The influence of interior mantle temperature on the structure of plumes: Heads for Venus, Tails for the Earth (original) (raw)
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Earth and Planetary Science Letters, 2007
The coexistence of Venusian highlands, attributed to long-lived axisymmetric mantle plumes, and uncompensated coronae, attributed to transient discrete mantle 'thermals', is difficult to reconcile with models of mantle convection under thermally steady-state conditions. However, cratering and geological studies indicate a uniformly young surface age (∼ 700 Myr) as well as a comparable timescale for resurfacing (∼ 100 to 400 Myr), possibly consistent with a recent lithospheric overturn and a transient mantle thermal regime. We use laboratory experiments on free and forced thermal convection at high Rayleigh number (Ra ∼ 10 7 ) in a variable viscosity fluid to investigate the steady-state and transient thermal regimes preceding and following such an overturn. From analyses of shadowgraph images and time series of global and local variations in temperature, basal heat flux and viscosity, we establish steady-state stagnant-and active-lid states and characterize two intermediate transient regimes. Flow in steady-state stagnant lid is in the form of intermittent thermals, consistent with published work. During the transition to active-lid convection the stagnant lid is stirred into the interior using a conveyor belt. Spreading of this cold fluid along the hot boundary leads to a transition to a "mixed mode" of flow from the hot boundary: approximately isoviscous thermals rise from the thermal boundary layer ahead of the advancing cold front and low viscosity plumes rise from behind the front, as a result of an enhanced temperature contrast. The longevity of this regime and the timescale for the transient depends on the rate of overturn (Pe) and the aspect ratio of the system (A). The magnitude of local temperature, viscosity and heat flux variations increases with Pe and can exceed steady-state values for active-lid convection. Additional numerical simulations show that the mixed mode regime will occur in the presence of internal heating, and for no-and free-slip boundaries. In contrast, the transition from active-lid to stagnant-lid convection is marked by a change from a flow composed of plumes and large-scale overturning motions to a regime dominated by rising and sinking thermals on a timescale of thermal diffusion. Applied to Venus, our results support a hypothesis that the contemporaneous coexistence of the Atla and Beta highlands regions with interspersed uncompensated coronae is consistent with a transient thermal regime following a lithospheric overturn. It is also expected that such coronae formed N 250 Myr after the uplift of the highlands. Implications of the thermal origin of coronae for Venusian mantle structure are also explored.
A mantle plume model for the equatorial highlands of Venus
Journal of Geophysical Research, 1991
The Equatorial Highlands of Venus consist of four main structures, Atla, Beta, Ovda, and Thetis regiones. Each has a circular to oval-shaped planform and rises 4-5 km above the mean planetary radius. These highlands are associated with long-wavelength geoid highs, with amplitudes ranging from 35 m at Ovda to 120 m at Atla. They also contain topographic valleys, interpreted as extensional rift zones, and Beta is known to contain shield volcanoes. These characteristics are all consistent with the Equatorial Highlands being formed by mantle plumes. An alternative model, in which Ovda and Thetis are interpreted as spreading centers analogous to terrestrial mid-ocean ridges, fails to explain most of the observed geoid anomalies and topography in these regions. Some smaller highlands, such as Bell Regio, Eistla Regio, and the Hathor/Innini/Ushas region, may also be plume related, but most coronae are unlikely to be the direct result of plume activity. We have modeled plumes using a cylindrical, axisymmetric finite element code and a depth-dependent, Newtonian rheology. We compare our model results with profiles of geoid and topography across Atla, Beta, Ovda, and Thetis; our best model fits are for Beta and Atla. Assuming whole mantle convection and that Earth and Venus have similar mantle heat flows, Venus must lack an Earth-like low-viscosity zone in its upper mantle in order satisfy the observed geoid and topography for these features. This conclusion is consistent with the long-wavelength admittance spectrum of Venus and with the observed differences in the slopes of the geoid spectra for the two planets. One explanation for the different viscosity structures of the two planets could be that the mantle of Venus is drier than Earth's mantle.
Experimental and observational evidence for plume-induced subduction on Venus
Nature Geoscience, 2017
Why Venus lacks plate tectonics remains an unanswered question in terrestrial planet evolution. There is observational evidence for subduction-a requirement for plate tectonics-on Venus, but it is unclear why the features have characteristics of both mantle plumes and subduction zones. One explanation is that mantle plumes trigger subduction. Here we compare laboratory experiments of plume-induced subduction in a colloidal solution of nanoparticles to observations of proposed subduction sites on Venus. The experimental fluids are heated from below to produce upwelling plumes, which in turn produce tensile fractures in the lithosphere-like skin that forms on the upper surface. Plume material upwells through the fractures and spreads above the skin, analogous to volcanic flooding, and leads to bending and eventual subduction of the skin along arcuate segments. The segments are analogous to the semi-circular trenches seen at two proposed sites of plume-triggered subduction at Quetzalpetlatl and Artemis coronae. Other experimental deformation structures and subsurface density variations are also consistent with topography, radar and gravity data for Venus. Scaling analysis suggests that this regime with limited, plumeinduced subduction is favoured by a hot lithosphere, such as that found on early Earth or present-day Venus.
