Continental Cratons Research Articles

Southwest Barents Sea rift basin evolution: comparing results from backstripping and time‐forward modelling

AbstractWe present tectonic models of progressive basin formation in the south‐west Barents Sea derived as part of the PETROBAR project (Petroleum‐related studies of the Barents Sea region). The basin architecture developed as a multi‐stage rift preceding the creation of the sheared/transtensional margin conjugate to NE Greenland. N‐ to NNE‐striking basins, with sediment thicknesses in places exceeding 15 km, are separated by basement highs. We use two basin analysis approaches, BMT™ backstripping and TecMod™time‐forward modelling, to determine stretching factors through time along the profile PETROBAR‐07. This 550 km‐long profile derived from wide‐angle reflection/refraction seismic data acquired in 2007, coincident with deep multichannel seismic reflection data. Detailed stratigraphic analysis of the reflection profile, in concert with a dense grid of 2D profiles tied to wells, provides timing and water depth constraint for the models. Velocity analysis of the wide‐angle data provides constraint on the cumulative crustal stretching. The north‐west trending cross‐section extends from continental craton, at the Varanger Peninsula, to within 16 km of the interpreted continent–ocean boundary. Rifting along the profile was episodic, with four distinct phases of basin formation during the Carboniferous, the Late Permian–Triassic, the Late Jurassic–Early Cretaceous and the Late Cretaceous–Eocene. Collectively, the basins exhibit a general trend of younging, narrowing, and deepening oceanward, suggesting a gradual focusing of rifting prior to final breakup. Cumulative stretching factors derived from BMT and TecMod correlate well with observed crustal thinning, and the two models provide uncertainty bounds for stretching factors for the separate rift phases. In contrast to orthogonally rifted margins, stretching is relatively minor immediately prior to transform breakup, with greater stretching occurring during earlier rift phases.

Structural and Coal-Controlled Characteristics of Boli Coal Basin

Boli coal basin has abundant coal resources and its structure shows southward protruding arcuate structure. There is difference about basement in western and eastern, so it can be further divided into continental craton basement in west and continental craton edge active zone basement in east. For this reason, the coal-bearing strata at the west is stable and its structure is relatively simple. On the contrary the eastern coal-bearing strata is less, poor-quality and thickness is unstable. There are two groups of faults and four folds that mainly control coal-bearing strata distribution. It can be divided into six coal-controlled structural styles in Boli coal basin based on system analysis and summarized about geological exploration data.

Temperature-dependent transient creep and dynamics of cratonic lithosphere

Large-scale mantle convection forms the upper boundary layer (lithosphere) where the vertical temperature drop is about 1300 K. Theoretical rheology and laboratory experiments with rock samples show that transient creep occurs while creep strains are sufficiently small. The transient creep is described by the temperature-dependent Andrade rheological model. Since plate tectonics allows only small deformations in lithospheric plates, creep of the lithosphere plates is transient whereas steady-state creep, described by non-Newtonian power-law rheological model, takes place in the underlying mantle. The solution of stability problem shows that the lithosphere is stable but small-scale convective oscillations are attenuated very weakly in regions of thickened lithosphere beneath continental cratons (subcratonic roots) where the thickness of the lithosphere is about 200 km. These oscillations create small-scale convective cells (the horizontal dimensions of the cells are of the order of the subcratonic lithosphere thickness). Direction of motion within the cells periodically changes (the period of convective oscillations is of the order of 3 × 108 yr). In this study, the oscillations of cratonic lithosphere caused by initial relief perturbation are considered. This relief perturbation is assumed to be created by overthrusting in orogenic belts surrounding cratons. The perturbation of the Earth's surface relief leads to a fast isothermal process of isostatic recovery. In the presence of vertical temperature gradient, vertical displacements, associated with the recovery process in the lithosphere interior, instantly produce the initial temperature perturbations exciting thermoconvective oscillations in the cratonic lithosphere. These small-amplitude convective oscillations cause oscillatory crustal movements which form sedimentary basins on cratons.

