Thrust belts

Thrust belts are long, narrow zones composed of many thrust faults that record significant horizontal displacements of the continental crust. Thrust faults are surfaces that place older rock above younger rock. Exploration of thrust belts has led to the discovery of vast oil and natural gas reserves.

Discovery of Thrust Belts

Thrust belts are arcuate zones in which the Earth's surface has been shortened in its horizontal dimension, primarily by faulting. The zones are expressed as mountain ranges hundreds of kilometers wide and thousands of kilometers long. They are composed of faulted sedimentary rock and are integral parts of the linear orogenic belts that mark the edge of every continent. Active thrust belts are areas of intense deformation, rugged topography, and frequent earthquakes. Thrust faults are the principal features of thrust belts and are gently inclined surfaces along which older rocks, originally near the bottom of a pile of sediments, are pushed up and over younger rocks at the top. The direction in which the rocks move is perpendicular to the length of the thrust belt. Thrust belts frequently merge with fold belts. Fold belts are similar to thrust belts except that the principal structures formed are folds several kilometers wide. Fold and thrust belts form along continental margins with convergent plate boundaries and record significant horizontal movements within the continental crust.

Thrust faults were first recognized in the mid-to-late nineteenth century in the Scottish Highlands. Before the middle of the nineteenth century, geologists working there believed that sedimentary strata were interbedded with metamorphic rocks. This interpretation arose from a lack of knowledge about the process of metamorphism. It is now known that heat transforms sedimentary rocks into metamorphic rocks during metamorphism. The transformation occurs gradually both in space and in time.

Rocks adjacent to each other should exhibit only slight differences in metamorphism. This implication led to the conclusion that interbedding the metamorphic rocks and the sedimentary strata in Scotland was unlikely. At about the same time, another significant discovery was made: The metamorphic rocks high in the sediment pile contained older fossils than the sedimentary strata below. Because the sedimentary sequence was not believed to be upside down, the idea of great gliding of the older, metamorphic rocks up and over the younger sedimentary strata was proposed. This idea of thrust faulting was quickly accepted and applied to other areas, including the United States, where, in the early twentieth century, Bailey Willis documented that rocks greater than 500 million years old were thrust above strata fewer than 100 million years old in Glacier National Park.

Characteristics of Thrust Belts

Thrust belts are elongate areas of faulted and relatively unmetamorphosed sedimentary strata that border linear zones of metamorphic and plutonic rock (a pluton is a body of rock that crystallizes from a liquid below the surface). Together, the thrust belt and the zone of metamorphic and plutonic rocks define two parts of an orogenic belt: the core of the orogenic belt, where temperatures were high enough to transform sedimentary strata into metamorphic rocks, and the thrust belt, where temperatures were too low for metamorphism. Deformation in the core of the orogenic belt is complex. In contrast, the deformation in the neighboring thrust belt, although still severe, is fairly simple to unravel. Thrust belts taper from thicknesses of 10,000 to 20,000 meters next to the metamorphic core to less than 5,000 meters at the opposite side, defining a wedge of faulted sedimentary strata inclined gently toward the core. Sediments closest to the core typically have been thrust farther than 100 kilometers.

Deep-water oceanic rocks are thrust above shallow-water marine strata in most thrust belts. The thrusts originate where the deep-water rocks were deposited and move toward the region of shallow marine sedimentation. The area toward which the thrusts move is termed the foreland and is away from the core of the orogenic belt, called the hinterland. The thrust faults are not planar surfaces of uniform orientation but are composed of a series of flat faults that parallel layering in the sedimentary rocks and ramp faults that cut obliquely across layering to join flat faults at different depths. The rocks above a ramp fault move upward, whereas rocks above a flat fault move horizontally relative to those below. The depth of the flat segments of a single thrust fault decreases toward the foreland, imparting a staircase shape to the thrust fault and forcing deep-water rocks to climb to the surface, away from the core of the orogenic belt. The sliding of rocks along a ramp produces folds of a characteristic shape, broad anticlines, and narrow synclines as the rocks bend to accommodate the motion.

