Many small backarc basins show subsidence rates significantly greater than those in major ocean basins of similar ages. In this study we test, by numerical modeling, the hypothesis that small backarc basins are formed by the same fundamental processes as major oceanic basins, i.e., by accretion, spreading and cooling, and that the rate of subsidence is modified and accelerated by lateral heat loss from the hot, subbasinal asthenosphere to the surrounding cooler continental ligthosphere. Bathymetric, seismic, and drilling data in the Marsili Basin, a small and young backarc basin in the central Mediterranean, are used to constrain the models. The results of this analysis confirm the above hypothesis. They further suggest that for actively spreading backarc basins, the most rapid subsidence occurs in a zone near the margin of the basin during the earliest stage of basin spreading, whereas after spreading terminates the location for rapid subsidence migrates towards the basin center. ¿American Geophysical Union 1991