosscut the Group I rocks as veinlets, indicating that Group II rocks are younger. Although harzburgites and lherzolite from Shingu have ordinary modal compositions, the constituent minerals have extraordinary chemical characteristics; low Mg and Cr and high Ti, Al and Fe³⁺. Fo values of olivine range from 91 to 77. Cr/(Cr + Al) atomic ratios of spinel are lower than 0.5 even in harzburgites. Fe³⁺/(Cr+Al+Fe³⁺) atomic ratios of spinel are sometimes over 0.1. TiO₂ contents of clinopyroxene often exceed 0.5 wt%. These characteristics are revealed when Group I rocks are veined or selvaged by Group 11 rocks; chemical compositions of minerals in peridotites systematically change forwards the latter. This strongly suggests that injections of melts with alkali basaltic affinity which had precipitated Group 11 rocks resulted in diffusion metasomatism on the Group I rocks.It is likely that the metasomatized peridotites are widespread underneath the areas where alkali basalt magmatism had fluorished, such as southwestern Japan. Some of Fe-rich lherzolite and harzburgite xenoliths reported in the literature are possibly metasomatites.

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Petrologie von Peridotit-Xenolithen in Lamprophyren von Shingu, Südwest-Japan: Hinweise auf die Herkunft Fe-reicher Mantel-Peridotite

Zusammenfassung In lamprophyrischen Gängen von Shingu kommen Xenolithe von Harzburgit, Lherzolith, Dunit and Wehrlit (= Gesteinsgruppe I) vor. Sie werden von einer Vielzahl von ultramaf-isch-mafischen Xenolithen mit Al- and Ti-reichem Klinopyroxen and/oder Kaersutit (Websterit, Klinopyroxenit, Kaersutit-Gestein, Gabbro and Anorthosit) (=Gesteinsgruppe II) begleitet, die die Xenolithe der Gruppe I häufig gangförmig durchkreuzen, was auf ein jü ngeres Alter der Gesteinsgruppe II hinweist. Obwohl die Harzburgite and Lherzolithe von Shingu übliche modale Mineralbestände aufweisen, sind die Mineralchemismen außergewöhnlich: Niedrige Mg- and Cr- and hohe Ti-, Al- and Fe³⁺-Gehalte. Die Fo-Gehalte von Olivin reichen von 91 bis 77. Die Cr/(Cr+Al)-Atom-Verhältnisse der Spinelle sind kleiner als 0,5, sogar in den Harzburgiten; die Fe³⁺/(Cr+Al+Fe³⁺)-Atom-Verhaltnisse teilweise größer als 0,1. Der TiO₂-Gehalt im Klinopyroxen ist meist über 0,5 Gew.%. Diese Charakteristika zeigen sich dort, wo die Gesteinsgruppe II die Gesteinsgruppe I durchschlägt oder kontaktiert. Der Mineralchemismus in den Peridotiten ändert sich dabei systematisch. Es wird vermutet, daß Schmelzinjektionen mit alkali-basaltischer Affinität, von denen die Gesteinsgruppe II herstammt, eine Diffusions-Metasomatose der Gesteinsgruppe I verursacht hat.Es wird angenommen, daß metasomatisierte Peridotite an der Basis von alkali-basaltischem Magmatismus weft verbreitet sind, wie zum Beispiel in Südwest-Japan. Einige in der Literatur aufscheinende Fe-reiche Lherzolith- and Harzburgit-Xenolithe sind möglicherweise metasomatisch entstanden.

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With 6 Figures     

Long-term temperature monitoring in a borehole drilled into the Nojima Fault, southwest Japan

Abstract Long-term monitoring of temperature distribution in an active fault zone was carried out using the optical fiber temperature-sensing technique. An optical fiber cable was installed in a borehole drilled into the Nojima Fault in Awaji Island, south-west Japan, and the temperature profile to a depth of 1460 m had been measured for 2.5 years (July 1997–January 2000). Although the obtained temperature records showed small temporal variations due to drifts of the measurement system all along the cable, local temperature anomalies were detected at two depths. One at around 80 m seems to correspond to a fracture zone and may be attributed to groundwater flow in the fracture zone. This anomaly had been stable throughout the monitoring period, whereas the other anomaly at around 500 m was a transient one. The water level in the borehole could be estimated from the diurnal temperature variations in the uppermost part of the borehole and may provide information on the hydrological characteristics of the fault zone, which is connected to the borehole through perforations on the casing pipe. Except for these minor variations, the temperature profile had been very stable for 2.5 years. The conductive heat flow calculated from this profile and the thermal conductivity measured on core samples increases with depth, probably resulting from errors in thermal conductivity due to sampling problems and/or from advective heat transfer by regional groundwater flow. Assuming that the middle part of the borehole (less fractured granite layer) is least affected by these factors, heat flow at this site is estimated to be approximately 70 mW/m².     

