Transform faults

Transform faults are plate boundaries at which the relative movement between two plates is parallel to the boundary. Transform faults occur in both continental and oceanic lithosphere.

There are two types of transform faults, depending on the sense of movement. This can usefully be determined by imagining yourself standing on one side of the plate boundary, looking towards it.  If the plate on the opposite side appears to move to your right, then the boundary is dextral or right-lateral. Alternatively, an imaginary ball-bearing trapped between the two plates would be rotated clockwise.

If the plate on the opposite side appears to move to your left, then the boundary is sinistral or left-lateral. Alternatively, an imaginary ball-bearing trapped between the two plates would be rotated counterclockwise.

The San Andreas Fault, the best-known example of a transform fault, is dextral. The Arava–Jordan fault system that passes through the Dead Sea, is an example of a sinistral transform fault.

Many transform faults are remarkably straight. Typically, the zone of deformed rocks along a transform fault is very narrow. However, small bends can occur on transform faults, and depending on their orientation, they may be restraining bends where the two sides of the plate push together slightly, or releasing bends, where the plates pull apart. In this second case, a small, rapidly subsiding pull-apart basin may form. Pull apart basins tend to trap sediment, and they may contain hydrocarbon deposits. The Los Angeles Basin in California is an example of a sedimentary basin formed in association with a transform fault. It contains substantial amounts of oil and natural gas. In Nova Scotia, Canada, the Stellarton Basin formed at a dextral transform fault that existed in the Paleozoic Era. Because Nova Scotia was in the tropics aat the time, tropical rain forest were able to deposit numerous peat layers, which became coal seams that were intensively mined in the 20th century, continuing to the present day at the time of writing (2023).

Convergent tectonic environments

Convergent tectonic environments are those where plates are converging. These are some of the most varied and dynamic environments, producing earthquakes, volcanoes, and a variety of resources that are mined by humans. Convergent environments are responsible for the highest mountains and the deepest ocean trenches on Earth. There are a number of types of convergent tectonic environments, depending on the presence of oceanic or continental crust on either the lower or the upper plate of the convergent system. We here examine four different permutations of continental and oceanic crust in convergent environments. At the end of this section, we will look at foreland basins, depressed areas of the lithosphere where a plate is held down by the isostatic effect of adjacent mountains.

Subduction zones and volcanic arcs

Subduction zones are sites where plates continuously converge. The subducted plate in most subduction zones carries oceanic crust, returning it through the subduction process back into the mantle.

Subudction zones produce the largest earthquakes, accounting for all the magnitude 9 (or higher) earthquakes recorded since the advent of modern seismology in the mid-20th century.

What happens to the subducted lower plate?

The rocks of the downgoing plate are subjected to metamorphism at high pressure, but relatively low temperature, because it takes millions of years for the thick slab of oceanic lithosphere to warm up to the temperature of the surrounding mantle.

Under such conditions, feldspar, usually one of the most abundant components of igneous rocks, becomes unstable. If there is water in the rocks of the downgoing slab, ferromagnesian minerals may absorb sodium from feldspar to produce a distinctive blue chain silicate mineral glaucophane. Such metamorphic rocks are known as blueschists. A second very distinctive metamorphic rock type occurs where high pressure conditions affect mafic rocks without water. Aluminum from feldspar combines with other silicates to form the red mineral garnet, while sodium dissolves in green pyroxene. The resulting red and green metamorphic rock is called eclogite. This mineral assemblage is only stable below about 45 kilometres depth, where the pressure is 1.2 GPa or higher.

Although both these metamorphic rock types have the same overall composition as mafic igneous rock like basalt and gabbro, they are much denser. They are even denser than typical mantle peridotite. It is therefore believed that the formation of eclogite plays a role in driving plate tectonics, because it makes the slabs of ocean floor that hang down below subduction zones are denser than the surrounding mantle, and therefore help to pull the subducted plate down..

Blueschist and eclogite may be brought back up to the surface under exceptional circumstances, particularly if subduction stops as a result of plate rearrangment. They are quite rare rocks but, when found, are clear indicators of ancient subduction zones.

