Bouguer Gravity Anomaly in the Andean Orogenic Belt and its Dynamic Implications for Regional Tectonic Evolution

Calculated Bouguer gravity anomalies from the Andean orogenic belt interpreted as derived from regional gravity data to aid understanding of the lithospheric structure and tectonic evolution of the belt.These anomalies reveal lithospheric structures distributed throughout the belt,including linear and circular structures.NE-trending structures reflect sinistral transpression across the northern part of the belt,and NW-trending structures represent dextral transtension in the southern part.These results are supported by gravity-anomaly patterns that demonstrate mantle flow in a trench-parallel direction both northward and southward away from the stagnation band that is beneath the subducting Nazca slab.This mantle flow has served as an important driving force in the evolution of the Andean orogenic belt.Features of the modified tectonic model of the Andean orogenic belt are consistent with the spatial variation in and interpretation of Bouguer gravity anomalies.

Active Depths of Main Faults in the Ying-Qiong Basin Investigated by Multi-Scale Wavelet Decomposition of Bouguer Gravity Anomalies and Power Spectral Methods

The Ying-Qiong Basin is located on the northwestern margin of the South China Sea and at the junction of the South China Block and the Indochina Block.It is characterized by complex geological structures.The existing seismic data in the study area is sparse due to the lack of earthquake activities.Because of the limited source energy and poor coverage of seismic data,the knowledge of deep structures in the area,including the spatial distribution of deep faults,is incomplete.Contrarily,satellite gravity data cover the entire study area and can reveal the spatial distribution of faults.Based on the wavelet multi-scale decomposition method,the Bouguer gravity field in the Ying-Qiong Basin was decomposed and reconstructed to obtain the detailed images of the first-to sixth-order gravitational fields.By incorporating the known geological features,the gravitational field responses of the main faults in the Ying-Qiong Basin were identified in the detailed fields,and the power spectrum analysis yielded the depths of 1.4,8,15,26.5,and 39 km for the average burial depths of the bottom surfaces from the first-to fifth-order detailed fields,respectively.The four main faults in the Yinggehai Basin all have a large active depth range:fault A(No.1)is between 5 and 39 km,fault B is between 26.5 and 39 km,and faults C and D are between 15 and 39 km.However,the depth of active faults in the Qiongdongnan Basin is relatively shallow,mainly between 8 and 26.5 km.

Refinement of Bouguer anomalies derived from the EGM2008 model,impact on gravimetric signatures in mountainous region:Case of Cameroon Volcanic Line,Central Africa

Global geopotential models have not included the very high frequencies of the Earth’s external gravity field.This is called omission error.This omission error becomes more important in mountainous areas(areas with highly variable topography).The work reported here consists in reducing the omission error in measurements of Bouguer gravity anomalies,by refining the global geopotential model EGM2008 using the spectral enhancement method.This method consists in computing the residual terrain effects and then coupling them to the gravimetric signal of the global geopotential model.To compute the residual terrain effects,we used the Residual Terrain Model(RTM)technique.To refine it required a reference surface(ETOPO1)developed up to degree 2190(the maximum degree of the EGM2008 model)and a detailed elevation model(AW3D30).Computation was performed with the TC program of the GRAVSOFT package.The topography of the study area was assumed to have a constant density of 2670 kg/m3.For the inner and outer zones,the respective integration radii of 10 km and 200 km have been chosen.We obtained very important RTM values ranging from−53.59 to 34.79 mGal.These values were added to the gravity anomalies grid of the EGM2008 model to improve accuracy at high frequencies.On a part of the Cameroon Volcanic Line and its surroundings(mountainous area),we made a comparison between the residual Bouguer anomalies before and after refinement.We report differences ranging from−37.40 to 26.40 mGal.We conclude that the impact of omission error on gravimetric signatures is observed especially in areas with high variable topography,such as on the Cameroon Volcanic Line and around the localities of Takamanda,Essu,Dumbo,and Ngambe.This finding illustrates the great influence that topography has on accurate measurement of these gravity anomalies,and thus why topography must be taken into account.We can conclude that in preparing a global geopotential model,a high resolution DTM must be used to decrease the omission error:the degree of expansion has to increase in order to take the higher frequencies into account.The refined Bouguer anomalies grid presented here can be used in addition to terrestrial gravity anomalies in the study area,especially in mountainous areas where gravimetric data are very sparse or nonexistent.

