Studies of free oscillations and body wave travel times have established that the one-way vertical S wave travel times through the upper mantle average about 5 s greater for oceans than for stable continental regions. This datum and the pure path dispersion data make it necessary to invoke significant differences between continental and oceanic shear velocity structures to depths exceeding 400 km. An analysis of the constraints on thermal profiles, including the constraints recently established by empirical geothermometry, indicates that temperature differences between the ancient shields and the ocean basins also extend deep into the mantle, with shields characterized by thermal gradients that are superadiabatic above 400 km. To at least 400-km depth beneath shields the dominant mechanism of heat transport is conduction, not advection. It is concluded that this entire region, termed here the continental tectosphere, translates coherently during lateral plate motions. It is proposed that the superadiabatic vertical temperature gradients beneath shields and the horizontal temperature gradients within the transition from shield to oceanic structures are stabilized by compositional gradients, the subcontinental mantle consisting of intrinsically less dense material. The existence of deep continental structure is apparently consistent with the dynamics of plate motions. It suggests that convection is not confined to the earth's upper mantle.
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