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1.
Summary. Models of shallow, global mantle circulation due to the accretion and subduction of lithospheric plates are formulated as potential theory problems on a sphere. Subducting and accreting plate boundaries represent sources and sinks respectively for the sublithospheric flow. Solutions, which are obtained by finite difference approximations, give the instantaneous flow velocities within the asthenosphere compatible with plate boundaries and relative plate motions. Results are presented for present-day plate boundaries and relative plate motions for the case of a uniform viscosity asthenosphere and for that of a low viscosity zone at the base of the lithosphere. These results are discussed in terms of available geophysical data. Some of the implications of a shallow, mantle-wide circulation are also considered.  相似文献   

2.
Flexure of subducted slabs   总被引：2，自引：0，他引：2  
The subducted lithosphere is regarded as a thin elastic plate that bends as a consequence of slab pull, the pressure of the asthenospheric flow induced by the subduction motion and the pressure exerted by the asthenospheric motion relative to the lithosphere. In westward subductions the latter factor enhances the slab pull, but in eastward subductions it opposes it. As a result, the subduction angle changes continuously with depth, following an elastic profile: it is smaller in eastward subductions and larger in those having a westward direction. The application of the model to 13 subducted slabs shows a good fit between the observed and the calculated shapes of the slabs.  相似文献   

3.
Summary . Frictional heating in upper mantle shear flows may lead to localized thermal runaway and partial melting in the asthenosphere, but only as the result of a finite-amplitude disturbance. A rigorous two-dimensional stability analysis shows that asthenospheric shear flows are stable to small-amplitude perturbations whether such flows are supercritical (shear stress decreases with increasing plate velocity) or subcritical (shear stress increases with increasing plate velocity). Disturbances which maintain a shear stress larger than the critical value for sufficiently long will lead to runaway. The response of the asthenosphere to events which do not satisfy this criterion must be determined by a non-linear analysis. Reasonable models of flow in the asthenosphere could be driven to runaway, at a superexponential growth rate, by sudden increases in shear stress of less than 10 bar. Disturbances resulting from plate collisions may maintain large enough stresses for sufficiently long times to initiate runaways, while stress changes associated with large earthquakes probably occur too rapidly to do so.  相似文献   

4.
Summary. A microplate is modelled as an elastic plate with two long strike-slip boundaries, lying over a Maxwell-type viscoelastic asthenosphere. The microplate is subjected to a constant and uniform shear strain rate by the opposite motions of two adjoining larger plates. After the occurrence of an earthquake at one of the microplate boundaries, the time evolution of shear stress at the other boundary is studied. It is found that stress build-up at the second boundary is delayed due to stress diffusion governed by the asthenosphere relaxation. Earthquake occurrence at this latter boundary would be delayed depending upon both the microplate width and the ratio between the Maxwell relaxation time of the asthenosphere and a characteristic time required for tectonic strain to recover rupture conditions. It turns out that the parameters which determine the occurrence of seismic activity along the microplate boundaries are more strictly constrained in the presence of a viscoelastic asthenosphere than in the case of an elastic half-pace model.  相似文献   

5.
Summary. The elegant geometrical rules of plate tectonics do not allow for a gradual shift in plate motion directions, or the gradual, as opposed to sudden, cessation of subduction. At the scale of the small plates in the NE Pacific, imperfections in boundary processes have a large effect on the net torque on the plates, and heavily influence the evolution of the geometry. In this area, the rotation of the spreading directions and the diminution of true subduction along the southern Canadian coast has not occurred by the sudden switching of plate motions from one stable condition to another. Instead, it appears as if the dominant factor for the evolution is the resistance of the ocean floor to formation of new, smoothly slipping transform faults. Compressive deformation of even young lithosphere is not only mechanically unlikely, but is not helpful to the particular configurations found in this area. Instead, a migrating shear zone and an episode of highly en echelon spreading along a new axis nearly perpendicular to the present Juan de Fuca ridge have resulted: the present Sovanco ridge was never a transform fault. Neither is the Nootka fault a shear zone, but the locus of stretching between plates whose motions are congruent at the Juan de Fuca ridge, but diverge toward the continental margin.  相似文献   

6.
Summary. The thickening plate theory proposed by Yoshii and Parker & Oldenburg for the oceanic lithosphere is extended to include the continental lithosphere. The theory is based on the assumption that the lithosphere—asthenosphere boundary is a solidus and that as a result solidification of the top of the asthenosphere is occurring. Observational data imply that the relationship between the plate thickness and basement age for the North American continent is y = 1.7 √ t + (50 ± 10), where y (km) is the plate thickness and t (Myr) is the basement age.
The theory is tested against changes with basement age of the observed surface heat-flow and seismic estimate of plate thickness. The following conclusions are inferred:
(1) The changes both of the observed heat flow and plate thickness with basement age are explained by this theory.
(2) The surface erosion and vertical distribution of radiogenic heat sources are important factors in controlling the thickening process of the continental lithosphere.
(3) The equality of the average surface heat-flow over the oceans and over the continents is a consequence of a faster release of latent heat at the lithosphere—asthenosphere boundary under the oceans, instead of a higher heat production in the continental crust.  相似文献   

