Subduction zones alter the upper plate in a variety of ways, including metasomatism via slab-derived fluids and accretion of exotic terranes. These processes should produce distinctive seismic signatures, as seismic velocities are sensitive to compositional variations. A new method is developed to estimates depths to interfaces and Poisson's ratio (Vp/Vs ratio) for dipping layered structures, by stacking teleseismic receiver functions. It also estimates crustal Vp/Vs and thickness. Using this method, we analyze a broadband data set from the central Alaska subduction zone, to seek evidence for mantle wedge composition. Crustal thickness varies from 27 to 45 km, and crustal Vp/Vs indicates an intermediate to mafic composition, expected for these accreted island arc terranes. Inversions for mantle structure confirm the presence of a 15--20 km thick low-velocity zone atop of the downgoing plate to 130 km depth, perhaps subducted crust of an exotic terrane. Vp/Vs in the mantle wedge allows for 15 ¿ 15% serpentinization where the slab is <80 km deep and the wedge is cold. Where the slab is deeper, Vp/Vs for the mantle wedge is unusually low, <1.7. This value is lower than predicted for any common mantle mineral and may indicate significant quantities of quartz, provided that past physical properties measurements are not grossly in error. These results suggest that the mantle wedge in subduction zones can occasionally differ substantially in bulk composition from normal subcrustal mantle, perhaps because collisions of exotic terranes cause tectonic mixing of crustal material into the mantle.