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1.
P and S velocity and attenuation estimates in the lower crust are obtained from a set of wide angle reflection–refraction profiles in the region of active tectonics at the NE edge of the Tibetan Plateau and discussed together with respect to similar data at its Himalaya–south Tibet edge.The quality factor is estimated in the lower half of the crust by accounting for the differential effect on amplitude–frequency observed between waves of different penetrations, and both in P and S modes. Attenuation values allow to exclude a significant proportion of partial melt and to estimate the homologous temperature, ratio of in situ to solidus absolute temperatures. The latter depend on the physical conditions being of dry, wet or dehydration melting, which are found different among the regions of the northern Bayan Har and northern Qang Tang boundaries between blocks, as well as the Tethyan–Himalayas, south of the Indus–Tsangpo suture. Their in situ temperatures differ also as estimated from their different Vp for a similar felsic composition.Joint measurement of several parameters, Vp, Vs, Qp and Qs reveals the composition, mineralogy, temperature and hydration conditions of the lower half of the thickened crust of Tibet that may be discussed in terms of evolution. The material presently in the thickened crust, even its lower part, has a felsic composition, upper to middle crustal lithology, and the temperature conditions estimated suggest that basic material that could have underlain it could be eclogitized and not appear anymore above the seismic Moho.Under northern Qang Tang, the felsic material in the lower half of the crust appears as hot and dry. Its burial may have occurred earlier or may have been moderate in the postcollisional phase. This is consistent with a model of indentation of the Qang Tang crust by an originally thinner Bayan Har crust to bring part of its crust to greater depth, suggested from imaging the crustal architecture. Under northern Bayan Har, the material in the lower half of the crust appears as felsic, at low temperature and not dry conditions. This is evidence that it has been transported from a shallower depth, and this recently enough not to be yet dehydrated and temperature equilibrated in a conductive geotherm. It supports a model of recent overriding of the middle crust of the north Kun Lun block to the north independently suggested from the image of crustal architecture. The Tethyan Himalayas case appears bracketed by these two cases in northern Tibet for Vp and temperature conditions, but shows highest attenuation in the lower crust that is colder but less dry than under northern Qang Tang.  相似文献   

2.
Various lines of evidence point to the lower crust as the source of the long-wavelength magnetic anomaly field measured by the POGO and Magsat satellites. Using seismically determined lower crust thicknesses and equivalent source inversion of the satellite anomaly data, magnetization for the lower crust for much of the United States has been calculated. The average magnetization for two hundred sixty-six 150 × 150 km areas is 3.5 A/m with a standard deviation of 1.1 A/m. These values are consistent with laboratory measurements of mafic-ultramafic rocks expected in the lower crust, and in agreement with previous estimates of lower crust magnetization based on long-wavelength aeromagnetic data. Average lower crust thickness for the same areas is 18.2 km (σ = 6.4 km). Thus, over large regions, it appears that variation in magnetization and variation in magnetic layer thickness contribute almost equally in causing the anomaly field variation at satellite altitude.  相似文献   

3.
Serpentinized Peridotite as Source of Aeromagnetic Anomalies   总被引：2，自引：0，他引：2  
The source of long-wavelength aeromagnetic anomalies appears to originate from the eartlrs deep crust. Constrained by previous studies on geochemical, petrologic analysis, the eclogite and serpentinized peridotite samples from drill hole ZK703 at Donghai in the western Sulu ultrahigh-pressure (UHP) terrane, East China, were unambiguously exhumed from the lower crust and the upper mantle, providing significant information about the magnetic properties of rocks at a deeper part of the crust. Results show that the serpentinization process favors the neoformation of nearly stoichiometric magnetite, resulting in the enhancement of its magnetization up to 8.6 A/m, which is sufficient enough to contribute to some magnetic anomalies. In contrast, eclogite samples have only weaker magnetization (generally less than 0.05 A/m) compared to serpentinized peridotite. Nevertheless, experiments under the lower crustal conditions are necessary to further support these conclusions.  相似文献   

