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1.
The evolution of sedimentary basins on a viscoelastic lithosphere: theory and examples 总被引：2，自引：0，他引：2
Christopher Beaumont 《Geophysical Journal International》1978,55(2):471-497
Summary. A systematic approach is suggested for modelling the development of sedimentary basins. The theory, which partitions basin formation into initiating and isostatic adjustment processes, is applicable to all modes of basin formation if these processes are linear, or can he represented with sufficient accuracy in an incrementally linear form.
The dynamics of regional isostatic adjustment are characterized by the Heaviside space-time Green functions for the response of elastic and viscoelastic (Maxwell) thin plate models of the lithosphere. It is shown, by convolving the Heaviside—Green functions with cylindrical surface loads, that the rate of isostatic adjustment on a viscoelastic lithosphere is a function of the wavelength of the surface load, long wavelengths being compensated most rapidly.
Six archetypal initiating processes for sedimentary basin development are presented. These processes are those responsible for the subsidence of the Earth's surface which creates a depression in which water and sediments collect. Isostatic amplification of subsidence by sediment and water loads is cast in the form of an integral equation with isostatic Heaviside—Green functions as kernel.
Specific examples, the basins that result from a graben initiating process, are compared with the largest scale structure of the North Sea Basin, a basin that is known to be underlain by a graben system. A model, in which a 50-km wide graben subsides exponentially with a time constant of 5 × 107 yr during the interval 180–100 Myr bp , is shown to be consistent with the largest scale structure of the North Sea Basin if the underlying lithosphere is viscoelastic with a flexural rigidity of ∼5 × 1025 Nm and relaxation time constant ∼ 106 yr. 相似文献
The dynamics of regional isostatic adjustment are characterized by the Heaviside space-time Green functions for the response of elastic and viscoelastic (Maxwell) thin plate models of the lithosphere. It is shown, by convolving the Heaviside—Green functions with cylindrical surface loads, that the rate of isostatic adjustment on a viscoelastic lithosphere is a function of the wavelength of the surface load, long wavelengths being compensated most rapidly.
Six archetypal initiating processes for sedimentary basin development are presented. These processes are those responsible for the subsidence of the Earth's surface which creates a depression in which water and sediments collect. Isostatic amplification of subsidence by sediment and water loads is cast in the form of an integral equation with isostatic Heaviside—Green functions as kernel.
Specific examples, the basins that result from a graben initiating process, are compared with the largest scale structure of the North Sea Basin, a basin that is known to be underlain by a graben system. A model, in which a 50-km wide graben subsides exponentially with a time constant of 5 × 10
2.
Patrick Wu 《Geophysical Journal International》1997,130(2):365-382
Previous investigations of the causal relationship between postglacial rebound and earthquakes in eastern Canada have focused on the mode of failure and the observed timing of the pulse of earthquake/faulting activity following deglaciation. In this study, the observational database has been extended to include observed orientations of the contemporary stress field and the rotation of stress since deglacial times. It is shown that many of these observations can be explained by a realistic ice history and a viscoelastic earth with a uniform 1021 Pa s mantle.
The effects of viscosity structure on the above predictions are also examined. It is shown that, since most of the above observations are found within the ice margin, they are not very sensitive to lithospheric thickness. Also, the inclusion of a 25 or 50 km ductile layer within the lithosphere will not decouple the seismogenic upper crust. High viscosity (1022 Pa s) in the lower mantle is rejected by the stress orientation and rotation observations. A low-viscosity (6 times 1020 Pa s) upper mantle with 1.6 times 1021 Pa s in the upper part of the lower mantle and 3 times 1021 Pa s in the lower part of the lower mantle below 1200 km depth has been found to give predictions that are in general agreement with the observations. 相似文献
The effects of viscosity structure on the above predictions are also examined. It is shown that, since most of the above observations are found within the ice margin, they are not very sensitive to lithospheric thickness. Also, the inclusion of a 25 or 50 km ductile layer within the lithosphere will not decouple the seismogenic upper crust. High viscosity (10
3.
