Transform faults

Transform faults occur along fracture zones found at the mid-oceanic ridges. The ridges are areas of erupting ultramafic lavas, which cause seafloor spreading, which, in turn, drives the moving lithospheric plates. Geologists have found that the offset ridges do not move in relation to each other and have been essentially fixed relative to the spreading sea floor.

Transform Fault Occurrence

Faults are regions of weakness or fractures in the earth's crust along which relative movement occurs. The simplest type of fault is the so-called normal fault, in which one block of crust is displaced vertically downward with respect to the other. A reverse or thrust fault is one in which the block is driven upward with respect to the other block. Yet another type of crustal displacement takes place horizontally, as one block slides laterally with respect to the other. These strike-slip or transcurrent faults are related to the most complex class of faults, known as transforms.

Because they are regions of crustal transformation, transform faults are found almost exclusively along the mid-oceanic ridges that nearly encircle the globe and along the boundaries of tectonic plates. The ridges are the sites of newly forming crustal materials composed of very dense magmas of relatively high iron and magnesium content. As the new crust forms, lava flows act to push the oceanic crust laterally away from the spreading ridge, as the sea floor spreads out at a rate of a few centimeters per year.

Mid–oceanic Ridge System

If one were able to view the mid-oceanic ridge system from orbit, it would quickly become apparent that the spreading centers do not occur along a smoothly continuous line but rather are broken into scores of offset ridges. The offset is marked by a fracture zone, which serves as the border between two spreading centers. Because the ridges are displaced, it was first believed that these fracture zones were simply transcurrent faults, along which right or left lateral displacement would be observed from opposing sides. The ridges are fixed with relation to each other and appear to have been so for long periods of geologic time. Clearly, a new type of faulting was being observed.

If one were to voyage to the bottom of the mid-Atlantic Ocean to view one of these faults, its highly unusual nature would become clear only after one had traveled hundreds of miles along its entire length. Starting at the west end of the transform fault, one would find that the crust is slowly moving toward the west on either side of the fracture zone. Because the crust is essentially moving in the same direction, earthquakes are rare events in this region.

Most transform faults are oriented at right angles to the mid-oceanic ridges. Typically, they extend from ridge to ridge (the active part of the fault); if spreading has been taking place for long periods of geologic time, fracture zones extend out from the ridge systems. These transforms and fracture zones are thought to be ancient areas of weakness in the crust that formed when the ocean basin (such as the Atlantic) began forming. The discontinuous nature of the ridge system is believed to be structurally ancient. When viewed globally, the spreading axis is offset from the earth's rotational axis, with the ridges corresponding to lines of longitude and the transforms roughly approximating latitudinal lines. Spreading rates are greatest at the equatorial regions of the globe.

Volcanic Eruptions and Earthquakes

The underwater ridge mountains are marked by a distinctive rift valley, centered along the range. Volcanic eruptions emanating from the rift create pillow-shaped lavas. The ridge is a boundary between the earth's lithospheric plates; it is a divergent boundary, for the sea floor is spreading laterally in opposite directions. A view of the ridge along the transform fault would reveal that the line of mountains is broken and offset. An observer passing over the high ridge on the north side of the fault would suddenly notice that the fault had taken on the appearance of a transcurrent fault. Movement on the south side of the fault is right lateral, and although the crust is traveling west, on the north side of the fault, the movement is toward the east. An observer on either side of the fault would see right lateral displacement.

Because crustal movement is in opposing directions between the offset ridges, earthquakes occur frequently in this area. The crust's relative motion is horizontal so that the focus or hypocenter (the actual point of rupture in the rock) of transform earthquakes is shallow—typically less than 70 kilometers deep, whereas trench earthquakes can be up to 700 kilometers deep. Magmatic eruptions and earthquakes offer convincing evidence that the earth's crustal plates are far from stationary. Past the offset ridge, crustal motion is once again in the same direction, and no lateral motion would be observed on opposite sides of the fracture. This transformation of crustal displacement along the shear zone is the derivation of the term “transform fault.”

While the Atlantic Ocean is a basin in which new crust is forming, causing the rifting of the continents and the growth of the ocean basin at a rate of a few centimeters per year, the opposite is the case in the Pacific Ocean basin. There, the transform faults are a bit more complex. An example would be the New Zealand fault, which terminates on both ends at subduction trenches rather than at ridges. New Zealand is seismically active, because the transform fault passes directly through both of the large islands.

