The northern Cordillera, which spans most of eastern Alaska and northwestern Canada, is a structurally complex mountain belt formed by a sequence of tectonic collisions that appended multiple terranes—rock formations with shared geologic histories—onto the western edge of North America. During the middle to Late Cretaceous, back-arc magmatism injected a series of mostly granitic intrusions into these terranes.
Paleomagnetic studies of igneous rocks suggest that much of the Cordilleran mountain building occurred hundreds or even thousands of kilometers south of the mountains’ current position. But this evidence conflicts with information from geological mapping, which to date has failed to locate the structures necessary to accommodate this considerable northward motion along the tectonic margin.
A topographic map showing the distribution of low-density rocks (in gray) in the northern Cordillera, the base of which defines the depth of a regional-scale crustal décollement (colored contours, where blue is deep and red is shallow) within the crust. Credit: Nathan Hayward, Geological Survey of Canada/Natural Resources Canada
Now Hayward presents new findings that may finally help resolve a major Cordilleran conundrum. Using a recently developed technique to invert regional three-dimensional gravity data, the author modeled the shape, distribution, and depth extent of low-density zones of rock predominantly associated with the northern Cordillera’s middle to Late Cretaceous granitic intrusions.
The results indicate these intrusions are truncated along their base by a décollement, a regional-scale fault along which a component of the proposed long-distance displacements could have occurred. The gravity modeling shows this décollement underlies the entire northern Cordillera and shallows eastward from a depth of 15 to 20 kilometers beneath the Selwyn Basin to about 11 kilometers beneath the Mackenzie Mountains at the Cordillera’s eastern edge. Using structural relationships, Hayward infers the décollement is mid-Cretaceous to Paleocene in age, being roughly concurrent with the mid-Cretaceous intrusions but older than the Eocene-aged Tintina fault that substantially displaces it.
Collectively, these findings suggest the newly discovered décollement may have played a major role in accommodating a component of Late Cretaceous displacement along North America’s northwest margin consistent with the “Baja British Columbia” hypothesis. Given this paper’s fresh insight into the northern Cordillera’s evolution, as well as the potential applicability of the author’s technique to a range of tectonic settings, this research is likely to generate substantial interest within the geophysical research community. (Tectonics, https://doi.org/10.1029/2018TC005295, 2019)
—Terri Cook, Freelance Writer
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