Implications for the lithospheric structure of Cambay rift zone,western India:Inferences from a magnetotelluric study

Broad-band and long-period magnetotelluric(MT) data were acquired along an east-west trending traverse of nearly 200 km across the Kachchh,Cambay rift basins,and Aravalli-Delhi fold belt(ADFB),western India.The regional strike analysis of MT data indicated an approximate N59°E geoelectric strike direction under the traverse and it is in fair agreement with the predominant geological strike in the study area.The decomposed transverse electric(TE)-and transverse magnetic(TM)-data modes were inverted using a nonlinear conjugate gradient algorithm to image the electrical lithospheric structure across the Cambay rift basin and its surrounding regions.These studies show a thick(～1-5 km) layer of conductive Tertiary-Mesozoic sediments beneath the Kachchh and Cambay rift basins.The resistive blocks indicate presence of basic/ultrabasic volcanic intrusives,depleted mantle lithosphere,and different Precambrian structural units.The crustal conductor delineated within the ADFB indicates the presence of fluids within the fault zones,sulfide mineralization within polyphase metamorphic rocks,and/or Aravalli-Delhi sediments/metasediments.The observed conductive anomalies beneath the Cambay rift basin indicate the presence of basaltic underplating,volatile(CO_2,H_2 O) enriched melts and channelization of melt fractions/fluids into crustal depths that occurred due to plume-lithosphere interactions.The variations in electrical resistivity observed across the profile indicate that the impact of Reunion plume on lithospheric structures of the Cambay rift basin is more dominant at western continental margin of India(WCMI) and thus support the hypothesis proposed by Campbell Griffiths about the plume-lithosphere interactions.     

Chronology of Magmatic Activity and Petrologic–Mineralogical Characteristics of Lavas of Kazbek Quaternary Volcano,Greater Caucasus

The paper presents detailed isotope-geochronological, geological, and petrologic–mineralogical data on lavas of one of the greatest Quaternary magmatic area in the Greater Caucasus, the Kazbek neovolcanic center, including polygenetic Kazbek stratovolcano and a number of subordinate volcanic cones in its vicinities. The research was conducted based on a representative collection of more than 150 geological samples that characterize most of the volcanic cones and lava flows of different age, some of which were known previously, and other were discovered by the authors. The high-precision K–Ar data obtained on these materials make it possible to reproduce the evolutionary history of youngest magmatism at the Kazbek center and evaluate the total duration of this evolution at ~450 ka. The magmatic activity was subdivided into four phases (at 460–380, 310–200, 130–90, and <50 ka) with long-lasting interludes in between. Because the latest eruptions occurred in the Kazbek vicinity in the Holocene, this volcano is regarded as potentially active. The volcanic rocks of the Kazbek center make up a continuous compositional succession of basaltic (trachy)andesite–(trachy)andesite–dacite and mostly belong to the calc–alkaline series. The principal petrographic characteristics of the rocks and the composition of their phenocryst minerals are determined, mineral assemblages of these minerals are distinguished in the lavas of different type, and the temperature of the magmatic melts is evaluated. A principally important role in the petrogenesis of the Kazbek youngest magmas is proved to have been played by fractional crystallization and replenishment of mafic melts in the magmatic chambers beneath the volcano, which resulted in their mixing and mingling with the residual dacite melt and the origin of high-temperature hybrid andesite lavas. The comprehensive geological studies, involving interpretation of high-resolution satellite images, allowed the authors to compile the first detailed (1: 25 000) volcanologic map of the Kazbek center and a geochronologic chart supplemented with a stratigraphic column, which illustrate the origin sequence of the volcanic vents and their lava flows, geological relations between them, as seen in reference geological sections, and variations in the composition of the magmatic products with time.     