Understanding the Meaning of the Positive Bouguer Anomaly of Waza (Northernmost Cameroon, Central Africa)

Gravity and magnetic data were analyzed in conjunction with available geological data to determine the origin of observed positive gravity anomaly at Waza and its possible relationship to mafic intrusions. The data analysis includes the construction of a Bouguer gravity anomaly, isostatic residual gravity anomaly, total magnetic gradient and Euler solutions maps, and two and one-half dimensional gravity models. A positive gravity anomaly that occurs in Waza has amplitude of 35 mGal and can be separated into two parts. The top of the disturbing body causing the anomaly cannot be deeper than 4 km. We believe the anomaly is probably caused by a body of basaltic rocks lying at the upper surface of the Precambrian basement and originate from a self-propagating disturbance of magmatic loads on a pre-existing zone of lithospheric weakness. Melt migration might have been aided by tectonic and flexural stresses, such that the intrusion is limited in extent to the melting region of the plume and did not reach the surface. The model calculated to satisfy the observed anomaly consists of a 2.5 Km-thick slab underlain by a 10.5 km deep column. This model is interpreted to represent a sheet or saucer-like mafic intrusion that has a thick deep feeder. Detailed geophysical studies would be necessary to locate any possible economic occurrences.

MULTI-SCALE DECOMPOSITION OF BOUGUER GRAVITY ANOMALY AND SEISMIC ACTIVITY IN NORTH CHINA

Bouguer gravity anomaly in North China is decomposed with multi scale decomposition technique of wavelet transform. Gravity anomalies produced by anomalous density bodies of various scales are revealed from surface to Moho. Characteristics of anomalies of different orders and corresponding structural features are discussed. The result shows that details of wavelet transform of different orders reflect the distribution features of rock density at different depths and in various scales. In most cases, the two sides of a fault especially a deep and large fault in North China differ greatly in rock density. This difference records the history of the formation and evolution of the crust. Deep structural setting for the \%M\%s≥7.0 strong earthquakes in this region is also discussed.

高精度重磁方法寻找隐伏矽卡岩型铁矿

位于华北克拉通东部的鲁中莱芜地区是矽卡岩型富铁矿重要产区,矿床主要产于矿山岩体与中奥陶统碳酸盐岩地层的接触带中。本文利用最新重磁测量成果,对矿山岩体外围西部石家泉—刘家庙一带重磁异常特征进行了细致研究,结合区内已知铁矿重磁场特征,圈定了深部找矿靶区。在成矿有利地段布设了大比例尺重磁剖面,以已知钻孔为约束条件,利用2.5D重磁联合反演技术,对重磁异常进行了定性及定量解释。解释结果为后续钻孔位置布设及深度预测提供了依据,该钻孔揭露了15.8 m厚的富铁矿,找矿效果显著,为今后本区进一步寻找矽卡岩型铁矿提供了重要指示依据。

理论自由空间重力异常处理在承德高原地区应用效果分析

布格重力校正方法中存在正常场校正的高程起算面误差、高程校正误差与地形和中间层校正误差,为避免传统布格重力校正方法中存在的误差影响,提高资料解释的准确性,本文采用了理论自由空间重力异常处理技术,对在承德御道口波状高原区的实测重力资料重新进行了处理。通过与布格重力异常的对比分析,对该方法进行了评价。研究结果表明,在波状高原地区,理论自由空间重力异常相较于布格重力异常能够较好地反映区域重力场的空间分布形态,对局部信息的展现也更加清晰、丰富,对构造形态表现更加合理、准确,根据不同的地质目的,合理选择布格校正和理论自由空间重力异常处理,可提升资料解释的可靠性。

Distribution law and susceptibility of geohazards across a gradient belt of the Western Sichuan Plateau