In Paper I (Breuer & Wolf 1995), a preliminary interpretation of the postglacial land emergence observed at a restricted set of six locations in the Svalbard Archipelago was given. The study was based on a simple model of the Barents Sea ice sheet and suggested increases in lithosphere thickness and asthenosphere viscosity with increasing distance from the continental margin.
In the present paper, the newly developed high-resolution load model. BARENTS-2, and land-uplift observations from an extended set of 25 locations are used to study further the possibility of resolving lateral heterogeneity in the upper mantle below the northern Barents Sea. A comparison of the calculated and observed uplift values shows that the lithosphere thickness is not well resolved by the observations, although values above 110 km are most common for this parameter. In contrast to this, there are indications of a lateral variation of asthenosphere viscosity. Whereas values in the range 1018 -1020 Pas are inferred for locations close to the continental margin, 1020 -1021 Pa s are suggested further away from the margin.
A study of the sensitivity of the values found for lithosphere thickness and asthenosphere viscosity to modifications of load model BARENTS-2 shows that such modifications can be largely accommodated by appropriate changes in lithosphere thickness, whereas the suggested lateral variation of asthenosphere viscosity is essentially unaffected. An estimate of the influence of the Fennoscandian. ice sheet leads to the conclusion that its neglect results in an underestimation of the thickness of the Barents Sea ice sheet by about 10 per cent. 相似文献
In the present paper, the newly developed high-resolution load model. BARENTS-2, and land-uplift observations from an extended set of 25 locations are used to study further the possibility of resolving lateral heterogeneity in the upper mantle below the northern Barents Sea. A comparison of the calculated and observed uplift values shows that the lithosphere thickness is not well resolved by the observations, although values above 110 km are most common for this parameter. In contrast to this, there are indications of a lateral variation of asthenosphere viscosity. Whereas values in the range 10
A study of the sensitivity of the values found for lithosphere thickness and asthenosphere viscosity to modifications of load model BARENTS-2 shows that such modifications can be largely accommodated by appropriate changes in lithosphere thickness, whereas the suggested lateral variation of asthenosphere viscosity is essentially unaffected. An estimate of the influence of the Fennoscandian. ice sheet leads to the conclusion that its neglect results in an underestimation of the thickness of the Barents Sea ice sheet by about 10 per cent. 相似文献
4.
Summary. The motion of a phase boundary in the Earth caused by temperature and pressure excitations at the Earth's surface is determined under a linear approximation. The solution is found as a sum of convolutions of pressure and temperature Green's functions with the corresponding excitations. The Green's functions are given under the form of Laplace transforms that can be inverted either by numerical evaluation of a branch cut integral or by inversion of a series expansion. This solution is a generalization of a solution previously derived by Gjevik. This latter solution is the first term in the series expansion. The relaxation times associated with the phase boundary motion are of the order of 105 –107 yr for the olivine—spinel phase transition and of 106 –107 yr for the basalt—eclogite transition. The linear approximation remains valid for long times only if the phase boundary moves slowly. 相似文献
5.
A simple new model for sudden lithospheric thinning that considers the crust to be stretched and the lower layer of the lithosphere to be partially stretched and partially mechanically eroded is proposed. This model allows calculation of the thermal field of the lithosphere during the initial warming phase and the surface uplift.
Application of this model to the Tuscan Basin explains the high regional heat flux density values (>100 mW m−2 ), the tectonic subsidence (about 1 km) and the average uplift (>400 m) observed in this region well. 相似文献
Application of this model to the Tuscan Basin explains the high regional heat flux density values (>100 mW m
6.