San Andreas Fault

The most famous of all transform faults forms a distinctive type of lithospheric plate boundary. Extending from the East Pacific Rise off the coast of western Mexico to the Mendocino fracture zone and the Juan de Fuca ridge system off the coast of Washington State is the 960-kilometer-long, 32-kilometer-deep system of strike-slip faults known as the San Andreas. The San Andreas fault forms a boundary between the northward-trending Pacific plate and the North American plate. Horizontal displacement along this huge fault system is estimated at 400 kilometers since its inception nearly 30 million years ago. The rate of displacement has been measured at an average of 3.8-6.4 centimeters annually.

How did this impressive plate boundary form, and how does it fit into the transform fault model? About 60 million years ago, the east Pacific was also home to a third plate, the Farallon, which eventually was subducted under the North American plate. The Farallon plate was pushed eastward into the North American plate by a spreading ridge system and trench that were eventually overridden by the continental plate. The remnants of this ridge system are found at the ends of the San Andreas fault. Because the Pacific plate's motion was northward, the ridge system was converted into a transform fault, characterized by its right lateral strike-slip displacement.

Triple Junctions

The complexity of three plate interactions, or triple junctions, explains the complex nature of transform fault systems such as the San Andreas. The remnants of the doomed Farallon plate are presently found as the Juan de Fuca plate, which is bounded by spreading ridges and transform faults to the west and a subduction zone where the plate is being inexorably pushed into the earth's upper mantle, under the North American plate. Inland, the Cascade volcanoes, most notably including Mount St. Helens, have their volcanic fires fueled by a rising magma plume that is generated by the melting of the subducted oceanic plate.

North of the Juan de Fuca plate is another large transform system similar to the San Andreas fault, called the Queen Charlotte transform. An extension of the San Andreas system, the Queen Charlotte fault begins to the south at the Juan de Fuca ridge but becomes inactive at its northern end, whereas the San Andreas is a ridge-ridge transform that is bordered by the East Pacific Rise to the south and Gorda Ridge to the north.

The great ridge system that was overridden by the continental plate is responsible for the faulted structure of the Basin and Range Province, and the triple junction of the plates may have given rise to the magma plume or “hot spot” that is responsible for the Snake River plain volcanics and the thermal activity at Yellowstone National Park. Clearly, understanding the evolution of transform faults such as the San Andreas and the Queen Charlotte is pivotal to the understanding of the complex mountain scenery of western North American.

Evidence from the Sea Floor

A revolution in the earth sciences had its germination in the ideas of the German meteorologist Alfred Wegener, who argued that the continents had once been joined in one supercontinent and had since drifted apart. Wegener's theories of continental drift were not taken seriously until evidence from the sea floor forced a rethinking of modern geology in the 1960's. Oceanographic research in the 1950's led to a new picture of the ocean basins. Far from the featureless abyssal plains they were once thought to be, the basins proved to be marked by dramatic mountain ranges characterized by rift valleys. In 1960, a Princeton University scientist, H. H. Hess, suggested that convection currents in the earth's upper mantle were driving volcanic eruptions along the rifts, causing the sea floor to spread and the continents to move apart.

The real breakthrough in understanding the ocean floor came as a result of numerous cross-Atlantic voyages by research vessels towing submerged magnetometers in an effort to measure the strength of the crustal rock's residual magnetism. As the basaltic magma erupts at the spreading rift, magnetite freezes out of the melt at 578 degrees Celsius, and the earth's magnetic field orientation is frozen into it. This temperature is known as the Curie temperature.

Magnetic Anomalies

When the first magnetic maps of the sea floor were produced, scientists groped for an explanation of the alternating nature of the field's polarity, which changed in a random pattern over short distances. Two research students at the University of Cambridge, Fred Vine and Drummond Matthews, solved the riddle of the magnetic anomaly stripes by proposing that the stripes represented reversals in the earth's magnetic field over time. Because the anomaly patterns were symmetrical with respect to the axis of the spreading centers, this was proposed as evidence in favor of the Hess model of seafloor spreading.

The huge fracture zones that appeared to offset the mid-oceanic ridges were mapped with magnetic anomaly measurements, and seismic data indicated that seismic activity was concentrated between the offset ridges along the fracture zones. Networks of seismic instruments also enabled geophysicists to study the direction of the spreading crustal movement, through geometrical solutions known as first motion studies or fault-plane solutions.

In 1965, Canadian geophysicist J. Tuzo Wilson dubbed the faults “transforms” because of the changing of relative motion along the length of the fracture. He reasoned that the faults were not causing the offset of the spreading ridges, but that the offset is what caused the appearance of a transcurrent fault between the ridges and an inactive segment, in which the sea floor was spreading in the same direction beyond the zone of offset.

