共查询到20条相似文献，搜索用时 15 毫秒
1.
Colin Brown 《Surveys in Geophysics》1994,15(2):123-157
This review paper selects key results from electromagnetic induction studies of a variety of distinctive tectonic phenomena in the top 200 km of the Earth. Its main theme is that electromagnetic data are essential for an understanding of tectonism involving partial melting, recycling of large volumes of fluids (CO2 and H2O) and underthrusting of metasedimentary rocks. The wide variety of tectonic regimes in which these processes are known to be important is reflected in the choice of case studies. A discussion of conductivity models for young oceanic lithosphere and asthenosphere is followed by results from induction studies across the S.E. Australian passive margin, the North American active margin, the Ryukyu Island-Arc and the Oregon Cascades continental arc. The importance of partial melting and free fluid movement i apparent in these regions. Terrain accretion and/or continent-continent collisions recorded at palaeosuture zones in Ireland, Germany and Scandinavia have left distinctive conductivity structures. These are often associated with grain-boundary graphite either in weakly-metamorphosed black shales in underthrust sedimentary basins or precipitated from CO2-rich fluids. They are discussed in the context of the evolution of mature continental crust. All of the case studies are based on experiments published since 1989 in which the electromagnetic results have been central to an integrated geophysical and geological interpretation. 相似文献
2.
3.
S. T. Balesta A. I. Farberov V. S. Smirnov A. A. Tarakanovsky M. I. Zubin 《Bulletin of Volcanology》1977,40(4):260-266
Crustal structure in a number of Kamchatka volcanic regions is deduced from geophysical data. Anomalous structure and physical properties of the crust are found beneath some volcanic groups. Beneath the Klyuchevskaya and Avachinskaya volcanic groups crustal layers have high elastic properties. There is a thick transition layer from the crust to the upper mantle which has lower clastic properties and electrical resistance. These data, supported by experimental investigations of elastic properties of xenoliths in volcanic rocks at high pressures and temperatures, enable the probable substance composition of the crustal layers to be defined. The feeding zones and magma chambers of individual volcanoes are deduced from anomalies in gravity, electrical conductivity and seismic wave propagation. 相似文献
4.
The continental crust is exposed in cross-section at numerous sites on the earth's surface. These exposures, which appear to have formed by obduction along great faults during continental collision, may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive Bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle. From an examination of five complexes which meet these criteria, it is concluded that the most prominent layering in the crust is not compositional but metamorphic. The lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks, respectively. Post-metamorphic granitic intrusions are common at intermediate to shallow levels. Position of discontinuities in refraction velocity, where present, commonly correspond to changes in composition or metamorphic grade with depth. The continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by COCORP. Field observations, coupled with geochemical data, indicate a complex evolution of the lower crust which can include anatexis, multiple deformation, polymetamorphism and reworking of older crustal material. The complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments. 相似文献
5.
Tectonic evolution of lower crustal rocks in an exposed magmatic arc section in the Hidaka metamorphic belt, Hokkaido, northern Japan 总被引：3，自引：0，他引：3
Abstract : The Hidaka metamorphic belt consists of an island-arc assembly of lower to upper crustal rocks formed during early to middle Paleogene time and exhumed during middle Paleogene to Miocene time. The tectonic evolution of the belt is divided into four stages, D0rs, D1, D2rs, and D3, based on their characteristic deformation, metamorphism, and igneous activity. The premetamorphic and igneous stage (D0) involves tectonic thickening of an uppermost Cretaceous and earliest Tertiary accretionary complex, including oceanic materials in the lower part of the complex. D1 is the stage of prograde metamorphism with increasing temperatures at a constant pressure during an early phase, and with a slight decrease of pressure at the peak metamorphic phase, accompanying flattening of metamorphic rocks and intrusions of mafic to intermediate igneous rocks. At the peak, incipient partial melting of pelitic and psammitic gneisses took place in the amphibolite–granulite facies transition zone, the melt and residuals cutting the foliations formed