The rheological law for the power law creep field of peridotite suggested by Chopra and Paterson (1981, 1984) is combined with theoretically modelled thermal and stress perturbations for continent-continent collisions (Bird et al., 1975; Bird, 1978a) to determine the effect of such collisions on the uppermost mantle below the zone of convergence. The model predicts a change from continental to oceanic geotherms during the oceanic subduction phase, followed by a rapid rise in differential stress during the collisional phase. These perturbations, when combined, cause a cyclical change in the steady state flow of the uppermost mantle ending with a re-equilibration of thermal and stress regimes. The thermal re-equilibration proceeds at a much slower rate, with a suggested total re-equilibration period of about 100 m.y. after collision has ceased. The combination of the flow law with the model predicts that the perturbations in the mantle are limited in depth with no changes in mantle behavior below about 100 km.