Paleostress orientation from 3-D orientation distribution of microcracks in quartz from the Cretaceous granodiorite core samples drilled through the Nojima Fault, south-west Japan   总被引：1，自引：0，他引：1  

Toru Takeshita  Koshi Yagi 《Island Arc》2001,10(3-4):495-505

Abstract The orientations of both healed extension microcracks and microcracks in quartz grains sealed mostly by carbonate minerals were measured from Cretaceous granodiorite core samples drilled along the Nojima Fault, southwest Japan. The preferred orientations of both healed and sealed microcracks consist of approximately three orthogonal sets, (components) A, B and C, in which A strikes NS–NW-SE and dips vertically, B strikes EW–NE-SW and dips vertically, and C is subhorizontal. Both the healed and sealed microcracks were possibly formed by hydraulic fracturing, and the successive release of tensile stress due to pore fluid overpressure in the principal stress directions could have caused this microcracking in mutually orthogonal directions. The quartz grains are also very moderately plastically deformed, which is indicated by the occurrence of kink bands and undulose extinction. The association of healed microcracks and kink bands in the quartz suggests that these microstructures formed under subgreenschist facies conditions (≈300°C) during hydrothermal activity that could have occurred immediately after the emplacement of granodiorite during the Late Cretaceous period. Based on both the preferred orientation of microcracks, and c-axis fabrics of kinked and unkinked grains (so called kink method), it is inferred that the σ₁-and σ₃-axis were oriented horizontally in NS–NW-SE and EW–NE-SW directions, respectively. The inferred paleostress field does not conform to the east–west-trending compression during the Quaternary period, but to the activation of EW–NE-SW-trending, left–lateral strike–slip faults during the Late Cretaceous period in southwest Japan.  相似文献   

华北石炭二叠纪煤系中I／S间层矿物的研究   总被引：1，自引：1，他引：1  

梁绍暹  王水利 《煤田地质与勘探》1995,23(6):12-14

华北石炭二叠煤系中的Ｉ／Ｓ间层粘土岩夹矸或粘土岩层，在太行山以东较为发育。Ｉ／Ｓ间层矿物的类型有累托石、ＩＳ有序间层矿物、Ｓ／Ｉ无序间层矿物，它们是在近岸泥岩沼泽或沼泽环境中由同期降落的火山灰蚀变而成，在酸性介质的影响下，形成ＩＳ有序间层矿物，在偏碱性的介质条件下，形成Ｓ／Ｉ无序间层矿物；高压异常区则形成累托石。  相似文献   

Evidence for a Neoproterozoic carbonate ramp on the northern edge of the Central African craton: relations with late Proterozoic intracratonic troughs     

Philippe Alvarez 《International Journal of Earth Sciences》1995,84(3):636-648

During Late Proterozoic times, the Archaean Central African craton was affected by trough faulting which led to the formation of grabens, the Sangha aulacogen being the main structure of this type in the studied area. This transverse basin connects with other basins on the northern and south-western borders of the craton. During the Cryogenian, this network of basins was filled with fluvio-deltaic and lacustrine periglacial deposits. The glacio-eustatic transgression in Neoproterozoic III (end-Proterozoic) times flooded extensive areas of shelf on the northern edge of the craton, leading to the development of carbonate sedimentation in a broad outer shelf environment associated with nearshore barriers and evaporitic lagoons. These facies are similar to those developed in the West Congolian Schisto-calcaire (shale-limestone) ramp succession. The North-Central African ramp succession (sediment slope) contains an example of tidal rhythmites in vertical accretion, which occurs beneath the barrier deposits on the subtidal outer shelf. Mathematical analysis of the bedding pattern yields a period of 29–30 days for the lunar month, a result which is in agreement with astrophysical evidence for this epoch (i.e. 650 Ma ago). Major subsidence and seismic activity on this gently sloping platform, associated with the proximity of the Sangha aulacogen, caused the triggering of carbonate turbidites and mass flow deposits. The proliferation of microbial mats under euphotic conditions on an extensive shelf led to the build-up of a carbonate platform. During early Neoproterozoic III times, the West Congolian and North-Central African ramps prograded northwards and southwards, respectively, into the Sangha aulacogen. The sea at that time was restricted to a long graben-like basin, while a remaining area of marine sedimentation persisted into the Palaeozoic. Thus the pattern of end-Proterozoic carbonate sedimentation on the borders of the Central African craton can be interpreted in terms of an overall gently sloping ramp model with progradation converging towards the Sangha aulacogen.  相似文献   

赣东北地区早前寒武纪超基性熔岩—层状侵入岩的初步研究   总被引：3，自引：1，他引：3  

吴根耀  符鹤琴 《大地构造与成矿学》1995,19(4):354-362

赣东北地区早前寒武纪超基性熔岩—层状侵入岩是扬子克拉通绿岩带的残块。科马提岩为橄榄质;稀土特征表明以Ⅱa型(重稀土亏损型中的轻稀土富集亚类)科马提岩为主,可能有Ⅲ型(重稀土富集型)科马提岩。(La/Sm)_N值>2说明其源区已被富轻稀土的液体或岩浆流体交代过。扬子克拉通的花岗—绿岩带地体的地球化学特征意味着晚太古代时它与南非克拉通有更密切的联系。  相似文献   