Compressible Convection in a Viscous Venusian Mantle
Journal of Geophysical Research, 1991
Finite element simulations of axisymmetric spherical shell compressible convection were carried out to investigate the effect of various surface boundary conditions in a Venustan mantle. We employed a thermal expansivity 0/ which decreased with depth, a uniform viscosity an order of magnitude greater than the Earth's, and zero and chondritic quantities of internal heating. As long as hot plumes from the core-mantle boundary were strong, the convection pauern was typical of that for variable 0/flow; that is, it was characterized by steady upflowing regions, unsteady collections of downflowing plumes, and large aspect ratio cells. Increases in the internal heating or the temperature T o at the top of the convecfing layer weakened the hot plumes and therefore decreased the width of the cells. A rigid surface increased the internal temperature and also decreased the width of convection cells. Extensive regions of subadiabaficity were found in the mantle. We compare our results with those for fully three-dimensional convection under similar conditions (Schubert et al., 1990). 15,551 L•rrcH AS• YUES: COMnU•S•ONAL CoswenoN • A V•SCOUS VESUS•AS MASTLE 1.5,.5.53
Geophysical Journal International, 1992
S U M M A R Y A variety of evidence suggests that at least some hotspots are formed by quasi-cylindrical mantle plumes upwelling from deep in the mantle. We model such plumes in cylindrical, axisymmetric geometry with depth-dependent, Newtonian viscosity. Cylindrical and sheet-like, Cartesian upwellings have significantly different geoid and topography signatures. However, Rayleigh number-Nusselt number systematics in the two geometries are quite similar. The geoid anomaly and topographic uplift over a plume are insensitive to the viscosity of the surface layer, provided that it is at least 1000 times the interior viscosity. Increasing the Rayleigh number or including a low-viscosity asthenosphere decreases the geoid anomaly and the topographic uplift associated with an upwelling plume. Increasing the aspect ratio increases both the geoid anomaly and the topographic uplift of a plume. The Nusselt number is a weak function of the aspect ratio, with its maximum value occurring at an aspect ratio of slightly less than 1.
Geophysical Research Letters, 2006
The subduction and stirring of cold oceanic lithosphere governs the thermal regime of the Earth's mantle. Whether upwelling mantle plumes are transient isoviscous thermals or long-lived low viscosity plumes depends on the magnitude of the resulting temperature variations in the thermal boundary layer at the base of the mantle. Previous laboratory experiments suggest that low viscosity ''Earthlike'' plumes occur where the hot thermal boundary layer (TBL) viscosity ratio, l h > O(10). Here, the results from two-dimensional numerical simulations, in which subduction is either forced from above or allowed to arise naturally show that: (1) a morphologic transition from upwellings in the form of isoviscous thermals to cavity plumes occurs where l h ! O(10) and is accompanied by a qualitative change in the temporal and spatial dynamics of the hot TBL; (2) this transition corresponds to a condition in which the velocity boundary layer (VBL) is concentrated within the basal part of the TBL for no-and free-slip boundaries; and (3) a regime in which l h ! O(10) can only occur if the total viscosity ratio across the convecting system, l T ! O(10 2). Our results support a recent conjecture that low viscosity mantle plumes in the Earth are a consequence of strong mantle cooling by plate tectonics. Moreover, Earthlike plume models may be inappropriate for explaining the origin of surface features on one plate planets such as Mars or Venus.
2004
We report results from analog laboratory experiments, in which a large-scale flow is imposed upon natural convection from a hot boundary layer at the base of a large tank of corn syrup. The experiments show that the subdivision of the convective flow into four regions provides a reasonable conceptual framework for interpreting the effects of large-scale flow on plumes. Region I includes the area of the hot thermal boundary layer (TBL) that is thinned by the large-scale flow, thereby suppressing plumes. Region II encompasses the critically unstable boundary layer where plumes form. Region III is the area above the boundary layer that is devoid of plumes. Region IV comprises the area of hot upwelling and plume conduits. Quantitative analysis of our experiments results in a scaling law for heat flux from the hot boundary and for the spatial extent of plume suppression. When applied to the Earth's core-mantle boundary (CMB), our results suggest that large-scale mantle flow, due to sinking lithospheric plates, can locally thin the TBL and suppress plume formation over large fractions of the CMB. Approximately 30% of heat flow from the core may be due to increased heat flux from plate-scale flow. Furthermore, CMB heat flux is non-uniformly distributed along the CMB, with large areas where heat flux is increased on average by a factor of 2. As a consequence, the convective flow pattern in the outer core may be affected by CMB heat-flux heterogeneity and sensitive to changes in plate-scale mantle flow. Because of plume suppression and dfocusingT of hot mantle from the CMB into zones of upwelling flow, plume conduits (hotspots) are expected to be spatially associated with lower-mantle regions of low seismic velocities, inferred as hot upwelling mantle flow. D