Transient creep and convective instability of the lithosphere

SUMMARY Laboratory experiments with rock samples show that transient creep, at which strain grows with time and strain rate decrease at constant stress, occurs while creep strains are sufficiently small. The transient creep at high temperatures is described by the Andrade rheological model. Since plate tectonics allows only small deformations in lithospheric plates, creep of the lithosphere plates is transient whereas steady-state creep, described by non-Newtonian power-law rheological model, takes place in the underlying mantle. At the transient creep, the effective viscosity, found in the study of postglacial flows, differs significantly from the effective viscosity, which characterizes convective flow, since timescales of these flows are very different. Besides, the transient creep changes the elastic crust thickness estimated within the power-law rheology of the lithosphere. Two problems of convective stability for the lithosphere with the Andrade rheology are solved. The solution of the first problem shows that the state, in which large-scale convective flow in the mantle occurs under lithospheric plates, is unstable and must bifurcate into another more stable state at which the lithospheric plates become mobile and plunge into the mantle at subduction zones. If the lithosphere had the power-law fluid rheology, the effective viscosity of the stagnant lithospheric plates would be extremely high and the state, in which large-scale convection occurs under the stagnant plates, would be stable that contradicts plate tectonics. The mantle convection forms mobile lithosphericplatesiftheeffectiveviscosityoftheplateisnottoomuchhigherthantheeffective viscosity of the underlying mantle. The Andrade rheology lowers the plate effective viscosity corresponding to the power-law fluid rheology and, thus, leads to instability of the state in which the plates are stagnant. The solution of the second stability problem shows that the state, in which the lithospheric plate moves as a whole with constant velocity, is stable but smallamplitude oscillations are imposed on this motion in regions of thickened lithosphere beneath continental cratons (subcratonic roots) where the thickness of the lithosphere is about 200 km. These oscillations create small-scale convective cells (the horizontal dimensions of the cells are of the order of the subcratonic lithosphere thickness). Direction of motion within the cells periodically changes (the period of oscillations is of the order of 10 8 yr). The small-amplitude convective oscillations cause small strains and do not destroy the thickening of the lithosphere beneath cratons. Thus, the transient creep of the lithosphere explains not only mobility of the lithospheric plates but longevity of subcratonic roots as well.

Mantle flow deflected by interactions between subducted slabs and cratonic keels

Subducting slabs can influence mantle flow, but the importance of neighbouring continental cratons is little understood. Geodynamical modelling, constrained by seismic data that identify regions of mantle flow beneath the Caribbean–South American Plate margin, shows that the deep-rooted South American craton acts to deflect and enhance mantle flow into a narrow channel.

Effects of seismic anisotropy on P-velocity tomography of the Baltic Shield

The upper mantle structure of Swedish part of Baltic Shield with its isotropic and anisotropic seismic velocity characteristics is investigated using telesesismic body waves (i.e. P waves and shear waves) recorded by the Swedish National Seismological Network (SNSN). Nonlinear high-resolution P and SV and SH wave isotropic tomographic inversions reveal velocity perturbations of ± 3 % down to at least 470 km below the network. Separate SV and SV models indicate several consistent major features, many of which are also consistent with P-wave results. A direct cell by cell comparison of SH and SV models reveals velocity differences of up to 4%. Numerical tests show that differences in the two S-wave models can only be partially caused by noise and limited resolution, and some features are attributed to the effect of large scale anisotropy. Shear-wave splitting and P-travel time residual analyses also detect anisotropic mantle structure. Distinct back-azimuth dependence of SKS splitting excludes single-layer anisotropy models with horizontal symmetry axes for the whole region. Joint inversion using both the P and S data reveals 3D self-consistent anisotropic models with well-defined mantle lithospheric domains. These domains of differently oriented anisotropy most probably retain fossil fabric since the domains' origin, supporting the idea of the existence of an early form of plate tectonics during formation of continental cratons already in the Archean. The possible disturbing effects of anisotropy on seismic tomography studies are investigated, and found to be potentially significant. P-wave arrival times were adjusted based on the estimates of mantle anisotropy, and re-inverted. The general pattern of the velocity-perturbation images was similar but changed significantly in some places, including the disappearance of a slab-like structure identified in the inversion with the original data. Thus the analysis demonstrates that anisotropy of quite plausible magnitude can have a significant effect on the tomographic images, and should not be ignored. If, as we believe, our estimates of anisotropy are reasonably correct, then the model based on the adjusted data should give a more robust and correct image of the mantle structure.

