Origin of a native sulfur chimney in the Kueishantao hydrothermal field, offshore northeast Taiwan

Analyses of rare earth and trace element concentrations of native sulfur samples from the Kueishantao hydrothermal field were performed at the Seafloor Hydrothermal Activity Laboratory of the Key Labo-ratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences. Using an Elan DRC II ICP-MS, and combining the sulfur isotopic compositions of native sulfur samples, we studied the sources and formation of a native sulfur chimney. The results show, when comparing them with native sulfur from crater lakes and other volcanic areas, that the native sulfur content of this chimney is very high (99.96%), the rare earth element (REE) and trace element constituents of the chimney are very low (ΣREE<21×10?9), and the chondrite-normalized REE patterns of the native sulfur samples are similar to those of the Kueishantao andesite, implying that the interaction of subseafloor fluid-andesite at the Kueishantao hydrothermal field was of short duration. The sulfur isotopic compo-sitions of the native sulfur samples reveal that the sulfur of the chimney, from H2S and SO2, originated by magmatic degassing and that the REEs and trace elements are mostly from the Kueishantao ande-site and partly from seawater. Combining these results with an analysis of the thermodynamics, it is clear that from the relatively low temperature (<116 ℃ ), the oxygenated and acidic environment is favorable for formation of this native sulfur chimney in the Kueishantao hydrothermal field.     

Chert–clastic sequence in the Cretaceous Shimanto Accretionary Complex on the central Kii Peninsula,SW Japan

Ocean plate stratigraphy (OPS) within an ancient accretionary complex provides important information for understanding the history of an oceanic plate from its origin at a mid‐ocean ridge to its subduction at a trench. Here, we report a recently discovered chert–clastic sequence (CCS) that comprises a continuous succession from pelagic sediments to terrigenous clastics and which constitutes part of the OPS in the Akataki Complex within the Cretaceous Shimanto Accretionary Complex on the central Kii Peninsula, SW Japan. As well as describing this sequence, we present U–Pb ages of detrital zircons from terrigenous clastic rocks in the CCS, results for which show that the youngest single grain and youngest cluster ages belong to the Santonian–Campanian and are younger than the radiolarian age from the underlying pelagic sedimentary rock (late Albian–Cenomanian). Thus, the CCS records the movement history of the oceanic plate from pelagic sedimentation (until the late Albian–Cenomanian) to a terrigenous sediment supply (Santonian–Campanian).     

Termination of intra‐arc rifting at ca 16 Ma in the Southwest Japan arc: The tectonostratigraphy of the Hokutan Group

Miocene intra‐arc rifting associated with the opening of the Japan Sea formed grabens in several areas in Southwest (SW) Japan, but the extensional tectonics of the arc are still not well understood. In this study, we first document the tectonostratigraphy of the Hokutan Group in the northwestern part of the Kinki district, and demonstrate the termination of extensional tectonics at ca 16.5 Ma, as inferred from grabens in the lower part of the group being unconformably overlain by sediments of the upper part. Second, we review early Miocene grabens in SW Japan to suggest that intra‐arc rifting was abandoned at ca 16 Ma, essentially simultaneously with the end of rotation of the SW Japan arc as evidenced by paleomagnetic studies. The lesser numbers of grabens and reduced thicknesses of graben fills suggest that extensional deformation of the SW Japan arc was significantly weaker than that of the Northeast (NE) Japan arc, which was broken into blocks, indicating various degrees of paleomagnetic rotation within NE Japan. The weak deformation has allowed paleomagnetic studies to infer the coherent rotation of the SW Japan arc.     

日本9．0级地震对新疆钻孔应变的影响

初步分析日本9．0级地震对新疆钻孔应变台站观测资料的影响,结果表明：该地震对新疆钻孔应变的影响与钻孔深度和震中距关系不大,这可能与钻孔所处的岩石性质等因素有关。     

Syntheses of the regional stress fields of the Japanese islands

The principal stresses in northern Honshu and in central-southwest Japan are synthesized on the basis of the ridge push, slab pull and across-arc variation of differential forces due to crust/plate structural variation. Assuming a more compressive north–south horizontal stress in central Japan-northern Honshu than that of southwest Japan, the calculated principal stress profiles explain the observed stress fields in these areas: namely, a strike–slip fault type for southwest-central Japan and a reverse fault type for northern Honshu, both having east–west σ_Hmax. Kyushu is characterized by the gradient of horizontal stresses both in the east–west and north–south directions, which cannot be explained by simple plate interactions or by crust/plate structural variation. Combined with other lines of evidence for existence of mantle upwelling in the East China Sea west of Kyushu, it is proposed that the stress gradient is produced by the viscous drag exerted by the flow spread laterally from the upwelling plume. The eastward movement of Kyushu and southwest Japan relative to Eurasia revealed by the recent Global Positioning System measurements conducted by the Geographical Survey Institute of Japan would be partly explained by this basal drag.     

