青藏高原西缘壳幔电性结构与断裂构造: 札达-泉水湖剖面大地电磁探测提供的依据

青藏高原西缘札达至泉水湖剖面大地电磁探测结果表明, 研究区被雅江缝合带、班公-怒江缝合带划分为3个构造区域, 由南至北分别为喜玛拉雅地体、冈底斯地体和羌塘地体.研究区内普遍存在中下地壳高导层, 高导层的顶面埋深起伏较大, 冈底斯内的高导层埋深大, 羌塘和喜玛拉雅地体内的高导层埋深较浅.在班公-怒江缝合带南侧高导层埋深最大, 班公-怒江缝合带南北两侧高导层埋深存在一个约20km的错动.冈底斯地体内的地壳高导层呈北倾形态, 南羌塘的地壳具有双高导层.沿剖面的上地壳存在多组规模不等、产状不同的电性梯度带或畸变带, 反映了沿剖面地区的缝合带与断裂带分布情况.根据电性结构特征, 推断了雅江缝合带、班公-怒江缝合带以及龙木措、噶尔藏布等主要断裂的构造特征与空间分布.      

Electrical Structure and Fault Features of Crust and Upper Mantle beneath the Western Margin of the Qinghai-Tibet Plateau： Evidence from the Magnetotelluric Survey along Zhada-Quanshui Lake Profile

The magnetotelluric (MT) survey along the Zhada (札达)-Quanshui (泉水) Lake profile on the western margin of the Qinghai (青海)-Tibet plateau shows that the study area is divided into three tectonic provinces by the Yalung Tsangpo and Bangong (班公)-Nujiang (怒江) sutures. From south to north these are the Himalayan terrane, Gangdise terrane, and Qiangtang (羌塘) terrane. For the study area, there are widespread high-conductivity layers in the mid and lower crust, the top layers of which fluctuate intensively. The high-conductivity layer within the Gangdise terrane is deeper than those within the Qiangtang terrane and the Himalaya terrane, and the deepest high-conductivity layer is to the south of the Bangong-Nujiang suture. The top surface of the high-conductivity layer in the south of the Bangong-Nujiang suture is about 20 km lower than that in the north of it. The high-conductivity layer within the Gangdise terrane dips toward north and there are two high-conductivity layers within the crust of the southern Qiangtang terrane. In the upper crust along the profile, there are groups of lateral electrical gradient zones or distortion zones of different scales and occurrence indicating the distribution of faults and sutures along the profile. According to the electrical structure, the structural characteristics and space distribution of the Yalung Tsangpo suture,Bangong-Nujiang suture, and the major faults of Longmucuo (龙木错) and Geerzangbu are inferred.     

Crustal Electrical Conductivity Structure of Southern Tibet from Magnetotelluric Survey

INTRODUCTIONAsatypicalintercontinent collisionarea ,Tibetanplateauisapresentlypopularregionnoticedbygeoscientistsallovertheworld ,sinceitisanidealandnaturallabforthestudyofthegeologicalevolutionoftheinnerlandmountain creatingzoneandoftheprocessofthecrust mantledeepdy namicsaswellasoftheinteractionbetweenthesurfaceandthecrustmovement.ThestudyoftheTibetanplateauevolutionisimportantnotonlyinsolvingsometheoreticalproblemsaboutcontinentaldynamicsandglobalchangesingeosciences ;butalsoinexplorin…     

青藏高原东南部地壳导电性结构与断裂构造特征——下察隅-昌都剖面大地电磁探测结果

根据2004年在青藏高原东南部完成的下察隅—昌都(1000线)宽频带大地电磁探测剖面数据研究高原东南部地壳导电性结构及断裂构造特征,这有助于推进印度与亚洲岩石圈碰撞、俯冲构造模式的研究。研究结果表明,沿剖面上地壳大范围分布的是规模不等的高阻体,电阻率大约在90～3000Ω.m,厚度由南向北增加,底界面的深度大约在5～30km变化。高阻层之下发现由不连续高导体构成的中地壳低阻层,其电阻率小于10Ω.m;其结构与青藏高原中、西部的壳幔高导体相似,但规模小得多,底面埋深也浅得多。沿剖面的上地壳存在多组规模不等、产状不同的横向电性梯度带或畸变带,它们反映了沿剖面地区地壳的断裂分布。通过与该区高精度重力资料对比,在重要的电性梯度带上,均存在布格重力低异常和负重力均衡异常。结合区域地质资料分析推断了嘉黎—然乌、班公—怒江和甲桑卡—赤布张错等主要断裂构造带的空间格局。     