Dynamic triggering: Stress modeling and a case study

[1] Changes in the static stress can trigger nearby earthquakes that occur within a few fault lengths from the causative event. Transient stresses caused by passage of surface waves commonly trigger events at remote distances, yet little is documented or understood about the processes and stresses necessary for remote triggering. To understand the causative stresses and environments behind remote, or dynamic, triggering, we must decipher the stresses caused by the passage of the surface waves in relation to the local stress field and fault conditions where the triggered events occur. In this study, we model the change in the stress field that the passing of Rayleigh and Love waves causes on a fault plane of arbitrary orientation relative to the direction of propagation of the waves, and we apply a Coulomb failure criterion to calculate the potential of these stress changes to trigger reverse, normal, or strike-slip failure. We compare these model results with data from dynamically triggered earthquakes in the Australian Bowen Basin, an area with low seismicity and mapped regional stress and that is at the margin of a stable continental craton. Our data analysis shows that for this region, surface waves arriving at 45° from the average strike direction are the most likely to trigger local seismicity. This agrees with our observations.

Lithospheric layering in the North American craton

How cratons-extremely stable continental areas of the Earth's crust-formed and remained largely unchanged for more than 2,500 million years is much debated. Recent studies of seismic-wave receiver function data have detected a structural boundary under continental cratons at depths too shallow to be consistent with the lithosphere-asthenosphere boundary, as inferred from seismic tomography and other geophysical studies. Here we show that changes in the direction of azimuthal anisotropy with depth reveal the presence of two distinct lithospheric layers throughout the stable part of the North American continent. The top layer is thick ( approximately 150 km) under the Archaean core and tapers out on the surrounding Palaeozoic borders. Its thickness variations follow those of a highly depleted layer inferred from thermo-barometric analysis of xenoliths. The lithosphere-asthenosphere boundary is relatively flat (ranging from 180 to 240 km in depth), in agreement with the presence of a thermal conductive root that subsequently formed around the depleted chemical layer. Our findings tie together seismological, geochemical and geodynamical studies of the cratonic lithosphere in North America. They also suggest that the horizon detected in receiver function studies probably corresponds to the sharp mid-lithospheric boundary rather than to the more gradual lithosphere-asthenosphere boundary.

Reactive infiltration of hydrous melt above the mantle transition zone

Seismic studies repeatedly image a low‐velocity layer overlying the mantle transition zone in tectonic settings ranging from subduction zones to continental cratons. This layer has been hypothesized to result from the presence of a dense partial melt formed by dehydration melting as relatively wet transition zone material is advected by convection currents into the low‐water‐solubility upper mantle. Here we examine this hypothesis by considering the dynamic infiltration of a low‐viscosity reactive hydrous melt into a high‐viscosity ambient solid. The thickness of the melt layer is strongly controlled by return flow induced in the surrounding viscous solid, and in steady state a dynamic equilibrium is achieved where the melt lens is restrained from collapse by the exterior mantle. Melt layers with thicknesses in excess of 10 km develop for a wide range of mantle parameters and develop on timescales equivalent to the lifespan of an ocean basin.

Long memory of mantle lithosphere fabric — European LAB constrained from seismic anisotropy

We present a uniform updated model of the European lithosphere–asthenosphere boundary (LAB) recalculated from data collected during our regional studies of seismic anisotropy and other tomographic experiments, and show results of mapping of large-scale domains of mantle lithosphere characterized by uniform fossil fabrics. Exploiting the long memory of the fabric of the deep continental lithosphere, we define the LAB as a boundary between a fossil anisotropy in the lithospheric mantle and an underlying seismic anisotropy related to present-day flow in the asthenosphere. Analysis of static terms of teleseismic P-wave travel time deviations shows that the LAB topography is more distinct beneath the Phanerozoic part of Europe than beneath its Precambrian part and deepens down to ∼ 220 km beneath the two Alpine roots, the South Carpathians and eastward of the Trans-European Suture Zone. Separation of the relative P residuals into static and directional-dependent terms, shear-wave splitting analysis and joint inversion of anisotropic parameters, reveals that the mantle lithosphere consists of domains with consistent olivine fabrics, which can be modelled by peridotite aggregates with plunging foliation or lineation. Changes of the fossil fabric allow us to map the domain boundaries within the mantle lithosphere. We propose processes which could create the observed fossil fabric as a consequence of successive subduction and accretion of micro-continent fragments outboard of continental cratons and a gradual stabilization of the LAB by a mantle flow after a detachment of the lower parts of subducting slabs. We advocate the importance of combining different methods and data in Earth structure studies and applying 3D approaches considering both seismic anisotropy and the general orientation of symmetry axes to construct more realistic models of large-scale lithospheric structure.

