Cratons - more details

Assuming the chemical boundary layer defines the base of the lithosphere, Archean lithosphere is 250-180 km deep, Proterozoic lithosphere is 180-150 km, and Phanerozoic lithosphere is 140-60 km.
30  However, there is "significant variability in cratonic root thickness" between different cratons as well as among those of similar age.18

Using maximum gradient in surface wave tomography models to define the base of the lithosphere, globally, cratonic lithosphere "may not extend beyond 200 km depth."34  "The analysis of heat-flow, mantle-xenolith and electrical conductivity data all indicate that the coherent, conductive part of continental roots (the seismically defined 'tectosphere') is at most 200-250 km thick."12  "The "Lehmann discontinuity", observed under continents at about 200-250 km, and the "Gutenberg discontinuity", observed under oceans at depths of about 60-80 km, may both be associated with the bottom of the lithosphere, marking a transition to flow-induced asthenospheric anisotropy."12

"The density of the lower parts (>180 km) of Archean and Proterozoic sections can approach that of Phanerozoic lithosphere," showing "a rapid increase in density over a short vertical distance."30

"Xenolith data indicate that cratonal lithosphere approached its modern thickness soon after the cratons stabilized and that the cratonal lithosphere above ~180 km depth has remained stable since then."32


in the upper mantle
"The evolution of the Hadean mantle from an initially molten state... would likely lead to some chemical heterogeneity in the upper mantle."
23  "The upper mantle is heterogenous in all parameters at all scales. The parameters include seismic scattering potential, anisotropy, mineralogy, major and trace element chemistry, isotopes, melting point, and temperature."1  "Many of the chemically distinct layers differ little in seismic properties."1

"Regional and depth differences in the water concentration of the upper mantle can vary by more than one order of magnitude."8

"The formation of iron-depleted cratons is a potential source [or iron] in the upper parts of the mantle."31

in the lithosphere
"The lithosphere is heterogeneous even over small distances."
17  Cratonic lithosphere varies substantially in temperature and seismic velocity.17  There are "large compositional variations in the cratonic lithosphere."10  Temperature and compositional variations produced a large range of density anomalies in cratonic subcrustal lithosphere.17

Regional differences in heterogeneity of the asthenosphere from harzburgite, lherzolite, eclogite "provide a diversity of melt productivity and crustal thickness in different places without requiring great variability in mantle temperature."1  "Large-scale chemical heterogeneity of basalts sampled along midocean ridge systems occur on length scales of 150 to 1400 km."1

Cratons have concentrations of diverse minerals, representing regional geochemical heterogeneities. The crust of most cratons are well-endowed with metals near the surface. There are different mixtures of metals from craton to craton: Kaapvaal is enriched in gold and platinum-group (PGE); Zimbabwe and Yilgarn in gold and tungsten; Sao Francisco in gold and Cu/Pb/Zn (base metals); Amazonian in gold and tin. Superior Province, Yilgarn, Zimbabwe have strong Au, Cu, Pb, Zn. Pilbara and Kaapvaal are enriched in siderophile elements Ni, Cr, and PGE in the crust and mantle; Amazonian, Leo-Man, Ntem, South China in Sn, W, U, Th.7

Iron has the strongest mineralogical influence on the density and velocity of mantle.
10  It appears that, at least in the upper mantle transition zone and the base of the lower mantle, chemical heterogeneity dominates, and "that variable iron content is an important component of this heterogeneity."31  "It is now clear that even small variations in iron content can produce large density variations in the deep mantle without significantly perturbing seismic velocities."31

"High degrees of melting that... established melt-depleted, Mg-rich and Ca, Al, Fe-poor cratonic mantle efficiently stripped incompatible elements, including water, from the upper mantle source."
5,8  "Dehydration can lead to several order of magnitude increase in viscosity, relative to mantle at the same pressure and temperature conditions." Cratonic keels could be the depleted residual from a large melting event.18  Water lowers the melting point of mantle rocks and allows melting at greater depths than in the absence of water.3

"Regional and depth differences in the water concentration of the upper mantle can vary by more than one order of magnitude."8  In the oceanic upper mantle, there is an abrupt transition from wet to dry conditions "at the relatively large depth of ~70 km."16  "Archean lithosphere contains less water than the oceanic mantle in the depth range between ~150 and ~250 km. Below ~250 km" they show similar conductivity.15

