Seismic refraction data from mature oceanic regions show remarkable consistency in crustal thickness between sites, yet the characteristics of different spreading centers show great variation. This leads to the paradox that the similarity in mature oceanic crust suggests that the igneous processes in crustal accretion are the same across a range of spreading rates but that the differences in the spreading centers themselves imply the opposite. We have developed a simple two dimensional model for crustal accretion at oceanic spreading centers by the episodic addition of sills at a high level within the crust, as implied by recent geophysical studies of spreading centers. Using our model, we have calculated velocity and temperature fields in the crust, including the effects of hydrothermal cooling. We show that oscillations in the temperature field due to episodicities of 200--20000 years have an effect localized to the region within 50--500 m of the intrusion. By assuming that all latent heat and excess specific heat introduced by the injection of sills is convected away by hydrothermal circulation, we estimate that the maximum heat transfer associated with hydrothermal cooling in the upper crust is of the order of 10 times greater than the conductive flux. We have calculated seismic velocities associated with the model temperature field in the crust, and we show that these are consistent with results from both seismic refraction and reflection experiments. The model may be applied to both fast and slow spreading ridges, and it accounts for the apparent increases in both the mean crustal temperature and the frequency of presence of melt with increasing spreading rate, hence resolving the paradox. We also show that the implications of this model are consistent with geological observations of layering in ophiolites. ¿ American Geophysical Union 1993