by flattening. In the deep crust, large amounts of S-type tonalite magma formed by crustal anatexis, intruded into the granulite facies gneiss zone and also into the upper levels of the metamorphic sequence during the subsequent stage. During D1 stage, mafic and intermediate magmas supplied and transported heat to form the arc-type crust and at the same time, the magmatic underplating caused extensional doming of the crust, giving rise to flattening and vertical uplifting of the crustal rocks. D2 stage is characterized by subhorizontal top-to-the-south displacement and thrusting of lower to upper crustal rocks, forming a basal detachment surface (décollement) and duplex structures associated with intrusions of S-type tonalite. Deformation structures and textures of high-temperature mylonites formed along the décollement, as well as the duplex structures, show that the D2 stage movement occurred under a N-S trending compressional tectonic regime. The depth of intra-crustal décollement in the Hidaka belt was defined by the effect of multiplication of two factors, the fraction of partial melt which increases downward, and the fluid flux which decreases downward. The crustal décollement, however, might have extended to the crust-mantle boundary and/or to the lithosphere and asthenosphere boundary. The subhorizontal movement was transitional to a dextral-reverse-slip (dextral transpression) movement accompanied by low-temperature mylonitization with retrograde metamorphism, the stage defined as D3. The crustal rocks from the basal décollement to the upper were tilted eastward on the N–S axis and exhumed during the D3 stage. During D2 and D3 stages, the intrusion of crustal acidic magmas enhanced the crustal deformation and exhumation in the compressional and subsequent transpressional tectonic regime. 相似文献
6.
Scott B. Smithson Jon A. Brewer S. Kaufman J.E. Oliver R.L. Zawislak 《Earth and Planetary Science Letters》1980,46(2):295-305
A COCORP deep crustal reflection profile across the Wind River uplift crosses exposed Archean rocks and resolves an unusual complex deep crustal structure at a depth of 24–31 km in an area where depth relations in Precambrian rocks can be inferred. The different levels of exposure across the beveled plunge of the Wind River uplift reveal supracrustal rocks at shallower levels with migmatites and pyroxene granulites at deeper levels. For the first time, deep crustal structure from reflection profiling may be interpreted in terms of exposed basement geology. A folded, multilayered deep structure shown by relfection data resembles multiply folded pyroxene granulite interlayered with granitic gneiss exposed in the central Wind River uplift; isoclinal folding is suggested in the folded layered seismic structure. Earlier seismic reflection studies suggested a simpler lower crust. These data indicate that lower crustal structure may have a complexity similar to deeply eroded Precambrian granulite-facies rocks. If this seismic feature represents folded metamorphic rocks, it seems unlikely that this Archean crust could have been thickened by underplating after 2.7 b.y. B.P. and the crust would have to be at least 30 km thich when this structure was formed. 相似文献
7.
A major electrical conductivity anomaly has been detected in Transdanubia (West Hungary) in the eastern part of the Drauzug-Bakony geological unit (DBU), a collision zone of the Alpine orogeny. Assuming the source of the conductivity anomaly to be a characteristic formation of the whole DBU, long period magnetotelluric (MT) measurements were made in two regions of the western part of the DBU, in the Gail valley and in the Karawanken. The general features of this formation were studied also in order to elucidate the cause of the Transdanubian conductivity anomaly. The conductivity anomaly detected by long period MT in the Western DBU lies much deeper (12–17 km) than in Transdanubia (5–9 km). As the anomaly in both regions is associated with a well-known tectonic zone, its origin was initially explained by ionic conduction, arising from the presence of pore fluids. The very low resistivity values, the great anisotropy, the effect of charges on the H-polarized MT curves, and the anomalous magnetic field variations also indicated an increase of electronic conduction, possibly caused by graphitic rocks or ores nearer to the surface.
In the Gail valley, audiomagnetotelluric measurements made in 1986 have led to the discovery of these standing blocks of graphitic shales lifted up gradually to the surface by tectonic forces from below the dolomites of the Carnic Alps. It is suggested that the same plate collision arranged the Paleozoic graphitic shales to narrow dikes in the Gail valley as well as in Transdanubia along the Balaton line and zones parallel to it. 相似文献
8.
