Results from the 1994 Aleutian Seismic experiment delineate basic oceanic arc crustal architecture, constrain magmatic flux rates and bulk arc composition, and address questions of continental crustal genesis. Here we present results from a transect across protocontinental crust of the westernmost Alaska Peninsula (line A3) and compare this structure to a purely oceanic arc transect farther west. Arc crustal structure is similar along these two transects. Magmatic accretion occurs at the top and bottom of preexisting oceanic crust as a 5- to 10-km-thick upper crustal carapace of low velocity (2--5.8 km s-1) volcaniclastics, flows and small plutons, and a mafic lower crustal underplate (~7.0 km s-1) of variable thickness, for a maximum arc crust thickness of ~25--30 km. Lateral lower crustal velocity gradients and high velocities (>7.5 km s-1) beneath the forearc suggest dominantly vertical lower crustal accretion above a focused melt source and a forearc underlain by little magmatic crust but rather partially intruded and/or serpentinized mantle. The ratio of upper to lower crustal volume is ~1, and the total arc crust volume implies a magmatic flux of ~67 km3 km-1 m.y.-1, more than twice previous estimates for this arc and global productivity. The crust is thinner and more mafic than continental crust, and it lacks a massive tonalitic upper crust characteristic of the continents. An interpreted accumulation of upper crustal carapace material at midcrustal depths on line A3 has a velocity of ~6.4 km s-1, suggesting an intermediate composition. Accretionary complex terranes consisting of accumulations of this type material would thus have bulk compositions similar to continental crust.