After one thrust moves older rocks up and over adjacent younger sediments, a new thrust parallel to the old one forms in the younger strata on the foreland side of the old thrust. Parallelism of the two thrusts allows the thrust slice (the rock between the two thrusts) to be tens of kilometers long. The horizontal distance between the old and new thrusts, usually hundreds of meters, defines the width of the thrust slice and depends primarily on the thickness of the pile of rocks being faulted. The new thrust then carries the younger strata and the old thrust toward the foreland and above still younger sediments. This process, called piggybacking, continues such that the thrust slices in the thrust belt are stacked like a series of cards. Because piggybacking is relatively straightforward, deformation in thrust belts is well understood. The cumulative horizontal displacement from piggybacking can be as large as 200 kilometers.

Décollement

The many thrusts in a thrust belt root into one flat fault at a depth of 10 to 20 kilometers, called the detachment surface or the décollement. Each separate thrust begins as a ramp fault that joins the décollement to a flat fault closer to the surface. The style of deformation above the décollement differs from that below. Thrusting occurs only above the décollement; rocks below, however, resist faulting and deform by changing their shape. The depth of the décollement commonly is controlled by a discontinuity that marks a change from brittle material above to ductile (changes shape without breaking when subjected to stress) below.

An example of a material that can be either ductile or brittle, depending on its temperature, is candle wax or paraffin. When warm paraffin is squeezed, it simply changes shape without breaking. Cold paraffin, however, fractures under compression. The warm paraffin is ductile; the cold paraffin is brittle. Rocks behave similarly. Such a transition from brittle to ductile material exists at depth in the continental crust where sedimentary strata lie above metamorphic or plutonic rocks. The sedimentary strata are brittle and are termed “cover.” Conversely, the metamorphic and plutonic rocks are ductile and are called “basement.” Because faulting in thrust belts is restricted to the brittle “cover” rocks, the deformation is known as thin-skinned. Thin-skinned thrusting significantly shortens the horizontal dimensions and thickens the vertical dimensions of the upper 10 to 20 kilometers of the continental crust. The effects of thin-skinned thrusting on basement rocks are largely unknown.

Active Thrust Belts

Active thrust belts are localized along the edges of continents and convergent plate boundaries. The shortening of the upper 10 kilometers of continental crust by thin-skinned thrusting provides a mechanism by which the horizontal compressive forces generated during the convergence of two plates can be accommodated. Essential to the formation of thrust belts are thick sequences of layered rock, such as those that accumulate in geosynclines. If the rock being deformed is not thick enough, it will not transmit the compressive forces necessary to develop thrust faults. Examples of active thrust belts include the foothills to the Himalayas in India and Nepal, where the Asian and Indian plates are colliding; the Andes of South America, where the Pacific and South American plates are converging; and the Transverse Ranges of California where the Pacific plate and North American plates are converging slightly. The Zagros fold and thrust belt in Iran is evolving between the Arabian and Eurasian plates. Similar conditions are occurring in folds and thrust belts in Pakistan and Taiwan. 

Large basins filled with sediment border all these continental margins. The Appalachian Mountains of Tennessee and North Carolina are an ancient thrust belt. There, slices of sediment deposited in geosynclines next to North America were thrust westward 225 million years ago as the continents of Africa and North America collided. The Appalachian Mountains are part of an orogenic belt greater than 3,000 kilometers long and almost 500 kilometers wide. In contrast to the mountains in Tennessee and North Carolina, the northern part of the Appalachian Mountains in Virginia and Pennsylvania is a fold belt. The folds are several kilometers wide and responsible for the valley and ridge topography of the northern Appalachian Mountains. Another ancient thrust belt is the Rocky Mountains of Idaho and Wyoming. It was there that piggybacking was first documented.

