Three-dimensional models of mantle flow across a low-viscosity zone: implications for hotspot dynamics (original) (raw)
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Geophysical Journal International, 1998
Because of their slow relative motion, hotspots, mainly in the Paci¢c, are often used as a reference frame for de¢ning plate motions. A coherent motion of all Paci¢c hotspots relative to the deep mantle may, however, bias the hotspot reference frame. Numerical results on the advection of plumes, which are thought to cause the hotspots on the Earth's surface, in a large-scale mantle £ow ¢eld are therefore presented. Bringing the results into agreement with observations also leads to conclusions regarding the viscosity structure of the Earth's mantle, as well as the sources and distribution of plumes.
Earth and Planetary Science Letters, 1988
The medium-wavelength geoid to depth anomalies ratio (GDR) at oceanic hotspot swells has been found to increase from --0.5 m/km to -5 m/km according to the age of the lithosphere they occur on. In order to interpret this trend, the geoid and topography anomalies associated with mantle convective plumes crossing a sublithospheric low viscosity zone (LVZ) have been derived from numerical models and a systematic investigation of the GDR dependence on the viscosity and depth extent of the LVZ, on the thickness and thermal structure of the lithosphere and on the Rayleigh number has been conducted. It is shown that, for viscosity drops across the base of the LVZ, greater than one order of magnitude, the GDR is strongly dependent on the depth of shallow interfaces such as the lithosphere/ athenosphere boundary and on the LVZ's thickness. Consequently, the empirical trend can be accounted for by the thickening of the lithosphere with age provided it occurs at the expense of a LVZ whose base is at a fixed depth (around 200 km). In such a frame, no significant variation with age of the LVZ's viscosity is required by the GDR data. Best fit with the empirical trend is found for a LVZ about 50 times less viscous than the underlying mantle. The mantle flow starts to fluctuate when the local Rayleigh number of the low-viscosity layer exceeds the Rayleigh number of the underlying mantle. The fluctuations are initiated in the upper boundary layer, in the diverging part of the plume, at a distance of a few hundreds of kilometers from the main ascending current. For viscosity contrasts in the range of 40-60, deduced from the present study, the conditions for the development of these small-scale instabilities are realized only where the lithosphere has not yet grown significantly downwards (ages < 50 Ma). These fluctuations are tentatively related to the small wavelength ( -200 km) geoid anomalies pattern observed in a young and hot area of the southwestern Pacific Ocean. This pattern could be the surface expression of a mantle plume rising just beneath the East Pacific Rise axis.
Journal of Geophysical Research: Solid Earth, 1994
Heat flow measurements through old seafloor demonstrate that the oceanic lithosphere is heated from below away from hot spot tracks. We reevaluate the hypothesis of small‐scale convection beneath the lithosphere with laboratory experiments in fluids whose viscosity depends strongly on temperature. Rayleigh numbers were between 106 and 108 and viscosity contrasts were up to 106. A layer of fluid was impulsively cooled from above, and a cold boundary layer grew at the top of the fluid layer. After a finite time, convective instabilities developed in the lowermost part of the boundary layer, while the upper part remained stagnant. The variation of surface heat flow as a function of time reflects the three‐dimensional nature of the flow and the presence of a thick lid. At viscosity contrasts greater than 103, this variation is very similar to what is observed on the oceanic lithosphere. For small times, heat flow follows the behavior of a half‐space cooled from above by conduction. Some...
The effect of spatial variations in viscosity on the structure of mantle flows
Izvestiya, Physics of the Solid Earth, 2006
According to an opinion widespread in the literature, high viscosity regions (HVRs) in the mantle always affect the structure of mantle flows, changing it in both the HVR itself and the entire mantle. Moreover, a simplified relation is often adopted according to which the flow velocity in the HVR decreases in inverse proportion to viscosity. Therefore, in order to treat a smoother value, some authors introduce a new variable equal to the product of the flow velocity and the viscosity value in a given place. On the basis of numerical modeling, this paper shows that HVRs of two types should be distinguished in the mantle. If an HVR is immobile, mantle flows actually do not penetrate it. If the viscosity increase is more than five orders, the HVR behaves as a solid and flow velocities within it almost vanish. However, if an HVR is free, it moves together with the mantle flow. Then, the general structure of flows changes weakly and flow velocities within the HVR become approximately equal to the average velocity of flows in the absence of the HVR. Horizontal layers and vertical columns differing in viscosity from the mantle behave as regions of the first type, whose flow velocities can differ by a few orders. However, even such large-scale regions as the continental lithosphere, whose viscosity is four to five orders higher than in the surrounding mantle, float together with continents at velocities comparable to mantle flows, i.e., behave as regions of the second type.
