The thermal regimes of continent-continent convergence zones are modelled by a finite difference technique, assuming that there is some subduction of continental crust. Gravity and heat flow profiles are generated from the thermal models. Subducted crust and slab remain cool except at the upper surface where frictional heating is important. Crustal rocks may be metamorphosed or melted by friction while the plates are converging or by radioactive self-heating more than 30 m.y. after the plates have stopped. In the former case, melting requires frictional shear stresses between 500 and 2000 bars at a plate velocity of 5 cm/yr. At lower velocities in the upper limit of frictional stress is greater. The total work performed in continental subduction may be minimized if the angle of underthrusting becomes more shallow, changing the location of subduction. A model for the geometry of oceanic and continental slabs in the Zagros Mountains is presented which satisfies gravity, heat flow, seismic, and geologic constraints. Continental underthrusting is beginning along a new fracture at the edge of the Persian Gulf, isolating the Arabian continental shelf from the subduction process. Slippage along this fault is Pleistocene and probably does not exceed 30 km. Subduction of continental crust at the crush zone is probably insignificant.