A RELIC PROTEROZOIC SUBDUCTION ZONE IN WESTERN CANADA - NEW EVIDENCE FROM SEISMIC-REFLECTION AND RECEIVER FUNCTION DATA

Lithoprobe seismic-reflection profiles in the Western Canada Sedimenta ry Basin reveal a prominent band of dipping reflections that appear to extend into the upper mantle, The NNE strike of these reflections, de termined from intersecting profiles, is parallel to the trend of the c a. 1.8 Ga Rimbey domain, an overlying magmatic belt. Receiver-function analysis of teleseismic data recorded at a nearby site shows evidence for a low-velocity zone (LVZ) beneath the Rimbey domain between 13 an d 16 km depth. The unusually low S-wave velocity of the LVZ (2.9 +/- 0 .2 km/s) suggests a composition similar to serpentinized peridotites t hat occur in ophiolites. Taken together, our data support a model of P roterozoic subduction beneath the Hearne province. We interpret the di pping reflections observed on the seismic profiles as a shear zone tha t developed during subduction of a SE-dipping slab, and the LVZ as a f ragment of oceanic lithosphere that was obducted to the overriding pla te.