Plate Tectonic Processes Research Articles

Coexisting divergent and convergent plate boundary assemblages indicate plate tectonics in the Neoarchean

The coexistence of divergent (spreading ridge) and convergent (subduction zone) plate boundaries at which lithosphere is respectively generated and destroyed is the hallmark of plate tectonics. Here, we document temporally- and spatially-associated Neoarchean (2.55–2.51 Ga) rock assemblages with mid-ocean ridge and supra-subduction-zone origins from the Angou Complex, southern North China Craton. These assemblages record seafloor spreading and contemporaneous subduction initiation and mature arc magmatism, respectively, analogous to modern divergent and convergent plate boundary processes. Our results provide direct evidence for lateral plate motions in the late Neoarchean, and arguably the operation of plate tectonics, albeit with warmer than average Phanerozoic subduction geotherms. Further, we surmise that plate tectonic processes played an important role in shaping Earth’s surficial environments during the Neoarchean and Paleoproterozoic.

Petrogenesis of the Late Carboniferous Trondhjemite in Central Inner Mongolia in North China and Constraints of Intra-Oceanic Subduction in the Southern Paleo-Asian Ocean

Intra-oceanic subduction is a fundamental process on Earth, the study of which can improve the understanding of plate tectonic processes and the history of continental growth. Here, we report on newly recognized trondhjemite in the north of Diyanmiao ophiolite belt in North China. The trondhjemite was found along the Erenhot-Hegenshan suture zone. U-Pb zircon dating revealed that the trondhjemite crystallized at 309 ± 2.1 Ma. The trondhjemite had a high amount of SiO2 (68.94–76.45 wt %), Al2O3 (13.37–15.90 wt %), and Sr (232–601 ppm); and a low amount of K2O (1.57–2.70 wt %), Y (6.91–9.39 ppm), Ni (1.10–4.19 ppm), and Cr (1.55–13.50 ppm). The Na2O/K2O ratios were 1.90–4.37. There was a lack of negative Eu anomalies. It was relatively enriched in large-ion lithophile elements (LILEs) such as Rb, Ba, K, and Sr; was depleted in high-field-strength elements (HFSEs) such as Nb, Ta, Ti, and P; and had low total rare-earth element (REE) contents (27.73–49.63 ppm) with distinct REE fractionation (chondrite-normalized (La/Yb)N of 5.76–10.52), which was similar to adakitic rocks formed by partial melting of subducted oceanic crust. The trondhjemite, together with Diyanmiao ophiolite (335.6 Ma), may have formed during the stages of intra-oceanic subduction, suggesting that in the Early Carboniferous–Late Carboniferous, the southern Paleo-Asian Ocean was in its subduction stage.

Kinematic Representations of Linear and Power‐Law Viscoelastic Deformation Through the Earthquake Cycle

AbstractViscoelastic deformation below the Earth's elastic crust is modulated by stresses generated by both plate tectonic and earthquake cycle processes. Rapid near‐fault deformation following large earthquakes has been interpreted as the signature of viscoelastic stress diffusion in the upper mantle and earthquake cycle models have been developed that integrated this effect throughout the duration of the earthquake cycle. Here we develop a surrogate method to approximate the upper crustal kinematics associated with time‐dependent viscoelastic deformation that does not require knowledge of nor provide constraints on subcrustal rheology. To do this we show how the effects of deformation in the viscoelastic upper mantle can be emulated by introducing a set of effective dislocations at crust‐mantle interface. The approach is shown to approximate both linear and power‐law viscoelastic rheologies and offers a way to accurately represent viscoelastic kinematics even in the case where upper mantle rheology is poorly constrained.