Kurt Lambeck 《Geophysical Journal International》1993,115(3):960-990
Observations of ice movements across the British Isles and of sea-level changes around the shorelines during Late Devensian time (after about 25 000 yr BP) have been used to establish a high spatial and temporal resolution model for the rebound of Great Britain and associated sea-level change. The sea-level observations include sites within the margins of the former ice sheet as well as observations outside the glaciated regions such that it has been possible to separate unknown earth model parameters from some ice-sheet model parameters in the inversion of the glacio-hydro-isostatic equations. The mantle viscosity profile is approximated by a number of radially symmetric layers representing the lithosphere, the upper mantle as two layers from the base of the lithosphere to the phase transition boundary at 400 km, the transition zone down to 670 km depth, and the lower mantle. No evidence is found to support a strong layering in viscosity above 670 km other than the high-viscosity lithospheric layer. Models with a low-viscosity zone in the upper mantle or models with a marked higher viscosity in the transition zone are less satisfactory than models in which the viscosity is constant from the base of the lithosphere to the 670 km boundary. In contrast, a marked increase in viscosity is required across this latter boundary. The optimum effective parameters for the mantle beneath Great Britain are: a lithospheric thickness of about 65 km, a mantle viscosity above 670 km of about (4-5) 1020 Pa s, and a viscosity below 670 km greater than 4 × 1021 Pa s. 相似文献
7.
North Atlantic Water (NAW) is an important source of heat and salt to the Nordic seas and the Arctic Ocean. To measure the transport and variability of one branch of NAW entering the Arctic, a transect across the entrance to the Barents Sea was occupied 13 times between July 1997 and November 1999, and hydrography and currents were measured. There is large variability between the cruises, but the mean currents and the hydrography show that the main inflow takes place in Bjørnøyrenna, with a transport of 1.6 Sv of NAW into the Barents Sea. Combining the flow field with measurements of temperature and salinity, this results in mean heat and salt transports by NAW into the Barents Sea of 3.9×1013 W and 5.7×107 kg s−1 , respectively. The NAW core increased in temperature and salinity by 0.7 °C yr−1 and 0.04 yr−1 , respectively, over the observation period. Variations in the transports of heat and salt are, however, dominated by the flow field, which did not exhibit a significant change. 相似文献
8.
Summary. Palaeomagnetic results are reported from 111 localities in meta- morphic rocks from the Southern Zone of the Lewisian Complex and 12 sites from similar terrain on the island of Lewis and Harris. These rocks were magnetized during slow uplift following the ca. 1800 Ma Laxfordian tectonic/ magmatic episode. The Southern Zone experienced diachronous uplift and there is a transition from predominantly positive NW remanence directions in the north to shallow negative NW directions in the south. More prolonged metamorphism in the south correlates with a transition from magnetite/ sulphide to magnetite/hematite assemblages. The relict Ruadh Mheallan zone relatively unaffected by Laxfordian tectonism preserves a (A1) remanence D = 82°, I = 65° which is sporadically recovered as high blocking temperature component in areas bordering this zone. Elsewhere, the migration of field directions is summarized as mean directions (A3) D = 323°, I = 44°, (A4) D = 314°, I = 14° and (A5) D = 313°, I = - 11° which appear to follow on sequentially from the predominant A2 direction ( D = 286°, I = 55°) observed in the Central Zone of the Lewisian Complex. The remanence directions are linked to a first approximation to the K-Ar hornblende ages and imply a migration of the ambient field direction through ca. 110° during an interval between 1 × 107 and 2 × 108 × yr at about 1600Ma; the weight of the evidence suggests that the interval represented is between 0.5 and 1 × 108 × yr.
The Lewisian A2—A5 directions yield palaeopoles which follow on from 1800–1700 Ma magnetizations from the Hudsonian terrains of Greenland and North America and overlap with the youngest record from these regions on the pre-drift reconstruction; collectively the data define part of a large apw loop. 相似文献
The Lewisian A2—A5 directions yield palaeopoles which follow on from 1800–1700 Ma magnetizations from the Hudsonian terrains of Greenland and North America and overlap with the youngest record from these regions on the pre-drift reconstruction; collectively the data define part of a large apw loop. 相似文献
9.
