Composition and development of the continental tectosphere
TL;DR: In this article, the Wilson cycle is used to balance the tectosphere by depleting the continental upper mantle in a basalt-like component, which stabilizes the old continental nuclei against convective disruption.
Abstract: Beneath the old continental nuclei are thick root zones which translate coherently during plate motions. These zones are apparently stabilised against convective disruption by the depletion of the continental upper mantle in a basalt-like component. Construction of this delicately balanced tectosphere is accomplished by the dynamic and magmatic processes of the Wilson cycle.
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TL;DR: In this paper, the authors show that the production of magmatically active rifted margins and the effusion of flood basalts onto the adjacent continents can be explained by a simple model of rifting above a thermal anomaly in the underlying mantle.
Abstract: When continents rift to form new ocean basins, the rifting is sometimes accompanied by massive igneous activity. We show that the production of magmatically active rifted margins and the effusion of flood basalts onto the adjacent continents can be explained by a simple model of rifting above a thermal anomaly in the underlying mantle. The igneous rocks are generated by decompression melting of hot asthenospheric mantle as it rises passively beneath the stretched and thinned lithosphere. Mantle plumes generate regions beneath the lithosphere typically 2000 km in diameter with temperatures raised 100–200°C above normal. These relatively small mantle temperature increases are sufficient to cause the generation of huge quantities of melt by decompression: an increase of 100°C above normal doubles the amount of melt whilst a 200°C increase can quadruple it. In the first part of this paper we develop our model to predict the effects of melt generation for varying amounts of stretching with a range of mantle temperatures. The melt generated by decompression migrates rapidly upward, until it is either extruded as basalt flows or intruded into or beneath the crust. Addition of large quantities of new igneous rock to the crust considerably modifies the subsidence in rifted regions. Stretching by a factor of 5 above normal temperature mantle produces immediate subsidence of more than 2 km in order to maintain isostatic equilibrium. If the mantle is 150°C or more hotter than normal, the same amount of stretching results in uplift above sea level. Melt generated from abnormally hot mantle is more magnesian rich than that produced from normal temperature mantle. This causes an increase in seismic velocity of the igneous rocks emplaced in the crust, from typically 6.8 km/s for normal mantle temperatures to 7.2 km/s or higher. There is a concomitant density increase. In the second part of the paper we review volcanic continental margins and flood basalt provinces globally and show that they are always related to the thermal anomaly created by a nearby mantle plume. Our model of melt generation in passively upwelling mantle beneath rifting continental lithosphere can explain all the major rift-related igneous provinces. These include the Tertiary igneous provinces of Britain and Greenland and the associated volcanic continental margins caused by opening of the North Atlantic in the presence of the Iceland plume; the Parana and parts of the Karoo flood basalts together with volcanic continental margins generated when the South Atlantic opened; the Deccan flood basalts of India and the Seychelles-Saya da Malha volcanic province created when the Seychelles split off India above the Reunion hot spot; the Ethiopian and Yemen Traps created by rifting of the Red Sea and Gulf of Aden region above the Afar hot spot; and the oldest and probably originally the largest flood basalt province of the Karoo produced when Gondwana split apart. New continental splits do not always occur above thermal anomalies in the mantle caused by plumes, but when they do, huge quantities of igneous material are added to the continental crust. This is an important method of increasing the volume of the continental crust through geologic time.
2,821 citations
TL;DR: For example, Hou et al. as mentioned in this paper show that a small increase in the mean elevation of the Tibetan Plateau of 1000 m or more in a few million years is required by abrupt tectonic and environmental changes in Asia and the Indian Ocean.