The depositional environment of the Koeris Formation in the Aggeneys-Gamsberg ore district, South Africa

Abstract The Koeris Formation in the Mesoproterozoic Aggeneys-Gamsberg ore district (South Africa) is located in the hanging wall of an unconformity, only metres above the giant Gamsberg and Big Syncline Pb-Zn ore deposits and potentially contains critical information on the metallogenesis and its geodynamic setting. We, therefore, conducted geochemical analyses of amphibolite and metasedimentary rocks of this formation in the core of the Gamsberg fold, the results of which shed new light on the plate tectonic processes after deposition of the sedimentary exhalative ore. Major elements previously used for the characterisation of the amphibolite and the discrimination of the plate tectonic setting of its protolith had been mobilised during retrograde metamorphic overprint and are thus of little tectonic significance. More informative are certain trace elements: The amphibolite shows a strong depletion in Nb, Ta and P, typical of subduction-related magmatism, whereas exceptionally strong enrichment in mobile elements like Rb and Ba indicate the participation of continental material (crust/sediment) in the enrichment of the mantle wedge. This is in perfect agreement with existing U-Pb age data, which attest formation of the mafic metavolcanic rocks of the Koeris Formation at the very end of the Okiepian orogenesis (1 210 to 1 180 Ma), when subduction was terminated. Generally, very high Zn and Pb contents speak for metasomatic interaction of the Koeris Formation with the Zn-and Pb-rich Gamsberg deposit prior to peak metamorphism during the Klondikean orogeny (1 040 to 1 020 Ma). The fact that especially the immobile trace elements (e.g. Nb, Ta) in all metasedimentary rocks mirror the chemical characteristics of the metavolcanic rocks indicates local amphibolite detritus as a main source of the sediments. We conclude that the metamorphic volcano-sedimentary sequence of the Koeris Formation is not, as previously assumed, the product of a large back-arc basin but was deposited in response to tectonic uplift in a small-scale intra- or inter-montane basin at the very end of the Okiepian orogeny. The thick amphibolite layers concentrated within this topographic depression may have played a crucial role in the conservation of the underlying ore. Therefore, the occurrence of the Koeris Formation may indicate favorable locations for further exploration.

Late Archean–Paleoproterozoic plate tectonics along the northern margin of the North China craton

AbstractThe North China craton of central Asia is a natural laboratory for investigating early Earth tectonic processes including subduction and large-scale horizontal crustal motions. However, it remains unclear how and when the North China craton formed from the amalgamation of several blocks and orogens including the Archean Western and Eastern blocks, the late Archean Central Orogenic Belt, and the Paleoproterozoic Northern Margin orogen. In this study, we integrated new and existing geological field observations, zircon and baddeleyite U-Pb geochronology, whole-rock geochemistry, and Sr-Nd-C-O isotope analyses along the northern margin of the North China craton to improve our understanding of its late Archean–Paleoproterozoic tectonic evolution. Observations show the Wuchuan ultramafic-mafic complex of the Northern Margin orogen contains a mixture of mylonite gneiss, serpentinized ultramafic blocks, and amphibolite pyroxenite xenoliths. Geochronology and geochemistry results suggest that the ca. 2.55–2.5 Ga development of the Wuchuan ultramafic-mafic complex was associated with subduction, which was followed by ca. 2.39 emplacement of lamprophyre dikes during continental rifting. Subsequent Paleoproterozoic subduction magmatism, continental collision, and post-orogenic rifting between the North China craton and southern margin of the Siberian craton are evidenced by ca. 2.0 Ga arc granitoids, ca. 1.87–1.85 Ga A-type, garnet-bearing granitoids and mafic dikes, and ca. 1.86 carbonatite. Ca. 2.0 Ga collision and ca. 1.87–1.85 Ga rifting were coincident with orogen-normal left-slip shear. Our findings show that the ca. 1.9 Ga North China-Siberia collision was a key event during the assembly of the Columbia supercontinent that occurred via modern plate-tectonic processes.

