Paleocurrent directions from paleomagnetic reorientation of magnetic fabrics in deep-sea sediments at the Antarctic Peninsula Pacific margin (ODP Sites 1095, 1101)

We have determined the azimuth of bottom-current flow in drift deposit sediments recovered at ODP Sites 1095 and 1101, Antarctic Peninsula, using paleomagnetic reorientation of anisotropy of magnetic susceptibility (AMS) ellipsoids. A total of 38 cores from the two ODP sites have been measured, providing spatial and directional information on the physical record of the ACC (Antarctic Circumpolar Current) in the Plio-Pleistocene. Declination and inclination of the paleomagnetic vector of each core segment were used to reorient the AMS principal axes to the geographic coordinates. The cores were reoriented using the measured direction of the characteristic remanent magnetization (ChRM) with respect to a common reference line for the core, from which we are able to determine the orientation of the paleocurrent flow for Sites 1095 (Drift 7) and 1101 (Drift 4) relative to the geographic coordinates. Both sites have paleocurrent directions trending ~ NW-SE, which in the former locality are parallel to a sediment wave field. Our study shows that a combination of magnetic fabric analysis and paleomagnetism allows deep-sea sedimentary fabric to be used as a long-term proxy of bottom-current flow history.     

应用模糊关系聚类法确定岩石组构要素优选方位

以往采用投影作图法确定岩石组构要素优选方位。本文依据模糊数学原理,提出解决该问题的一种新方法,它通过计算夹角余弦,建立组构要素模糊关系矩阵,做模糊聚类分析,直接确定优选方位。该方法也适用于节理统计与分组。     

南沙群岛海区NS97-13柱样沉积物磁化率各向异性研究

通过南海南沙群岛海区NS97-13柱样磁化率各向异性系统测试、统计计算和测年等分析,给出了沉积物的磁化率椭球几何形态,重建了研究海区的古水流方向、沉积物的沉积形式。研究结果表明:(1)压扁型椭球(E>1)的择优方向位于磁性线理-磁性叶理直线的上方,夹角β的范围为0°<β≤33.14°,拉长型椭球(E<1)的择优方向位于磁性线理-磁性叶理直线的下方,夹角α的范围为33.14°<α<56.86°,指示磁性颗粒沉降时受动力作用的大小及其受力方向不同;(2)由磁化率各向异性参数(磁性线理-各向异性度,磁性叶理-各向异性度)确定的沉积物有改造沉积、再沉积和稳定沉积三种形式,前两种形式掺杂在稳定沉积层中的不同部位,构成部分混合沉积;(3)最大磁化率和中间磁化率的轴向分布表征研究海区距今近62 ka以来古水流方向主要为南北和北东-南西向,总磁化率记录了Heinrich层中的冷事件H1—H6。     

Differential rotations in the Oman Ophiolite: paleomagnetic evidence from the southern massifs

New paleomagnetic data from 11 sites in layered gabbros and lava flows from the Oman Ophiolite indicate stable, early remagnetizations and suggest that the southern portion of the ophiolite (the Wadi Tayin, Sumail, Nakhl-Rustaq and Haylayn massifs) is relatively unrotated since detachment near the paleoridge. The gabbros possess a magnetization carried by a combination of primary and secondary magnetites derived from hydrothermal alteration. Evidence from positive tilt tests, constancy of remanence directions in differing magnetic mineralogies and agreement with previous paleomagnetic data, however, suggests that this remagnetization was acquired early – analogous to the remagnetization of the V₂ volcanic series. Thus, the evidence implies that the southern portion of the ophiolite has been primarily translated from the paleoridge since the time of V₂ remagnetization, and 120° of clockwise rotation affecting the northern Oman Ophiolite is internal to the ophiolite, rather than a combination of internal and global rotation as previously hypothesized. Given this evidence, we propose a simplified model of a rapid, active microplate rotation of a portion of the ophiolite resulting from spreading at an EPR-type propagating ridge at a high angle to the previous spreading direction. Paleomagnetic data from this and previous studies can be well explained by a rapidly rotating microplate, similar to the kinematic evolution documented for the Juan Fernandez microplate in the modern setting.     