Abstract: Convective removal of lower lithosphere beneath the Tibetan Plateau can account for a rapid increase in the mean elevation of the Tibetan Plateau of 1000 m or more in a few million years. Such uplift seems to be required by abrupt tectonic and environmental changes in Asia and the Indian Ocean in late Cenozoic time. The composition of basaltic volcanism in northern Tibet, which apparently began at about 13 Ma, implies melting of lithosphere, not asthenosphere. The most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere. The initiation of normal faulting in Tibet at about 8 (± 3) Ma suggests that the plateau underwent an appreciable increase in elevation at that time. An increase due solely to the isostatic response to crustal thickening caused by India's penetration into Eurasia should have been slow and could not have triggered normal faulting. Another process, such as removal of relatively cold, dense lower lithosphere, must have caused a supplemental uplift of the surface. Folding and faulting of the Indo-Australian plate south of India, the most prominent oceanic intraplate deformation on Earth, began between about 7.5 and 8 Ma and indicates an increased north-south compressional stress within the Indo-Australian plate. A Tibetan uplift of only 1000 m, if the result of removal of lower lithosphere, should have increased the compressional stress that the plateau applies to India and that resists India's northward movement, from an amount too small to fold oceanic lithosphere, to one sufficient to do so. The climate of the equatorial Indian Ocean and southern Asia changed at about 6–9 Ma: monsoonal winds apparently strengthened, northern Pakistan became more arid, but weathering of rock in the eastern Himalaya apparently increased. Because of its high altitude and lateral extent, the Tibetan Plateau provides a heat source at midlatitudes that should oppose classical (symmetric) Hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons. For the simple case of axisymmetric heating (no dependence on longitude) of an atmosphere without dissipation, theoretical analyses by Hou, Lindzen, and Plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal (monsoonlike) circulation than such a source centered on the equator, but only if heating exceeds a threshold whose level increases with the latitude of the heat source. Because heating of the atmosphere over Tibet should increase monotonically with elevation of the plateau, a modest uplift (1000–2500 m) of Tibet, already of substantial extent and height, might have been sufficient to exceed a threshold necessary for a strong monsoon. The virtual simultaneity of these phenomena suggests that uplift was rapid: approximately 1000 m to 2500 m in a few million years. Moreover, nearly simultaneously with the late Miocene strengthening of the monsoon, the calcite compensation depth in the oceans dropped, plants using the relatively efficient C4 pathway for photosynthesis evolved rapidly, and atmospheric CO2 seems to have decreased, suggesting causal relationships and positive feedbacks among these phenomena. Both a supplemental uplift of the Himalaya, the southern edge of Tibet, and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the Himalaya that, in turn, enhanced extraction of CO2 from the atmosphere. Thus these correlations offer some support for links between plateau uplift, a downdrawing of CO2 from the atmosphere, and global climate change, as proposed by Raymo, Ruddiman, and Froehlich. Mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts, but might also play an important role in altering regional and global climates.
1,753 citations
TL;DR: The crustal growth and stabilization of the North China Craton (NCC) relate to three major geological events in the Precambrian: (1) a major phase of continental growth at ca. 2.9-2.7 Ga, (2) the amalgamation of micro-blocks and cratonization at 2.5-3.5 Ga, and (3) Paleoproterozoic rifting-subduction-accretion-collision tectonics and subsequent high-grade granulite facies metamorphism-granitoid mag
1,320 citations
1,170 citations
TL;DR: In this paper, the authors presented a method for the inversion of waveform data for the three-dimensional distribution of seismic wave velocities, applied to data from the global digital networks (International Deployment of Accelerometers, Global Digital Seismograph Network).
Abstract: A method is presented for the inversion of waveform data for the three-dimensional distribution of seismic wave velocities. The method is applied to data from the global digital networks (International Deployment of Accelerometers, Global Digital Seismograph Network); the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths. The moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity. A global model is constructed for shear wave velocity, expanded up to degree and order 8 in spherical harmonics, and described by a cubic polynomial in depth for the upper 670 km of the earth's mantle. Although no a priori information is incorporated, the model predictions reproduce much of what is known about the dispersion of mantle waves, for example, high phase velocities for shields, low velocities at ridges, and a strong degree 2 pattern for Rayleigh waves. Since the method makes use of complete waveforms, overtone data are also included. It is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently. The model shows that shields and ridges are major features in the depth interval 25–250 km. The ridges of the southern Pacific and the larger shields persist to 350 km, but the SouthEast Indian Rise is underlain by a high-velocity anomaly at this depth, as is much of the Mid-Atlantic Ridge. At 450–650 km the major features are a broad region of high velocities incorporating South America, much of the South Atlantic and parts of West Africa, a broad region of low velocities in the central and eastern Pacific, high velocities in the western Pacific, and a low-velocity anomaly beneath the Red Sea and the Gulf of Aden. In the absence of a crustal correction, degrees 2 and 3 show a high positive correlation with the geoid; paradoxically, this is largely destroyed when the distribution in crustal thickness is taken into account. Spherical harmonic degrees 4–7 show a significant negative correlation.
1,143 citations
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TL;DR: In this article, the transform fault concept is extended to a spherical surface, where the motion of one block relative to another block may then be described by a rotation of a rigid crustal blocks relative to the other block.
