In-situ wafer curvature measurements were performed to determine the mechanical stress in thermally grown SiO2 films on Si during 4 MeV Xe ion irradiation at various temperatures in the range from 90 to 575 K. Radiation induced viscous flow is observed and the radiation induced viscosity is determined at various temperatures. It ranges from 2.9 x 1023 at Pa ion/cm2 90-300 K to 1.6X 1023 Pa ion/cm2 at 500 K. Both its magnitude and temperature dependence can be explained in terms of a phenomenological model in which stress relaxation takes place in locally heated, mesoscopic regions of low viscosity, centered around individual ion tracks. According to this model, stress relaxation occurs in ~ 10 ps and within ~ 3 nm of the ion track.