庐枞火山岩盆地及其外围重、磁场特征

为探测长江中下游成矿带庐江-枞阳白垩纪火山岩盆地的深部构造和地壳结构,2007年初在庐枞火山岩盆地进行了以深反射地震剖面探测为主的,新一轮重力、磁力和大地电磁剖面测量。作者在前人研究的基础上对庐枞火山岩盆地及其外围重、磁场特征进行了研究。作者首先分析长江中下游地区重、磁场的分布特征,然后以区域重、磁场特征为背景来认识庐枞地区重、磁场的分布特征。研究各类地质体的物性参数是开展地球物理解释的前提,文中收集并分析了前人对庐枞地区的岩石物性的较为系统研究成果。为了提取重、磁异常的特征,文中对重、磁异常进行了位场转换和图像处理。利用新的深反射地震剖面探测和大地电磁剖面研究成果,采用定性和定量解释方法对庐枞地区重、磁场的分布特征进行了研究并提出新的认识。庐枞火山岩盆地深部存在隐伏的磁性强的中碱性岩类是产生区域磁异常的主要原因。庐枞火山岩盆地下部火成岩所侵入的地层向盆地东南方向延伸,盆地的西北边界向东南方向倾斜。而在罗河断裂带以西没有火成岩存在。亦即庐枞火山岩盆地是一个沿北东向罗河断裂向东发育的非对称火山盆地。另外,在庐枞火山岩盆地西部边缘罗河深部存在切穿莫霍面的断裂带沿北东向延展数十千米。     

青藏高原申扎-双湖剖面岩石圈电性结构特征及其含义

为了研究班公湖-怒江缝合带的壳幔电性结构及构造特征,并为其俯冲极性提供电性约束,对青藏高原中部申扎-双湖大地电磁测深剖面进行全面数据处理分析,获得了可靠的二维电性结构模型,研究表明:沿剖面上地壳分布的是规模不等的高阻体,底面埋深在10~25 km变化,高阻层之下发现由不连续的高导体构成的中下地壳高导层.通过对电性结构的分析,认为班公湖-怒江特提斯洋的俯冲消亡极性可能是双向的,随后拉萨-羌塘地体碰撞带处的上地壳高阻体发生拆沉,以上两次动力学事件可能共同作用于缝合带处的壳幔高导体,同时北拉萨地体的壳幔高导体还可能体现了构造作用、岩浆活动和成矿作用之间的关系.     

Structure of the Northwestern Boundary of the Middle Amur Sedimentary Basin (Russian Far East) from Audiomagnetotelluric Sounding Data

This paper reports the data on the structure of the northwestern boundary of the Middle Amur sedimentary basin which were obtained after resumption of audiomagnetotelluric soundings. The geoelectric sections for two profiles across the basin strike are constructed, the sedimentary cover and basement structures are studied, and their electrical properties are determined. The compared data of the earlier and present studies show low-resistivity zones of 50–150 Ohm m beneath high-resistivity rocks of 500–1000 Ohm m in the northwestern part of the Ul’dura–Churki uplift at a 2–4 km depth, which are absent in the southwestern part of the uplift. This fact is related to strike-slip–overthrust processes due to graben formation in the area of junction with the Jiamusi–Bureya Massif in the Kur fault zone (YYilan–Yitong branch of the Tan-Lu fault zone) and also to global left-lateral strike-slip displacements and volcanic activity.     

Chemical composition,volatile components,and trace elements in melts of the Karymskii volcanic center,Kamchatka, and Golovnina volcano,Kunashir Island: Evidence from inclusions in minerals

