Open AccessThe effect of a shallow low‐viscosity zone on the mantle flow, the geoid anomalies and the geoid and depth‐age relationships at fracture zones
Author(s) -
Robinson Elizabeth M.,
Parsons Barry,
Driscoll Mavis
Publication year - 1988
Publication title -
geophysical journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0952-4592
DOI - 10.1111/j.1365-246x.1988.tb01386.x
Subject(s) - geoid , geology , geophysics , viscosity , mantle (geology) , mechanics , physics , thermodynamics , measured depth
SUMMARY Using a 2‐D finite element method, we studied the flow that is driven by the horizontal temperature gradient at a fracture zone and calculated the resulting geoid and topography anomalies. Assuming a three‐layered viscosity structure for the upper mantle consisting of a conducting lid overlying a low‐viscosity channel which in turn overlies a unit‐viscosity layer extending to the base of the upper mantle, we studied the effect of varying (1) the viscosity contrast between the fluid layers, (2) the Rayleigh number based on the viscosity of the bottom layer, and (3) the thickness of the low‐viscosity channel. The flow at first downwells immediately adjacent to the fracture zone on the older side. Its time scale and characteristic wavelength depend on the viscosity and depth of the top layer, which is nearest the largest temperature gradient in the fluid. After the initiation of the driven flow, additional flows are generated away from the fracture zone through shear and thermal coupling and boundary‐layer instabilities. Eventually, however, the flow extends throughout the fluid, so that the time scales and the characteristic wavelengths of the flow depend on the thickness and viscosity of both layers. When the Rayleigh number that is based on the viscosity of the top layer and the depth of both fluid layers is less than 10 6 , the geoid anomalies are dominated by the convective signal. When this Rayleigh number exceeds 10 6 , the geoid anomalies retain a step across the fracture zone out to large ages. We compared our results with geoid anomalies over the Udintsev fracture zone and found that the predicted geoid anomalies are in good agreement with the observed anomalies when the viscosity of (at least) the top layer is greater than one order of magnitude less than post‐glacial rebound values (<10 20 Pas). However, we also compared the calculated topographic steps with those predicted by the average depth‐age relationships observed in the oceans and found that a viscosity contrast of at least one order of magnitude at a depth in the uppermost mantle is required to produce realistic topographic steps.