Cratonic Lithosphere Research Articles

Edge-driven convection and thinning of craton lithosphere: Two-dimensional thermal-mechanical modeling

There are usually abrupt changes in lithospheric thickness at the boundaries between ancient cratons and adjacent young mobile belts. Lateral variations in temperature and density between the two can trigger small-scale mantle convection (edge-driven convection, EDC). Here, we use two-dimensional thermal-mechanical simulations to explore the EDC caused by the lithospheric step between a craton and a mobile belt, and its role in the thinning of the craton lithosphere. The results show that the impact of EDC on a craton depends on the properties of the craton lithosphere and on their contrasts with the adjacent mobile belt, given the same initial condition. When the craton lithospheric density is relatively large, a high-strength craton has strong ability to resist EDC, and craton lithospheric thinning is limited to the edge. In contrast, the ability of a low-strength craton to resist EDC is weak, and the craton lithosphere is gradually eroded by the downward flow, eventually leading to large-scale thinning of the cratonic lithosphere. When the craton lithospheric density is relatively small, regardless of the strength of the cratonic lithosphere, the craton can well withstand the impacts of EDC. In this case, upwelling flow drives cratonic lithosphere materials to the base of the mobile belt, and lithospheric thinning only occurs at the edge of the craton lithosphere. The Archean North China Craton (NCC) was surrounded by Phanerozoic mobile belts, and its lithospheric thinning first occurred on the northern and eastern margins, a process that lasted for a long time. We suggest that EDC has played an important role in lithospheric thinning of the NCC, in particular the initiation of lithospheric thinning, but we cannot rule out the additional contributions from other mechanisms.

Magnetotelluric images of Paleoproterozoic accretion and Mesoproterozoic to Neoproterozoic reworking processes in the northern São Francisco Craton, central-eastern Brazil

Broadband and long period magnetotelluric (MT) data were collected along an east-west oriented, 580-km-long profile across the northern São Francisco Craton where extensive Proterozoic and Phanerozoic sedimentary cover and lack of deep-probing geophysical surveys have prevented to establish unequivocally the cratonic character of the Archean-Paleoproterozoic lithosphere. Following dimensionality analyses, the MT dataset was interpreted using both 2-D and 3-D inversion procedures. The near-surface structure is better resolved in the 2-D model due to its finer resolution. A huge upper crustal conductor is found all along the shallow early Neoproterozoic Irecê Basin in the central domain of the craton, extending laterally for approximately 150 km and restricting signal propagation below the basin. Its high conductance is explained by a combination of high porosity and high fluid salinity in the sedimentary package. Another upper crustal conductor is observed on the west side of the profile, interpreted as fractured metasedimentary rocks of the Rio Preto belt thrusted on top of the craton basement during Neoproterozoic marginal collision. The 3-D model explains significantly better the measured data related to deep structure. Contrary to what is expected for a stable cratonic block, the geoelectric model shows pronounced electrical complexity and heterogeneity, an indication that the cratonic lithosphere was multiply reworked in the past by tectonothermal events. Different lithospheric resistive blocks bounded by major conductive zones are identified. Constrained by geochemical and isotopic data, these vertical conductive interfaces are interpreted as cryptic suture zones due to large-scale amalgamation of continents and microcontinents leading to the assembly of the São Francisco Craton in the Paleoproterozoic. The conductivity enhancement is more likely explained by emplacement of sulfides along previous suture zones during mafic magmatism. At upper mantle depths, high conductivity observed below most of the profile indicates that metasomatism or refertilization processes with incompatible elements caused by the Paleoproterozoic subducting slabs and Mesoproterozoic to Neoproterozoic upwelling of deep fluids and melts reworked this portion of the craton mantle.

Variation of melting processes and magma sources of the early Deccan flood basalts, Malwa Plateau, India

The Deccan flood basalts cover large parts of India and represent one of the largest volcanic provinces on Earth. We present geochemical and isotopic data on a magnetically defined stratigraphic section of lavas from the northern portion of the Deccan, the Malwa Plateau, which are dated at ∼67.5 Ma. These are therefore some of the oldest Deccan lavas, erupted before the Main Deccan units further to the south. Although the younger Malwa Plateau lavas have undergone significant crustal contamination by at least two distinct crustal components, the older, more alkalic lavas are less contaminated and have similar isotope and trace element compositions to Réunion oceanic intraplate basalts. Our data therefore provide geochemical evidence for a contribution of the Réunion hotspot to the generation of the early Deccan flood basalts. Most lavas reflect fractional crystallization and assimilation of less than 10% of a crustal melt but samples with the highest SiO2 and Sr isotope compositions underwent more extreme crustal contamination. The composition of the least contaminated basalts from the Malwa Plateau suggests that the degree of partial melting increased significantly from the base to the top of the section. Most Malwa Plateau basaltic magmas formed by <5% partial melting of garnet peridotite in the Réunion mantle plume whereas the degree of melting in the Main Deccan Volcanic Province increased threefold and included MORB sources. Together, these geochemical variations indicate increasing degrees of melting between 68.5 and 65.4 Ma and a variation of the mantle sources from Réunion plume mantle to an increasing contribution from depleted upper mantle. These changes probably reflect lithospheric erosion by the mantle plume within ∼2 million years that allows melting of entrained upper mantle material. The geochemical data thus agree with geophysical models of significant thinning of cratonic lithosphere during the passage across mantle plumes. The magma evolution from alkaline to tholeiitic basalts observed in the Deccan flood basalts resembles that observed in early oceanic plume-related volcanism like Hawaii.