Across a gradient belt of the Western Sichuan Plateau,geohazards have seriously limited economic and social development.According to incomplete statistics,15,673 geohazards have been recorded in the study area.In order to mitigate the threat of geohazards to human engineering activities in the region,an overall understanding of the distribution pattern of geohazards and susceptibility assessment are necessary.In this paper,a gradient belt of the Western Sichuan Plateau and its zoning criteria were defined.Subsequently,on the basis of relief amplitude,distance to faults,rainfall,and human activities,three indicators of endogenic process were introduced:Bouguer gravity anomaly gradient,vertical deformation gradient,and horizontal deformation gradient.Thereafter,the distribution patterns of geohazards were investigated through mathematical statistics and ArcGIS software.By randomly selecting 10,449 hazards,a geohazard susceptibility map was generated using the Information Value(IV)model.Finally,the IV model was validated against 5224 hazards using the Area Under Curve(AUC)method.The results show that 47.6%of the geohazards were distributed in the zone of steep slope.Geohazards showed strong responses to distance to faults,human activities,and annual rainfall.The distribution of geohazards in the gradient belt of the Western Sichuan Plateau is more sensitive to vertical internal dynamics factors(such as vertical deformation gradient and Bouguer gravity anomaly gradient)without any apparent sensitivity to horizontal internal dynamics factors.The areas of high and very-high risk account for up to 32.22%,mainly distributed in the Longmenshan and Anning River faults.According to the AUC plot,the success rate of the IV model for generating the susceptibility map is 76%.This susceptibility map and geohazard distribution pattern can provide a reference for geological disaster monitoring,preparation of post-disaster emergency measures,and town planning.

Structural Setting of the South-West Cameroon Using Satellite Potential Field Derived from SGG-UGM-2 Gravity Data

This study aims to improve knowledge of the structure of southwest Cameroon based on the analysis and interpretation of gravity data derived from the SGG-UGM-2 model. A residual anomaly map was first calculated from the Bouguer anomaly map, which is strongly affected by a regional gradient. The residual anomaly map generated provides information on the variation in subsurface density, but does not provide sufficient information, hence the interest in using filtering with the aim of highlighting the structures affecting the area of south-west Cameroon. Three interpretation methods were used: vertical gradient, horizontal gradient coupled with upward continuation and Euler deconvolution. The application of these treatments enabled us to map a large number of gravimetric lineaments materializing density discontinuities. These lineaments are organized along main preferential directions: NW-SE, NNE-SSW, ENE-WSW and secondary directions: NNW-SSE, NE-SW, NS and E-W. Euler solutions indicate depths of up to 7337 m. Thanks to the results of this research, significant information has been acquired, contributing to a deeper understanding of the structural composition of the study area. The resulting structural map vividly illustrates the major tectonic events that shaped the geological framework of the study area. It also serves as a guide for prospecting subsurface resources (water and hydrocarbons). .

胶东地区重磁场特征及地质解释

本文系统统计了山东胶东地区各地层、岩浆岩的磁化率、剩余磁化强度及密度等物性参数特征,从地质、物性两个角度对区域重磁场特征进行了综合分析。研究结果表明:重、磁场总体走向呈NE—NNE,宏观上反映了胶东地区的地层分布、构造格架和岩浆活动;重力场强度变化较大,跨越(-40~40)×10^(-5)m/s^(2)区间,在胶莱盆地南北两侧表现为重力低,盆地内则反映为重力高;磁场以低缓磁场为主,北、中部场强低、变化小,东南部场强高、变化大。胶东地区重磁场特征反映了该区南部、北部岩浆活动较强烈,中部壳层变薄,基底构造以近EW向、局部构造呈NE向,以及较高密度、较强磁性的古老结晶基底呈带、呈块分布的总体格局。

四川长宁及周缘重力数据融合及深浅构造特征研究

高精度地球物理数据是获取地壳内部物质变化的基础,2019年6月17日四川长宁发生MS6.0级地震,近期该区域多次发生5级左右地震,且区域内地震的发震诱因存在争议。本研究以实测重力剖面数据为基准,首先寻找重力场模型数据与实测重力剖面数据之间的映射关系式;然后对重力场模型数据进行修正,采用双线性插值法获取研究区内高精度网格数据;以长宁MS6.0地震震中为中心点,提取4条重力剖面并采用密度反演获取跨长宁MS6.0地震震中的地壳密度差变化特征。研究结果表明:通过重力场模型数据与实测重力数据间的映射关系式计算所获取的融合数据精度小于5%;长宁及周边震群发震区域对应三叠纪地层且震群多沿背斜、向斜发生;长宁MS6.0地震位于剩余布格重力异常0~30 mGal的正低值区域和密度反演高低转换带上0.048~0.058 g/cm^(3)的正低值区域,该区域位于长宁背斜上的大地湾断层与双河背斜轴部隐伏断层的交汇处。本研究可为多源重力数据融合提供技术基础,服务于以动力学为基础的地震物理预测。

Bouguer gravity anomaly of the Moon from CE-1 topography data:Implications for the impact basin evolution

In this study,the terrain correction for lunar free-air gravity anomaly (FAGA) is calculated in spherical coordinates based on the global topography data detected by the laser altimeter on Chang'E-1 (CE-1). The obtained lunar Bouguer gravity anomaly (BGA) reveals density irregularities of the interior mass. BGA is important in characterizing the mascon basins. According to the BGA of the Moon,the South Pole-Aitken (SPA) basin is considered the largest mascon basin on the Moon,and the feature of BGA in the basin implies the impacting direction. Further,the mascon basins seem to be classified into two types,Type Highland and Type Plain. For the mascon basins of Type Highland the dense materials mainly come from the shallow crust,which are associated with the basalt deposits. The other type,Type Plain,includes mascon basins whose major dense materials may be located deep at the litho-sphere,corresponding to the uplifted mantle.