The transition from continental to lithospheric breakup recorded in proto-oceanic crust: Insights from the NW South China Sea

AbstractThe formation of a new plate boundary and creation of the first oceanic crust, two of the most important processes of plate tectonics, still remains little understood. While older studies used to assumed a sharp ocean-continent boundary between continent and ocean, recent studies suggest a progressive oceancontinent transition (OCT) between unequivocal continental and oceanic crusts. In the latter view, breakup is not instantaneous but a lasting phase, which raises questions about the nature of the OCT basement and the processes operating between continental and lithospheric breakup. Based on detailed observations of high-quality and yet unpublished reflection seismic data, we describe and interpret the characteristic structures of the NW-South China Sea OCT and their relationship with overlying syn-breakup phase sediments. We show that the OCT displays a transition from fault-dominated rifting to magma-dominated seafloor spreading. On its continent-ward side, the OCT is made of hybrid crust where tectonic thinning of continental crust is compensated by syn-extensional magmatic thickening. Oceanward, the hybrid crust evolves into a fully magmatic but fault-dominated proto-oceanic crust, and finally turns into a mature Penrose-type oceanic crust. Relying on the growth structures observed in the syn-breakup sedimentary sequences and magmatic additions, we propose a kinematic restoration of the breakup phase. We suggest out-of-sequence flip-flop faulting to explain the switch from asymmetrical, fault-dominated-extension, to fully magmatic and largely symmetrical syn-extension accretion recorded in the syn-breakup sedimentary sequences overlying the OCT.

A refined study of Paleoproterozoic high-pressure granulite-facies metamorphism in the Kongling complex of northern Yangtze block

The Paleoproterozoic era is a critical tuning period for the transitions in global geodynamic regimes and plate tectonic processes. However, detailed pressure–temperature–time (P–T–t) paths of Paleoproterozoic metamorphic rocks worldwide are scarce, which hinders our understanding of deep thermal state and relevant tectonic processes at that time. Here, we identified a prolonged Paleoproterozoic (from ca. 2.05 Ga to ca. 2.0 Ga) metamorphism by zircon U–Pb geochronology of mafic granulite in the Kongling Complex of the northern Yangtze Block. The metamorphic zircon grains/domains show distinct weakly to strongly fractionated heavy rare earth elements (HREE), corresponding to competitive growth with or without garnet during the metamorphism. Mineral assemblages of orthopyroxene + plagioclase are distinctly found both within and around (as corona) garnet, indicating high-temperature decompression. P–T estimations from mineral compositions, chemical potential modeling and phase equilibrium modeling together yield a clockwise path with peak metamorphism at 10.0 ± 0.3 kbar and 860 ± 30 °C followed by decompression to 6.0 ± 0.5 kbar and 840 ± 40 °C in a period of about 50 Myr. This contradicts a Paleoproterozoic ultrahigh-pressure collision in northern Yangtze Block and may exemplify the assembly of micro-continents during formation of the Paleoproterozoic Columbia (Nuna) supercontinent. According to the available P-T paths of worldwide Paleoproterozoic metamorphism, we propose a typical warm subduction/collision in Paleoproterozoic plate tectonic regime.

Evolution of Earth's tectonic carbon conveyor belt.

Concealed deep beneath the oceans is a carbon conveyor belt, propelled by plate tectonics. Our understanding of its modern functioning is underpinned by direct observations, but its variability through time has been poorly quantified. Here we reconstruct oceanic plate carbon reservoirs and track the fate of subducted carbon using thermodynamic modelling. In the Mesozoic era, 250 to 66 million years ago, plate tectonic processes had a pivotal role in driving climate change. Triassic-Jurassic period cooling correlates with a reduction in solid Earth outgassing, whereas Cretaceous period greenhouse conditions can be linked to a doubling in outgassing, driven by high-speed plate tectonics. The associated 'carbon subduction superflux' into the subcontinental mantle may have sparked North American diamond formation. In the Cenozoic era, continental collisions slowed seafloor spreading, reducing tectonically driven outgassing, while deep-sea carbonate sediments emerged as the Earth's largest carbon sink. Subduction and devolatilization of this reservoir beneath volcanic arcs led to a Cenozoic increase in carbon outgassing, surpassing mid-ocean ridges as the dominant source of carbon emissions 20 million years ago. An increase in solid Earth carbon emissions during Cenozoic cooling requires an increase in continental silicate weathering flux to draw down atmospheric carbon dioxide, challenging previous views and providing boundary conditions for future carbon cycle models.