4.
A synthesis of crustal thickness estimates was made recently utilizing available field, geochemical, seismicity, shear wave velocity and gravity data in the Philippines. The results show that a significant portion of the Philippine archipelago is generally characterized by crust with a thickness of around 25 to 30 kilometers. However, two zones, which are made up of a thicker crust (from 30 to 65 km) have also been delineated. The Luzon Central Cordillera region is characterized by thick crust. Another belt of thickened crust is observed in the Bicol-Negros-Panay-Central Mindanao region. This paper examines the interplay of tectonic and magmatic processes and their role in modifying Philippine arc crust. The processes, which could account for the observed crustal thicknesses, are presented. The contributions of magmatic arcs as compared to the contribution of the emplacement and accretion of ophiolite complexes to crustal thickness are also discussed.  相似文献   

5.
Rates of generation and growth of the continental crust   总被引：4，自引：1，他引：3  
Models for when and how the continental crust was formed are constrained by estimates in the rates o crustal growth. The record of events preserved in the continental crust is heterogeneous in time with distinctive peaks and troughs of ages for igneous crystallisation, metamorphism, continental margin and mineralisation. For the most part these are global signatures, and the peaks of ages tend to b associated with periods of increased reworking of pre-existing crust, reflected in the Hf isotope ratios o zircons and their elevated oxygen isotope ratios. Increased crustal reworking is attributed to periods o crustal thickening associated with compressional tectonics and the development of supercontinents Magma types similar to those from recent within-plate and subduction related settings appear to hav been generated in different areas at broadly similar times before ~3.0 Ga. It can be difficult to put th results of such detailed case studies into a more global context, but one approach is to consider when plate tectonics became the dominant mechanism involved in the generation of juvenile continental crust The development of crustal growth models for the continental crust are discussed, and a number o models based on different data sets indicate that 65%-70% of the present volume of the continental crus was generated by 3 Ga. Such estimates may represent minimum values, but since ~3 Ga there has been reduction in the rates of growth of the continental crust. This reduction is linked to an increase in th rates at which continental crust is recycled back into the mantle, and not to a reduction in the rates a which continental crust was generated. Plate tectonics results in both the generation of new crust and it destruction along destructive plate margins. Thus, the reduction in the rate of continental crustal growth at ~3 Ga is taken to reflect the period in which plate tectonics became the dominant mechanism b which new continental crust was generated.  相似文献   

6.
鄂尔多斯地块南缘地应力测量研究   总被引：2，自引：2，他引：0  
利用水压致裂法得到的地应力测试数据对鄂尔多斯地块南缘地壳浅部地应力分布规律及断层活动性进行研究。结果表明:（1）两个水平主应力随深度线性增长,应力梯度分别为0.032和0.021,在测量深度域内水平和垂直应力的关系为SH > Sh > SV,该应力状态有利于断层发生逆断层活动,与1556年华山大地震的发震正断层的性质不同;（2）研究区的最大水平主应力方向为北南-北北西向,与区域速度矢量场方向一致,与其他资料解译的区域构造应力方向有一些差异,主要是受鄂尔多斯地块周缘断层活动的影响;（3）利用Mohr-Coulomb准则及Byerlee定律,摩擦系数取0.6~1.0,对研究区的地应力状态进行分析,发现鄂尔多斯地块南缘的测点未达到或超过地壳破裂极限状态,不存在断层失稳或地震等其他形式的地壳活动,处于较稳定地壳应力状态;（4）实测数据为该区补充了新的地应力测量资料,研究结果为该区工程设计及建设、构造应力场数值模拟提供了边界条件,对于该区地质灾害评价、地壳稳定性以及大陆动力学的研究具有重要意义。   相似文献   

7.
Geologic mapping in the Bolivian Andes and balanced cross-section construction permit the determination of bounds on the amount of crustal shortening that has occurred in the mountain belt. Assumptions are carefully selected in the cross-section interpretations so that a precise minimum is calculated, as well as larger, more plausible estimates. The minimum bound on crustal shortening within the Cordillera Oriental and Subandean Zone is 210 km. Relaxation of specific assumptions yields estimates of 325 and 670 km; independent estimates cluster in the range of 300 to 350 km. The estimates are used to evaluate the contribution of crustal shortening to the present crustal thickness in the central Andes, and, by inference, that of magmatic addition. The minimum estimate of crustal shortening accounts for at least two-thirds of the present cross-sectional area, whether the entire crust across the width of the mountain belt is considered, or just the sedimentary wedge within the Cordillera Oriental and Subandean Zone. Magmatic addition is volumetrically less important in thickening the crust. Consideration of the deformation in the Altiplano indicates that crustal shortening has been an important process there as well. The balance between magmatic and crustal shortening in creating the thickened crust also may be affected by other processes. Tectonic erosion may augment the thickening, suggesting that magmatic addition would make an even smaller volumetric contribution. Strike-slip faulting or delamination (for shortening estimates greater than 300 km) may thin the crust laterally or vertically, respectively; these processes either allow greater amounts of magmatic addition or accommodate larger amounts of shortening. The shortening that has occurred across the mountain belt has been driven neither by magmatic intrusion nor by continental collision; it has occurred in response to subduction of only oceanic lithosphere.  相似文献   