中国东北地区岩石圈有效弹性厚度及构造涵义研究

基于WGM2012布格重力异常数据,获取中国东北地区岩石圈有效弹性厚度T e分布。采用小波多尺度分解方法,反演区域不同深度地质体横向差异。结合地震重点监视地区(依舒断裂带北段地区、扎兰屯地区、长白山地区)地震活动特征,分析岩石圈深部构造与地震活动之间的关系。结果表明,依舒断裂带北段地区T_(e)约为15.5 km,岩石抗变形能力较弱,浅部密度横向差异较明显,震源深度较浅;扎兰屯地区T_(e)约为24.5 km,岩石抗变形能力较强,深部密度横向差异较大,震源深度较深;长白山地区T_(e)约为11.5 km,受西太平洋板块俯冲作用下的热地幔动力作用,地震活动相对较活跃。

关于水准测量高差重力异常改正方法的探讨

为了实现在没有条件实测重力和检索布格重力异常数据库的地区进行水准测量高差的重力异常改正,满足水准测量闭合差的限差要求,本文利用多种重力场模型计算的空间重力异常和不同插值方法得到的布格重力异常,对某隧道水准测量高差进行重力异常改正。根据误差传播定律计算重力异常中误差,分析三种不同正常重力平均值的计算公式对水准测量闭合差的影响。实例计算结果表明,在所有外业限差满足要求,但水准测量闭合差仍然超限的情况下,通过利用重力场模型和布格重力异常计算的重力异常改正对高差测量值进行改正后,两种方法均能使闭合差满足限差要求,部分布格重力异常插值方法优于重力场模型。同时验证三种重力异常改正的近似公式对结果影响并不大,可基于实际情况采用不同的公式进行计算。以上结论可为地形起伏较大地区的水准测量重力异常改正计算提供参考。

似大地水准面与大地水准面转换分析

传统的似大地水准面与大地水准面近似转换在地形复杂的山区难以满足精度需求。本文基于似大地水准面与大地水准面严密转换方法,计算珠峰地区转换改正数值大小,分析似大地水准面与大地水准面转换的空间变化。结果表明,严密方法计算的转换值变化范围为-3.270~-0.119 m,其中地形位差改正极值达到1.272 m,重力梯度改正极值达到-0.138 m,这两项改正影响幅度较大,在山区需顾及;严密方法计算和近似计算结果都与地形高度具有一定的相关性,但前者结果相较于后者更为平滑,在布格重力异常与地形变化的共同影响下,严密方法局部特征与地形高度负相关。

北部海岸带(台州以北)航磁推断的基底构造特征

北部海岸带(台州以北)横跨华北地块、下扬子地块和华夏地块三个构造单元,区域地质构造复杂,岩浆活动强烈,断裂发育。本文以最新编制的航磁资料为主,结合航空重力、区域地质和实测岩石物性资料等,对北部海岸带(台州以北)进行了航磁综合解释。依据化极航磁ΔT反映的东西分带和南北分块的特点,详细分析了磁场面貌、特征及其与区域构造的对应关系,展现了不同特征的磁场是构造、岩浆活动以及地层分布等信息的综合体现,并进一步对基底岩相构造进行了划分和研究。郯庐断裂带以西的渤海海岸带,以团块状正磁异常为特征,基底由太古宇迁西群、鞍山群、泰山群及古元古界变质岩构成,以东为变化负磁异常区,磁场由鞍山群、胶东群、辽河群及粉子山群变质岩系及部分中酸性岩浆岩引起;南黄海海岸带的团块状正磁异常与太古宇变质岩有关,基底由元古宇变质岩构成;苏南-勿南沙和浙闽沿海地区的剧烈变化磁异常带,是被中新生代岩浆岩强烈改造了的元古宇弱磁性变质基底;东海海岸带是以北北东向展布的正负相间磁异常带为特征,变质基底由元古宇陈蔡群的变质岩构成,正磁异常带主要是岩浆岩带的反映。基底岩相构造在航空重磁场上的特征,对区内构造单元划分及地质演化具有重要的参考价值。

