Magma mingling in the Tungho area,Coastal Range of eastern Taiwan

Complex rocks, consisting of different lithologic breccias and sediments in the Tungho area of the southern Coastal Range, eastern Taiwan, were formed by magmas and magma–sediment mingling. Based on field occurrences, petrography, and mineral and rock compositions, three components including mafic magma, felsic magma, and sediments can be identified. The black breccias and white breccias were consolidated from mafic and felsic magma, respectively. Isotopic composition shows these two magmas may be from the same source. Compared to the white breccias, the black breccias show clast-supported structures, higher An values in plagioclase, higher contents of MgO, CaO, and Fe₂O₃ and lower SiO₂, greater enrichment in the light rare earth elements (LREE), and depletion in the heavy rare earth elements (HREE). The white breccias show matrix-supported blocks and mingling with tuffaceous sediments to form peperite. Physical and chemical evidence shows that the characteristics of these two components (mafic and felsic magmas) are still apparent in the mingled zone. According to their petrography, mafic and felsic magmas did not have much time for mingling. White intrusive structures and black flow structures show that mingling occurred before they solidified. Finally, the occurrence of mingling between magmas and sediments suggests that the mingling has taken place at the surface and not in the magma chamber.     

Tectonics and sedimentation around Kashinosaki Knoll: A subducting basement high in the eastern Nankai Trough

Abstract   When seamounts and other topographic highs on an oceanic plate are subducted, they cause significant deformation of the overriding plate and may act as asperities deeper in the seismogenic zone. Kashinosaki Knoll (KK) is an isolated basement high of volcanic origin on the subducting Philippine Sea Plate that will soon be subducted at the eastern Nankai Trough. Seismic reflection imaging reveals a thick accumulation of sediments (∼1200 m) over and around the knoll. The lower portion of the sedimentary section has a package of high-amplitude, continuous reflections, interpreted as turbidites, that lap onto steep basement slopes but are parallel to the gentler basement slopes. Total sediment thickness on the western and northern slopes is approximately 40–50% more than on the summit and southeastern slopes of KK. These characteristics imply that the basal sedimentary section northwest of KK was deposited by infrequent high-energy turbidity currents, whereas the area southeast of KK was dominated by hemipelagic sedimentation over asymmetric basement relief. From the sediment structure and magnetic anomalies, we estimate that the knoll likely formed near the spreading center of the Shikoku Basin in the early Miocene. Its origin differs from that of nearby Zenisu Ridge, which is a piece of the Shikoku Basin crust uplifted along a thrust fault related to the collision of the Izu–Bonin arc and Honshu. KK has been carried into the margin of the Nankai Trough, and its high topography is deflecting Quaternary trench turbidites to the south. When KK collides with the accretionary prism in about 1 My, the associated variations in sediment type and thickness around the knoll will likely result in complex local variations in prism deformation.     

Large aperture seismic imaging at a convergent margin: Techniques and results from the Costa Rica seismogenic zone

In March and April 1995 a cooperative German, Costa Rican, and United States research team recorded onshore-offshore seismic data sets along the Pacific margin of Costa Rica using the R/V Ewing. Off the Nicoya Peninsula we used a linear array of ocean bottom seismometers and hydrophones (OBS/H) with onshore seismometers extending across much of the isthmus. In the central area we deployed an OBS/H areal array consisting of 30 instruments over a 9 km by 35-km area and had land stations on the Nicoya Peninsula adjacent to this marine array and also extending northeast on the main Costa Rican landmass. Our goal in these experiments was to determine the crustal velocity structure along different portions of this convergent margin and to use the dense instrument deployments to create migrated reflection images of the plate boundary zone and the subducting Cocos Plate. Our specific goal in the central area was to determine whether a subducted seamount is present at the location of the 1990, M 7 earthquake off the Nicoya Peninsula and can thus be linked to its nucleation. Subsequently we have processed the data to improve reflection signals, used the data to calculate crustal velocity models, and developed several wide-aperture migration techniques, based on a Kirchhoff algorithm, to produce reflection images. Along the northern transect we used the ocean bottom data to construct a detailed crustal velocity model, but reflections from the plate boundary and top and bottom of the subducting Cocos plate are difficult to identify and have so far produced poor images. In contrast, the land stations along this same transect recorded clear reflections from the top of the subducting plate or plate boundary, within the seismogenic zone, and we have constructed a clear image from this reflector beneath the Nicoya shelf. Data from the 3-D seismic experiment suffer from high-amplitude, coherent noise (arrivals other than reflections), and we have tried many techniques to enhance the signal to noise ratio of reflected arrivals. Due to the noise, an apparent lack of strong reflections from the plate boundary zone, and probable structural complexity, the resulting 3-D images only poorly resolve the top of the subducting Cocos Plate. The images are not able to provide compelling evidence of whether there is a subducting seamount at the 1990 earthquake hypocenter. Our results do show that OBS surveys are capable of creating images of the plate boundary zone and the subducting plate well into the seismogenic zone if coherent reflections are recorded at 1.8 km instrument spacing (2-D) and 5 km inline by 1 km crossline spacing for 3-D acquisition. However, due to typical high amplitude coherent noise, imaging results may be poorer than expected, especially in unfavorable geologic settings such as our 3-D survey area. More effective noise reduction in acquisition, possibly with the use of vertical hydrophone arrays, and in processing, with advanced multiple removal and possibly depth filtering, is required to achieve the desired detailed images of the seismogenic plate boundary zone.     

Compositional relations of coexisting orthopyroxene,pigeonite and augite in a tholeiitic andesite from Hakone Volcano

The compositions of five different coexisting pyroxenes hypersthene, pigeonite and augite in groundmass and bronzite and augite of phenocryst in a tholeiitic andesite from Hakone Volcano, Japan have been determined by the electron probe microanalyser. It is shown that there is a compositional gap of about 25 mole per cent CaSiO₃ between groundmass pigeonite and augite, compared with 35 per cent CaSiO₃ between phenocrystic augite and bronzite. Subcalcic augite or pigeonitic augite was not found. The groundmass augite, which occurs only as thin rims of pigeonite and hypersthene, is less calcic and more iron-rich than the phenocryst augite. It is also shown that the groundmass pigeonite is 3–4 mole per cent more CaSiO₃-rich than the coexisting groundmass hypersthene. The Fe/(Mg + Fe) ratios of these coexisting hypersthene and pigeonite are about 0.31 and 0.33, respectively. It is suggested from these results that a continuous solid solution does not exist between augite and pigeonite of the Fe/(Mg + Fe) ratio at least near 0.3 under the conditions of crystallization of groundmass of the tholeiitic andesite. It is suggested from the Mg-Fe partition and the textural relation that the groundmass augite crystallized from a liquid more iron-rich than that from which groundmass hypersthene and pigeonite crystallized.     

中国内陆高原正极性雷电的观测实验研究

叙述了１９９７年夏季中日两国科学家在甘肃平凉地区合作进行的有关正极性雷电的综合观测实验情况,包括地布局,所采用的技术手 取得的主要结果。实验中采用人工引发雷电技术并取得了两次成功,其中一次是利用“空中解发”方式,即火箭拖带的细钢丝下端不直接接地,而是通过一段绝缘尼龙线与接地的另一段细钢丝相连。用这种方式可以更真实地再现自然雷电的初始过程。实验中观测到了空中导线两端先导的双向传输现象;电流、电场变化     

地闪回击的微秒级辐射场特征及近地面连接过程分析

利用１μｓ时间分辨率的慢天线电场变化仪在甘肃中川地区雷暴过程中测量得到的大量地闪辐射波形地地闪回击辐射场特征及回击的慢前沿过程进行分析,发现１８次正地闪和８５次负地闪产在周前沿过程上升时间为１９．２μｓ和９．４μｓ,８４次负地闪继后回击的前沿过程为４．３ μｓ。ＹＹ ＵＤＡ Ｏ ３．１μ;     

Year-to-Year and interdecadal variability in the activity of intraseasonal fluctuations in the Northern Hemisphere wintertime circulation

Summary Interannual variability in the activity of fluctuations with subseasonal time scales is investigated based upon observed data of the extratropical Northern Hemisphere circulation over the recent 38 winters. Their activity is represented in the root mean square (RMS) field of filtered geopotential height in which the fluctuations with time scales between 10 days and a season are retained. The singular value decomposition (SVD) was applied to the covariance matrix between the seasonal mean and RMS fields for the 500-hPa height.The leading SVD mode for the north Pacific represents the strong relationship between the polarity of the Pacific/North American (PNA) pattern in the seasonal-mean anomalies and the amplitude of a meridionally-oriented dipole-like oscillation within the season. It tends to be more active when the seasonal-mean jet stream is strongly diffluent over the central Pacific than when the jet is extended zonally across the Pacific. The leading SVD mode for the north Atlantic is indicative of stronger intraseasonal fluctuations near Greenland in the presence of anticyclonic seasonal-mean anomalies associated with the North Atlantic Oscillation (NAO).The intraseasonal variability in the extratropics is strongly correlated with the underlying sea surface temperature (SST) anomalies, and that in the north Pacific also exhibits significant but rather weak correlation with SST anomalies in the equatorial Pacific. The activity of the atmospheric intraseasonal fluctuations is found to be modulated in accordance with interdecadal variability in the seasonal-mean circulation and SST.On leave from Department of Earth & Planetary Physics, University of Tokyo.With 12 Figures     

Determination of Ge, As, Se and Te in Silicate Samples Using Isotope Dilution-Internal Standardisation Octopole Reaction Cell ICP-QMS by Normal Sample Nebulisation

A method for the determination of Ge, As, Se and Te in silicate samples using isotope dilution-internal standardisation (ID-IS) octopole reaction cell (ORC) ICP-QMS by normal sample nebulisation was developed. The method does not involve either hydride generation or ion exchange. Germanium, Se and Te were determined by isotope dilution (ID), and As was determined by ID-IS. A silicate sample with an added Ge-Se-Te spike was digested with an HF-HNO₃-HBr mixture, dried, re-dissolved with HF and the supernatant liquid was directly aspirated into an ORC-ICP-QMS instrument with He or H₂ gas. No matrix effects were observed down to a dilution factor (DF) of ∼ 70 for Ge, Se and Te and DF of ∼ 1000 for As, which resulted in 3s detection limits in silicates of 2, 1, 0.1 and 4 ng g⁻¹, respectively. Advantages of the method are the simple sample introduction as well as a capability of determining S, Ti, Zr, Nb, Mo, Sn, Sb, Hf and Ta by ID-IS-ICP-QMS/SFMS from the same solution. Furthermore, the total sample solution consumption was only 0.253 ml with DF = 2000. Therefore, only a 0.13 mg test portion was required. To demonstrate the applicability of this technique, Ge, As, Se and Te in eight silicate reference materials were determined, as well as S, Ti, Zr, Nb, Mo, Sn, Sb, Hf and Ta in four carbonaceous chondrites.     

The effect of back pressure on the contact evolution of a close binary system

The contact condition which includes the effect of pressure in common envelope is derived and employed in the evolutionary calculation of a close binary system in contact phase. The result indicates that the effect is negligibly small and the equality of surface potentials of both components is a sufficiently good approximation for the contact condition.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.