"Since the 1990s, geologists have recognized with increasing certainty that mantle minerals can hold substantial amounts of water. This implies that the oceans may no longer be the main water reservoir of Earth."3  "Water does not have to be fluid to be stored in the deep Earth. It dissolves as hydroxyl (OH-) in anhydrous minerals such as olivine, pyroxenes, garnet, and their high-pressure forms."3

"The main constituents of the upper mantle are olivine and orthopyroxene (enstatite)."22  "Laboratory measurements indicate that olivine, the primary constituent of the upper mantle, is significantly weakened by the presence of water."8  "The influence of water on viscosity depends on the concentration of water in olivine." "The viscosity of olivine aggregates is reduced by a factor of ~140 in the presence of water at a confining pressure of 300 MPa."16

"The high water solubilities in aluminous orthopyroxene at low pressure and temperature will effectively "dry out" olivine, and this may also contribute to a stiffening of the lithosphere."22  "Water storage capacity in Earth"s shallow mantle is controlled by orthopyroxene, a less abundant phase than olivine, because water solubility in this phase is more than 2 orders of magnitude higher than in olivine."3  "Aluminum is known to greatly enhance water solubility in orthopyroxene."22  "The water contents in aluminous pyroxenes are strikingly high, reaching values close to 1 weight % at low pressures and temperatures." "The high water contents appear to be intrinsic to the pyroxenes."22

The asthenospheric "low-velocity zone usually begins at a depth of 60-80 km below the oceans and ends around 220 km.  Below continental shields, the upper boundary is depressed to 150 km."22  "The asthenosphere coincides with a zone where the water solubility in mantle minerals has a pronounced minimum.  The minimum is due to a sharp decrease of water solubility in aluminous orthopyroxene with depth, whereas the water solubility in olivine continuously increases with pressure.  Melting in the asthenosphere may therefore be related not to volatile enrichment but to a minimum in water solubility, which causes excess water to form a hydrous silicate melt."22  "The top of the low-velocity zone is very sharp and well defined, whereas the lower boundary is more diffuse and difficult to locate."  "Toward the lower boundary of the asthenosphere, the decrease in melt fraction will be more gradual, reflecting the gradual increase of water solubility in olivine and orthopyroxene."22

Residue from crust formation is about 5 times the volume of continental crust.  Residue left after harzburgite segregation is about twice the volume of the lithospheric mantle.
2  "A global estimate is that one volume of granitic magma and three volumes of granulitic restite result from melting four volumes of amphibolitic compositions." "Thus, dehydration melting has the double advantage of explaining granitic magma formation and crustal differentiation without the intervention of any external fluid flux."33

"Granitoid magmas formed during intracrustal melting leave dense residues" that may founder. This leaves a "layer in the upper lithosphere, which is richer in clinopyroxene and... Fe-rich olivine than normal cratonic lithosphere."4  "An olivine + clinopyroxene + garnet restite left in the lower crust following melting... gave rise to granitoid magmas." The density of the restite is greater than surrounding crust "and should accumulate at the top of the lithospheric mantle."4  Neutral buoyancy level is not supportable. A single pluton has various types of magma with different densities. Finding different density magmas at the same level contradicts the existence of a neutral buoyancy level. "Successive magma pulses stopped at a similar level, whatever their density."33

"Oceanic mantle has also experienced melt extraction, although not to the very high extents seen in cratonic mantle."20  "Small amounts of melt can be produced at depths between ~115 and 60 km beneath mid-ocean ridges."16  "The chemical boundary layer beneath oceans originates by melt extraction at mid-ocean ridges."20  "Because the solubility of water in melt is 2 to 3 orders of magnitude greater than that in mantle minerals, the Mid-Ocean Ridge Basalt (MORB) melting process can effectively "dry out" the mantle."16

"Evidence does not, in general, require or favor localized high temperatures at hotspots. The absence of heat-flow and thermal anomalies at hotspots implies the presence of athermal mechanisms to explain melting and geochemical anomalies. Ocean island-like basalts are far more widely distributed than just along linear island chains, indicating that melting conditions are more widespread than assumed in the plume model."1

"Melt depletion... leaves the mantle lithosphere depleted in radioactive heat-producing elements and in water... low temperature and low water content of cratonic mantle lithosphere can lead it to have a viscosity as much as 3-4 orders of magnitude higher than warm, wet asthenospheric mantle."5

"Compared to convecting mantle, mantle keels are highly depleted in Ca and Al and to a lesser extent Fe.  Ca and Al... stabilize clinopyroxene and garnet, both of which are the first minerals to be exhausted during... partial melt extraction.  The depletion in Ca and Al is believed to be the result of extensive melt extraction (20-40 wt.% partial melt)."20  Experiments indicate that cratonic mantle peridotites have had 30-50% melt extracted.19

