Interpretation of Geological and Gravity Data from the Bamiléké Plateau (West-Cameroon): Implication for the Understanding of Its Underground Lithotectonic Geometry

The Bamiléké Plateau represents a key domain in the understanding of the geodynamics associated with the central Cameroon shear. The study aimed to highlight the subsurface architecture of the plateaus basement setting with focus on high potential areas for hydrogeological and mining development projects. To this end, geological field observations were carried out. Since the structures sought were near-surface, a separation approach based on the upward continuation method was applied to the Bouguer anomaly grid. A set of processing techniques, including vertical derivative or DZ, analytical signal or SA and categorization of gravity signatures, was applied to generate the residual map. The synthesis geological model, obtained from analysis and interpretation of the various transformed maps and 2.5D modeling of two gravity profiles P1 and P2 highlights the following features: 1) intrusions of steep-sided granitic batholiths from Dschang to Bandjoun (profile P1), increasing in width from NW (Dschang) to SE (Bandjoun);2) larger volume batholiths with moderate sides located at Bafang and Bangangté (profile P1). These plutonic massifs were weakened by brittle deformation, which favored the emplacement of phonolite or anorthosite dykes within them. The emplacement of these dykes was accompanied by compressional faults with high dip between Dschang and Bandjoun and extensional faults with medium dip between Bafang and Bangangté. These fault zones (trending N85E to N95E) are ideal for hydrogeological investigations in a basement setting, as well as a series of dyke networks that could potentially be preferred zones for the circulation and accumulation of useful substances. The resulting geological sections P1 and P2 highlight the influence of granitic intrusions in the geological system of the study area, as well as the structural control associated with the various dyke intrusions. All the models obtained can serve as fundamental references for hydrogeological and mining exploration project on the Bamiléké Plateau.

Zircon U-Pb Age,Geochemical,and Sr-Nd-Pb-Hf Isotopic Constraints on the Time Frame and Origin of Early Cretaceous Mafic Dykes in the Wuling Mountain Gravity Lineament,South China

In view of the importance of mafic dyke swarms and their contribution to current scientific problems relating to South China,herein,we present the findings of studies on twenty–five representative mafic dykes cropping out in Hunan Province and Guangxi Zhuang Autonomous Region,within the southern Wuling Mountain gravity lineament,China.These results include new zircon LA-ICP-MS U-Pb age,whole rock geochemical,Sr-Nd-Pb isotopic,and zircon Hf isotopic data for these dykes.The dykes formed between 131.5±1.2 and 121.6±1.1 Ma,and have typical doleritic textures.They fall into the alkaline and shoshonitic series,are enriched in light rare earth elements(LREE),some large ion lithophile elements(LILE;e.g.,Rb,Ba,and Sr),Th,U,and Pb,and are depleted in Nb,Ta,Hf,and Ti.Moreover,the dolerites have high initial 87 Sr/86 Sr ratios(0.7055–0.7057),negativeεNd(t)and zirconεHf(t)values(-14.8 to-11.9,-30.4 to-14.9),and relatively constant initial Pb isotopic ratios(that are EM1-like,16.77–16.94,15.43–15.47,and 36.84–36.92 for 206 Pb/204 Pb,207 Pb/204 Pb,and 208 Pb/204 Pb,respectively).These results indicate that the dykes were likely derived from magma generated through low-degree partial melting(1.0%–10%)of an EM1-like garnet–lherzolite mantle source.The parental magmas fractionated olivine,clinopyroxene,plagioclase,and Ti-bearing phases with negligible crustal contamination,during ascent and dyke emplacement.Several possible models have been proposed to explain the origin of Mesozoic magmatism along the Wuling Mountain gravity lineament.Herein we propose a reasonable model for the origin of these mafic dykes,involving the collision between the paleo-Pacific Plate and South China,which led to subsequent lithospheric extension and asthenosphere upwelling,resulting in partial melting the underlying mantle lithosphere in the Early Cretaceous,to form the parental magmas to the WMGL mafic dykes,as studied.