Impacts of fossil anisotropy on the electric and permeability anisotropy of highly fossiliferous limestone: a case study

The Middle Eocene Lutetian Samalut formation is among the best examples of anisotropic fossiliferous rocks in Egypt, where the effect of the anisotropic Nummulite Gizehensis fossils on the petrophysical behavior can be traced. The Samalut formation has been sampled and studied at Wadi Feiran in SW Sinai. Petrographically, it is composed of two microfacies; Nummulitic packstone and Fusulinid mudstone. Tight cementation by micro to pseudosparite, aggrading neomorphism and compaction with increasing load pressure are the most important porosity-reducing factors. The anisotropy of the fossil content (λ_F), due to shape and orientation, and its effect on the petrophysical properties were assigned by measuring the lengths of the longest and shortest axes. Petrophysically, both microfacies are characterized by low porosity values (1.47 ≤ \({\emptyset _{{\text{He}}}}\) ≤ 5.29%). The formation resistivity factor (F) and permeability (k) were measured in the horizontal and vertical directions (parallel and perpendicular to the bedding plane, respectively). The studied samples are characterized by high to very high formation resistivity factor (190?≤?F?≤?8938) and relatively very low permeability (0.012?≤?k?≤?0.110 md). The studied samples are characterized by fair to medium electric anisotropy ‘λ_E’, which is attributed to a relatively medium to fair degree of electric foliation. It has been shown that, the fossil shape anisotropy and orientation ‘λ_F’ (1.5?≤?λ_F?≤?3.5) is the main contributor for the electric and permeability anisotropy that corrected for the same porosity value (1.61?≤?λ_EC?≤?2.25 and 1.03?≤?λ_kC?≤?2.04; respectively). Foliation of the studied microfacies has been contributed to the orientation of the fossil remains parallel to the bedding plane. The anisotropy degree is relatively greater for the Nummulitic packstone microfacies than that of the Fusulinid mudstone. The present study refers to the possible anisotropic effect of fossil content (due to shape and orientation) on the petrophysical properties of the studied rocks which may be extended to the anisotropy of reservoir rocks on the bedding scale.     

Peculiarities of the magmatism and tectonics of the Knipovich Ridge

Petrological-geochemical data were obtained for intrusive rocks (gabbroids) recovered on the eastern flank of the Knipovich Ridge by deep-sea site 344 (DSDP, Leg 238). It was found that these rocks are similar to basalts and basaltic glasses studied in the adjacent sections of the ridge rift zone [7, 8]. This indicates that the intrusive rocks and erupted lavas are comagmatic. The gabbroids, basalts, and their quenched glasses were derived by differentiation in different-depth chambers and feeder channels. The petrochemical features of the gabbros and basalts (low Fe content, oxidized magnetic minerals) caused their weak magnetic properties. Owing to the multidirectional movements of the oceanic blocks, the bodies of the intrusive and effusive rocks their lost strict linearity and produced the mosaic anomalous magnetic field of the Knipovich Ridge.     

中太平洋海山玄武岩的岩石学特征与年代

玄武岩在中太平洋海山分布广泛, 且普遍存在风化蚀变现象.对测区内玄武岩样品的岩石学特征研究表明, 玄武岩具斑状结构, 基质以间粒结构为主.斑晶矿物为基性斜长石、单斜辉石和伊丁石化橄榄石.基质矿物为斜长石、辉石、橄榄石、铁矿物、磷灰石、玻璃及次生矿物等. 各海山玄武岩同属碱性玄武岩系列, 并为钾质类型玄武岩, 它们的形成基本上处于同一地质构造环境中.稀土元素分析结果为轻稀土富集型, 轻重稀土在岩石形成过程中发生了不同程度的分异作用.CM5海山玄武岩比其他海山年代相对较老（60 Ma）.     