"A common index of melt extraction is the Mg# (the molar ratio Mg/(Mg+Fe)), which is a measure of the relative proportion of Fe to Mg."20  "Convecting mantle is more "fertile" in terms of meltable components and is characterized by Mg#s of ~0.88-0.89. Cratonic mantle keels are largely depleted of meltable components (clinopyroxene and garnet) and are characterized by average Mg#s of ~0.92-0.93."19,20  The shallowest portion of the residual column is the most depleted (has the highest Mg#).19

High degrees of melt extraction remove dense clinopyroxene and garnet, as well as Fe, leaving a less dense residue.20  Highly melt-depleted peridotites are less dense than "fertile convecting mantle" due to less clinopyroxene and garnet mode, as well as lower Fe in olivine and orthopyroxene.19  Cratonic peridotites are depleted in clinopyroxene, Ca, Fe, Al compared to fertile asthenospheric mantle.19  Low FeO and MgO in Si-enriched peridotites could indicate the addition of a Si-rich component, such as orthopyroxene, which is in Si-rich peridotites.19

Xenolith data from Kaapvaal and Slave cratons indicate "less chemical depletion in deep lithospheric roots." For Kaapvaal garnet lherzolites, "the deeper (>140 km), high-temperature samples are 1-2% denser than the highly depleted samples from shallower (<140 km) depth." "Xenolith samples from the Slave craton indicate... a denser, more iron-rich layer below 145 km." "Thus, evidence... points to dense lithospheric roots below 140 km."24

Individual cratons:
In the Kaapvaal craton, there was an episode of orogeny, followed by a craton-wide overprinting of granitoid magmatism attributed to intracrustal melting.
25  As much as 40% of the crust may have been re-melted during the event.25

"The thickness of the Moho transition zone" is "less than 0.5 km and the maximum variation in crustal thickness" is "less than 1 km. The flat and almost perfectly sharp Moho, together with the absence of a mafic lower crust, suggests large-scale crustal reworking... between crustal formation and the time of cratonic stabilization."29

"The southern Kaapvaal craton... is a prototypical example of Archean tectosphere,... and extensively studied as a result."
29  The lowermost crust must be intermediate rather than mafic in composition.29  This appears to be characteristic of much of the Kaapvaal craton.29

Speculations on the assembly of the Kaapvaal crust "typically involve extensive collisional accretion of island arcs and microcontinental blocks to form nuclear continental masses."  Yet this "may be expected to produce a complicated mosaic of varying Moho structures and diverse crustal lithologies."29  A plausible explanation for what is actually found is "that a large volume of the Kaapvaal crust has been re-melted on a regional scale since its formation. Such large thermal events mainly involve the lower crust."29  A study of the Vredefort dome indicated "that as much as 40% of the crust, chiefly the lower crust, was re-melted" "during a craton-wide thermal event at 3.11 Ga."  "The very large degree of crustal melting proposed... is sufficient to form something resembling an "ocean" of melt in the cratonic lower crust near the crust-mantle boundary.  The magmatic differentiation and layering accompanying the crystallization of that lower crustal melt "ocean" is one possible means for producing... a flat and sharp Moho."29

Western Superior
Like other cratons, the Western Superior is thought to have accreted from island arcs and microcontinents.  Yet there is a "remarkable absence at mid-crustal and lower-crustal depths of a lateral density segmentation inherited from the accretionary or collisional stages of the Western Superior."
28  Also generally absent are "significant topographic features at the interface between the middle and the lower crustal layers, which is a first-order seismic velocity discontinuity."28

Study of a section of the Western Superior craton shows that in much of the section "significant mass anomalies occur only in the upper crust and at the crust-mantle boundary." "The upper crustal density heterogeneities are within the first 10 km of the crust, and suggest the absence of important lithological contrasts at larger depths, both within and between the different Western Superior subprovinces."28

The absence of mass anomalies at deep crustal levels may be related to mass redistribution processes resulting from "a major episode of intracrustal softening and crustal differentiation", "indicated by voluminous late orogenic  granitic magmatism" around 2.71-2.66 Ga.28  "Thermal softening of the crust..." may have led to "not only the transfer and emplacement at high crustal levels of low-density felsic material extracted from partially molten mid-crustal to lower-crustal rocks, but also the foundering of mafic intrusions."28

