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 with the objective of characterizing underlying mantle stratigraphy. Coherent P to S wave conversions are identified by simultaneously deconvolving seismograms as functions of epicentral distance and along theoretical moveout curves corresponding to possible mantle phases. Clear P(D)s conversions are observed from the 410 and 660 km discontinuities at times that are only slightly faster than those predicted from the IASP91 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 mantle underlying the Slave craton, and adjacent Wopmay orogen and Taltson magmatic zone. Considered in light of other geophysical and geological evidence, these results suggest that the root underlying the Slave province has been modified along its margins and may remain intact only toward a central core. Another important result involves the observation of a P(D)s conversion from a negative velocity contrast interface at approximately 360 km depth. It and a similar phase, observed on the Kaapvaal craton, would appear not to be directly related to tectospheric structure, but may originate at the top of a layer containing a dense silicate partial melt just above the mantle transition zone.