Deep Crustal Electrical Signatures of Eastern Dharwar Craton, India

Wide band magnetotelluric (MT) investigations were carried out along a profile from Kavali in the east to Anantapur towards west across the Eastern Ghat Granulite Terrain (EGGT), Eastern Dhanvar Craton (EDC) and a Proterozoic Cuddapah Basin. This 300 km long profile was covered with 20 stations at an interval of 12–18 km. The MT data is subjected to robust processing, decomposition and static shift correction before deriving a 2-D model. The model shows a resistive crust (−10,000–30,000 ohm-m) to a depth of 8–10 km towards west of the Cuddapah basin. The mid crust is less resistive (about 500 ohm-m) and the lower crust with a slight increase in resistivity (about 1,500 ohm-m) in the depth range of 20–22 km. The resistivity picture to the east of the Cuddapah basin also showed a different deep crustal structure. The resistivity of upper crust is about 5,000 ohm-m and about 200 ohm-m for mid and lower crust. The sediment resistivity of Cuddapah basin is of the order of 15–20 ohm-m. MT model has shown good correlation with results from other geophysical studies like deep seismic sounding (DSS), gravity and magnetics. The results indicate that the lower crustal layers are of intermediate type showing hydrous composition in Eastern Dhanvar Craton.     

Magnetotelluric images of the crust and mantle in the southwestern Taurides, Turkey

Inversion of the magnetotelluric data across the southwestern Taurides reveals two subzones of crust with varying thicknesses: conductive lower crust (<75 Ω m), overlain by resistive (>350 Ω m) upper crust, with four resistive cores (>2000 Ω m) separated by three relatively conductive vertical zones. The first and second vertical zones coincide with surface faults interpreted in Anatolia, such as Fethiye Burdur Fault Zone. The third one is the most conductive and lies in continuity with the Strabo Fault Zone in the Mediterranean Sea. A hypocentral cross section of earthquakes along the profile shows more dense seismic activity in the second resistive core where the conductive crust is not present beneath it. The depth of the crust/upper mantle boundary varies between 30 and 50 km and has an undulating character. The resistivity of the upper mantle reaches 500–1000 Ω m.     

Crustal structure measurements near FRAM II in the pole abyssal plain

An extensive refraction profiling program was carried out during the FRAM II experiment (March–May, 1980) in the eastern Arctic Ocean. Two structural areas were covered: north of the ice camp (86°N, 24°W) into the basin of the Pole Abyssal Plain and south onto the flanks of the Morris Jesup Rise. Digital multichannel data on an 800 by 800 m, 24 channel hydrophone array and a single 2-component ocean bottom seismometer (OBS) were recorded for offsets from 2.5 to 100 km. Arrival times, amplitudes and phase velocities of the seismic signals recieved on the hydrophone array were determined using high resolution array processing. From these measurements and the OBS data, preliminary velocity structural models of the crust have been derived. For the purposes of this paper, 2 refraction lines have been analyzed, a 40 km line on a flat region of the Pole Abyssal Plain and an 86 km line on a slightly dipping region taken as the drifting ice camp shoaled on the Morris Jesup Rise. These preliminary analyses yield a sedimentary layer with a gradually increasing velocity 1.5–2 km thick. This cover overlays a crust with a thin layer 2 (< 1 km) and yields a total ocean bottom to mantle thickness of 4–7 km.     