7.
We present a new surface velocity field for Baja California using GPS data to test the rigidity of this microplate, calculate its motion in a global reference frame, determine its relative motion with respect to the North American and the Pacific plates, and compare those results to our estimate for Pacific–North America motion. Determination of Pacific Plate motion is improved by the inclusion of four sites from the South Pacific Sea Level and Climate Monitoring Project. These analyses reveal that Baja California moves as a quasi-rigid block but at a slower rate in the same direction, as the Pacific Plate relative to North America. This is consistent with seismic activity along the western edge of Baja California (the Baja California shear zone), and may reflect resistance to motion of the eastern edge of the Pacific Plate caused by the 'big bend' of the San Andreas fault and the Transverse Ranges in southern California.  相似文献   

8.
A two-layer lithospheric stretching model that includes the effects of decompression melting was used to estimate the deformation and thermal evolution of the Queen Charlotte Basin, British Columbia. The basin contains up to 6 km of Tertiary fill and is postulated to have been formed during a transtensional stage of Cenozoic plate motion between the Pacific and North American plates. Several models of basin formation have been proposed to explain the sediment distribution, contemporaneous volcanism and high present-day heat flow. We used bathymetry, Tertiary sediment thickness and crustal thickness to calculate the amount of stretching in the crust and lower lithosphere, and the volume of melt generated during advection of mantle rocks. A second set of calculations traced the thermal evolution of the sediments and lithosphere, and we show maps of estimated present-day heat flow and sediment maturity. This study differs significantly from previous work in the use of gridded data that provide coverage over a large region and permit lateral variations in lithospheric deformation and thermal properties to be clearly defined, a difficult quest in studies based on single-point or profile data. In addition, the use of crustal thickness, derived from a regional interpretation of gravity data and constrained by seismic refraction results, as an input allows reliable estimates of extension to be made despite recent deformation of sedimentary strata in Hecate Strait. We present results for a model which used a prerift crustal thickness of ≈34 km and a short rifting period from 25 to 20 Ma. This model infers that significant thinning occurred beneath south-western Hecate Strait and southern Queen Charlotte Sound, and several kilometres of igneous crust were added at these sites, without requiring elevated asthenospheric temperatures prior to extension. Net lithospheric extension is surprisingly uniform within the basin and averages 76%, or ≈50 km, across the margin. This amount is consistent with other estimates of extension and may provide information useful in refining models of plate motion along this margin.  相似文献   

9.
Physical models of subduction investigate the impact of regional mantle flow on the structure of the subducted slab and deformation of the downgoing and overriding plates. The initial mantle flow direction beneath the overriding plate can be horizontal or vertical, depending on its location with respect to the asthenospheric flow field. Imposed mantle flow produces either over or underpressure on the lower surface of the slab depending on the initial mantle flow pattern (horizontal or vertical, respectively). Overpressure promotes shallow dip subduction while underpressure tends to steepen the slab. Horizontal mantle flow with rates of 1–10 cm yr−1 provides sufficient overpressure on a dense subducting lithosphere to obtain a subduction angle of  ∼60°  , while the same lithospheric slab sinks vertically when no flow is imposed. Vertical drag force (due to downward mantle flow) exerted on a slab can result in steep subduction if the slab is neutrally buoyant but fails to produce steep subduction of buoyant oceanic lithosphere. The strain regime in the overriding plate due to the asthenospheric drag force depends largely on slab geometry. When the slab dip is steeper than the interplate zone, the drag force produces negative additional normal stress on the interplate zone and tensile horizontal stress in the overriding plate. When the slab dip is shallower than the interplate zone, an additional positive normal stress is produced on the interplate zone and the overriding plate experiences additional horizontal compressive stress. However, the impact of the mantle drag force on interplate pressure is small compared to the influence of the slab pull force since these stress variations can only be observed when the slab is dense and interplate pressure is low.  相似文献   