8.
A 3D interpretation of the newly compiled Bouguer anomaly in the area of the “Dead Sea Rift” is presented. A high-resolution 3D model constrained with the seismic results reveals the crustal thickness and density distribution beneath the Arava/Araba Valley (AV), the region between the Dead Sea and the Gulf of Aqaba/Elat. The Bouguer anomalies along the axial portion of the AV, as deduced from the modelling results, are mainly caused by deep-seated sedimentary basins (D > 10 km). An inferred zone of intrusion coincides with the maximum gravity anomaly on the eastern flank of the AV. The intrusion is displaced at different sectors along the NNW–SSE direction. The zone of maximum crustal thinning (depth 30 km) is attained in the western sector at the Mediterranean. The southeastern plateau, on the other hand, shows by far the largest crustal thickness of the region (38–42 km). Linked to the left lateral movement of approx. 105 km at the boundary between the African and Arabian plate, and constrained with recent seismic data, a small asymmetric topography of the Moho beneath the Dead Sea Transform (DST) was modelled. The thickness and density of the crust suggest that the AV is underlain by continental crust. The deep basins, the relatively large intrusion and the asymmetric topography of the Moho lead to the conclusion that a small-scale asthenospheric upwelling could be responsible for the thinning of the crust and subsequent creation of the Dead Sea basin during the left lateral movement. A clear segmentation along the strike of the DST was obtained by curvature analysis: the northern part in the neighbourhood of the Dead Sea is characterised by high curvature of the residual gravity field. Flexural rigidity calculations result in very low values of effective elastic lithospheric thickness (t e < 5 km). This points to decoupling of crust in the Dead Sea area. In the central, AV the curvature is less pronounced and t e increases to approximately 10 km. Curvature is high again in the southernmost part near the Aqaba region. Solutions of Euler deconvolution were visualised together with modelled density bodies and fit very well into the density model structures. An erratum to this article can be found at  相似文献   

9.
H. Laubscher 《地学学报》1990,2(6):645-652
Gravity surveys of the past century established that mountains have roots, seismic refraction lines shot in the second half of this century confirmed the downbulge of the Moho under the Alps, and recent reflection traverses provided new details on the behaviour of crustal layers in the deep part of the Alps. However, geophysical data are ambiguous geologically. For models of the root in terms of rock distribution to be tectonophysically acceptable, they must be the retrodeformable result of kinematic sequence that fits the geological surface data. For a cross-section through the Swiss Alps based on refraction data and somewhat modified by the recent reflection traverses, a kinematic model compatible with large-scale geological data may be obtained by the superposition of three Neogene phases with alternating vergence. Although Alpine collision is largely dextrally compressive in the central Alps, the N-S component may be discussed in a cross-section. Particularly puzzling geophysical features include a high-velocity body in the middle crust and the disappearance of the layered foreland crust in the root. In order to account for these phenomena, it is proposed that the crustal root is interpreted as the result of complex reshuffling of middle and lower crustal masses as well as large-scale phase transformations. The mid-crustal highvelocity body is interpreted as a delaminated section of the lower crust of the Adria plate that was wedged into the middle crust of the Alps in the middle Miocene. The disappearance of the foreland lower crust is attributed to eclogitization attendant on the subduction of continental crust. Material balance estimates suggest that during Alpine collision large volumes of continental crust have disappeared through subduction.  相似文献   

