Continental rift zones

Continental rift zones are places where the continental crust is stretched and thinned. Distinctive features include active volcanoes and long, straight valley systems formed by normal faults. Continental rifting, in some cases, has evolved into the breaking apart of a continent by seafloor spreading to form a new ocean.

Characteristics of Rift Zones

Continental rift zones are areas where the continental crust has been stretched and thinned. They are characterized by long valleys bounded by faults (rift valleys), active volcanoes within and adjacent to the rift valleys, earthquakes, hot springs, and other manifestations of unusually high temperatures near the Earth's surface. Rift zones are sometimes regions of high elevation, so the rift valleys' margins are high mountain ranges. Continental rifts are an expression at the surface of hot, partially molten rock in the mantle buoyantly rising beneath a continent.

Continental rifts are commonly linear valley systems that trend at a high angle to the direction in which crust has been stretched. Linear rifts include the Rio Grande Rift in New Mexico and Colorado, the Rhine Valley in northern Europe, and the East African Rift System (EARS) in Ethiopia, Kenya, Mozambique, and Tanzania. The EARS microplate named Victoria began rotating in the opposite direction of other nearby microplates in the late 2010s. Scientists cite movement in the lithosphere as the cause of this abnormality. Other continental rift zones are broad areas of alternating linear valleys and mountain ranges, such as the Basin and Range Province of western North America.

Grabens and Half-Grabens

The basic architectural unit in the upper crust of continental rift zones is the half-graben. Half-graben valleys collect sediment eroded from the adjacent, relatively uplifted fault block. The resulting sedimentary accumulations are wedge-shaped, thicker at the place of greatest subsidence adjacent to the bounding fault, and gradually thinner away from the fault. Some rift valleys are bounded by normal faults on both sides to form grabens rather than half-grabens. In most cases, however, the amount of slip is much larger across one of the two bounding faults, and the smaller fault is generally interpreted to represent a minor modification of the basic half-graben form. The size of a half-graben basin is determined by the length and amount of slip across the main normal fault that formed the basin. Sizes vary, but a typical major half-graben basin in a continental rift zone is fifty to 200 kilometers long and twenty to fifty kilometers across. Slip across the bounding fault of a large half-graben is typically several kilometers and may exceed ten kilometers.

Linear continental rift zones are chains of half-grabens, linked end to end, with one to three half-grabens occurring side by side across the rift. The linked half-grabens define a major valley system along which a large river system commonly develops, such as the Rio Grande and the Rhine River. The bounding normal faults of the end-linked half-grabens commonly alternate in the dip direction, so that the asymmetry of the half-graben basins reverses from one half-graben to the next down the rift valley. Broad rift zones such as the Basin and Range Province are also composed of half-grabens. In these areas, however, the half-grabens are arrayed side-to-side, such that the rift zone may be ten or more half-graben units wide, as well as being linked at the end. In contrast to the reversing asymmetry of end-linked half-grabens, laterally adjacent half-grabens commonly have the same asymmetry.

Normal slip-along faults stretch the crust horizontally. Estimates of the amount of stretching across continental rifts vary from a few kilometers across linear rifts such as the Rhine Valley to hundreds of kilometers across broad rift zones such as the Basin and Range. The higher estimates of extension predict extreme thinning of the crust if the crust does not change volume in the process. Although the crust in rift zones is thinner than normal (usually about 25 to 30 kilometers), it generally is too thick to be consistent with constant-volume stretching of the amount indicated by surface observations.

Rift-Zone

The discrepancy in crustal thickness is probably explained by the addition of new rock to the crust during rifting, resulting from intrusion and extrusion of magma derived from the upwelling mantle below. A significant fraction of rift-zone volcanism is basalt, which represents new crustal rock extracted from the mantle by partial melting. The amount of mantle-derived magma trapped within the crust to form intrusions probably greatly exceeds the amount erupted at the surface, so that although the amount of new crust formed during rifting may be quite large, the precise amount is not yet known.

