Establishing a high-resolution surface sediment chronology with multiple dating methods – Testing 137Cs determination with Nurmijärvi clastic-biogenic varves

The uppermost 50 cm of sediments from the 23 m deep lacustrine basin of Nurmijärvi, a lake in south-central Finland, are composed of the clastic-biogenic type of varves. We use multiple dating methods to render a sediment chronology that is as precise and accurate as possible for future paleoenvironmental research on this high-fidelity sedimentary archive. The 250-year-long varve chronology contains a chronological error of ±2%, as estimated based on repeated varve counts. The varve-based chronology was verified with the vertical distribution of ¹³⁷Cs by comparing variations in the paleomagnetic relative declination of the surface sediments with instrumental observations and a calculated representation of declination features in Finland. A comparison of the sediment cesium content using two gamma counters, different sediment pretreatments, and normalization of the ¹³⁷Cs concentration with masses and accumulation rates of different sediment components revealed that the AD 1986 fallout peak from the Chernobyl accident is the dominant feature in the studied sediments, regardless of the sample pretreatment or normalizing procedure. The vertical distribution of the AD 1986 peak in sediments with clastic-biogenic varves is fairly narrow, thus providing an accurate age-depth marker for the recent deposits. However, the Nurmijärvi section also reveals a ca. 10-year-long period of catchment-to-basin ¹³⁷Cs mobilization and redeposition after the AD 1986 fallout, as well as significant downwards diffusion of the Chernobyl-derived cesium in the sediment. This study highlights the ways in which the quality of surface sediment chronology needs to be considered and secured prior to comparisons between paleoproxy records and instrumental observations.     

湘西沅麻盆地早白垩世红层古地磁学研究及其对川东褶皱带新生代构造变形的指示

川东褶皱带作为华南板块中部的"侏罗山式"褶皱,开始形成于晚古生代.自白垩纪晚期开始,受太平洋板块和印度板块对欧亚大陆挤压的影响,这一构造带乃至华南板块中部又叠加了新的构造变形.但是,目前对于川东褶皱带白垩纪以来的构造演化缺乏足够的认识.位于川东褶皱带东侧、雪峰造山带西麓的沅麻盆地形成于早白垩世.晚白垩世以来,沅麻盆地与...     

羌北地块中-晚侏罗世雁石坪群古地磁新结果

本文报道青藏高原羌北地区中-晚侏罗世雁石坪群古地磁新结果.对采自青海省格尔木市唐古拉山乡雁石坪剖面(33.6°N, 92.1°E)11个灰岩采点(118块)和10个碎屑岩采点(99块)定向样品系统古地磁学研究表明,大部分样品的退磁曲线具有双分量特征.低温分量方向在地理坐标系下较为集中,应该为地层褶皱之后的黏滞剩磁.高温特征剩磁分量方向可分为两类:(1)索瓦组(J₃s)和布曲组(J₂b)灰岩,以磁铁矿为主要载磁矿物,高温特征剩磁分量(D_s=355.7°,I_s=42.1°,k=58.2,α₉₅=6°)可通过99%置信度的褶皱检验.(2)雪山组(J₂x)和雀莫错组(J₂q)碎屑岩,以赤铁矿、磁铁矿为主要载磁矿物,高温特征剩磁分量(D_s=3.3°,I_s=28.9°,k=30.7,α₉₅=8.9°)可通过95%置信度的倒转检验和99%置信度的褶皱检验.两组分量都应该是岩石形成时的原生剩磁信息.碎屑岩组的磁倾角比灰岩组偏低13°左右,其剩磁方向很可能存在着与压实作用相关的剩磁倾角变浅的状况.本文取灰岩组平均磁化方向作为雁石坪群的原生剩磁分量,获得羌北地区雁石坪群古磁极位置:80.0°N,295.2°E(dp/dm=7.4/4.5).古地磁结果表明,羌北-昌都地区晚石炭-晚二叠世期间位于南纬中低纬度地区,早三叠世以后开始大规模北向漂移,至中-晚侏罗世已到达24.3°N.其快速北向运动主要发生在早三叠至早侏罗世期间(3500 km左右),与现今位置相比中晚侏罗世之后的北向迁移总量为900 km左右.     

