An array of seismographs was deployed over the central Trans-Hudson Orogen from July 1991 to January 1992 and from October 1994 to July 1996 with the objective of characterizing subcrustal lithospheric structure in a region of diamondiferous kimberlite occurrence using tomographic imaging techniques. The two-dimensional array was located in south central Saskatchewan and consisted of 17 stations with an average spacing of 100 km. We obtained relative travel time residuals for 321 teleseismic events and inverted them for subcrustal velocity variations. The ray coverage affords resolution from 60 to 400 km depth. Our results reveal heterogeneities in mantle velocity that deviate by up to ¿1.5% from the iasp91 Earth model. The most pronounced low-velocity anomaly is quasi-cylindrical, 120 km in diameter and extends to ~ 220 km depth. This feature is partly surrounded by a region of high velocity which penetrates to slightly greater depths. Cretaceous diamondiferous kimberlites and high concentrations of kimberlitic minerals in glacial tills occur above or near the rims of the low velocity anomalies. In addition, correlations exist between a long-wavelength gravity low and the high-velocity region, as well as between high heat flow and low mantle velocities in the southern portions of the study area. Taken together, these observations are consistent with the interpretation of the imaged anomalies as due to thermomechanical erosion of the lithospheric keel of the Sask craton during the Cretaceous by plume activity or Rayleigh-Taylor-like instability within the asthenosphere. The diamondiferous kimberlites are viewed as a direct consequence of this process. Low levels of heterogeneity below 250 km depth are interpreted to be indicative of effective homogenization in a convecting asthenosphere. ¿ 1998 American Geophysical Union