Seismic mapping of crustal structures beneath the Indus-Yarlung Suture, Tibet

Records of densely spaced shots along the Sino-US reflection line INDEPTH II at offsets between 70 and 130 km parallel to the main profile provide an image of the crust straddling the Indus-Yarlung suture. The major features are prominent reflections at about 20 km depth beneath and extending out to about 20–30 km north and south of the surface exposure of the suture, and north-dipping reflectors north of the suture. Various interpretations for the reflections are possible. (i) They represent a decollement, possibly of the Gangdise thrust system. In this scenario, the surface expression of the Gangdise thrust as mapped in eastern south Tibet is a splay with the decollement continuing southwards and either ending as a blind thrust or ramping up as one of the thrusts within the northernmost Tethyan shelf sequence. (ii) The reflections represent fabrics within gneisses, partly obliterated by intrusions reaching various levels of the crust. The reflection bands may be interpreted in terms of deformation or sedimentary structures belonging to the Indian crust, the accretionary complex, and the basement of the Gangdise belt. The intrusions could be related to the Tethyan leucogranites south of the suture (Rinbung leucogranite), and to the Gangdise magmatic arc to the north of the suture. (iii) The reflections represent a fortuitous coincidence of different features north and south of the suture. South of the suture, the reflections may record the basement–cover interface of the Indian crust or a thrust system in the Tethyan shelf. North of the suture, they may comprise different levels within the Gangdise belt and its basement. Although it is not possible to discriminate between the suggested scenarios without additional information, the seismic mapping points to the importance of post-collisional (Oligocene–Miocene) tectonics, which reshaped the suture.     

Crustal evolution and tectonics of the Archean Bundelkhand craton, Central India

Magnetotelluric studies over the Bundelkhand craton indicates a high resistivity sub-structure, typically observed in the Archean-Proterozoic regions. The geoelectric section shows a single high resistivity layer in the northern part of the craton, extending from surface to a depth of about 60 km and a three layered resistivity structure overlying a conductive bottom in its southern part. The geological studies reported earlier have delineated an EW trending zone of ultramafic rocks, called the Bundelkhand tectonic zone (BTZ), which marks the divide between the two electrical resistivity patterns. The geoelectric structure is broadly indicative of a northward dipping tectonic fabric in this region which conforms to the Himalayan subduction, to the immediate north of this craton. However this observation cannot explain the findings from geochemical, isotope analysis and geological studies, suggesting possible vertical block movements in the region, which are also indicated in the Bouguer gravity studies. The geoelectric structure beneath the Vindhyan group to the south shows low resistivities even up to 60 km, suggesting that the Bundelkhand craton which is characterized by high resistivity rocks, does not extend to the south beneath the Vindhyans, as was believed by the earlier researchers. A low resistivity body with an extremely high conductance of about 100,000 Siemens is delineated at the mid crustal depths beneath the exposed Bijawars south of Bundelkhand craton. The causative factors behind this low resistivity are not immediately apparent, but some possibilities are discussed here.     

Crust structure and composition in the southern Siberian craton (influence zone of Baikal rifting), from magnetotelluric data

The magnetotelluric (MT) profile traverses the southeastern edge of the Siberian craton and the adjacent Paleozoic Olkhon collision zone, both being within the influence area of the Baikal rifting. The processed MT data have been integrated with data on the crust structure and composition, as well as with magnetic, gravity, and seismic patterns. Large resistivity lows are interpreted with reference to new geothermal models of rifted crust in the Baikal region. The northwestern and southeastern flanks of the profile corresponding, respectively, to the craton and the collision zone differ markedly in the crust structure and composition and in the intensity of rifting-related processes, the difference showing up in the resistivity pattern. The high-grade metamorphic and granitic crust of the craton basement in the northwestern profile flank is highly resistive but it includes a conductor (less than 50 ohm · m) below 16–20 km and a nearly vertical conductive layer in the upper crust. The crust in the southeastern part, within the collision zone, is lithologically heterogeneous and heavily faulted. High resistivities are measured mainly in the upper crust composed of collisional plutonic and metamorphic complexes. Large and deep resistivity lows over the greatest part of the section are due to Cenozoic activity and rift-related transcrustal faults that vent mantle fluids constantly recharged from deeper mantle.     