10.
A.P Singh  D.M Mall   《Tectonophysics》1998,290(3-4):285-297
In 1967 a major earthquake in the Koyna region attracted attention to the hitherto considered stable Indian shield. The region is covered by a thick pile of Deccan lava flows and characterized by several hidden tectonic features and complex geophysical signatures. Although deep seismic sounding studies have provided vital information regarding the crustal structure of the Koyna region, much remains unknown. The two available DSS profiles in the region have been combined along the trend of Bouguer gravity anomalies. Unified 2-D density modelling of the Koyna crust/mantle suggests a ca. 3 km thick and 40 km wide high velocity/high density anomalous layer at the base of the crust along the coastline. The thickness of this anomalous layer decreases gradually towards the east and ahead of the Koyna gravity low the layer ceases to be visible. Based on the seismic and gravity data interpretation in the geodynamical/rheological boundary conditions the anomalous layer is attributed to igneous crustal accretion at the base of the crust. It is suggested that the underplated layer is the imprint of the magmatism caused by the deep mantle plume when the northward migrating Indian plate passed over the Reunion hotspot.  相似文献   

11.
下地壳麻粒岩包体波速测定及其地质意义   总被引：5，自引：0，他引：5       下载免费PDF全文
下地壳岩石的波速测定可以提供有关下地壳物质组成及物理性质的重要信息。但是，由于下地壳难于直接采样，因此常常利用目前出露地表的古老麻粒岩地体样品来代表下地壳，但所获结果常常与实际情况不符，由新生代玄武岩携带到地表的下地壳包体则为我们提供有关下地壳的直接信息。本文对采自河北汉诺坝武岩中的下地壳包体进行了密度和高温高压波速测定，下地壳由不同类型的麻粒岩相岩石组成。其密度和波速有较大的差异，因此所获波速不能直接与地震测深得到的下地壳波速进行对比，而需要进行波速的温度、压力校正，并进行具体的分析，本文得到下地壳包体的原地波速6.13-6.90km/s与该区地震资料得到的结果十分接近，并探讨了下地壳低速层的可能成因。  相似文献   

12.
A change in the polarity of magnetization with depth in the 2.45 Ga Matachewan dyke swarm is used to document vertical crustal movements that occurred at 1.9–2.3 Ga along the Kapuskasing Structural Zone, a 500-km-long fault zone that transects the Archean Superior Province of Canada. At shallow crustal levels dykes have a primary magnetization dominantly of one polarity, but at greater depths (20 km down) a polarity change occurs associated with the growth of exsolved magnetite in feldspar due to slow crustal cooling after cessation of Matachewan igneous activity. Regions of the dyke swarm with one dominant polarity are separated from those with opposite polarity by major faults. Using this polarity distribution and associated variations in the intensity of feldspar clouding and hydrous alteration, maps of the southern Superior Province are produced that display regional crustal tilting on which are superimposed more local fault-bounded blocks associated with the Kapuskasing zone. Some of these blocks have been recognized for the first time as a result of this study.The paleomagnetic work has also shown that the Matachewan swarm is regionally distorted both within and north of the Kapuskasing zone, and originally had a more radial disposition. This widespread distortion suggests that the lower crust was still relatively ductile at the time of deformation, perhaps due to high heat flow associated with the waning stages of the Matachewan mantle plume beneath.  相似文献   

13.
The deep crustal structure of eastern Dharwar craton has been investigated through τ-p extremal inversion of P-wave travel times from a network of seismographs recording quarry blasts. Travel times have been observed in the distance range 30–250 km in a laterally homogeneous lithospheric segment Main features of the inferred velocity-depth relationship include: (a) 29 km thick combined upper and middle crust velocity varying from 6 km/s to 7 km/s, with no observable velocity discontinuity in this depth range; (b) a lower crust (∼ 29–41 km) with velocity increasing from 7.0 to 7.3 km/s; (c) an average upper mantle velocity of 8.1 km/s; and (d) presence of a 12 km thick high velocity crustal layer (7.4 – 7.8 km/s) in the depth range 41–53 km, with a distinct velocity gradient marking a velocity increase of 0.4 km/s. The anomalous 53 km thick crust is viewed as a consequence of magmatic underplating at the base of the crust in the process of cratonization of the eastern Dharwar craton during late Archaean. The underplated material reflects here with the velocity of 7–3 to 7–8 km/s below the depth of 40 km. Our proposition of magmatic underplating is also supported by the presence of large scale I-granitoid, a product of partial melting of the upper mantle material.  相似文献   

