Back to Search Start Over

The effect of a shallow low viscosity zone on mantle convection and its expression at the surface of the earth

Authors :
Barry Parsons.These calculations also predict an asymptotic heat flow on old ocean floor which is higher than the plate model and between 50 and 55 mW/m2 . This value agrees with measurements of heat flow on old seafloor in the Atlantic. In conclusion, we prefer an approximate model for the viscosity structure of the upper mantle which initially has a 125 km thick low viscosity zone that represents a viscosity contrast of two orders of magnitude. The viscosity contrast decreases as the plate ages to one order of magnitude or less by 130 m.y., and the low viscosity zone may also thicken with age. Finally, the Rayleigh number of the upper mantle is at least 105 and may be as large as 107 . With this model, the evolution of the surface plates would initially involve small scale convection which is driven by shear coupling to instabilities downstream and to small scale convection associated with fracture zones. This convective flow would begin at close to 5 m.y. and remain confined to the low viscosity zone until nearly 40 m.y.. As this convective flow cools the upper mantle beneath the low viscosity zone, longer wavelength convection begins throughout the upper (or whole) mantle, and the heat transport from the longer wavelength convection flattens the depth-age curve and may form swells.
Woods Hole Oceanographic Institution.
Joint Program in Marine Geology and Geophysics.
Massachusetts Institute of Technology. Dept. of Earth, Atmospheric, and Planetary Sciences.
Robinson, Elizabeth M
Barry Parsons.These calculations also predict an asymptotic heat flow on old ocean floor which is higher than the plate model and between 50 and 55 mW/m2 . This value agrees with measurements of heat flow on old seafloor in the Atlantic. In conclusion, we prefer an approximate model for the viscosity structure of the upper mantle which initially has a 125 km thick low viscosity zone that represents a viscosity contrast of two orders of magnitude. The viscosity contrast decreases as the plate ages to one order of magnitude or less by 130 m.y., and the low viscosity zone may also thicken with age. Finally, the Rayleigh number of the upper mantle is at least 105 and may be as large as 107 . With this model, the evolution of the surface plates would initially involve small scale convection which is driven by shear coupling to instabilities downstream and to small scale convection associated with fracture zones. This convective flow would begin at close to 5 m.y. and remain confined to the low viscosity zone until nearly 40 m.y.. As this convective flow cools the upper mantle beneath the low viscosity zone, longer wavelength convection begins throughout the upper (or whole) mantle, and the heat transport from the longer wavelength convection flattens the depth-age curve and may form swells.
Woods Hole Oceanographic Institution.
Joint Program in Marine Geology and Geophysics.
Massachusetts Institute of Technology. Dept. of Earth, Atmospheric, and Planetary Sciences.
Robinson, Elizabeth M
Publication Year :
2010

Abstract

Thesis (Ph. D.)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1987.<br />Includes bibliographical references (v.2, leaves 309-317).<br />Many features of the oceanic plates cannot be explained by conductive cooling with age. A number of these anomalies require additional convective thermal sources at depths below the plate: mid-plate swells, the evolution of fracture zones, the mean depth and heat flow relationships with age and the observation of small scale (150-250 km) geoid and topography anomalies in the Central Pacific and Indian oceans. Convective models are presented of the formation and evolution of these features. In particular, the effect of a shallow low viscosity layer in the uppermost mantle on mantle flow and its geoid, topography, gravity and heat flow expression is explored. A simple numerical model is employed of convection in a fluid which has a low viscosity layer lying between a rigid bed and a constant viscosity region. Finite element calculations have been used to determine the effects of (1) the viscosity contrast between the two fluid layers, (2) the thickness of the low viscosity zone, (3) the thickness of the conducting lid, and (4) the Rayleigh number of the fluid based on the viscosity of the lower layer. A model simple for mid-plate swells is that they are the surface expression of a convection cell driven by a heat flux from below. The low viscosity zone causes the top boundary layer of the convection cell to thin and, at high viscosity contrasts and Rayleigh numbers, it can cause the boundary layer to go unstable. The low viscosity zone also mitigates the transmission of normal stress to the conducting lid so that the topography and geoid anomalies decrease. The geoid anomaly decreases faster than the topography anomaly, however, so that the depth of compensation can appear to be well within the conducting lid. Because the boundary layer is thinned, the elastic plate thickness also decreases and, since the low viscosity allows the fluid to flow faster in the top layer, the uplift time decreases as well. We have compared the results of this modeling to data at the Hawai<br />by Elizabeth M. Robinson.<br />Ph.D.

Details

Database :
OAIster
Notes :
2 v. (320 leaves), application/pdf, English
Publication Type :
Electronic Resource
Accession number :
edsoai.on1141647324
Document Type :
Electronic Resource

Tools

Authors Abstract Subjects Details