A fold-and-thrust belt forms when an oceanic plate subducted under a continental plate. The plate tectonic model for the Sevier orogenic belt (by Ron Blakey) illustrates the geometry. A mountain chain forms about 100 kilometers inland from the edge of the continental plate with volcanoes, large intrusions of granite, and thrust sheets. The typical thrust sheet is a packet of sedimentary rocks with a thickness that can range from a fraction of a kilometer to several kilometers, lateral dimensions ranging up to tens of kilometers, and displacement distance or distance traveled that can be a hundred or more kilometers. Sedimentary rocks commonly have beds that are very weak--typically shale, but sometimes gypsum-- which serve as glide horizons.

The fold-and-thrust belt provides some of the most dramatic images of the Northern Rocky Mountains. Thrust sheets with Paleozoic-age limestone are strong and resistant to erosion. Where these are thrust over soft, Cretaceous-age shale, erosion has created impressive mountain landscapes, in particular, the Rocky Mountain Front. Travelers driving west across the prairies can not fail to be impressed by the sudden appearance of snow-capped mountains looming high above the plains. Check out some of these images.

All of the rocks shown in these photos are out-of-place. They have all moved to the east northeast from the place where they were deposited, so that now old rocks overlie young rocks with a thrust fault between. The animation by Jim Sears vividly depicts this eastward displacement and horizontal shortening.

It is one thing to look at Chief Mountain on the east side of Glacier Park (see link above) and realize that the block of rock forming the high cliffs has probably moved 30 kilometers. It is quite another thing to look at the profile or cross section across the entire width of the Cordillera and realize that everything (on the surface) has moved eastwards.

Canada's Lithoprobe Project ran seismic lines across southern British Columbia, parallel to the U.S. border, to determine the deep structure of the Cordillera. The eastern seismic lines are close enough to northwestern Montana that the results are directly applicable, particularly to the area between Glacier Park and the Yaak. The Lithoprobe results are available in several formats. A brochure showing a cross section 1100 kilometers long can be obtained free of charge from the Lithoprobe Project. A low resolution image shows the subduction zone and adjacent area near Vancouver Island. The best images available on the Internet are a series of 7 interpreted seismic profiles or cross sections which are segments of the 1100 kilometer cross section. First find the location of the profiles (listed below) on the Lithoprobe map of Southern British Columbia. As you step through the sequence (going from west to east), you need to remember what the previous cross sections looked like. Scroll down past the seismic profile to the interpreted cross section (with colors). Click on parts of the cross section for explanations or select topics listed below the cross section.

New thrust slices are continually added to the base of the crust at the margin of the continent (west of Vancouver Island). These new thrust slices push previously accreted slices inland along the base of the crust. Thus, on Vancouver Island the top of the crust is much older than the bottom part of the crust. Young (i.e. less than 40 million year old) thrust slices penetrated under the Insular Belt and under the western part of the Coastal Belt.

The cross section contains two superterranes. The Intermontane superterrane was accreted onto the margin of the continent in the Jurassic and the Insular/Coastal superterrane in the Late Cretaceous.

As thrust sheets moving eastwards "piled up" large fold-shaped structures called antiforms were created. The Nicola structure is the largest is one of the metamorphic core complexes. Injection of granites.

East of Nicola antiform there is a reversal of shear sense on the thrust faults so that the upper sheet moves east. This is clearly gravity driven, allowing the high mountains to spread laterally towards the plains much like a ball of silly putty will slowly flow out to form a thin "puddle".

The ease with which thrust sheets move and the great distance traveled are a result of the boundary zones behaving a very weak, almost lubricating material. Fault gouge commonly contains clay minerals. Montmorillonite (also called bentonite) is especially slippery and effective at lubricating fault planes.

The current motion of the Juan de Fuca plate is oblique to the Washington coast. Reconstruction of plate motions in the Cretaceous and Early Tertiary also shows oblique motion by the Farallon plate, moving to the northeast as it was being subducted under North America. The oblique motion was resolved into a perpendicular and a parallel component. The perpendicular component was responsible for thrusting away from the continent. The stress and fault reorientation caused by this effect is documented in a study of the Washington coast.

Montana is located far inland from the continental margin. Thus the effects are "far-field" effects. The major effects are accretion of superterrane in Late Cretaceous and increased thrusting when the rate of subduction increased. The rate of subduction peaked at the end of the Paleocene and fell sharply in the Eocene due to changes in plate motions. Thrusting abruptly ceased at about 55 m.y. and as discussed in a later section. A period of crustal extension followed.

The high topography of the mountains is built up on the ocean side of the range by adding new thrust sheets to the bottom of the stack, by large granitic intrusions, and by deforming the basement rocks. On the foreland side of the mountain range the slice above (rather than below) a thrust fault moves further eastward away from the ocean.

In Montana we have spectacular examples of thrust faults. In the image looking north into Glacier Park from Marias Pass on U.S. Highway 2, hard, resistant Precambrian-age (about 1.4 billion years old) sedimentary rocks have been thrust over soft, weak Cretaceous-age (about 100 million years old) shale. This is the famous Lewis Thrust where older rocks have been shoved over younger rocks. The glide horizon is shown by the green arrow. The highly deformed nature of the shale in the thrust zone has been studied using X-ray techniques and the scanning electron microscope.

Chief Mountain on the eastern boundary of Glacier National Park appears in many geology textbooks because it is an erosional remnant of the thrust sheet--an isolated block of Precambrian rocks sitting on top of soft Cretaceous shale. The rocks forming Chief Mountain moved approximately 30 or 40 kilometers to the northeast on the Lewis Thrust Fault.

The aerial photo of the Rocky Mountain Front (actually called the Sawtooth Range), just north of the Sun River Gorge, shows 6 thrust slices. Each of the light-colored mountain ridges is the same Mississippian-age Madison limestone. The unit is repeated 6 times as imbricate slices.

The Lewis Thrust at Crowsnest Mountain north of Crowsnest Pass in Alberta is a dramatic scene.

A kink fold in a thrust sheet is shown in this photo of the Canadian Rockies.

This style of deformation, in which the sedimentary rocks are sheared off of the basement and move as thrust sheets whereas the deeply-buried basement is hot enough to deform plastically, is called "thin-skinned tectonics" by American geologists.

The period of active thrusting in Montana has been dated as from 72 to 58 million years ago.

Plate Tectonics of Montana (Page 10 of 14)