Melt inclusions were examined in phenocrysts in basalt, andesite, dacite, and rhyodacite from the Karymskii volcanic center in Kamchatka and dacite form Golovnina volcano in Kunashir Island, Kuriles. The inclusions were examined by homogenization and by analyzing glasses in more than 80 inclusions on an electron microscope and ion microprobe. The SiO₂ concentrations in the melt inclusions in plagioclase phenocrysts from basalts from the Karymskii volcanic center vary from 47.4 to 57.1 wt %, these values for inclusions in plagioclase phenocrysts from andesites are 55.7–67.1 wt %, in plagioclase phenocrysts from the dacites and rhyodacites are 65.9–73.1 wt %, and those in quartz in the rhyodacites are 72.2–75.7 wt %. The SiO₂ concentrations in melt inclusions in quartz from dacites from Golovnina volcano range from 70.2 to 77.0 wt %. The basaltic melts are characterized by usual concentrations of major components (wt %): TiO₂ = 0.7–1.3, FeO = 6.8–11.4, MgO = 2.3–6.1, CaO = 6.7–10.8, and K₂O = 0.4–1.7; but these rocks are notably enriched in Na₂O (2.9–7.4 wt % at an average of 5.1 wt %, with the highest Na₂O concentration detected in the most basic melts: SiO₂ = 47.4–52.0 wt %. The concentrations of volatiles in the basic melts are 1.6 wt % for H₂O, 0.14 wt % for S, 0.09 wt % for Cl, and 50 ppm for F. The andesite melts are characterized by high concentrations (wt %) of FeO (6.5 on average), CaO (5.2), and Cl (0.26) at usual concentrations of Na₂O (4.5), K₂O (2.1), and S (0.07). High water concentrations were determined in the dacite and rhyodacite melts: from 0.9 to 7.3 wt % (average of 15 analyses equals 4.5 wt %). The Cl concentration in these melts is 0.15 wt %, and those of F and S are 0.06 and 0.01 wt %, respectively. Melt inclusions in quartz from the dacites of Golovnina volcano are also rich in water: they contain from 5.0 to 6.7 wt % (average 5.6 wt %). The comparison of melt compositions from the Karymskii volcanic center and previously studied melts from Bezymyannyi and Shiveluch volcanoes revealed their significant differences. The former are more basic, are enriched in Ti, Fe, Mg, Ca, Na, and P but significantly depleted in K. The melts of the Karymskii volcanic center are most probably less differentiated than the melts of Bezymyannyi and Shiveluch volcanoes. The concentrations of water and 20 trace elements were measured in the glasses of 22 melt inclusions in plagioclase and quartz from our samples. Unusually high values were obtained for Li concentrations (along with high Na concentrations) in the basaltic melts from the Karymskii volcanic center: from 118 to 1750 ppm, whereas the dacite and rhyolite melts contain 25 ppm Li on average. The rhyolite melts of Golovnina volcano are much poorer in Li: 1.4 ppm on average. The melts of the Karymskii volcanic center are characterized by relative minima at Nb and Ti and maxima at B and K, as is typical of arc magmas.     

Geoelectric investigation of the Hellenic subduction zone using long period magnetotelluric data

The strongest evidence up to date for a subduction zone in the Hellenic region is a clearly identified Wadati-Benioff zone below the central Aegean Sea, to a maximum depth of 180 km. Alternative seismic tomography models suggest that subduction process continues deeper than the Wadati-Benioff zone to a maximum depth of at least 600 km. So far the lack of deep electrical studies in the region impeded scientists from imposing other control factors than seismic to the proposed models for the Hellenic Subduction Zone (HSZ). A Long Period Magnetotelluric (LMT) study was carried out in the southern part of the Greek mainland to study the deep electrical characteristics of the HSZ and examine whether prominent modelled features correlate with structures identified by the seismic methods. The study comprised collection, processing and modelling of magnetotelluric (MT) data in the period range 100–10000 s from ten sites located along a 250 km NE–SW trending profile. The dimensionality of the data was examined at a pre-modelling stage and it was found that they do not exhibit three-dimensional (3-D) features. The latter enabled to construct both one-dimensional (1-D) and two-dimensional (2-D) models. The proposed geoelectric model for HSZ was based on 2-D modelling, since it had better maximum depth resolution of about 400 km, and revealed structures not detected by 1-D modelling attempts. The model structure which was related to the African and Euro-Asian lithosphere is relatively resistive (> 800 Ω-m) and has an average thickness of 150–170 km. Although the bottom of the lithosphere is adequately resolved, the Wadati-Benioff zone that delineates the top of the subducting lithospheric slab is not identified by any electrical feature. The modelled structure associated with the subducting part of the African lithosphere penetrates a relatively conductive (< 200 Ω-m) asthenosphere with a dip angle of 42°. Intermediate electrical resistivities (200–800 Ω-m) are attributed to the ascending melting part of the lithosphere below the region of the Hellenic Volcanic Arc (HVA) and to a dipping zone below the south-western part of the profile, at 170–220 km depths.     

