LITHOSPHERIC STRUCTURE IN NORTHWESTERN CANADA FROM LITHOPROBE P- AND S-WAVE REFRACTION PROFILES: A SYNTHESIS

The LITHOPROBE refraction/wide-angle reflection (R/WAR) experiment included data acquisition along three corridors extending from the Archean Slave craton to the present Pacific basin in NW Canada. P- and S-wave velocity (Vp and Vs) models are combined to produce crustal cross sections of values of Poisson’s ratio (σ). The Vp, Vs and σ values are related to rock types through comparisons with laboratory data. σ values of 0.27 for eastern domains of the Paleoproterozoic Wopmay Orogen imply an accreted terrane of island arc or plateau affinity extending throughout the crust. In the western domains, values of σ drop to about 0.23. Such values continue westward for 700 km into the Foreland and Omineca belts of the northern Cordillera as a westward thinning wedge, providing independent evidence that much of the crust from east of the Cordilleran deformation front to the accreted Stikinia terrane of the Intermontane Belt consists of metasedimentary rocks, as inferred from coincident near-vertical incidence seismic reflection studies. Consistent with its derivation as a volcanic arc terrane, Stikinia shows σ values of 0.27-0.25. Upper mantle velocity and ratio values beneath the Slave craton indicate an ultramafic peridotitic composition. In the western Wopmay Orogen, low Vp, Vs and σ indicate the presence of pyroxenite in the upper mantle, which can be related to a Proterozoic subduction episode. The LITHOPROBE results combined with those from earlier studies permit generation of a 2000-km-long lithospheric velocity model and interpreted cross section. The seismic data identify a variety of orogenic styles ranging from thin- to thick-skinned accretion in the Cordillera and crustal-scale tectonic wedging associated with both Paleoproterozoic and Mesozoic collisions. Despite the variety of ages, orogenic styles and tectono-magmatic deformations that are spanned by the seismic corridors, the Moho remains remarkably flat and shallow (33-36 km) across most of the transect. Significant variations only occur at major tectonic boundaries. Laterally variable crustal velocities are consistently slower beneath the Cordillera than beneath the cratonic crust. This is consistent with the high temperatures (800-900 °C) required by the slow upper mantle velocities (7.8-7.9 km/s) observed beneath much of the Cordillera.