大地电磁探测结果显示,青藏高原的中下地壳普遍存在大规模的高导层,其电阻率低于10 Ωm,远低于稳定构造区地壳的平均电阻率值.通过对可能形成地壳内大规模低阻异常地质原因的分析认为,青藏高原地壳中的高导层不可能是由金属矿、石墨层或者单纯的含盐水流体引起的,而很可能是由于地壳岩石的部分熔融或者是部分熔融与含盐水流体共同导致的.这些高导层的形成是与板块运动等动力学过程密切相关的.地壳内的高导层可能是印度板块和亚洲板块俯冲的电性痕迹,其成因与板块俯冲过程中由于摩擦生热导致的岩石部分熔融和脱水作用有关,也可能与岩石圈拆沉造成幔源物质上涌有关.沿高原内主要缝合带均存在东西向连续分布的大规模高导体,有可能是青藏高原下地壳物质向东"逃逸"的证据;其中班公-怒江缝合带可能是最重要的物质运移"通道". 相似文献
9.
G. Barberi A. Gervasi G. Neri B. Orecchio 《Physics of the Earth and Planetary Interiors》2004,147(4):297-314
27,646 P- and 15,025 S-wave readings obtained from 2238 earthquakes and 84 artificial sources were used to perform tomographic inversion of P velocity and VP/VS ratio in the crust of Calabrian Arc by Thurber’s inversion algorithm. For this investigation a seismic database with more than twelve-thousand events was built, including all local earthquake data recorded between 1978 and 2001 at all stations of the national and local networks in south Italy. Spread Function computations and checkerboard and restore tests proved higher accuracy of velocity estimates in the upper 40 km beneath Calabrian Arc compared to previous investigations in the same area. The obtained three-dimensional velocity model furnished remarkable improvement of hypocenter locations of the global earthquake dataset (RMS reduction of 38% respect to 1D locations) and greater accuracy in the definition of microplates and tectonic units in the study region. Velocity domains evidenced by our tomography correspond to tectonic units locally identified with geological methods by previous investigators and allow us to better detail their shape and geometry at depth. In particular, at a depth of about 20 km beneath Calabria we detected the deep contact between the overthrusting Tyrrhenian crust and the subducting Ionian slab, improving the accuracy of the current subduction model of the Calabrian Arc region. 相似文献
10.
M. PERIC 《Geophysical Prospecting》1981,29(2):274-287
The laterites in Burundi, which are formed by weathering of ultrabasic rocks, show a complete profile with the following horizons: canga, the ferruginous crust capping, ferralite, consisting essentially of iron hydroxides, and saprolite, which contains a large quantity of hydrosilicate minerals. Nickel bearing minerals occur in the saprolite and the lower portion of ferralite. Resistivity well-logging and resistivity sounding indicated that the electrical properties of rocks depend upon their composition: Canga and ferralite showed high resistivities of 6,500 Ωm and 800 Ωm, respectively. The resistivity of saprolite was found to be much lower, between 10 Ωm and 20 Ωm. The laterite is underlain by resistive peridotite. The chargeability of saprolite was found to be lower than that of the upper horizons and the bedrock. Electrolytic conductivity of laterite, which depends on the geometry of the deposit, was found to be low, because the laterite contains moisture and ground water, which are highly resistive. The relatively high conductivity of saprolite is caused by nickeliferous hydrosilicates, which exhibit the electrical properties of clay minerals, with an apparent maximum conductivity of 0.25 S/m. The conductivity of saprolite corresponds to a concentration between 30% and 50% of conductive silicate minerals distributed in the pore space of deposit. A nickel enrichment of up to 6% was estimated from the resistivity of the saprolite. Prospecting for laterites by electrical sounding showed that the development of laterite horizons in a nickel deposit correlates with the surface morphology of weathered ultrabasic massif. Thus the method can be used in preliminary exploration of such deposits. 相似文献
11.
