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1.
summary . The recent Indonesian earthquake (1977 August 19; Ms =8.0) makes possible a direct determination of Rayleigh-wave phase velocities at very long periods (200 to 300 s) over a pure shield path (College, Alaska-State College, Pennsylvania) by the two-station method. Results are in good agreement with previous models derived by regionalization of great-circle phase velocities and discussed in a previous paper (Okal, Paper I). Great-circle phase velocities computed at College and State College are correctly predicted by the models derived in Paper I, and further substantiate one of its main conclusions: no substantial structural differences between oceans and continents at depths greater than 240 km, are necessary to account for the presently available seismic data.  相似文献   

2.
The triangular Songpan-Ganzi flysch terrane exposes a Triassic turbidite sequence with an average thickness of ca. 8 km. The sediments may have been accumulated in a remnant Paleo-Tethyan ocean bounded by the converging North China, South China, and the Qiangtang terrane from three sides, or a back-arc basin with an oceanic basement created during the Triassic closure of the Paleo-Tethyan ocean. To differentiate the two competing models, we systematically reviewed the available provenance data that include U–Pb detrital zircon ages at the basin scale, paleocurrent directions, sandstone petrography, and heavy-mineral assemblages from the Triassic Songpan-Ganzi basin samples. We use the Kolmogorov–Smirnov tests to differentiate competing hypotheses for detrital-zircon provenance interpretations and DZmix modelling to quantify relative contributions of detrital zircon from all potential source areas for the Triassic Songpan-Ganzi deposits. The most important result of this work is that the Songpan-Ganzi basin had a stable and locally derived source system: the western, central and eastern sub-basins were mainly sourced from the north whereas the easternmost and southeastern sub-basins were mainly sourced from westernmost South China (i.e., the Longmen Shan area) and the Qiangtang terrane. The stability of the source areas around the Songpan-Ganzi basin throughout the Triassic is most compatible with the remnant ocean model that predicts a long-lived marine basin with a pre-Triassic oceanic/continental basement trapped between converging continental blocks during the Triassic.  相似文献   

3.
Rifted margins are created as a result of stretching and breakup of continental lithosphere that eventually leads to oceanic spreading and formation of a new oceanic basin. A cornerstone for understanding what processes control the final transition to seafloor spreading is the nature of the continent‐ocean transition (COT). We reprocessed multichannel seismic profiles and use available gravity data to study the structure and variability of the COT along the Northwest subbasin (NWSB) of the South China Sea. We have interpreted the seismic images to discern continental from oceanic domains. The continental‐crust domain is characterized by tilted fault blocks generally overlain by thick syn‐rift sedimentary units, and underlain by fairly continuous Moho reflections typically at 8–10 s twtt. The thickness of the continental crust changes greatly across the basin, from ~20 to 25 km under the shelf and uppermost slope, to ~9–6 km under the lower slope. The oceanic‐crust domain is characterized by a highly reflective top of basement, little faulting, no syntectonic strata and fairly constant thickness (over tens to hundreds of km) of typically 6 km, but ranging from 4 to 8 km. The COT is imaged as a ~5–10 km wide zone where oceanic‐type features directly abut or lap on continental‐type structures. The South China margin continental crust is cut by abundant normal faults. Seismic profiles show an along‐strike variation in the tectonic structure of the continental margin. The NE‐most lines display ~20–40 km wide segments of intense faulting under the slope and associated continental‐crust thinning, giving way to a narrow COT and oceanic crust. Towards the SW, faulting and thinning of the continental crust occurs across a ~100–110 km wide segment with a narrow COT and abutting oceanic crust. We interpret this 3D structural variability and the narrow COT as a consequence of the abrupt termination of continental rifting tectonics by the NE to SW propagation of a spreading centre. We suggest that breakup occurred abruptly by spreading centre propagation rather than by thinning during continental rifting. We propose a kinematic evolution for the oceanic domain of the NWSB consisting of a southward spreading centre propagation followed by a first narrow ridge jump to the north, and then a younger larger jump to the SE, to abandon the NWSB and create the East subbasin of the South China Sea.  相似文献   