Abstract: The transform fault concept is extended to a spherical surface. The earth's surface is considered to be made of a number of rigid crustal blocks. It is assumed that each block is bounded by rises (where new surface is formed), trenches or young fold mountains (where surface is being destroyed), and great faults, and that there is no stretching, folding, or distortion of any kind within a given block. On a spherical surface, the motion of one block (over the mantle) relative to another block may then be described by a rotation of one block relative to the other block. This rotation requires three parameters, two to locate the pole of relative rotation and one to specify the magnitude of the angular velocity. If two adjacent blocks have as common boundaries a number of great faults, all of these faults must lie on ‘circles of latitude’ about the pole of relative rotation. The velocity of one block relative to the other must vary along their common boundary; this velocity would have a maximum at the ‘equator’ and would vanish at a pole of relative rotation.
The motion of Africa relative to South America is a case for which enough data are available to critically test this hypothesis. The many offsets on the mid-Atlantic ridge appear to be compatible with a pole of relative rotation at 62°N (±5°), 36°W (±2°). The velocity pattern predicted by this choice of pole roughly agrees with the spreading velocities determined from magnetic anomalies. The motion of the Pacific block relative to North America is also examined. The strike of faults from the Gulf of California to Alaska and the angles inferred from earthquake mechanism solutions both imply a pole of relative rotation at 53°N (±3°), 53°W (±5°). The spreading of the Pacific-Antarctic ridge shows the best agreement with this hypothesis. The Antarctic block is found to be moving relative to the Pacific block about a pole at 71°S (±2°), 118°E (±5°) with a maximum spreading rate of 5.7 (±0.2) cm/yr. An estimate of the motion of the Antarctic block relative to Africa is made by assuming closure of the Africa-America-Pacific-Antarctica-Africa circuit and summing the three angular velocity vectors for the cases above.
1,106 citations
TL;DR: In this paper, the authors used plate theory to calculate the temperature distribution in the lithosphere thrust beneath island arcs, and to determine the flow and the stress elsewhere in the mantle, and demonstrated that earthquakes are restricted to those regions of the mantle which are colder than a definite temperature.
Abstract: Summary Plate theory has successfully related sea floor spreading to the focal mechanisms of earthquakes and the deep structure of island arcs. It is used here to calculate the temperature distribution in the lithosphere thrust beneath island arcs, and to determine the flow and the stress elsewhere in the mantle. Comparison with observations demonstrates that earthquakes are restricted to those regions of the mantle which are colder than a definite temperature. The flow and the stress heating in the mantle can maintain the high heat flow anomaly observed behind island arcs. Plate theory also suggests a new approach to the convection problem. The most obvious mechanism causing surface motion is the force on the plates due to the sinking lithosphere. This does not appear to be the way in which the motions are maintained. However, the input of large volumes of cold material can control convection and cause general downward movements in the mantle near island arcs. This input of cold lithosphere must cease when the island arc tries to consume a continent, since the light continental crust cannot sink through the denser mantle. Attempts to assimilate continental crust in this way can produce fold mountains, and also permit a rearrangement of convection cells.
988 citations
TL;DR: In this article, Pollack et al. constructed a global map of lithospheric thickness based on the regional variation of surface heat flow, geotherms, and lithosphere thickness, and identified the lid as synonymous with the lithosphere.
969 citations
TL;DR: In this article, the authors consider that continental collision is followed by crustal thickening, to accommodate further plate convergence, with ensuing partial melting of the lower crust, resulting in a dry refractory lower crust consisting of pyroxene granulites and anor-thosites.
Abstract: Extensive terranes of basement reactivation are interpreted as resulting from crustal thickening following continental collision. It is suggested that terranes, such as the Grenville Province and much of the Variscan orogenic belt in Europe, have their modern analog in the Tibetan Plateau. The Tibetan Plateau is underlain by a continental crust between 60 and 80 km thick and is characterized by extensive high-potash Neogene vulcanism. Following T. H. Green's arguments that partial melting of a dioritic lower crust may yield potassic granitic liquids and refractory anorthositic residues, we consider that continental collision is followed by crustal thickening, to accommodate further plate convergence, with ensuing partial melting of the lower crust. At high structural levels, silicic-potassic ignimbrites are extruded in intermontane basin-horst terranes, with subjacent granite plutons. At deeper levels, a dry refractory lower crust consisting of pyroxene granulites and anor-thosites is generated.
848 citations