"The Slave is a small craton, ~700 x 500 km in exposed areal extent, bounded by Paleoproterozoic belts to the south, east, and west."
6  Ultradepleted garnet and spinel peridotites are in a shallow layer, above normal Archean SCLM, with a sharp boundary at 145+5 km.11

"The late tectonic evolution of Archean cratons, such as the Slave, is complex and involves extensive rifting, magmatism, compressional deformation, and metamorphism." "The Slave"s Neoarchean orogenesis is characterized by high temperature-low pressure metamorphic conditions (HT-LP) and the intrusion of voluminous granitoid plutons within a short time interval."6

"Extensive plutonism", "crustal melting and associated HT-LP metamorphism argue for widespread mantle heat input to the crust."6  An "intense craton-wide "granite bloom" suggests a widespread thermal disturbance, the exact cause of which remains speculative."6

"Post-2.64 Ga structures are dominated by a least three regional folding events at shallow to mid-crustal levels," recording "large horizontal shortening."6

China has 3 cratons: Tarim, Sino-Korea (or North China), and Yangtze, plus intervening deformation zones.  Tarim has relatively low heat flow, very low temperature at the Moho (500), and relatively thick lithosphere (>160 km).  Yangtze has heat characteristics like Tarim and thickness of 140 km.  Heat flow in the Sino-Korean craton is like the other two in the western part, but high in the eastern part.  The eastern part has 700 Moho temperature and a thin (<80 km) lithosphere.  The Tarim and Yangtze have a strong upper crust and upper mantle sandwiching a relatively weak lower crust, whereas the eastern Sino-Korean craton shows a strong upper crust and weak upper mantle.

"Data suggest that the Yangtze craton has a widespread Archean basement, overlain by shallow crust partially reworked in Proterozoic time.  The major Mesoproterozoic event appears to have largely involved remelting of the Archean basement rocks."36

"The basement of the [North China] craton can be divided into two distinct blocks, named the Eastern Block and Western Block, separated by a 100-300 km wide crustal boundary zone, defined as the Central Zone."35  The Eastern and Western blocks "underwent regional metamorphism at ~2.5 Ga, shortly after formation."  The evidence does "not support a continental collisional model for the formation of the basement rocks in the Eastern and Western blocks".  They were "formed through the interaction of mantle-derived magmas with pre-existing lithosphere."35

"The late Archean basement in the Eastern Block is dominated by tonalitic-trondhjemitic-granodioritic gneiss domes" which are "generally circular, elliptical, or oval..., 10-50 km in diameter... with ~2.5 Ga syntectonic granites in the cores of the domes."  They "are separated by linear belts of supracrustal rocks."35  Mineral assemblages in the eastern block possibly came from "intrusion and underplating of large amounts of mantle-derived magmas."35  "Sub-crustal peridotites from eastern China are predominantly shallow-facies mantle rocks (i.e. spinel facies 75-80 km)," similar to "lithosphere found beneath tectonically active continent or modern ocean basins (~200 Ma)."9

"The Western Block has a basement characterized by late Archean rocks in the northwest, flanked to the southeast by Paleoproterozoic khondalite belts.  Early and middle Archean rocks have not been reported from the Western Block."  It has a "lithological assemblage, structural style and metamorphic history similar to those of the Eastern Block."35
The later (Paleoproterozoic) mineral assemblages reflect "a continental collisional environment."

"The Central Zone" is "a roughly north-south trending belt and is separated from the Eastern and Western blocks by major faults.  The zone consists of reworked Archean basement and late Archean to Paleoproterozoic sedimentary and igneous rocks metamorphosed in subgreenschist to granulite facies."35  The Central Zone has linear belts, mainly NNE-SSW trending ductile shear zones "related to Phanerozoic-style collisional tectonics."35

The North China craton has "fragments of ancient oceanic crust, melanges, high-pressure granulites, retrograded eclogites and crustal-scale ductile shear zones in the central zone of the craton.  These discoveries make the central zone of the craton distinct from the eastern and western zones, in which the basement is dominated by Archean tonalitic-trondhjemitic-granodioritic domiform batholiths tectonically interdigitated with minor supracrustal rocks."  The central zone "underwent granulite facies metamorphism at ~1.85 Ga... involving isothermal decompression, reflecting a continental collisional environment.  In contrast, "the cratonic blocks "experienced granulite facies metamorphism at ~2.5 Ga."35