4.
This paper describes a method for determining Moho depth, lithosphere thinning factor (γ= 1 − 1/β) and the location of the ocean–continent transition at rifted continental margins using 3-D gravity inversion which includes a correction for the large negative lithosphere thermal gravity anomaly within continental margin lithosphere. The lateral density changes caused by the elevated geotherm in thinned continental margin and adjacent ocean basin lithosphere produce a significant lithosphere thermal gravity anomaly which may be in excess of −100 mGal, and for which a correction must be made in order to determine Moho depth accurately from gravity inversion. We describe a method of iteratively calculating the lithosphere thermal gravity anomaly using a lithosphere thermal model to give the present-day temperature field from which we calculate the lithosphere thermal density and gravity anomalies. For continental margin lithosphere, the lithosphere thermal perturbation is calculated from the lithosphere thinning factor (γ= 1 − 1/β) obtained from crustal thinning determined by gravity inversion and breakup age for thermal re-equilibration time. For oceanic lithosphere, the lithosphere thermal model used to predict the lithosphere thermal gravity anomaly may be conditioned using ocean isochrons from plate reconstruction models to provide the age and location of oceanic lithosphere. A correction is made for crustal melt addition due to decompression melting during continental breakup and seafloor spreading. We investigate the sensitivity of the lithosphere thermal gravity anomaly and the predicted Moho depth from gravity inversion at continental rifted margins to the methods used to calculate and condition the lithosphere thermal model using both synthetic models and examples from the North Atlantic.  相似文献   

5.
The deep seismic reflection profile Western Approaches Margin (WAM) cuts across the Goban Spur continental margin, located southwest of Ireland- This non-volcanic margin is characterized by a few tilted blocks parallel to the margin. A volcanic sill has been emplaced on the westernmost tilted block. The shape of the eastern part of this sill is known from seismic data, but neither seismic nor gravity data allow a precise determination of the extent and shape of the volcanic body at depth. Forward modelling and inversion of magnetic data constrain the shape of this volcanic sill and the location of the ocean-continent transition. The volcanic body thickens towards the ocean, and seems to be in direct contact with the oceanic crust. In the contact zone, the volcanic body and the oceanic magnetic layer display approximately the same thickness. The oceanic magnetic layer is anomalously thick immediately west of the volcanic body, and gradually thins to reach more typical values 40 km further to the west. The volcanic sill would therefore represent the very first formation of oceanic crust, just before or at the continental break-up. The ocean-continent transition is limited to a zone 15 km wide. The continental magnetic layer seems to thin gradually oceanwards, as does the continental crust, but no simple relation is observed between their respective thinnings.  相似文献   

6.
Summary. A long seismic refraction profile was carried out between southern Israel and Cyprus. The seismic energy was generated by 33 sea shots each of 0.8 t explosives and was recorded by land stations in Israel and Cyprus and by ocean bottom seismographs deployed along the profile.
The results showed that the continental crust of southern Israel thins towards the Mediterranean underneath a northward thickening sedimentary cover. Cyprus is underlain by a 35 km thick continental crust thinning south-wards and extending to Mt Eratosthenes. Between Mt Eratosthenes and the Israel continental shelf the crystalline crust is composed of high velocity (6.5 km s-1)material and is about 8 km thick. It is covered by 12–14 km of sediments and may represent a fossil oceanic crust.  相似文献   

7.
Summary. The 300 km ECORS - Bay of Biscay profile was carried out along the Aquitaine shelf and comprised a complete set of experiments including zero-offset and 7.5 km constant offset vertical seismic reflection and six expanding spread profiles. Large offset recordings were fundamental for the definition of the layered lower crust and the Moho, while ESPs provided decisive complementary information for the geological interpretation. These data show a strong variation in crustal thickness from about 20 km beneath the rifted Parentis basin, a failed arm of the oceanic Bay of Biscay, up to 35 km to the north below the Armorican shelf, in the Hercynian domain, and to the south below the Cantabria shelf, in the vicinity of the Pyrenean deformation front. The results have important implications for the behaviour of the crust during the formation of rifted sedimentary basins and during continental collision.  相似文献   