Geophysicists mapping the magnetic anomalies were aided by the work of terrestrial geologists, who were able to identify magnetic reversal patterns in terrestrial lavas that were identical to those found in oceanic rocks. Radiometric dating established a magnetic anomaly time scale that would allow scientists to determine the ages of rocks laterally out from the ridges and hence to deduce the rates of seafloor spreading. Armed with the paleomagnetic data and with Wilson's notion of transform faults as directional guides, scientists were able to show that the Atlantic Ocean had indeed been opening at a rate of a few centimeters per year and that the landmasses of western Europe and eastern North America were once essentially in contact.

Identifying Transform Faults

Wilson and others went on to study the more complex faults of the eastern Pacific. The San Andreas fault had been identified long before as a transcurrent fault. The new paradigm of plate tectonics placed it in a global tectonic scale; the fault is now accepted by most as a kind of transform fault that connects to spreading ridge centers to the north and south.

While faults such as the New Zealand Alpine and San Andreas are fairly accessible to scientists, most of the planet's transform faults lie below thousands of meters of ocean. Echo sounding, radar, sonar seismographic, and magnetic anomaly maps have helped scientists to locate the earth's transform faults. The 1978 Seasat mission radar mapped the ocean floor from space, producing detailed information on the globe's mid-oceanic ridge and fault system. In addition, deep-sea submersibles have been piloted to the rifts, where eruptions of lavas and fault displacements have been observed directly.

Plate Tectonics

The theory of plate tectonics in the 1960's caused a revolution in the study of geology, geophysics, and even paleontology. Post-World War II oceanographic research led to the discovery that the mid-oceanic ridges are sites of newly forming crust and resulted in Wilson's explanation of transform faults as ancient fracture zones offsetting the spreading ridges.

With a vast supply of paleomagnetic, seismic, geologic, and other evidence at their disposal, geologists have been able to reconstruct earth history and the relative motions of the continents, using transform faults as directional guides and magnetic reversals to determine the rates of plate movement. Like any successful theory, plate tectonics is elegantly simple; it explains nearly all the earth's diverse landforms and rock formations. Changes in continental distribution may also be linked to the extinction events that punctuate the geologic time scale.

Geologists' understanding of the interaction between ridge systems, subduction trenches, and transforms has led to a better understanding of the paleogeography of the American West and how that complex mountainous region of active faults and recent volcanic activity came into being. Aside from contributing to the fundamental understanding of earth processes, plate tectonics theory explains forces that can influence human lives. Active plate boundaries are the sites of earthquake and volcanic activity. In California, millions of people live and work astride one of the world's largest transform faults, the San Andreas. Residents of Vancouver and Seattle are similarly threatened by the offshore Queen Charlotte Islands fault. On the other side of the Pacific Ocean, New Zealand is nearly bisected by the New Zealand Alpine fault, making it a seismically active region. Whenever human beings decide to build their homes and cities near these moving regions of the earth's crust, the possibility of geologic catastrophe is very real.

Principal Terms

Curie temperature: the temperature below which minerals can retain ferromagnetism

divergent boundary: the boundary that results where two plates are moving apart from each other, as is the case along mid-oceanic ridges

ferromagnetic: relating to substances with high magnetic permeability, definite saturation point, and measurable residual magnetism

fracture zone: the entire length of the shear zone that cuts a generally perpendicular trend across a mid-oceanic ridge

hypocenter: the initial point of rupture along a fault that causes an earthquake; also known as the focus

magnetic anomalies: patterns of reversed polarity in the ferromagnetic minerals present in the earth's crust

mid-ocean ridge: a long, broad, continuous ridge that lies approximately in the middle of most ocean basins

rift valley: a region of extensional deformation in which the central block has dropped down with relation to the two adjacent blocks

seafloor spreading: the hypothesis that oceanic crust is generated by convective upwelling of magma along mid-ocean ridges, causing the sea floor to spread laterally away from the ridge system

transcurrent fault: a fault in which relative motion is parallel to the strike of the fault (that is, horizontal); also known as a strike-slip fault

Bibliography

Condie, Kent C. Plate Tectonics and Crustal Evolution. 4th ed. Oxford: Butterworth Heinemann, 1997. An excellent overview of modern plate tectonics theory that synthesizes data from geology, geochemistry, geophysics, and oceanography. Of special interest is Chapter 6, on seafloor spreading, and Chapter 9's treatment of the Cordilleran system, including a discussion of the evolution of the San Andreas fault. A very helpful tectonic map of the world is enclosed. The book is nontechnical and suitable for a college-level reader. Useful “suggestions for further reading” follow each chapter.

