Plug flow in the Earth's asthenosphere (original) (raw)
Related papers
Kinematic models of large-scale flow in the Earth's mantle
Journal of Geophysical Research, 1979
It is likely that the motions of the lithospheric plates strongly influence the accompanying large-scale flow in the earth's mantle. In order to isolate and understand the effects of the plate motions on the large scale flow, we calculate simple kinematic models in which the observed plate motions and geometries are imposed as boundary conditions; thermal buoyancy forces are ignored and the flow is determined by the mass flux imposed by the moving plates. With the assumption of a layered, radially symmetric, Newtonian rheology, the flow accompanying the observed plate motions is calculated analyticairy. The flow is in general complicated, but for models in which whole-mantle flow occurs, the dips of subduction zones are predicted remarkably well by the flow models. Circulation does not form simple closed cells and flow under slowly moving plates is not closely related to the direction of plate motion. There is little return flow under the Pacific unless the lithosphere is essentially decoupled from the mantle by an extremely weak low viscosity layer. Only viscosity contrasts between the upper and lower mantle of three to four orders of magnitude are sufficient to confine flow to the upper mantle. For a wide range of mantle viscosity structures, significant mixing between the upper and lower mantle is expected over geologic time. On the basis of these models, the sou,rce rock for oceanic ridge basalt probably lies below the asthenosphere. A comparison of the magnitude of the deviatoric stress with theoretical flow mechanism calculations and with models of postglacial rebound indicate that the assumption of Newtonian viscosity is plausible. The drag on the lithosphere is compared to that calculated assuming simple shear. On a regional basis, the stresses can differ by up to 90 ø , e.g., in eastern North America. Small plates have higher apparent drag coefficients than larger ones. The normal stress at the base of the lithosphere is excessive in models in which the flow is confined to the upper 700 km of the mantle unless the lithosphere is decoupled from the mantle by an extremely low viscosity layer.
2013
The textbook view is that the asthenosphere is the place beneath the tectonic plates where competing temperature and pressure effects on mantle rheology result in the lowest viscosity region of Earth's mantle. We think the sub-oceanic asthenosphere exists for a different reason, that instead it is where rising plumes of hot mantle stall and spread out beneath the strong tectonic plates. Below this plumefed asthenosphere is a thermal and density inversion with cooler underlying average-temperature mantle. Here we show several recent seismic studies that are consistent with a plume-fed asthenosphere. These include the seismic inferences that asthenosphere appears to resist being dragged down at subduction zones, that a sub-oceanic thermal inversion $ 250-350 km deep is needed to explain the seismic velocity gradient there for an isochemical mantle, that a fast 'halo' of shear-wave travel-times surrounds the Hawaiian plume conduit, and that an apparent seismic reflector is found $ 300 km beneath Pacific seafloor near Hawaii. We also present 2D axisymmetric and 3D numerical experiments that demonstrate these effects in internally consistent models with a plume-fed asthenosphere. If confirmed, the existence of a plume-fed asthenosphere will change our understanding of the dynamics of mantle convection and melting, and the links between surface plate motions and mantle convection.
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
Asthenospheric flow and asymmetry of the East Pacific Rise, MELT area
Journal of Geophysical Research, 2002
Although the Pacific and Nazca plates share the East Pacific Rise (EPR) as a boundary, they exhibit many differing characteristics. The Pacific plate subsides more slowly and has more seamounts than the Nazca plate. Both the seismic and magnetotelluric components of the Mantle ELectromagnetic and Tomography Experiment (MELT) found pronounced asymmetry in mantle structure across the spreading axis near 17°S. The Pacific (west) side has lower S-wave velocities, exhibits greater shear wave splitting, and is more electrically conductive than the Nazca (east) side. These results suggest asymmetric mantle flow and melt distribution beneath the EPR. To better understand the causes for these asymmetric properties, we construct numerical models of melting and mantle flow beneath a midocean ridge migrating to the west over a fixed mantle. Although the ridge is migrating to the west, the migration has little effect on the upwelling rates, requiring a separate mechanism to create the asymmetry. Models that produce asymmetric melting with a temperature anomaly require large (>100°C) excess temperatures and may not be consistent with the observed subsidence and crustal thickness. A possible mechanism for creating asymmetry without a temperature anomaly is across-axis asthenospheric flow, possibly driven by pressures created by upwelling beneath the Pacific Superswell to the west. Pressure-driven asthenospheric flow follows the base of the lithosphere, extending the upwelling region to the west as it follows the thinning lithosphere toward the axis, and shutting off melting as it crosses the axis and encounters an increasingly thick lithosphere to the east.