An Analytic Model For A Mantle Plume Fed By A Boundary Layer
Geophysical Journal International, 1987
An approximate analytical solution for flow in a mantle plume of constant radius, viscosity, and density contrast is obtained in cylindrical coordinates. The differential equations for vertical velocity of the mantle surrounding the plume and for topography are homologous to the equation for flexure of an elastic plate. Although the model is too simple t o be fully applicable to the Earth, one can conclude that the vertical velocity in the mantle changes significantly away from plumes, that the viscosity of the plume is important for controlling flow rate, and that the long-wavelength geoid anomalies are sensitive to the viscosity of the surrounding mantle. The first induced upwelling away from a plume is quite weak and unlikely to control the spacing of plumes.
Depth-dependent rheology and the horizontal length scale of mantle convection
Journal of Geophysical Research, 2006
Numerical simulations show that depth-dependent viscosity can increase the wavelength of mantle convection. The physical mechanism behind this phenomenon and its robustness with respect to model parameters remain to be fully elucidated. Toward this end, we develop theoretical heat flow scalings for a convecting fluid layer with depthdependent viscosity. Bottom and internally heated end-members are considered. For the former, the viscosity structure consists of a high-viscosity central region bounded from above and below by horizontal low-viscosity channels. For internally heated cases, only a surface low-viscosity channel is present. Theoretical scalings derived from boundary layer theory show that depth-dependent rheology lowers the lateral dissipation associated with steady state convective rolls, allowing longer aspect ratio cells to form as the viscosity contrast between the channels and the central region is increased. The maximum cell aspect ratio is estimated from the condition that the pressure gradients that drive lateral flow in the channels do not become so large as to inhibit vertical flow into the channels. Scaling predictions compare favorably to results of numerical simulations for steady state cells. As the Rayleigh number driving convection is increased, small-scale boundary layer instabilities begin to form. This increases lateral dissipation within the channels and the preferred cell aspect ratio decreases as a result. Internally heated simulations show that a near-surface high-viscosity layer, an analog to tectonic plates, can suppress these small-scale instabilities. This allows a low-viscosity channel to maintain large aspect ratio cells for Rayleigh numbers approaching that of the present-day Earth.
Journal of …, 2000
Layered viscosity, temperature-dependent viscosity, and surface plates have an important effect on the scale and morphology of structure in spherical models of mantle convection. We find that long-wavelength structures can be produced either by a layered viscosity with a weak upper mantle or temperature-dependent viscosity even in the absence of surface plates, corroborating earlier studies. However, combining the layered viscosity structure with a temperature-dependent viscosity results in structure with significantly shorter wavelengths. Our models show that the scale of convection is mainly controlled by the surface plates, supporting the previous two-dimensional studies. Our models with surface plates, layered and temperaturedependent viscosity, and internal heating explain mantle structures inferred from seismic tomography. The models show that hot upwellings initiate at the core-mantle boundary (CMB) with linear structures, and as they depart from CMB, the linear upwellings quickly change into quasi-cylindrical plumes that dynamically interact with the ambient mantle and surface plates while ascending through the mantle. A linear upwelling structure is generated again at shallow depths (<200 km) in the vicinity of diverging plate margins because of the surface plates. At shallow depths, cold downwelling sheets form at converging plate margins. The evolution of downwelling sheets depends on the mantle rheology. The temperature-dependent viscosity strengthens the downwelling sheets so that the sheet structure can be maintained throughout the mantle. The tendency for linear upwelling and downwelling structures to break into plume-like structures is stronger at higher Rayleigh numbers. Our models also show that downwellings to first-order control surface plate motions and the locations and horizontal motion of upwellings. Upwellings tend to form at stagnation points predicted solely from the buoyancy forces of downwellings. Temperature-dependent viscosity greatly enhances the ascending velocity of developed upwelling plumes, and this may reduce the influence of global mantle flow on the motion of plumes. Our results can explain the anticorrelation between hotspot distribution and fast seismic wave speed anomalies in the lower mantle and may also have significant implications to the observed stationarity of hotspots.
Influence of low viscosity asthenosphere on mantle flows
Izvestiya, Physics of the Solid Earth, 2006
Numerical simulation in recent years has revealed that the cold lithosphere, whose viscosity is three to four orders of magnitude higher than that of the underlying mantle, behaves during mantle convection as a stagnant lid. On the basis of model calculations, this paper shows how convection changes over to this regime with increasing viscosity. Spatially fixed high viscosity inclusions and those moving with the convective flow have fundamentally different effects on the structure of convective flows. The model calculations indicate that anomalously low viscosity asthenospheric regions also lead to a specific regime of convection. With a decrease in the viscosity by more than three orders of magnitude, a further reduction in the viscosity of the region ceases to influence the structure of convection in the outer region. The boundary with this region behaves as a freely permeable boundary. In the low viscosity asthenospheric region itself, autonomous convection can arise under certain conditions. PACS numbers: 91.45.Fj