Geoarchaeological context of Holocene subsidence at the Ferry Berth Lapita site,Mulifanua, Upolu,Samoa

The 1973 discovery of an underwater archaeological site during dredging for a ferry landing at Mulifanua on Upolu raised important unanswered questions about the prehistory of Samoa, particularly the evolution of Holocene shorelines and relative local sea levels. A cultural horizon yielding Lapita potsherds, the only decorated Lapita assemblage yet found in Samoa and dating to ca. 2.8 ka, lies at a depth of 2.25 m below modern sea level beneath a capping of cemented paleobeachrock. With fluctuating hydro-isostatic sea level taken into account, the sherd occurrence implies subsidence of a former coastline by ca. 4 m at a mean rate of 1.4 mm/yr. Shoreline features on both Upolu and nearby Savai'i are fully compatible with bulk Holocene subsidence. We attribute the observed subsidence to downflexure of the lithosphere from volcano loading centered on the Savai'i locus of historic volcanism, and conclude that any other Lapita sites that may exist in Samoa have subsided by a comparable amount. Although the Samoan linear volcanic chain resembles other Pacific hotspot tracks where active volcano loading is confined to their southeastern ends, the most voluminous Holocene eruptions in Samoa have occurred on Savai'i at the northwestern end of the exposed island chain. Samoan volcanism has evidently been influenced by lateral flexure of the Pacific plate as it moves past the northern extension of the Tonga subduction zone, and the active volcanism is apparently controlled by a longitudinal rift, which transects both Upolu and Savai'i and is superimposed upon older volcanic edifices that may record earlier hotspot volcanism. Early archaeological sites in American Samoa display a variable record of subsidence and possible uplift apparently related to volcano loading in the Manu'a Islands and possibly to the location of Tutuila in a position to be affected by uparching of lithosphere between downflexures beneath more active volcanic islands to the east and west. Bulk subsidence of Upolu and Savai'i may be the fundamental cause of damaging coastal erosion in modern Samoa. © 1998 John Wiley & Sons, Inc.     

福建浦城毛洋头铀矿床地质特征与找矿前景

浦城毛洋头铀矿床产于福罗山—大坑火山盆地内,盆地基底为上元古界麻源群大金山组变质岩系和燕山早期高溪复式花岗岩体。受断裂控制,矿体形态复杂,矿体群带具有向主干断裂侧伏,同时又向火山通道中心侧伏的"双侧伏"特点。中酸-酸性火山岩是有利的铀源层,构造产状变异部位、不同岩性接触界面,为铀成矿的有利富集地段。     

Tephrochronology of recent events in the Andean Range (northern Patagonia): spatial distribution and provenance of lacustrine ash layers in the Nahuel Huapi National Park