Lithospheric Structure and Evolution of Southern Africa: Constraints From Joint Inversion of Rayleigh Wave Dispersion and Receiver Functions

AbstractWe conduct a joint inversion of teleseismic receiver functions and Rayleigh wave phase velocity dispersion from both ambient noise and earthquakes using data from 79 seismic stations in southern Africa, which is home to some of the world's oldest cratons and orogenic belts. The area has experienced two of the largest igneous activities in the world (the Okavango dyke swarm and Bushveld mafic intrusion) and thus is an ideal locale for investigating continental formation and evolution. The resulting 3‐D shear wave velocities for the depth range of 0–100 km and crustal thickness measurements show a clear spatial correspondence with known geological features observed on the surface. Higher than normal mantle velocities found beneath the southern part of the Kaapvaal craton are consistent with the basalt removal model for the formation of cratonic lithosphere. In contrast, the Bushveld complex situated within the northern part of the craton is characterized by a thicker crust and higher crustal Vp/Vs but lower mantle velocities, which are indicative of crustal underplating of mafic materials and lithospheric refertilization by the world's largest layered mafic igneous intrusion. The thickened crust and relatively low elevation observed in the Limpopo belt, which is a late Archean collisional zone between the Kaapvaal and Zimbabwe cratons, can be explained by eclogitization of the basaltic lower crust. The study also finds evidence for the presence of a stalled segment of oceanic lithosphere beneath the southern margin of the Proterozoic Namaqua‐Natal mobile belt.

Elemental and radiogenic isotope perspective on formation and transformation of cratonic lower crust: Central Slave craton (Canada)

Kimberlite-borne granulite xenoliths provide rare insights into the age, chemical composition and tectonothermal evolution of the otherwise largely inaccessible deep cratonic crust. The formation and transformation of the lower continental crust (LCC) beneath the central Slave craton (Canada) is here illuminated using whole-rock trace-element and Sr-Nd isotope compositions of nine metabasaltic (MBG), one gabbroic (MGG) and two metasedimentary/hybrid (MSG) granulite xenoliths. On the one hand, published sulphide Re-Os and a few zircon U-Pb data indicate that at least a portion of the LCC beneath the central Slave craton has a Palaeoarchaean origin (∼3.3 Ga), which apparently coincides with a period of juvenile crust and deep lithospheric mantle formation during plume impingement beneath the pre-existing cratonic nucleus. On the other hand, enrichment in Li, Sr, LREE, Pb and Th, but relative depletion in Ti, Hf and HREE, suggest formation of (picro)basaltic protoliths by partial melting of a subduction-modified garnet-bearing source, Crystallisation in the crust after fractionation of plagioclase is inidicated by their Sr and Eu negative anomalies, which are complementary to the positive anomalies in the MGG. Samarium-Nd isotopes in MBG and MGG show large scatter, but fall on Neo- or Mesoarchaean age arrays. These elemental systematics are suggested to fingerprint deserpentinisation fluids plus small amounts of sedimentary melt as the main contaminants of the mantle source, supporting the operation of at least regional and transient subduction at 3.3 Ga. Evidence for quasi-coeval plume impingement and subduction beneath the central Slave craton in the Mesoarchaean is reconcilable in a dynamic regime where vertical tectonics, though waning, was still active and plate interactions became increasingly important. Unradiogenic 87Sr/86Sr (down to 0.7017) is consistent with significant loss of Rb and probably other heat-producing elements (K, Th, U) plus H2O during Neoarchaean metamorphism, which helped to enhance LCC viscosity and stabilise the cratonic lithosphere.