Volcanic arcs and active continental margins

Initially, the rocks of the downgoing plate typically contain quite large amounts of water, locked up in hydrous minerals formed on the ocean floor. As they are metamorphosed into eclogite, these rocks lose water, which flows into the overlying mantle. As explained in the section on igneous rocks  the addition of water to mantle peridotite causes partial melting, forming magma that rises to the surface and is responsible for the formation of a volcanic arc on the upper plate of the subduction zone. As you might expect, those volcanoes are rather different from those formed at mid-ocean ridges, because the magmas in subduction zones contain significant amounts  of water together with traces of material derived from the downgoing slab. That causes them to evolve somewhat differently as they rise towards the surface. Typically they form intermediate magma, and the water they contain  makes them more explosive than mid-ocean ridge volcanoes. Typical subduction-zone volcanoes are conical stratovolcanoes like Mount Fuji in Japan or Mount Rainier in Washington State, USA. Two examples in Indonesia, Krakatau and Tambora, were responsible for huge eruptions in the 19th century CE that put so much tephra into the atmosphere that they significantly cooled Earth’s climate for several years. A similarly powerful eruption of Mount Pinatubo in the Philippines in 1991 also had a detectable cooling effect on the climate.

The overlying plate, above the subduction zone, experiences slightly different phenomena depending on whether it is continental or oceanic.

Oceanic Island Arcs

In the situation where the overlying plate is oceanic, it typically carries a curved chain of volcanoes known as an island arc. There are many examples around the West Pacific. Island arc volcanoes erupt mafic magma (formed by partial melting of peridotite), but they also erupt intermediate magma. The intermediate compositions probably result from the long journey that these magmas have taken (typically about 100 km) from their source in the mantle wedge. As the magma it rises towards the surface under the influence of water, some crystals of ferromagnesian minerals settle out, leaving the magma richer in silica, giving it a more intermediate composition. This process of magma differentiation is probably very important in the overall evolution of the Earth’s crust. Most geologists who have worked on the overall evolution of the Earth think that island-arc formation this is one of the main process is whereby the Earth’s silica-rich continental crust has formed over time.

Continental volcanic arcs and active continental margins

Where the upper plate in a subduction zone is continental, as occurs at the western edge of South America and parts of North America, volcanoes tend to be even more silica rich, resulting in intermediate to felsic compositions. This results both because of the long journey that the magma takes to the surface (allowing plenty of time for differentiation) and also because the magma may melt parts of the silica-rich continental crust that it passes through. Intermediate to felsic volcanic activity has produced the famous volcanoes of the Cascade Range in western North America, including Mt. St. Helen’s, Mt. Rainier, and the former Mt. Mazama in Oregon, which exploded about 8000 years ago forming Crater Lake, and covering the Pacific northwest with tephra, some of which is preserved as far north as Edmonton, Alberta.

The western margins of both the North and South American continents are also plate boundaries. These continental margins are therefore known as active continental margins. In addition to the volcanic activity at the surface, a lot goes on beneath the surface in these areas. As you might expect, many of the volcanoes sit above bodies of magma that cool down before reaching the surface to produce igneous intrusions. The magma typically contains a lot of water, and hot water penetrates into the surrounding rocks, carrying with it dissolved metals, which may be deposited within the pore spaces to produce mineral deposits. One very important category of deposit is called porphyry copper. Porphyry is a name for an intrusive igneous rock that formed only a short distance below the surface; it contains a mix of coarse crystals (recording slow cooling of the magma while it was deep down) and finer grains formed by more rapid cooling of the remaining magma near the surface. Copper is present in these deposits mainly as sulphides. Some porphyry deposits also contain silver, gold, and molybdenum. One of the challenges in extracting copper from these rocks (as is also true of volcanic massive sulphide deposits in oceanic crust) is that the sulphur has to be removed by reacting the ore with oxygen, which generates the acidic gas sulphur dioxide, a major contributor to acid rain.

It’s clear from the map of North and South America that active continental margins are typically mountain belts. These mountains are produced by shortening of the continental crust in response to convergent plate movement. When we look at rocks  that have been exposed by erosion in active continental margins, it’s common to see metamorphic rocks with fabrics, and sedimentary strata that have been shortened by folds and faults. Faults produced by shortening are typically thrust faults, sloping faults where the upper block of rock (the hanging wall)  has moved up and over the lower block (the footwall). Typically this means that they place older strata on top of younger, in violation of the principle of superposition.

The formation of mountains by shortening of the crust is known as orogenesis, from Greek words meaning mountain formation, and the belt of mountains formed by this process is known as an orogen. In North Americal the orogen along the Pacific continental margin is known as the Cordillera or Cordilleran orogen. In South America the corresponding mountain belt along the active continental margin is called the Andes. Orogens formed like the Andes, as a result of continuous subduction of oceanic lithosphere, often for tens of millions of years, beneath an active continental margin, are sometimes called Cordilleran or Andean orogens.

