Everything about The Rocky Mountain Trench totally explained
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's 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 25 km 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, doesn't 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.
Northern Rocky Mountain Trench
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.
Southern Rocky Mountain Trench
The Southern Rocky Mountain Trench was created mainly by Cenozoic-aged extension (
normal faulting). What little strike-slip movement that's found in the southern trench isn't 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's 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.
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