Hydrothermal sediments associated with a relict back-arc spreading center in the Shikoku Basin, recovered from the Nankai accretionary prism, Japan

The hemipelagic mudrocks of the Nankai accretionary prism, Japan, contain hydrothermal deposits associated with a relict spreading center in the Shikoku Basin. Initial work on core samples from Ocean Drilling Program site 808 found several samples with elevated concentrations of calcium, magnesium, iron and manganese, at depths of between 1060 and 1111 m below sea floor. However, the origin of these sediments was uncertain, due to a lack of data. There was no recorded evidence of whether these elevated concentrations were present throughout this interval of core, or if they were present as discrete layers with the background hemipelagic mudrocks in between. In the present study the core was resampled, and the sediments with anomalous chemical compositions were found to be present in discrete layers. This fact, along with a detailed interpretation of their geochemistry, has allowed them to be identified as hydrothermal sediments, associated with the relict spreading center in the Shikoku Basin. The lower (older) two layers display a chemical composition typical of umbers, while the upper (younger) two layers are metalliferous mudrocks typical of deposits found further from the spreading center.     

华北地区与日本海沟地震的相关性分析

据板块运动及构造动力的因果关系研究了我国华北地区６级以上与日本海沟地区７级以上地震活动的关系，结果表明，两个地区群体地震活动在时间轴上的不均匀分布及起伏具有一定的同步性，且华北地区的群体地震活动的开始时间晚于日本海沟地区。     

日本列岛及中国东南沿海地震对中国大陆地震活动的影响估计

１９９４－１９９５年初日本列岛及中国东南沿海发生一组大地震，其中包括引起重大破坏的阪神大地震。从地震横向迁移角度对这些大地震集中发生对大陆地震的影响进行了分析。大震引起的应变波动传播速度馒、周期长、能量大、影响距离和深度大，其波峰可能触发地震。据Ｂｏｔｔ和Ｄｅａｎ的非弹性介质应变波运动方程解释了从海沟到内陆的大震迁移，指出１９１８年以来迁移序列目前对山西地震带及贺兰山地震带有影响；１９９４－１９９５年的地震序列在２０２０年左右将对渤海──郯庐断裂产生影响，２０３０年左右对华北平原产生影响；目前可能激发广东、福建沿海发生中等地震。     

Anatomy and genesis of a subduction-related orogen: A new view of geotectonic subdivision and evolution of the Japanese Islands

Abstract The Japanese Islands represent a segment of a 450 million year old subduction-related orogen developed along the western Pacific convergent margin. The geotectonic subdivision of the Japanese Islands is newly revised on the basis of recent progress in the 1980s utilizing microfossil and chronometric mapping methods for ancient accretionary complexes and their high-P/T metamorphic equivalents. This new subdivision is based on accretion tectonics, and it contrasts strikingly with previous schemes based on‘geosyncline’tectonics, continent-continent collision-related tectonics, or terrane tectonics. Most of the geotectonic units in Japan are composed of Late Paleozoic to Cenozoic accretionary complexes and their high-PIT metamorphic equivalents, except for two units representing fragments of Precambrian cratons, which were detached from mainland Asia in the Tertiary. These ancient accretionary complexes are identified using the method of oceanic plate stratigraphy. The Japanese Islands are comprised of 12 geotectonic units, all noted in southwest Japan, five of which have along-arc equivalents in the Ryukyus. Northeast Japan has nine of these 12 geotectonic units, and East Hokkaido has three of these units. Recent field observations have shown that most of the primary geotectonic boundaries are demarcated by low-angle faults, and sometimes modified by secondary vertical normal and/or strike-slip faults. On the basis of these new observations, the tectonic evolution of the Japanese Islands is summarized in the following stages: (i) birth at a rifted Yangtze continental margin at ca 750–700 Ma; (ii) tectonic inversion from passive margin to active margin around 500 Ma; (iii) successive oceanic subduction beginning at 450 Ma and continuing to the present time; and (iv) isolation from mainland Asia by back-arc spreading at ca 20 Ma. In addition, a continent-continent collision occurred between the Yangtze and Sino-Korean cratons at 250 Ma during stage three. Five characteristic features of the 450 Ma subduction-related orogen are newly recognized here: (i) step-wise (not steady-state) growth of ancient accretionary complexes; (ii) subhorizontal piled nappe structure; (iii) tectonically downward-younging polarity; (iv) intermittent exhumation of high-P/T metamorphosed accretionary complex; and (v) microplate-induced modification. These features suggest that the subduction-related orogenic growth in Japan resulted from highly episodic processes. The episodic exhumation of high-P/T units and the formation of associated granitic batholith (i.e. formation of paired metamorphic belts) occurred approximately every 100 million years, and the timing of such orogenic culmination apparently coincides with episodic ridge subduction beneath Asia.     

永清浅牛6井动水位异常与远震关系分析

通过分析浅牛６井４次突跳异常特征及与日本海５次地震的关系,认为这４次突跳异常分别属于日本海５次地震前的“场兆”异常。这４次异常为板块边缘的构造活动引起井孔所处区域构造应力场发生变化,从而引起含水层下部的含油气层中气体上涌所致。