Study on Evolution of Gravity Fieldand Earthquake Prediction in theNorth-south Seismic Belt and theEastern Qinghai-Xizang Block

The relation between the dynamic evolution feature of gravity field and strong seismicity is studied. The result shows that the regional gravity field variation enjoys inhomogeneity of spatial and temporal distribution and gravity change in different regions. It may be resulted from active faults and seismogenic process, and may be due to microdynamic activity of regional strain energy, which might be accumulated or released in different stages, and there exists transformation process of stress.

Determination of the Structural Lineaments in the Kribi-Campo-Ma’an Area from a Multi-Scale Analysis of Gravity Data Using the HGM and Euler 3D Deconvolution Approaches

With the aim to produce a detailed structural map and then enhance the geological information of the south-western Cameroon, a field gravity study is carried out in the Kribi-Campo-Ma’an area (SW Cameroon). This study area is assumed to undergo pending active tectonics as witnessed by several earthquakes [1]. Besides, the area has been affected by some shears that gave rise to many major faults including the Kribi-Campo Fault (KCF). This N-S lineament is an active fault corresponding to the NW margin of the CC [2] [3] and related to the development of the Kribi-Campo basin. The gravity data analysis enables to highlight many deeply-seated structural features trending in the NE, NNE, N-S, NNW, NW and E-W directions in this region. The NW-SE trend is more strongly developed than the other identified trends and could have deeply affected the major NNE structure. The NNE-SSW to N-S fractures and faults which are local relays of the Kribi-Campo fault are related to the Kribi shear zone. The fairly good clustering observed on local maxima of the horizontal gradient magnitude enables to suggest that the contacts outlined have vertical to subvertical dips. However, a probable interpretation of these features is that they are vertical to subvertical faults which follow the aforementioned directions. The location of the study area in the gulf of Guinea suggests that it sustains tectonics related both to the equatorial and the south Atlantic oceans opening.

Characteristics of satellite-gravity variations in the North-South Seismic Belt before the 2013 Lushan earthquake

To study the characteristics of gravity variations in and near the North-South Seismic Belt before the 2013 Lushan earthquake,we used the geopotential-field models based on monthly data of the RI.~5 GRACE satellite to calculate the gravity changes. Here we present the patterns of annually cumulative variation, differentiatial variation and secular trend, as well as the continuous time-series at 4 characteristic sites during 2004 -2012. The result shows that the anomalous positive-to-negative transition zone, in which the epicenter of the 2008 Wenchuan earthquake was located, did not show any new gravity change before the Lushan earthquake, though located in the same zone.

Lineaments Extraction from Gravity Data by Automatic Lineament Tracing Method in Sidi Bouzid Basin (Central Tunisia): Structural Framework Inference andHydrogeological Implication

The gravity method may be used in the exploration of deep sedimentary basins. It allows the structuring and the lateral and vertical extent of sedimentary fill to be determined. This study has concerned a qualitative and quantitative gravity analysis of Sidi Bouzid Basin in Central Tunisia. Bouguer anomaly analysis and Gravity data filtering allows us to emphasize the structures affecting the basin. The Automatic Lineament Tracing method helps to quantify the different gravity responses of faults located in the shallow and deep sedimentary sections and in the basement. The elaborated structural map of the study area constitutes a useful document for rationalizing the future groundwater exploration in the arid area of central Tunisia since it shows faults dipping and deep hydrogeologic sub-basin delineation.