Magnetotelluric soundings and crustal architecture at Century mine,northern Australia

Magnetotelluric soundings were obtained along two traverse lines to the north and west of the Century mine in northwest Queensland. The survey was designed to cross the Termite Range Fault, a major structure on the Lawn Hill Platform, and to provide insights into the crustal-scale architecture that may have controlled the location of this world-class zinc deposit. The projected surface trace of the Termite Range Fault is coincident with a major change in resistivity character that extends to a significant depth. A relatively flat-lying, stacked series of resistive/conductive layers occurs on the northeastern side of the fault , while on the southwestern side the resistive/conductive layers are much less evident. The major contrast in resistivity is interpreted as due to a steep northeast-dipping Termite Range Fault that may extend to 20 km depth. To the southwest of the Termite Range Fault, a second major fault, the Riversleigh Lineament, is inferred from geology and gravity data, although there is no corresponding resistivity contrast seen across this fault in the magnetotelluric-derived model. This fault is interpreted as a buried structure, as distinct from the reactivated Termite Range Fault, and the two faults together may have created a wide damage zone (with an associated strike change) in the crust. A regional-scale 3D geological model of the Lawn Hill Platform provides a basis for correlating the resistive/conductive layers with major lithological units in the area. The stacked layers in the 2D resistivity inversion model of the Termite Range Fault hangingwall are reasonably well correlated with lithological units, particularly in the near-surface. A key point is that although similar geological units occur on either side of the Termite Range Fault, the contrasting electrical properties of these units are pronounced and their source is not well constrained; increased carbonaceous material in the Termite Range Fault hangingwall units is implied. In addition, there is a strong gradient in the Bouguer gravity field in the region of the Termite Range Fault and Riversleigh Lineament structures. This gradient provides supporting evidence for a northeast-facing fault structure in the basement and cover architecture. Newly acquired seismic data in the area has yet to be evaluated and compared with the magnetotelluric model. These results demonstrate an important role for magnetotelluric soundings in determining resistivity contrasts relating to the configuration of geological units and the architecture of deep-seated mineralising faults.     

VRANCEA99—the crustal structure beneath the southeastern Carpathians and the Moesian Platform from a seismic refraction profile in Romania

The VRANCEA99 seismic refraction experiment is part of an international and multidisciplinary project to study the intermediate depth earthquakes of the Eastern Carpathians in Romania. As part of the seismic experiment, a 300-km-long refraction profile was recorded between the cities of Bacau and Bucharest, traversing the Vrancea epicentral region in NNE–SSW direction.

The results deduced using forward and inverse ray trace modelling indicate a multi-layered crust. The sedimentary succession comprises two to four seismic layers of variable thickness and with velocities ranging from 2.0 to 5.8 km/s. The seismic basement coincides with a velocity step up to 5.9 km/s. Velocities in the upper crystalline crust are 5.9–6.2 km/s. An intra-crustal discontinuity at 18–31 km divides the crust into an upper and a lower layer. Velocities within the lower crust are 6.7–7.0 km/s. Strong wide-angle PmP reflections indicate the existence of a first-order Moho at a depth of 30 km near the southern end of the line and 41 km near the centre. Constraints on upper mantle seismic velocities (7.9 km/s) are provided by Pn arrival times from two shot points only. Within the upper mantle a low velocity zone is interpreted. Travel times of a PLP reflection define the bottom of this low velocity layer at a depth of 55 km. The velocity beneath this interface must be at least 8.5 km/s.

Geologic interpretation of the seismic data suggests that the Neogene tectonic convergence of the Eastern Carpathians resulted in thin-skinned shortening of the sedimentary cover and in thick-skinned shortening in the crystalline crust. On the autochthonous cover of the Moesian platform several blocks can be recognised which are characterised by different lithological compositions. This could indicate a pre-structuring of the platform at Mesozoic and/or Palaeozoic times with a probable active involvement of the Intramoesian and the Capidava–Ovidiu faults. Especially the Intramoesian fault is clearly recognisable on the refraction line. No clear indications of the important Trotus fault in the north of the profile could be found. In the central part of the seismic line a thinned lower crust and the low velocity zone in the uppermost mantle point to the possibility of crustal delamination and partial melting in the upper mantle.     

