We present structural data from several underthrust sequences in southwest Alaska that suggest that the deformation regime beneath accretionary prisms is characterized by a subvertical &sgr;1 and regionally extensive layer-parallel shearing. These results constrast markedly with the results obtained from the more shallow parts of accretionary prisms and fold and thrust belts where &sgr;1 plunges gently seaward or toward the deformation front. On the basis of Mohr circle diagrams, we propose that these different stress directions are consistent only with a model in which the two regimes are separated by a very weak d¿collement and calculated coefficient of friction values that range from 0.25 to 0.08. These values are significantly lower than coefficient of friction values calculated for the basal d¿collement in the Taiwan fold and thrust belt (e.g.,¿0.2 (Barr and Dahlen, 1989a)) and they are also lower than laboratory friction measurements for most rocks (e.g., 0.6--0.8 (Byerlee, 1978)). Consequently, these low values suggest the presence of rocks with unusual material properties (e.g., montmorillonite (Logan and Rauenzahn, 1987)) or anomalous fluid pressures along the d¿collement. On the basis of the occurrence of shale diapirs, the distribution of early veins, and the absence of montmorillonite in the underthrust sequences, we propose that there was an anomalously high fluid pressure along the d¿collement fault instead of there being such a low coefficient of friction. These results emphasize the importance of dewatering and fluid flow in controlling the position of the d¿collement, which in turn determines how the accretionary prism grows. In essence, it is the development of a weak, overpressured d¿collement within a thick sediment pile that allows plate convergence to be partitioned into offscraping and underthrusting regimes. ¿American Geophysics Union 1990