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21.
塔里木地块二叠纪古地磁结果及二叠纪中国大陆的重建 总被引：2，自引：0，他引：2
在塔里木盆地西北缘下二叠统的三个剖面中采集的标本,获得了两个古地磁极:一个获自灰岩剖面(λ=65°N,φ=163°E,K=128,A_(?5)=4°,n=11).另一个获自两个玄武岩剖面(λ=55°N,φ=172°E,K=66,A_(95)=4°,n=21).结合原先已发表的数据,塔里木地块二叠纪古地磁场为λ=61°N,φ=177°E.K=98.A_(95)=9°,N=4.塔里木地块古纬度比中朝、扬子地块高得多,认为这些地块是分开的大陆地块.晚二叠世.塔里木、西伯利亚、中朝、扬子地块之间发生过重要的相对运动表明.地质上推断的晚古生代碰撞、并合作用在中亚及东亚的复合构造演变才刚刚开始. 相似文献
22.
采用主成分分析方法、线性谱法和LINFIND方法，分离了塔里木盆地中生代岩石的多磁成分，并采用岩石磁学和褶皱检验等方法研究了剩磁稳定性．对于塔里木盆地三叠系、侏罗系的岩石其携磁矿物以磁铁矿为主，具有两组次生磁性成分和一组原生剩磁成分；原生剩磁成分的解阻温度为550℃，而两组次生成分的解阻温度分别为175－250℃和300一375℃，可能为生物剩磁和次生氧化形成的剩磁．白垩纪岩石的携磁矿物以亦铁矿为主，同样具有两组次生剩磁和一组原生剩磁；两组次生剩磁分别为现代地磁场的粘滞剩磁和岩石变形过程中形成的构造剩磁．塔里木盆地中生代早期极移不明显，处于一个相对平静时期；侏罗纪至白垩纪盆地则主要表现有一定规模的南移，伴有顺时针旋转运动。盆地白垩纪的古纬度与现在纬度相比，仍存在17°－20°左右的纬度等，这一纬度差是通过白垩纪以后塔里木板块的北向漂移和板块北部造山带的压缩及边界的大型走滑作用来缩小；另外，压实作用也可能是由垩纪磁倾角变低的一个原因。 相似文献
23.
The purpose of this research is to demonstrate the possibility of applying one geophysical method to studying untraditional systems as is the case with Portland-cement-based materials. The research demonstrates how conventional paleomagnetic methodology can be employed in studying the mode of magnetic recording in present-day industrial materials. Portland-cement admixtures such as fly ashes and furnace slags should be discriminated, because those particles interact in soils and sediments in nature. Moreover, a better undertanding of magnetic remanent acquisition in model materials can serve to improve the interpretation of magnetic remanent acquisition in natural rocks formed a long time ago. The magnetic constituents of Portland-cement paste and mortar acquire a magnetic remanence due to their alignment with the earth's magnetic field at the casting place. This magnetization can be measured using ordinary paleomagnetic techniques. The alignment of the individual magnetic particles accounts for the intensity of the magnetic remanence, which can be increased by adding water and by vibration before setting and hardening. Blast furnace slag admixtures also add to the enhancement of the intensity of remanence. The magnetization of Portland-cement-based materials shows a near linear relationship with the water /cement (w/c) ratios employed in the experimental work; the w/c ratios range between 0.2 – 0.6 in pastes and 0.3 – 0.6 in mortar. Stable remanent magnetization was obtained during the first seven days of setting and hardening, a period necessary for magnetic particles to become locked parallel to the earth's magnetic field. The stability of magnetic remanence predicts the usefulness of the methodology in studying the properties of Portland cement and particularly in the control of iron-bearing admixtures. 相似文献
24.
Vietnam's Central Highlands in Pleiku feature numerous volcanic craters (i.e. maars) formed over 0.2 Ma ago that contain continuous high-resolution lacustrine sediment records extending from the Late Pleistocene through the Holocene. Their evolution and sedimentary proxies are potentially powerful archives for interpretation of East-Asian monsoon variability across Vietnam, as well as for local to regional changes in landcover. The focus of this study is to develop a reliable chronology for the Biển Hồ 25-m-long lake sediment sequence in Vietnam's Central Highlands. We present a combined sediment chronology extending back to 55 ka cal BP that is based on 137Cs surface sediment chronology, paleomagnetic dating, and 47 14C dates distributed evenly throughout the sequence. Based on sedimentary facies analysis and a smooth age-depth model based on 137Cs and 14C data, we infer a long-term stable rate of sedimentation in Biển Hồ lake without unconformities or erosional hiatuses, except for the uppermost section (ca. 100 years) where human activities increase the rate of sedimentation significantly. The Biển Hồ sequence thus provides a complete and uninterrupted record of the lake's depositional history. The present contribution is a geochronologic opening salvo for future contributions from Biển Hồ lake featuring multi-disciplinary evidence for longer-term paleoclimate and environmental changes. 相似文献
25.
