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1.
The deep structure of the gabbro–anorthosite–rapakivi granite (“AMCG-type”) Korosten Pluton (KP) in the northwestern Ukrainian Shield was studied by 3-D modelling of the gravity and magnetic fields together with previous seismic data. The KP occupies an area of ca. 12,500 km2 and comprises several layered gabbro-anorthositic intrusions enveloped by large volumes of rapakivi-type granitoids. Between 1.80 and 1.74 Ga, the emplacement of mafic and associated granitoid melts took place in several pulses. The 3-D geophysical reconstruction included: (a) modelling of the density distribution in the crust using the observed Bouguer anomaly field constrained by seismic data on Moho depth, and (b) modelling of the magnetic anomaly field in order to outline rock domains of various magnetisation, size and shape in the upper and lower crust. The density modelling was referred to three depth levels of 0 to 5, 5 to 18, and 18 km to Moho, respectively. The 3-D reconstruction demonstrates close links between the subsurface geology of the KP and the structure of the lower crust. The existence of a non-magnetic body with anomalously high seismic velocity and density is documented. Most plausibly, it represents a gabbroic stock (a parent magma chamber) with a vertical extent of ca. 20 km, penetrating the entire lower crust. This stock has a half-cylindrical shape and a diameter of ca. 90 km. It appears to be connected with a crust–mantle transitional lens previously discovered by EUROBRIDGE seismic profiling. The position of the stock relative to the subsurface outlines of the KP is somewhat asymmetric. This may be due to a connection between the magmatism and sets of opposite-dipping faults initially developed during late Palaeoproterozoic collisional deformation in the Sarmatian crustal segment. Continuing movements and disturbances of the upper mantle and the lower crust during post-collisional tectonic events between 1.80 and 1.74 Ga may account for the long-lived, recurrent AMCG magmatism.  相似文献   

2.
We present new results on the structure resulting from Palaeoproterozoic terrane accretion and later formation of one of the aulacogens in the East European Platform. Seismic data has been acquired along the 530-km-long, N–S-striking EUROBRIDGE'97 traverse across Sarmatia, a major crustal segment of the East European Craton. The profile extends across the Ukrainian Shield from the Devonian Pripyat Trough, across the Palaeoproterozoic Volyn Block and the Korosten Pluton, into the Archaean Podolian Block. Seismic waves from chemical explosions at 18 shot points at approximately 30-km intervals were recorded in two deployments by 120 mobile three-component seismographs at 3–4 km nominal station spacing. The data has been interpreted by use of two-dimensional tomographic travel time inversion and ray trace modelling. The high data quality allows modelling of the P- and S-wave velocity structure along the profile. There are pronounced differences in seismic velocity structure of the crust and uppermost mantle between the three main tectonic provinces traversed by the profile: (i) the Pripyat Trough is a ca. 4-km-deep sedimentary basin, fully located in the Osnitsk–Mikashevichi Igneous Belt in the northern part of the profile. The velocity structure is typical for a Precambrian craton, but is underlain by a ca. 5-km-thick lowest crustal layer of high velocity. The development of the Pripyat Trough appears to have only affected the upper crust without noticeable thinning of the whole crust; this may be explained by a rheologically strong lithosphere at the time of formation of the trough. (ii) Very high seismic velocity and Vp/Vs ratio characterise the Volyn Block and Korosten Pluton to a depth of 15 km and probably also the lowest crust. The values are consistent with an intrusive body of mafic composition in the upper crust that formed from bimodal melts derived from the mantle and the lower crust. (iii) The Podolian Block is close to a typical cratonic velocity structure, although it is characterised by relatively low seismic velocity and Vp/Vs ratio. A pronounced SW-dipping mantle reflector from Moho to at least 70 km depth may represent the Proterozoic suture between Sarmatia and Volgo–Uralia, the structure from terrane accretion, or a later shear zone in the upper mantle. The sub-Moho P-wave seismic velocity is high everywhere along the profile, with the exception of the area above the dipping reflector. This velocity change further supports a plate tectonic origin of the dipping mantle reflector. The profile demonstrates that structure from Palaeoproterozoic plate tectonic processes are still identifiable in the lithosphere, even where younger metamorphic equilibration of the crust has taken place.  相似文献   

