Continental structures

The continental crust is layered on a large scale and can, in most places, be divided into upper granitic and lower gabbroic layers. A variety of rock types and structures are superimposed on this compositional layering. Thus, the crust is a mosaic of geological terranes that have been assembled to form the present continents.

Crustal

The continental crust contains rocks of many different ages that are complexly related to one another. These rocks were formed and emplaced by various geological processes operating from the earliest Precambrian (approximately 4.6 billion years ago) to the present. As such, the continental crust preserves the most complete history of the development of the Earth. Most of what can be seen of the continents is limited to the surface outcrops and information from some deep mines and drill holes. These observations have shown that the continents consist of a veneer of sediments and sedimentary rocks overlying a complex basement of igneous and metamorphic rocks.

On a large scale, the continental crust appears to be a horizontally layered mass of variable thickness. The base of the crust is defined by the Mohorovičić discontinuity, the boundary where the rapid increase in seismic wave velocity marks the beginning of the mantle. The depth to the Mohorovičić discontinuity, and therefore the continental crustal thickness, varies between fifteen and eighty kilometers. The continents are thickest under the great mountain systems and appear thinnest where the crust is submerged beneath sea level along continental margins or where it has been subjected to rifting. Over much of the interior of the continents, the Mohorovičić discontinuity appears to undulate gently, yielding crustal thicknesses between twenty-five and forty-five kilometers.

In most places, seismic refraction studies have shown that below the sedimentary veneer, the continental crust is divided into two, three, or four layers defined by crustal discontinuities. In general, the lower crust has a higher seismic wave velocity and a composition that is more mafic than the upper crust. Comparisons of seismic velocities suggest that the lower crust is gabbroic. In contrast, the upper crust has the composition of granite or granodiorite. While the terms used for the crustal layers are igneous, scientists think most of these rocks have been metamorphosed. Two kinds of observations confirm the existence of this layering. First, in some mountain ranges, more than ten kilometers of uplift of early Precambrian rocks has occurred. The rocks exposed in these eroded mountains are granodioritic to dioritic gneisses, similar to those deduced from the seismic studies. Second, independent geophysical studies, including seismic refraction, gravity and magnetic anomalies, and heat-flow measurements, confirm the trend to more basic rocks in the deep crust.

Origin of Crustal Layering

The occurrence of denser (gabbroic) material at the base and lighter (granitic) rock at the top of the crust is consistent with what is expected based on the segregation of materials by gravity. While the mechanism by which this gravitational segregation developed is not known with certainty, major possibilities that have been considered include remobilization, partial melting, and upward migration of magmas throughout much of geologic time. The low melting fractions form magmas, which migrate upward and crystallize, resulting in rocks that are usually lighter. Thus, igneous processes may lead to the concentration of the less dense granitic fraction to the top of the crust, which is consistent with the distribution of radioactive elements deduced from heat flow and measures of natural radioactivity. The second idea proposes that the crust has gradually thickened with time because of subcrustal deposition of basic material out of the mantle. The last materials to come from the mantle might be denser and more gabbroic than the first, more granitic, upper layers. Whatever the cause, some horizontal layering occurs in all areas of the continental crust. In places, three layers will change gradually into two or into four over distances of a hundred kilometers. In other cases, more sudden lateral changes occur.

Considerable detail on the structure of continental layering has been derived from deep seismic reflection studies. These studies have shown the existence of thick sections of layered rock within the Precambrian crust. These layered rocks have been interpreted as piles of volcanic and sedimentary rock that have been metamorphosed and preserved within the crust. Elsewhere, major reflectors appear to represent thrust faults within the upper or middle sections of the crust, attesting the ability of tectonic events to affect more than the uppermost crustal layers.

