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1.
The Central India Tectonic Zone(CITZ) marks the trace of a major suture zone along which the south Indian and the north Indian continental blocks were assembled through subduction-accretioncollision tectonics in the Mesoproterozoic.The CITZ also witnessed the major,plume-related,late Cretaceous Deccan volcanic activity,covering substantial parts of the region with continental flood basalts and associated magmatic provinces.A number of major fault zones dissect the region,some of which are seismically active.Here we present results from gravity modeling along five regional profiles in the CITZ, and combine these results with magnetotelluric(MT) modeling results to explain the crustal architecture. The models show a resistive(more than 2000Ω·m) and a normal density(2.70 g/cm~3) upper crust suggesting\ dominant tonalite-trondhjemite-granodiorite(TTG) composition.There is a marked correlation between both high-density(2.95 g/cm~3) and low-density(2.65 g/cm~3) regions with high conductive zones (<80Ω·m) in the deep crust.We infer the presence of an interconnected grain boundary network of fluids or fluid-hosted structures,where the conductors are associated with gravity lows.Based on the conductive nature,we propose that the lower crustal rocks are fluid reservoirs,where the fluids occur as trapped phase within minerals,fluid-filled porosity,or as fluid-rich structural conduits.We envisage that substantial volume of fluids were transferred from mantle into the lower crust through the younger plume-related Deccan volcanism,as well as the reactivation,fracturing and expulsion of fluids transported to depth during the Mesoproterozoic subduction tectonics.Migration of the fluids into brittle fault zones such as the Narmada North Fault and the Narmada South Fault resulted in generating high pore pressures and weakening of the faults,as reflected in the seismicity.This inference is also supported by the presence of broad gravity lows near these faults,as well as the low velocity in the lower crust beneath regions of recent major earthquakes within the CITZ. 相似文献
2.
A 1000-km-long lithospheric transect running from the Variscan Iberian Massif (VIM) to the oceanic domain of the Northwest African margin is investigated. The main goal of the study is to image the lateral changes in crustal and lithospheric structure from a complete section of an old and stable orogenic belt—the Variscan Iberian Massif—to the adjacent Jurassic passive margin of SW Iberia, and across the transpressive and seismically active Africa–Eurasia plate boundary. The modelling approach incorporates available seismic data and integrates elevation, gravity, geoid and heat flow data under the assumptions of thermal steady state and local isostasy. The results show that the Variscan Iberian crust has a roughly constant thickness of 30 km, in opposition to previous works that propose a prominent thickening beneath the South Portuguese Zone (SPZ). The three layers forming the Variscan crust show noticeable thickness variations along the profile. The upper crust thins from central Iberia (about 20 km thick) to the Ossa Morena Zone (OMZ) and the NE region of the South Portuguese Zone where locally the thickness of the upper crust is <8 km. Conversely, there is a clear thickening of the middle crust (up to 17 km thick) under the Ossa Morena Zone, whereas the thickness of the lower crust remains quite constant (6 km). Under the margin, the thinning of the continental crust is quite gentle and occurs over distances of 200 km, resembling the crustal attitude observed further north along the West Iberian margins. In the oceanic domain, there is a 160-km-wide Ocean Transition Zone located between the thinned continental crust of the continental shelf and slope and the true oceanic crust of the Seine Abyssal Plain. The total lithospheric thickness varies from about 120 km at the ends of the model profile to less than 100 km below the Ossa Morena and the South Portuguese zones. An outstanding result is the mass deficit at deep lithospheric mantle levels required to fit the observed geoid, gravity and elevation over the Ossa Morena and South Portuguese zones. Such mass deficit can be interpreted either as a lithospheric thinning of 20–25 km or as an anomalous density reduction of 25 kg m−3 affecting the lower lithospheric levels. Whereas the first hypothesis is consistent with a possible thermal anomaly related to recent geodynamics affecting the nearby Betic–Rif arc, the second is consistent with mantle depletion related to ancient magmatic episodes that occurred during the Hercynian orogeny. 相似文献
3.
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. 相似文献
4.