Response of a Rheologically Stratified Lithosphere to Subduction of an Indenter‐Shaped Plate: Insights Into Localized Exhumation at Orogen Syntaxes

AbstractThis study investigates the influence of the 3‐D geometry of a down‐going plate, the rheological structure of the upper plate, and the migration of the overriding plate toward the trench in relation to the overall subduction velocity on the exhumation pattern in orogen syntaxes. Using a thermomechanical numerical code (DOUAR), we analyze the strain localization, rock uplift, and exhumation response of a rheologically stratified continental lithosphere to subduction of a convex‐upward‐shaped indenter. The models consider three thermorheological lithospheric profiles that determine the degree of mechanical coupling between the upper crust and lithospheric mantle. These models include a strong, cratonic lithosphere; a weaker, younger (and hotter) continental plate; and an intermediate case. The strongly coupled case predicts a localization of high rock uplift rates along narrow linear bands crossing the entire model domain parallel to the trench. In contrast, in a weakly coupled lithosphere, rock uplift is concentrated within a curved ellipse region of anomalously high exhumation rates located above the indenter apex. The aspect ratio of the localized area of rapid rock uplift is controlled by the initial width of the rigid indenter and the relationship between boundary velocities. In particular, the combination of little or no upper plate migration with a narrow indenter causes a nearly circular region (~100‐km diameter) of rapid exhumation that resembles the pattern of thermochronometer ages observed in orogen syntaxes such as the Southeast Alaska and the Olympic Mountains of the Cascadia subduction zone (western USA).

The subduction of the west Pacific plate and the destruction of the North China Craton

While a general concensus has recently been reached as to the causal relationship between the subduction of the west Pacific plate and the destruction of the North China Craton, a number of important questions remain to answer, including the initial subduction of west Pacific plate beneath the eastern Asian continent, the position of west Pacific subduction zone during the peak period of decratonization (i.e., Early Cretaceous), the formation age of the big mantle wedge under eastern Asia, and the fate of the subducted Pacific slab. Integration of available data suggests that the subduction of the western Pacific plate was initiated as early as Early Jurrasic and the subduction zone was situated to 2,200 km west of the present-day trench in the Early Creataceous, as a result of eastward migration of the Asian continent over a distance of ca. 900 km since the Early Cretaceous. The retreat of the subducting west Pacific plate started ∼145 Ma ago, corresponding to the initial formation of the big mantle wedge system in the Early Cretaceous. The subduction of the Pacific slab excerted severe influence on the North China Craton most likely through material and energy echange between the big mantle wedge and overlying cratonic lithosphere. The evolution history of the west Pacific plate was reconstructed based on tectonic events. This allows to propose that the causes of phases A and B for the Yanshanian orogeny were respectively related to rapid low-angle subduction and to lowering subduction angle of the west Pacific plate. At ca. 130–120 Ma, the subduction of the west Pacific plate was characterized by increasing subducting angle, slab rollback and rapid trench retreat, leading to the final stagnation of the subducting slab within the mantle transition zone. This process may have significantly affected the physical property and viscosity of the mantle wedge above the stagnant slab, resulting in non-steady mantle flows. The ingression of slab-released melts/fluids would significantly lower the viscosity of the mantle wedge and overlying lithosphere, inducing decratonization. This study yields important bearings on the relationship between the subduction of the west Pacific plate and the evolution of the lithospheric mantle beneath the North China Craton.

Kimberlite and carbonatite dykes within the Premier diatreme root (Cullinan Diamond Mine, South Africa): New insights to mineralogical-genetic classifications and magma CO2 degassing

The ca. 1153 Ma Premier kimberlite pipe on the Kaapvaal craton has been intruded by late-stage kimberlite and carbonatite magmas forming discrete 0.5 to 5 m wide dykes within the lower diatreme. On the basis of petrography and geochemistry, the fresh kimberlite dykes represent archetypal monticellite phlogopite kimberlite of Group-1 affinity. Their mineral compositions, however, show marked deviations from trends that are typically considered as diagnostic for Group-1 kimberlite in mineralogical-genetic classification schemes for volatile-rich ultramafic rocks. Groundmass spinel compositions are transitional between magnesian ulvöspinel (a Group-1 kimberlite hallmark feature) and titanomagnetite trends, the latter being more diagnostic for lamproite, orangeite (formerly Group-2 kimberlite), and aillikite. The Premier kimberlite dykes contain groundmass phlogopite that evolves by Al- and Ba-depletion to tetraferriphlogopite, a compositional trend that is more typical for orangeite and aillikite. Although high-pressure cognate and groundmass ilmenites from the Premier hypabyssal kimberlites are characteristically Mg-rich (up to 15 wt% MgO), they contain up to 5 wt% MnO, which is more typical for carbonate-rich magmatic systems such as aillikite and carbonatite. Manganese-rich groundmass ilmenite also occurs in the Premier carbonatite dykes, which are largely devoid of mantle-derived crystal cargo, suggesting a link to the kimberlite dykes by fractionation processes involving development of residual carbonate-rich melts and fluids. Although mineralogical-genetic classification schemes for kimberlites and related rocks may provide an elegant approach to circumvent common issues such as mantle debris entrainment, many of the key mineral compositional trends are not as robust for magma type identification as previously thought.Utilizing an experimentally constrained CO2-degassing model, it is suggested that the Premier kimberlite dykes have lost between 10 and 20 wt% CO2 during magma ascent through the cratonic lithosphere, prior to emplacement near the Earth's surface. Comparatively low fO2 values down to −5.6 ∆NNO are obtained for the kimberlite dykes when applying monticellite and perovskite oxybarometry, which probably reflects significant CO2 degassing during magma ascent rather than the original magma redox conditions and those of the deep upper mantle source. Thus, groundmass mineral oxybarometry may have little value for the prediction of the diamond preservation potential of ascending kimberlite magmas.After correction for olivine fractionation and CO2-loss, there remains a wide gap between the primitive kimberlite and carbonatite melt compositions at Premier, which suggests that these magma types cannot be linked by variably low degrees of partial melting of the same carbonated peridotite source in the deep upper mantle. Instead, fractionation processes produced carbonate-rich residual melts/fluids from ascending kimberlite magma, which led to the carbonatite dykes within Premier pipe.