14.
Shallow and deep sources generate a gravity low in the central Iberian Peninsula. Long-wavelength shallow sources are two continental sedimentary basins, the Duero and the Tajo Basins, separated by a narrow mountainous chain called the Spanish Central System. To investigate the crustal density structure, a multitaper spectral analysis of gravity data was applied. To minimise biases due to misleading shallow and deep anomaly sources of similar wavelength, first an estimation of gravity anomaly due to Cenozoic sedimentary infill was made. Power spectral analysis indicates two crustal discontinuities at mean depths of 31.1 ± 3.6 and 11.6 ± 0.2 km, respectively. Comparisons with seismic data reveal that the shallow density discontinuity is related to the upper crust lower limit and the deeper source corresponds to the Moho discontinuity. A 3D-depth model for the Moho was obtained by inverse modelling of regional gravity anomalies in the Fourier domain. The Moho depth varies between a mean depth of 31 km and 34 km. Maximum depth is located in a NW–SE trough. Gravity modelling points to lateral density variations in the upper crust. The Central System structure is described as a crustal block uplifted by NE–SW reverse faults. The formation of the system involves displacement along an intracrustal detachment in the middle crust. This detachment would split into several high-angle reverse faults verging both NW and SE. The direction of transport is northwards, the detachment probably being rooted at the Moho.  相似文献   

15.
Studies of supercontinental cycle are mainly concentrated on the assembly, breakup and dispersal of supercontinents, and studies of continental crustal growth largely on the growth and loss (recycling) of the crust. These two problems have long been studied separately from each other. The Paleozoic–Mesozoic granites in the Central Asian Orogenic Belt have commonly positive Nd values, implying large-scale continental crustal growth in the Phanerozoic. They coincided temporally and spatially with the Phanerozoic Pangea supercontinental cycle, and overlapped in space with the P-wave high-V anomalies and calculated positions of subducted slabs for the last 180 Ma, all this suggests that the Phanerozoic Laurasia supercontinental assembly was accompanied by large-scale continental crustal growth in central Asia. Based on these observations, this paper proposes that there may be close and original correlations between a supercontinental cycle, continental crustal growth and catastrophic slab avalanches in the mantle. In this model we suggest that rapid continental crustal growth occurred during supercontinent assembly, whereas during supercontinental breakup and dispersal new additions of the crust were balanced by losses, resulting in a steady state system. Supercontinental cycle and continental crustal growth are both governed by changing patterns of mantle convection.  相似文献   

16.
Geochronological data, major and trace element abundances, Nd and Sr isotope ratios, δ18O whole rock values and Pb isotope ratios from leached feldspars are presented for garnet-bearing granites (locality at Oetmoed and outcrop 10 km north of Omaruru) from the Damara Belt (Namibia). For the granites from outcrop 10 km N′ Omaruru, reversely discordant U–Pb monazite data give 207Pb/235U ages of 511±2 Ma and 517±2 Ma, similar to previously published estimates for the time of regional high grade metamorphism in the Central Zone. Based on textural and compositional variations, garnets from these granites are inferred to be refractory residues from partial melting in the deep crust. Because P–T estimates from these xenocrystic garnets are significantly higher (800°C/9–10 kbar) than regional estimates (700°C/5 kbar), the monazite ages are interpreted to date the peak of regional metamorphism in the source of the granites. Sm–Nd garnet–whole rock ages are between 500 and 490 Ma indicating the age of extraction of the granites from their deep crustal sources. For the granites from Oetmoed, both Sm–Nd and Pb–Pb ages obtained on igneous garnets range from 500 to 490 Ma. These ages are interpreted as emplacement ages and are significantly younger than the previously proposed age of 520 Ma for these granites based on Rb/Sr whole rock age determinations. Major and trace element compositions indicate that the granites are moderately to strongly peraluminous S-type granites. High initial 87Sr/86Sr ratios (>0.716), high δ18O values of >13.8‰, negative initial Nd values between −4 and −7 and evolved Pb isotope ratios indicate formation of the granites by anatexis of mid-crustal rocks similar to the exposed metapelites into which they intruded. The large range of Pb isotope ratios and the lack of correlation between Pb isotope ratios and Nd and Sr isotope ratios indicate heterogeneity of the involved crustal rocks. Evidence for the involvement of isotopically highly evolved lower crust is scarce and the influence of a depleted mantle component is unlikely. The crustal heating events that produced these granites might have been caused by crustal thickening and thrusting of crustal sheets enriched in heat-producing elements. Very limited fluxing of volatiles from underthrust low- to medium-grade metasedimentary rocks may have also been a factor in promoting partial melting. Furthermore, delamination of the lithospheric mantle and uprise of hot mantle could have caused localized high-T regions. The presence of coeval A-type granites at Oetmoed that have been derived at least in part from a mantle source supports this model.  相似文献   