二叠纪末期的全球淹没事件   总被引：9，自引：0，他引：9       下载免费PDF全文

陈中强 《沉积与特提斯地质》1995,(3):34-39

本文根据华南层序地层学分析，着重研究了晚二叠世的淹没事件。研究表明，晚二叠世海平面变化曲线不同于Ｃ．Ａ．Ｒｏｓｓ和Ｊ．Ｒ．Ｒｏｓｓ（１９８７）的海平面变化曲线。根据华南地区和世界其它地区的古生物分析和层序地层分析，晚二叠世海平面变化结论如下：（１）二叠纪末期，全球发生海泛淹没事件，海平面急剧上升，而不是下降；（２）晚二叠世晚期的海进开始于吴家坪末期，至长兴期末期（二叠纪末期）达到高峰时期。  相似文献   

恒山绿岩带的构造特征—晚太古代大陆裂谷作用证据   总被引：3，自引：0，他引：3  

李江海  钱祥麟 《华北地质矿产杂志》1995,10(2):181-189

恒山太古宙绿岩带呈断片或复式向形穿插于片麻岩基底内，绿岩带地层连续性较好，可以恢复出完整的沉积－火山地层序列。主要经历二期变形，早期逆冲褶皱变形和晚期右旋走滑变形。为绿帘角闪岩相－角闪岩相变质。构造恢复表明，绿岩带的形成明显晚于基底，并整合其上，且在其演化过程中对基底产生叠加影响，最合适的构造环境为大陆裂谷，在大陆裂谷闭合过程中。绿岩带及克拉通基底曾发生双向的逆冲作用，基底发生重新活动，并且受到开裂时形成的正断层的控制。当裂谷闭合到一定程度，由于受北部刚性基底的制约，横向挤压不再能调节压应力时，发生了右旋走滑变形，并使早先构造线向ＮＥＥ－ＥＷ向偏转。恒山绿岩带对应于五台山区、太行山北段、灵丘南山等地的五台群，有力地证明绿岩带形成于太古代末期－早元古代陆内裂谷环境，代表华北克技通演化过程中的重要事件，标志着晚太古代华北克拉通已达到相当大的规模，并发生ＮＥ向的刚性破裂。  相似文献   

广东沿海的晚更新世沙坝   总被引：6，自引：0，他引：6  

刘尚仁 《地理科学》1995,15(3):281-288

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The smallest of the largest: a new look at possible dwarfing in sauropod dinosaurs     

Jianu  Coralia-Maria  Weishampel  David B. 《Geologie en Mijnbouw》1999,78(3-4):335-343

Franz Baron Nopcsa recognised that the Late Cretaceous titanosaurid sauropod from Transylvania, Magyarosaurus dacus, was much smaller than the better known sauropods from the Upper Jurassic of the United States and even from Late Cretaceous deposits elsewhere in the world. In keeping with his biogeographic interpretations, Nopcsa viewed this difference as a consequence of body-size effects via island habitation. We present a preliminary re-evaluation of Nopcsa's claims and their heterochronic consequences using two approaches, viz. (1) regression analysis of humeral data as a means of establishing patterns in body size among titanosaurids, and (2) optimisation of humeral data onto titanosaurid cladograms to evaluate evolutionary trends within the clade. Our regression analysis is based on twenty species distributed among fourteen genera of neosauropods and uses length and mid-shaft mediolateral width of the humerus of presumed fully adult forms, and of growth samples that consist of postnatal to adult individuals. Linear regression analysis suggests that, among adult neosauropods, M. dacus appears to be represented by the smallest individuals; that M. dacus humeri appear to be more similar to those of subadults than to adults of other taxa; and that this juvenile morphology may constitute dwarfing in M. dacus by paedomorphosis. In order for these regressions to reflect their evolutionary context more fully, we also present optimisation analyses of humeral form within Titanosauroidea. Although many aspects of the phylogeny of this clade are relatively poorly resolved, preliminary results of our analyses are consistent with the conclusion that M. dacus was a heterochronic dwarf.  相似文献   

陕西黄土高原晚更新世环境变化   总被引：14，自引：3，他引：11  

赵景波  黄春长 《地理科学》1999,19(6):565-569

岐山剖面孢粉分析表明,黄土高原东南部晚更新世植被与气候变化可分为７个阶段和１３个小阶段。其中末次冰期的３个冷干阶段和２个温湿阶段被分别命名为秦家寨,岐山,坡头冰阶和尚家坡,杨家湾间冰阶。冰阶的植被以温带森林草原为主,间冰阶的植被以落叶阔叶林为主。  相似文献   

天山乌鲁木齐河源分选环的发育特征   总被引：1，自引：1，他引：1  

刘耕年  刘岳峰 《冰川冻土》1993,15(3):474-480

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