8.
The Massif Central, the most significant geomorphological unit of the Hercynian belt in France, is characterized by graben structures which are part of the European Cenozoic Rift System (ECRIS) and also by distinct volcanic episodes, the most recent dated at 20 Ma to 4000 years BP. In order to study the lithosphere-asthenosphere system beneath this volcanic area, we performed a teleseismic field experiment.
During a six-month period, a joint French-German team operated a network of 79 mobile short-period seismic stations in addition to the 14 permanent stations. Inversion of P -wave traveltime residuals of teleseismic events recorded by this dense array yielded a detailed image of the 3-D velocity structure beneath the Massif Central down to 180 km depth. The upper 60 km of the lithosphere displays strong lateral heterogeneities and shows a remarkable correlation between the volcanic provinces and the negative velocity perturbations. The 3-D model reveals two channels of low velocities, interpreted as the remaining thermal signature of magma ascent following large lithospheric fractures inherited from Hercynian time and reactivated during Oligocene times. The teleseismic inversion model yields no indication of a low-velocity zone in the mantle associated with the graben structures proper. The observation of smaller velocity perturbations and a change in the shape of the velocity pattern in the 60–100 km depth range indicates a smooth transition from the lithosphere to the asthenosphere, thus giving an idea of the lithosphere thickness. A broad volume of low velocities having a diameter of about 200 km from 100 km depth to the bottom of the model is present beneath the Massif Central. This body is likely to be the source responsible for the volcanism. It could be interpreted as the top of a plume-type structure which is now in its cooling phase.  相似文献   

9.
Long-period geomagnetic data can resolve large-scale 3-D mantle electrical conductivity heterogeneities which are indicators of physiochemical variations found in the Earth's dynamic mantle. A prerequisite for mapping such heterogeneity is the ability to model accurately electromagnetic induction in a heterogeneous sphere. A previously developed finite element method solution to the geomagnetic induction problem is validated against an analytic solution for a fully 3-D geometry: an off-axis spherical inclusion embedded in a uniform sphere. Geomagnetic induction is then modelled in a uniform spherical mantle overlain by a realistic distribution of oceanic and continental conductances. Our results indicate that the contrast in electrical conductivity between oceans and continents is not primarily responsible for the observed geographic variability of long-period geomagnetic data. In the absence of persistent high-wavenumber magnetospheric disturbances, this argues strongly for the existence of large-scale, high-contrast electrical conductivity heterogeneities in the mid-mantle. Lastly, for several periods the geomagnetic anomaly associated with a mid-mantle spherical inclusion is calculated. A high-contrast inclusion can be readily detected beneath the outer shell of oceans and continents. A comparison between observed and computed c responses suggests that the mid-mantle contains more than one order of magnitude of lateral variability in electrical conductivity, while the upper mantle contains at least two orders of magnitude of lateral variability in electrical conductivity.  相似文献   

10.
The waveform inversion method described in Woodhouse & Dziewonski (1984) was modified to retrieve regional scale 3-D heterogeneities by using the minor arc part of seismograms. The lateral heterogeneities are expanded horizontally into blocks (10°× 10°) and radially into Legendre polynomials up to order 3 (0–670 km), and thus the results show much fine details of lateral variation than previous global scale studies. We assumed that the heterogeneities produce the perturbation of eigenfrequencies which are the minor arc average of local eigenfrequency shift. We applied the method to the upper mantle beneath the Atlantic Ocean and its environments. Care was taken about the weighting of the data set. We found that the fit of each seismogram became better when the weighting of each seismogram is proportional to the inverse of initial data residuals. Resolution is good in the triangular region surrounded by South America, Europe, and North America. Resolution is not good in the South Atlantic because of the poor path coverage. Depth resolution is not clear, because of the use of Legendre polynomials, though the results suggest a broad half-width of the order of 200 km or more. We found some similarities between previous global studies and our results. For example, low velocities beneath the East Pacific Rise, Chile Rise and Azores triple junction and a high velocity Canadian shield are obtained. However, there are also differences; the high-velocity zone beneath the Brazilian shield at shallow depth is not a prominent feature in this study. Instead, we found a somewhat unexpected feature near the Romanche and Vema fracture zones where shallow positive anomalies exist. Smoothed results calculated by the spherical harmonic expansion are also shown for the purpose of comparison with global studies.  相似文献   