17.
青藏高原东部的隆升机制一直都是地学界的研究热点,研究学者们提出和发展了多种岩石圈变形模型,而存在多种模型的主要原因之一是对青藏高原东部地壳及岩石圈结构认识不足。本文主要针对SinoProbe-02项目横跨龙门山断裂带、全长400多公里的宽角、折射地震数据及重力数据进行联合反演和综合解释。研究结果表明,龙门山及邻近地区地壳结构可明确划分为上地壳、中地壳和下地壳。上地壳上层为沉积层,龙门山断裂带以西大部分区域被三叠纪复理岩覆盖,而在龙日坝断裂与岷江断裂之间出现了密度为2.7g/cm3的高速异常体;向东靠近龙门山地区,沉积层厚度逐渐减薄。中地壳速度变化不均一,而且变形强烈;若尔盖盆地和龙门山断裂带下方出现明显低速带;中地壳在龙门山西侧厚度加厚,在岷江断裂下方和四川盆地靠近龙门山断裂带地区附近厚度达到最大。莫霍面整体深度从东往西增厚,最厚可达56 km。本次研究得到的地壳结构和密度分布分析结果表明现有的地壳厚度和物质组成不足以支撑龙门山及邻近地区目前所达到的隆升高度,因此四川盆地刚性基底西缘因挤压作用产生的弯曲应力也是该地区抬升的重要条件之一。  相似文献   

18.
The evolution of an active continental margin is simulated in two dimensions, using a finite difference thermomechanical code with half-staggered grid and marker-in-cell technique. The effect of mechanical properties, changing as a function of P and T, assigned to different crustal layers and mantle materials in the simple starting structure is discussed for a set of numerical models. For each model, representative P–T paths are displayed for selected markers. Both the intensity of subduction erosion and the size of the frontal accretionary wedge are strongly dependent on the rheology chosen for the overriding continental crust. Tectonically eroded upper and lower continental crust is carried down to form a broad orogenic wedge, intermingling with detached oceanic crust and sediments from the subducted plate and hydrated mantle material from the overriding plate. A small portion of the continental crust and trench sediments is carried further down into a narrow subduction channel, intermingling with oceanic crust and hydrated mantle material, and to some extent extruded to the rear of the orogenic wedge underplating the overriding continental crust. The exhumation rates for (ultra)high pressure rocks can exceed subduction and burial rates by a factor of 1.5–3, when forced return flow in the hanging wall portion of the self-organizing subduction channel is focused. The simulations suggest that a minimum rate of subduction is required for the formation of a subduction channel, because buoyancy forces may outweigh drag forces for slow subduction. For a weak upper continental crust, simulated by a high pore pressure coefficient in the brittle regime, the orogenic wedge and megascale melange reach a mid- to upper-crustal position within 10–20 Myr (after 400–600 km of subduction). For a strong upper crust, a continental lid persists over the entire time span covered by the simulation. The structural pattern is similar in all cases, with four zones from trench toward arc: (a) an accretionary complex of low-grade metamorphic sedimentary material; (b) a wedge of mainly continental crust, with medium-grade HP metamorphic overprint, wound up and stretched in a marble cake fashion to appear as nappes with alternating upper and lower crustal provenance, and minor oceanic or hydrated mantle interleaved material; (c) a megascale melange composed of high-pressure and ultrahigh-pressure metamorphic oceanic and continental crust, and hydrated mantle, all extruded from the subduction channel; (d) zone represents the upward tilted frontal part of the remaining upper plate lid in the case of a weak upper crust. The shape of the P–T paths and the time scales correspond to those typically recorded in orogenic belts. Comparison of the numerical results with the European Alps reveals some similarities in their gross structural and metamorphic pattern exposed after collision. A similar structure may be developed at depth beneath the forearc of the Andes, where the importance of subduction erosion is well documented, and where a strong upper crust forms a stable lid.  相似文献   