Tephrochronology is a powerful tool for dating sedimentary sequences, especially in Patagonia, where a large number of active volcanoes have produced frequent historical eruptions. Short lacustrine sedimentary sequences were extracted from the lakes Moreno Oeste and Ton?ek (Nahuel Huapi National Park). Seventeen volcanic ash layers were identified in both cores, ²¹⁰Pb and ¹³⁷Cs were used for dating techniques, and historical volcanic records were employed for correlation. White pumice and glass shards from the tephras were characterised by measuring major and trace element contents by instrumental neutron activation analysis. Two volcanic sources – the Cordón Caulle and the Calbuco volcanoes – were recognised as the prevailing systems that impacted the area in the past 800–1000 a. The Calbuco volcano was mainly responsible for the nine tephras identified in the Lake Ton?ek sequence. Four of these nine tephras also contained material from the Cordón Caulle complex, and could be interpreted as composite tephras or possible reworked ones. The Calbuco volcano showed predominant influences in the Lake Moreno Oeste sequence. Six of the eight tephras identified in such a sequence were from the Calbuco volcano, and three of these six contained material from the Cordón Caulle complex. The other two were from the Cordón Caulle complex, with contributions from the Calbuco volcano in one of them. These findings show that the high frequency of volcanic events in the study region demands an accurate characterisation of the products generated by each source, as well as an evaluation of their spatial distribution, to obtain a consistent framework for dating recent environmental changes. Copyright © 2010 John Wiley & Sons, Ltd.     

江苏三处旧石器遗址中发现的火山玻璃

火山灰年代学是判断第四纪沉积物年龄的有效手段。文章报告在江苏三处旧石器遗址（和尚墩、放牛山和将军崖）上部堆积中发现的火山玻璃及初步研究结果。和尚墩遗址的火山玻璃主要出现在两个层位: 上层距地表约45cm,火山玻璃的水合层很薄,年代有可能晚于K Ah火山（7.3ka）;  下层距地表约70cm,火山玻璃的水合过程已经完成,时代稍老,大致与日本晚更新世火山同期,至少相当于AT火山,年代等于/大于25ka。遗址中的火山玻璃分为pm型和bw型。形态、屈折率及微量包含物的研究表明,火山玻璃的来源可能与日本的火山喷发无关。江苏旧石器遗址中发现的火山玻璃对判断中国东部地区更新世沉积物时代以及埋藏其中的旧石器遗存年代具标志意义。     

安庆-贵池矿集区及邻区深部电性结构研究

长江中下游地区经历了多期次的地质构造演化具有丰富的矿产资源,对重要矿集区及其邻区的深部电性结构进行研究具有重要意义。通过对穿过安庆—贵池矿集区的一条宽频带大地电磁测深长剖面数据进行分频段以及分区反演,构建了覆盖大别造山带至下扬子地块东缘的二维电性结构模型。发现矿集区的深部电性结构与邻区的构造单元具有显著差异,大别造山带和江南隆起带与浙赣凹陷之间的地壳整体表现为高阻特征,而下扬子坳陷和江南隆起带之间存在岩石圈上地幔尺度的高导电异常体并且与地壳浅部的高导体相连。安庆—贵池矿集区的成矿机制主要为燕山期陆内俯冲以及早白垩时期的伸展作用,矿集区下地壳加厚、拆沉和上地壳丰富的断裂系统起到了重要的控矿作用。     

宽甸火山群地质遗迹的地质特征与特殊性

宽甸地处营口-宽甸古隆起东部北缘,在距今0.12~8 Ma间,火山多次喷发留下了特殊的火山群地质遗迹.宽甸火山群的多期喷发,产生多种岩石组成的火山岩,火山岩中含有大量上地幔橄榄岩包体与丰富的巨晶矿物以及特殊的火山群地质地貌具有追溯地质历史的重大科学研究价值和观赏价值.     

