Fully two-dimensional analytic boundary layer solutions are used to model the thermodmechanical structure of the oceanic upper mantle when a shallow horizontal return flow helps balance the lithospheric transport of mass from ridge to trench. The following are all incorporated in the solutions: horizontal and vertical advection of heat, vertical heat conduction, viscous dissipation, adiabatic heating and cooling, buoyancy, and the pressure- and temperature-dependent nonlinear rheology of olivine. Depth profiles of horizontal and vertical velocities, temperature, and shear stress are calculated for several ages of ocean floor. Such solutions are used to construct accurate isotherm and streamline patterns within the rigid lithosphere and high-shear, return flow asthenosphere of the oceanic upper mantle boundary layer. Ocean floor topography is inferred from the thermal contraction of the cooling lithosphere and asthenosphere and from the adverse horizontal pressure gradient required by the dynamics to drive the shallow return flow. The latter effect would be dominant if a shallow return flow did occur in the earth's upper mantle. Solutions which provide adequate fits to the ocean floor bathymetry data for the Pacific plate can be found if the activation volume for the creep of olivine is small, i.e., about 11 cm3/mol, and if the deep mantle temperature T is high, i.e., if T is about 1500¿C at depths of 100--200 km (depending on age) for dry olivine or about 1400¿C at similar depths for wet olivine. The upper mantle temperatures required for a shallow return flow to be compatible with bathymetry data imply partial melting in the lower lithosphere and upper asthenosphere from the ridge axis to ages of about 10 m.y. Shear stresses at the base of the rigid lithosphere generally range from about 1 to several tens of bars, and horizontal adverse pressure gradients at great depth vary from about 10 to 100 mbar/km for wide variations in the extent of return flow and deep mantle temperature. For a relatively cold deep mantle, shear stresses in the lower lithosphere and asthenosphere decrease almost linearly with depth, implying horizontal pressure gradients essentially constant with depth. For a hot deep mantle, shear stress versus depth profiles show curvature in the lower lithosphere and upper asthenosphere and a linearly decreasing pattern at greater depths. Under such circumstances, horizontal pressure gradients exhibit considerable depth variation including the possibility of a reversal in sign between the top and the bottom of the asthenosphere.