Seismic anisotropy of the mantle lithosphere beneath the Swedish National Seismological Network (SNSN)

Body-wave analysis — shear-wave splitting and P travel time residuals — detect anisotropic structure of the upper mantle beneath the Swedish part of Fennoscandia. Geographic variations of both the splitting measurements and the P-residual spheres map regions of different fabrics of the mantle lithosphere. The fabric of individual mantle domains is internally consistent, usually with sudden changes at their boundaries. Distinct backazimuth dependence of SKS splitting excludes single-layer anisotropy models with horizontal symmetry axes for the whole region. Based upon joint inversion of body-wave anisotropic parameters, we instead propose 3D self-consistent anisotropic models of well-defined mantle lithosphere domains with differently oriented fabrics approximated by hexagonal aggregates with plunging symmetry axes. The domain-like structure of the Precambrian mantle lithosphere, most probably retaining fossil fabric since the domains' origin, supports the idea of the existence of an early form of plate tectonics during the formation of continental cratons already in the Archean. Similarly to different geochemical and geological constraints, the 3D anisotropy modelling and mapping of fabrics of the lithosphere domains contribute to tracking plate tectonics regimes back in time.

Instantaneous dynamics of the cratonic Congo basin

Understanding the formation mechanisms of cratonic basins provides an examination of the rheological, compositional and thermal properties of continental cratons. However, these mechanisms are poorly understood because there are few currently active cratonic basins. One cratonic basin thought to be active is the Congo basin located in equatorial Africa. The Congo basin is coincident with a large negative free‐air gravity anomaly, an anomalous topographic depression, and a large positive upper mantle shear wave velocity anomaly. Localized admittance models show that the gravity anomaly cannot be explained by a flexural support of the topographic depression at the Congo. We analyze these data and show that they can be explained by the depression of the Congo basin by the action of a downward dynamic force on the lithosphere resulting from a high‐density object within the lithosphere. We formulate instantaneous dynamic models describing the action of this force on the lithosphere. These models show that the gravity and topography of the Congo basin are explained by viscous support of an anomalously dense region located at 100 km depth within the lithosphere. The density anomaly has a magnitude within the range of 27–60 kg m−3 and is most likely compositional in origin. Our models do not provide a constraint on the lithospheric viscosity of the Congo craton because the shallow location of the anomaly ensures strong coupling of the anomaly to the surface regardless of viscosity structure.

