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71.
Eighty-nine basaltic lava flows from the northwest wall of Haleakala caldera preserve a concatenated paleomagnetic record of portions of the Matuyama-Brunhes (M-B) reversal and the preceding Kamikatsura event as well as secular variation of the full-polarity reversed and normal geomagnetic field. They provide the most detailed volcanic record to date of the M-B transition. The 24 flows in the transition zone show for the first time transitional virtual geomagnetic poles (VGPs) that move from reverse to normal along the Americas, concluding with an oscillation in the Pacific Ocean to a cluster of VGPs east of New Zealand and back finally to stable polarity in the north polar region. All but one of the 16 Kamikatsura VGPs cluster in central South America. The full-polarity flows, with 40Ar/39Ar ages spanning a total of 680 kyr, pass a reversal test and give an average VGP insignificantly different from the rotation axis, with standard deviation consistent with that for other 0-5 Ma lava flows of similar latitude. Precise 40Ar/39Ar dating consisting of 31 incremental heating experiments on 12 transitional flows yields weighted mean ages of 775.6±1.9 and 900.3±4.7 ka for the M-B and Kamikatsura transitional flows, respectively. This Matuyama-Brunhes age is ∼16 kyr younger than ages for M-B flows from the Canary Islands, Tahiti and Chile that were dated using exactly the same techniques and standards, suggesting that this polarity transition may have taken considerably longer to complete and been more complex than is generally believed for reversals. 相似文献
72.
We present new 40Ar/39Ar data for sanidine and biotite derived from volcanic ash layers that are intercalated in Pliocene and late Miocene astronomically dated sequences in the Mediterranean with the aim to solve existing inconsistencies in the intercalibration between the two independent absolute dating methods. 40Ar/39Ar sanidine ages are systematically younger by 0.7-2.3% than the astronomical ages for the same ash layers. The significance of the discrepancy disappears except for the upper Ptolemais ashes, which reveal the largest difference, if an improved full error propagation method is applied to calculate the absolute error in the 40Ar/39Ar ages. The total variance is dominated by that of the activity of the decay of 40K to 40Ar (∼70%) and that the amount of radiogenic 40Arp in the primary standard GA1550 biotite (∼15%). If the 40Ar/39Ar ages are calculated relative to an astronomically dated standard, the influence of these parameters is greatly reduced, resulting in a more reliable age and in a significant reduction of the error in 40Ar/39Ar dating.Astronomically calibrated ages for Taylor Creek Rhyolite (TCR) and Fish Canyon Tuff (FCT) sanidine are 28.53±0.02 and 28.21±0.04 Ma (±1 S.E.), respectively, if we start from the more reliable results of the Cretan A1 ash layer. The most likely explanation for the large discrepancy found for the younger Ptolemais ash layers (equivalent to FCT of 28.61 Ma) is an error in the tuning of this part of the sequence. 相似文献
73.
~(40)Ar/~(39)Ar dating of shear deformation of the Xianshuihe fault zone in west Sichuan and its tectonic significance 总被引:6,自引:1,他引:6
ZHANG Yueqiao CHEN Wen & YANG Nong . Institute of Geomechanics Chinese Academy of Geological Sciences Beijing China . Institute of Geology Chinese Academy of Geological Sciences Beijing China . Institute of Geology Geophysics Chinese Academy of Sciences Beijing China 《中国科学D辑(英文版)》2004,47(9)
TheNW-SEstrikingXianshuihefaultzoneslicesthesoutheasternTibetanPlateauandconnectssoutheastwardwiththeAnninghe-Zemuhe-Xiaojiangfaultzone,whichformahuge,activesinistralstrike-slipfaultzone(fig.1).ThisfaultzoneisanimportantseismicfaultineastTibet[1-5].EarthquakegeologystudiesandoffsetpatternsofyounggeologicalfeatureshaveshownthatlateQuaternarysinistralsliprateoftheXianshuihefaultzonereaches13mm/a[1,2].TheXianshuhefaultzoneconsistsoftwomainbranches,theDaofufaultbranchinthewestandtheXianshuih… 相似文献
74.
