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.

Magmatism at rift zones: The generation of volcanic continental margins and flood basalts

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.

Mantle dynamics, uplift of the Tibetan Plateau, and the Indian Monsoon

/pdf/global-continental-and-ocean-basin-reconstructions-since-200-huopwd2ldv.pdf

Global continental and ocean basin reconstructions since 200 Ma

Rare earth elements (REE) include the lanthanide series elements (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) plus Sc and Y. Currently these metals have become very critical to several modern technologies ranging from cell phones and televisions to LED light bulbs and wind turbines. This article summarizes the occurrence of these metals in the Earth's crust, their mineralogy, different types of deposits both on land and oceans from the standpoint of the new data with more examples from the Indian subcontinent. In addition to their utility to understand the formation of the major Earth reservoirs, multi-faceted updates on the applications of REE in agriculture and medicine including new emerging ones are presented. Environmental hazards including human health issues due to REE mining and large-scale dumping of e-waste containing significant concentrations of REE are summarized. New strategies for the future supply of REE including recent developments in the extraction of REE from coal fired ash and recycling from e-waste are presented. Recent developments in individual REE separation technologies in both metallurgical and recycling operations have been highlighted. An outline of the analytical methods for their precise and accurate determinations required in all these studies, such as, X-ray fluorescence spectrometry (XRF), laser induced breakdown spectroscopy (LIBS), instrumental neutron activation analysis (INAA), inductively coupled plasma optical emission spectrometry (ICP-OES), glow discharge mass spectrometry (GD-MS), inductively coupled plasma mass spectrometry (including ICP-MS, ICP-TOF-MS, HR-ICP-MS with laser ablation as well as solution nebulization) and other instrumental techniques, in different types of materials are presented.

Rare earth elements:A review of applications, occurrence, exploration, analysis, recycling, and environmental impact

The distribution of intraplate earthquakes and of igneous rocks postdating continental rifting is summarized and placed into a plate tectonic framework for the following continental areas: eastern and central North America, Africa, Australia, Brazil, Greenland, Antarctica, Norway, Spitsbergen, India, and the margins of the Red Sea and Gulf of Aden. In continents, intraplate earthquakes tend to be concentrated along preexisting zones of weakness within areas affected by the youngest major orogenesis that predates the opening of the present oceans. Many preexisting zones of weakness (including fault zones, suture zones, failed rifts, and other tectonic boundaries), particularly those near continental margins, were reactivated during the early stages of continental separation. In contrast, intraplate shocks rarely occur within the older oceanic lithosphere or within the interiors of ancient cratonic blocks of the continents. In several continental areas, rocks and tectonic features postdating the opening of present-day oceans, including carbonatites, kimberlites, other alkalic rocks, mafic dikes, and ring dikes, as well as some of the largest intraplate shocks, seem to be located preferentially along old zones of weakness near the ends of major oceanic transform faults that were active in the early opening of adjacent oceans. In several places, alkaline magmatism and earthquakes extend several hundred kilometers inland from the ends of oceanic transform faults (but not necessarily with the same strike as the transform fault). Major preexisting zones of weakness that are oriented subparallel to the directions of relative continental separation appear to control the locations of transform faults that develop in a new ocean. In some instances, alkaline magmatism persisted along reactivated features of this type for as long as 100 m.y. after the initial stages of continental fragmentation. Most kimberlites in South Africa seem to have been emplaced along preexisting zones of weakness that were reactivated during the early opening of the South Atlantic. The type of intraplate magmatism appears to be related to the thickness of the lithosphere. Unlike oceanic transform faults where large horizontal movements have occurred, reactivated zones of weakness in continents usually appear to have been the sites of only relatively small displacement. Seismic activity and alkaline magmatism may be controlled by deep fractures that penetrated the entire lithosphere to tap asthenospheric sources of magma. Seismic activity along these zones seems to occur in response to the present-day stress regime, which is not necessarily the same as that which was active during the emplacement of the alkaline rocks. Other intraplate shocks are concentrated along old zones of weakness that are subparallel to continental margins. Such shocks are found in the Appalachians, northeastern and northern Greenland, Norway, Great Britain, Spitsbergen, northern Canada, and Australia. These zones of weakness were also reactivated during continental separation in either the Mesozoic or the Cenozoic. Evidence is now mounting for Cretaceous and Cenozoic deformation along some of these features. Although not many focal mechanism solutions or in situ measurements of stress are available for intraplate areas, horizontal compressive stresses appear to be present today in many of the pre-Mesozoic orogenic belts that were reactivated by continental rifting. This evidence, as well as examples of Cenozoic thrust faulting, indicates that the stress field has changed since rifting commenced. High compressive stresses, the absence of earthquakes in Antarctica, their near absence along the margins of the Gulf of Mexico, and the much lower levels of activity in the oceanic lithosphere adjacent to most continents argue against mere sedimentary loading and the cooling of the oceanic lithosphere as the main source of stress that is reactivating faults of these older fold belts. The large compressive stresses and the uplift found in many continental areas adjacent to continental margins may be caused by a deep-seated source in the mantle of long wavelength or by stresses transmitted in the lithosphere. These effects may be related to either the cooling and underplating of the continental lithosphere adjacent to continental margins, large tractions on the base of the lithosphere in shield areas, stress concentrations related to marked changes in the age and thickness of the lithosphere, convective motions of the mantle beneath these areas, or those regions acting like broad zones of weakness that are being compressed between adjacent areas of greater strength. During the fragmentation of a supercontinent, multibranched rifting usually follows the youngest zone of previous orogenesis and as much as possible avoids passing through old cratonic areas where the lithosphere is thick, cold, and strong. Rift junctions seem to be related to the preexisting mosaic of cratons and younger belts of deformation rather than to a motive force involving mantle plumes. Likewise, many zones of unusually high magmatic activity, i.e., hot spots, appear to be related to nodes or junctions in this mosaic pattern. Thus these hot spots appear to be passive features rather than the surficial expression of mantle plumes. Major transform faults that are active during the early opening of an ocean also tend to develop where the margins of the older cratons undergo an abrupt change in strike. During the early development of an ocean the preexisting mosaic of structural elements within the thick continental lithosphere may result in large normal forces across some plate margins, leaky transform faulting, and localized stress concentrations. The early directions of sea floor spreading and of transforming faulting may be altered by these boundary forces and by the geometrical constraints imposed in separating old cratonic blocks. These constraints are relaxed once old, thick lithosphere is no longer in contact across long transform faults. Since these early directions are strongly influenced by the preexisting tectonic framework and may not coincide with the direction of the forces driving the plates apart, early transform faults may have components of extension (or compression) along them in addition to strike slip motion. A small component of extension may be responsible for the formation of volcanic ridges and seamount chains such as the Walvis ridge, Rio Grande rise, and New England seamount chain. These features predate the marked change in the strike of transform faulting that occurred in the North and South Atlantic about 80 m.y. ago as thin oceanic lithosphere finally came in contact across large oceanic transform faults. Several zones of intraplate magmatism in the surrounding continents also ceased at that time.