Kimberlites and aillikites as probes of the continental lithospheric mantle

Although the mantle xenoliths carried by kimberlites are the source of much of our information about the composition of the mantle beneath the continents, the compositions of kimberlites themselves have received little attention for the information they carry about the nature of the lithospheric mantle. This neglect in part reflects their common fragmental, contaminated, and hybrid nature, but also the pervasive view that Group-I kimberlites are sourced in the underlying asthenosphere. Insight into the nature of kimberlites and their relationship to the other alkaline ultramafic rocks, such as aillikites, olivine lamproites, and meimechites, can be obtained by comparing their major element compositions in a way that treats their carbonate content as a primary magmatic phase. Group-I kimberlites and aillikites contain significant magmatic carbonate and their compositions fall to the Si-poor side of the composition of olivine. Group-I kimberlite can be distinguished from aillikite on the basis of Fe content, but there appears to be a gradation between these two end-members. In contrast, olivine lamproites and meimechites contain relatively little primary magmatic carbonate and have compositions that are more Si-rich than olivine. Pearce element ratio analysis assuming P as a conserved element indicates that much of the major element variation in hypabyssal kimberlites can be explained by variable amounts of olivine and orthopyroxene in proportions (∼ 70/30) similar to that of cratonic mantle xenoliths. Much of the olivine is present as xenocrysts, but the orthopyroxene is occult and has presumably been assimilated. The fact that individual fields of alkaline ultramafic rocks are characterized by uniform Fe and Ti contents that can be mapped on a regional scale suggests that the major element composition of these unusual rocks, and Group-I kimberlites in particular, is a reflection of the continental lithospheric mantle with which they have interacted. The association of Fe-rich aillikitic magmas with zones of cratonic rifting, and the requirements of Fe–Mg partitioning indicate that they form deeper than Group-I kimberlites, near the intersection of the 1350 °C mantle adiabat with the CO 2 mantle solidus, at pressures of 8 +GPa. According to our working model, Group-I kimberlites are produced by the influx and reaction of carbonate-rich magmas with the highly magnesian harzburgites of the lithospheric mantle beneath continental cratons. In non-cratonic environments, these rising carbonate-rich magmas evolve into aillikites because of the lower Mg # of the asthenospheric mantle. They rarely reach the surface, however, but become the enriched component incorporated into higher-degree basaltic melts. An inverse correlation between diamond grade and kimberlite Fe and Ti contents may simply reflect the fact kimberlites with the lowest Ti contents have interacted with the most depleted and reduced harzburgites of the lithospheric mantle, where diamonds are most likely to be encountered.

PROSPECCIÓN MAGNETOTELÚRICA DE ACUÍFEROS Y ESPESOR BASÁLTICO EN CARLOS PELLEGRINI, PROVINCIA DE SANTA FE (ARGENTINA)

Three magnetotelluric soundings were performed in C. Pellegrini, Santa Fe Province determining a West-East profile of 5926 m of long. The aim of the study was the estimation of possible aquifers underneath a basaltic cover. The basaltic thickness in this area of the Argentina corresponding to the Chacoparanense Basin was also estimated. These lava overfloors reached a great extension (1,2 million km2) and were contemporary with the continental aperture. Results allowed the identification of four stratigraphic formations. The first one has low resistivity of 2 to 4 Ωm (except the first 100 m with about 10 Ωm; which corresponds to modern sediments). This formation is present up to 700 m depth westwards of the profile and 850 m eastwards. The second formation is a basaltic layer, (Serra Geral Fm), with 680 m thickness in the West and 390 m and East of the profile, and resistivities between 50 Ωm to 150 Ωm. The following formation corresponds to sedimentary rocks showing a resistivity of 1-7 Ωm, where different sedimentary horizons can be discriminated; its top is approximately at 1200 m depth and the base reaches the range of depth of 2400 m to 3000 m. Within this think layer, a basal cover of 30 Ωm can be distinguished with approximately 200 m thickness, following the geometry of what can be interpreted as crystalline basement. Finally a last block has resistivities higher than 100 Ωm and it may be a massive formation corresponding to Precambrian age, as part of the continental craton.

PROSPECCIÓN MAGNETOTELÚRICA DE ACUÍFEROS Y ESPESOR BASÁLTICO EN CARLOS PELLEGRINI, PROVINCIA DE SANTA FE (ARGENTINA)

Three magnetotelluric soundings were performed in C. Pellegrini, Santa Fe Province determining a West-East profile of 5926 m of long. The aim of the study was the estimation of possible aquifers underneath a basaltic cover. The basaltic thickness in this area of the Argentina corresponding to the Chacoparanense Basin was also estimated. These lava overfloors reached a great extension (1,2 million km2) and were contemporary with the continental aperture. Results allowed the identification of four stratigraphic formations. The first one has low resistivity of 2 to 4 Ωm (except the first 100 m with about 10 Ωm; which corresponds to modern sediments). This formation is present up to 700 m depth westwards of the profile and 850 m eastwards. The second formation is a basaltic layer, (Serra Geral Fm), with 680 m thickness in the West and 390 m and East of the profile, and resistivities between 50 Ωm to 150 Ωm. The following formation corresponds to sedimentary rocks showing a resistivity of 1-7 Ωm, where different sedimentary horizons can be discriminated; its top is approximately at 1200 m depth and the base reaches the range of depth of 2400 m to 3000 m. Within this think layer, a basal cover of 30 Ωm can be distinguished with approximately 200 m thickness, following the geometry of what can be interpreted as crystalline basement. Finally a last block has resistivities higher than 100 Ωm and it may be a massive formation corresponding to Precambrian age, as part of the continental craton.