Spatial Analysis of Gravity Data in the Basement of the Yaoundé-Yoko Area from the Global Gravity Model: Implication on the Sanaga Fault (South-Cameroon)

In this work, gravity anomalies from the XGM2016 global gravity model are used to study the basement of the Yaounde, Yoko area. The aim is to locate the characteristic tectonic faults and to characterize the geometry of the basement of these localities in order to improve the knowledge of the structural and tectonic basement of the study area. Numerical filters (vertical gradient, horizontal gradient, upward continuation) and Euler deconvolution were applied to the gravity anomalies respectively for qualitative and quantitative analysis. The results of the qualitative analysis allowed us to establish the lineament map of the study area;ranging from 0 to 35 km depth. For the quantitative analysis, the work is done in two parts: 1) highlighting the distribution of depths of geological structures in the basement of the study area;2) 2D1/2 modeling of geological structures to highlight the geometry of the basement of Yaounde, Yoko area. Thus, from five suitably selected profiles, the established models reveal the presence of eight blocks of geological structures of different densities and analyze their implications on the Sanaga Fault. Moreover, the models show that the positive anomalies characteristics for the Sanaga Fault reflect the anomalous character due to the strong dominance of the shale intrusion in the basement.

Gravity anomaly before the Leshan M5.0 earthquake?

The North-South Seismic Belt was analyzed using gravity observation data from 2011 to 2015, and the nontidal analysis results show that there was a nonlinear gravity change at both the Chengdu and Guza seismostations one month before the Leshan M5.0 earthquake.

Geophysical lineaments of Western Ghats and adjoining coastal areas of central Kerala,southern India and their temporal development

A family of geophysical lineaments have been identified in ~15,000 km^2 in central Kerala between9 30'N to 10 45'N and 76 00'E to 77°30′E,integrating geophysical data with geological and geomorphological features. The characteristics of these lineaments in the magnetic and gravity fields and their derivatives have been analysed. The evolution of the lineaments has been traced to the temporal phases of global evolution of the region. A group of these faults have formed by reactivation of the deep-seated distensional fractures associated with and accompanying dyke emplacements during the episodic breakup of Gondwana at ~90 and 65 Ma under distinctive mantle thermal regimes. It is possible that reactivation of these distensional faults may have started during the cooling interval of time between the two distensions in the 90 and 65 Ma and post 65 Ma periods and later in the Cenozoic, when the lineaments were enlarged to their present dimension, possibly under the influence of forces that led to the uplift of the western Ghats. These may extend down to the crust-mantle interface. A cluster of youngergeophysical lineaments has been generated by reactivation along the weak planes of transformation of the charnockitic rocks of the Precambrian. They seem to have a strike-slip character. They are devoid of any dyke association and were formed on a cold crust. They may be confined to the upper-middle crust.They were generated in the high intensity intra-plate palaeo-stress fields of the triple forces arising from(1) the back-thrust from the Himalayan Collision;(2) the impact of epeirogenic forces and related isostatic uplift of the Western Ghats and(3) the flexural isostatic uplifts due to surface loads of late Mesozoic basaltic lavas and Cenozoic sedimentation in the coastal rifted basins in late Cenozoic, probably in the time span of 20 Ma to the present, when the palaeostress fields were most intense.

Qualitative Interpretation of Gravity and Aeromagnetic Data in West of Tikrit City and Surroundings, Iraq

The qualitative interpretation of anomalies of the gravity and the magnetic fields at the west of Tikrit City and surroundings, Iraq, was carried out. According to the tectonic framework of Iraq, the study area is classified as a part of the stable shelf unit. The polynomial technique was employed to separate the residual-regional anomalies as a first step in the qualitative interpretation. Bouguer gravity anomaly map did not exhibit any clear closures. The magnetic maps showed remarkable high closure in the central eastern part of the study area. The regional gravity and magnetic fields values decline towards the eastern parts of the maps (Mesopotamian Basin) in consistent with the increase in the sedimentary cover (basement depth). The residual gravity and magnetic maps displayed several positive and negative closures. The gravity and magnetic lineaments were depicted and superimposed on the tectonic map of the study area. The rose diagram showed that the dominant orientation of the gravity and magnetic lineaments was toward NE-SW.