Summary. The viscoelastic response of the Earth to the mass displacements caused by late Pleistocene deglaciation and concomitant sea level changes is shown to be capable of producing the secular motion of the Earth's rotation pole as deduced from astronomical observations. The calculations for a viscoelastic Earth yield a secular motion in the direction of 72° W meridian which is in excellent agreement with observed values. The average Newtonian viscosity and the relaxation time obtained from polar motion data are about (1.1 ± 0.6)1023 poise (P) and 104 (1 ± 0.5) yr. The non-tidal secular acceleration of the Earth can also be attributed to the viscoelastic response to deglaciation and results in an independent viscosity estimate of 1.6 × 1023 P with upper and lower limits of 1.1 × 1023 and 2.8 × 1023 P. These values are in agreement with those based on the polar drift analysis and indicate an average mantle viscosity of 1–2 × 1023 P. 相似文献
10.
Janna Riisager Mireille Perrin & Pierre Rochette 《Geophysical Journal International》1999,138(2):590-596
From a large collection of Ethiopian flood basalts (~30 Myr old) sampled for magnetostratigraphy, 4 0 Ar/3 9 Ar geochronology and geochemical investigations, 47 samples were selected in order to test their suitability for Thellier palaeointensity experiments. Only 17 samples from eight individual flows yielded reliable palaeointensity estimates, with flow-mean virtual dipole moments ranging from 3.0 to 10.5 × 102 2 A m2 .
A critical review of the Oligocene palaeointensity data set, including these new Ethiopian data, indicates an Oligocene mean virtual dipole moment of 5.1 ± 2.5 × 102 2 A m2 for the complete data set. After applying mild selection criteria, the reduced data set yields a mean value of only 4.6 ± 1.9 × 102 2 A m2 . This value is significantly lower than the present-day field strength but is higher than the Mesozoic dipole low mean field. This low Oligocene field might be in agreement with the high palaeosecular variation and rather high non-dipole field invoked around 30 Ma. However, the Oligocene data set is largely dependent on the palaeointensity determinations from Armenia, obtained mainly from baked contacts, which show amazingly low dispersion at both flow and between-flow levels. More data are needed to reduce the weight of these determinations on the mean value and avoid a possible bias. 相似文献
A critical review of the Oligocene palaeointensity data set, including these new Ethiopian data, indicates an Oligocene mean virtual dipole moment of 5.1 ± 2.5 × 10
11.
The earthquake swarm that struck Shadwan Island at the entrance of the Gulf of Suez in 2001 August included 408 events. Almost all of these events (94 per cent) were microearthquakes and only 6 per cent had small measurable magnitudes (5.0 > M L ≥ 3.0) . Most of the earthquakes were weak and followed each other so closely in time that they could not be identified at more distant stations. The fault plane solutions of the strongest events of the swarm show almost identical focal mechanisms, predominately normal faulting with a significant sinistral strike-slip component for nodal planes trending NW–SE. A comparison with the mechanisms of the 1969 and 1972 events which took place 20 km north of the swarm epicentral region shows similarities in faulting type and orientation of nodal planes. The azimuths of T -axes determined from focal mechanisms in this study are oriented in the NNE–SSW direction. This direction is consistent with the present-day stress field derived from borehole breakouts in the southern Gulf of Suez and the last phase of stress field changes in the Late Pleistocene, as well as with recent GPS results.
The source parameters of the largest ( ML ≥ 3.0) events of the 2001 August Shadwan swarm have been estimated from the P -wave spectra of the Egyptian National Seismograph Network (ENSN). Averaging of the values obtained at different stations shows relatively similar source parameters, including a fault length of 0.65 ≤ L ≤ 2 km , a seismic moment of 7.1 × 1012 ≤ Mo ≤ 3.0 × 1014 N m and a stress drop of 0.4 ≤Δσ≤ 10 bar. 相似文献
The source parameters of the largest ( M
12.
D. J. Webb 《Geophysical Journal International》1982,70(1):261-271
Summary. A model of the tides in a hemispherical ocean is used to investigate the effect of changes in the Earth's rotation rate on the power dissipated by the ocean tides. The results obtained are then used in an idealized astronomical model to investigate how they affect the history of the Earth—Moon system.
