Mantle stratigraphy and evolution of the Slave province

Abstract

A data set of 1033 three‐component, P wave seismograms from five broadband stations at the Yellowknife Array is assembled to investigate mantle structure below the southern Slave province in Canada's Northwest Territories. Following wave field decomposition, seismograms are source‐normalized through simultaneous deconvolution to estimate the near‐receiver impulse response as a function of epicentral distance and back azimuth. Images of impulse response reveal a well‐developed mantle stratigraphy, anisotropic in part, extending from the Mohorovicic discontinuity to the transition zone, A layer of depth‐localized anisotropy (±5%), termed H, is situated between ∼70 and 80 km depth with an average shear velocity comparable to that of the ambient mantle and a sharp upper boundary less than 100 m in transition width. The absence of free surface crustal reverberations on the transverse component affords a window into the upper mantle between 100 and 200 km depth. A sequence of at least two layers between 120–150 km depth, collectively termed X, is most clearly evident to the north and is underlain by a second structure L which dips from 170 km in the west to 230 km into the center of the Slave province. The deepest interface above the transition zone W marks a shear velocity inversion near 350 km depth whose signature is restricted to the SV component signalling a dominantly isotropic response. Consideration of these observations in light of data acquired in a recent LITHOPROBE seismic reflection traverse and in petrological studies of kimberlite xenoliths prompts speculation into the role of subduction in craton stabilization. It is suggested that the proto‐Slave craton was assembled through processes of shallow subduction resulting in a near‐horizontal mantle stratigraphy (i.e., H, X) both compositional and rheological in nature. Interpretation of L as the continuation of dipping reflectors on the seismic reflection profile argues for a final phase of craton assembly involving oblique underplating of subducted lithosphere in the Proterozoic. Subsequent modification of the lithosphere, as manifest by Phanerozoic kimberlite volcanism, may be related to W if an interpretation as the top of a layer containing a dense silicate melt fraction is invoked.