MANTLE STRATIGRAPHY AND EVOLUTION OF THE SLAVE PROVINCE

A data set of 1033 three-component, P wave seismograms from five broad band stations at the Yellowknife Array is assembled to investigate man tle structure below the southern Slave province in Canada's Northwest Territories. Following wave field decomposition, seismograms are sourc e-normalized through simultaneous deconvolution to estimate the near-r eceiver impulse response as a function of epicentral distance and back azimuth. Images of impulse response reveal a well-developed mantle st ratigraphy, anisotropic in part, extending from the Mohorovicic discon tinuity to the transition zone. A layer of depth-localized anisotropy (+/-5%), termed H, is situated between similar to 70 and 80 lan depth with an average shear velocity comparable to that of the ambient mantl e and a sharp upper boundary less than 100 m in transition width. The absence of free surface crustal reverberations on the transverse compo nent affords a window into the upper mantle between 100 and 200 km dep th. A sequence of at least two layers between 120-150 km depth, collec tively 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 dominan tly isotropic response. Consideration of these observations in light o f data acquired in a recent LITHOPROBE seismic reflection traverse and in petrological studies of kimberlite xenoliths prompts speculation i nto the role of subduction in craton stabilization. It is suggested th at the proto-Slave craton was assembled through processes of shallow s ubduction 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 pr ofile argues for a final phase of craton assembly involving oblique un derplating of subducted lithosphere in the Proterozoic. Subsequent mod ification of the lithosphere, as manifest by Phanerozoic kimberlite vo lcanism, may be related to W if an interpretation as the top bf a laye r containing a dense silicate melt fraction is invoked.