Using the tidal model it is found that at rotation rates higher than that of the present Earth, the power dissipated by the semi-diurnal tides in the ocean drops off rapidly as a result of the increased tidal frequency. Thus if the Earth's rotation rate is doubled from its present value, then the rate of energy dissipation in the ocean is reduced to approximately one-third of its present value and the tidal torque is reduced by a factor of about 6.
The present value for secular acceleration of the Moon, calculated from the results of the tidal model is -30.5 arcsec century-2 . Using this value in the astronomical model, which has the Moon and Sun in circular orbits above the equator, and assuming that the tidal torque is independent of the tidal frequency, the Gerstenkorn event is predicted to have occurred 1.3 × 109 yr ago.
When the astronomical model is run with a torque determined at all times from the tidal model, the reduction in the energy dissipated early in the history of the system, leads to a Gerstenkorn date of 5.3 × 109 yr ago. However, dissipation within the solid earth is found to be important early in the history of the system and when this effect is included it gives a date for the Gerstenkorn event of 3.9 × 109 yr ago. 相似文献
Using the tidal model it is found that at rotation rates higher than that of the present Earth, the power dissipated by the semi-diurnal tides in the ocean drops off rapidly as a result of the increased tidal frequency. Thus if the Earth's rotation rate is doubled from its present value, then the rate of energy dissipation in the ocean is reduced to approximately one-third of its present value and the tidal torque is reduced by a factor of about 6.
The present value for secular acceleration of the Moon, calculated from the results of the tidal model is -30.5 arcsec century
When the astronomical model is run with a torque determined at all times from the tidal model, the reduction in the energy dissipated early in the history of the system, leads to a Gerstenkorn date of 5.3 × 10
13.
Gravity Anomalies and Flexure of the Lithosphere along the Hawaiian-Emperor Seamount Chain 总被引：1，自引：0，他引：1
Simple models for the flexure of the lithosphere caused by the load of the Hawaiian-Emperor Seamount Chain have been determined for different values of the effective flexural rigidity of the lithosphere. The gravity effect of the models have been computed and compared to observed free-air gravity anomaly profiles in the vicinity of the seamount chain. The values of the effective flexural rigidity which most satisfactorily explain both the amplitude and wavelength of the observed profiles have been determined. Computations show that if the lithosphere is modelled as a continuous elastic sheet, a single effective flexural rigidity of about 5 × 1029 dyne-cm can explain profiles along the Hawaiian-Emperor Seamount Chain. If the lithosphere is modelled as a discontinuous elastic sheet an effective flexural rigidity of about 2 × 1030 dyne-cm is required. Since the age of the seamount chain increases from about 3 My near Hawaii to about 70 My near the northernmost Emperor seamount these results suggest there is apparently little decrease in the effective flexural rigidity of the lithosphere with increase in the age of loading. This suggests the lithosphere is rigid enough to support the load of the seamount chain for periods of time of at least several tens of millions of years. Thus the subsidence of atolls and guyots along the chain is most likely to be regional in extent and is unlikely to be caused by an inelastic behaviour of the lithosphere beneath individual seamounts. 相似文献
14.
Continental drift and true polar wandering 总被引：1，自引：0，他引：1
S. R. Dickman 《Geophysical Journal International》1979,57(1):41-50
Summary . Evidence in the form of 75 yr of ILS data is accumulating which suggests that true polar wander may be currently taking place. It seems likely that true wander of some magnitude must always accompany plate motions, but the extrapolated ILS rate is an order of magnitude larger than the rate of true polar wander deduced from palaeomagnetic data over the past 55 Myr. The conflict between palaeomagnetic and latitude data provides the motivation for investigating one possible excitation of polar wander, the mass redistribution which accompanies continental drift.
