Fold belts

Fold belts are linear regions of the earth's crust that have been squeezed into folds and cut by faults, similar to the way a loose rug on a floor may be wrinkled when pushed at one end. They form some of the most spectacular and scenic mountain chains, including the Alps, the Himalaya, and the Rocky Mountains. The compressive forces that produce fold belts are generated when large slabs of the earth's crust converge.

Formation of Fold Belts

Fold belts are one of the most common types of mountain belts. They form scenic and spectacular young mountains such as the Alps, the Himalaya, and the Rocky Mountains, as well as older mountains, such as the Appalachians. Compared with volcanic mountains, which form by the eruption of molten rock at the earth's surface, fold belts are produced when the earth's crust is squeezed by compressive forces into a series of folds and faults.

Fold belts were first recognized in the mid-nineteenth century in the Swiss canton of Glarus. Here, Arnold Escher von der Linth found evidence that the earth's crust had been heaved up and moved laterally for scores of kilometers, an unbelievable concept for its time. Subsequent Alpine geologists modified the details of Escher's interpretation, but by the late nineteenth century, the concept that some mountains are composed of great folds and laterally displaced rocks was generally accepted. Thus, the discipline of structural geology was born out of the discovery of fold belts.

Theory

The formulation of the theory of plate tectonics in the 1960s provided an explanation for the location and origin of fold belts. Plate tectonics is based on the concept that the earth's crust and upper mantle are divided into a mosaic of large slabs, or “plates,” that are in constant motion relative to one another. Plates can move away from each other, as along mid-ocean spreading ridges. They can also laterally slide past each other, as along the San Andreas fault in California. Plates can also move toward each other, with one plate sliding beneath the other. This latter process is called subduction, which can occur in three ways. First, dense, heavy oceanic basalt can subduct beneath lower-density (lighter) continental crust, as along the West Coast of South America. Second, oceanic crust can subduct beneath more buoyant oceanic crust, typified by the Mariana Islands in the western Pacific Ocean. Third, continental plates—which, because of their mutual low density, have difficulty subducting beneath each other—can collide and hence push the crust up into large mountains.

Fold belts are associated with all three subduction processes, but the largest are formed by continent-continent collisions. For example, the Appalachian Mountains were created more than 200 million years ago by the continent-continent collision of North America with Western Europe and North Africa, and the geologically young Himalaya are being uplifted today by the collision of India with southern Asia.

When subduction occurs by one of these three processes, fold belts can form in two general localities. Assume that there are two plates (plate A and plate B) moving together and that plate A is subducted beneath plate B. The point where plate A bends down and begins to be subducted beneath plate B is called the trench. As plate A slides deeper and deeper beneath plate B, it is heated by the earth's internal heat and eventually undergoes partial melting. The molten rock, or magma, rises through the overlying plate B and eventually forms a chain of volcanoes called a magmatic arc. Fold belts may be formed on the trench side of the magmatic arc, in which case they are called fore-arc fold belts, or they may form in plate B behind the arc, when they are called back-arc fold belts. The geological character of these two settings is drastically different, resulting in very different fold belts.

Types of Fold Belts

Fore-arc fold belts contain an abundance of deep-water oceanic sediment and sometimes the upper part of oceanic crust that was scraped off the subducting plate (plate A) as it descended into the subduction zone beneath plate B. In addition, fore-arc fold belts may contain accreted terranes, or masses of rock that were too large to be subducted. Examples of fore-arc fold belts include the Olympic Mountains of Washington State, the island of Barbados in the lesser Antilles, and the island of Java in Indonesia.

In contrast, back-arc fold belts form farthest from the trench on the back side of a magmatic arc. These fold belts are usually composed of well-stratified, shallow marine and nonmarine rocks and do not normally contain accreted terranes. Back-arc fold belts are known for their tremendous oil and gas production, such as in southwestern Wyoming and Alberta, Canada. One of the best known back-arc fold belts is the Rocky Mountain system of western North America, which formed between 150 million and 55 million years ago when a large subduction zone was located along the West Coast. The Rocky Mountain system forms the topographic backbone of the North American continent, extending more than 8,000 kilometers (4,971 miles) in length from the Brooks Range in northern Alaska, through western Canada, western Montana, western Wyoming, eastern Idaho, central Utah, southern Arizona, and into Mexico. The Rocky Mountain fold belt is also called the fold and thrust belt, or overthrust belt.