3.
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   

4.
The complete gravity data set from France and part of the neighboring countries has been analyzed to compute the topography of the Moho undulations. This work is based on an improved filtering technique and an appropriate assumed density contrast between the crust and the upper mantle. Comparison with deep seismic refraction data reveals that this relief map expresses the continuity and geometry of the Moho undulations better than the sparsely distributed seismic refraction data in France. This gravity Moho map, though may not give absolute depths at places, provides a far better correlation with surface geology than the result from other geophysical techniques. Four domains have been recognized: (a) the Alpine domain where all the Moho undulations are concentric with the Alps; (b) the Armorican domain in which all the undulations are north-west/south-east oriented; (c) the Pyrenean domain, in which the undulations are parallel with the Mountain chain; and (d) the Massif Central Domain which does not show clear structural orientation because of the influence of the strong heat flow located at the lower crust/upper mantle interface. Study of the topography and of the superficial structures associated with these undulations reveals that the undulations delineated in the Alpine Domain result from the Tertiary compression which shaped the Alps. The Armorican Domain was first created during the Lower to Middle Cretaceous opening of the Bay of Biscay. It is now slightly affected by the Tertiary to Quaternary closure of this Bay. The Pyrenean Domain was successively shaped by the Lower Cretaceous oblique opening of the Bay of Biscay and by the Upper Cretaceous to Eocene northward displacement of Spain. Comparison between the Moho undulations map and the stress map of France reveals that most of the undulations are perpendicular to the actual shortening directions. This observation suggests that the Mesozoic, Cenozoic and Quaternary stress directions were roughly the same. Massif Central is characterized by the convergence of these three sets of undulations. Its Post-Oligocene uplift was probably the result of the converging stresses recognized in the three surrounding domains. When the Moho undulations and the topography are compared, two types of periodic crustal instabilities can be recognized. One corresponds to the buckling of the crust developed under compression, the other to boudinage which was associated with extension. Both phenomena show a typical wavelength of 200–250 km which is in agreement with the results of the actual physical and numerical modeling currently available.  相似文献   

5.
Gravity signals from the lithosphere in the Central European Basin System   总被引：1，自引：0，他引：1  
We study the gravity signals from different depth levels in the lithosphere of the Central European Basin System (CEBS). The major elements of the CEBS are the Northern and Southern Permian Basins which include the Norwegian–Danish Basin (NDB), the North-German Basin (NGB) and the Polish Trough (PT). An up to 10 km thick sedimentary cover of Mesozoic–Cenozoic sediments, hides the gravity signal from below the basin and masks the heterogeneous structure of the consolidated crust, which is assumed to be composed of domains that were accreted during the Paleozoic amalgamation of Europe. We performed a three-dimensional (3D) gravity backstripping to investigate the structure of the lithosphere below the CEBS.Residual anomalies are derived by removing the effect of sediments down to the base of Permian from the observed field. In order to correct for the influence of large salt structures, lateral density variations are incorporated. These sediment-free anomalies are interpreted to reflect Moho relief and density heterogeneities in the crystalline crust and uppermost mantle. The gravity effect of the Moho relief compensates to a large extent the effect of the sediments in the CEBS and in the North Sea. Removal of the effects of large-scale crustal inhomogeneities shows a clear expression of the Variscan arc system at the southern part of the study area and the old crust of Baltica further north–east. The remaining residual anomalies (after stripping off the effects of sediments, Moho topography and large-scale crustal heterogeneities) reveal long wavelength anomalies, which are caused mainly by density variations in the upper mantle, though gravity influence from the lower crust cannot be ruled out. They indicate that the three main subbasins of the CEBS originated on different lithospheric domains. The PT originated on a thick, strong and dense lithosphere of the Baltica type. The NDB was formed on a weakened Baltica low-density lithosphere formed during the Sveco-Norwegian orogeny. The major part of the NGB is characterized by high-density lithosphere, which includes a high-velocity lower crust (relict of Baltica passive margin) overthrusted by the Avalonian terrane. The short wavelength pattern of the final residuals shows several north–west trending gravity highs between the Tornquist Zone and the Elbe Fault System. The NDB is separated by a gravity low at the Ringkøbing–Fyn high from a chain of positive anomalies in the NGB and the PT. In the NGB these anomalies correspond to the Prignitz (Rheinsberg anomaly), the Glueckstadt and Horn Graben, and they continue further west into the Central Graben, to join with the gravity high of the Central North Sea.  相似文献   