本文分析了天山山区近40 a来年降水变化的基本特征,并与南疆、北疆进行了比较,所得的主要结果如下:(1)天山山区在年降水量干湿变化阶段上与北疆的相似性强于南疆。(2)年降水量的空间分布的同步变化性以北疆为最好,南疆最差,天山山区居中,而年降水量的空间分布的反向变化性,以天山山区为最大,北疆最小,南疆居中。(3)天山山区与南疆从60年代到90年代,年降水均表现出了持续的增加的趋势,北疆年降水从60年代到90年代,除70年代外,不断增多。 相似文献
75.
云南北衙矿区石英正长斑岩岩体在空间上与金、铅锌矿体共生。红泥塘岩体地表岩石正长石的40Ar-39Ar坪年龄和等时线年龄为25.89±0.13Ma和25.72±0.7Ma,万洞山岩体地表以下382m钻孔中岩石的正长石坪年龄和等时线年龄为25.53±0.25Ma和25.50±0.07Ma,分别为两个岩体的形成年龄。但是,万洞山岩体地表团块状白云母的坪年龄和等时线年龄为32.50±0.09Ma和32.34±0.04Ma,为白云母的结晶年龄,也可能是主岩的结晶年龄 相似文献
76.
吉林六批叶沟金矿床绢云母40Ar-39Ar 快中子活化法定年研究 总被引:6,自引:0,他引:6
在六批叶沟金矿石中,精选出与金矿物同期生成的绢云母作为样品,采用40Ar-39Ar快中子活化法测年,获得8个一致相连的平坦型绢云母年龄谱,tp=(190.28±0.30) Ma,氩同位素计算得等时线年龄为(189.98±0.58) Ma,两者完全吻合.等时线截距为(296.9±10.0) Ma,与大气氩的该值(295.5 Ma)相比较,两者完全吻合,表明该样品出自未受明显后期热力作用影响的非扰动体系.锁定六批叶沟金矿床主要成矿年龄为190 Ma左右,属早侏罗世的燕山早期. 相似文献
77.
78.
79.
关中盆地水资源可持续开发利用对策研究--以渭南市为例 总被引:4,自引:0,他引:4
以关中盆地东部的渭南市为例,概述了水资源的分布特征及其开发利用现状。认为水资源开发利用中存在水资源浪费严重,重复利用率低;行政区块限制,水资源调配不够;不合理开采地下水,出现负环境效应;地下水人工调控力度不够,不能有效的促进生态环境良性循环;对地下水资源的研究、规划和管理滞后等五个方面的主要问题。针对存在的主要问题,提出了六项水资源可持续开发利用对策,包括节水对策、开源对策、改水对策、调水对策、增补对策以及管理对策。 相似文献
80.
The basement of Sardinia represents a nearly complete sectionof a segment of the Variscan belt that experienced a polyphasetectono-metamorphic evolution and Barrovian metamorphism. Thisbasement is well suited to investigate the relationship betweentectono-metamorphic evolution and argon isotope records in whitemica. Micaschists from the garnet zone (maximum T of up to 520560°C)contain two texturally and chemically resolvable generationsof white mica: (1) deformed celadonite-rich flakes, defininga relict S1 foliation preserved within the main S2 foliationor enclosed in rotated albite porphyroblasts; (2) celadonite-poorwhite micas aligned along the main S2 foliation. The S1 foliationdeveloped earlier and at a deeper crustal level with respectto that at which the thermal peak was reached. From the staurolitezone (T of up to 590625°C) to the sillimanite + K-feldsparzone, white mica is nearly uniform in composition (muscovite)and is predominantly aligned along the S2 foliation or is oflater crystallization. In situ 40Ar39Ar laser analysesof white mica yielded ages of 相似文献