Windowed f-k spectra of a three-dimensional wavefield excited by a point source in a two-dimensional multilayered elastic medium

When full 3-D modelling is too costly or cumbersome, computations of 3-D elastic wave propagation in laterally heterogeneous, multilayered 2-D geological structures may enhance considerably our ability to predict strong ground motion for seismological and engineering purposes. Towards this goal, we extend the method based on the combination of the thin-layer finite-element and boundary-element methods (TLFE-BEM) and calculate windowend f - k spectra of the 3-D wavefield. The windowed f - k spectra are spatially localized spectra from which the local properties of the wavefield can be extracted. The TLFE-BEM is particularly suited for calculating the complete wavefield where surface waves are dominant in multilayered media. The computations are performed in the frequency domain, providing the f - k spectra directly. From the results for the 3-D wavefield excited by a point source in a 2-D multilayered, sloped structure, it can be said that the phase velocity of the fundamental-mode Rayleigh wave in a laterally heterogeneous multilayered medium, estimated from the windowed f - k spectra, varies with the location of the point source. For the model calculated in this article, the phase velocity varies between the value for the flat layered structure of the thick-layer side and that for the structure just under the centre of the window. The exact subsurface structure just under the centre of an array in a laterally heterogeneous medium cannot be obtained if we use the f - k spectral analysis assuming a flat layered structure.     

Occurrence of Spadella cephaloptera During One Year at Ischia Island (Gulf of Naples)

Abstract. The life cycle of the benthic arrowworm, Spadella cephaloptera , which inhabits the Posidonia meadow at Ischia (Gulf of Naples, Italy), was examined by a one-year systematic sampling survey and by laboratory culture. The seasonal fluctuations in abundance and the reproductive characteristics indicate that there were two generations of S. cephaloptera in one year; these generations have a different life span. The habitat of S. cephaloptera depends on the growth of the Posidonia meadow; the influence of different factors on the S. cephaloptera population, especially of the Posidonia vagile fauna, is discussed.     

Temporal variation of the partial pressure of CO2 in surface seawater of Iyo Nada in the Seto Inland Sea,Japan

The temporal variation of the partial pressure of carbon dioxide in surface seawater of Iyo Nada in the western Seto Inland Sea was measured by infrared absorption of carbon dioxide (CO₂) in air equilibrated with seawater. The variation of from January to late May in 1994 was relatively small (from 270 to 340 atm). on the contrary, the measured in September 1993 ranged from 200 to 450 atm. It seems that the relative high observed in the north and the south of Iyo Nada in September were caused by destruction of a stratification owing to tidal currents at straits between Hiroshima Bay and Iyo Nada and Hayasui Straits, respectively. The low observed in the central Iyo Nada in September was ascribed to a tidal front where high concentrations of chlorophyll-a and a relation between and chlorophyll-a in negative sense were found. Except the observation in the north of Iyo Nada in September, the in the surface seawater of Iyo Nada were lowe than the atmospheric (about 367 atm). The results obtained by this work and Kimotoet al. (1993) lead to the conclusion that Iyo Nada acts as a sink of the atmospheric CO₂ on the average.     

A real-time observation network of ocean-bottom-seismometers deployed at the Sagami trough subduction zone,central Japan

We installed a real-time operating regional observation network of Ocean-Bottom-Seismometers, connected to an electro-optical fiber communication cable, at the Sagami trough subduction zone, just south of the Tokyo metropolitan area, central Japan. The network, called ETMC, has six seismic observation sites at approximately 20 km spacing. In addition, there are three tsunami observation sites along the ETMC network to monitor the propagation process of tsunamis around the Sagami trough region.The on-line data from the ETMC has been improving the detection capability of smaller-magnitude earthquakes even at areas close to the margin of the trough. The ETMC data analyzing system, which has a function of real-time digital filtering for each seismic channel, can read the arrival times of P- and S-waves precisely, constraining well the automatic on-line hypocenter locations. The network has been providing useful information regarding the bending and downgoing process of the Philippine sea plate at the Sagami trough subduction zone.The pressure sensors of the installed network have a detection capability of tsunami wave trains with an amplitude of less than 1 cm. For example, the sensors recorded the full time history of tsunami wave trains, with mm order resolution, originating from a tsunami earthquake with 5.7 M_W and the tsunami magnitude of 7.5 occurred near Tori Shima (Tori Is.) of the Izu-Bonin Is. arc on September 4, 1996. The maximum amplitude of the tsunami signals on the trough-floor was approximately 1 cm (P-P), in contrast with approximately 20 cm (0-P) at a coastal site on Izu-Oshima, near the trough. Also, the pressure sensors observed tsunamis due to a large tsunami earthquake (7.1 M_W) at the northern New Guinea, on July 17, 1998.