利用接收函数H-κ-c叠加方法约束武陵山重力梯度带地壳结构

贯穿中国内地南北的大兴安岭-太行山-武陵山重力异常梯度带被认为是东亚重要的岩石圈分界线,本文利用布设在武陵山重力梯度带的43个国家地震局固定台站和10个流动宽频台站的观测数据,采用远震P波接收函数方法,计算并挑选了共计12 739条高质量的远震P波接收函数,综合其中7个台站H-κ叠加和46个台站H-κ-c叠加的结果,并结合前人结果获得了研究区的莫霍(Moho)界面起伏形态以及地壳平均波速比(v_(P)/v_(S))分布和通过H-κ-c叠加方法获得相应台站下方地壳各向异性。结果表明研究区地壳厚度在30~52 km之间,其中最厚的大巴山区域超过50 km,最薄的雪峰山以东地区仅约30 km,整体特征表现为西厚东薄。Moho界面梯度变化最大的区域,北起秦岭-大巴山东侧一带,沿着江汉盆地与武陵隆起的盆山耦合处,南至江南造山带中段北侧;地壳平均波速比整体的分布特点表现为:高值(>1.81)普遍分布在武陵山重力梯度带以西地区,而低值(<1.75)分布以东地区以及江南造山带内部;地壳各向异性同样在武陵山重力梯度带两侧存在明显差异,以东地区快波最大极化方向为近E-W向,而以西地区则表现为NE-SW向。最后,我们推测武陵山重力梯度带附近及以东地区存在普遍的下地壳拆沉现象。

西太平洋俯冲板块对中国东北构造演化的影响及其动力学意义

中国东北地区在古生代期间以众多微陆块的拼合以及古亚洲洋的闭合为特征,其后又经历了中-新生代太平洋构造域及中生代蒙古—鄂霍茨克构造域的叠加与改造,以致东北地区的构造行迹显得极为复杂,而大兴安岭重力梯级带及其西部地区构造演化是否与西太平洋俯冲有关仍然存在争议.本研究利用分布于中国东北、华北地区以及韩国、日本等部分台网所接收的近震与远震走时数据获得了中国东北地区壳幔精细的三维P波速度结构.成像结果显示,太平洋板块持续西向俯冲,俯冲板片的前缘停滞在大兴安岭—太行山重力梯度带以东区域的地幔转换带之中;长白山火山区上地幔存在着显著的低速异常体,推测西太平洋板块的深俯冲脱水导致了上地幔底部岩石的熔点降低,从而形成了大范围的部分熔融物质上涌.通过分析上地幔的速度结构,我们认为由于太平洋板块的大规模西向深俯冲,在大地幔楔中发生板片脱水、低速热物质上涌等复杂的地球动力学过程;俯冲板片前缘带动上地幔中不均匀分布的地幔流强烈作用于上部的岩石圈,这对东北地区深部壳幔结构乃至大兴安岭重力梯级带的形成、演化有着重要的影响.

大兴安岭-太行山重力梯度带以西的第四纪火山活动

大兴安岭—太行山重力梯度带以西的第四纪火山活动自北往南贯穿兴蒙造山带和华北克拉通2个大地构造单元,可以分为北、中、南三部分,展现规模不等的火山群:北部诺敏河火山群和绰尔河—哈拉哈河火山群,中部阿巴嘎火山群和达里诺尔火山群,南部大同火山群和乌兰哈达火山群。这些火山群主要由单成因火山组成,少则几十座,多则200余座,如阿巴嘎火山群向北延入蒙古达里干加,构成亚洲东部面积最大的新生代火山区。因此,大兴安岭—太行山重力梯度带以西的第四纪火山成为探讨大陆内部火山活动构造背景与成因机制的重要场所,也是获取大兴安岭—太行山重力梯度带东西两侧和兴蒙造山带与华北克拉通2个大地构造单元岩石圈差异演化信息的重要窗口。本文介绍了这些火山的分布、构造背景和成因机制。