Cox, Allan, and R. B. Hart. Plate Tectonics: How It Works. Palo Alto, Calif.: Blackwell Scientific, 1986. A valuable treatment of the geometrical relationships and movements of the earth's lithospheric plates. Designed for the reader who has a basic qualitative knowledge of plate tectonics but who wishes to learn more, particularly about quantitative analysis of plate movements. Filled with easy-to-follow exercises that demonstrate plate motions, particularly those associated with transform faults.

Doyle, Hugh A. Seismology. New York: John Wiley, 1995. A good introduction to the study of earthquakes and the earth's lithosphere. Written for the layperson, the book contains many useful illustrations.

Kearey, Philip, Keith A. Klepeis, and Frederick J. Vine. Global Tectonics. 3rd ed. New York: Wiley-Blackwell, 2009. A great overview of tectonics, Chapter 4 has a section covering transform faults. Written for the college student.

Lambert, David, et al. The Field Guide to Geology. 2d ed. New York: Facts on File, 2007. For the beginning student of geology, this reference work is filled with marvelous diagrams that make the concepts easy to understand. Suitable for any level of reader.

McClay, Kenneth R. Thrust Tectonics. London: Chapman and Hall, 1992. This collection of papers was presented as part of the Thrust Tectonics Conference held at Royal Holloway and Bedford New College, University of London, in 1990. The advanced nature of the collection makes this most useful for the college student.

Mitra, Shankar, et al., eds. Structural Geology of Fold and Thrust Belts. Baltimore: Johns Hopkins University Press, 1992. A good discussion of physical geology focusing on the structure and processes of thrust faults and folds. Illustrations, bibliography, and index.

Redfren, Ron. The Making of a Continent. New York: Times Books, 1983. Richly illustrated with dramatic photographs, this book is a lucid discussion of plate tectonics with respect to the continent of North America. Contains excellent explanations of seafloor spreading and transforms, along with a section on the San Andreas fault.

Shea, James H., ed. Plate Tectonics. New York: Van Nostrand Reinhold, 1985. A collection of classic and key scientific papers, mostly from the 1960's, that together constitute a sweeping overview of plate tectonic geology. Of special interest are papers by Fred Vine and J. Tuzo Wilson on the magnetic anomalies off Vancouver Island and a paper by L. R. Sykes on transform faults at the mid-oceanic ridges. With chapter introductions by the editor, the work is suitable for a college-level reader with an interest in the history of plate tectonics theory.

Shepard, Francis P. Geological Oceanography. New York: Crane, Russak, 1977. Chapter 2 addresses seafloor spreading and faulting of the oceanic crust. Photographs, diagrams, and supplementary reading lists augment the text, which is suitable for a beginning geology or oceanography student.

Sullivan, Walter. Continents in Motion. 2d ed. New York: American Institute of Physics, 1993. Dedicated to Harry Hess and Maurice Ewing, two late pioneers of plate tectonics theory, this is the classic popular work on moving crustal plates. Well-written explanations of transform faults and their roles in seafloor spreading and a discussion of the San Andreas fault are included in the highly readable text.

Tarbuck, Edward J., Frederick K. Lutgens, and Dennis Tasa. Earth: An Introduction to Physical Geology. 10th ed. Upper Saddle River, N.J.: Prentice Hall, 2010. This basic geology textbook contains a section on rock deformation. It provides diagrams to illustrate the various types of fault and the concepts of strike and dip, and it includes a photograph of an overthrust formation. Review questions and a list of key terms conclude the chapter. For high school and college-level reader. It has excellent illustrations and graphics.

Wilson, J. Tuzo, ed. Continents Adrift and Continents Aground. San Francisco: W. H. Freeman, 1976. Selected, classic readings from Scientific American are introduced with commentary by Wilson, a leading figure in the history of plate tectonics theory. Chapter 2 deals with seafloor spreading and transform faults with a classic article by Don L. Anderson on the San Andreas fault. Suitable for a general audience. Provides a historical perspective of plate tectonics. Contains a bibliography.

Wyllie, Peter J. The Way the Earth Works: An Introduction to the New Global Geology and Its Revolutionary Development. New York: John Wiley & Sons, 1976. Wyllie's book has a very informative section on transform faults and earthquake studies. An extensive list of suggested readings augments the text, which is suitable for a college-level reader.

Young, Patrick. Drifting Continents, Shifting Seas. New York: Franklin Watts, 1976. A good entry-level discussion of plate tectonics theory, written by a journalist with a knack for simplifying complex concepts. Contains a brief glossary, indexed with a bibliography. Suitable for high school readers.