11.
Summary. The Lg phase has been shown previously to be a collection of higher-mode surface waves guided by the continental crust (Knopoff, Schwab & Kausel). A simple scaling between continental and oceanic crustal thicknesses suggests that a search for an oceanic Lg phase should be made in the period range from 1 to 2s. In a search for SH polarized Lg arrivals over oceanic paths, we found that in addition to the fundamental mode, seismo-grams at relatively short ranges in the Pacific showed the presence of only the first higher mode with group velocities on the steep portion of the dispersion curve rather than at the group velocity minimum as expected. Numerical model analysis indicates that, contrary to the continental case, there is no strong confluence of stationary phases of higher-mode crustal waves in the appropriate period range to produce Lg wave packets; this is due to small but significant differences in scaled crustal structures. Further, lateral variations in the thickness of oceanic sediments are sufficient to scatter most of the crustal surface-wave energy within a relatively short distance. Even were this thickness uniform, attenuation in the sediments would be strong enough to absorb the Lg stationary phases in a short distance.  相似文献   

12.
Summary. The stretching and thinning of the continental crust, which occurs during the formation of passive continental margins, may cause important changes in the velocity structure of such crust. Further, crust attenuated to a few kilometres' thickness, can be found underlying 'oceanic' water depths. This paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness. A single seismic refraction line shot over thinned continental crust as part of the North Biscay margin transect in 1979 was studied in detail. Tau— p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure. This was confirmed when synthetic seismograms were calculated. The thinned continental crust (β± 7.0) exhibits a two-gradient structure in the non-sedimentary crust with velocities between 5.9 and 7.4 km s−1; an upper 0.8 s−1 layer overlies a 0.4 s−1 layer. No layer comparable to oceanic layer 3 was detected. The uppermost mantle also contains a low-velocity zone.  相似文献   

13.
Summary. We have analysed a thirty-six day recording of the natural electric and magnetic field variations obtained on the deep ocean floor north-east of Hawaii. The electromagnetic fields are dominated by tides which have an appreciable oceanic component, especially in the east electric and north magnetic components. The techniques of data analysis included singular value decomposition (SVD) to remove uncorrelated noise. There are three degrees of freedom in the data set for periods longer than five hours, indicating a correlation of the vertical magnetic field and the horizontal components, suggesting source field inhomogeneity. Tensor response functions were calculated using spectral band averaging with both SVD and least squares techniques and rotated to the principal direction. One diagonal component, determined mainly by the north electric and east magnetic fields, is not interpretable as a one-dimensional induction phenomenon. The other diagonal term of the response function indicates a rapid rise in conductivity to 0.05 mho m−1 near 160 km. No decrease in conductivity below this depth is resolvable. Polarization analysis of the magnetic field indicates moving source fields with a wavelength near 5000 km. Model studies suggest that the two dimensionality in the response function may be caused by motion in the ionospheric current system.  相似文献   

14.
Summary. NORSAR recordings of Rayleigh waves generated by presumed nuclear explosions on central and southern Novaya Zemlya and in northwestern Siberia have been studied. Using a frequency time analysing technique and correcting for presumed known dispersion effects across the Baltic Shield, dispersion curves for two different paths across the southern part of the Barents Sea were obtained. The curves are very unusual in that they give extremely low velocities even for periods up to 20 s. For the path to the middle part of the island, the inversion of the data gives a model with sediments and consolidated sediments down to 25 km, followed by a 15-km thick basaltic layer and an upper mantle with a P velocity as low as 7.9 km/s. For the path to the southern part of Novaya Zemlya the data inversion gives a somewhat different model with sediments and consolidated sediments down to 8 km, followed by a 17-km thick zone with velocities close to granitic and a 15-km thick layer with basaltic velocities. Again the upper-mantle P velocity is only 7.9 km/s. Other indications of lateral inhomogeneities in the Barents Sea are obtained by utilizing the array's capability to determine the angle of approach of seismic waves. It is demonstrated that reflections both from inhomogeneities in the Barents Sea and the continental margin off Norway can be detected. For waves from the southern end of the island, a reflection from a strong discontinuity close to the direct path to the middle part of the island is found, whereas signals from this area include a reflected wave possibly coming from the edge of the Svalbard platform.  相似文献   