Structure of the lithosphere along the deep seismic sounding profile: Tien Shan—Pamirs—Karakorum—Himalayas

During 1973–1977, as part of the International Geodynamic Project, some seismic investigations of the Earth's crust have been carried out by geotraverses of the Tien Shan—Pamirs—Karakorum—Himalayas. The seismic data obtained together with other geophysical information, allow the construction and interpretation of the lithospheric section through the Pamirs-Himalayas structure. This section includes thick crust with complex layering, supra-asthenospheric and asthenospheric layers of the upper mantle. The thickness of the Earth's crust increases from 50–55 km in the north, in the Ferghana depression (Tien Shan), to 70–75 km in the south, near the Karakul Lake (Northern Pamir). It varies within 60–65 km for the Central and Southern Pamir, Karakorum and the Inner Himalayas. Its thickness is least (35–37 km) in the south, under the outer margin of the Himalayan foredeep. Extreme gravity minima and depressions on the geoid surface correspond to the regions with maximum thickness of the Earth's crust. The centers of the disturbing masses on the geoid surface are located in the vicinity of the asthenosphere's upper layer; this determines the effect of the whole lithospheric layer, including its asthenospheric layer, at intense changes of gravity anomalies. The asthenospheric upper layer is recorded at a depth of about 120 km, its base at a depth of 200 km, in the northern and southern regions, and 300 km in its central part (Southern Pamir, Karakorum). In the middle asthenospheric layer, wave velocities decrease to 7.5 km/sec, under the base they increase to 8.4 km/sec and reach 9.4 km/sec at a depth of about 400 km. In the supra-asthenospheric layer of the upper mantle, longitudinal and shear wave-velocities slightly increase (by less than 0.1 km/sec) towards its base.     

青藏高原东北缘岩石圈密度与磁化强度及动力学含义

利用横贯柴达木盆地南北的格尔木—花海子剖面岩石圈二维P波速度结构以及地震波速度与介质密度之间的关系,建立了该剖面岩石圈二维密度结构与二维磁化强度的初始模型。依据重磁同源原理,在柴达木盆地重、磁异常的二重约束下完成了重磁联合反演,获得了该剖面岩石圈二维密度结构与二维磁化强度分布。结果表明:柴达木盆地地壳厚度沿测线变化较大,平均厚度约60km。在柴达木盆地南缘地壳厚约50km,达布逊湖附近地壳最厚为63km左右,大柴旦附近地壳较薄,为50km左右。柴达木盆地的地壳纵向上可分为三层,即上地壳、中地壳与下地壳。位于盆地中部的中、下地壳分别发育大范围的壳内低密度体,并处于上地幔隆起的背景之上;横向上可将盆地分成南北两个部分,分界在达布逊湖附近。整个剖面结晶基底埋深变化也很大,在达布逊湖附近为12km,在昆仑山北缘基底几乎出露地表。结晶基底的展布形态与地壳底界,即莫霍面呈近似镜像对称。综合研究认为,柴达木盆地的岩石圈结构存在着明显的南北差异,其分界在达布逊湖的北面。在盆地南部,岩石圈介质横向变化较小,各层介质分布正常;在盆地的北侧,岩石圈结构特别在中、下地壳和上地幔顶部横向上发生了变化。壳内低密度体的存在意味着柴达木盆地具有较热的岩石圈和上地幔,加之基底界面与莫霍面的镜像对称分布,形成与准噶尔盆地和塔里木盆地的构造差异。多种地球物理参数所揭示的地壳上地幔结构及其横向变化特点为柴达木盆地构造演化及青藏高原北部边界的地球动力学研究提供了岩石圈尺度的地球物理证据。     

Electrical structure of the crust in South Eastern Tibet

As part of the Sino-French earth science programme in Tibet, magnetotelluric and geomagnetic soundings were carried out in the southwestern part of Tibet. Eight magnetotelluric sounding sites and nineteen geomagnetic stations were occupied along roughly north-south profiles. The magnetotelluric profile runs from Lhozag to the south of Yangbajain to the north. The geomagnetic profile runs from Lang Kartse to Gulu. These experiments indicate a conductive layer at about 25 km in depth. This layer corresponds to the isotherm 1100°C. Shallow conductive structures related to the presence of significant partial melting were evidenced to the south of the Zangbo suture. A conductive structure was also evidenced at about 15 km in depth below the Nyainquentanglha range. This structure may correspond to partial melting due to the presence of fluids at the dipping plane between two overthrusting crustal blocks.     

