Abstract
Convection on a scale smaller than the horizontal dimensions of lithospheric plates can be produced by instabilities in the upper or lower boundary layer of the larger mantle flow. Small-scale convection associated with the upper boundary layer should pro duce a detectable gravity signal and affect the rate of cooling and subsidence of the lithosphere. I describe here a numerical model of small-scale convection in a fluid of variable viscosity. The results indicate that recently observed gravity anomalies1 showing a pat tern of highs and lows aligned in the direction of oceanic plate motion may be the result of small-scale mantle flow. The convective flow must begin in the first 6 Myr of lithospheric cooling to produce the observed signals, which is not inconsistent with constraints on the viscosity of the mantle. The calculated trend for the subsidence of the ocean floor is found to be almost linear with (time)1/2 even when small-scale convection has significantly changed the rate of subsidence. For average shallow asthenospheric viscosities of ∼1018 Pa s, the model subsidence can match data for the oceans2,3 and reproduce the magnitude and wavelength of the observed gravity anomalies.
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Buck, W. When does small-scale convection begin beneath oceanic lithosphere?. Nature 313, 775–777 (1985). https://doi.org/10.1038/313775a0
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DOI: https://doi.org/10.1038/313775a0
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