6.
The crustal structure of the central Eromanga Basin in the northern part of the Australian Tasman Geosyncline, revealed by coincident seismic reflection and refraction shooting, contrasts with some neighbouring regions of the continent. The depth to the crust-mantle boundary (Moho) of 36–41 km is much less than that under the North Australian Craton to the northwest (50–55 km) and the Lachlan Fold Belt to the southeast (43–51 km) but is similar to that under the Drummond and Bowen Basins to the east.The seismic velocity boundaries within the crust are sharp compared with the transitional nature of the boundaries under the North Australian and Lachlan provinces. In particular, there is a sharp velocity increase at mid-crustal depths (21–24 km) which has not been observed with such clarity elsewhere in Australia (the Conrad discontinuity?).In the lower crust, the many discontinuous sub-horizontal reflections are in marked contrast to lack of reflecting horizons in the upper crust, further emphasising the differences between the upper and lower crust. The crust-mantle boundary (Moho) is characterised by an increase in velocity from 7.1–7.7 km/s to a value of 8.15 + 0.04 km/s. The depth to the Moho under the Canaway Ridge, a prominent basement high, is shallower by about 5 km than the regional Moho depth; there is also no mid-crustal horizon under the Canaway Ridge but there is a very sharp velocity increase at the Moho depth of 34 km. The Ridge could be interpreted as a horst structure extending to at least Moho depths but it could also have a different intra-crustal structure from the surrounding area.The sub-crustal lithosphere has features which have been interpreted, from limited data, as being caused by a velocity gradient at 56–57 km depth with a low velocity zone above it.Because of the contrasting crustal thicknesses and velocity gradients, the lithosphere of the central Eromanga Basin cannot be considered as an extension of the exposed Lachlan Fold Belt or the North Australian Craton. The lack of seismic reflections from the upper crust indicates no coherent accoustic impedance pattern at wavelengths greater than 100 m, consistent with an upper crustal basement of tightly folded meta-sedimentary and meta-volcanic rocks. The crustal structure is consistent with a pericratonic or arc/back-arc basin being cratonised in an episode of convergent tectonics in the Early Palaeozoic. The seismic reflections from the lower crust indicate that it could have developed in a different tectonic environment.  相似文献   

7.
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   

8.
Seismic reflection and refraction data were collected west of New Zealand's South Island parallel to the Pacific–Australian Plate boundary. The obliquely convergent plate boundary is marked at the surface by the Alpine Fault, which juxtaposes continental crust of each plate. The data are used to study the crustal and uppermost mantle structure and provide a link between other seismic transects which cross the plate boundary. Arrival times of wide-angle reflected and refracted events from 13 recording stations are used to construct a 380-km long crustal velocity model. The model shows that, beneath a 2–4-km thick sedimentary veneer, the crust consists of two layers. The upper layer velocities increase from 5.4–5.9 km/s at the top of the layer to 6.3 km/s at the base of the layer. The base of the layer is mainly about 20 km deep but deepens to 25 km at its southern end. The lower layer velocities range from 6.3 to 7.1 km/s, and are commonly around 6.5 km/s at the top of the layer and 6.7 km/s at the base. Beneath the lower layer, the model has velocities of 8.2–8.5 km/s, typical of mantle material. The Mohorovicic discontinuity (Moho) therefore lies at the base of the second layer. It is at a depth of around 30 km but shallows over the south–central third of the profile to about 26 km, possibly associated with a southwest dipping detachment fault. The high, variable sub-Moho velocities of 8.2 km/s to 8.5 km/s are inferred to result from strong upper mantle anisotropy. Multichannel seismic reflection data cover about 220 km of the southern part of the modelled section. Beneath the well-layered Oligocene to recent sedimentary section, the crustal section is broadly divided into two zones, which correspond to the two layers of the velocity model. The upper layer (down to about 7–9 s two-way travel time) has few reflections. The lower layer (down to about 11 s two-way time) contains many strong, subparallel reflections. The base of this reflective zone is the Moho. Bi-vergent dipping reflective zones within this lower crustal layer are interpreted as interwedging structures common in areas of crustal shortening. These structures and the strong northeast dipping reflections beneath the Moho towards the north end of the (MCS) line are interpreted to be caused by Paleozoic north-dipping subduction and terrane collision at the margin of Gondwana. Deeper mantle reflections with variable dip are observed on the wide-angle gathers. Travel-time modelling of these events by ray-tracing through the established velocity model indicates depths of 50–110 km for these events. They show little coherence in dip and may be caused side-swipe from the adjacent crustal root under the Southern Alps or from the upper mantle density anomalies inferred from teleseismic data under the crustal root.  相似文献   