Magnetotelluric images of the crustal structure of Chyulu Hills volcanic field, Kenya

Electromagnetic experiments were conducted in 1995 as part of a multidisciplinary research project to investigate the deep structure of the Chyulu Hills volcanic chain on the eastern flank of the Kenya Rift in East Africa. Transient electromagnetic (TEM) and broadband (120–0.0001 Hz) magnetotelluric (MT) soundings were made at eight stations along a seismic survey line and the data were processed using standard techniques. The TEM data provided effective correction for static shifts in MT data. The MT data were inverted for the structure in the upper 20 km of the crust using a 2-D inversion scheme and a variety of starting models. The resulting 2-D models show interesting features but the wide spacing between the MT stations limited model resolution to a large extent. These models suggest that there are significant differences in the physical state of the crust between the northern and southern parts of the Chyulu Hills volcanic field. North of the Chyulu Hills, the resistivity structure consists of a 10–12-km-thick resistive (up to 4000 Ω m) upper crustal layer, ca. 10-km-thick mid-crustal layer of moderate resistivity (50 Ω m), and a conductive substratum. The resistive upper crustal unit is considerably thinner over the main ridge (where it is ca. 2 km thick) and further south (where it may be up to 5 km thick). Below this cover unit, steep zones of low resistivity (0.01–10 Ω m) occur underneath the main ridge and at its NW and SE margins (near survey positions 100 and 150–210 km on seismic line F of Novak et al. [Novak, O., Prodehl, C., Jacob, A.W.B., Okoth, W., 1997. Crustal structure of the southern flank of the Kenya Rift deduced from wide-angle P-wave data. In: Fuchs, K., Altherr, R., Muller, B., Prodehl, C. (Eds.), Structure and Dynamic Processes in the Lithosphere of the Afro-Arabian Rift System. Tectonophysics, vol. 278, 171–186]). These conductors appear to be best developed in upper crustal (1–8 km) and middle crustal (9–18 km) zones in the areas affected by volcanism. The low-resistivity anomalies are interpreted as possible magmatic features and may be related to the low-velocity zones recently detected at greater depth in the same geographic locations. The MT results, thus, provide a necessary upper crustal constraint on the anomalous zone in Chyulu Hills, and we suggest that MT is a logical compliment to seismics for the exploration of the deep crust in this volcanic-covered basement terrain. A detailed 3-D field study is recommended to gain a better understanding of the deep structure of the volcanic field.     