19.
The southern segment of the seismic profile EUROBRIDGE—EUROBRIDGE-97 (EB'97)—located in Belarus and Ukraine, crosses the suture zone between two main segments of the East European Craton—Fennoscandia and Sarmatia—as well as Sarmatia itself. At the initial stage of our study, a 3-D density model has been constructed for the crust of the study region, including the major part of the Osnitsa–Mikashevichi Igneous Belt (OMIB) superimposed by sediments of the Pripyat Trough (PT), and three domains in the Ukrainian Shield—the Volhyn Domain (VD) with the anorthosite–rapakivi Korosten Pluton (KP), the Podolian Domain (PD), and the Ros–Tikich Domain (RTD). The model comprises three layers—sediments with maximum thickness (6 km) in the PT and two heterogeneous layers in the crystalline crust separated at a depth of 15 km. 3-D calculations show the main features of the observed gravity field are caused by density heterogeneities in the upper crust. Allocation of density domains deeper than 15 km is influenced by Moho topography. Fitting the densities here reveals an increase (up to 2960 kg m−3) in the modelled bodies accompanied by a Moho deepening to 50 km. In contrast, a Moho uplift to a level of 35–37 km below the KP and major part of the PT is associated with domains of reduced densities. An important role for the deep Odessa–Gomel tectonic zone, dividing the crust into two regions one of basically Archean consolidation in the west (PD and RTD) and one of Proterozoic crust in the east (Kirovograd Domain)—was confirmed.2-D density modelling on the EB'97 profile shows that in the upper crust three main domains of different Precambrian evolution—the OMIB (with the superimposed PT), the VD with the KP, and the PD—can be distinguished. Deeper, in the middle and lower crust, layered structures having no connection to the surface geology are dominant features of the models. Least thickness of the crust was obtained below the KP. Greatest crustal thickness (more than 50 km) was found below the PD, characterised also by maximum deviation of velocity/density relation in the rocks from a standard one. The velocity and density models along the EB'97 profile have been interpreted together with inferred Vp/Vs ratios to estimate crustal composition in terms of SiO2 content. In the course of the modelling, the status of the PD as a centre of Archean granulitic consolidation has been confirmed. The crustal structure of the anorthosite–rapakivi KP is complex. For the first time, a complicated structure for the lower crust and lower crust–upper mantle transition zone beneath the KP has been determined. The peculiarities of the crustal structure of the KP are quite well explained in terms of formation of rapakivi–anorthosite massifs as originating from melt chambers in the upper mantle and lower crust. An important role for the South Pripyat Fault (SPF), repeatedly activated during Proterozoic–Palaeozoic times, has been ascertained. At the subplatform stage of crustal evolution the SPF was, probably, a magma channel facilitating the granitic intrusions of the KP. In the Palaeozoic the fault was reactivated during rifting in the PT.  相似文献   

20.
There is an ongoing debate about the tectonic evolution of southeast Australia, particularly about the causes and nature of its accretion to a much older Precambrian core to the west. Seismic imaging of the crust can provide useful clues to address this issue. Seismic tomography imaging is a powerful tool often employed to map elastic properties of the Earth's lithosphere, but in most cases does not constrain well the depth of discontinuities such as the Mohorovi?i? (Moho). In this study, an alternative imaging technique known as receiver function (RF) has been employed for seismic stations near Canberra in the Lachlan Orogen to investigate: (i) the shear-wave-velocity profile in the crust and uppermost mantle, (ii) variations in the Moho depth beneath the Lachlan Orogen, and (iii) the nature of the transition between the crust and mantle. A number of styles of RF analyses were conducted: H-K stacking to obtain the best compressional–shear velocity (V P /V S) ratio and crustal thickness; nonlinear inversion for the shear-wave-velocity structure and inversion of the observed variations in RFs with back-azimuth to investigate potential dipping of the crustal layers and anisotropy. The thick crust (up to 48 km) and the mostly intermediate nature of the crust?mantle transition in the Lachlan Orogen could be due to the presence of underplating at the base of the crust, and possibly to the existing thick piles of Ordovician mafic rocks present in the mid and lower crust. Results from numerical modelling of RFs at three seismic stations (CAN, CNB and YNG) suggest that the observed variations with back-azimuth could be related to a complex structure beneath these stations with the likelihood of both a dipping Moho and crustal anisotropy. Our analysis reveals crustal thickening to the west beneath CAN station which could be due to slab convergence. The crustal thickening may also be related to the broad Macquarie volcanic arc, which is rooted to the Moho. The crustal anisotropy may arise from a strong N–S structural trend in the eastern Lachlan Orogen and to the preferred crystallographic orientation of seismically anisotropic minerals in the lower and middle crust related to the paleo-Pacific plate convergence.  相似文献   

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