The existence of a thin low-viscosity layer beneath the lithosphere

Craig, Claire Harvey; McKenzie, Dan

doi:10.1016/0012-821x(86)90008-7

Cited by 99 publications

(59 citation statements)

References 13 publications

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“…McKenzie, 1984). The Although the above model is preferred because it is consistent with the results of other studies (Forsyth, 1977;Weilandt and Knopoff, 1982;McKenzie, 1984;Craig and McKenzie, 1986;Robinson et a1., 1987b;Robinson and Parsons, 1987), this study cannot uniquely constrain the Rayleigh number, low viscosity layer thickness or viscosity contrast, because these parameters often have similar effects on the observables. For example, the Rayleigh number could be greater than 10 6 .…”

Section: Discussionsupporting

confidence: 77%

“…Since the temperature gradient between lithosphere of differing age across the fracture zone will drive convection in a viscous Earth, the variability in the geoid anomalies and the geoid-age relationships at fracture zones may also be explained by the convective flow underneath them (Craig and McKenzie, 1986).…”

Section: Overviewmentioning

confidence: 99%

“…Third, Craig and McKenzie (1986) have studied convection at fracture zones, which is induced by the horizontal temperature gradient at the base of the oceanic lithosphere. This decoupling requires a region of the mantle such as a low viscosity zone whose physical properties mitigate the transmission of shear stress between the two flow regimes.…”

Section: This Thesismentioning

confidence: 99%

“…Much evidence suggests that a low viscosity zone exists beneath the surface plates from (1) theoretical calculations of the viscosity structure with depth in the oceanic mantle (Turcotte and Oxburgh, 1967;Parmentier, 1978;F1eitout and Yuen, 1984;Buck and Parmentier, 1986), (2) experimental results on the presence of melt in olivine and studies of melt production in the upper mantle (Cooper and Koh1stedt, 1984;McKenzie, 1984), (3) seismic studies of the uppermost oceanic mantle (Anderson and Sammis, 1970;Solomon, 1972;Forsyth, 1977;Wei1andt and Knopoff, 1982), and (4) studies of the evolution of the thermal structure at fracture zones (Craig and McKenzie, 1986;Robinson et al, 1987b, see Chapter 4). Chapter 2) showed that apparently shallow depths of compensation can result from convective processes in the mantle when a low viscosity zone exists in the uppermost mantle.…”

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confidence: 99%

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

Robinson¹

1987

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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 caiculations 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 Hawaii, Bermuda, Cape Verde and Marquesas swells, and have found that it can reproduce their 3 observed anomalies. The viscosity contrasts that are required range from 0.2-0.01, which are in agreement with other estimates of shallow viscosity variation in the upper mantle. Also, the estimated viscosity contrast decreases as the age of the swell increases. This trend is consistent with theoretical estimates of the variation of such a low viscosity zone with age.Fracture zones juxtapose segments of the oceanic plates of different ages and thermal structures. The flow induced by the horizontal temperature gradient at the fracture zone initially downwells immediately adjacent to the fracture zone on the older side, generating cells on either side of the plume. The time scale and characteristic wavelength of this flow depends initially on the viscosity near the largest temperature gradient in the fluid which, in our model, is the viscosity of the low viscosity layer. They therefore depend on .both the Rayleigh number and the viscosity contrast between the laye...

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Section: Discussionsupporting

confidence: 77%

Section: Overviewmentioning

confidence: 99%

Section: This Thesismentioning

confidence: 99%

mentioning

confidence: 99%

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

Robinson¹

1987

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“…The domain height (and hence asthenospheric depth) for the models presented in this section and section 5 is 120 km, which is probably an underestimate by at least a factor of 2 of the actual asthenospheric thickness [e.g., Craig and McKenzie, 1986]. The magnitude of Y shown in Figure 10 is small relative to that arising from thermal or compositional gradients, and it would even smaller for larger h A .…”

Section: −1mentioning

confidence: 82%

Porosity‐driven convection and asymmetry beneath mid‐ocean ridges

Katz

2010

Geochem Geophys Geosyst

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[1] Seismic tomography of the asthenosphere beneath mid-ocean ridges has produced images of wave speed and anisotropy that are asymmetric across the ridge axis. These features have been interpreted as resulting from an asymmetric distribution of upwelling and melting. Using computational models of coupled magma/mantle dynamics beneath mid-ocean ridges, I show that such asymmetry should be expected if buoyancy forces contribute to mantle upwelling beneath ridges. The sole source of buoyancy considered here is the dynamic retention of less dense magma within the pores of the mantle matrix. Through a scaling analysis and comparison with a suite of simulations, I derive a quantitative prediction of the contribution of such buoyancy to upwelling; this prediction of convective vigor is based on parameters that, for the Earth, can be constrained through natural observations and experiments. I show how the width of the melting region and the crustal thickness, as well as the susceptibility to asymmetric upwelling, are related to convective vigor. I consider three causes of symmetry breaking: gradients in mantle potential temperature and composition and ridge migration. I also report that in numerical experiments performed for this study, the fluid dynamical instability associated with porosity/shear band formation is not observed to occur.

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Water in Hawaiian peridotite minerals: A case for a dry metasomatized oceanic mantle lithosphere

Peslier

Bizimis

2015

Geochem Geophys Geosyst

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The distribution of water concentrations in the oceanic upper mantle has drastic influence on its melting, rheology, and electrical and thermal conductivities and yet is primarily known indirectly from analyses of OIB and MORB. Here, actual mantle samples, eight peridotite xenoliths from Salt Lake Crater (SLC) and one from Pali in Oahu in Hawaii were analyzed by FTIR. Water contents of orthopyroxene, clinopyroxene, and the highest measured in olivine are 116-222, 246-442, and 10-26 ppm weight H 2 O, respectively. Although pyroxene water contents correlate with indices of partial melting, they are too high to be explained by simple melting modeling. Mantle-melt interaction modeling reproduces best the SLC data. These peridotites represent depleted oceanic mantle older than the Pacific lithosphere that has been refertilized by nephelinite melts containing <5 weight % H 2 O. Metasomatism in the Hawaiian peridotites resulted in an apparent decoupling of water and LREE that can be reconciled via assimilation and fractional crystallization. Calculated bulk-rock water contents for SLC (50-96 ppm H 2 O) are on the low side of that of the MORB source (50-200 ppm H 2 O). Preceding metasomatism, the SLC peridotites must have been even drier, with a water content similar to that of the Pali peridotite (45 ppm H 2 O), a relatively unmetasomatized fragment of the Pacific lithosphere. Moreover, our data show that the oceanic mantle lithosphere above plumes is not necessarily enriched in water. Calculated viscosities using olivine water contents allow to estimate the depth of the lithosphere-asthenosphere boundary beneath Hawaii at 90 km.

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The existence of a thin low-viscosity layer beneath the lithosphere

Cited by 99 publications

References 13 publications

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

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

Porosity‐driven convection and asymmetry beneath mid‐ocean ridges

Water in Hawaiian peridotite minerals: A case for a dry metasomatized oceanic mantle lithosphere

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