The Snake River Plain experiment revisited. Relationships between a Farallon plate fragment and the transition zone

We reconsider the analysis of the 93 PASSCAL-OREGON SRP experiment data. Th e techniques applied to the processing of Pds waves, which are P to S conve rsions at seismic discontinuities at depth d in the receiver region, differ from those employed by Dueker & Sheehan [1997]. We perform global move-out corrections and migration, taking advantage of the alignment of seismicity along the great circle described by the array (South America, and the Kuri l and Aleutians islands). We do not detect a deep signature of a mantle plu me. The 410 km discontinuity exhibits a fragmented aspect, which prevents u s from reliably estimating the transition zone thickness. The 660 km discon tinuity is clearly more visible, and largely deflected in a large part of t he profile. We associate this anomalous behaviour and the noisy aspect of t he discontinuities with the presence of a Farallon plate fragment.