15.
Summary . In this paper we present laboratory measurements of compressional and shear wave velocities of a diverse suite of gabbroic rocks collected from the walls of the Mid-Cayman Spreading Centre with DSRV Alvin. The degree of deformation and alteration affecting these gabbros is quite variable, and we believe that they are typical of plutonic rocks emplaced at shallow levels (upper portion of seismic layer 3 and shallower) of the oceanic crust. The compositional and textural variations are reflected in the wide range of laboratory velocities which span most of the range of seismic velocities reported for oceanic and ophiolite rock samples including basalts, gabbros, ultramafics, and their altered derivatives. Based upon the laboratory velocities and the geological setting of the Mid-Cayman gabbros, it is argued that no unique lithology, except anhydrous peridotite, can be unequivocally identified in the oceanic lithosphere from seismic velocity data alone. Furthermore, these data allow for the possibility of considerable lithologic heterogeneity within portions of the oceanic crust at the scale of a few centimetres to a few hundred metres. Such heterogeneities would go unrecognized because seismic refraction studies mask these variations resulting in a picture of apparent uniformity.  相似文献   

16.
New multichannel seismic reflection data were collected over a 565 km transect covering the non-volcanic rifted margin of the central eastern Grand Banks and the Newfoundland Basin in the northwestern Atlantic. Three major crustal zones are interpreted from west to east over the seaward 350 km of the profile: (1) continental crust; (2) transitional basement and (3) oceanic crust. Continental crust thins over a wide zone (∼160 km) by forming a large rift basin (Carson Basin) and seaward fault block, together with a series of smaller fault blocks eastwards beneath the Salar and Newfoundland basins. Analysis of selected previous reflection profiles (Lithoprobe 85-4, 85-2 and Conrad NB-1) indicates that prominent landward-dipping reflections observed under the continental slope are a regional phenomenon. They define the landward edge of a deep serpentinized mantle layer, which underlies both extended continental crust and transitional basement. The 80-km-wide transitional basement is defined landwards by a basement high that may consist of serpentinized peridotite and seawards by a pair of basement highs of unknown crustal origin. Flat and unreflective transitional basement most likely is exhumed, serpentinized mantle, although our results do not exclude the possibility of anomalously thinned oceanic crust. A Moho reflection below interpreted oceanic crust is first observed landwards of magnetic anomaly M4, 230 km from the shelf break. Extrapolation of ages from chron M0 to the edge of interpreted oceanic crust suggests that the onset of seafloor spreading was ∼138 Ma (Valanginian) in the south (southern Newfoundland Basin) to ∼125 Ma (Barremian–Aptian boundary) in the north (Flemish Cap), comparable to those proposed for the conjugate margins.  相似文献   

17.
Summary. A large set of published great circle Rayleigh wave phase velocities in the period range 125–350s is used to compare three recent tectonic models (Okal, Lévêque, Jordan). Prior to any regionalization, the symmetry property of the great circle integrals is used to obtain a lower limit of the signal/noise ratio in the data. It turns out that the signal is responsible for at least 30 per cent of the data variance in the period range 175–300s.
A standard regression method is applied for computing the'pure path'velocities and the model efficiency is derived from a variance analysis. It is shown that, even at great depth, none of the three models explains more than 60 per cent of the energy due to the long-wavelength lateral heterogeneities (λ6500 km).
The three models have nearly the same efficiency for explaining the short-period data ( T ∼ 125s). Between 200 and 300s, the higher performance of Okal's model indicates that it is important to separate the subduction zones from the other orogenic zones. By perturbing the lateral extension of the subduction zones, it comes out that they constitute on both sides of the subducting slabs wider anomalies than often assumed, suggesting large downgoing flows. On the contrary, the effect of surface features such as marginal seas are restricted to a close region in front of the trenches.
Finally, the anomalous ellipticity values deduced directly from great circle data are partly explained by a coupling between tectonics and ellipticity.  相似文献   