The mass redistribution arises mainly because of the contrasting density structure of oceanic and continental regions. The change in the inertia tensor resulting from 106 yr of plate motions is found to be negligibly small; even consideration of episodic plate movements, anelasticity, or a decoupled lithosphere cannot boost the effect to the ILS rate of polar wander. These conclusions are strengthened by the fact that any one of several absolute plate velocity models, based on extremely diverse assumptions, yields the same results.
In contrast, preliminary findings regarding the effect of Pleistocene deglaciation activities on the inertia tensor reveal that such non-isostatic phenomena may have a large influence on polar wander. 相似文献
The mass redistribution arises mainly because of the contrasting density structure of oceanic and continental regions. The change in the inertia tensor resulting from 10
In contrast, preliminary findings regarding the effect of Pleistocene deglaciation activities on the inertia tensor reveal that such non-isostatic phenomena may have a large influence on polar wander. 相似文献
15.
Surface velocities in parts of the India–Asia collision zone are compared to velocities calculated from equations describing fluid flow driven by topographically produced pressure gradients. A good agreement is found if the viscosity of the crust is ∼1020 Pa s in southern Tibet and ∼1022 Pa s in the area between the Eastern Syntaxis and the Szechwan Basin. The lower boundary condition of the flow changes between these two areas, with a stress-free lower boundary in the area between the Szechwan basin and the Eastern Syntaxis, and a horizontally rigid but vertically deformable boundary where strong Indian lithospheric material underlies southern Tibet. Deformation maps for olivine, diopside and anorthite show our findings to be consistent with laboratory measurements of the rheology of minerals. Gravitationally driven flow is also suggested to be taking place in the Indo–Burman Ranges, with a viscosity of ∼1019 –1020 Pa s. Flow in both southern Tibet and the Indo–Burman Ranges provides an explanation for the formation of the geometry of the Eastern Himalayan Syntaxis. The majority of the normal faulting earthquakes in the Tibetan Plateau occur in the area of southern Tibet which we model as gravitationally spreading over the Indian shield. 相似文献
16.
The dynamical origin of subduction zone topography 总被引：1，自引：0，他引：1
Summary. Subduction zones are expressed topographically by long linear oceanic trenches flanked by a low outer rise on the seaward side and an island arc on the landward side. This topographic structure is reflected in free air gravity anomalies, suggesting that much of the topography originates from dynamical forces applied at the base of the crust. We have successfully reproduced the general topographic features of subduction zones by supposing that the stresses generated by the bending of the viscous lower lithosphere as it subducts are transmitted through the thin elastic upper portion of the lithosphere. The trench is due to a zone of extensional flow (associated with low pressure) in the upper part of the viscous lithosphere.
The stresses in the subducting slab are computed using a finite element technique, assuming a Maxwell viscoelastic constitutive relation. Various dips (10 to 90°) were investigated, as well as depth dependent and non-Newtonian (power law, n = 3) viscosities. Observed subduction zone dimensions are well reproduced by these models. The effective viscosity required at mid-depth in the lithosphere is about 6 × 1022 P. This low value is probably due to the stress dependence of the effective viscosity. However, these models also show that the topography of the subduction zone depends primarily upon the geometry of the subducting slab (dip, radius of curvature of the bend) rather than upon its rheology. Shear stresses beneath the trench reach maxima of approximately 50 MPa. An interesting feature of some solutions is a dynamically supported bench or platform between the trench and island arc. 相似文献
The stresses in the subducting slab are computed using a finite element technique, assuming a Maxwell viscoelastic constitutive relation. Various dips (10 to 90°) were investigated, as well as depth dependent and non-Newtonian (power law, n = 3) viscosities. Observed subduction zone dimensions are well reproduced by these models. The effective viscosity required at mid-depth in the lithosphere is about 6 × 10
17.
Summary. Some lavas of Early Carboniferous age from Scotland and Derbyshire have been analysed for palaeofield magnitude by the application of a new correction to data obtained by the Shaw method. Its application has yielded field magnitude values from data which had previously been rejected. A modern pottery specimen and recent lavas from Sicily and Westman Island have also been analysed.
