Proterozoic Prism Arrests Suspect Terranes: Insights into the Ancient Cordilleran Margin from Seismic Reflection Data

The Canadian Cordillera is one of the principal regions from which the hypothesis of exotic, accreted, or suspect terranes was developed a few decades ago. In an important modification to this hypothesis, new field mapping and seismic reflection profiling reveal a vast volume of Proterozoic strata of largely North American affinity along the margin, leaving little room for the suspect terranes. The Proterozoic strata, deposited in at least three distinct periods between 1.85 and 0.54 Ga, form a reflective tectono-depositional prism or wedge that has a volume greater than a million cubic kilometers, extends over 1000 kilometers in length, and makes up most of the crust of the Canadian Cordillera. The suspect terranes apparently grounded upon and were arrested by this metamorphosed sedimentary prism during a complex interplay of thrusting and strike-slip displacements 190-170 Ma. Thus, the Cordilleran suspect terranes, excepting Stikinia, have shallow roots only a few kilometers deep, with no deep crust or mantle attached.