盆地深层火山岩磁异常的增强方法

通过模型正演分析发现,盆地深层火山岩产生的磁异常不是很大,表现为叠加在强背景之上的次级异常。基于积分迭代延拓方法提出了增强盆地深层火山岩磁异常信息的新方法—积分迭代延拓平化曲。该方法不仅能起到增强盆地深层火山岩磁异常的作用,而且还能起到消除火山岩埋深不同对磁异常形态和大小的影响,以达到均衡磁异常的目的。将该方法应用到松辽盆地的深层火山岩预测中见到了明显的应用效果。     

Magnetic fabric results from DSDP Holes 380A (Black Sea) and 524 (South Atlantic) sediment cores: A case study for the comparison between low and high field torque magnetometer measurements

Magnetic fabric (anisotropy) measurements have been made on samples of Quaternary sediments from DSDP Hole 380A (Black Sea) and Tertiary and Cretaceous nannofossil oozes, marls and volcanoclastic sandstones from the Southeast flank of the Walvis Ridge at Hole 524, recovered during Legs 42B and 73 of the International Program of Ocean Drilling (IPOD). The fabric was determined by means of both a low field and a high field torque magnetometer.The overall results from the low field measurements are similar to those from the high field measurements. The results from both sets of measurements indicate the existence of a NE-SW lineation at Hole 380A and a NW-SE lineation at Hole 524. For Hole 380A some of the individual parameters show slight differences in values for the low field and high field torque measurements while for Hole 524 these individual parameters compare favourably and indicate an approximately one-to-one correspondence. In general the fabric is better defined at Hole 524 than at Hole 380A. Anisotropy measurements sometimes were found to be greatly affected by the shape of the sample. This apparent shape-effect appears to be more prevalent in the high field than in the low field measurements. In addition, for Holes 380A and 524 some samples show deformational-style fabric characteristics on one instrument and depositional-style characteristics on the other. The reasons for these differences and how the effects on the anisotropy data can be removed or minimised are described in this paper.     

An experimental study of velocities under pressure for ancient oceanic rocks

Abstract

Seismic velocities of Indus suture rock types from Dras‐Sanko‐Kargil, Kashmir Himalaya, as a function of pressure up to 10 kbar were studied. The high‐pressure measurements on the rocks reflect the depthwise increase in velocity, and in general they help in better understanding and better interpretation of the regional rocks in terms of their geological observations. An attempt has been made at correlation with ocean‐dredged samples, and it was found that the ultramafics, gabbros, and dykes are compatible with oceanic rocks and other ophiolite sequences, whereas metavolcanics are incompatible, suggesting the dismembered nature of Indus ophioli‐tes.     

陆相断陷湖盆沉积物磁组构特征及环境意义--以泥河湾盆地为例

选取陆相湖盆泥河湾盆地的郝家台和东谷坨两剖面,共取样２８６块,研究喾物磁组构参数特征及其环境意义。结果表明,沉积物磁组构参数的变化与沉积物粒度参数、磁化率的变化密切相关,蕴含着环境变化的信息。磁化率各向异性度（Ｐ）、磁线理（Ｌ）、沉积物磁化率的高值和细粒物质含量的低值指示湖水平面下降,气候较为干燥;相反,则说明湖水平面上升,气候较为湿润。据此,将泥河湾盆地沉积过程中湖水平面的变化过程划分为２个大的     

安徽宣城第四纪网纹红土的磁组构特征及其意义

安徽宣城第四纪网纹红土剖面黄棕色土层和网纹层的K、P、E、F、L、q等磁组构参数特征显示,i3>60°样品的磁组构特征有效降低了后期风化作用的影响,可近似代表黄棕色土层和网纹层的原生磁组构特征。黄棕色土层的原生磁组构特征显示它形成于沉积动力强度弱而稳定、事件沉积偶尔发生的沉积环境;该层磁组构参数的平均值及组合参数特征均与风成黄土-古土壤沉积的相应特征十分吻合,表明其原始沉积环境与风尘沉积环境一致,从而为黄棕色土层的风积成因提供了沉积动力特征上的证据。网纹层形成于沉积动力强度大、稳定性相对较差的沉积环境;该层的磁组构特征与风成沉积的相应特征差别显著,但与部分水成沉积的相应特征有近似之处,结合该层物质组成上的风成特性,初步推断网纹层可能为早期的风积物经后期水流改造的产物。     