Intraplate seismicity, reactivation of preexisting zones of weakness, alkaline magmatism, and other tectonism postdating continental fragmentation

The intraplate deformation in the central Indian Ocean basin is a well-known example of a deviation from an axiom of plate tectonics: that of rigid plates with deformation concentrated at plate boundaries. Here we present multichannel seismic reflection profiles which show that high-angle reverse faults in the sediments of the central Indian Ocean extend through the crust and possibly into the uppermost mantle. The dip of these faults, which we believe result from the reactivation of pre-existing faults formed at the spreading centre, is ˜40° in the basement, which is consistent with the distribution and focal mechanisms of earthquakes on faults now forming at spreading centres. This style of deformation, coupled with the observation of large earthquakes in the mantle lithosphere, indicates that brittle failure of the oceanic lithosphere may nucleate in the vicinity of the brittle/ductile transition and propagate through the crust.

/pdf/fault-reactivation-in-the-central-indian-ocean-and-the-1beuqfze3g.pdf

Fault reactivation in the central Indian Ocean and the rheology of oceanic lithosphere

Since Paleozoic time, the development of sedimentary basins on the continental margin of southern Africa has been controlled by the structures formed during the breakup of Gondwanaland. In Mozambique, the earliest rift (∼180 m.y. B.P.), between East and West Gondwana, produced a north-south–trending series of large horsts and grabens which were buried beneath detritus from the Limpopo and Zambezi river systems. Oceanward sediment dispersion was controlled by the Mozambique Ridge. This stage of continental breakup coincided with the establishment of marine conditions in the older, epicontinental basins which lay over the present-day Agulhas Bank and off the Transkei and Natal coasts (Outeniqua, St. Johns, and Durban basins). When West Gondwana broke up (125 to 130 m.y. B.P.), a large sediment wedge (Orange Basin) was initiated on the west coast of southern Africa by discharge from the Orange River and associated rivers onto a downfaulted, tensional-formed margin. At the same time, a large transform fault (Agulhas fracture zone) truncated the Outeniqua to Durban Basins as the Falkland Plateau separated from south and east Africa. These movements resulted in the formation of new ocean basins and the enlargement of older adjacent ones. Subsequent major sea-level movements are attributable to epeirogenic/eustatic events which are possibly related to variations in world-wide ocean-ridge spreading rates. Most variations in sediment accumulation rates are related to the distribution of marginal traps rather than differences in detrital discharge rates from the major river systems.