再论印度与亚洲大陆何时何地发生初始碰撞

印度与亚洲大陆碰撞形成了喜马拉雅造山带.该造山带是当今固体地球科学研究的重点和热点,是建立新的大陆动力学理论的最佳天然实验室.印度与亚洲大陆碰撞时限是正确认识和理解该造山带形成与演化、高原隆升的动力学过程等的起点.近南北向陆陆碰撞的最直接证据是碰撞带两侧块体在古纬度上的相互重叠.本文拟通过对相关古地磁资料的分析,结合近年来在拉萨地块南缘林子宗群火山岩和沉积岩夹层上获得的最新古地磁结果,探索当今古地磁数据所限定的印度和亚洲大陆发生初始碰撞的时间和古地理位置.结果表明,拉萨地块林子宗群形成时期(约64~44 Ma)古亚洲大陆最南缘的古地理位置(~10°N)限定了印度与亚洲大陆的初始碰撞最可能发生在65~50 Ma之间;如果以由印度洋海底地形所限定的东冈瓦纳大陆裂解前的印度板块形状为大印度模型,则印度与亚洲大陆的初始碰撞很可能发生在60~55 Ma之间.     

拉萨地块和羌塘地块多个二叠纪基性岩及一个古特提斯海山的初步古地磁结果:对模拟东冈瓦纳晚古生代裂解的暗示

Detachment of the sliver-like Cimmerian terrane from eastern Gondwana in the Early Permian triggered mafic volcanism in many parts of the rift zone. To understand this tectonic episode we have carried out paleomagnetic investigations on mafic volcanic for-mations that were erupted on key terranes that now form part of Tibet. Specifically, we will present data from sections near Lhasa City (central Lhasa block) and Tuotuohe (central Qiangtang Block) as well as near Gyanyima (Paleotethyan sea-mount) that was emplaced onto the floor of Palaeotethys during the Late Permian. Paleomagnetic plots from each location will be used for tectonic calculations. Our new data will be used to evaluate regional scale models con-cerned with how the Cimmerian terranes in southern and SE Asia (from Iran-Tibet-SW China-Myanmar- Thailand-Sumatra) formerly abutted eastern Gond-wana.     

黑龙江东部白垩纪-古近纪古地磁初步结果及其构造意义

陆内块体旋转是周边构造环境和深部构造活动相互作用的结果.前人研究表明华北东部和俄罗斯远东地区晚中生代以来的块体旋转样式,很可能以牡丹江断裂为界发生了显著变化.进一步对牡丹江断裂两侧块体晚中生代以来的块体旋转样式的限定,有助于正确理解这一差异旋转的机制.对采自黑龙江省东部白垩纪和古近纪岩石的(51个采点)古地磁学研究表明...     

Evidence for significant clockwise rotations of the Korean Peninsula during Cretaceous

In an effort to evaluate the Cretaceous magnetostratigraphy for the Korean Peninsula and to establish the tectonic coherence of its various elements, we collected paleomagnetic data from 121 samples from 20 sites within the Chilgok Formation (108.3–109.9 Ma) in the Gyeongsang Basin. Together with previously published data, we evaluate the results from a total of 163 sites in the basin.We combine our age model with results from recent stratigraphic, paleomagnetic and radiometric geochemical studies. In this study, we found that two distinct declination shifts decrease with younging direction, indicating two clockwise rotational events of the Korean Peninsula with respect to the Eurasia continent. The earlier event took place during 130–100 Ma (Phase I, newly termed “Goguryeo Disturbance”) and a later one during 80–50 Ma (Phase III, belonging to “Bulguksa Orogeny”). The mean rotation rate in the interval from 115.2 to 103.8 Ma (Phase I) is about 0.74°/Ma, while the rate from 90.9 to 79.8 Ma (Phase II) is 0.19°/Ma. Based on paleolatitude change during Phase I, we infer that the Korean Peninsula (eastern part of the Sino-Korea Block) migrated southward about 300 km after the complete amalgamation of the Sino-Korea Block into the Eurasian continent resulting in N–S compression within the Korean peninsula and Manchuria. Large-scale strike-slip faults (e.g., Tan-Lu Fault, Okcheon Boundary Fault) were probably rejuvenated in the Sino-Korea Block during Phase I.     

