UNUSUAL MANTLE POISSON'S RATIO, SUBDUCTION, AND CRUSTAL STRUCTURE IN CENTRAL ALASKA

Abstract

Subduction zones alter the upper plate in a variety of ways, including metasomatism via slab-derived fluids and accretion of exotic terTanes. 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 confmn 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 slab deeper,