9.
David E. James  Fenglin Niu  Juliana Rokosky   《Lithos》2003,71(2-4):413-429
High-quality seismic data obtained from a dense broadband array near Kimberley, South Africa, exhibit crustal reverberations of remarkable clarity that provide well-resolved constraints on the structure of the lowermost crust and Moho. Receiver function analysis of Moho conversions and crustal multiples beneath the Kimberley array shows that the crust is 35 km thick with an average Poisson's ratio of 0.25. The density contrast across the Moho is 15%, indicating a crustal density about 2.86 gm/cc just above the Moho, appropriate for felsic to intermediate rock compositions. Analysis of waveform broadening of the crustal reverberation phases suggests that the Moho transition can be no more than 0.5 km thick and the total variation in crustal thickness over the 2400 km2 footprint of the array no more than 1 km. Waveform and travel time analysis of a large earthquake triggered by deep gold mining operations (the Welkom mine event) some 200 km away from the array yield an average crustal thickness of 35 km along the propagation path between the Kimberley array and the event. P- and S-wave velocities for the lowermost crust are modeled to be 6.75 and 3.90 km/s, respectively, with uppermost mantle velocities of 8.2 and 4.79 km/s, respectively. Seismograms from the Welkom event exhibit theoretically predicted but rarely observed crustal reverberation phases that involve reflection or conversion at the Moho. Correlation between observed and synthetic waveforms and phase amplitudes of the Moho reverberations suggests that the crust along the propagation path between source and receiver is highly uniform in both thickness and average seismic velocity and that the Moho transition zone is everywhere less than about 2 km thick. While the extremely flat Moho, sharp transition zone and low crustal densities beneath the region of study may date from the time of crustal formation, a more geologically plausible interpretation involves extensive crustal melting and ductile flow during the major craton-wide Ventersdorp tectonomagmatic event near the end of Archean time.  相似文献   

10.
A combined gravity map over the Indian Peninsular Shield (IPS) and adjoining oceans brings out well the inter-relationships between the older tectonic features of the continent and the adjoining younger oceanic features. The NW–SE, NE–SW and N–S Precambrian trends of the IPS are reflected in the structural trends of the Arabian Sea and the Bay of Bengal suggesting their probable reactivation. The Simple Bouguer anomaly map shows consistent increase in gravity value from the continent to the deep ocean basins, which is attributed to isostatic compensation due to variations in the crustal thickness. A crustal density model computed along a profile across this region suggests a thick crust of 35–40 km under the continent, which reduces to 22/20–24 km under the Bay of Bengal with thick sediments of 8–10 km underlain by crustal layers of density 2720 and 2900/2840 kg/m3. Large crustal thickness and trends of the gravity anomalies may suggest a transitional crust in the Bay of Bengal up to 150–200 km from the east coast. The crustal thickness under the Laxmi ridge and east of it in the Arabian Sea is 20 and 14 km, respectively, with 5–6 km thick Tertiary and Mesozoic sediments separated by a thin layer of Deccan Trap. Crustal layers of densities 2750 and 2950 kg/m3 underlie sediments. The crustal density model in this part of the Arabian Sea (east of Laxmi ridge) and the structural trends similar to the Indian Peninsular Shield suggest a continent–ocean transitional crust (COTC). The COTC may represent down dropped and submerged parts of the Indian crust evolved at the time of break-up along the west coast of India and passage of Reunion hotspot over India during late Cretaceous. The crustal model under this part also shows an underplated lower crust and a low density upper mantle, extending over the continent across the west coast of India, which appears to be related to the Deccan volcanism. The crustal thickness under the western Arabian Sea (west of the Laxmi ridge) reduces to 8–9 km with crustal layers of densities 2650 and 2870 kg/m3 representing an oceanic crust.  相似文献   