Basaltic volcanism commonly occurs with eruptions of more silica-rich rocks, particularly rhyolite. The silica-rich rocks are believed to have formed from the melting of the continental crust by the heat carried into it by the mantle-derived basalt. This “bimodal” association of basalt and rhyolite has been considered a distinctive characteristic of continental rifts. However, studies have documented important exceptions. For example, much of the volcanism during rifting in the Basin and Range Province formed rocks of intermediate composition (specifically dacite) rather than a bimodal suite. The intermediate volcanic rocks are formed mainly by mixing basaltic and rhyolitic magmas. Therefore, the appearance of bimodal and intermediate volcanism depends on whether the basaltic magma and rhyolitic magma remain separate or are mixed. Modern research focuses on this mixing and what causes or prevents it.

Although it has been established that a close relationship exists between crustal stretching and volcanism, the nature of the relationship is controversial. Two main possibilities have been presentedthe active rift model and the passive rift model. It is uncertain if one of these models, or a combination of them, is correct. In the active rift model, upwelling in the asthenosphere causes rifting of the lithosphere. Hot, partially molten mantle rock rises buoyantly beneath a continental plate, releasing basaltic magma, which, in turn, rises into and through the plate. The heat from the basaltic magma warms the lithosphere, causing it to expand, resulting in the uplift of the Earth's surface in that area. Over geologic time, the heated lithosphere behaves roughly like a fluid, spreading out from the elevated area. In the upper crust, this spreading occurs by normal faulting.

In the passive rift model, stretching of the lithosphere causes the upwelling of the mantle and the basaltic volcanism. The lithosphere thins as it is stretched, allowing the underlying asthenosphere to well up passively beneath. The upwelling reduces the pressure in the asthenosphere because the weight of the overlying thinned lithosphere is less than that of lithosphere of normal thickness. Because melting temperatures decrease as pressure decreases, the pressure decrease induces melting of the asthenosphere and basaltic volcanism.

Continental drift was proposed in 1912 by German meteorologist Alfred Wegener to explain how continents moved over time. Modern-day geologists use the science of plate tectonics to describe this movement. Continental rifts are commonly thought of as features that lie within a lithospheric plate. In some instances, however, continental rifting evolves into seafloor spreading, breaking the plate in two and forming a new ocean basin at the site of the rift. Therefore, continental rifting might be viewed as the beginning of continental drift. In other cases, however, rifts cease activity without causing continental breakup, sometimes called “failed rifts.” There is a critical threshold at which a change occurs from continental rifting to seafloor spreading, but the nature of this threshold is still obscure.

The North Atlantic Ocean and its margins form a classic example of the results of this rift-to-drift process. At the beginning of the Triassic period, about 245 million years ago, there was no North Atlantic Ocean. The continents of North America, Europe, and Africa were joined together to form a part of the supercontinent Pangaea. During the Triassic, a broad continental rift zone formed that was similar to the modern Basin and Range Province of western North America. After twenty or thirty million years of rifting, the continent began to break apart, and seafloor spreading began, first between Africa and North America and later between Europe and North America. The record of continental rifting before the North Atlantic Ocean formed is left in the continental margins that surround the ocean. The continental shelf areas that surround the Atlantic are underlain by continental crust that contains numerous half-graben rift basins of the Triassic period.

Study of Rift Zones

Continental rifts and the processes that form them have been studied by virtually every geological, geochemical, and geophysical technique available. Applications of these techniques fall into three main categories: field studies of surface geology, laboratory analysis of samples collected in the field, and geophysical field studies. The study of any natural phenomenon starts with basic fieldwork, in this case, making maps of the rock bodies and their interrelationships as exposed at the surface. These data allow the geologist to draw inferences about the three-dimensional arrangement of rock bodies and the geologic history that led to that arrangement.