The composite airborne total intensity map of the Southern Granulite Terrain (SGT) at an average elevation of 7000' (≈ 2100 m) shows bands of bipolar regional magnetic anomalies parallel to the structural trends suggesting the distribution of mafic/ultramafic rocks that are controlled by regional structures/shear zones and thrusts in this region. The spectrum and the apparent susceptibility map computed from the observed airborne magnetic anomalies provide bands of high susceptibility zones in the upper crust associated with known shear zones/thrusts such as Transition Zone, Moyar-Bhavani and Palghat-Cauvery Shear Zones (MBSZ and PCSZ). The quantitative modelling of magnetic anomalies across Transition Zone, MBSZ and PCSZ suggest the presence of mafic rocks of susceptibility (1.5-4.0 × 10−3 CGS units) in upper crust from 8-10 km extending up to about 21-22 km, which may represent the level of Curie point geotherm as indicated by high upper mantle heat flow in this section.Two sets of paired gravity anomalies in SGT and their modelling with seismic constraints suggest gravity highs and lows to be caused by high density mafic rocks along Transition Zone and Cauvery Shear Zone (CSZ) in the upper crust at depth of 6-8 km and crustal thickening of 45-46 km south of them, respectively. High susceptibility and high density rocks (2.8 g/cm3) along these shear zones supported by high velocity, high conductivity and tectonic settings suggest lower crustal mafic/ultramafic granulite rocks thrusted along them. These signatures with lower crustal rocks of metamorphic ages of 2.6-2.5 Ga north of PCSZ and Neoproterozoic period (0.6-0.5 Ga) south of it suggest that the SGT represents mosaic of accreted crust due to compression and thrusting. These observations along with N-verging thrusts and dipping reflectors from Dharwar Craton to SGT suggest two stages of N-S directed compression: (i) between Dharwar Craton and northern block of SGT during 2.6-2.5 Ga with Transition Zone and Moyar Shear towards the west as thrust, and (ii) between northern and southern blocks of SGT with CSZ as collision zone and PCSZ as thrust during Neoproterozoic period (0.6-0.5 Ga). The latter event may even represent just a compressive phase without any collision related to Pan-African event. The proposed sutures in both these cases separate gravity highs and lows of paired gravity anomalies towards north and south, respectively. The magnetic anomalies and causative sources related to Moyar Shear, MBSZ and PCSZ join with those due to Transition Zone, Mettur and Gangavalli Shears in their eastern parts, respectively to form an arcuate-shaped diffused collision zone during 2.6-2.5 Ga.Most of the Proterozoic collision zones are highlands/plateaus but the CSZ also known as the Palghat Gap represents a low lying strip of 80-100 km width, which however, appears to be related to recent tectonic activities as indicated by high upper mantle heat flow and thin crust in this section. It is supported by low density, low velocity and high conductive layer under CSZ and seismic activity in this region as observed in case of passive rift valleys. They may be caused by asthenospheric upwarping along pre-existing faults/thrusts (MBSZ and PCSZ) due to plate tectonic forces after the collision of Indian and Eurasian plates since Miocene time. 相似文献
5.
Relation between electrical resistivity and earthquake generation in the crust of West Anatolia, Turkey 总被引：1，自引：0，他引：1
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. 相似文献
6.
This article reviews the electrical conductivity structures of the oceanic upper mantle, subduction zones, and the mantle transition zone beneath the northwestern Pacific, the Japanese Islands, and continental East Asia, which have particularly large potential of water circulation in the global upper mantle. The oceanic upper mantle consists of an electrically resistive lid and a conductive layer underlying the lid. The depth of the top of the conductive layer is related to lithospheric cooling in the older mantle, whereas it is attributable to the difference in water distribution beneath the vicinity of the seafloor spreading-axis. The location of a lower crustal conductor in a subduction zone changes according to the subduction type. The difference can be explained by the characteristic dehydration from the subducting slab in each subduction zone and by advection from the backarc spreading. The latest one-dimensional electrical conductivity model of the mantle transition zone beneath the Pacific Ocean predicts values of 0.1–1.0 S/m. These values support a considerably dry oceanic mantle transition zone. However, one-dimensional electrical profiles may not be representative of the mantle transition zone there, since there exists a three-dimensional structure caused by the stagnant slab. Three-dimensional electromagnetic modeling should be made in future studies. 相似文献
7.
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 总被引：10，自引：0，他引：10
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。 相似文献
8.