Thermal-chemical conditions of the North China Mesozoic lithospheric mantle and implication for the lithospheric thinning of cratons

Cratons are the most ancient parts of continents that are underlain by thick, cold, old and refractory lithospheric roots. However, how cratonic roots remain stable for billions of years and become remobilized later is still not well understood. The eastern North China Craton (NCC) is the best region to illuminate this issue because of its well-known lithospheric thinning and decratonization during the Mesozoic–Cenozoic. The thinning mechanism is debated because of limited constraints on the thermal-chemical conditions (lithology and P–T–H2O–fo2) of the Archean lithospheric mantle before and during its removal. Here, we provide constraints on these thermal-chemical conditions for the Archean lithospheric mantle beneath the eastern NCC during its extensive thinning in the form of whole-rock chemical and Sr–Nd–Pb isotopic compositions and mineral (especially olivine) chemistry of the Early Cretaceous primitive basalts (MgO > 10 wt.%) from Yixian and Sihetun in the western Liaoning Province.Our data support a model in which the Yixian and Sihetun basalts were derived from metasomatized Archean lithospheric mantle under shallow (∼50–60 km), hot (∼1,290–1,350°C) conditions. This indicates the existence of a relict (∼25 km) of the Archean lithospheric mantle during the Early Cretaceous, supporting gradual or episodic erosion of the eastern NCC lithospheric mantle. Furthermore, the NCC lithospheric mantle was not only widely rehydrated (>1,000 ppm H2O) but also highly oxidized (Δlogfo2FMQ=+1.5∼+1.9 at 1.7–2.0 GPa) during its extensive thinning. Such rehydration and oxidization are demonstrated to be closely related to wet upwelling from the Mantle Transition Zone (MTZ) triggered by the deep subduction of the Paleo-Pacific oceanic slab in the period ∼200–125 Ma. We emphasize that the water released from the upwelling MTZ component and associated hydrous melt influx played a key role in the lithospheric thinning of the eastern NCC by oxidizing the lithospheric mantle and lowering its melting point, which led to redox melting, promoting the erosion of cratonic lithosphere. Our study provides key evidence for the role of deep volatile cycling from the MTZ in modifying thermal-chemical conditions and in the lithospheric thinning of cratons.

Long‐Period Rayleigh Wave Phase Velocity Tomography Using USArray

AbstractSeismic models show that the shallow mantle beneath the contiguous United States is characterized by low velocities west of the Rocky Mountain Front, high velocities beneath the North American craton, and moderate wave speeds along the eastern seaboard. However, numerous questions remain, including the distribution of temperature and partial melt in the mantle that may explain the slower velocities in the west; the origin and depth extent of the western edge of the cratonic lithosphere; and explanations for slower velocities observed along the eastern seaboard. To investigate these questions, we have measured teleseismic Rayleigh wave phase and amplitude at USArray stations for periods of 25–180 s and calculated phase velocity using the Helmholtz tomography method. Notably, the new long‐period maps represent an opportunity to resolve mantle structure at sublithospheric depths and show differences relative to shorter periods in both the distribution of slow velocities in the west and the extent of fast velocities in the central United States. The maps are then used as a basis for comparing six recent 3‐D shear velocity models of the contiguous United States, from which we predict phase velocities. While predictions from surface‐wave models agree best with our observations at short and intermediate periods, the longest‐period maps are most similar in amplitude to body‐wave models.

