UPPER-MANTLE STRATIGRAPHY BENEATH THE SOUTHERN SLAVE CRATON

A large teleseismic data set comprising 724 broadband, three-component P-wave seismograms has been compiled for the southern Slave craton wi th the objective of characterizing underlying mantle stratigraphy. Coh erent P to S wave conversions are identified by simultaneously deconvo lving seismograms as functions of epicentral distance and along theore tical moveout curves corresponding to possible mantle phases. Clear P( D)s conversions are observed from the 410 and 660 km discontinuities a t times that are only slightly faster than those predicted from the IA SP91 model, and over 1.0 s slower than corresponding times observed at other stations on the Canadian Shield and the south African Kaapvaal craton. The P(D)s times show very little azimuthal variation, implying an absence of major lateral velocity variations in the lithospheric m antle underlying the Slave craton, and adjacent Wopmay orogen and Talt son magmatic zone. Considered in light of other geophysical and geolog ical evidence, these results suggest that the root underlying the Slav e province has been modified along its margins and may remain intact o nly toward a central core. Another important result involves the obser vation of a P(D)s conversion from a negative velocity contrast interfa ce at approximately 360 km depth. It and a similar phase, observed on the Kaapvaal craton, would appear not to be directly related to tectos pheric structure, but may originate at the top of a layer containing a dense silicate partial melt just above the mantle transition zone.