The Pontides are characterized by a series of Mesozoic-Cenozoic fold belts comprising a N-vergent foreland fold and thrust belt in the Western Pontides and a concave, upward-shaped fold belt in the Eastern Pontides. The curvature of the fold belt follows the Caucasus which may imply a phase of oroclinal bending. In order to test whether the fold curvature represents a phase of oroclinal bending, a paleomagnetic study has been carried out in the Eastern Pontides on late Cretaceous and middle Eocene volcanic and sedimentary rocks from 29 sites. Rock magnetic studies reveal medium-temperature components with an unblocking temperature of 400–580 °C, indicating pseudo-single domain titanomagnetite as the most abundant carrier of magnetic remanence in the middle Eocene rocks studied here. In the upper Cretaceous rocks, a high-temperature component with an unblocking range of 580–650 °C was isolated. Stepwise thermal and alternating field demagnetization isolated two components of remanent magnetization in middle Eocene rocks comprising a low unblocking temperature/coercivity component near the present field direction and a characteristic remanent magnetization (ChRM) component of Ds = 332.3°, Is = 49.9° (k = 33.3, α95 = 9.2°, N = 15 sites). A positive fold test at a 95% confidence level and a reversal test indicate a primary magnetization. Component analysis of the upper Cretaceous rocks identifies a stable ChRM Ds = 160.3°, Is = −45.0°, (k = 85.6, α95 = 6.0°, N = 8 sites) following removal of secondary remanence. Their ChRM direction passes fold and reversal tests at a 95% confidence level. Both the upper Cretaceous and middle Eocene paleomagnetic data from the Eastern Pontides and the Lesser Caucasus clearly demonstrate evidence of oroclinal bending that occurred contemporaneouslywith the convergence between Arabia and Eurasia in the Paleocene. 相似文献
26.
通过对柴达木地块天峻县组合玛地区晚二叠世13个采点的系统古地磁测定,揭示了一组高温特征剩磁分量.实验结果表明,采样剖面获得的晚二叠世古地磁结果具有正、反极性,其特征剩磁方向为:Dg=333.7°,Ig=37.3°,Kg=35.4,N=9,α95 =8.8;Ds=333.9°,Is=41.7°,Ks=69.9,α95 =6.2°,相对应的古地磁极位置为:64.0°N,342.4°E,A95=5.9°,古纬度为24.0°N.这一高温分量通过了倒转检验,我们认为这一高温特征剩磁分量很可能代表了研究区晚二叠世时期的原生特征剩磁.通过对比塔里木地块晚石炭-晚二叠世古地磁结果,发现两块体在晚石炭世存在明显的古纬度差(16.6±9.3°),而在晚二叠世其古纬度差(3.5±5.4°)在古地磁误差范围内并没有明显差别,从构造意义上说,说明柴达木地块在晚二叠世已是塔里木地块的一部分,结合地质资料,认为柴达木地块在晚二叠世时古地理位置处于塔里木地块的南缘或西南缘,这表明柴达木/塔里木地块间的古阿尔金断裂的形成时代不可能早于晚石炭世时,很可能形成于晚二叠世以后. 相似文献
27.
古地磁欧拉极及对华北地块的应用 总被引：2，自引：0，他引：2
本文详细讨论了古地磁欧拉最佳小圆拟合法的基本原理和实际方法，并提出了提高拟合速度和优度的措施，同时利用此方法对前人取得的华北地块中石炭世至晚侏罗世这一时间的古地磁视极移曲线进行了最佳小圆拟合。 相似文献
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29.
为研究四川攀西（攀枝花-西昌）地区大陆演化,在该区及邻区采集了中生代岩石标本。用热退磁和交变场退磁方法对59个采点的619块样品做了磁清洗处理。古地磁结果满足褶皱检验,对精度参数K小于10的采点或有后期重磁化的样品未纳入统计处理。 古地磁结果表明,攀西断隆、凉山断陷、断凹属扬子地块的西缘,早三叠世时位于北纬13°左右,至今北移了千余公里,上三叠纪后北移速度明显减慢;其西邻盐源断陷自早三叠世以来南移数百公里,无论古纬度或古方位与东邻地块的结果有较明显差异。 相似文献
30.
研究了西藏吉隆—沃马盆地龙骨沟剖面新近纪沉积环境,作了古地磁年代学研究,认为喜马拉雅山北坡新生代断陷盆地发育始于7.2MaB.P., 3.2MaB.P.湖盆萎缩消亡,标志着喜马拉雅山地区在7.2MaB.P.和3.2MaB.P.发生过强烈的隆升事件.沃马盆地龙骨沟剖面所含三趾马动物群化石层年龄大约为7.0～6.7MaB.P.,隐示着此时青藏地区三趾马生活区与当时的华北平原三趾马生活区有着大体相当的地理、气候环境.之后由于喜马拉雅山持续抬升,断陷盆地下沉,并在5.9～3.6MaB.P.期间湖盆面积最为广阔.青藏高原抬升而华北平原沉降, 中国西部地区地形高于东部,东西部气候环境发生重大差异.3.6MaB.P.由于青藏地区持续强烈隆升,西部地区河流切穿古老湖盆,3.20MaB.P.吉隆—沃马湖盆萎缩,于1.7MaB.P.逐渐消失,进入侵蚀切割阶段. 相似文献