11.
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   

12.
Interpretation of a long-range seismic refraction line in Saudi Arabia has shown that beneath the Arabian Shield velocity generally increases with depth, from about 6 km s−1 at the surface to about 7 km s−1 at the top of the crust-mantle transition zone. The base of this transition zone (Moho) occurs at 37–44 km in depth. Intracrustal discontinuities can also be recognized, the most important being in the 10–20 km-depth range and separating the upper from the lower crust. Laterally, the variations in the intracrustal discontinuities and the total crustal thickness can be correlated with previously defined tectonic regions. Beneath the Red Sea shelf and coastal plain the crust, including 4 km of sediments, is only 15–17.5 km thick. With the aid of both seismic and gravity data an abrupt, steeply dipping transition from the crust of the Red Sea shelf and coastal plain to that of the Arabian Shield has been derived. With a jump of more than 20 km in Moho depth, this appears to be the major discontinuity between the Red Sea depression and the Arabian continental shield.  相似文献   

13.
The large-scale seismic refraction and wide-angle reflection experiment POLONAISE'97 together with LT-7 and TTZ profiles carried out with the most modern techniques gave a high resolution of crustal structure of the Trans-European Suture Zone (TESZ) in NW and central Poland. The results of seismic investigations show the presence of relatively low velocity rocks (Vp < 6.1 km/s) down to a depth of 20 km beneath the Polish Basin (PB), and a high velocity lower crust (Vp = 6.8–7.3 km/s). The crustal thickness in the TESZ is intermediate between that of the East European Craton (EEC) to the northeast (40–45 km) and that of the Variscan crust (VB) to the southwest ( 30 km). Velocities in the uppermost mantle are relatively high (Vp = 8.25–8.45 km/s). The crust is three-layered with substantial differences in the velocities and thickness of individual layers. The area of the TESZ in NW and central Poland can be divided into at least two crustal blocks (terranes), called here Pomeranian Unit (PU, in the northwest) and Kuiavian Unit (KU, in the southeast). The postulated boundary between KU and PU is rather sharp at particular levels of the crust. Velocity distribution in the middle and lower crystalline crust in the TESZ area resemble values recognized in the EEC area, the fundamental difference being the much smaller thickness of both these layers. Our hypothesis/speculation is that the attenuated lower and middle crust of the TESZ belong to proximal terranes built of the EEC crust detached in the southeast and re-accreted to the EEC due to the process of anti-clockwise rotation of the Baltica paleocontinent during the Ordovician–Early Silurian.  相似文献   

14.
Seismic velocities under confining pressures to 10 kbar have been measured for rocks of the Ivrea—Verbano and Strona—Ceneri Zones of northern Italy, a metamorphic complex thought to represent a cross-section of the continental crust and crust—mantle boundary. Laboratory-determined compressional wave velocities for schists and gneisses of the amphibolite facies found in the upper levels of the section (having an average density of 2.74 g/cm3) average 6.45 km/sec at pressures between 6 and 10 kbar. These increase with depth to values greater than 7.1 km/sec for amphibolites and rocks of the amphibolite—granulite facies transition and to 7.5 km/sec. (average density 3.06 g/cm3) in intermediate and mafic granulite facies rocks near the base of the section. Compressional wave velocities then abruptly increase to 8.5 km/sec in ultramafic complexes near the Insubric Line. Regional geophysical surveys show that Pg is 6.0 km/sec (density of 2.7 g/cm3), P* is 7.2–7.4 km/sec (density of 3.1 g/cm3) and Pn is 8.1 km/sec, values which are in agreement with the laboratory data when effects of temperature are taken into consideration. Estimated thicknesses of exposed rock units are in reasonable agreement with thicknesses determined for crustal layers in seismic refraction experiments. The agreement between the regional crustal structure and the laboratory-determined values of velocity and density provides strong evidence for the hypothesis that the rocks of this metamorphic complex represent a cross-section of the continental crust of the Po Basin.Using the Ivrea—Verbano and Strona—Ceneri sequence as a model of the continental crust, the crust of northern Italy is shown to consist of a thick series of metamorphic rocks with greenschist facies rocks occupying the uppermost levels. These grade downward into amphibolite facies gneisses and schists with occasional granitic intrusives. The Conrad discontinuity is marked by a change from silicic and intermediate amphibolite facies gneisses to intermediate and mafic granulite facies rocks in which hydrous minerals diminish in abundance and thus represents a distinct transition in terms of both composition and metamorphic grade. The lower crust is dominated by a heterogeneous series of mafic and metapelitic rocks in the granulite facies. Importantly, metasedimentary rocks of intermediate silica content found in the complex can have compressional wave velocities equivalent to velocities in mafic rocks suggesting that the lower continental crust everywhere is not necessarily mafic in composition. Ultramafic complexes near the Insubric Line may represent the upper mantle of the continent and their setting suggests that the continental crust-upper mantle boundary is sharp and is not isochemical.  相似文献   

