On the possible role of chemical boundary layers in regulating the thermal thickness of continents and oceans
Abstract
One of the most important observations made during the early developments of plate tectonic theory was that the depth of the seafloor initially (for the first ~70 Ma) increases with the square root of the crust's age and that the accompanying heat flow decreases as the inverse of the square root of age. It was subsequently shown that the sqrt(t) relationships could be explained by approximating the growth of the upper thermal boundary layer (TBL: the boundary layer over which the mode of heat transfer changes from advective to conductive) of the Earth's convecting interior by an infinite half-space conductive cooling model. This model, which we term the boundary layer model, predicts that the thickness of the TBL increases monotonically with the square root of seafloor age according to the relationship, L = sqrt(4kt), where L is the thermal thickness, t is time, and k is thermal diffusivity. For a thermal diffusivity of ~30 km2/Ma, this relationship takes the form, 11 sqrt(t), where L is in km and t is in Ma. By accounting for thermal contraction and the decrease in thermal gradient across the growing TBL, the evolution of seafloor depth and heat flow with time follow accordingly. The success of the boundary layer model in linking the kinematics of seafloor spreading to the depth and heatflow of <70 Ma old lithosphere represents the strongest evidence so far that plate tectonics and convection are linked. However, the boundary layer model breaks down at 70 Ma after which the heat flow and seafloor depth saturate at constant values. At the same time, it appears from seismic studies that the TBL thickness also saturates at a maximum value of 90-100 km. Many models have been proposed to explain the discrepancy between the predictions of the boundary layer model and the geologic features of post-70 Ma lithosphere. Most of these models, however, are difficult to test. Here, we present a testable model that explains the evolution of oceanic TBLs by invoking a pre-existing chemical boundary layer (CBL). We base this hypothesis on a growing understanding of the deep thermal and compositional structure of continents. Continents are underlain by a thick melt-depleted and dehydrated mantle layer, the former resulting in buoyancy and the latter resulting in increased viscosity. Radiogenic isotopic studies indicate that these CBLs do not significantly deform over billion year timescales, implying that on the timescales of mantle convection, such CBLs act as rigid lids resting on top of and separated from the convecting mantle. For this reason, the upper TBL of the convecting mantle therefore consists of a purely conductive layer (represented by the rigid CBL) and a convective sub-layer (CS-L), which lies just beneath the CBL and represents the actively convecting part of the TBL. We show using petrologic and geodynamic arguments that the thickness of the CBL beneath continents may limit the thickness of the convective sublayer and accordingly, the thickness of continental TBLs. Petrologic observations require that the seafloor also be underlain by a melt-depleted and dehydrated mantle layer, albeit thinner than that beneath continents. The base of this layer roughly coincides with the thermal thickness at which the boundary layer model breaks down. By analogy with our continental studies, we suggest that the presence of a CBL beneath oceanic crust may also be responsible for maintaining a constant thickness of oceanic TBLs beyond ~70 Ma. A future test of this hypothesis would be to seismically map whether there exists a crossover between the CBL and TBL beneath oceans and, if so, whether the crossover occurs at 70 Ma.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.T21B..04L
- Keywords:
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- 3640 Igneous petrology;
- 3670 Minor and trace element composition;
- 8102 Continental contractional orogenic belts;
- 8120 Dynamics of lithosphere and mantle: general