Rodolfo A. Tamayo JR Graciano P. Yumul JR René C. Maury Hervé Bellon Joseph Cotten Mireille Polvé Thierry Juteau and Cliff Querubin 《Island Arc》2000,9(4):638-652
Abstract Amphibolites unconformably overlain by a metasedimentary sequence of quartz-muscovite-feldspar-kyanite schists, metagraywackes and epidote-bearing amphibolites occur in the northern portion of the south-western Zamboanga metamorphic basement complex, western Mindanao. These amphibolites (here identified as the Mount Dansalan amphibolites) display relict magmatic textures inherited from cumulate gabbro protoliths. Bulk-rock major and trace-element data are consistent with this hypothesis. Together with the chemistry of relict igneous clinopyroxenes, they indicate a magmatic arc-related signature for the gabbro protoliths. Geochemical data allow us to identify various sources for the associated metasediments: the gabbro themselves for the metagraywackes and a continental basement for the quartz-muscovite-feldspar-kyanite schists. Both sources contributed to the genesis of the epidote-amphibolite metasediments. The compositions of the metamorphic mineral assemblages suggest that the rocks have undergone metamorphism at temperatures ranging from 550°C to 700°C and pressures probably in the range of 5–9 kbar. 40 K–40 Ar isotopic study of amphibole separates from the Mount Dansalan samples document a metamorphic event dated at 24.6 ± 1.4, 22.2 ± 1.4 and 21.2 ± 1.2 Ma. Our results are in agreement with plate tectonic models which describe the south-western Zamboanga metamorphic basement as a continental terrane. However, its evolution was not as simple as it was usually considered. In particular the basement incorporated slivers of magmatic arc crust, which cannot be unambiguously related to any of the Tertiary arcs documented in the area. 相似文献
12.
This paper reports Re-Os and Nd isotopes of black shales at the bottom of Lower Cambrian from the northern Tarim Basin and traces source materials of the black shales through isotopes. The average Re/Os, 187Re/188Os, and 187Os/188Os ratios of the black shales at the bottom of Lower Cambrian from the Tarim Basin are 7.18, 5.6438, and 1.9616, respectively. These isotopic ratios suggest the crustal sources of the black shales. The εNd(0) value is -13.17, the εNd(540 Ma) value is -7.32 and the Nd model ages are 1.535 Ga. These parameters in the black shales are quite consistent with those from the basement rocks. Based on the Re-Os and Nd isotopic characteristics of the black shales, we conclude that the continental crust mainly composed of basement rocks is the source material of the black shales. Through comparison of these isotopic parameters with those from the Yangtze Platform, it is clear that the Re-Os isotopic characteristics in the black shales from the Tarim and Yangtze platforms are quite different, which maybe indicates the differences in depositional settings between two platforms. These Re-Os isotopic data provide us with constraints to analyze the genetic relation between the two platforms. 相似文献
13.
Are some of the deep crustal conductive features found in SW Iberia caused by graphite? 总被引：1，自引：0，他引：1
Fernando A. Monteiro Santos António Mateus Jaume Pous 《Earth and Planetary Science Letters》2002,201(2):353-367
Recent results obtained from two-dimensional inversion of magnetotelluric (MT) data acquired in SW Iberia reveal high-conductive features at the middle-lower levels of the crust. The top of these anomalous structures correlates very well with the depth (10-13 km) of an important seismic interface that has been interpreted as a regional detachment horizon. Very shallow and relatively narrow conductors in the Ossa Morena Zone appear to correspond to small-scale fluid-deposited graphite systems in the preorogenic metasedimentary sequences. Some of the midcrustal conductors can be ascribed to graphite-bearing thrust zones, the formation of graphite with variable crystallinity being a consequence of Variscan shearing processes. Deep-seated conductors are tentatively interpreted as a result of relatively continuous, highly crystalline grain-boundary graphite films presumably preserved in basement, granulite(?) rocks. Assuming that graphite occurs as interconnected films, calculations indicate that a fraction of 0.006-0.02% of this accessory mineral is enough to explain the range of the electrical resistivity estimated on the basis of MT models. The role of graphite on the thermal behaviour of the crust is also discussed. The results show that low contents of graphite do not significantly change the thermal behaviour of earth materials. 相似文献
14.