The Archean Tanzanian craton sits between the eastern and western branches of the East African Rift.
34  It is in the middle of the East African Plateau, and has an area of over 350,000 km2 and an average elevation of 1260 m.  It is composed of greenstone belts and granitoids more than 2500 Myr old.34  The Tanzanian craton has a mean thickness (of lithosphere) of about 170+20 km.34  "The lithosphere of the Tanzanian craton appears to have remained stable throughout its... history."34

"The stable craton has endured numerous collisional events resulting in a complex pattern of mobile belts surrounding its perimeter."34

"The volume of igneous basalt in the eastern branch of the East African Rift system, estimated at greater than 900,000 km3, far exceeds that of the western branch."34  Study results show "that an upper mantle plume, centered beneath the Tanzanian cratonic lithosphere, provides the buoyancy required for uplift of the East African Plateau."34

"Low-velocity zones beneath cratonic lithophere are not uncommon."  They appear below the Kaapvaal craton, Canadian Shield, and Australian shield.  "The low-velocity anomalies below these lithospheric roots, however, do not reach shear wave velocities below 4.4 km/s.  By comparison, the dramatic excursion to β=4.20+0.05 km/s at 200-250 km below the surface of the Tanzanian craton is unique.  These velocities are lower than velocities found at comparable depths beneath the East Pacific Rise spreading center and require unusually high temperatures and perhaps partial melting."34

Wyoming craton formed at 2.5-2.9 Ga during extensive magmatism. The result was simultaneous crustal extraction and a mantle residuum forming a subcratonic mantle root.
26  The center of Wyoming craton has ~50 km-thick crust and probable mafic 15-20 km-thick, dense lower crust.26

The Wyoming craton's Archean crust is bounded on 3 sides by Proterozoic collisional orogenic belts: Great Falls tectonic zone in the north, the Dakota part of the Trans-Hudson Orogen in the east, and the Cheyenne belt in the south.  To the west are Proterozoic and Archean terranes.26  Wyoming craton collided with the continental block to the west, forming high-pressure granulites in the Teton Range.26

There are Proterozoic mafic dykes throughout the Wyoming craton. At least some are from craton-wide extension.26

Western Australia
"The Yilgarn craton largely consists of volcanic and sedimentary rocks (greenstones) and granites that formed between 3.0 and 2.6 Ga and were metamorphosed at low-grade."
27  "All indicate intense tectonic, volcanic, plutonic, and metamorphic activity between 2.78 and 2.63 Ga."27  "During the 1960s and 1970s the greenstones were considered to represent remnants of primordial crust that was deformed during the diapiric emplacement of steep-sided granite batholiths. In the 1980s most models interpreted the greenstones and granites as products of rifting within older continental crust."  A 1993 model saw "a major episode of plate tectonic activity which swept together... a number of diverse crustal fragments... to form the Yilgarn craton."27

The 3.6 to 2.8 Ga "Pilbara granite-greenstone terrain consists of ovoid outcrops of granitoid rocks, mainly granite, granodiorite, and tonalite, separated by narrow belts of steeply dipping greenstones."27

The Pilbara and Yilgarn joined 2.0-1.8 Ga "along the Capricorn Orogen to form the West Australian craton."27  "Major episodes of continental rifting are recorded by the eruption of the late Archean flood basalts on the Pilbara craton and east-west dikes cutting the Yilgarn craton."27

"The Amazonian craton was initially a single microcontinent which was split during a rifting episode into the northern (Guyana) and southern (Guapore) shields, and resulted in the development of the Amazon and Solimoes basins."
14  Moderately high velocities located beneath these basins extend down to 100 km depth.14  "A strong positive gravity anomaly... suggest[s] that shallow ultrabasic bodies were emplaced beneath the rift basins."14  "Ultramafic magmatism in rifted areas resulted from melting of the mantle through decompression, due either to tectonic extension or upwelling of a mantle plume."14

Umkondo Igneous Province
An extensive magmatic event formed the Umkondo Igneous Province, an area of ~2.0 x 10
6  km2 on the Kalahari craton.  "Enormous volumes of tholeiitic magma were emplaced... in a narrow time frame at ca. 1112-1106 Ma."13  Yet "A clear plume signature is lacking in most of the Umkondo rocks studied to date."13  "Most of the tholeiitic magmas in the province may have been derived from melting of continental lithospheric mantle."13

The Umkondo Province formed after the main, largely buried, orogenesis, inferred to have occurred along the western margin of the Kalahari craton.13

* * * * * * * * * * *

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