Shear Wave Splitting Discloses Two Episodes of Collision‐Related Convergence in Western North America

AbstractSeismic anisotropy imposes first‐order constraints on the strain history of crust and upper mantle rocks. In this study, we analyze the mantle seismic anisotropy of the Western Canada Sedimentary Basin using a new shear wave spitting data set consisting of 1,333 teleseismic arrivals from 82 seismic stations. The resulting 332 high‐quality measurements yield a regional mean apparent splitting time (i.e., the magnitude of anisotropy) of 1.1 ± 0.3 s and an average fast orientation (i.e., the direction of anisotropy) of 54.6° ± 17.2°, which favor a two‐layer anisotropic model based on the 90° back azimuthal periodicity in both parameters. The northeast trending fast orientations dominate the lower layer at lithospheric depths and are approximately parallel to the present‐day absolute plate motions (APMs; i.e., &lt;35°) due to the active asthenospheric flow. On the other hand, deviations from the APMs along the Canadian Rocky Mountain foothills could reflect disrupted mantle flow surrounding a southwestward migrating cratonic lithosphere. Also revealed are two elongated upper‐layer anisotropic anomalies in the lithosphere that are spatially correlated with Moho depths. Their characteristics suggest frozen‐in anisotropy imprinted along two convergent boundaries: (1) the Paleoproterozoic Snowbird Tectonic Zone that separates northeast (north) from northwest (south) fast directions and (2) the foothills of the Rocky Mountains that exhibit northeast trending orientations consistent with those of the APMs, maximum crustal stress, and electromagnetic anisotropy. Compressions associated with the Cordilleran orogenesis could be responsible for the spatial changes in the shear wave anisotropy from the foothills to the cratonic interior.

Mapping the Gawler Craton–Musgrave Province interface using integrated heat flow and magnetotellurics

The approximate position of a terrane boundary between the Archean–Mesoproterozoic Gawler Craton and the Meso–Neoproterozoic Musgrave Province, central Australia, is currently inferred from magnetic and gravity anomalies. However, its precise location, structural architecture and tectonic attributes are poorly understood. Thick overlying sediments of the Neoproterozoic Officer Basin restrict outcropping basement, and the number of basement-intersecting drill holes across the proposed boundary is limited. The Gawler Craton–Musgrave Province interface is critical to interpreting the tectonic relationship between these adjacent terranes, providing insights into the mechanism of Australian Proterozoic assembly. To assist its geophysical identification, we present 14 new heat flow estimates from the Nawa and Mount Woods Domains of the Gawler Craton, along with the first heat flow value calculated from the eastern Musgrave Province. These estimates are integrated with a magnetotelluric (MT) profile that transects the study area. The new datasets show that a transition exists from heat flow values of 68–89 mW m−2 in the eastern Musgrave Province and northern Nawa Domain, through to higher heat flow values of 93–112 mW m−2 in the southern Nawa and Mount Woods Domains. This transition corresponds to a highly resistive region located between two subvertical conductive bodies observed in the MT data; seismic interpretations from the Gawler Craton–Officer Basin–Musgrave Province–Amadeus Basin (GOMA) profile indicate that it corresponds to a region located between the Sarda Bluff Fault and Karari Shear Zone within the Nawa Domain. We therefore argue that the transition from Musgrave Province to Gawler Craton lithosphere can be mapped on the basis of higher heat flow and electrical conductivity, providing sufficient contrast to constrain this interface more accurately than previous geophysical models based on magnetic and gravity data alone.

