LITHOPROBE NEAR-VERTICAL TO WIDE-ANGLE SEISMIC PROFILES ACROSS THE NORTHERN CANADIAN CORDILLERA: COMPARISON AND INTERPRETATION

LITHOPROBE recorded more than 2000 km of coincident multichannel reflection (MCS) and refraction/wide-angle reflection (R/WAR) seismic data across northwestern Canada between 1996 and 2000. Comparison of the complementary data sets enhances interpretation of lithospheric structural models. Here, we highlight some of the interpretations across the western craton and northern Cordillera, including the region of the Rocky Mountains.

East of the Cordilleran deformation front, the R/WAR models clearly define the Western Canada Sedimentary Basin and Proterozoic Fort Simpson basin, which overly the Fort Simpson magmatic arc. Over the same region, the MCS data show coherent layered reflectivity with a distinct change in character at the Foreland Belt. Below the eastern Foreland Belt (northern Rockies), high upper crustal velocities correlate with prominent MCS reflections, indicating Proterozoic middle crustal rocks thrust over older strata of the Fort Simpson basin. Strong MCS reflectivity that is tied to outcrops of Proterozoic rocks extends as a tapering wedge westward from the northern Rockies into the western part of the orogen below the collage of accreted terranes of the Intermontane Belt. The velocity models do not show an equivalent taper in their contour values. The MCS and R/WAR profiles indicate that most accreted terranes are thin flakes thrust onto the Proterozoic wedge. The only exception is Stikinia, the major terrane comprising the Intermontane Belt, which includes a whole crustal section. The crustal-scale, strike-slip Tintina fault divides ancestral from accreted North America. The fault is clearly identified on the MCS sections by zones lacking reflectivity whereas its character in the velocity models is evident but subtle. In the Omineca Belt east of the Tintina fault, both the R/WAR and MCS data indicate a complex, laterally variable structure throughout the upper crust. Low velocities in the upper crust and prominent MCS reflective characteristics clearly define the Bowser Basin overlying Stikinia. Moho reflectivity from R/WAR and MCS data sets provides consistent responses. Strong wide-angle Moho reflections below the Intermontane Belt correlate with distinct MCS Moho reflections. In contrast, the Moho below the northern Rockies exhibits weak wide-angle and MCS reflections.