The structure is different from the fold-and-thrust belt. Whereas in the fold-and-thrust belt the sedimentary rocks were sheared off the basement and moved as independent sheets and the hot, weak basement rocks deformed in a ductile manner; in the Laramide ranges the sedimentary rocks remain firmly attached to the underlying basement. The basement was cold and strong and failed by faults rather than deforming plastically. Thrust faults extend down through the basement to depths of over 25 kilometers, which allowed large blocks of basement to be upfaulted or downfaulted. The sedimentary rocks were passively folded over the thrust faults in structures called drape folds. (See the second and third photos in the collection of images from the Big Horn Mountains of Wyoming taken by Ball State University.)

The main Laramide ranges are arranged in rows that are oriented east-west. (I've added the Bear's Paw Mountains to the list below because it is underlain by a large dome-shaped structure, similar to the Little Belt Mountains and the Black Hills, but with lower structural relief. It is obscured by Eocene-age lava flows and gravity slide features.)

Block-Faulted, Basement-Cored Laramide Ranges in MT and WY

The amount of uplift (on deep-seated thrust faults) decreases to the north and to the east. Thus, in the table above the Wind River Range has the greatest amount of uplift, whereas uplift of the Bear's Paw Mountains is relatively small.

The development of Larmide ranges in Wyoming is shown in tectonic maps for the Early, Middle, and Late Paleocene (66 - 58 million years ago). Most of Wyoming and the eastern two thirds of Montana is a mosaic of fault-bounded basement blocks that have all moved with respect to one another. This style of tectonics is called "thick-skinned tectonics" by American geologists because it involves the basement. John Rogers at Yale University likened the movement of the basement blocks to blocks of ice in an Alaskan river during spring break-up and termed the style as "ice-flow tectonics". The animation by Jim Sears also shows the motion of the Laramide ranges in Wyoming. As shown in the animation, the plate convergence which created the fold-and-thrust belt also compressed and moved the Laramide ranges. However, thick-skinned tectonics results from a different geometry of the subducted plate.

The most common model for subduction of an oceanic plate under the margin of a continent has the subducted slab descending at a 45 degree angle into the asthenosphere. This is a good model for the northern Andes in South America and many other places in the world. However, under central Chile and Argentina (from 26 to 33 degrees south), the subducted plate has a shallow angle of descent. An east-west cross section at latitudes between 24 and 25 degrees south showing the flat-slab geometry is available on the Internet. (Select image 5). (The cross section at 30 degrees south, which best shows the flat-slab geometry, has been published, but an Internet version has not been released yet.) Isolated, basement-cored Laramide-style mountain ranges--the Sierra Pampeaneas--in Argentina are located above the flat portion of the subducted Nazca plate. A well-developed fold-and-thrust belt form where the oceanic plate descends at a 45 degree angle.

This correlation of isolated, Laramide-style mountain ranges in South America and the flat-slab geometry of the subducted plate is one of the important tools for interpreting plate tectonics in Montana and Wyoming. Traction applied to the bottom of Montana and Wyoming lithosphere as the subducted Farallon plate "scraped its underside" caused horizontal shortening of the crust. Horizontal shortening, which in Wyoming was approximately 15% and less in Montana, occurred mostly by movement on thrust faults in the basement. In the table of Laramide ranges above the horizontal shortening (and vertical relief) was greatest in the Wind River Range in the southwest and least in the northeast (Porcupine dome and Bear's Paw Mountains).

Bird recently inverted paleomagnetic data and data on fault movements in Wyoming and Montana to obtain the direction of regional shortening. The northeasterly displacement direction he found is parallel to the direction for the Farallon plate moving under Wyoming lithosphere as determined by the reconstruction of plate motions (by Engebretson et al.)

The two mechanisms for flat-slab subduction are buoyancy and trench roll-back. Buoyancy is the explanation why the Juan de Fuca plate has such a shallow dip under Oregon, Washington, and southern British Columbia. New lithosphere is generated several hundred kilometers to the west and is being subducted under the margin of the continent. The young lithosphere is relatively hot and buoyant and does not sink very easily. Hence, the flat subduction angle. This buoyancy explanation has also been used for the southern Andes, where subduction of an oceanic ridge or a submarine plateau was cited as the cause for the flat portion of the subducted Nazca plate.

The second explanation considers the motion of the continental plate with respect to the global hotspot frame of reference. (Hotspots are considered to be relatively stationary, especially compared to the tectonic plates.) A subducted plate is a sheet descending edge-on in a very viscous fluid.

In the simple model of an oceanic plate being subducted under a continental plate (as shown in the first diagram), the subducted plate descends at an angle of around 45 degrees. However, if the continental plate moves too rapidly to the left in this diagram, then it attempts to override the trench, which is the site at which the oceanic plate bends and disappears into the depths. Rather than overriding the trench, the continental plate simply overrides part of the flat portion of the oceanic plate, the trench moves to the left so that it is always in front of the advancing continent, and a flat-lying portion of the subducted slab is created. The flat-slab geometry is shown in a computer model (Figure a) by Ulrich Christensen. This flat-lying portion of the subducted slab can "scrape the underside" of the continental plate, creating a horizontal shortening over a large area. This is the mechanism considered to be the cause for the Laramide ranges. Peter Bird at the University of California at Los Angeles has successfully modeled this process with a very large computer program using finite element analysis. (See articles published in 1985, 1988, and 1998.)

There is evidence for both mechanisms for flat-slab subduction. The North American plate did move rapidly westward during the Late Cretaceous and Paleocene. (See maps generated at GEOMAR site or see Blakey's maps for ....). According to Engebretson et al (1985), the Farallon plate split into two parts at about ?? million years with the northern part called the Kula plate. The Farallon-Kula ridge passed under the western U.S. and must have had some effects. Assessing the contribution of each of these two mechanisms is difficult because the Kula plate has completely disappeared and only small fragments of the Farallon plate remain--the Juan de Fuca plate and the Nazca plate.

Plate Tectonics of Montana (Page 11 of 14)