准噶尔盆地基底构造

我们已完成了穿越准噶尔盆地及其周边地区的I-I、II-II、III-III、IV-IV和额敏—哈密剖面5条综合地球物理剖面。通过综合研究,初步了解准噶尔盆地及邻近地区的地球动力学问题:准噶尔盆地基底由北部的乌伦古地体和南部的玛纳斯地体组成。两者的分界为西西北方向的滴水泉—三个泉缝合线。其西部与北东向Dalbutte缝合带相连,东部与北西向的Cranamary缝合带相连。准噶尔盆地北部的乌伦古地体基底为双层构造,上层为泥盆系和下石炭统组成的褶皱基底,大致表现为北厚(3~5 km)、南薄(1~2 km)。缝合线以南的玛纳斯地体为单层基底,即新元古代结晶基底。准噶尔盆地地壳厚度为44~52 km,北薄南厚。周边山区地壳厚度高于盆地地区。盆地及邻近地区地壳分为上、中、下层,并且中地壳一般较薄。盆地地区的地壳存在多条深断裂。南北方向发育了6条主要深断裂,分别为红车、德伦山、石溪、呼图壁、彩南和阜康。这些断层倾角较大,向上延伸至上地壳下部,向下切入地壳基底界面。壳内水平构造和构造面无明显垂向断层,似有“开放断层”特征。这些断层是上地幔物质挤入地壳的良好通道。此外,该地区还有两条主要的横向深断层。一是北西西走向的滴水泉—三个泉深断裂,它向南倾斜,具有逆断层性质,可能会破坏滴水泉—三个泉缝合带。另一条是近东西向的昌吉—玛纳斯深断裂,向南倾斜,主要发育在中下地壳,具有逆断层性质。这些深断裂对盆地构造发育具有一定的控制作用。准噶尔盆地西部的莫霍面基本连续地延伸到了天山的莫霍面,并且后者的莫霍面深度明显大于前者。但是,盆地东部的莫霍面与博格达山脉的莫霍面并不连续。前者以叠加关系延伸到后者之下,表明盆地东部的地壳向博格达山脉俯冲。这有助于解释天山东部构造活动相对减弱而博格达山脉向北推高的构造地貌现象。周边准噶尔盆地具有挤压盆地-山地构造耦合格局,尤其是南部边界东部博格达—准东盆地的山地-盆地构造耦合。现在将准噶尔盆地与吐哈盆地分开的博格达山脉是年轻的、仍在上升的山脉。博格达山的隆升是印支运动以来多次推覆造山运动的反映,其现貌是新近纪以来新构造运动的结果。准噶尔盆地盖层发育经历了3个阶段:与天山和松潘—甘孜造山带形成有关的二叠纪—三叠纪前陆盆地阶段,区域压缩较弱的侏罗纪—早始新世陆内坳陷阶段,以及新近纪晚期以来与天山抬升有关的活化前陆盆地阶段。     

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.     

华北中部岩石圈电性结构——应县—商河剖面大地电磁测深研究

20 0 1年, 沿着山西应县到山东商河, 重新布置大地电磁测深剖面进行研究.采用现代先进的大地电磁数据处理技术和快速松弛二维反演方法获得该剖面二维电性结构模型, 从而充分展示了华北地区岩石圈电性结构的特点.从电性特征上讲, 华北岩石圈以太行山前断裂为界划分为东、西两区, 东区为低阻区, 西区为高阻区.在东区, 上地壳电性结构基本与华北裂谷系的隆、坳构造格局相对应, 岩石圈的电导最高达3× 104 S, 远远大于强烈活动的安第斯山岩浆弧区和西藏高原岩石圈的电导.这里, 在构造连接部位的地壳中有不连续的高导体存在, 电导率大约0.1~ 0.8S/m.在西区, 太行山和恒山的岩石圈为高阻块体, 表现出稳定大陆区岩石圈导电性结构的特点.但恒山高阻块体之下发现一组向西缓倾的高导层, 其电导率为0.0 4~0.2 5S/m, 顶面在2 0km深处, 底面深度大约40km.      

Relation between electrical resistivity and earthquake generation in the crust of West Anatolia, Turkey

In this paper, we present a relation between the earthquake occurrence and electric resistivity structures in the crust, in West Anatolia and the Thrace region of Turkey. The relationship between magnetotelluric georesistivity models and crustal earthquakes in West Anatolia, during a period from 1900 to 2000, is investigated. It is found that most of the large crustal earthquakes occurred in and around the areas of the highest electrical resistivity in the upper crust, although rare small magnitude earthquakes are observed in some parts of the conductive lower crust in West Anatolian extensional terrain. The high-resistivity zones may represent rocks that are probably mechanically strong enough to permit sufficient stress to accumulate for earthquakes to occur in western Anatolia and the Thrace region. However, some recent studies state that the generation of a large earthquake is not only a pure mechanical process, but is closely related to fluid existence. We also reviewed recent world-wide researches including results from the Anatolian data for the first time and discussed all general findings in combination. Our findings show that the boundary between the resistive upper crust and the conductive lower crust correlates well with the cutout depth of the seismicity in West Anatolia and Thrace. This boundary is also attributed to the fluid bearing brittle–ductile transition zone in world literature. Fluid migration from the conductive lower crust to the resistive upper crust may contribute the seismicity in resistive zones. Alternatively, the upper–lower crust boundary may act as a stress concentrator and fluids may help to release strain energy in brittle parts of lower crust, by small magnitude earthquakes, whereas they may help in focusing strain in mechanically strong and electrically resistive zones for large earthquakes to occur.