COMPARISON OF TELESEISMIC AND MAGNETOTELLURIC RESPONSES ON THE 
SLAVE CRATON

ALAN G. JONES, David Snyder, Jessica Spratt (Geological Survey of Canada), 
Michael Bostock, Charly Bank (University of British Columbia), Alan Chave,
Rob Evans (Woods Hole Oceanographic Institution)

The Archean Slave craton in northwestern Canada is an ideal natural laboratory 
for investigating lithosphere formation and evolution processes, and has become 
an international focus of broad geoscientific investigation following the discovery 
of economic diamondiferous kimberlite pipes. In particular, teleseismic and
deep-probing magnetotelluric (MT) data acquired over the last 5 years are 
yielding valuable information on the craton's sub-continental lithospheric 
mantle (SCLM) that can be compared with geochemical information derived from 
analyses of mantle xenoliths.

Teleseismic studies from Autumn 1996 to Spring 1998 comprised ten stations 
distributed across the craton anchored by the Yellowknife Seismic Array.
P-wave tomography revealed that the central part of the craton has the fastest
velocities throughout the whole lithospheric mantle. Receiver function analyses
showed a relatively uniform crustal thickness, with somewhat thinner crust to the
northwest (<40 km) compared to the southeast (>40 km). Shear wave splitting
analyses shows a generally NE-SW fast split wave direction, but with perturbations
that may be taken to support a three-part zonation of the Slave's SCLM.
Further telesesimic measurements are currently taking place on the craton.

The MT studies conducted from Fall 1996 to Summer 2000 on four
different projects have virtually covered the Slave in its entirety with
measurements. The principle result from these surveys is the discovery of an
unexpected and remarkable anomaly in electrical conductivity, collocated 
with the kimberlite field, which is modeled as a spatially confined 
upper-mantle region of low resistivity (<30 ohm·m) at depths of 80-100+ km
within the SCLM. We currently interpret this anomaly as due to dissolved 
hydrogen or carbon in graphite form. This geophysically anomalous upper-mantle
 region is also spatially coincident with a geochemically defined ultradepleted
harzburgitic layer. The tectonic processes that emplaced this structure are possibly 
related to the lithospheric 
subduction and trapping of overlying oceanic mantle at 2630-2620 Ma. 

This paper will present a comparison of the teleseismic and magnetotelluric
results, demonstrating the advantages of combined interpretation.