The nature of synmetamorphic fluids and their flow is examined in the granulitic lower crust of Madagascar, part of a Precambrian crustal-scale network of vertical ductile shear zones. Based on three independent data sets - field and satellite mapping, C-, O- and H-isotope geochemistry and gravimetry - this crust is divided into three zones: outside of shear zones, minor shear zones (<140 km long and 7 km wide), and major shear zones (>350 km long and 20–35 km wide). The major shear zones are rooted in and are controlled by the mantle. They tapped mantle-derived CO2 with carbon fluxes of the same order of magnitude as oceanic ridge degassing. One major shear zone shows abundant phlogopite-diopside-apatite-calcite mineralizations (a well known paragenesis in mantle metasomatism) due to mantle-fluid infiltration and their interaction with the crust. Carbonatitic magmas possibly collected in the major shear zones at the base of the crust and may be the source for CO2 upwellings as well as other metasomatic agents. Small-scale minor shear zones were controlled by crustal deformation processes and focused crustally-derived H2O-rich fluids. Pervasive fluid circulation was restricted to the vicinity (< 100 m) of synmetamorphic plutons. Fluid absent conditions dominate everywhere else. Mantle-CO2 flushing is not required for granulite genesis but is a consequence of the high associated heat flux. Fluid transfer at the mantle/crust interface is controlled by the tectonic setting and the associated geothermal gradient. The C- and O- isotope systematics of metamorphosed carbonates sampled on a regional scale within a known petrological and structural framework are shown to be of great help to identify the distribution of major fluid-rock interaction processes associated with plate tectonics. 相似文献
9.
With the super-wide band magnetotelluric sounding data of the Jilong (吉隆)-Cuoqin (措勤) profile (named line 800) which was completed in 2001 and the Dingri (定日)-Cuomai (措迈) profile (named line 900) which was completed in 2004,we obtained the strike direction of each MT station by strike analysis,then traced profiles that were perpendicular to the main strike direction,and finally obtained the resistivity model of each profile by nonlinear conjugate gradients (NLCG) inversion. With these two models,we described the resistivity structure features of the crust and the upper mantle of the center-southern Tibetan plateau and its relationship with Yalung Tsangpo suture: the upper crust of the research area is a resistive layer with resistivity value range of 200-3 000 ?·m. The depth of its bottom surface is about 15-20 km generally,but the bottom surface of resistive layer is deeper in the middle of these two profiles. At line 900,it is about 30 km deep,and even at line 800,it is about 38 km deep. There is a gradient belt of resistivity at the depth of 15-45 km,and a conductive layer is beneath it with resistivity even less than 5 ?·m. This conductive layer is composed of individual conductive bodies,and at the south of the Yalung Tsangpo suture,the conductive bodies are smaller with thickness about 10 km and lean to the north slightly. However,at the north of the Yalung Tsangpo suture,the conductive bodies are larger with thickness about 30 km and also lean to the north slightly. Relatively,the conductive bodies of line 900 are thinner than those of line 800,and the depth of the bottom surface of line 900 is also shallower. At last,after analyzing the effect factors to the resistivity of rocks,it was concluded that the very conductive layer was caused by partial melt or connective water in rocks. It suggests that the middle and lower crust of the center-southern Tibetan plateau is very thick,hot,flabby,and waxy. 相似文献
10.
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. 相似文献
11.
K. E. Robertson G. S. Heinson D. H. Taylor S. Thiel 《Australian Journal of Earth Sciences》2017,64(3):385-399
The magnetotelluric (MT) method was used to image the crust and upper mantle beneath the Delamerian and Lachlan orogens in western Victoria, Australia. During the Cambrian time period, this region changed from being the extended passive margin of Proterozoic Australia into an Andean-style convergent margin that progressively began to accrete younger oceanic terranes. Several broadband MT transects, which were collected in stages along coincident deep (full crust imaging) seismic reflection lines, have now been combined to create a continuous 500 km east–west transect over the Delamerian–Lachlan transition region in the Stawell Zone. We present the electrical resistivity structure of the lithosphere using both 3D and 2D inversion methods. Additionally, 1D inversions of long-period AusLAMP (Australian Lithospheric Architecture Magnetotelluric Project) MT data on a 55 km regionally spaced grid were used to provide starting constraints for the 3D inversion of the 2D profile. The Delamerian to Lachlan Orogen transition region coincides with the Mortlake Discontinuity, which marks an isotopic discontinuity in Cenozoic basalts, with higher strontium isotope enrichment ratios in the Lachlan Orogen relative to the Delamerian Orogen. Phase tensor ellipses of the MT data reveal a distinct change in electrical resistivity structure near the location of the Mortlake Discontinuity, and results of 3D and 2D inversions along the MT profile image a more conductive lower crust and upper mantle beneath the Lachlan Orogen than the Delamerian Orogen. Increased conductivity is commonly ascribed to mantle enrichment and thus supports the notion that the isotope enrichment of the Cenozoic basalts at least partially reflects an enriched mantle source rather than crustal contamination. Fault slivers of the lower crust from the more conductive Lachlan region expose Cambrian boninites and island arc andesites indicative of subduction, a process that can enrich the mantle isotopically, and also electrically, by introducing carbon (graphite) and water (hydrogen). 相似文献
12.
《Gondwana Research》2010,17(3-4):545-562
This article reviews the electrical conductivity structures of the oceanic upper mantle, subduction zones, and the mantle transition zone beneath the northwestern Pacific, the Japanese Islands, and continental East Asia, which have particularly large potential of water circulation in the global upper mantle. The oceanic upper mantle consists of an electrically resistive lid and a conductive layer underlying the lid. The depth of the top of the conductive layer is related to lithospheric cooling in the older mantle, whereas it is attributable to the difference in water distribution beneath the vicinity of the seafloor spreading-axis. The location of a lower crustal conductor in a subduction zone changes according to the subduction type. The difference can be explained by the characteristic dehydration from the subducting slab in each subduction zone and by advection from the backarc spreading. The latest one-dimensional electrical conductivity model of the mantle transition zone beneath the Pacific Ocean predicts values of 0.1–1.0 S/m. These values support a considerably dry oceanic mantle transition zone. However, one-dimensional electrical profiles may not be representative of the mantle transition zone there, since there exists a three-dimensional structure caused by the stagnant slab. Three-dimensional electromagnetic modeling should be made in future studies. 相似文献
13.
Crustal memory and basin evolution in the Central European Basin System—new insights from a 3D structural model 总被引：1，自引：2，他引：1
The Central European Basin System (CEBS) is composed of a series of subbasins, the largest of which are (1) the Norwegian–Danish Basin (2), the North German Basin extending westward into the southern North Sea and (3) the Polish Basin. A 3D structural model of the CEBS is presented, which integrates the thickness of the crust below the Permian and five layers representing the Permian–Cenozoic sediments. Structural interpretations derived from the 3D model and from backstripping are discussed with respect to published seismic data. The analysis of structural relationships across the CEBS suggests that basin evolution was controlled to a large degree by the presence of major zones of crustal weakness. The NW–SE-striking Tornquist Zone, the Ringkøbing-Fyn High (RFH) and the Elbe Fault System (EFS) provided the borders for the large Permo–Mesozoic basins, which developed along axes parallel to these fault systems. The Tornquist Zone, as the most prominent of these zones, limited the area affected by Permian–Cenozoic subsidence to the north. Movements along the Tornquist Zone, the margins of the Ringkøbing-Fyn High and the Elbe Fault System could have influenced basin initiation. Thermal destabilization of the crust between the major NW–SE-striking fault systems, however, was a second factor controlling the initiation and subsidence in the Permo–Mesozoic basins. In the Triassic, a change of the regional stress field caused the formation of large grabens (Central Graben, Horn Graben, Glückstadt Graben) perpendicular to the Tornquist Zone, the Ringkøbing-Fyn High and the Elbe Fault System. The resulting subsidence pattern can be explained by a superposition of declining thermal subsidence and regional extension. This led to a dissection of the Ringkøbing-Fyn High, resulting in offsets of the older NW–SE elements by the younger N–S elements. In the Late Cretaceous, the NW–SE elements were reactivated during compression, the direction of which was such that it did not favour inversion of N–S elements. A distinct change in subsidence controlling factors led to a shift of the main depocentre to the central North Sea in the Cenozoic. In this last phase, N–S-striking structures in the North Sea and NW–SE-striking structures in The Netherlands are reactivated as subsidence areas which are in line with the direction of present maximum compression. The Moho topography below the CEBS varies over a wide range. Below the N–S-trending Cenozoic depocentre in the North Sea, the crust is only 20 km thick compared to about 30 km below the largest part of the CEBS. The crust is up to 40 km thick below the Ringkøbing-Fyn High and up to 45 km along the Teisseyre–Tornquist Zone. Crustal thickness gradients are present across the Tornquist Zone and across the borders of the Ringkøbing-Fyn High but not across the Elbe Fault System. The N–S-striking structural elements are generally underlain by a thinner crust than the other parts of the CEBS.The main fault systems in the Permian to Cenozoic sediment fill of the CEBS are located above zones in the deeper crust across which a change in geophysical properties as P-wave velocities or gravimetric response is observed. This indicates that these structures served as templates in the crustal memory and that the prerift configuration of the continental crust is a major controlling factor for the subsequent basin evolution. 相似文献
14.