Asthenospheric Shear Controls Global Patterns of Intraplate Volcanism
2010
Convection in the Earth's mantle, manifested at the surface as plate tectonics, generates the majority of Earth's volcanism via rifting and subduction. Intraplate volcanism, occurring away from plate boundaries, has not been attributed to global-scale convection, but instead to a variety of localized processes such as upwelling plumes, lithospheric cracking, sub-lithospheric drips, and small-scale convection. Despite these proposed local controls, the mantle's interior heat ultimately drives both intraplate volcanism and global convection, which suggests a causal link. We pursue this link using a mantle flow model to estimate the magnitude of shearing deformation that occurs in the low-viscosity asthenosphere beneath the tectonic plates. We found, with high statistical confidence, that recent continental and oceanic intraplate volcanism preferentially occurs above asthenosphere experiencing abnormally rapid shear. Basaltic volcanism in western North America, eastern A...
Tectonophysics, 2017
Earth's continents drift in response to the force balance between mantle flow and plate tectonics and actively change the plate-mantle coupling. Thus, the patterns of continental drift provide relevant information on the coupled evolution of surface tectonics, mantle structure and dynamics. Here, we investigate rheological controls on such evolutions and use surface tectonic patterns to derive inferences on mantle viscosity structure on Earth. We employ global spherical models of mantle convection featuring self-consistently generated plate tectonics, which are used to compute time-evolving continental configurations for different mantle and lithosphere structures. Our results highlight the importance of the wavelength of mantle flow for continental configuration evolution. Too strong short-wavelength components complicate the aggregation of large continental clusters, while too stable very long wavelength flow tends to enforce compact supercontinent clustering without reasonable dispersal frequencies. Earth-like continental drift with episodic collisions and dispersals thus requires a viscosity structure that supports long-wavelength flow, but also allows for shorter-wavelength contributions. Such a criterion alone is a rather permissive constraint on internal structure, but it can be improved by considering continental-oceanic plate speed ratios and the toroidal-poloidal partitioning of plate motions. The best approximation of Earth's recent tectonic evolution is then achieved with an intermediate lithospheric yield stress and a viscosity structure in which oceanic plates are ∼10 3 × more viscous than the characteristic upper mantle, which itself is ∼100-200× less viscous than the lowermost mantle. Such a structure causes continents to move on average ∼(2.2 ± 1.0)× slower than oceanic plates, consistent with estimates from present-day and from plate reconstructions. This does not require a low viscosity asthenosphere globally extending below continental roots. However, this plate speed ratio may undergo strong fluctuations on timescales of several 100 Myr that may be linked to periods of enhanced continental collisions and are not yet captured by current tectonic reconstructions.
Observational hints for a plume-fed, suboceanic asthenosphere and its role in mantle convection
Journal of Geophysical Research, 1995
An asthenosphere layer which is entirely fed from below by plumes and which loses equal mass by accretion to the overlying oceanic lithosphere and at subduction zones may play a critical role in shaping the form of mantle convection. In this study we discuss geochemical, seismic, and geoid/depth evidence for lateral flow within this type of asthenosphere. In particular, we suggest that there are large-scale layered, horizontal flow structures that connect upward plume input beneath hotspots to near-ridge regions of increased asthenosphere accretion into the growing oceanic lithosphere. Lateral asthenosphere flow is also shaped by oceanic subducfion zones, with a partial return flow from trenches, and by deep continental roots that are migrating barriers to asthenosphere flow. This alternative paradigm offers relatively simple explanations for several puzzles about mantle convection, for example, the low mantle heat flow beneath continents. It also offers an explanation for why mid-ocean ridges appear to be passive features that migrate with little geochemical or morphological change with respect to the lower mantle and seem to be uncoupled from large-scale mantle flow, while in contrast, trenches appear to be strongly coupled to mantle-thick regions of fast (colder) seismic velocity anomalies.
Top-driven asymmetric mantle convection
in Foulger, G.R., Lustrino, M., and King, S.D., eds., The Interdisciplinary Earth: A Volume in Honor of Don L. Anderson: Geological Society of America Special Paper 514 and American Geophysical Union Special Publication 71,, 2015
The role of decoupling in the low-velocity zone is crucial for understanding plate tectonics and mantle convection. Mantle convection models fail to integrate plate kinematics and thermodynamics of the mantle. In a first gross estimate, we computed at >300 km3/yr the volume of the plates lost along subduction zones. Mass balance predicts that slabs are compensated by broad passive upwellings beneath oceans and continents, passively emerging at oceanic ridges and backarc basins. These may correspond to the broad low-wavespeed regions found in the upper mantle by tomography. However, west-directed slabs enter the mantle more than three times faster (~232 km3/yr) than in the opposite east- or northeast-directed subduction zones (~74 km3/yr). This difference is consist ent with the westward drift of the outer shell relative to the underlying mantle, which accounts for the steep dip of west-directed slabs, the asymmetry between flanks of oceanic ridges, and the directions of ridge migration. The larger recycling volumes along west-directed subduction zones imply asymmetric cooling of the underlying mantle and that there is an “easterly” directed component of the upwelling replacement mantle. In this model, mantle convection is tuned by polarized decoupling of the advecting and shearing upper boundary layer. Return mantle flow can result from passive volume balance rather than only by thermal buoyancy-driven upwelling.