A continental crustal model and its geothermal implications 总被引：1，自引：0，他引：1
The following crustal model based on realistic estimates of metamorphic rock volumes and H2O content is proposed as a basis for geothermal calculations: (1) a surface zone of intermediate metamorphic rocks containing granitic intrusions and grading downward into (2) a more felsic migmatite zone, (3) a lower crustal zone of approximately andesitic composition crystallized in granulite or possibly amphibolite facies. Heat production values and thickness for the three zones are 3 HGU, 5 HGU, 0.5–1.5 HGU and 8, 8, and 18 km respectively. If the surface heat flow is 1.2 HFU, the model predicts a temperature of only 407°C at the Moho and an upper mantle heat flow of 0.3–0.5 HFU. The low temperatures resulting from this model rule out a seismic low-velocity zone in the crust produced by thermal effects. 相似文献
15.
H. Jödicke 《Surveys in Geophysics》1992,13(4-5):381-407
In the present discussion on the origin of highly conducting layers in the middle and lower crust, water (brines) and graphite are widely accepted as the two most promising explanations. In this review, petrophysical and petrological aspects are summarized aiming at an understanding of how the indispensable interconnectedness of these highly conducting phases within a normally insulating rock matrix may be established and maintained under conditions of the upper and the lower crust. Accordingly, interpretation of conductivity models will be conclusive in cases of conductance values reaching several thousand S for layers at deeper crustal level since they can not be explained other than by the existence of metaanthracitic or graphitic rocks. In cases of low to moderate conductance values as often observed, e.g., at the transition zone from the upper to the lower crust, interpretation may remain ambiguous and may need further multidisciplinary investigations. 相似文献
16.
Great strides have been made in understanding the upper part of the crust by in-situ logging in, and laboratory experiments on core recovered from super-deep bore-holes such as the KTB. These boreholes do not extend into the lower crust, and can contribute little to the elucidation of mechanisms that produce the high electrical conductivities that are commonly observed therein by magneto-telluric (MT) methods. Laboratory studies at simulated lower crustal conditions of temperature, pressure and saturation, on electrolyte saturated rocks thought to have been derived from the lower crust, have not been possible up until now due to their experimental difficulty. It is necessary to subject electrolyte-saturated rock samples to independently controlled confining and pore-fluid pressure, which implies that the rock be sleeved in some impermeable but deformable material, that can withstand the very high temperatures required. Metals are the only materials capable of being used, but these cause great difficulties for cell sealing and conductivity measurement. In this paper we describe recent breakthroughs in experimental work, specifically the development of two new types of sophisticated metal/ceramic seal, and a conductivity measurement technique that enables the measurement of saturated rock conductivity in the presence of a highly conducting metallic sleeve. The advances in experimental technique have enabled us to obtain data on the electrical conductivity of brine saturated basic, acidic and graphite-bearing rocks at lower crustal temperatures and raised pressures. These data have facilitated the comparison of MT derived crustal electrical conductivity profiles with profiles obtained from laboratory experiments for the first time. Initial modelling shows a good agreement between laboratory derived and MT derived profiles only if the mid-crust is composed of amphibolite pervaded by aqueous fluids, and the lower crust is composed of granulite that is saturated with aqueous fluids and/or contains interconnected grain surface films of graphite. The experimental data are consistent with a three layer crust consisting of an aqueous fluid saturated acidic uppermost layer, above an aqueous fluid saturated amphibolite mid-crust, and a granulite lowermost crust, which may or may not be saturated with aqueous fluids, but if not, requires the presence of an additional conduction mechanism such as conduction through thin graphite films. 相似文献
17.
通过在大别造山带东部横穿超高压变质带的一条NNE向剖面大地电磁测深资料的分析解释，获得了关于沿剖面的地壳上地幔二维电性结构，显示北淮阳与大别地块是电性差异显著的构造单元，它们之间的界面与晓天—磨子潭断裂对应；晓天—磨子潭断裂倾向北，在中上地壳层位出现错动解耦现象；从地表向深处可划分出4个主要电性层：地表风化层、中上地壳高阻层、壳内相对高导层以及上地幔层；大别地块内中、上地壳层位以高阻层为主，与高压-超高压变质岩分布区对应，高阻层最厚处在岳西—英山之间；在大别地块内，推测存在燕山期花岗质岩浆活动的通道，它们造成了超高压变质岩的进一步抬升，同时影响了大别地块内存在的壳内相对高导层的分布，壳内相对高导层在层位上相差较大. 相似文献
18.
