NON-MAGNETIC, REFLECTIVE PROTEROZOIC CRUST OF THE ACCRETIONARY CANADIAN CORDILLERAN MARGIN

The Canadian Cordillera is considered an important example of an accretionary margin, in this case characterized by suspect terranes recognized 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 km³, extends over 1000 km in length, and makes up most of the crust of the Canadian Cordillera. The Cordilleran suspect terranes, excepting Stikinia, have shallow roots only a few kilometers deep, with no deep crust or mantle attached. Detailed seismic stratigraphy, studies comparing bulk P- and S-wave seismic velocity (Poisson's ratio), and most recently, analysis of high-altitude (4 km) aeromagnetic data all confirm the original interpretation of the large, wedge-shaped prism composed primarily of Proterozoic sediments. Synthetic seismograms calculated using logs of an exposed Muskwa Assemblage section near Fort Nelson, B.C., closely matches the observed reflections on LITHOPROBE SNORCLE line 2b that can be followed nearly to the Moho. S-wave velocities and especially P-wave/S-wave velocity ratios modeled from the associated refraction survey records independently reveal a westward-tapering wedge that coincides closely with the one defined by increased reflectivity. The subdued nature of the long-wavelength (>100 km) magnetic field indicates that almost the entire crust of the Cordillera inboard of the Coast Belt is largely non-magnetic. This interpretation is clearest beneath the Yukon where the accreting prism probably formed within an embayment along the western Laurentian continental margin. Further south beneath British Columbia, crystalline basement of ancestral North America may occupy a significant portion of the crust, probably in the general form of basin and ranges that resulted from the late Neoproterozoic breakup of western Laurentia and Siberia.