Petrological, mineralogical and geochemical peculiarities of Archaean cratons

The most outstanding features of Archaean cratons are their extraordinary thickness and enduring longevity. Seismically, Archaean cratonic fragments are sharply-bounded deep roots of mechanically strong, cold lithospheric mantle, clearly distinguishable from non-cratonic lithosphere. Rhenium-depletion of deep cratonic xenolith whole rocks and sulphide inclusions in diamond indicate that melting was broadly coeval with formation of the overlying proto-cratonic crust, which was of limited mechanical strength.A very important process of proto-cratonic development was vertical crustal reorganisation that eventually yielded a thermally stable, cratonised crust with a highly K-U-Th-rich uppermost crust and much more depleted deeper crust. Clastic sedimentary rocks available for geochemical study are predominantly found in the youngest parts of supracrustal stratigraphies and over-represent the highly evolved rocks that appeared during cratonisation. Vertical crustal reorganisation was driven by crustal radiogenic heat and emplacement of proto-craton-wide, incubating and dense supracrustal mafic and ultramafic volcanic rocks. Statistical analysis of these cover sequences shows a preponderance of basalt and a high abundance of ultramafic lavas with a dearth of picrite. The ultramafic lavas can be grouped into Ti-enriched and Ti-depleted types and high pressure and temperature experimental data indicate that the latter formed from previously depleted mantle at temperatures in excess of 1700 °C.Most mantle harzburgite xenoliths from cratonic roots are highly refractory, containing very magnesian olivine and many have a high modal abundance of orthopyroxene. High orthopyroxene mode is commonly attributed to metasomatic silica-enrichment or a non-pyrolitic mantle source but much of the excess silica requirement disappears if melting occurred at high pressures of 4–6 GPa. Analysis of experimental data demonstrates that melting of previously depleted harzburgite can yield liquids with highly variable Si/Mg ratios and low Al2O3 and FeO contents, as found in komatiites, and complementary high Cr/Al residues. In many harzburgites, there is an intimate spatial association of garnet and spinel with orthopyroxene, which indicates formation of the Al-phase by exsolution upon cooling and decompression. New and published rare earth element (REE) data for garnet and orthopyroxene show that garnet has inherited its sinusoidal REE pattern from the orthopyroxene. The lack of middle-REE depletion in these refractory residues is consistent with the lack of middle- over heavy-REE fractionation in most komatiites. This suggests that such pyroxene or garnet (or precursor phases) were present during komatiite melting. In the Kaapvaal craton, garnet exsolution upon significant cooling occurred as early as 3.2 Ga and geobarometry of diamond inclusions from ancient kimberlites also supports cool Archaean cratonic geotherms. This requires that some mantle roots have extended to 300 to possibly 400 km and that early cratons must have been much larger than 500 km in diameter.We maintain that the Archaean-Proterozoic boundary continues to be of geological significance, despite the recognition that upper crustal chemistry, as sampled by sedimentary rocks, became more evolved from ca. 3 Ga onwards. The boundary coincides with the disappearance of widespread komatiite and marks the end of formation of typical refractory cratonic lithosphere. This may signify a fundamental change in the thermal structure of the mantle after which upwellings no longer resulted in very high temperature perturbations. One school of thought is that the thermal re-ordering occurred at the core-mantle boundary whereas others envisage Archaean plumes to have originated at the base of the upper mantle. Here we speculate that Archaean cratonic roots may contain remnants of older domains of non-convecting mantle. These domains are potential carriers of isotope anomalies and their base could have constituted a mechanical and thermal boundary layer. Above laterally extensive barriers, emerging proto-cratons were protected from the main mantle heat loss. The eventual collapse of these mechanical barriers terminated very high temperature upwellings and dismembered portions of the barrier were incorporated into the cratonic mantle during the final Neoarchaean ‘superplume’ event. The surviving cratons may therefore preserve biased evidence of geological processes that operated during the Archaean.

Structure of the Lithosphere Beneath the Barotse Basin, Western Zambia, From Magnetotelluric Data

AbstractA magnetotelluric survey in the Barotse Basin of western Zambia shows clear evidence for thinned lithosphere beneath an orogenic belt. The uppermost asthenosphere, at a depth of 60–70 km, is highly conductive, suggestive of the presence of a small amount of partial melt, despite the fact that there is no surface expression of volcanism in the region. Although the data support the presence of thicker cratonic lithosphere to the southeast of the basin, the lithospheric thickness is not well resolved and models show variations ranging from ~80 to 150 km in this region. Similarly variable is the conductivity of the mantle beneath the basin and immediately beneath the cratonic lithosphere to the southeast, although the conductivity is required to be elevated compared to normal lithospheric mantle. In a general sense, two classes of model are compatible with the magnetotelluric data: one with a moderately conductive mantle and one with more elevated conductivities. This latter class would be consistent with the impingement of a stringer of plume‐fed melt beneath the cratonic lithosphere, with the melt migrating upslope to thermally erode lithosphere beneath the orogenic belt that is overlain by the Barotse Basin. Such processes are potentially important for intraplate volcanism and also for development or propagation of rifting as lithosphere is thinned and weakened by melt. Both models show clear evidence for thinning of the lithosphere beneath the orogenic belt, consistent with elevated heat flow data in the region.

Moho Depth Variations From Receiver Function Imaging in the Northeastern North China Craton and Its Tectonic Implications