15.
Crustal studies within the Japanese islands have provided important constraints on the physical properties and deformation styles of the island arc crust. The upper crust in the Japanese islands has a significant heterogeneity characterized by large velocity variation (5.5–6.1 km/s) and high seismic attenuation (Qp=100–400 for 5–15 Hz). The lateral velocity change sometimes occurs at major tectonic lines. In many cases of recent refraction/wide-angle reflection profiles, a “middle crust” with a velocity of 6.2–6.5 km/s is found in a depth range of 5–15 km. Most shallow microearthquakes are concentrated in the upper/middle crust. The velocity in the lower crust is estimated to be 6.6–7.0 km/s. The lower crust often involves a highly reflective zone with less seismicity, indicating its ductile rheology. The uppermost mantle is characterized by a low Pn velocity of 7.5–7.9 km/s. Several observations on PmP phase indicate that the Moho is not a sharp boundary with a distinct velocity contrast, but forms a transition zone from the upper mantle to the lower crust. Recent seismic reflection experiments revealed ongoing crustal deformations within the Japanese islands. A clear image of crustal delamination obtained for an arc–arc collision zone in central Hokkaido provides an important key for the evolution process from island arc to more felsic continental crust. In northern Honshu, a major fault system with listric geometry, which was formed by Miocene back arc spreading, was successfully mapped down to 12–15 km.  相似文献   

16.
New gravity data from the Adamawa Uplift region of Cameroon have been integrated with existing gravity data from central and western Africa to examine variations in crustal structure throughout the region. The new data reveal steep northeast-trending gradients in the Bouguer gravity anomalies that coincide with the Sanaga Fault Zone and the Foumban Shear Zone, both part of the Central African Shear Zone lying between the Adamawa Plateau and the Congo Craton. Four major density discontinuities in the lithosphere have been determined within the lithosphere beneath the Adamawa Uplift in central Cameroon using spectral analysis of gravity data: (1) 7–13 km; (2) 19–25 km; (3) 30–37 km; and (4) 75–149 km. The deepest density discontinuities determined at 75–149 km depth range agree with the presence of an anomalous low velocity upper mantle structure at these depths deduced from earlier teleseismic delay time studies and gravity forward modelling. The 30–37 km depths agree with the Moho depth of 33 km obtained from a seismic refraction experiment in the region. The intermediate depth of 20 km obtained within region D may correspond to shallower Moho depth beneath parts of the Benue and Yola Rifts where seismic refraction data indicate a crustal thickness of 23 km. The 19–20 km depths and 8–12 km depths estimated in boxes encompassing the Adamawa Plateau and Cameroon Volcanic Line may may correspond to mid-crustal density contrasts associated with volcanic intrusions, as these depths are less than depths of 25 and 13 km, respectively, in the stable Congo Craton to the south.  相似文献   

17.
Wide-angle seismic and gravity data across the Narmada-Son lineament (NSL) in central India are analyzed to determine crustal structure, velocity inhomogeneities and hence constrain the tectonics of the lineament. We present the 2-D crustal velocity structure from deep wide-angle reflection data by using a ray-trace inverse approach. The main result of the study is the delineation of fault-bounded horst raised to a subsurface depth (1.5 km) and the Moho upwarp beneath the NSL. The crust below the basement consists of three layers with velocities of 6.45–6.7, 6.2–6.5 and 6.7–6.95 km/s and interface depths of about 5.5–8.7, 14–17 and 18–23 km along the profile. The low-velocity (6.2–6.5 km/s) layer goes up to a depth of 5 km and becomes the thickest part (13 km), while the overlying high-velocity (6.45–6.7 km/s) layer becomes the thinnest (3 km) and upper boundary lies at a depth of 1.5 km beneath the NSL. The overall uncertainties of various velocity and boundary nodes are of the order of ±0.12 km/s and ±1.40 km, respectively. The up-lifted crustal block and the up-warping Moho beneath the NSL indicate that the north and south faults bounding the NSL are deeply penetrated through which mafic materials from upper mantle have been intruded into the upper crust. Gravity modeling was also undertaken to assess the seismically derived crustal features and to fill the seismic data gap. The lateral and vertical heterogeneous nature of the structure and velocity inhomogeneities in the crust cause instability to the crustal blocks and played an important role in reactivation of the Narmada south fault during the 1997 Jabalpur earthquake.  相似文献   

