Snake river plain experiment revisited. Relationships between a Farallon plate fragment and the transition zone

摘要

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