河北阳原新生代玄武岩中两类辉石岩包体的矿物学和地球化学特征

位于大兴安岭-太行山重力梯度带西侧的阳原新生代玄武岩中含有大量橄榄岩和辉石岩包体。辉石岩包体含有绿色的Cr辉石岩和黑色的Al辉石岩包体两种类型,均为二辉岩,且多数样品中单斜辉石含量高于斜方辉石。岩石结构和主微量元素组成说明阳原辉石岩是高压堆晶体。其EMI型Sr-Nd同位素特征,明显不同于寄主玄武岩,暗示两者没有成因联系,而可能代表古老岩浆事件的产物。综合文献资料发现重力梯度带西侧的辉石岩包体大多具有富集同位素特征,而东侧的样品以亏损同位素组成为主。这种空间上的成分差异暗示重力梯度带分隔两个不同的地幔域,也可能与重力梯度带两侧岩石圈地幔形成时代不同有关。

太行山重力梯度带的形成与华北岩石圈减薄的时空差异性有关

通过对比华北太行山重力梯度带两侧新生代玄武岩及其中幔源包体的成分,发现:(1)华北东、西部新生代玄武岩具有相反的演化趋势,说明新生代以来西部岩石圈逐渐减薄,而东部岩石圈逐渐加厚;(2)西部岩石圈地幔组成相对复杂,年龄多为晚太古代-元古代;而东部岩石圈地幔组成相对单一,年龄多为现代值,少数为元古代;(3)西部壳幔过渡带较厚而东部较薄,反映两地不同的岩浆底侵作用程度．华北岩石圈组成的空间不均一性可能与岩石圈减薄过程的时空差异有关．岩相古地理分析说明太行山重力梯度带的雏形形成于早白垩世,与华北中生代岩浆活动的高峰相吻合．由于岩浆作用与岩石圈减薄作用密切相关,因此认为华北岩石圈减薄的时空不均一性是形成太行山重力梯度带的重要机制．

应用远震有限频率层析成像反演中国东北地区上地幔P波三维速度结构

选取黑龙江、吉林、辽宁、内蒙古区域地震台网,以及NECESSArray流动台阵记录的223个远震事件的波形资料,采用多道互相关方法得到了22569个P波相对走时数据,并计算了相应的走时灵敏度核,应用有限频率层析成像反演得到中国东北地区上地幔600 km以上的P波三维速度结构模型,利用检测板评估了反演结果的分辨率。结果表明,松辽盆地下方80~200 km的深度上呈主体的低速异常,与这一地区上地幔浅部的高地温值和低密度的特征相互对应,可能暗示了部分熔融的地幔。南北重力梯度带两侧的速度结构明显不同,这一差异可以延伸到200 km以下,表明在中国东北地区南北重力梯度带有可能是一条上地幔内部结构的变化带,或是深部结构的分界线。长白山火山区下呈大范围的低速异常,并可从上地幔浅部延伸到地幔转换带中,推测此低速异常可能反映了地幔转换带内上涌的热物质,上涌的原因则主要是受到太平洋板块俯冲运动的作用。

中国东部及邻区晚中生代伸展拆离构造——综述与新认识

伸展构造是大陆岩石圈减薄破坏在上部浅层的一种重要表现。其中,低角度伸展拆离构造为中国东部以及邻区晚中生代地壳伸展变形的显著构造变形样式,是研究区域岩石圈浅层构造变形与深部减薄过程关系的重要窗口。结合相关的几何学、运动学、年代学等资料的综述分析,本文对中国东部及邻区低角度伸展拆离构造时空展布、运动学极性、应变机制进行了系统论述。在空间展布上,这些伸展拆离构造在东亚大陆的分布不均一,指示岩石圈的不均匀伸展减薄作用。在形成时代上,伸展拆离活动主要集中在两个时代区间:140–125Ma为伸展拆离构造在中下地壳初始形成期,在水平伸展作用主导下,以简单剪切为主;125–110Ma为伸展拆离构造快速隆升剥露期,在垂向挤压的主导下,以纯剪切为主。相关的几何学和运动学标志表明这些伸展构造总体具有在NW–SE方向上单向低角度伸展拆离运动学特征,反映它们总体受NW–SE向伸展应力场作用控制。以大兴安岭—太行山—武陵山重力梯度带为界,东、西两侧区域伸展活动均具有从NW向SE传播的趋势,且这种趋势西部较东部明显,表现为不同的动力学背景特征。东部主要受到古太平洋板块后撤影响,而西部可能与蒙古—鄂霍茨克洋闭合之后,增厚地壳发生重力崩塌而引发区域大规模伸展活动以及同时发生的秦岭—大别构造带陆内造山后应力伸展的联合作用相关。