Temperature and salinity history of the Precambrian ocean: implications for the course of microbial evolution

The temperature and salinity histories of the oceans are major environmental variables relevant to the course of microbial evolution in the Precambrian, the “age of microbes”. Oxygen isotope data for early diagenetic cherts indicate surface temperatures on the order of 55–85 °C throughout the Archean, so early thermophilic microbes (as deduced from the rRNA tree) could have been global and not just huddled around hydrothermal vents as often assumed. Initial salinity of the oceans was 1.5–2× the modern value and remained high throughout the Archean in the absence of long-lived continental cratons required to sequester giant halite beds and brine derived from evaporating seawater. Marine life was limited to microbes (including cyanobacteria) that could tolerate the hot, saline early ocean. Because O 2 solubility decreases strongly with increasing temperature and salinity, the Archean ocean was anoxic and dominated by anaerobic microbes even if atmospheric O 2 were somehow as high as 70% of the modern level. Temperatures declined dramatically in the Paleoproterozoic as long-lived continental cratons developed. Values similar to those of the Phanerozoic were reached by 1.2 Ga. The first great lowering of oceanic salinity probably occurred in latest Precambrian when enormous amounts of salt and brine were sequestered in giant Neoproterozoic evaporite basins. The lowering of salinity at this time, together with major cooling associated with the Neoproterozoic glaciations, allowed dissolved O 2 in the ocean for the first time. This terminated a vast habitat for anaerobes and produced threshold levels of O 2 required for metazoan respiration. Non-marine environments could have been oxygenated earlier, so the possibility arises that metazoans developed in such environments and moved into a calcite and silica saturated sea to produce the Cambrian explosion of shelled organisms that ended exclusive microbial occupation of the ocean. Inasmuch as chlorine is a common element throughout the galaxy and follows the water during atmospheric outgassing, it is likely that early oceans on other worlds are also probably so saline that evolution beyond the microbial stage is inhibited unless long-lived continental cratons develop.

Cratonic mantle roots, remnants of a more chondritic Archean mantle?

The Earth's continents are cored by Archean cratons underlain by seismically fast mantle roots descending to depths of 200 + km that appear to be both more refractory and colder than the surrounding asthenospheric mantle. Low-temperature mantle xenoliths from kimberlite pipes indicate that the shallow parts of these cratonic mantle roots are dominated by refractory harzburgites that are very old (3 + Ga). A fundamental mass balance problem arises, however, when attempts are made to relate Archean high-Mg lavas to a refractory restite equivalent to the refractory lithospheric mantle roots beneath Archean cratons. The majority of high-Mg Archean magmas are too low in Al and high in Si to leave behind a refractory residue with the composition of the harzburgite xenoliths that constitute the Archean mantle roots beneath continental cratons, if a Pyrolitic primitive mantle source is assumed. The problem is particularly acute for 3 + Ga Al-depleted komatiites and the Si-rich harzburgites of the Kaapvaal and Slave cratons, but remains for cratonic harzburgites that are not anomalously rich in orthopyroxene and many Al-undepleted komatiites. This problem would disappear if fertile Archean mantle was richer in Fe and Si, more similar in composition to chondritic meteorites than the present Pyrolitic upper mantle of the Earth. Accepting the possibility that the Earth's convecting upper mantle has become poorer in Fe and Si over geologic time not only provides a simpler way of relating Archean high-Mg lavas to the lithospheric mantle roots that underlie Archean cratons, but could lead to new models for the nature Archean magmatism and the lower mantle sources of modern hot-spot volcanism.