南冲绳海槽及其邻域的磁性基底与地壳结构

研究区域的基底断裂构造十分发育。EW、NEE和NE向基底大断裂宏观上构成和控制区域地质构造的基本格架;NS、NNW和NW向断裂多为张扭性平移断层,它们对基本格架起强烈的分割破坏和错断作用,形成和控制次一级区域差异性的构造运动和岩浆活动特征。东海陆架边缘隆起带磁性基底埋深一般为3～4km.基底由前中新世变质岩系及不同时期形成的火成岩类组成。地壳厚度为24-28km,为大陆地壳。南冲绳海槽盆地磁性基底埋深一般为5～7km,可划分出4个次一级盆地。基底极可能由前中新世以来海槽张裂运动所形成的玄武岩层或部分变质岩系组成,靠近陆坡坡脚处可能包括前第三纪的老地层。地壳厚度为15～20km,地壳为亚大洋型。     

南海东部表层沉积物中普通角闪石和磁铁矿的特征及其成因

采用电子探针测试技术和晶体化学-成因分类图解法对南海东部表层沉积物中普通角闪石和磁铁矿的化学成分进行研究,结果表明,普通角闪石属于钙质角闪石组,主要类型有镁角闪石、钙镁闪石质角闪石、镁绿钙闪石质角闪石、浅闪石质角闪石、铁闪石质角闪石,均为中酸性岩浆成因。磁铁矿化学成分中TiO2、V2O5、Al2O3、MnO、MgO等标型组分的含量与岩浆成因的磁铁矿相似,其成因图解表明既有中酸性岩浆成因的也有基性岩浆成因的磁铁矿。普通角闪石和磁铁矿的成因性质与其共存的矿物特征和研究区的区域地质背景相吻合。     

泥质潮坪磁组构特征及其意义研究

在１９８８年－１９９３年期间，对河北歧口及上海南汇海岸带泥质潮坪现行滩面积浦西曹杨新村近代亚潮坪沉积剖面进行了沉积动力环境与沉积动力环境与沉积磁组构特征测量分析。现行开敞海岸同物源潮坪沉积磁且构参量量值特征显示出，其Ｋ值高滩为高值，低潮滩次之，中潮滩为低。亚近开敞海岸泥质潮坪沉积剖面磁组构测量结果；Ｋ值是区分潮坪沉积动力活动带的高分辨率标志；Ｐ、Ｌ、Ｆ、ｑ是分析潮坪沉积结构的分辨率标志；ｑ是分析潮     

马里亚纳弧前区锥形海山岩石学及成因

锥形海山是马里亚纳弧前区非火山成因的海山。它是由俯冲的太平洋板块对仰冲板块的上推作用,地壳下的上地幔物质底辟侵入的结果。海山顶部的岩石是由蛇纹石化超镁铁质方辉橄榄岩组成。其主要矿物成分为斜方辉石、橄榄石、尖晶石、磁铁矿、蛇纹石和碳酸盐。手标本呈显晶质块状构造,薄片中呈粒状、残留状、筛网状及斑状变晶结构。岩石化学特征表现为:低Si和Al,高M/F值,含水,亏损了所有过渡金属。岩石的成因属超基性岩浆直接结晶,后经海水的参入,在热液作用下普遍发生蛇纹石化和碳酸盐化。     

Carbonate cement fabrics displayed: A traverse across the margin of the Bahamas Platform near Lee Stocking Island in the Exuma Cays

Consolidated to friable carbonate rocks found in the Lee Stocking Island area in the Exuma Cays include: (1) reef rock, (2) channel stromatolites, (3) shallow-water hardgrounds, (4) beachrock rimming the islands and (5) Pleistocene bedrock.

The most common cement fabrics observed are: aragonitic fibers, which include acicular fan-druse and square-tipped coarse fibers cementing beachrock and stromatolites; and an isopachous needle-fiber rim cementing hardgrounds and stromatolites.

Less common are high-Mg calcite bladed textures of the reef rock and stromatolites. Two types of blades are present: the more common stubby variety of either high-Mg or low-Mg calcite, and an elongated variety of high-Mg calcite which was found in only three beachrock samples.

Aragonitic micrite envelopes usually surround grains in beachrock, hardgrounds and stromatolites, but only in association with fibrous cement. An aragonitic crust cements the surfaces of lime mud beds of the tidal channel, while a high-Mg calcite cryptocrystalline cement occurs in all the rock types. Calcified algal filaments of high-Mg calcite, from the abundant green and blue-green algae in the area, are a primary cement in stromatolites and a secondary cement in hardgrounds and beachrock. A low-Mg calcite equant spar cements the Pleistocene samples and is associated with meteoric diagenesis and cementation of the Pleistocene surface.