Continental Breakup and the Development of Post-Paleozoic Sedimentary Basins around Southern Africa

Long-wavelength (100–300 km) folding in the central Indian Ocean associated with the diffuse plate boundary separating the Indian, Australian, and Capricorn plates is Earth's most convincing example of organized large-scale lithospheric deformation. To test the timing and mechanics of this deformation as implied by plate- kinematic and deformation models, we present a new analysis of the seismic stratigraphy of the Bengal Fan sediments. This analysis shows that the folding of the oceanic lithosphere was multiphase, with major events occurring in the Miocene (8.0–7.5 Ma), Pliocene (5.0–4.0 Ma), and Pleistocene (0.8 Ma). The Miocene phase was the most intense and involved deformation of an area south of 1°S, whereas in the Pliocene the activity shifted northward. In the final phase (Pleistocene), the activity was focused in the equatorial region. No evidence was found for deformation prior to 8.0–7.5 Ma. The spatial extent of the Pleistocene folding event overlaps the Pliocene and/or Miocene folding events and coincides with both the area of most active faulting and the zone of greatest historical seismicity. The seismic data show that the timing of reverse faulting, and thus more significant shortening of the lithosphere, generally coincided with the phases of folding, but there are examples of folding of the oceanic lithosphere without associated reverse faulting.

/pdf/evidence-for-multiphase-folding-of-the-central-indian-ocean-10no87rcx6.pdf

Evidence for multiphase folding of the central Indian Ocean lithosphere

Multichannel seismic reflection profiles collected from the intraplate deformation area in the Central Indian Ocean Basin are used to describe brittle structures produced under compressive stress. Reflectors within oceanic basement are divided into four types: north-dipping and south-dipping reverse faults that are interpreted as reactivations of structures formed at the spreading-centre as outward and inward dipping faults respectively; lower-angle, north-dipping reflectors that probably represent new faults or faults just initiating; and sub-horizontal reflectors within the uppermost crust that are interpreted as hydrothermal alteration fronts. Within the sedimentary cover upwards fault propagation shows the faults steepening from c . 40° just above basement to near vertical and is preceded by sediment folding. A fractal analysis of faulting suggests two fault populations possibly reflecting different criteria for brittle failure. Measurements of north-south crustal shortening indicate a shortening rate of 2.5 (± 0.9) mm a −1 , which is at the lower end of predictions from plate motions, but significant enough to recognize this area as a diffuse plate boundary. The formation of long-wavelength basement undulations and the reactivation of fracture zones and ridge-parallel fault fabrics are linked in a unified tectonic model driven by the high level of intraplate compressive stress in the area. There is little evidence from the seismic profiles for intraplate deformation starting before the widespread unconformity dated as 7 Ma.

/pdf/seismic-reflection-images-of-intraplate-deformation-central-r87hmxpdfa.pdf

Seismic reflection images of intraplate deformation, central Indian Ocean, and their tectonic significance

The diffuse deformation zone in the central Indian Ocean is the classical example of distributed deformation of the oceanic lithosphere, where shortening between the Indian and Capricorn plates is manifest as reverse faulting (5–10-km-spaced faults) and long-wavelength (100–300 km) folding. The onset of this deformation is commonly regarded as a key far-field indicator for the start of major uplift of the Himalayas and Tibet, some 4000 km further to the north, due to increased deviatoric stresses within the wider India-Asia area. There has been disagreement concerning the likely timing for the onset of deformation between plate-motion inversions and seismic reflection–based studies. In the present study, fault displacement data from seismic-reflection profiles within the central Indian Ocean demonstrate that compressional activity started much earlier than previously thought, at around 15.4–13.9 Ma. From reconstructions of fault activity histories, we show that 12% of the total reverse fault population had been activated, and 14% of the total strain accumulated, prior to a sharp increase in the deformation rate at 8.0–7.5 Ma. There is no evidence for any regional unconformity before 8.0–7.5 Ma; early shortening was accommodated by activity on single isolated fault blocks. Total strain estimates derived are more variable and complex than those predicted from plate inversion, and they do not show simple west to east increase.

/pdf/early-pre-8-ma-fault-activity-and-temporal-strain-2kq94fgset.pdf

R.A. Scrutton

Papers

Fault reactivation in the central Indian Ocean and the rheology of oceanic lithosphere

Continental Breakup and the Development of Post-Paleozoic Sedimentary Basins around Southern Africa

Evidence for multiphase folding of the central Indian Ocean lithosphere

Seismic reflection images of intraplate deformation, central Indian Ocean, and their tectonic significance

Early (pre–8 Ma) fault activity and temporal strain accumulation in the central Indian Ocean