Two linear granite belts in the central-western North China Craton and their implication for Late Neoarchaean-Palaeoproterozoic continental evolution

Abstract Precambrian granitoid rocks are widely distributed in the North China Craton (NCC). The dominant granitic rocks in the central-western NCC are trondhjemitic, tonalitic and granodioritic (TTG) gneisses, as well as some calc-alkaline granitic gneisses and minor charnockites. Two linear granite belts in this area are different from other early Precambrian granitic rocks in the NCC in terms of their occurrence and chemistry; the Liang-Tu (LT) garnet granite belt and the Sanggan (SG) granite belt. The LT belt occurs along two large-scale ductile shear zones in metamorphosed Al-rich sedimentary country rocks (khondalitic series). In the central part of the LT belt are metagabbroic granulite lenses that were originally mafic dykes. In the outer part of the belt, garnet granites grade into khondalitic rocks. LT belt granites have geochemical characteristics of S-type granites. The SG granite belt is ∼400 km long and composed of many deformed gneissic granitic bodies and granitic sills and veins with/without gneissosity. They contain various lenses including high-pressure granulites and retrograded eclogites. The SG granite belt trends east-northeast, parallel to the regional gneissosity of the country rocks. Major and trace element geochemistry of these granites shows characteristics of syn-collisional and post-collisional granite. The country rocks beside the Sanggan granite belt are different, including the Huai’an-Fengzhen terrain (HFT) to the northwest and the Wutai-Fuping terrain (WFT) to the southeast. The LT and SG granite belts formed within an old continental craton and an old orogenic zone between two blocks, respectively.

Supercontinents in Earth History

Understanding the formation of cratons and orogenic belts is critical to the modelling of supercontinental assemblies. Continental cratons began to assemble by 3000 Ma or possibly earlier. The oldest assembly, Ur, was followed by Arctica at ∼2500 Ma and Atlantica at ∼2000 Ma. These three continental blocks apparently remained coherent until the breakup of Pangea. Nearly all of earth's continental blocks were assembled into one large landmass during at least three times in earth history. The oldest assembly comparable in size to Pangea was probably Columbia, which formed at ∼1800 Ma and began to rift at ∼1500 Ma. Columbia was followed by Rodinia, which lasted from ∼1100 Ma to 700 Ma. East and West Gondwana combined to form Gondwana at ∼500 Ma, and it joined with Laurasia to form Pangea at ∼250 Ma.

Buoyancy of the continental upper mantle

The thermal boundary layer beneath continental cratons extends into the Earth's mantle to depths of at least 200 km. It has been proposed that chemical depletion of the lithospheric mantle during partial melting offsets the effect of increased density from conductive cooling, resulting in neutral buoyancy with respect to the underlying asthenosphere. Mineral compositions of garnet peridotite xenoliths in the Kaapvaal craton give equilibration temperatures and pressures that define a continental conductive geotherm intersecting a mantle adiabat with a potential temperature of 1300°C at ∼60 kbar. We calculated normative densities for a “low‐temperature” garnet and spinel peridotite xenolith suite using Mg#. At their temperatures and pressures of equilibration, all the low‐temperature peridotites are positively buoyant with respect to the convecting mantle, which is inconsistent with the hypothesis of a neutrally buoyant thermal boundary layer. To account for the possibility that pressure, temperature, and mineral proportions may have varied over time, equilibrium solidus mineral assemblages for the low‐temperature xenoliths over a range of pressures and temperatures were generated by free energy minimization using the program Perplex (http://www.perplex.ethz.ch). The equilibrium solidus densities for xenolith compositions along a 40 mW/m2 conductive geotherm were compared with the density of pyrolite along a mantle adiabat with a potential temperature of 1300°C. These density calculations show that most of the xenoliths are positively buoyant with respect to asthenospheric mantle (“pyrolite”) at their temperatures and pressures of equilibration, confirming the results from the normative density calculations. Also, at the onset of the accretion of the Kaapvaal craton, when the thermal boundary layer was thinner and hotter than today, these Kaapvaal peridotites would have been positively buoyant with respect to the convecting mantle at shallower pressures. Therefore we propose that peridotite at the base of the Kaapvaal “plate,” in the mantle thermal boundary layer, was and is positively buoyant. Combined with evidence from the geoid and geomorphology, which suggests that cratons are isostatically compensated and neutrally buoyant, our results imply that there must be dense layers within cratonic crust or upper mantle that offset the positive buoyancy of depleted cratonic mantle peridotites.