AbstractA detailed knowledge of the crustal thickness in the northeastern North China Craton (NCC) is important for understanding the unusual Phanerozoic destruction of the craton. We achieve this goal by employing a 2‐D wave equation‐based migration method to P receiver functions from 198 broadband seismic stations, using Ps conversions and surface‐reflected multiples. By combining receiver function images along 19 profiles, we constructed a high‐resolution Moho depth model for the northeastern NCC. The results present dominant E‐W Moho depth variations similar to previous observations and new regional N‐S variations beneath both sides of the North‐South Gravity Lineament. To the west, while a deeper Moho (∼42 km) appears in the interior of the Trans‐North China Orogen, a relatively shallow Moho (∼38 km) exists in the northern margin of the Trans‐North China Orogen to western NCC. To the east, the crust beneath the Yan Mountains in the marginal area is thicker (∼32–40 km) than that (∼26–32 km) beneath the Bohai Bay Basin in the craton interior, and the Moho further shallows from NE (∼32 km) to SW (∼26 km) within the basin. Along with other observations, we conclude that the dominant E‐W difference may have been associated with the Paleo‐Pacific plate subduction under eastern Asia since the Mesozoic. The newly observed complex N‐S variations may have reflected the structural heterogeneity of the cratonic lithosphere inherited since the formation of the NCC in the Paleoproterozoic, or spatially uneven effects on the cratonic lithosphere of subsequent thermotectonic events during the long‐term evolution of the craton, or both.

Magnetotelluric investigation of lithospheric electrical structure beneath the Dharwar Craton in south India: Evidence for mantle suture and plume-continental interaction

Broad-band and long period magnetotelluric measurements made at 63 locations along ∼500 km long Chikmagalur-Kavali profile, that cut across the Dharwar craton (DC) and Eastern Ghat Mobile Belt (EGMB) in south India, is modelled to examine the lithosphere architecture of the cratonic domain and define tectonic boundaries. The 2-D resistivity model shows moderately conductive features that intersperse a highly resistive background of crystalline rocks and spatially connect to the exposed schist belts or granitic intrusions in the DC. These features are therefore interpreted as images of fossil pathways of the volcanic emplacements associated with the greenstone belt and granite suite formation exposed in the region. A near vertical conductive feature in the upper mantle under the Chitradurga Shear Zone represents the Archean suture between the western and eastern blocks of DC. Although thick (∼200 km) cratonic (highly resistive) lithosphere is preserved, significant part of the cratonic lithosphere below the western DC is modified due to plume-continental lithosphere interactions during the Cretaceous–Tertiary period. A west-verging moderately conductive feature imaged beneath EGMB lithosphere is interpreted as the remnant of the Proterozoic collision process between the Indian land mass and East Antarctica. Thin (∼120 km) lithosphere is seen below the EGMB, which form the exterior margin of the India shield subsequent to its separation from East Antarctica through rifting and opening of the Indian Ocean in the Cretaceous.

Dynamics of Metasomatic Transformation of the Rocks of the Lithospheric Mantle and Earth’s Crust in Deep-Fault Zones Controlling the Siberian Platform Trap Magmatism

Abstract —For a multirate approximation, we have determined the dynamics of rock heating by a magmatic-fluid flow in a flat permeable zone cutting the cratonic lithosphere of the Siberian Platform from a magma chamber at a depth of 50 km to the Earth’s surface. This dynamics is compared with the dynamics of infiltration metasomatism in a three-layer lithosphere section: (1) harzburgitic mantle (depth 50–40 km), (2) crystalline basement (39–7 km), whose composition was simulated by the section of rocks hosting the skarn deposits of the Aldan Shield, and (3) platform cover (6–0 km), with its simplified rock compositions specified on the basis of the rock compositions in the southern and northern parts of the trap area of the Siberian Platform. Numerical modeling of the metasomatic transformation of rocks was performed in a multireservoir flow reactor, using the Selektor software. The initial composition of fluids in a magmatic source varied from highly reduced (water–methane) to ordinary (water–acid) (lg pO2 from –13.0 to –12.0). The obtained balances of the interacting phases show no significant change in the mass of aluminosilicate rocks in the mantle and Earth’s crust sections and a significant loss of their mass under replacement of carbonate and sulfate deposits.

Lithospheric modification by extension and magmatism at the craton-orogenic boundary: North Tanzania Divergence, East Africa

We present a joint analysis of newly acquired gravity and teleseismic data in the North Tanzanian Divergence, where the lithospheric break-up is at its earliest stage. The impact of a mantle upwelling in more mature branches of the East African Rift has been extensively studied at a lithospheric scale. However, few studies have been completed that relate the deep-seated mantle anomaly detected in broad regional seismic tomography with the surface deformation observed in the thick Archaean Pan-African suture zone located in North Tanzania. Our joint inversion closes the gap between local and regional geophysical studies, providing velocity and density structures from the surface down to ca. 250 km depth with new details. Our results support the idea of a broad mantle upwelling rising up to the lithosphere and creating a thermal modification along its path. However, our study clearly presents an increasing amplitude of the associated anomaly both in velocity and density above 200 km depth, which cannot be solely explained by a temperature rise. We infer from our images the combined impact of melt (2–3 per cent), composition and hydration that accompany the modification of a thick heterogenous cratonic lithosphere are a response to the hot mantle rising. The detailed images we obtained in density and velocity assert that Archaean and Proterozoic units interact with the mantle upwelling to restrict the lithosphere modifications within the Magadi–Natron–Manyara rift arm. The composition and hydration variations associated with those units equilibrate the thermal erosion of the craton root and allow for its stability between 100 and 200 km depth. Above 80 km depth, the crustal part is strongly affected by intruding bodies (melt and gas) which produces large negative anomalies in both velocity and density beneath the main magmatic centres. In addition to the global impact of a superplume, the velocity and density anomaly pattern suggests a 3-D distribution of the crust and mantle lithospheric stretching, which is likely to be controlled by inherited fabrics and enhanced by lateral compositional and hydration variations at the Tanzanian craton-orogenic belt boundary.