10.
Geomagnetic field motions of Holocene secular variations are investigated using a separation method. The palaeomagnetic secular variations from Britain, North America and Australia have been subjected to maximum-entropy method analyses. Based on the results of spectral analyses, the secular variations are separated by band-pass filters into low-frequency components, generally including the period band 1800-3600 yr, and high-frequency components, generally including the period band 1000-1200 yr. There is an interval, from 4200 to 1700 yr BP, which shows clockwise rotational motions in the low-frequency components of all three sites. Westward drifting of geomagnetic fields may be globally dominant. Swinging or elliptical looping motions constrained to a certain direction were observed in the low-frequency components of the British data. The time duration for the persistence of the swinging motion constrained to a certain direction was 3500 years or so, which could be the lifetime of an oscillating stationary field. The duration of the transitional motion was 1000-1300 years, which may indicate the recurrence time of a stationary field.  相似文献   

11.
Summary. A dynamical theory of flow in the asthenosphere is developed by treating it as a thin layer of Newtonian viscous fluid, pushed horizontally by the dense sinking slabs attached to some plates and by the differences in heating under continental and oceanic crust. Two-dimensional lubrication theory is adapted to give the world-wide effects of inflow to the asthenosphere at the slabs, outflow to the plates at mid-ocean ridges, and the horizontal forces applied at the slabs. The resulting torque balance for the plates is better than any previously published, and leads one to conclude that the asthenosphere's viscosity is least at the top.  相似文献   

12.
金明德 《地理学报》1996,51(2):142-146
本文从力学原理出发，讨论了由于地球自转，大陆板块受到一个沿纬圈指向东的主矩和指向赤道的主矢作用，致使板块向赤道作俯冲运动。  相似文献   

13.
Secular polar motion has been recorded in ILS data over the past 75 years, an amount greater by a factor of ten than the 'true polar wandering' deduced from paleomagnetic data. In this work, the possibility that the secular trend is an observational artifact of the continental drift of the ILS stations is directly examined by consideration of several absolute plate velocity models earlier proposed by Minster et al. (1974), Kaula (1975), and Solomon, Sleep & Richardson (1975). The assumptions underlying those models are discussed; in general, the absolute velocity models are more likely to be valid when geologically short timescales are considered.
The corrections to the ILS data due to the stations' motion fail by an order of magnitude to explain the ILS trend; even by taking into account possible plate hyperactivity and non-rigidity, the corrections could explain no more than 30 per cent of the trend. The corrections are small because the absolute plate velocities of North America and Eurasia are small and primarily east—west. Consequently, the rotation pole is undergoing significant motion of its own relative to the surface of the Earth.
The Kimura z term found by the ILS observations provides an independent means of estimating the relative motion between Eurasia and North America. It also contains other geophysical information; the 7.5-yr periodicity discovered by Naito & Ishii (1974) may be widespread.
Lastly, tectonically induced changes in the zenith direction, such as at Mizusawa, are probably too small to be detected, contrary to earlier proposals.  相似文献   

14.
Apparent polar wander in the mean-lithosphere (= no-net-rotation = no-net-torque uniform drag) reference frame is compared with apparent polar wander in the hotspot reference frame over the past 100 Myr. Palaeo-magnetic poles and plate rotations previously used to determine an apparent polar wander path for the hotspot reference frame are here used to determine an apparent polar wander path in the mean-lithosphere reference frame. We find that the two paths are similar, especially for Late Cretaceous time, when a 10°–20° shift of the pole occurred. To first-order the hotspots and lithosphere (as a whole) moved in unison relative to the palaeomagnetic axis during Late Cretaceous time. A non-dipole field explanation for the apparent shift can probably be excluded. However, either motion of the time-averaged geomagnetic axis relative to the spin axis or polar wandering could have caused this shift, the latter being the more likely explanation.  相似文献   

15.
A paleomagnetic study of the Mull lava succession   总被引：1，自引：0，他引：1  
Summary. A paleomagnetic study has been made of a succession of 139 non-overlapping basaltic lavas, representing 91 per cent of the longest remaining succession of flows associated with the Paleogene Mull volcano. All the lavas have experienced considerable hydrothermal alteration, probably at up to several million years after initial magnetization and frequently with alteration to the opaque minerals and the production of new potentially magnetic phases. The question of whether directional remagnetization has taken place while preserving within-unit directional consistency and discreteness of unit mean direction is discussed. Extensive directional remagnetization is excluded as an explanation for the data. If stable directions obtained by alternating field remanence cleaning coincide with original TRM directions then a mean pole position for all temporally independent lava directions from the British Tertiary igneous province is at 71.9° N, 167.2° E, with k:22 and α95:3.0°. This pole is significantly different from the geographic pole. If the difference in palaeomagnetic and geographic poles is interpreted in terms of absolute plate motion, then 2010 km of northwards motion of the western part of the Eurasian Plate, at 3.7 cm/yr, has taken place over the last 55 Myr. This motion has implications for the geological history of the Arctic and for the complexity of mantle motions.  相似文献   

