The thickness of Earth's mechanical lithosphere is poorly defined. To investigate whether rheology controls the thickness of the overriding plate's mechanical lithosphere in subduction zones, the thermal structure was modelled numerically assuming a temperature dependent mantle viscosity. It was found that the overriding lithosphere was ablated such that very high temperatures reached close to the surface near the apex of the wedge corner, leading to unrealistically high heat flow. Since temperature dependent rheology clearly does not control the thickness of the mechanical lithosphere, we suggest that it is instead controlled by buoyancy. The source of buoyancy we assume is compositional, e.g., buoyant crust. Two end-member models with crustal thickness of 10 and 70 km respectively were then undertaken, these had lower heat flow. This work supports the assumption of some earlier workers (e.g. Plank and Langmuir, 1988) who equated the mechanical lithosphere with the crust of the overriding plate. ¿ 1999 American Geophysical Union