M. Malinowski A. ela
niewicz M. Grad A. Guterch T. Janik CELEBRATION Working Group 《Tectonophysics》2005,401(1-2):55-77
The CELEBRATION 2000 together with the earlier POLONAISE'97 deep seismic sounding experiments was aimed at the recognition of crustal structure in the border zone between the Precambrian East European Craton (Baltica) and Palaeozoic Europe. The CEL02 profile of the CELEBRATION family is a 400-km long SW–NE transect, running in Poland from the Upper Silesia Block (USB), across the Małopolska Block (MB) and the Trans-European Suture Zone (TESZ) to the East European Craton (EEC). The structure along CEL02 was interpreted using both 2D tomography and forward ray-tracing techniques as well as 2D gravity modelling.The crustal thickness along CEL02 varies from 32–35 km in the USB to 45–47 km beneath the TESZ and the EEC. The USB is a clearly distinctive crustal block with the characteristic high velocity lower crust (7.1–7.2 km/s), interpreted as a fragment of Gondwana. The Kraków–Lubliniec Fault is a terrane boundary produced by soft docking of the USB with the MB. The Małopolska crust fundamentally differs from the USB and has a strong connection with Baltica. It is a transitional, 150- to 200-km wide unit composed of the extended Baltican lower crust and the overlying low velocity (5.15–5.9 km/s) Neoproterozoic metasediments in the up to 18-km thick upper crust. The Łysogóry Unit has its crustal structure identical with that of Małopolska, thus it is connected with Baltica and cannot be interpreted as a Gondwana-derived terrane. Higher velocity and density bodies found below the Mazovia–Lublin Graben at a depth of 12 km and at the base of the lower crust, might be a result of mantle-derived mafic intrusions accompanying the extension of Baltica. By the preliminary 2D gravity modelling, we have reconfirmed the need for considering the increased TESZ mantle density in comparison to the EEC and USB mantle. 相似文献
15.
N. Vieira da Silva A. Mateus F.A. Monteiro Santos E.P. Almeida J. Pous 《Tectonophysics》2007,445(1-2):98
In SW Iberian Variscides, the boundary between the South Portuguese Zone (SPZ) and the Ossa Morena Zone (OMZ) corresponds to a major tectonic suture that includes the Beja Acebuches Ophiolite Complex (BAOC) and the Pulo do Lobo Antiform Terrane (PLAT). Three sub-parallel and approximately equidistant MT profiles were performed, covering a critical area of this Palaeozoic plate-tectonic boundary in Portugal; the profiles, running roughly along an NE–SW direction, are sub-perpendicular to the main Variscan tectonic features. Results of the three-dimensional (3-D) modelling of MT data allow to generate, for the first time, a 3-D electromagnetic imaging of the OMZ–SPZ boundary, which reveals different conductive and resistive domains that display morphological variations in depth and are intersected by two major sub-vertical corridors; these corridors coincide roughly with the NE–SW, Messejana strike–slip fault zone and with the WNW–ESE, Ferreira–Ficalho thrust fault zone. The distribution of the shallow resistive domains is consistent with the lithological and structural features observed and mapped, integrating the expected electrical features produced by igneous intrusions and metamorphic sequences of variable nature and age. The development in depth of these resistive domains suggests that: (1) a significant vertical displacement along an early tectonic structure, subsequently re-taken by the Messejana fault-zone in Late-Variscan times, has to be considered to explain differences in deepness of the base of the Precambrian–Cambrian metamorphic pile; (2) hidden, syn- to late-collision igneous bodies intrude the meta-sedimentary sequences of PLAT; (3) the roots of BAOC are inferred from 12 km depth onwards, forming a moderate resistive band located between two middle-crust conductive layers extended to the north (in OMZ) and to the south (in SPZ). These conductive layers overlap the Iberian Reflective Body (evidenced by the available seismic reflection data) and are interpreted as part of an important middle-crust décollement developed immediately above or coinciding with the top of a graphite-bearing granulitic basement. 相似文献
16.