安徽泥河玢岩铁矿电磁法探测试验

玢岩铁矿是我国重要的铁矿床类型之一,主要产出在富钠质的安山玢岩和闪长玢岩内或接触带中,由于其围岩多为火成岩,给常规的地球物理探测造成诸多影响,很难确定矿体位置及产状,找矿难度较大。电磁法具有勘探深度大、分辨率高等特点,是探测玢岩铁矿可能的重要手段,为评估电磁法在玢岩铁矿探测中的应用效果,作者在物性统计的基础上,结合已知地质资料,构建了玢岩铁矿理论电性模型,通过模型正演及反演,探讨了音频大地电磁、可控源音频大地电磁、瞬变电磁和复电阻率法刻画玢岩铁矿的分辨率和效果。在此基础上,以安徽省泥河铁矿为例,开展了4种电磁法探测试验,获得了泥河铁矿区三维电性结构模型,该模型反映了地层、岩体、构造的基本信息,虽然不能直接指明矿体位置,但是通过圈定闪长玢岩岩体隆起范围,为找矿工作提供了重要线索。试验结果表明,高质量的数据采集、精细的数据处理和有效的反演方式,是电磁法寻找玢岩铁矿重要保障,泥河铁矿的电磁法探测实例可以为长江中下游玢岩铁矿找矿工作提供指导。     

Coupled basin evolution and late-stage metamorphic core complex exhumation in the southern Basin and Range Province, southeastern Arizona

Records of lithospheric extension and mountain-range uplift are most continuously contained within syntectonic sedimentary rocks in basins adjacent to large structural culminations. In southeastern Arizona, metamorphic core complexes form mountain ranges with the highest elevations in the region, and supposedly much less extended terranes lie at lower elevations. Adjacent to the Santa Catalina-Rincon metamorphic core complex, within the Tucson Basin, stratigraphic-sequence geometries evident in a large suite of 2-D seismic reflection data suggest a two-phase basin-evolution model controlled by the emplacement and subsequent uplift of the core complex. In its earliest stage, Phase I of basin formation was characterized by extensive faults forming relatively small-scale proto-basins, which coalesced with the larger basin-bounding detachment fault system. Synextensional sedimentation within the enlarging basin is evidenced by sediment-growth packages, derived from adjacent footwall material, fanning into brittle hanging-wall faults. During this phase, volcanism was widespread, and growth packages contain interbedded sediments and volcanic products but, paradoxically, no mylonitic clasts from the adjacent metamorphic core complex. Phase II of basin evolution begins after a significant tectonic hiatus and consists of a symmetric deepening of the central basin with the introduction of mylonitic clasts in the basin fill. This is coupled with the activation of a series of high-angle normal faults ringing the core complex. These observations suggest a two-phase model for metamorphic core complex evolution, with an initial stage of isostatic core complex emplacement during detachment faulting that resulted in little topographic expression. This was followed, after a significant tectonic hiatus, by late-stage exhumation and flexural uplift of the Santa Catalina-Rincon metamorphic core complex through younger high-angle faulting. Moreover, the geometry of upper basin fill units suggests an extremely low effective elastic thickness in the region and that flexural uplift of the core complex induced asymmetric transfer of ductile mid-crustal rocks from beneath the subsiding Tucson Basin to the uplifting mountain range.     

Electrical structures of the source area of the 1999 Chi-Chi, Taiwan, earthquake: Spatial correlation between crustal conductors and aftershocks

Wide-band magnetotelluric (MT) data were collected on an east–west profile, approximately perpendicular to the local strike of the Chelungpu thrust, through the hypocentral area of the Chi-Chi earthquake for imaging the seismogenic structure. MT data were then inverted for two-dimensional resistivity models plus best-fitting static shift parameters using a nonlinear conjugate gradient algorithm that minimizes the sum of the normalized data misfits and the smoothness of the model. As shown in the inverted 2D resistivity models presented in this paper, an electrical conductor beside the hypocenter of the Chi-Chi earthquake indicates that deep-crustal fluids may participate in the rupture process of the Chi-Chi earthquake. A striking spatial correlation between the crustal conductor and occurrence of aftershocks beneath the Chelungpu fault suggests a postseismic pore pressure adjustment ongoing after the mainshock. Additionally, the hypocenter exhibits an electrical resistive zone, consistent very well with a predicted compact zone from a crustal deformation and transient fluid flow modeling.     