E. B. Lebedev B. N. Ryzhenko H. Burkhardt A. V. Zharikov I. A. Roshchina N. N. Kononkova S. R. Zebrin 《Izvestiya Physics of the Solid Earth》2014,50(3):367-377
Recent geophysical studies have revealed zones with anomalously low seismic velocities and increased electric conductivity in the middle part of the Earth’s crust. It is suggested that these zones can probably be related with the variations in the porosity and permeability of the rocks and the presence of fluids, and that they can explain the existence of crustal waveguides. In order to verify this model, we carried out an experimental study. Among the studied metamorphic processes, the acid metasomatose most completely explains the silicification of rocks. The present work considers the physico-chemical causes of the redeposition of quartz from the solutions, the influence of the pH of the solutions on the mass transfer and deposition of quartz during silicification, and alteration of the physical properties of the rocks. Silicification leads to the increase in density, change in microtexture, and cementation of rocks. It also causes the change in seismic velocities in the Earth’s crust. 相似文献
19.
The occurrence of ultrahigh pressure (UHP) minerals, such as coesite and diamond in crustal rocks in orogenic belts suggests that a huge amount of continental crust can be subducted to man-tle depth during the continental-continental collision[1—6]. This… 相似文献
20.
Xiaozhi Yang 《Surveys in Geophysics》2011,32(6):875-903
Electromagnetic measurements have demonstrated that the lower continental crust has remarkable electrical anomalies of high
conductivity and electrical anisotropy on a global scale (probably with some local exceptions), but their origin is a long-standing
and controversial problem. Typical electrical properties of the lower continental crust include: (1) the electrical conductivity
is usually 10−4 to 10−1 S/m; (2) the overlying shallow crust and underlying upper mantle are in most cases less conductive; (3) the electrical conductivity
is statistically much higher in Phanerozoic than in Precambrian areas; (4) horizontal anisotropy has been resolved in many
areas; and (5) in some regions there appear to be correlations between high electrical conductivity and other physical properties
such as seismic reflections. The explanation based on conduction by interconnected, highly conductive phases such as fluids,
melts, or graphite films in grain boundary zones has various problems in accounting for geophysically resolved electrical
conductivity and other chemical and physical properties of the lower crust. The lower continental crust is dominated by mafic
granulites (in particular beneath stable regions), with nominally anhydrous clinopyroxene, orthopyroxene, and plagioclase
as the main assemblages, and the prevailing temperatures are mostly 700–1,000°C as estimated from xenolith data, surface heat
flow, and seismic imaging. Pyroxenes have significantly higher Fe content in the lower crust than in the upper mantle (peridotites),
and plagioclase has higher Na content in the lower crust than in the shallow crust (granites). Minerals in the lower continental
crust generally contain trace amounts of water as H-related point defects, from less than 100 to more than 1,000 ppm H2O (by weight), with concentrations usually higher than those in the upper mantle. Observations of xenolith granulites captured
by volcano-related eruptions indicate that the lower continental crust is characterized by alternating pyroxene-rich and plagioclase-rich
layers. Experimental studies on typical lower crustal minerals have shown that their electrical conductivity can be significantly
enhanced by the higher contents of Fe (for pyroxenes), Na (for plagioclase), and water (for all minerals) at thermodynamic
conditions corresponding to the lower continental crust, e.g., to levels comparable to those measured by geophysical field
surveys. Preferred orientation of hydrous plagioclase, e.g., due to ductile flow in the deep crust, and alternating mineral
fabrics of pyroxene-rich and plagioclase-rich layers can lead to substantial anisotropy of electrical conductivity. Electrical
conductivity properties in many regions of the lower continental crust, especially beneath stable areas, can mostly be accounted
for by solid-state conduction due to the major constituents; other special, additional conduction mechanisms due to grain
boundary phases are not strictly necessary. 相似文献