Geologic Terranes

The continental crust is a mosaic of subcontinental, geologic terranes with different ages, rock types, rock structures, and geologic histories. For many years, this mosaic was recognized in the continental shields, where age determinations on the outcropping rocks allowed the division of the exposed Precambrian rocks into provinces. Within the Canadian Shield of North America, the structures of the younger provinces crosscut earlier structures, and the igneous and metamorphic processes associated with the development of the younger province rework the rocks in older provinces. A classic example occurs in Quebec and in eastern Ontario, where there is a juxtaposition between the early Superior Province, dating from 2.8 to 2.5 billion years before the present, and the linear, northeast-trending Grenville Province, dating from 1.2 to 0.9 billion years before the present. While studies of the shields had demonstrated the existence of crustal provinces, much of the crustal mosaic on the craton was hidden beneath sedimentary rocks of Paleozoic and younger ages. Scientists aided by geophysical surveying recognize many provinces within the continental interiors, based on the character of gravity and magnetic anomalies and on observed crustal layering.

Along the margins of continents or within modern mountain ranges, continental structures have been studied extensively. These regions have been most affected by the most recent cycle of seafloor spreading and plate motions. Thus, geologists have found structures within the continents related to divergent plate motions, involving normal faulting and volcanic activity. To convergent motions, leading to thrust faulting, volcanic activity, metamorphism, and batholith formation; and to transform boundaries such as that found along the San Andreas fault in California.

Many “foreign” or “suspect” geologic terranes have been found within the younger mountain belts. These foreign terranes are often geologically and structurally distinct from adjacent terranes. These terranes are similar to the Precambrian provinces of the continental crust in that they have been assembled with other crustal components to form the modern continents. The boundaries between the foreign terranes or between crustal provinces are similar in that they often involve thrust faults that may affect a significant thickness of the crust and that may even displace the Mohorovičić discontinuity and involve the mantle.

Rift Systems

Continental structures indicate that continents have been assembled over a long period and that the process is ongoing. The latest cycle of plate motions tells scientists, however, that continents are subject to forces and processes that tend to disassemble them as well. While these forces often lead to a continental breakup, they are only partially successful in many cases. In these cases, scars are left on the continental crust, recording the event. In general, the disruption of the continent is marked by thinning or rifting of the crust and by injection of material from the mantle.

Where the continental breakup is incomplete or stalled, the rift zone and injected igneous rocks are preserved as bodies of dense, highly magnetic rock that cut across the layering of the continental crust. One such zone, which extends within the midcontinent of the United States from Lake Superior through Minnesota and Iowa on into Kansas, is the largest continuous section of a Precambrian rift system that has been largely disrupted by later tectonic events. Other failed rift systems include faults and intrusions in the lower Mississippi River Valley and in Kentucky. Such failed rift systems usually involve the intrusion into the crust of large volumes of mafic igneous rock. Such dense intrusive masses may have considerable influence on later geologic events, such as the formation of depositional basins and the location of inland seas.

Gravity and Magnetic Anomalies

The structure of continents has been defined by geologic mapping, radiometric age determinations, remote-sensing techniques, and geophysical methods. By far, the predominant methods of characterizing the deep crust or areas covered with sedimentary rocks are the geophysical methods, primarily gravity anomalies, magnetic anomalies, and seismic refraction and reflection profiles.

Gravity anomalies allow scientists to investigate the mass distribution in the subsurface. Observations are made with a gravimeter, which measures values as small as 0.00001 centimeter per second squared (0.01 milligal), which is approximately one part in one hundred million of the Earth's total gravity. Surveys are done where elevation and locations are well known, and the data are evaluated by standard equations. The gravity data allow one to model the densities, depths, sizes, and shapes of rock bodies in the subsurface.

Magnetic surveys may involve measurement of the total magnetic field or simply the strength of the field in the vertical direction. The magnetometer commonly measures values of the magnetic field of one part in fifty thousand of the Earth's field. The magnetic anomaly allows investigation of the magnetic susceptibility of rock in the subsurface. The susceptibility is the property of a material that causes it to reinforce or to move into a magnetic field the way a nail will move toward a magnet. The permanent magnetic effect of the rocks, termed the remanence, can also be evaluated as is done for the rocks created at the crests of the mid-ocean ridges. Data are corrected for the natural variation of the Earth's field as a function of location and are evaluated by a standard set of equations that allow the scientist to characterize bodies at depth. The magnetic method is especially good for the study of the crust because the mantle has very low values of the magnetic properties and does not influence the data greatly.