In the twenty-first century, scientists continue studying thrust belts from the surface and using other methods. The use of Uranium–lead (U-Pb) dating in conjunction with structural and substructural analysis allows scientists to better understand the evolution of thrust belts. This method has been employed in the Jura Mountains in the European Alps. The evolutional study of thrust and fold belts has also allowed scientists to understand the stages of their structural development better. Finally, researchers have increasingly looked to the study of far older mountain ranges to understand the dynamics of fold and thrust belts in the twenty-first century. The expiration of folds and thrust belts continues to expand thanks to innovative technology such as satellite imagery and the ability to travel to areas once considered too challenging to reach. Three-dimensional structural modeling, field mapping, cross-section balancing, and 2D-kinematic modeling have allowed for a more complete structural analysis of folds and thrust belts. 

Principal Terms

continental crust: the upper 50 kilometers of the Earth below continents; it is composed primarily of rock that is lighter than that which makes up oceanic crust

continental margin: the edge of a continent next to an ocean basin; it is both exposed on land and submerged below water

faulting: the process of fracturing the earth such that rocks on opposite sides of the fracture move relative to each other; faults are the structures produced during the process

folding: the process of bending horizontal layers of rock so that they dip; folds include anticlines, which are arches, and synclines, which are shaped like the letter U or V

geosyncline: a major depression in the surface of the Earth where sediments accumulate; geosynclines lie parallel to the edges of continents and are long and narrow

orogenic belt: a mountain belt composed of a core of metamorphic and plutonic rocks and an adjacent thrust belt

Bibliography

Cubas, N., et al. “Prediction of Thrusting Sequence Based on Maximum Rock Strength and Sandbox Validation.” Trabajos de Geologia, vol. 29, 2009, pp. 189-195.

Finkl, Charles W., editor. The Encyclopedia of Field and General Geology. New York: Van Nostrand Reinhold, 1988.

Hammerstein, James A., et al. "Fold and Thrust Belts: Structural Style, Evolution and Exploration – an Introduction." Geological Society, London, Special Publications, vol. 490, 2020, doi.org/10.1144/SP490-2020-81. Accessed 23 July 2024.

Hatcher, Robert D., Jr. Structural Geology: Principles, Concepts, and Problems. 2d ed., Englewood Cliffs, N.J.: Prentice-Hall, 1995.

Lacombe, Olivier, et al., editors. Thrust Belts and Foreland Basins. Berlin: Springer, 2007.

Lee, Fang, et al. "Stress Evolution of Fault-and-thrust Belts in 2D Numerical Mechanical Models." Frontiers in Earth Science, vol. 12, 2024, p. 1415139, doi.org/10.3389/feart.2024.1415139. Accessed 21 July 2024.

Madritsch, Herfried, et al. "Reconstructing the Evolution of Foreland Fold-And-Thrust Belts Using U-Pb Calcite Dating: An Integrated Case-Study From the Easternmost Jura Mountains (Switzerland)." Tectonics, vol. 43, no. 5, 2024, p. e2023TC008181, doi.org/10.1029/2023TC008181. Accessed 21 July 2024.

Nemcok, M., S. Schamel., and R. Gayer. Thrust Belts; Structural Architecture, Thermal Regimes and Petroleum Systems. New York: Cambridge University Press, 2005.

Plummer, Charles C., and Diane H. Carlson. Physical Geology. 13th ed., Boston: McGraw-Hill, 2009.

Poblet, J., and R. J. Lisle. Kinematic Evolution and Structural Styles of Fold-and-Thrust Belts, Special Publication 349. London: Geological Society of London, 2011.

Rao, Gang, et al. "Editorial: Active Fold-and-Thrust Belts: From Present-Day Deformation to Structural Architecture and Modelling." Frontiers in Earth Science, vol. 9, 2021, p. 816157, doi.org/10.3389/feart.2021.816157. Accessed 21 July 2024.

Sharma, R. S. Cratons and Fold Belts of India. New York: Springer, 2009.

Short, N. M., and R. W. Blair. Geomorphology from Space: A Global Overview of Regional Landforms. NASA SP-486. Washington, D.C.: National Aeronautics and Space Administration, 1986.

Skinner, Brian F., et al. The Dynamic Earth: An Introduction to Physical Geology. 5th ed., New York: John Wiley & Sons, 2006.