Crustal structure and lithology of the northern Canadian Cordillera: alternative interpretations of SNORCLE seismic reflection lines 2a and 2b

Abstract

Seismic reflectors at shallow crustal levels recorded in SNORCLE (Slave – Northern Cordillera Lithospheric Evolution) lines 2a and 2b in the northern Canadian Cordillera can be reconciled with many stratigraphic and structural elements known from geological mapping. Clearly evident examples are the crustal penetrating, Northern Rocky Mountain Trench (NRMT) Fault, the several kilometres thin slices of Slide Mountain (Sylvester Allochthon) and Cache Creek terranes, and the northward dipping Hotailuh Fault. Interpretations of lithological successions and structures at deeper crustal levels are more speculative. Of fundamental significance is a southwesterly tapering wedge of continental crust overlain in its distal part by the allochthonous terrane Stikinia and, northeast of Thibert Fault, by a thick, probably structurally thickened sequence of sedimentary rocks and the extensive granitic Cassiar batholith. The top of the wedge is interpreted to be a tectonic accretion surface resulting from collision of Stikinia with Cache Creek Terrane and Ancestral North America. The mid-crust accretion surface has an apparent southwest dip, whereas the surface expression of accretion is recorded in part by detached, northeast-dipping faults, such as the King Salmon and Hotailuh faults. East of the NRMT, the lithology of more than 20 km of the middle and lower crust is either layered crystalline basement, Mesoproterozoic and Neoproterozoic sedimentary strata, or a combination of the two. West of the trench this zone of continental crust tapers westward; crystalline basement probably predominates in both zones. In the western part of line 2a, the allochthonous terrane Stikinia occupies the full crust thickness.