华北克拉通重力梯度带两侧晚中生代火山岩地球化学特征对比研究及其对岩石圈减薄的时空制约

对华北克拉通重力梯度带东西两侧的河北承德袁家庄和内蒙古四子王旗白脑包出露的晚中生代火山岩进行了地球化学对比研究。全岩主量元素、微量元素和Sr-Nd-Hf同位素数据表明,袁家庄火山岩和白脑包火山岩具有相似的地球化学特征。这些火山岩轻、重稀土强烈分异,富集大离子亲石元素(如Rb,Ba,Th,U,K等),亏损高场强元素(如Nb,Ta,Zr,Hf,Ti等);其同位素具有高(87Sr/86Sr)i,低εNd(t)和低εHf(t)的特征,因此认为袁家庄和白脑包火山岩具有相似的源区,即均来源于被交代的古老岩石圈地幔的部分熔融。结合文献资料,作者认为,这些晚中生代火山岩是华北克拉通岩石圈减薄的直接产物,中生代时期华北克拉通岩石圈的减薄不仅仅局限于重力梯度带以东地区,以西的部分地区也有发生。

Lineament Characterization and Their Tectonic Significance Using Gravity Data and Field Studies in the Al-Jufr Area,Southeastern Jordan Plateau

Lineaments in the southeastern Jordan plateau are mapped using gravity data and field studies in order to understand the tectonic origin of these lineaments, especially in relation to the Dead Sea transform （DST） and the Red Sea opening. Four sets trending E-W, NW-SE, NE-SW, and N-S are identified in gravity data. Field studies generally reveal similar orientations. Field and gravity studies indicate that most of the lineaments are extensional features that correspond to normal faults. Most of these were subsequently reactivated into strike-slip shear fractures. The NW-SE and N-S lineaments represent dilatational fractures. The N-S trending lineaments are the oldest. The E-W lineaments form conjugate shear fractures and are younger than the N-S lineaments. These conjugate shear fractures are also older than other set of conjugate shear fractures oriented NE-SW. The evolution of all these fractures is attributed to the DST and the Red Sea spreading. Kinematic and dynamic analysis of the two, older and younger, pairs of conjugate strike-slip fractures revealed, respectively, broadly NW-SE and N-S oriented transpressional stress with corresponding transtensional stress oriented NE-SW and E-W.

The subduction of the Copiapó aseismic ridge,is the causing of the formation of metallic minerals deposits in north of Chile and Argentina?

The results obtained in this work evince that the metallic mineral deposits located in the northern region of the Chilean-Pampean flat slab(in northern Chile and north-western Argentina),at approximately 27°30’S,would be related to the subduction of the Copiapo aseismic ridge.The analysis of the gravity anomalies and vertical gravity gradient allows inferring a deflection and truncation of the main trend of the Andean structures at the extrapolated zone of the Copiapo ridge beneath South America.Thus,the general NNE-trend of the Andean structures are rotated locally to an ENE-strike within the area of the Ojos del Salado-San Buena Ventura lineament.We explain that this anomalous behavior of the gravity derived anomalies is related to the deformational effects imprinted by the ridge subduction.Regions with a low subduction angle(<30° to horizontal)are related to large mineralization due to fluids released by dehydration of the subducting crust.In addition,a higher degree of mantle melting could be produced by a thicker oceanic crust.Therefore,we interpret that the processes associated to the subduction of the Copiapo aseismic ridge(emplaced on a thickened oceanic crust due to a local compensation of the seamounts)are the cause of formation and emplacement of big metallic mineral deposits in this region of Chile and Argentina.