18.
The POLONAISE'97 (POlish Lithospheric ONset—An International Seismic Experiment, 1997) seismic experiment in Poland targeted the deep structure of the Trans-European Suture Zone (TESZ) and the complex series of upper crustal features around the Polish Basin. One of the seismic profiles was the 300-km-long profile P2 in northwestern Poland across the TESZ. Results of 2D modelling show that the crustal thickness varies considerably along the profile: 29 km below the Palaeozoic Platform; 35–47 km at the crustal keel at the Teisseyre–Tornquist Zone (TTZ), slightly displaced to the northeast of the geologic inversion zone; and 42 km below the Precambrian Craton. In the Polish Basin and further to the south, the depth down to the consolidated basement is 6–14 km, as characterised by a velocity of 5.8–5.9 km/s. The low basement velocities, less than 6.0 km/s, extend to a depth of 16–22 km. In the middle crust, with a thickness of ca. 4–14 km, the velocity changes from 6.2 km/s in the southwestern to 6.8 km/s in the northeastern parts of the profile. The lower crust also differs between the southwestern and northeastern parts of the profile: from 8 km thickness, with a velocity of 6.8–7.0 km/s at a depth of 22 km, to ca.12 km thickness with a velocity of 7.0–7.2 km/s at a depth of 30 km. In the lowermost crust, a body with a velocity of 7.20–7.25 km/s was found above Moho at a depth of 33–45 km in the central part of the profile. Sub-Moho velocities are 8.2–8.3 km/s beneath the Palaeozoic Platform and TTZ, and about 8.1 km/s beneath the Precambrian Platform. Seismic reflectors in the upper mantle were interpreted at 45-km depth beneath the Palaeozoic Platform and 55-km depth beneath the TTZ.

The Polish Basin is an up to 14-km-thick asymmetric graben feature. The basement beneath the Palaeozoic Platform in the southwest is similar to other areas that were subject to Caledonian deformation (Avalonia) such that the Variscan basement has only been imaged at a shallow depth along the profile. At northeastern end of the profile, the velocity structure is comparable to the crustal structure found in other portions of the East European Craton (EEC). The crustal keel may be related to the geologic inversion processes or to magmatic underplating during the Carboniferous–Permian extension and volcanic activity.  相似文献   


19.
We present results from a seismic refraction experiment on the northern margin of the Guayana Shield performed during June 1998, along nine profiles of up to 320 km length, using the daily blasts of the Cerro Bolívar mines as energy source, as well as from gravimetric measurements. Clear Moho arrivals can be observed on the main E–W profile on the shield, whereas the profiles entering the Oriental Basin to the north are more noisy. The crustal thickness of the shield is unusually high with up to 46 km on the Archean segment in the west and 43 km on the Proterozoic segment in the east. A 20 km thick upper crust with P-wave velocities between 6.0 and 6.3 km/s can be separated from a lower crust with velocities ranging from 6.5 to 7.2 km/s. A lower crustal low velocity zone with a velocity reduction to 6.3 km/s is observed between 25 and 25 km depth. The average crustal velocity is 6.5 km/s. The changes in the Bouguer Anomaly, positive (30 mGal) in the west and negative (−20 mGal) in the east, cannot be explained by the observed seismic crustal features alone. Lateral variations in the crust or in the upper mantle must be responsible for these observations.  相似文献   

20.
A few long-range airborne magnetic profiles flown at an altitude of 7.5 km a.s.l. across the Indian shield are analysed and interpreted in terms of magnetization in the lower crust. The wavelengths of the crustal anomalies are in the range of 51–255 km and this is used to separate them from signals originating at shallow depths. Spectral analysis of these profiles provided a maximum depth of 34–41 km for the long-wavelength anomalies and 9–10 km for the shallow sources identified as Mohorovic̆ić discontinuity and the basement respectively. The magnetic “high” recorded in satellite observations over the Indian shield is interpreted as due to a bulge of 3–4 km in the Moho under the Godovari graben, with a magnetization of 200 nT in the direction of the Earth's present-day magnetic field. Similarly the magnetic lows observed over the Himalaya are interpreted in terms of thickening of the granitic part of the crust from 18 to 23.5 km with a magnetization contrast of 200 nT in the direction of the Earth's present-day magnetic field.  相似文献   

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