CONSTRUCTIONAL VS. DESTRUCTIONAL PROCESSES IN A COLLISIONAL OROGEN: THE CASE OF THE CANADIAN ROCKIES

The Canadian Rockies are often implicitly regarded to be constructional mountains created during the Laramide Orogeny. But while the original elevation and structures are Laramide in age, the mountainous relief itself is destructional, and post-orogenic. The present mountain physiography is the result of 55-60 million years of post-orogenic differential erosion, in which more resistant rocks have been left at higher elevations than less-resistant rocks.

Numerous lines of evidence suggest that the mountains and foothills have lost several kilometers of overburden since the end of the Laramide Orogeny, requiring that the local relief of the mountains and foothills that we see is erosional in origin. Further evidence of erosional origin is the adjustment of physiography to lithology: the mountains have high relief because the exposed sub-Mesozoic rocks can hold up high, steep slopes, whereas the foothills have low relief because the underlying Mesozoic rocks are less competent.

Mesozoic rocks originally blanketed what is now the thrust and fold belt, and physiography of the belt at the end of Laramide (EOL) time (60-55 Ma) depended on whether the Mesozoic cover had been stripped off to reveal the underlying, competent, Paleozoic/Proterozoic rocks. A reconstruction of the thrust and fold belt using critical-taper theory generally agrees with reconstructions from earlier stratigraphic and paleothermometry studies: what are now the front ranges in the eastern Rocky Mountains were still covered with Mesozoic rocks at the EOL. Hence that part of the belt must have comprised a high-elevation upland of relatively low relief. The “main ranges” in the western part of the belt were in part stripped of Mesozoic cover by EOL time, and were more mountainous. Generation of modern relief in the front ranges, including the escarpment at the mountain front, had to await stripping of Mesozoic rocks and incision of rivers into harder substrates in post-Laramide time.