Collisional orogens

In the previous sections we have distinguished two different types of subduction zone based on whether there is continental or oceanic lithosphere in the upper plate of the system, when the lower plate is oceanic. What happens if the lower plate in a subduction zone contains a continent? Because continental crust is much less dense than oceanic crust, it’s buoyant and resists being subducted because of isostasy. The downgoing continental crust may jam up the subduction zone, causing a range of mountains, a collisional orogen, to grow over a few million years. Once again, we can distinguish two possibilities based on the nature of the upper plate. If the upper plate carries an oceanic island arc, then the process is described as continent-arc collision. If on the other hand the upper plate is continental, then the process is called continent-continent collision.

Arc-continent collision

Arc-continent collision is taking place in at least two places in the western Pacific Ocean region at the present day.  In Taiwan, a former island arc is colliding with the passive continental margin of China, building a new orogen on the previously passive continental margin. The area is an active earthquake belt, as thrust faults are shortening the crust of Taiwan. In Papua New Guinea, the northern passive continental margin of Australia started colliding with an oceanic island arc in the late Cenozoic Era at about 12 Ma. A complex series of collisions has thrust a large ophiolite, a slab of oceanic lithosphere, onto the continental margin. As the plates continued to converge, the buoyant Australian continental margin could not be subducted because of isostasy, so new subduction has started to pull the Pacific Ocean floor under the Australian to the south, converting it into an active continental margin.

Closely similar events occurred along the eastern margin of North America , but about 470 million years ago in the early Paleozoic Era. In western Newfoundland, a slab of oceanic lithosphere forms the Bay of Islands ophiolite, which was thrust onto the margin of ancient North America during an continent-arc collision.

Ophiolites in orogens preserve the same types of mineral deposits as are found in the ocean basins, particularly including volcanic massive sulphide deposits. (See the section on spreading centres for more information.)

Continent-continent collision

Continent-continent collision occurs when continental lithosphere arrives at both sides of a subduction zone. Continent-continent collision has been responsible for the largest orogen on the modern Earth, the Himalaya, together with the Tibetan Plateau which lies to its north. Collision is believed to have doubled the thickness of the continental crust beneath the Himalaya and the Tibetan Plateau, beneath which the Moho occurs at about 70 km depth.  As in other types of orogen, this thickening is achieved in part by the motion of thrust faults, and movement on these faults leads to damaging earthquakes.

Thickening of continental crust, especially sedimentary rocks containing water, can lead to partial melting, as wet rocks are subjected to increasing temperatures and pressures. The resulting magma is felsic, and produces large plutons of granite. Additional melting can happen when the lithosphere slab breaks off and hot asthenosphere rises into the space beneath the continental crust.

Felsic intrusive rocks in collision zones are sometimes are mined for rare metals including lithium, a metal of increasing strategic importance for the production of batteries.

Foreland basins

Wherever mountains are formed in an orogen they have an isostatic effect on the adjacent plate, pushing the plate down. When this plate carries continental crust, a deep trough may be formed, called a foreland basin. Sedimentation in foreland basins is typically rapid, because a lot of material is eroded from the adjacent mountains. Initially these sediments are typically deposited by underwater flows of turbidity currents, but eventually, if sedimentation catches up with subsidence and fills the foreland basin to sea-level, turbidites may be overlain by shallow-marine sediments or river deposits.

Foreland basins have a very characteristic wedge shape in cross-section, deepening steadily towards the mountains. A long-lived orogen like the North American Cordillera may incorporate slices of its own foreland basin as it grows.

In various parts of the world, foreland basins contain major deposits of coal, or reservoirs of oil and natural gas. For example, in North America, the Paleozoic foreland basin of the Appalachian mountain range contains major coal deposits. The Mesozoic foreland basin of the Cordillera, known as the Western Canada Sedimentary Basin, overlies an older passive continental margin and contains large amounts of oil and natural gas. At least 400,000 oil and gas wells have been drilled in Alberta alone.

Cratons and supercontinents

Cratons

Continental areas that have been stable since about 500 Ma are known as cratons. Where igneous and metamorphic rocks are exposed at the surface, cratons are known as shields. Portions covered with sedimentary rock are known as continental platforms.

Large igneous provinces

Compared with plate margins, cratons in plate interiors are relatively slow-moving tectonic environments, but they are not without important resources.  Hotspots in oceanic lithosphere (such as the one beneath Hawai’i)  are mentioned in the section on plate movement. However, mantle plumes can also rise  beneath continental crust, where they are responsible for the development of large igneous provinces: masses of magma producing intrusions and volcanoes within continents. Some large igneous provinces have eventually evolved into rifts. Others seem to have died out when the source of heat was removed.

