As I understand it, the outer portion of the Earth can be divided into the crust and mantle on chemical grounds (separated by the Moho), or into the lithosphere and asthenosphere based on mechanical properties. The lithosphere is relatively strong and includes the crust and the upper area of the mantle. The asthenosphere below is relatively weak and deforms by diffusion creep due to the high temperatures, though the lithosphere/asthenosphere boundary is gradual (since it is temperature dependent).
However, isostatic compensation for mountain ranges on continents etc. requires the mantle directly below the crust to flow. However, this is part of the lithosphere and is relatively strong - I would expect that isostatic compensation occurs for the lithosphere with respect to the asthenosphere. Is it the case that although relatively strong, this area of the lithosphere (the mantle just below the crust) is still sufficiently plastic to allow isostatic compensation of the crust?