Cement precipitation coincides with the path of the cool oceanic water from Exuma Sound as it warms and loses CO₂ and moves up onto the bank near Lee Stocking with the incoming tide. Cryptocrystalline cement is the first and commonest cement forming to the seaward while platformward, fibrous cements become predominant. As suggested by their crystal size and location on the shelf margin, we think that the reef rock cryptocrystalline material are the fastest forming of the cements, where the incoming oceanic water is more saturated with respect to calcium carbonate and undergoes the most significant warming. The rate of the warming and degassing process is thought to increase in the tidal channel though the cementation rate is thought to fall slightly in response to a reduced availability of calcium carbonate. On the platform interior further warming and degassing are believed to cause cement precipitation and the development of hardgrounds, but these may form at a slower rate than that of the margin, though this rate is still quite high. Cementation gradients occur from the tidal channel to the intertidal zones of: (1) west Norman's Pond Cay, where cement fabric suggests a reduced calcium carbonate availability, and (2) west Lee Stocking Island, where a change in mineralogy suggests a change in water chemistry.

Thus, a sequence of cement fabrics and mineralogies can be traced. Micritic textures occur in a more seaward position; fine, fibrous aragonite fibers in a more lagoonal and levee position; and coarser aragonite fibers and Mg-calcite cements in the intertidal and supratidal position. This sequence is thought to track the evolution of the water mass.     

Rock magnetic properties of the fine-grained sediment on the outer shelf of the East China Sea: implication for provenance

The rock magnetic properties of surface sediments from the fine-grained depositional area on the outer shelf of the East China Sea, an area surrounded by sands, were investigated with a view to providing information on the sediment provenance. Multiple magnetic parameters such as magnetic susceptibility, anhysteretic remanent magnetization, saturation remanent magnetization (SIRM), coercivities of SIRM, and S ratios (relative abundance of low-coercivity magnetic minerals) were measured for all 179 surface samples, and partial representative samples were examined for their magnetic hysteresis parameters, temperature dependence of magnetic susceptibility and X-ray diffraction spectra. Our research indicates that the magnetic mineralogy is dominated by magnetite with a small amount of hematite and is primarily of a pseudo-single-domain to multidomain nature with a detrital origin. The magnetic concentration in the study area generally decreases to the east or southeast accompanied by magnetic particle fining to the east or northeast. The geographic pattern of magnetic properties is most reasonably explained by a major source of sediment jointly from the erosion of the old Huanghe River deposit and the discharge of the Changjiang River. The rock magnetic data facilitate understanding of the transport mechanism of fine-grained sediments in the outer shelf of the East China Sea.     

Spectral analyses of offshore magnetic data for computing depths to the basement rocks

This paper deals with the computation of the depth to the magnetic interface from the total magnetic field intensity values obtained along profiles in the offshore area between Vengurla and Rajapur bay off the West Coast of India, by the application of one dimensional spectral analysis method.From the response characteristics of the total field magnetic values it is surmised that the magnetic interface may be identified with the Deccan trap series of rocks which are likely to form the magnetic basement rocks in this area.The plot of depths to the basement rocks as obtained from spectral analysis indicates that the basement is sloping towards outer edge of the Continental Shelf though not uniformly.     

东海陆架区中北部前第三系基底综合地球物理研究

从地震、钻井资料出发,结合周边地质特征,推断东海陆架区中北部存在中生界和古生界地层。结合地震资料,正演消除了海底和新生界密度不均匀在重力场上的影响,定量反演重、磁力异常并求取了重力基底和磁性基底。重力基底的特征在东西分带的大背景下表现出“靠陆侧南北分块,靠洋侧东西分带”的特点。西湖凹陷区的磁性基底与重力基底埋深大致相同,不一致地方可能是受岩体的影响。西湖凹陷以西地区,包括海礁凸起等单元,总体出现磁性基底埋深比重力基底埋深深2km左右,初步解释为古生界地层的反映。中生界地层厚度的分布也具有东西分带和南北分块特点。西湖凹陷西缘的中生界地层厚度大,其中带中生界虽然含有一定的火山岩或火山碎屑岩,但沉积岩更多,这对下一步的油气勘探有利。