Eastern Anatolia consisting of an amalgamation of fragments of oceanic and continental lithosphere is a current active intercontinental contractional zone that is still being squeezed and shortened between the Arabian and Eurasian plates. This collisional and contractional zone is being accompanied by the tectonic escape of most of the Anatolian plate to the west by major strike-slip faulting on the right-lateral North Anatolian Transform Fault Zone (NATFZ) and left-lateral East Anatolian Transform Fault Zone (EATFZ) which meet at Karlıova forming an east-pointing cusp. The present-day crust in the area between the easternmost part of the Anatolian plate and the Arabian Foreland gets thinner from north (ca 44 km) to south (ca 36 km) relative to its eastern (EAHP) and western sides (central Anatolian region). This thinner crustal area is characterized by shallow CPD (12–16 km), very low Pn velocities (< 7.8 km/s) and high Sn attenuation which indicate partially molten to eroded mantle lid or occurrence of asthenospheric mantle beneath the crust. Northernmost margin of the Arabian Foreland in the south of the Bitlis–Pötürge metamorphic gap area is represented by moderate CPD (16–18 km) relative to its eastern and western sides, and low Pn velocities (8 km/s). We infer from the geophysical data that the lithospheric mantle gets thinner towards the Bitlis–Pötürge metamorphic gap area in the northern margin of the Arabian Foreland which has been most probably caused by mechanical removal of the lithospheric mantle during mantle invasion to the north following the slab breakoff beneath the Bitlis–Pötürge Suture Zone. Mantle flow-driven rapid extrusion and counterclockwise rotation of the Anatolian plate gave rise to stretching and hence crustal thinning in the area between the easternmost part of the Anatolian plate and the Arabian Foreland which is currently dominated by wrench tectonics. 相似文献
17.
Late Quaternary activity and dextral strike-slip movement on the Karakax Fault Zone, northwest Tibet 总被引：1，自引：0，他引：1
Field observations and interpretations of satellite images reveal that the westernmost segment of the Altyn Tagh Fault (called Karakax Fault Zone) striking WNW located in the northwestern margin of the Tibetan Plateau has distinctive geomorphic and tectonic features indicative of right-lateral strike-slip fault in the Late Quaternary. South-flowing gullies and N–S-trending ridges are systematically deflected and offset by up to ~ 1250 m, and Late Pleistocene–Holocene alluvial fans and small gullies that incise south-sloping fans record dextral offset up to ~ 150 m along the fault zone. Fault scarps developed on alluvial fans vary in height from 1 to 24 m. Riedel composite fabrics of foliated cataclastic rocks including cataclasite and fault gouge developed in the shear zone indicate a principal right-lateral shear sense with a thrust component. Based on offset Late Quaternary alluvial fans, 14C ages and composite fabrics of cataclastic fault rocks, it is inferred that the average right-lateral strike-slip rate along the Karakax Fault Zone is ~ 9 mm/a in the Late Quaternary, with a vertical component of ~ 2 mm/a, and that a M 7.5 morphogenic earthquake occurred along this fault in 1902. We suggest that right-lateral slip in the Late Quaternary along the WNW-trending Karakax Fault Zone is caused by escape tectonics that accommodate north–south shortening of the western Tibetan Plateau due to ongoing northward penetration of the Indian plate into the Eurasian plate. 相似文献
18.
