HR: 1340h
AN: T23C-0564    [Abstracts]
TI: Origin of Relief in a Collisional Orogen: the Case of the Canadian Rockies
AU: * Osborn, G
EM: osborn@ucalgary.ca
AF: Geology and Geophysics Dept., University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4 Canada
AU: Stockmal, G
EM: gstockmal@nrcan.gc.ca
AF: Geological Survey of Canada, 3303-33 St. NW, Calgary, AB T2L 2A7 Canada
AU: Haspel, R
EM: rnhaspel@ucalgary.ca
AF: Geology and Geophysics Dept., University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4 Canada
AB: Contrary to popular assumption, the mountainous local relief of the Canadian Rockies probably did not originate with the thrusting that characterized the Laramide Orogeny; rather, that relief is a post-orogenic phenomenon. The magnitude of present local relief in the Rockies thrust-and-fold belt (TFB) depends the relative erosional resistance of rocks exposed at the surface. The mountains consist of high ridges of well-indurated Paleozoic and Proterozoic rocks carried by thrust faults, alternating with valleys developed in softer Mesozoic clastic rocks, whereas in the foothills, where the less-resistant Mesozoic rocks are at the surface, relief is subdued. Mesozoic rocks originally blanketed the entire width of what is now the TFB, and local physiography of the belt at the end of Laramide (EOL) time must have depended on whether the Mesozoic rocks had been erosionally removed to expose the underlying, resistant Paleozoic rocks. A reconstruction of the TFB using critical-taper theory generally agrees with reconstructions from earlier stratigraphic and paleothermometry studies: what are now the front ranges in the eastern Rockies were still covered with Mesozoic rocks at the EOL. Hence this part of the belt may have comprised a high-elevation upland of relatively low relief, perhaps broadly similar to the Tibetan Plateau. Generation of modern relief, including the escarpment at the mountain front, had to await stripping of Mesozoic rocks and incision of rivers into harder substrates in post-EOL time. In the 'main ranges' in the western part of the TFB, thrusting and consequent erosion began earlier. Mesozoic cover was partly stripped off by EOL time, and probably locally in early Cretaceous time, so that part of the belt probably displayed 'mountainous' relief then. But the evolution there probably was the same, from original high-elevation, low-relief plateau to later rugged mountains.
DE: 8175 Tectonics and landscape evolution
SC: Tectonophysics [T]
MN: Fall Meeting 2005