The effect of P–T on the reaction between tonalitic melt and mantle lherzolite at 2–4 GPa and implications for evolution of North China Cratonic Lithosphere and generation of High Mg# andesite

To assess the effects of reaction factors (mainly P and T) on the mineral assemblage and chemical composition of residual minerals and reacted melt in the reaction between andesitic melt and mantle peridotite, we conducted experimental reactions between tonalitic melt and mantle lherzolite using a DS-3600 six-anvil apparatus at 2–4 GPa and 1250 °C–1400 °C. The reaction products contain two types of reaction vein: orthopyroxene(r) [r means this mineral generated in melt-peridotite reaction and located at reaction rim] + garnet(r) and orthopyroxene(r). The mineral assemblages of the reaction veins are controlled by the reaction P and T, which can be determined via the empirical discriminant function: Y = 3.33 × 10−6 T2–2.17 × 10−3 T − 0.5 − P (T in °C, P in GPa; Y ≥ 0: Opx veins form; Y < 0: Opx + Grt veins form). These two types of reaction vein match those found in mantle xenoliths in Mesozoic and Cenozoic magmas from the North China Craton (NCC), indicating that they were formed by similar processes. The melt–peridotite reaction (including P and T) also influences the chemical composition of the residual minerals in the experimental products, with the major element variations being similar to those of NCC lithospheric mantle evolution from Palaeozoic to Cenozoic. Based on the experimental results, olivine in xenoliths from depths of ~100 km (~3 GPa) has higher SiO2 and lower NiO and MnO contents than olivine in xenoliths from depths of ~130 km (~4 GPa), and orthopyroxene in xenoliths from depths of ~100 km has higher Mg#, higher CaO content, and lower NiO content than orthopyroxene in xenoliths from depths of ~130 km. The reacted-mantle xenoliths contain clinopyroxene that originated from cold (<1300 °C) and deep (~135 km) lithospheric mantle. The P–T conditions also control the chemical composition of the reacted melt. An increasing reaction pressure increases the SiO2 content, whereas an increasing reaction temperature reduces the SiO2 content. In combination with the varying Na2O and K2O contents of the parental magma, the reacted melt can yield almost all types of intermediate magma with high Mg# (Mg# = mol[Mg/(Mg + ∑Fe)], Mg# > 0.45) character. This variation in major element of the reacted melt agrees with field observations of Mesozoic high-Mg# andesitic rocks from the NCC, indicating similar causes of the variation in the major element contents. With respect to the trace element content of the reacted melt, a garnet content of 15%–20% produces a right deviation of the heavy rare earth element partition curve (chondrite-normalized) and substantially elevates the Sr/Y and LaN/YbN ratios, indicating an adakite-like reacted melt.

An oxygen isotope test for the origin of Archean mantle roots

The origin of the peridotites that form cratonic mantle roots is a central issue in understanding the history and survival of Earth’s oldest continents. A long-standing hypothesis holds that the unusual bulk compositions of some cratonic peridotites stem from their origin as subducted oceanic serpentinite, dehydrated during subduction to form rigid buoyant keels (Schulze, 1986; Canil and Lee, 2009). We present oxygen isotope data from 93 mantle peridotites from five different Archean cratons to evaluate their possible origin as serpentinites. Cratonic mantle peridotite shows remarkably uniform δ18O values, identical to modern MORB-source mantle, that do not vary with bulk rock Si-enrichment or Ca-depletion. These data clearly conflict with any model for cratonic lithosphere that invokes serpentinite as a protolith for cratonic peridotite, and place additional constraints on cratonic mantle origins. We posit that the uniform δ18O was produced by sub-arc and/or MOR depletion processes and that the Si-enriched nature of some samples is unlikely to be related to slab melt infiltration. Instead, we suggest a peridotitic source of Si-enrichment, derived from ascending mantle melts, or a water-fluxed depleted mantle. These variably Si-enriched, cratonic mantle protoliths were then collisionally compressed into the thick cratonic roots that have protected Earth’s oldest continental crust for over 2.5 Gyr.