Folded Rocks and Faults

The internal anatomy and style of deformation associated with fold belts has been studied extensively for more than a century, yet there is still much that scientists do not understand. Fold belts are, by their nature, composed of highly deformed rocks that make geological interpretations difficult at best. The most common geological structures encountered in fold belts are folded rocks. Folding is generally the result of compressional forces that squeeze the rock in ridges and valleys, or anticlines and synclines, respectively. Anticlines are formed when rock layers are arched upward, whereas synclines are downfolds, or depressions. The name “anticline” refers to the flanks of a fold that are inclined away from each other, while the flanks of a syncline are inclined toward each other. The process of folding may be envisioned by imagining a flat rug lying on a flat floor; if the rug is pushed at one end, it will be wrinkled into a series of anticlines and synclines; however, the floor beneath the rug will not be wrinkled. The interface between the rug and the floor is called the detachment surface, or décollement, and is located in a weak rock layer such as shale in many fold belts.

In addition to folds, fold belts are commonly cut by faults. Faults are narrow zones of brittle fracture that displace rock layers in lieu of folds. The detachment surface between the rug and floor in the previous example is a type of fault. In addition, faults may cut up from the detachment surface and displace sections of the rug (like tearing the rug). These faults are called thrust faults, and they commonly displace deeply buried, older rocks over younger rocks beneath the fault. The Lewis thrust fault, which forms Glacier National Park in northwest Montana, is one of the world's classic thrust faults. Thrust faults and folds are intimately linked in fold belts. As thrust faults move, they tend to cut upward at a steep angle across stiff, hard rock formations such as sandstone. Such areas are called ramps. As the rock layers move up and over a ramp, they become folded into an anticline called a ramp-anticline or fault-bend-fold. There are often several stiff layers within a stratigraphic section, so the profile of most thrust faults resembles a staircase with many ramps or steps. The flat regions between ramps are simply called flats, or treads, and synclines are commonly formed above these areas.

Study of Fold Belts

Fold belts offer spectacular exposures of contorted rocks on the face of a mountain that provide much information and inspiration to the field geologist. Geologists have traditionally studied fold belts by constructing detailed geological maps in the field. Field mapping has always been the first step in understanding any geological phenomenon. A structural geologist (one who studies deformed rocks) must first understand the geometry of deformed rocks—that is, the shapes and attitudes into which they have been deformed—which is best done by constructing a detailed geological map through fieldwork or from aerial photographs. Computerized images and photographs from space satellites are also very useful in mapping large regions. Next, the geologist must describe the relative motions of rock layers that occurred to produce the structures; this step is called “kinematic analysis.” Finally, once the geometry and kinematic sequence are understood, the geologist may ponder the forces and stresses that created the deformation. This step, called dynamic analysis, is often the goal of a structural investigation. Forces and stresses are often, but not always, associated with movement of the earth's large plates. Fold belts have contributed much to the understanding of how plates move. In turn, plate tectonics has offered an explanation for the origin of great fold belts.

Fold belts are of interest to the layperson for two basic reasons. First, fold belts form some of the earth's most spectacular scenery; the Alps, the Himalaya, the Rocky Mountains, and the Appalachian Mountains are all examples of fold belts. More practically, fold-belt mountains have nurtured many resources that are valuable, if not essential, to civilization, such as water, timber, animals for domestication and meat, and natural resources from the rocks themselves (for example, gold, copper, and oil). Most of the earth's great rivers—such as the Indus, Ganges, and Brahmaputra of India, the Rhone and Rhine of Europe, and the Columbia and Missouri-Mississippi of North America—flow from snowfields high in fold-belt mountains. Mountains have thus been a source of renewing resources for humankind.

Principal Terms

anticline: a folded structure created when rocks arch upward; the limbs of the fold dip in opposite directions and the oldest rocks are exposed in the middle of the fold

décollement: the detachment surface beneath a fold belt, usually located in a weak layer of rock such as shale; the maximum depth to which the rocks are folded and faulted

flat: that portion of a thrust fault where the fault is subparallel to adjacent layers of rock; also called treads, flats are usually located in weak rocks such as shale or evaporite (salt, anhydrite, gypsum)

ramp: that portion of a thrust fault where the fault cuts across a layer of relatively stiff rock at a higher angle than does the rest of the fault

syncline: a folded structure created when rocks are bent downward; the limbs of the fold dip toward one another, and the youngest rocks are exposed in the middle of the fold

thrust fault: a fault that is inclined at a low angle (less than 45 degrees), with rocks above the fault having moved up and over younger rocks below the fault; such faults are generally the result of compressive forces

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