Field geologists' ability to formulate detailed and predictive interpretations from surface observations has been expanded greatly by new and improved laboratory measurement techniques. Potentially applicable techniques span all Earth science because tectonic analysis of a region involves synthesizing all pertinent observations in a single integrated framework. Of particular importance are paleontology and isotopic geochronology, which provide information about the timing of geological events and involve petrological and geochemical techniques that are used to infer the depth, rock type, and pressure and temperature at the source regions of rift-related volcanic rocks.

Most geophysical field techniques used in the study of continental rifts utilize seismology, the study of the way sound waves pass through the Earth. Earthquake seismologists analyze the spatial distribution of earthquakes to find which areas are tectonically active at present. Seismic refraction studies, using both earthquakes and artificial explosions as sound sources, are used to determine the thickness of the crust and its large-scale structure. Seismic reflection studies use artificial sound sources such as explosions or specially designed vibrator trucks to provide more detailed information about the internal structure of the crust.

The best approach in tectonic research, in studying continental rifts or any other type of feature, is to blend all these sources of information into an integrated scheme. For example, the three-dimensional structure of the crust can be inferred best by combining seismic reflection and refraction data with data regarding variations in the strength of the Earth's gravity field (which reflects variations in rock density at depth) and with surface geologic mapping.

Significance

Modern continental rift zones present significant seismic and volcanic hazards. The largest earthquakes along continental rift zones generally measure 7 to 7.5 on the Richter scale. They are, therefore, smaller than the magnitude eight or greater earthquakes that occur along large strike-slip faults, such as the 1906 San Francisco earthquake along the San Andreas fault, or the magnitude nine or greater earthquakes that occur along subduction zones, such as the 1960 earthquake in Peru. A large rift-zone earthquake is nevertheless capable of producing great destruction in areas near the epicenter, especially if buildings are not built to withstand earthquake stresses. Damaging earthquakes in Greece, for example, reflect continental rifting in the area of the Aegean Sea.

Continental rifts have been the sites of some of the Earth's largest explosive volcanic eruptions. Major eruptions from large rift-related volcanic centers can be literally thousands of times larger than the May 1980 eruption of Mount St. Helens. Because no historic eruption of this size has occurred anywhere on the Earth, it is hard to estimate how much damage would result from such an eruption. At least three, and perhaps more, such large explosive volcanic centers in the western United States were the sources of huge explosive eruptions within the last one million years and are still volcanically active (Yellowstone National Park in Wyoming, Long Valley in California, and the Valle Grande in northern New Mexico). Similar active centers are present on other continents.

Ancient continental rift zones are important sites of metallic mineral deposits, mainly formed at or near the explosive volcanic centers just mentioned. A large portion of the gold, silver, copper, lead, and zinc deposits in the Basin and Range Province of the western United States formed beneath or adjacent to rift-related volcanic centers. Petroleum accumulations are found in some half-graben basins in continental rift zones, such as the Great Basin, the Rhine Valley in Germany, and the Pannonian Basin in Hungary and Romania.

Principal Terms

asthenosphere: a layer in the upper mantle beneath the lithosphere that behaves as a fluid, permitting the overlying plates to move

crust: the outer layer of the Earth, composed of silica-rich, low-density rock, which in continental areas ranges from about twenty-five to seventy kilometers in thickness

fault: a large fracture or system of fractures across which relative movement of rock bodies has occurred

fault slip: the direction and amount of relative movement between the two blocks of rock separated by a fault

footwall: the rock body located below a nonvertical fault

graben: a roughly symmetrical crustal depression formed by the lowering of a crustal block between two normal faults that slope toward each other

half-graben: an asymmetrical structural depression formed along a single normal fault as the downthrown block tilted toward the fault

hanging wall: the rock body located above a nonvertical fault

lithosphere: the outer shell of the Earth, including both the crust and the upper mantle, which behaves rigidly over periods of thousands to millions of years

lithospheric plates: segments of the lithosphere that are similar in size to continents; these plates form a mosaic that covers the Earth's surface

mantle: the iron- and magnesium-rich, silica-poor part of the Earth beneath the crust

normal fault: a fault across which slip caused the hanging wall to move downward relative to the footwall

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