Gravity and magnetic data are often analyzed together for a region. The combination of the physical properties of density and the magnetic susceptibility restricts the possible rock types rather well. The patterns, or “fabrics,” of magnetic and gravity anomalies have been used to define different crustal provinces. The coincidence of large gravity and magnetic anomalies has been one of the best ways to locate and evaluate dense mafic volcanic rocks and intrusions associated with the crustal rift zones.

Seismic Investigations

Seismic investigations have provided the most detailed information about the structure of the continental crust. Two methods have been used, and both involve the use of a seismometer with a number of sensors (or phones) and a seismic energy source.

The first involves measuring the arrival times of seismic wave energy from a distant source. In this method, termed seismic refraction profiling, a series of phones that will detect ground vibrations is laid out in a straight array. The seismic energy may come from an earthquake or a human-made source, but in either case it travels through the Earth at a velocity characteristic of the rocks through which it passes. The time that it takes the seismic wave to reach the individual phones on the array is analyzed graphically or by a simple set of equations. The velocity structure and layering of the subsurface is then determined. These layer velocities may be interpreted in terms of specific rock types that have been measured in the laboratory.

The second seismic method involves reflection of the seismic waves off of surfaces at depth. Seismic reflection profiling is done with a shorter array of seismic phones and typically uses an artificial energy source. The times for waves to reflect back to the surface are measured and are translated into depths using the rock velocities. In general, more detail can be seen in layered regions by using the reflection method. Conversely, in areas with few layers, refraction may give sufficient information.

An additional method for the study of crustal structure uses remote sensing. Remote sensing is the use of electromagnetic radiation from ultraviolet to infrared and of radar wavelengths to survey the surface. Observations are taken from satellites or from aircraft and may be processed by computer. The resulting photographs or computer images are studied to characterize the Earth's surface. One of the primary discoveries of this method has been the existence of long linear features, called lineaments, on the Earth's surface. Some lineaments are only vaguely seen, while others are very obvious. The lineaments often cut across rocks of different ages and types and may truncate major topographic features. Comparison of lineament maps with magnetic and gravity maps shows good correlations in many areas. The lineaments appear to be related to deep basement structures, or zones of weakness. Throughout geologic time, the reactivation of these old zones may have affected the overlying rocks and geologic processes, whatever their types or ages. These basement structures have also served as conduits for ore-forming solutions through time, and thus lineament analysis is used by many in the exploration for natural resources.

Economic Significance

Although the upper sections of the continental crust can be studied through standard geologic mapping and borehole analysis, the largest part of the crust is understood through several geophysical techniques. Primary among them are studies based on heat-flow, seismic, gravity, and magnetic observations. These studies have demonstrated that the continental crust is strongly layered, with the lighter and lowest melting components toward the top of the crust. This chemical separation tells scientists that most of the economically valuable elements, such as the precious and base metals as well as the energy-related resources, have already been concentrated upward.

Geophysical studies have also allowed scientists to locate lines of weakness within the mosaic structure of the continents. Basement structures include large vertical fault systems that cut through much of the continental crust. Occasionally, these structures may be reactivated by modern-day geologic conditions, causing seismic activity far from the margins of the major plates. The historic earthquakes in the Mississippi Valley are of this type. In addition, the existence of the deep basement faults may be important in the localization of ore deposits. Economic geologists have found that many large ore districts or metal provinces are related to lineaments apparently caused by basement structures.

Principal Terms

basement: a term that refers to the crystalline, usually Precambrian, igneous and metamorphic rocks that occur beneath the sedimentary rock on the continents

continental shield: the oldest exposed Precambrian rocks that form the nuclei of the continents

craton: the part of the continent that is covered with a variable thickness of sedimentary rock but that has not been affected by mountain building

crustal discontinuity: a boundary within the crust that is detected by a change in the velocity of seismic waves and that results from the different densities of crustal layers

geologic terrane: a crustal block with a distinct group of rocks and structures resulting from a particular geologic history; assemblages of terranes form the continents

Mohorovičić discontinuity: the boundary between the crust and the upper mantle that was first defined by a rapid change in seismic velocities; it separates low-density crust from the denser mantle

rifting: a process of crustal extension or separation that is accomplished by a series of faults involving down-dropped blocks in the central portion, forming a large valley

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