Rocky Mountain Trench

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The Rocky Mountain Trench, also called "the valley of a thousand peaks," is a physiographic feature extending ~1600 km (995 mi) from Flathead Lake, Montana, to the British Columbia-Yukon border. Although some of its topography has been carved into glacial valleys, it is primarily a by-product of faulting. It separates the Rocky Mountains on its east from the Columbia Mountains and the Cassiar Mountains on its west. It also skirts part of the McGregor Plateau area of the Nechako Plateau subarea of the Interior Plateau of British Columbia. It is up to 25km wide, peak-to-peak, and varies in depth, but is clearly visible from the air and discernible from high up on any of the mountain ridges lining it.

It is used by four major river basins: the Columbia, Fraser, Peace and Liard's. Three reservoirs of the Columbia River Treaty fill much of its length today - Lake Koocanusa, Lake Kinbasket and Lake Williston. Rivers that use the trench are the Kootenay River, the Columbia River, Canoe River, Fraser River, Parsnip River, Finlay River, Fox River, and the Kechika River. The Kechika and Fox are part of the Liard system, the Parsnip and Finlay and part of the Peace River system. The Canoe River is a short tributary of the Columbia system, draining into Lake Kinbasket. The Kootenay River is a tributary of the Columbia, joining it near Trail B.C. after a quick trip through the United States as the Kootenai River. The Kootenai River, however, does not follow the trench but exits it southwest (as the Lake Koocanusa reservoir to the Libby Dam).

The Rocky Mountain Trench can be divided into the Northern Rocky Mountain Trench and Southern Rocky Mountain Trench by a break in the valley system at ~54°N near Prince George, British Columbia. The northern portion of the trench is dominated by strike-slip faulting while the southern part of the trench was created by normal faults. Despite differences in timing and faulting styles of the northern and southern portions, they were aligned with each other because faulting for both was controlled by a pre-existing, west-facing, deep basement ramp with over 10 km of vertical offset.

The Northern Rocky Mountain Trench is closely aligned with the Tintina Trench near the British Columbia-Yukon border, and the two could arguably be classified as one and the same. The Tintina Trench extends through the Yukon into Alaska. Right-lateral strike-slip fault movement on the Tintina-Northern Rocky Mountain Trench may have begun during the middle Jurassic. The fastest rates of slip probably occurred during two pulses in the middle Cretaceous and early Cenozoic, respectively, with the latter probably occurring during the Eocene. Between 750 km to > 900 km of total right-lateral movement has occurred, of which 450 km of offset has occurred since the mid-Cretaceous. The end result is that terrains to the west of the fault system have moved toward the north. In the context of plate tectonics, strike-slip movement on the Tintina-Northern Rocky Mountain Trench is also related to strike-slip movement along the San Andreas Fault, the extension of the Basin and Range Provence, and other extensional or strike-slip fault systems in western North America.

The Southern Rocky Mountain Trench was created mainly by Cenozoic-aged extension (normal faulting). What little strike-slip movement that is found in the southern trench is not considered significant. The extensional faulting was nonetheless substantial, having extended as deep as 13.5 km (8.39 mi). The southern trench also differs from the northern trench in that it is more sinuous and is asymmetrical in cross-section (perpendicular to its length). The western side of the Southern Rocky Mountain Trench is more subdued and irregular than the east side. During late Paleozoic to Mesozoic time, rapid sediment deposition and subsidence to the west transitioned in the area of the modern Rocky Mountain Trench into a stable continental shelf in the east. The Nevadan Orogeny destroyed the western wedge of sedimentary rocks during Jurassic to middle Cretaceous time, thrusting them up into metamorphic fold belts. Currently, strata on either side of the Southern Rocky Mountain Trench consist mainly of Precambrian and Paleozoic metasedimentary and sedimentary rocks. Within the trench are unconsolidated Cenozoic sandstones and conglomerates.

The aforementioned basement ramp along which orogeny-related thrust faulting and subsequent strike-slip and normal faulting occurred is probably associated with the ancient continental shelf of Paleozoic and Mesozoic time.

[edit] References

For a more in-depth geological overview, the following sources are recommended:

• Bally, A.W., Gordy, P., and Stewart, G. 1966. Structure, seismic data, and orogenic evolution of the southern Canadian Rockies. Bulletin of Canadian Petroleum Geology, v. 14, pp. 337–381.
• Gabrielse, H. 1985. Major dextral transcurrent displacements along the Northern Rocky Mountain Trench and related lineaments in north-central British Columbia. Geological Society of America Bulletin, v. 96, pp. 1-14.
• Henderson, G.C. 1959. A summary of the regional structure and stratigraphy of the Rocky Mountain Trench in Holland, S.S., chairman, Symposium on the Rocky Mountain Trench. Canadian Mining and Metallurgical Bulletin, v. 565, no. 62, pp. 322-327.
• Leech, G.B. 1959. The southern part of the Rocky Mountain Trench, in Holland, S.S., chairman, Symposium on the Rocky Mountain Trench. Canadian Mining and Metallurgical Bulletin, v. 565, no. 62, pp. 327-333.
• van der Velden, A.J. and Cook, F.A. 1996. Structure and tectonic development of the southern Rocky Mountain Trench. Tectonics, v. 15, no. 3, pp. 517-544.