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81.
对1970年以来西藏地区139组5级以上地震序列类型进行统计及特征分析,发现西藏地区中强以上地震序列以主—余型和孤立型为主,约占79.1%。随主震震级的增加,主—余型所占比例逐渐增加,孤立型和多震型所占比例减少。绝大多数地震的最大震级余震在主震后3个月内发生。西藏地区多震型地震主要发生西藏中强以上地震活跃时段,因此在西藏及邻区5级以上地震频度较高时需注意多震型地震的发生。从地震序列类型空间分布来看,孤立型地震和主—余型地震大多发生在构造相对单一的地区,而多震型地震则大多发生在构造复杂且存在多组构造交汇的区域。 相似文献
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扁铲侧胀试验在浅海钙质土力学特性评价中的应用 总被引:2,自引:0,他引:2
利用扁铲侧胀仪对海底浅层钙质土进行原位测试,能够一次获得多个评价土性的试验参数。试验表明,由于波浪循环动荷载的作用使得海底表层(深1~2 m)钙质砂的密实度比其紧邻的下部偏大,以珊瑚礁为主要胶结物的钙质土,内部具有大的孔隙或松散堆积体结构是造成海底浅层钙质砂表层与浅部土体的扁铲侧胀压力明显变化的主要原因。扁铲试验结果与现场标准贯入试验具有较好的相关性,基于Seed公式推导的适合于计算水下平均动应力比的简化公式能够较好地用于对不同地震烈度下海底浅层钙质土液化可能性的判别。 相似文献
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Shao-Yi Huang Yue-Gau Chen George S. Burr Manoj K. Jaiswal Yunung Nina Lin Gongming Yin Jingwei Liu Shujun Zhao Zhongquan Cao 《Quaternary Research》2014
We present a reconstructed lithologic column compiled from a series of lacustrine outcrops along a tributary of the Nyang River, a major tributary of the Yarlung-Tsangpo in southeast Tibet. The deposits were preserved between terraces at altitudes of 2950–3100 m asl. The stratigraphic record features at least two sets of coarsening-upward sequences depicting episodic aggradation and progradation of a glacially dammed lake related delta. Recognized facies changes illustrate the evolution cycles of depositional environments from pro-delta, delta front, to delta plain. Radiocarbon and optically stimulated luminescence dates reveal an aging-downward trend in stratigraphic order and provide an approximate timeline for the formation of glacially dammed lakes in late Pleistocene. This result reflects that the Zelunglung Glacier had progressively advanced to block the Yarlung-Tsangpo river and the dam materials had stepwise stacked up to an altitude of 3095 m asl during Marine Oxygen Isotope Stages 4 to 2. 相似文献
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A combined magnetic fabric and paleomagnetic study has been carried out on the siliciclastic rocks gathered from a stratigraphic cross-section through the Nanpanjiang Basin, South China, in an attempt to extract the paleoflow information preserved in and, thus, constrain the possible origins of these clastic rocks. The sediments used for this study were formed by sediment-gravity flows along the southern margin of the South China block in the Middle Triassic time (ca. 245–228 Ma). The results show a normal distribution of both low field magnetic susceptibility values and natural remanent magnetization intensities, which along with the monotonic detrital framework mode, mainly comprising quartz and lithic particles, may suggest a single provenance involved in deposition of these clastic deposits. Anisotropy of magnetic susceptibility (AMS) analysis acquires primarily the sedimentary magnetic fabrics, which, in this study, reveal paleoflow directions ranging from NNW to ENE with an overall mean orientation of NE. Demagnetization on a part of samples isolates a characteristic remanent component averaged at D = 44.8°, I = 16.9°, κ = 9.7, α95 = 6. 5°, n = 55, corresponding to a paleolatitude N8.6° and a clockwise rotation of ca. 45° since the Middle Triassic for the studied cross-section. This mean direction passes fold tests and is consistent with the reference direction expected from the South China block at the 95% confidence level. Restoring this ∼45° declination renders an overall northward paleoflow, which, combined with other evidence, suggests a southern provenance for these sediments during deposition in the Middle Triassic time. In terms of the early Mesozoic plate framework of southeastern Asia, a tectonic scenario is proposed here, whereby the nearly N–S convergence of the Indochina and South China blocks and its related Indosinian orogeny in the Middle Triassic caused the formation of the Nanpanjiang foreland basin, which was filled by voluminous detritus shed from the uplifted orogenic belt on its southern side. 相似文献
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西藏吉塘花岗岩地球化学特征及成因 总被引:7,自引:2,他引:5
沿澜沧江结合带发育一条长达1000余千米的印支期澜沧花岗岩带,吉塘岩体位于该带北段,产出在吉塘以西约3km,轴向NNW,侵位于古元古代吉塘群变质岩系中,长70km,宽2~10km、出露面积341km2。主要由花岗闪长岩和二长花岗岩组成,Rb-Sr等时线年龄值为220Ma。本文对吉塘岩体开展了系统的岩石地球化学特征分析。研究结果表明吉塘岩体为过铝质钙碱性S型花岗岩,吉塘岩体在岩石化学组成上,SiO2含量63%~71%,平均66.5%,Na2O含量2.8%~4.2%,K2O含量1.7%~3.4%,铝饱和指数A/CNK=1.09~1.48,CIPW标准矿物中刚玉1.5%~5.6%,微量元素组成类似于片麻岩质的中地壳。(87Sr/86Sr)i值为0.7280~0.7395,εNd(i)为-14.1~-16.2,指示岩浆源区为地壳物质,成因上与俯冲作用无关,Nd同位素亏损地幔模式年龄(tDM)在2.0Ga左右,与昌都陆块变质基底古元古代吉塘群的原始形成年代相当,岩体Sr、Nd同位素组成也与变质基底吉塘群片麻岩一致,综合分析认为,岩浆源岩为变质杂砂岩成分的古元古代吉塘群片麻岩,其原岩建造主要是火山弧成因的英安岩或英安质火山碎屑岩,微量元素组成指示为黑云母脱水熔融。分析认为岩浆形成于地壳加厚增温环境下的地壳深熔作用,构造上与板块碰撞后的环境相联系。支持澜沧江构造带为冈瓦纳与扬子大陆边缘多岛弧系统的边界即古特提斯主洋盆的观点。澜沧江洋的闭合时间早于澜沧花岗岩带的形成年龄220Ma,根据构造带上相关研究成果,倾向于认同碰撞时间在280Ma左右。 相似文献