18.
Large Igneous Provinces (LIP) are of great interest due to their role in crustal generation, magmatic processes and environmental impact. The Agulhas Plateau in the southwest Indian Ocean off South Africa has played a controversial role in this discussion due to unclear evidence for its continental or oceanic crustal affinity. With new geophysical data from seismic refraction and reflection profiling, we are able to present improved evidence for its crustal structure and composition. The velocity–depth model reveals a mean crustal thickness of 20 km with a maximum of 24 km, where three major units can be identified in the crust. In our seismic reflection records, evidence for volcanic flows on the Agulhas Plateau can be observed. The middle crust is thickened by magmatic intrusions. The up to 10 km thick lower crustal body is characterized by high seismic velocities of 7.0–7.6 km s−1. The velocity–depth distribution suggests that the plateau consists of overthickened oceanic crust similar to other oceanic LIPs such as the Ontong-Java Plateau or the northern Kerguelen Plateau. The total volume of the Agulhas Plateau was estimated to be 4 × 106 km3 of which about 10 per cent consists of extruded igneous material. We use this information to obtain a first estimate on carbon dioxide and sulphur dioxide emission caused by degassing from this material. The Agulhas Plateau was formed as part of a larger LIP consisting of the Agulhas Plateau itself, Northeast Georgia Rise and Maud Rise. The formation time of this LIP can be estimated between 100 and 94 (± 5) Ma.  相似文献   

19.
Further evidence for oceanic excitation of polar motion   总被引：3，自引：0，他引：3  
While the role of the atmosphere in driving variations in polar motion is well established, the importance of the oceans has been recognized only recently. Further evidence for the role of the oceans in the excitation of polar motion is presented. To estimate the equatorial excitation functions, χ 1 and χ 2 , for the ocean, we use velocity and mass fields from a constant-density ocean model, driven by observed surface wind stresses and atmospheric pressure, for the period 1993–1995; comparison with similar functions derived from a more complex density-stratified ocean model indicates the effectiveness of the simple constant-density modelling approach. Corresponding atmospheric excitation functions are computed from NCEP/NCAR re-analyses. Results indicate significant improvements in the agreement with the observed polar motion excitation when the simulated oceanic effects are added to atmospheric excitation. Correlations between the polar motion and the geophysical signals at periods of 15–150 days increase from 0.53 to 0.80 and from 0.75 to 0.88 for χ 1 and χ 2 , respectively. The oceanic signals are particularly important for seasonal variations in χ 1 (correlation increases from 0.28 to 0.85 when oceanic excitation is included). A positive impact of the oceans on more rapid polar motion is also observed, up to periods as short as 5 days. The sensitivity of the results to different forcing fields and different amounts of friction in the oceans is also discussed.  相似文献   

20.
In this study we image crustal structure beneath a magmatic continental rift to understand the interplay between crustal stretching and magmatism during the late stages of continental rifting: the Main Ethiopian Rift (MER). The northern sector of this region marks the transition from continental rifting in the East African Rift to incipient seafloor spreading in the southern Red Sea and western Gulf of Aden. Our local tomographic inversion exploits 172 broad-band instruments covering an area of 250 × 350 km of the rift and adjacent plateaux. The instruments recorded a total of 2139 local earthquakes over a 16-month period. Several synthetic tests show that resolution is good between 12 and 25 km depth (below sea level), but some horizontal velocity smearing is evident along the axis of the Main Ethiopian Rift below 16 km. We present a 3-D P -wave velocity model of the mid-crust and present the first 3-D Vp / Vs model of the region. Our models show high P -wave velocities (6.5 km s−1) beneath the axis of the rift at a depth of 12–25 km. The presence of high Vp / Vs ratios (1.81–1.84) at the same depth range suggest that they are cooled mafic intrusions. The high Vp / Vs values, along with other geophysical evidence, suggest that dyking is pervasive beneath the axis of the rift from the mid-crustal depths to the surface and that some portion of partial melt may exist at lower crustal depths. Although the crustal stretching factor across the Main Ethiopian Rift is ∼1.7, our results indicate that magma intrusion in narrow zones accommodates a large proportion of extensional strain, with similarities to slow-spreading mid-ocean ridge processes.  相似文献   

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