16.
Summary. A simple, analytical model for mantle convection with mobile surface plates is presented. Our aim is to determine under what conditions free convection can account for the observed plate motions, and to evaluate the thermal structure of the mantle existing under these conditions. Boundary layer methods are used to represent two-dimensional cellular convection at large Rayleigh and infinite Prandtl numbers. The steady-state structure consists of cells with isentropic interiors enclosed by thermal boundary layers. Lithospheric plates are represented as upper surfaces on each cell free to move at a uniform speed. Buoyancy forces are concentrated in narrow rising and decending thermal plumes; torques imparted by these plumes drive both the deformable mantle and overlying plate. Solutions are found for a comprehensive range of cell sizes. We derive an expression for the plate speed as a function of its length, the mantle viscosity and surface heat flux. Using mean values for these parameters, we find that thermal convection extending to 700 km depth can move plates at 1 cm yr-1, while convection through the whole mantle can move plates at 4–5 cm yr-1. Analysis of the steady-state temperature field, for the case of heating from below, shows that the upper thermal boundary layer develops a complex structure, including an 'asthenosphere' defined by a local maximum in the geotherm occurring at depths of 50–150 km.  相似文献   

17.
The mechanism of lithospheric thinning of the North China Craton (NCC) remains controversial. To constrain the mechanism, this study investigated the upper-mantle deformation pattern of the craton by measuring shear wave splitting at the cratonic edge. The results, derived from data recorded at 47 stations, reveal a complex pattern of mantle deformation. Inside the eastern craton, the majority of fast direction trends SE–NW parallel to the tectonic extension direction accompanying with the lithospheric thinning. At the cratonic edge, 15 stations with only null splitting results indicate undetectable anisotropy beneath the stations. This may be due to upwelling or chaotic ascension of mantle flow. To the north, off the craton, large delay times and variation of splitting parameter with backazimuth are generated by the combination of lithospheric and asthenospheric anisotropy. Based on comparison of the splitting results and the predicted ones by the compelling models, it is likely that lithospheric delamination dominated the lithospheric thinning at the north edge of the NCC during the Mesozoic to Cenozoic.  相似文献   

18.
S receiver functions from 67 broad-band seismic stations in the western United States clearly reveal the existence of a mantle discontinuity with velocity reduction downward, which we interpret as the lithosphere–asthenosphere boundary (LAB). The average depth of the LAB is ∼70 km. The boundary is relatively sharp with an overall sharpness of less than 20 km. The boundary is more prominent south of the Mendocino Triple Junction, where the Farallon Plate has completely subducted. This may indicate partial melts at the base of the lithosphere caused by the upwelling of the asthenospheric flow through the slab window. A double low velocity zone is observed at base of the lithosphere beneath southern Sierra Nevada, implying a second melting zone at a depth of ∼100 km, well correlated with previous studies of lithospheric delamination in the area.  相似文献   

19.
Summary. The mean gravity profiles, across Central and South America and Eurasia, in the direction normal to the subduction zone are deduced from the Gem 10B gravity model. They have a typical shape: a maximum close to the trench, a negative slope towards the interior of the plate over a 3000 km wide distance, usually followed by a local maximum. It is found that large convective cells driven by the heat sink of the sinking slab have an associated gravity signal having such a typical shape. A detailed comparison between observed and theoretical data supports this point of view and thus constrains the possible structure of the convective flow under these plates.  相似文献   

20.
Summary. In order to contribute to the resolution of the problem of the plate tectonic character of the Caribbean, a palaeomagnetic study has been carried out on some Jamaican igneous rocks. Sixteen Late Cretaceous intrusives and lavas and one Late Miocene lava sampled in five sites have been investigated. Because of widespread maghaemitization of the predominantly large-grained deuteric class 1 titanomagnetites, some difficulty was experienced in identifying stable directions of magnetization in the Cretaceous rock units. Using thermal demagnetization technique, two distinct directions of magnetization were obtained, significantly different from those observed in contemporaneous North American rocks. Nine units yield a palaeomagnetic pole at 143.8°W, 44.1°N, referred to as 'normal', while seven units yield'equatorial'poles situated both east and west of Jamaica. It is not possible to decide which of the two directions of magnetization reflects the Late Cretaceous geomagnetic field in Jamaica, but from other evidence they appear to merit palaeotectonic interpretation. They are consistent with the plate tectonic behaviour of the Caribbean since Late Cretaceous and its motion from the southwest relative to the present day frame of reference, with a simultaneous large anticlockwise horizontal rotation of Jamaica. Late Miocene lavas, containing high-coercivity magnetic material, yield closely grouped directions giving a palaeopole at 152.4°W, 73.3°N, supporting the deductions made from the Cretaceous data.  相似文献   

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