Structure of the upper mantle below soouth-central Saskatchewan from teleseismic travel-time inversion

An array of three-component seismographs was deployed over the central Trans-Hudson Orogen from October 1994 to July 1996 with the objective of characterizing subcrustal lithospheric structure below an area with diamondiferous kimberlite occurance. The twodimensional array consisted of nine broad-band and eight short-period stations at approximately 100 km spacing. Most stations were situated on the Phanerozoic cover of southern Saskatchewan and located above the Glennie domain and adjacent domains as inferred from aeromagnetic anomalies. Additional data came from a feasibility test conducted in the fall of 1991 and from one permanent station run by the Incorporated Research Institutions for Seismology (IRIS). Relative travel-time residuals for 331 teleseismic events were obtained and inverted for slowness variations using a tomography code by VanDecar (1991). The ray coverage allows for depth resolution from 60 km to 400 km below the array. Our results show heterogeneities in mantle slowness above 220 to 270 km that deviate by ±2% from the iasp91 radial Earth model. The maximum slowness anomaly is a quasi-cylindrical vertical body of 120 km diameter having its maximum at (52.8°N, 105.9°W, 100km depth) and extending from 60 km depth to 220 km depth. The minimum slowness anomaly is kidney shaped and has its peak value at (51.8°N, 103.9°W, 170km depth). Cretaceous diamondiferous kimberlites and high concentrations of kimberlitic minerals in glacial tills occur above or near the rims of the low velocity anomalies. These are interpreted as remnants of a mantle plume conduit or Rayleigh-Taylor-like instability in the uppermost mantle. The correlation of the fast anomaly with a long-wavelength gravity low suggests a lithospheric keel, consistent with the presence of the proposed Saskatchewan craton. In the south, a correlation between low mantle velocities and high heat flow corroborates the interpretation of the low velocity anomalies as being primarily thermal in origin. Low levels of heterogeneity below 220 to 270 km depth are interpreted as being representative of the convecting asthenosphere. Our preferred interpretation of the imaged anomalies is the thermomechanical erosion of the lithospheric keel of the Saskatchewan craton during the Cretaceous. Diamondiferous kimberlites are a direct consequence of this process. Material from deeper levels of the mantle now trapped in the lithosphere explains the low velocity anomalies, whereas the remnants of the lithospheric keel give rise to high velocity anomalies. Our analyses indicate a lithospheric thickness of 220 to about 270 km below the Trans-Hudson Orogen.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat