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The great width of the Cordillera in the western United States and the role of flat-slab subduction in causing volcanism and tectonic shortening over a large area require special explanation. This section discusses the causes and effects of flat-slab subduction.
Volcanism is part of the activity of a convergent plate margin, especially one in which an oceanic plate is subducted under a continental plate. A chain of volcanoes forms about 100 kilometers inland from the trench. (The Andes Mountains along the west coast of South America is a good example.) The volcanoes mark the point (in the diagram) where rocks on the subducting slab (or being carried down with the descending slab) start to melt. The molten magma, typically of basaltic composition, rises up through the asthenosphere, the mantle portion of the lithosphere, and into the lower crust.
The basaltic magma may accumulate and "pool" for a while in the lower or middle crust and cause partial melting in nearby rocks, forming granitic magma. The granitic magma may rise to the surface and form explosive volcanoes and lava flows of rhyolite or the magma may crystallize at depth forming smaller plutons or gigantic batholiths. The basaltic magma may rise to the surface forming shield volcanoes and lava flows of basalt.
A common scenario is for the basaltic and granitic magmas to mix, forming a magmas with intermediate compositions--called andesite when cooled.
Several factors make the pattern of volcanism in the Western U.S. more complicated than a single line of volcanoes parallel to the coast. Exotic terranes accreted to the western margin of the continent moved the trench (and thus the position of the volcanic arc) westward. However, accreted terranes were shoved eastward (or northeastward) by horizontal compression and shortening. Rapid westward motion of North America caused the roll-back of the trench and shift the locus of volcanism. Transform faults carried accreted terranes in British Columbia towards Alaska. The result is a very wide area in the Western U.S. with volcanic activity.
Migration of the Onset of Volcanism to the Northeast
The following discussion focuses on the pattern of volcanism in Montana and Idaho. If one starts a line from Boise, Idaho and extends it to the northeast to the Bear's Paw Mountains in north central Montana and then projects the location of the various bodies of igneous rock onto this line using the (radiometric) age as the vertical axis; then a definite pattern of volcanism is apparent. Volcanism started at about 100 million years ago in the southeast end of the profile. (Ash from volcanoes on the Idaho/Oregon border was deposited in Montana starting shortly after 100 m.y.) The onset of volcanic activity becomes progressively younger to the northeast. By 65 m.y., it had reached the Little Rocky Mountains near the Canadian border in north central Montana.
Onset of Volcanism
Southwest Idaho
Southwest Montana
Boulder batholith
Elkhorn Mountains volcanics
Livingston volcanics
Adel Mountains volcanics
Judith Mountains
Little Rocky Mountains
Magma Gap
From about 58 million years to about 54 million years ago there was a general cessation of volcanic activity throughout the Idaho-Montana area (except for the Little Rockies) following by almost synchronous volcanic eruptions over much of the area. The intense volcanic activity lasted from about 54 to 48 m.y.
Eocene Igneous Flare-Up
Challis volcanics
Lowland Creek volcanics
Absaroka volcanics
Crazy Mountains
Little Belt Mountains
Highwood Mountains
Sweetgrass Hills
Bear's Paw Mountains
Both thin-skinned and thick-skinned tectonics involved horizontal shortening which continued up until about 54 m.y. at which time the shortening abruptly ceased.
The timing and pattern of tectonics and volcanism is explained by the changing configuration of plate subduction. With the breakup of the supercontinent Pangea and the formation of the North Atlantic Ocean, North America moved westward. The convergence rate between the Farallon plate and the North American plate increased from about 100 m.y. to about 55 m.y. The convergence rate peaked at about 60 m.y.
Flat-Slab Geometry and Trench Rollback
A simple kitchen experiment demonstrates the largest driving force for plate tectonics. If you heat parafin wax in a pan on the stove and then let it cool, a thin layer or "skin" of solidified wax will form on the surface of the melted wax. Let the patch of solid wax floating on the surface grow until it is about a quarter or a third of the area of the pan. Take a pencil and very gently poke one edge of the "skin" so that it is submerged under melted wax. Suddenly the entire skin will follow the submerged edge and disappear into the melted wax, as neatly as if the sheet had slid down a slot. The solidified wax is denser than the melted wax. Thus there is a gravity instablity with heavy material on top of lighter material. The gravity driven subduction restores stability to the system.
Christensen's flat-slab subduction model has the subducted slab descending through the asthenosphere until it hits a dense layer and flattens out. The model also applies to a second mechanism--one in which rapid motion of the continental plate toward the trench causes the continental plate to override the oceanic plate with the result that the position of the trench "rolls back" in front of the advancing continental plate and the oceanic plate "scrapes the underside" of the continental plate.
When the rate of roll-back is slow, then there is hot asthenosphere between the plates along the flat-lying portion. [Subducted rocks with low melting points become molten and the zone of volcanic activity keeps extending eastwards with respect to the North American plate.] However, when the westward motion of the the North American plate (and the westward shift or roll-back of the trench) was very rapid, then the subducted plate "scraped the underside" of the North American plate, no hot asthenosphere was between, most or all volcanism was extinguished above the flat-lying portion. With shear-traction applied to the North American plate, this was time of greatest horizontal shortening. When the velocity of North America (and the convergence rate between the Farallon plate and the North American plate) slowed down, the subducted Farallong plate was no longer forced up against the underside of the overlying North American plate. There was a decoupling (or delamination). First fluids that triggered melting, then hot asthenosphere came between the plates. The result was a nearly synchronous eruption of volcanoes over vast portions of Montana and parts of Idaho, Wyoming, and South Dakota.
There was similar behavior in Colorado, but with a shift in the timing. Since there are no Laramide ranges in Alberta and no significant volcanism, this means that the flat subducted slab did not extend much further than the Bear's Paw Mountains. [Actually Pinhorn Butte, part of the Sweetgrass Hills is just across the border in Alberta. Buried dikes, presumable Eocene in age, found under Cypress Hills of southwestern Saskatchewan.]
The difference in behavior between Montana and Alberta can be attributed to the Kula Plate. The Farallon plate broke into two parts at about 80 m.y. with the northern part called the Kula Plate. Since the Kula Plate was subducted under Western Canada and Alaska and has completely disappeared and only a small remnant of the Farallon plate remains today (off the coast of Washington, Oregon, and southern British Columbia), it is difficult to precisely locate the Farallon/Kula boundary. However, the difference in behavior is explained if the Farallon plate had a flat portion under Montana, but there was no flat-lying Kula plate under Alberta.
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Plate Tectonics of Montana (Page 12 of 14)