The Cordillera in northern Canada is underlain by westward tapering layers that can be followed from outcrops of Proterozoic strata in the Foreland belt to the lowermost crust of the orogenic interior, a distance of as much as 500 km across strike. They are interpreted as stratified Proterozoic rocks, including ∼1.8–0.7 Ga supracrustal rocks and their basement. 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 were acquired as part of the Lithoprobe Slave‐Northern Cordillera Lithospheric Evolution (SNORCLE) transect. In the Mackenzie Mountains of the eastern Yukon, the layering in Line 3 is visible between 5.0 and 12.0 s (∼15 to 36 km depth). It is followed southwestward for nearly 650 km (∼500 km across strike) and thins to less than 1.0 s (∼3.0–3.5 km thickness) near the Moho at the Yukon‐Alaska international boundary. In the northern Rocky Mountains of British Columbia, the upper part of the layering on Line 2 correlates with outcrops of Proterozoic (1.76–1.0 Ga) strata in the Muskwa anticlinorium. At this location, the layering is at least 15 km thick and is followed westward then southward into the middle and lower crust for ∼700 km (∼300 km across strike). It disappears as a thin taper at the base of the crust ∼150 km east of the coast of the Alaskan panhandle. The only significant disruption in the layering occurs at the Tintina fault zone, a late to postorogenic strike‐slip fault with up to 800 km of displacement, which appears as a vertical zone of little reflectivity that disrupts the continuity of the deep layering on both profiles (∼300 km apart). The base of the layered reflection zone coincides with the Moho, which exhibits variable character and undulates in a series of broad arches with widths of ∼150 km. In general, the mantle appears to have few reflections. However, at the southwest end of Line 3 near the Alaska‐British Columbia border, a reflection dips eastward from ∼14.0 s to ∼21.0 s (∼45 to 73 km depth) beneath exposed Eocene magmatic rocks. It is interpreted as a relict subduction surface of the Kula plate. Our interpretation of Proterozoic layered rocks beneath most of the northern Cordillera suggests a much different crustal structure than previously considered: (1) Ancient North American crust comprising up to 25 km of metamorphosed Proterozoic to Paleozoic sediments plus 5–10 km of pre‐1.8 Ga crystalline basement projects westward beneath most of the northern Canadian Cordillera. (2) The lateral (500 km by at least 1000 km) and vertical (up to 25 km) extent of the Proterozoic layers and their internal deformation are consistent with a long‐lived margin for northwestern North America with alternating episodes of extension and contraction. (3) The detachments that carry deformed rocks of the Mackenzie Mountains and northern Rocky Mountains are largely confined to the upper crustal region above the layering. (4) Accreted terranes include thin klippen that were thrust ...
Abstract. We present geochemical and isotopic data for Nisutlin assemblage metasedimentary rocks and Anvil assemblage greenstones from the Teslin tectonic zone of the northern Canadian Cordillera. This study aims to establish the tectonic setting of formation for the sedimentary and basaltic protoliths of these highly deformed and metamorphosed rocks and thereby place constraints on the origin of these enigmatic rocks for which differing tectonic models have been proposed.
The Lithoprobe Slave Northern Cordillera Lithospheric Evolution (SNORCLE) study across northwestern North America, in combination with related crustal studies, has been synthesized into an 1800 km long cross section of the lithosphere that is constrained by high-resolution geophysical data (seismic reflection, refraction, electromagnetic, potential fields) and detailed bedrock geology. The cross section offers one of the longest "continuous" profiles of the continental lithosphere anywhere in the world that is constrained by combined geophysical measurements and electromagnetic properties and exposed bedrock geological relationships. The primary conclusion of the study is that, during all major orogenic episodes recorded from Archean to present in that part of Earth's lithosphere, the crust, and perhaps much of the mantle, was reorganized and redistributed rather than being differentiated from the mantle at the time of orogenesis. The observed subsurface geometries of relict subduction zones, accretion boundaries, and magmatic arcs all lead to the inference that the crust includes a dominant proportion of reworked material. A similar conclusion appears applicable for the origin of subcrustal lithosphere in the region, i.e., that much of the lithosphere, whether Archean in the Slave Province or Proterozoic in the Cordillera, is old and thus that the amount of "new" lithosphere added to the plate during orogenesis is surprisingly small. A corollary is that many accreted rocks at surface that record orogenic complexity are detached from their originally underlying lithosphere and were emplaced upon unrelated crust and mantle during deformation.
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