岩心裂缝重定向的古地磁方法研究

裂缝信息对于低渗透储层的勘探开发具有重要意义。本文在深入探讨利用古地磁方法进行岩心裂缝重定向原理的基础上,详细介绍了岩心裂缝重定向的工作方法。针对垂直及平行于岩心中心轴的两种样品加工方式,推导出可直接利用退磁结果表达的裂缝方向的公式,并将这一结果应用于鄂尔多斯盆地一口有裂缝井的岩心,取得较好的应用效果。研究结果表明,利用古地磁方法进行岩心裂缝重定向具有操作简便、成本较低、且精度相对较高的特点。本文研究结果对于鄂尔多斯盆地的裂缝系统研究具有参考意义。     

Paleomagnetic evidence for a large counterclockwise rotation of northern Greece prior to the Tertiary clockwise rotation

Abstract

This study aims at unravel the geotectonic evolution of northern Greece prior to the already established Tertiary clockwise rotation. Therefore, Mesozoie sediments, Early Mesozoie ophiolites and Carboniferous granites were sampled. While the metamorphosed and/or too weakly magnetized limestones had to be rejected, the gabbros and serpentinites of the 80 km long Chalkidiki belt (40.4°N, 23.3”E), and the granites of the northern Pelagonian zone (40.8°N, 21.2°E) have yielded similar results interpretable in terms of geoleetonies. In both areas the demagnetizing process has revealed a poh phased magnetic evolution.

The oldest magnetizations, labelled M (D=311°, I=20°, a₉₅, = 15°; VGP: 37°N, 272.5°, for the ophiolites; D=320.5°, I = 26°, a₉₅ =11°; VGP : 46°N, 264.5”E, for the granites) are interpreted as overprints acquired in Late Jurassic-Cretaceous times. The younger magnetizations, called C2 (D = 66°, I = 28°, a₉₅ = 9°; VGP : 28°N, 117°E, in the ophiolites ; D=64°, I = 2° a₉₅, = 11°; VCP : 20°N, I28°E, in the granites) are Tertiary overprints. Northeasterly C’ directions with negative inclinations (and conversely) are considered as overprints empiaceli prior to the Ca magnetizations ; they are interpreted as due to a barkthrusting of the ophiolilic belt of Chalkidiki and of the N. Pelagonian granitic belt, during the Early - Middle Tertiary convergence phase. The large deviation from the M to the C₂ directions, also observed by other authors in Mesozoic volcanics and sediments, results from a counterclockwise rotation of the Hellenides, probably in the Late Cretaceous as it is the case for the counterclockwise rotations of the western Mediterranean microplates. The deviation from the C₂ to the present field direction is due to a clockwise rotation of all Hellenic zones, probably in several phases.     

扬子地块古生代大地构造演化及古地磁学研究综述

本文简要地论述了扬子地块古生代大地构造研究现状，扬子与华夏地块之间的华南裂谷盆地于奥陶纪初发生收缩，直到加里东末期基本封闭，古生代扬子地块与华北地块之间的秦岭海槽以拉张为主，扬子地块西缘古生代期间处于相对稳定的构造环境，古生代末进入特提斯构造域，发生活化。