An average virtual dipole moment (VDM, Smith) of 2.5 ± 0.4 × 1022 A m2 was determined for the Early Carboniferous. This is only 30 per cent of the present-day VDM. 相似文献
An average virtual dipole moment (VDM, Smith) of 2.5 ± 0.4 × 10
18.
We have analysed the fundamental mode of Love and Rayleigh waves generated by 12 earthquakes located in the mid-Atlantic ridge and Jan Mayen fracture zone. Using the multiple filter analysis technique, we isolated the Rayleigh and Love wave group velocities for periods between 10 and 50 s. The surface wave propagation paths were divided into five groups, and average group velocities calculated for each group. The average group velocities were inverted and produced shear wave velocity models that correspond to a quasi-continental oceanic structure in the Greenland–Norwegian Sea region. Although resolution is poor at shallow depth, we obtained crustal thickness values of about 18 km in the Norwegian Sea area and 9 km in the region between Svalbard and Iceland. The abnormally thick crust in the Norwegian Sea area is ascribed to magmatic underplating and the thermal blanketing effect of sedimentary layers. Maximum crustal shear velocities vary between 3.5 and 3.9 km s−1 for most paths. An average lithospheric thickness of 60 km was observed, which is lower than expected for oceanic-type structure of similar age. We also observed low shear wave velocities in the lower crust and upper mantle. We suggest that high heat flow extending to depths of about 30 km beneath the surface can account for the thin lithosphere and observed low velocities. Anisotropy coefficients of 1–5 per cent in the shallow layers and >7 per cent in the upper mantle point to the existence of polarization anisotropy in the region. 相似文献
19.
Volume, heat and salt transport by the West Spitsbergen Current 总被引：1，自引：0，他引：1
During the summer of 2000 (June-July) 14 CTD and ADCP transects perpendicular to the West Spitsbergen Current and along the western border of the Barents Sea were made. The measurements covered the area between 69° 43'and 80° N and 01° and 20° E. The main purpose was to follow changes in volume, heat and salt content of Atlantic Water (AW) on its way north. The strongest and most stable flow of AW was located along the continental slope where northward flowing currents exceeding 40 cm/sec were measured. A few weaker northward branches were also found to the west of the slope. South-directed currents were recorded between them and eddy-like mesoscale structures were commonly observed. Measured by vessel-mounted acoustic Doppler current profiler (VM-ADCP), the net northward transport of AW volume in the upper 136 m layer decreased from nearly 6 Sv at the southernmost transect to below 1 Sv at a latitude of 78° 50'N. Similarly, heat transport drops from about 173 TW to about 9 TW and relative salt transport (over 34.92 psu) from 980 × 103 kg/sec to 14 × 103 kg/sec. Transport in the southern direction prevails at the transect located between 79° 07'and 79° 30'N. The calculated baroclinic geostrophic transport of AW volume, heat and salt in the upper 1000 m layer behaves similarly. East-directed transport dominates at the Barents Sea boundary while westward flow prevails on the western side of the West Spitsbergen Current. 相似文献
20.
Permo-Carboniferous (Kiaman) palaeointensity results from the western Bohemian Massif, Germany 总被引：1，自引：0，他引：1
Thellier palaeointensity analyses were carried out on samples from three quartz porphyry intrusions located within the western part of the Bohemian Massif close to the German-Czech Republic border. These intrusives have been dated at 280-275 Ma. which places them in the main part of the Permo-Carboniferous (Kiaman) Reversed Superchron (P-CRS). This age is consistent with the mean characteristic remanence direction calculated at D/ I = 204/- 1, giving a VGP at 36N, 341E. Palaeointensity values range from 6.6 μT to 22.1 μT, with a mean value of 12.0 ± 1.3 μT, which has a corresponding VDM value of (3.0 ± 1.2) × 1022 A m2 . This suggests that the geomagnetic field strength recorded by the quartz porphyries is only 37 per cent of the current value, supporting recent studies that report a field strength of between 25 and 40 per cent of the present-day value during the P-CRS. 相似文献