Seafloor magnetotelluric (MT) data were collected at seven sites across the Hawaiian hot spot swell, spread approximately evenly between 120 and 800 km southwest of the Hawaiian-Emperor island chain. All data are consistent with an electrical strike direction of 300°, aligned along the seamount chain, and are well fit using two-dimensional (2D) inversion. The major features of the 2D electrical model are a resistive lithosphere underlain by a conductive lower mantle, and a narrow, conductive, ‘plume’ connecting the surface of the islands to the lower mantle. This plume is required; without it the swell bathymetry produces a large divergence of the along-strike and across-strike components of the MT fields, which is not seen in the data. The plume radius appears to be less than 100 km, and its resistivity of around 10 Ωm, extending to a depth of 150 km, is consistent with a bulk melt fraction of 5–10%.A seismic low velocity region (LVR) observed by Laske et al. [Laske, G., Phipp Morgan, J., Orcutt, J.A., 1999. First results from the Hawaiian SWELL experiment, Geophys. Res. Lett. 26, 3397–3400] at depths centered around 60 km and extending 300 km from the islands is not reflected in our inverse model, which extends high lithospheric resistivities to the edge of the conductive plume. Forward modeling shows that resistivities in the seismic LVR can be lowered at most to 30 Ωm, suggesting a maximum of 1% connected melt and probably less. However, a model of hot subsolidus lithosphere of 102 Ωm (1450–1500 °C) within the seismic LVR increasing to an off-swell resistivity of >103 Ωm (<1300 °C) fits the MT data adequately and is also consistent with the 5% drop in seismic velocities within the LVR. This suggests a ‘hot, dry lithosphere’ model of thermal rejuvination, or possibly underplated lithosphere depleted in volatiles due to melt extraction, either of which is derived from a relatively narrow mantle plume source of about 100 km radius. A simple thermal buoyancy calculation shows that the temperature structure implied by the electrical and seismic measurements is in quantitative agreement with the swell bathymetry. 相似文献
19.
The Moho topography is strongly undulating in southern Scandinavia and northeastern Europe. A map of the depth to Moho shows similarities between the areas of the Teisseyre–Tornquist Zone (TTZ) in Poland and the Fennoscandian Border Zone (FBZ), which is partly coinciding with the Sorgenfrei–Tornquist Zone (STZ) in Denmark. The Moho is steeply dipping at these zones from a crustal thickness of approximately 32 km in the young Palaeozoic Platform and basin areas to approximately 45 km in the old Precambrian Platform and Baltic Shield. The Moho reflectivity (PMP waveform) in the POLONAISE'97 refraction/wide-angle seismic data from Poland and Lithuania is variable, ranging from ‘sharp’ to strongly reverberating signals of up to 2 s duration. There is little or no lower crustal wide-angle reflectivity in the thick Precambrian Platform, whereas lower crustal reflectivity in the thin Palaeozoic Platform is strongly reverberating, suggesting that the reflective lower crust and upper mantle is a young phenomena. From stochastic reflectivity modelling, we conclude that alternating high- and low-velocity layers with average thicknesses of 50–300 m and P-wave velocity variations of ±3–4% of the background velocity can explain the lower crustal reflectivity. Sedimentary layering affects the reflectivity of deeper layers significantly and must be considered in reflectivity studies, although the reverberations from the deeper crust cannot be explained by the sedimentary layering only. The reflective lower crust and upper mantle may correspond to a zone that has been intruded by mafic melts from the mantle during crustal extension and volcanism. 相似文献
20.
Analysis of multichannel seismic data from the continental margin off Svalbard between the Senja and Spitsbergen fracture zones suggests that the transition between continental and oceanic crust is located at or close to the Hornsund Fault Zone. In the Late Paleocene/Early Eoeene (57 m.y.) the region between Svalbard and Northeast-Greenland was subjected to regional shear movements associated with a transform system between the young Lofoten-Greenland Basin and the Arctic Ocean. Approximately 50 m.y. ago the spreading axis migrated to the northeast creating a deep basin north of the Greenland-Senja Fracture Zone forming the passive margin between Bear Island and 76.5°N. North of 76.5°N the regional transform was maintained. At the time of the main reorganization of relative plate motion (36 m.y.) the northern margin evolved. A continental fragment was possibly cut off from the Svalbard margin forming a small microcontinent. The microcontinent appears as the submarine ridge which has been associated with the Hovgaard Fracture Zone. It is suggested that the sediments west of the Hornsund Fault Zone are not older than Eocene in the south and mid-Oligocene in the north. The position of the spreading axis has greatly influenced the margin sedimentation. 相似文献