准噶尔盆地及其周缘地区晚古生代火山机构分布与发育环境分析

本文收集了北疆地区已报道的145处晚古生代火山机构信息.其中,准噶尔盆地周缘32处,主要集中分布于博罗科努山、博格达山以及克拉美丽山,包括破火山口、火山穹窿、火山通道(火山颈)等多种火山机构类型;准噶尔盆地盆内火山机构85处,主要分布在西北缘克百断裂带和盆地腹部的四处凹陷(三南凹陷、东道海子凹陷、滴水泉凹陷和五彩湾凹陷)及七处凸起(白家海凸起、石西凸起、夏盐凸起、三个泉凸起、滴北凸起、滴南凸起和北三台凸起).盆内火山机构分布主要受海西期断裂系控制,大致沿着NE、近EW两个方向的断裂展布,并在断裂交汇部位最为发育.由于后期改造,晚古生代火山机构普遍遭受剥蚀,且发生强烈变形和风化、淋滤改造,野外识别主要依靠残留地貌特征、火山岩相变化趋势及引爆角砾岩等特征岩性;地震识别则主要依靠地震切片、属性分析及构造趋势面分析等手段.准噶尔盆地晚古生代火山岩年龄集中于340～320Ma,300～295 Ma,分别对应东、西准噶尔岛弧俯冲时期.自早石炭世至晚石炭世,准噶尔盆地及邻区火山活动具有自水下向水上、深水向浅水、陆缘向陆内转换的变化趋势.     

Magnetotelluric investigation of lithospheric electrical structure beneath the Dharwar Craton in south India:Evidence for mantle suture and plume-continental interaction

Broad-band and long period magnetotelluric measurements made at 63 locations along ～500 km long Chikmagalur-Kavali profile,that cut across the Dharwar craton(DC)and Eastern Ghat Mobile Belt(EGMB)in south India,is modelled to examine the lithosphere architecture of the cratonic domain and define tectonic boundaries.The 2-D resistivity model shows moderately conductive features that intersperse a highly resistive background of crystalline rocks and spatially connect to the exposed schist belts or granitic intrusions in the DC.These features are therefore interpreted as images of fossil pathways of the volcanic emplacements associated with the greenstone belt and granite suite formation exposed in the region.A near vertical conductive feature in the upper mantle under the Chitradurga Shear Zone represents the Archean suture between the western and eastern blocks of DC.Although thick(～200 km)cratonic(highly resistive)lithosphere is preserved,significant part of the cratonic lithosphere below the western DC is modified due to plume-continental lithosphere interactions during the Cretaceous—Tertiary period.A west-verging moderately conductive feature imaged beneath EGMB lithosphere is interpreted as the remnant of the Proterozoic collision process between the Indian land mass and East Antarctica.Thin(～120 km)lithosphere is seen below the EGMB,which form the exterior margin of the India shield subsequent to its separation from East Antarctica through rifting and opening of the Indian Ocean in the Cretaceous.     

太康隆起东段地热资源远景区调查评价与研究

应用大地电磁测深法(MT)对太康隆起东段深部地层结构及隐伏断裂进行探测研究,共布设了4条MT剖面,获得了97个测深点,并对所获得的MT数据的二维偏离度、构造走向等进行计算和分析,采用非线性共轭梯度(NLCG)二维反演方法对TE和TM模式的数据进行联合反演,得到了可靠的地下介质二维地电模型。研究结果表明,商丘凸起电性结构纵向上具有典型的分层特征,从上到下可分为三层,即低阻层-中高阻层-高阻层。根据该电性结构模型,结合研究区域重力、航磁、地震及区域地质资料,绘制了商丘凸起基岩地质图,同时根据深部基底隆起形成的高阻异常与区域地温场高值区具有较好的对应关系,据此圈定了2处地热异常远景区。