北部海岸带(台州以北)航磁反映的断裂和岩性构造特征

北部海岸带(台州以北)横跨华北地块、下扬子地块和华夏地块三个构造单元,区域地质构造复杂,岩浆活动强烈,断裂发育。本文以最新编制的航磁资料为主,结合航空重力、区域地质和实测岩石物性资料等,对区内的岩浆岩航磁异常特征及分布规律、断裂进行划分和研究;圈定各类侵入岩和火山岩数百处,岩浆活动主要集中于中新生代,以燕山期为主,受深大断裂控制;断裂主要以NE向、NNE向及NW向为主,间有SN向、EW向及弧形断裂存在,前者对区域构造、岩浆活动等有明显的控制作用;并利用天然地震震中分布和GPS速度场等信息初步探讨了断裂活动性。研究成果为服务于国家重大战略需求和海岸带综合地质调查,解决重大地质问题,实现海陆统筹的目标和服务于北部海岸带(台州以北)经济区规划、城市群规划选址、重大工程建设等提供了参考依据。

胶东招远-平度断裂地球物理异常特征及其控矿条件研究

招远-平度断裂是胶东金矿集区三条超大型控矿断裂(三山岛、焦家及招远-平度断裂)之一。受地表第四系覆盖影响,金矿体空间赋存位置与主断裂及其次级断裂地表出露位置的耦合关系尚存问题。通过开展精细化处理区域重力和航空磁测数据,如空间域不同高度延拓(100 m、200 m和500 m)及垂向一阶导数等方法,发现招远-平度断裂地表出露位置与深部地球物理梯度带存在位移差,且位移差向断裂倾向方向南东侧移动趋势愈发明显,进一步证明招远-平度断裂深部倾向南东,倾角由浅至深逐渐变缓,局部变化剧烈的地质推论。同时,开展地表高精度1/10000比例尺的重力结果显示,重力异常曲线自西向东整体呈快速上升趋势,局部存在凹陷区,表明花岗质碎裂岩及绢英岩化花岗质碎裂岩密度(2.63 g/cm^(3))介于玲珑二长花岗岩(2.57 g/cm^(3))和古元古界(2.82 g/cm^(3))之间,含矿层位与围岩存在较大密度差,金矿体主要赋存于花岗质碎裂岩及绢英岩化花岗质碎裂岩层位的倾角由陡变缓或由缓变陡的转折部位,可分别发现石英脉型金矿和蚀变岩型金矿。

满都拉—卡楚加的长剖面地壳介质密度分布及深部结构特征探榷

通过对南起中国内蒙古满都拉、向北经蒙古国、直抵北端俄罗斯贝加尔湖北卡楚加全长1320 km长重力剖面的布格重力异常数据进行处理和解析,依其构建了沿剖面的二维地壳密度结构模型,并详细分析了沿剖面的地壳底界面(Moho界面)展布的深部构造特征,及剖面辖域内的5个次级局部构造单元的重力异常场、地壳密度结构、界面起伏及断裂构造分布的特征.期望此研究结果能为该剖面跨越的中亚造山带东部地区的地壳结构、各次级构造单元的界域与关联方面的进一步研究提供相关的重力场依据.

宜川—泰安剖面的密度结构、构造特征和地震活动

文中利用宜川—泰安重力剖面的相对联测和同址GNSS测量数据获得沿线的布格重力异常、剩余密度相关成像和地壳密度结构,可以看出:剖面的布格异常变化范围为-(161.7~5.5)×10^(-5)m/s^(2),自西向东总体呈上升趋势,在临汾盆地有明显“凹陷”,可能与上地壳存在低密度构造有关;剖面地壳密度分布具有明显的不均匀性,以太行山重力梯级带为界,西部密度较高,东部密度较低,中下地壳存在低密度体,可能是上地幔物质上涌岩石含部分高温流体和熔融岩体所致;不同构造块体的密度差异明显,在断裂带分布区域均存在密度差,说明密度分布与构造有一定关系;地壳介质的不均匀性和地震活动相关:鄂尔多斯断块的地壳介质相对均匀,不容易积累能量,地震活动水平低;山西断陷带和华北平原断块则相反,具有显著的不均匀性,地震活动频繁。