PRECAMBRIAN CRUST AND LITHOSPHERE BENEATH THE NORTHERN CANADIAN CORDILLERA DISCOVERED BY LITHOPROBE SEISMIC REFLECTION PROFILING

The Northern Canadian Cordillera is underlain by westward tapering layers that can be followed from exposed Proterozoic strata in the Foreland Belt to the lower crust up to 500 km across strike. The layering was discovered on two new deep seismic reflection profiles in the Yukon (line 3; ~650 km) and northern British Columbia (line 2; ~1245 km in two segments) that are part of the LITHOPROBE Slave - Northern Cordillera Lithospheric Evolution (SNORCLE) transect. In the eastern Yukon, the zone is visible between 5.0 and 12.0 s (~15 - 36 km depth), projects west for nearly 650 km (~ 500 km across strike), and thins to ~1.0 s (~3.0-3.5 km) near the Moho at the Alaska border. In northern British Columbia, where the reflection zone correlates to outcrops of Proterozoic strata on the east, the layering is at least 15 km thick. It projects west into the lower crust for about 700 km (~300 km across strike) and disappears as a thin taper at the Moho beneath accreted rocks of Stikinia; the eastern boundary of which is thus interpreted as a crustal-scale tectonic wedge above the layering. A consequence of this geometry is that most detachments of the Mackenzie Mountains and northern Rocky Mountains are confined to the upper crust. Large structures involving thick sequences of Proterozoic strata, however, have been uplifted along detachments near the Moho. The only major disruption in the layered zone occurs at the dextral Tintina fault, a vertical zone of little reflectivity on both profiles. The base of the layering is the reflection Moho, which is likely pre-Cordilleran because structures in the deep layering are truncated by Cordilleran structures above, and are listric below. Accreted terranes east of Stikinia, including Yukon-Tanana, Slide Mountain, Quesnellia, and Cache Creek, are confined to the upper crust above pericratonic North American strata, but may thicken westward as the underlying layered zone thins. South of the disappearance of the layering, line 2 crosses deformed Cretaceous rocks of the Skeena fold belt, beneath which older Mesozoic and Paleozoic rocks of Stikinia are deformed in large structures, some with amplitudes of 10-15 km that are not visible on the surface. On the west end of line 3, a reflection dips east from about 15.0 s to about 20.0 s (about 47 - 69 km) that is interpreted as a relict Kula plate subduction surface.