Magmas in these provinces are predominantly mafic, because they are derived by partial melting of the mantle as it rises adiabatically and experiences lowered pressure. Mafic magma rises into the crust where it cools gradually. Crystals may grow and sink within the magma. Because the crystallizing minerals have different overall composition from the magma, there may be subtle variations in the composition of the magma. The settling crystals record these changes by forming layers on the floor of the magma chamber, rather like the layers typical of sedimentary rocks, but quite different in composition. Once cooled, these bodies of magma form layered igneous intrusions.

In some layered igneous intrusions, minerals such as chromite, a chromium oxide, are highly concentrated in dark layers.  One example, the Bushveld intrusion in South Africa, contains over half of the Earth’s chromium reserves, together with very large reserves of platinum group elements. Chromium is an important component of many types of steel, as it both hardens the metal and makes it rust-resistant. Platinum group elements have major uses in catalytic converters (used to convert poisonous carbon monoxide to carbon dioxide in car exhaust systems) and in jewelry.

Another type of igneous rock that is associated with stable continental plate interiors is kimberlite. Kimberlite is a fragmental rock produced in violent volcanic eruptions that are clearly sourced from deep in the mantle, because the fragments include ultramafic rocks and minerals that can only form at very high pressure. They are humanity’s main source of diamonds. No kimberlite eruptions have ever been recorded in real time, but kimberlite pipes are known from most of the Earth’s continental areas, and their shape has been reconstructed from data collected during diamond mining operations. The most recent erupted in the Igwisi Hills in Tanzania at about 12 ka, young enough to have been observed by humans but too old for there to be any known historical record. The next youngest kimberlite occurred at about 30 Ma.

When not subject to igneous activity from mantle plumes, cratons typically show a history of gentle subsidence. Sedimentary strata may be deposited on the top of a craton, forming a continental platform, and relatively thin sedimentary basins may form in these areas. A notable example is the Proterozoic Athabasca Basin on the Canadian Shield, which is a major source of the element uranium. Uranium is present in small amounts throughout the shield, particularly in felsic igneous rocks. However, it may become concentrated in clastic sediments that were derived by erosion of the shield, particularly at the boundaries between oxidised and reduced sedimentary rocks. Uranium is mined by humans because of its radioactive properties. The more radioactive isotopes are concentrated for human uses. During the period 1945–1965 CE this was primarily for weapons. Subsequently, larger amounts of uranium have been extracted for electricity generation in nuclear reactors. The mining of uranium involves significant health hazards from radioactivity, which have been disporportionately borne by Indigenous communities over its relatively short history. Other environmental hazards from the use of uranium include accidents at nuclear facilities (including those at Chernobyl in Ukraine in 1986 and in Fukoshima in Japan as a result ot the Tohoku earthquake in 2011). An additional challenge is the disposal of nuclear waste: because of the long half-lives of uranium isotopes, reactor fuel remains radioactive long after the energy output has declined below what is useful for electricity generation. However, producing energy from uranium avoids the inevitable greenhouse gas impact associated with burning hydrocarbons. Balancing the environmental and health impacts of these different energy sources on the spheres of the Earth is a major challenge facing humanity.

Supercontinents and the history of the crust

Our review of plate tectonics has shown that the oceanic lithosphere, including the oceanic crust, is continuously being recycled into the asthenosphere and lower mantle by the process of subduction, which consumes lithosphere at the same rate that it is formed at spreading centres. As a result of this continuous recycling, no in-place ocean floor is older than about 200 Ma. The only fragments of older ocean floor are those preserved as ophiolites in orogens.

In contrast, continental crust, once formed, is never completely subducted. (In a few orogens we find evidence from high-pressure minerals that continental crust may have been pulled deeper than 100 km, but the ‘attempted subduction’ of continental crust only lasted a few million years before isostasy brought it back to the surface again. As a result, continental crust, once forms, remains at the surface indefinitely, and the continental crust contains a much longer (though more intermittent) record of Earth history, going back to about 4.0 Ga, the age of the Acasta Gneiss in the Northwest Territories of Canada.

The amount of continental crust has probably increased through geologic time, though whether this accumulation of continental crust was steady or irregular is a current controversy in Earth science. What is known, is that at various times in Earth history, the majority of the Earth’s continental crust has become clumped together in a single supercontinent. 

Before this time, there is a great deal of controversy about the nature of the tectonic system. There was undoubtedly a tectonic cycle in the Archean, but whether it was the same or different from modern plate tectonics is a matter of debate. There appear to be systematic differences in the characteristics of Archean rocks suggesting that the tectonic system was different. For example, Archean felsic intrusive rocks typically contain less potassium and more sodium than modern granites, and passive continental margins, characterized by thick accumulations of shallow-water shelf sediment, are unknown before about 3 Ga. The Archean Earth must have looked very different from the present-day Earth, but the exact nature of the Archean tectonic system is a subject of active research among Earth scientists.