Unrest at Cayambe Volcano revealed by SAR imagery and seismic activity after the Pedernales subduction earthquake, Ecuador (2016)

For the first time in decades, a sudden increase in seismicity has been observed and monitored at Cayambe volcano in Ecuador, in 2016. This seismic unrest, which occurred a few months after the April 2016, Mw 7.8 Pedernales subduction earthquake, has raised many questions, especially as there is no record of recent eruptions at Cayambe volcano. Here we analyze a time series of 104 images from Sentinel 1 (SAR) data spanning the period 2014–2018 using the NSBAS processing chain in order to quantify surface deformation around this potentially explosive and ice-covered volcano. We evidence a large-scale uplift reaching a maximum mean displacement rate of about 0.44 cm/yr in Line of Sight. This uplift is mainly due to a significant and sudden acceleration of the deformation pattern, focused on the SE flank starting in November 2016. We model this signal as related to magma emplacement at around 6 km depth below the summit, with a sudden volume influx of about 2.6 million m3. The inflation and surface deformation pattern is concomitant in time with two Mw 3 seismic events recorded at Cayambe volcano in November 2016 and is consistent with the location at the summit of the volcano-tectonic (VT) seismic swarm from September 2016 onwards. This VT swarm follows an earlier swarm in June 2016 located on the northern side of the volcano and closer to a western branch of the Chingual-Cosanga-Pallatanga-Puna (CCPP) fault system, that plays a major role in accommodating plate tectonic processes in South America. We thus propose that static stress changes from the Pedernales megathrust earthquake triggered magma ascent below the Cayambe volcano, through reactivation of the CCPP fault system. Finally, volcanic hazards around Cayambe ice-capped volcano, previously dormant for the last 300 years, should be reassessed in light of this recent unrest.

Seismic evidence for lithospheric boudinage and its implications for continental rifting

Abstract The continental rifting that precedes the breakup of a continent and the formation of a new ocean basin is one of the key processes of plate tectonics. Although often viewed as a two-dimensional process, rifted margins exhibit significant variations along strike. We document along-strike variations developed during the ca. 200–160 Ma continental rifting that formed the margins of the Gulf of Mexico ocean basin. Rayleigh-wave ambient noise tomography reveals a zone of high and low seismic velocity resembling large scale geologic boudins in the mantle lithosphere of the northwestern Gulf of Mexico margin. These features become progressively less prominent eastward following the transition from a magma-poor to a magma-rich passive margin. We infer that mantle refertilization and thickness of the pre-rift lithosphere control deformation style and the along-strike variations in continental rifting. Our results also suggest that deformation during rifting produces long-lived features that persist long after breakup and, therefore, can be used to study rifted margins globally.

Geological history and supercontinent cycles of the Arctic

AbstractThe geological history of the Arctic is constrained within the framework of the assembly and breakup of three supercontinents. The first of these was preceded by the crystallization of the oldest dated rocks on Earth and consolidation of the Arctic region’s Archean cratons between 2.82 and 2.54 Ga. Following the emplacement of regional mafic dike swarms between 2.51 and 2.03 Ga, the cratons were amalgamated into the Nuna (Columbia) supercontinent between 2.0 and 1.6 Ga, and the distribution of low-thermalgradient eclogite (indicative of continental subduction) and ophiolite (indicative of obduction of oceanic crust onto a continental margin) suggests that diagnostic plate-tectonic processes were well in place by the early Paleoproterozoic. Basin formation, flood basalts, and dike swarms are features of the partial(?) breakup of Nuna (Columbia) by 1.5–1.27 Ga. The extent to which specific dike swarms led to continental breakup and a rift-to-drift transition remains unclear. Assembly of the second supercontinent (Rodinia, 1.4–0.9 Ga) is recorded by a network of Grenvillian and Sveconorwegian collisional orogenic belts. Prominent features of Rodinia breakup (780–615 Ma) in the Arctic are extensive dike swarms and regional-scale glacial-periglacial deposits associated with the Sturtian (717–661 Ma) and Marinoan (ca. 645 ± 6 to ca. 635 Ma) snowball Earth glaciations. Assembly of the third supercontinent, Pangea, between 600 Ma and ca. 250 Ma, was accomplished through stitching of four orogens in the Arctic (Timan-Varanger, Caledonian, Ellesmerian, and Urals-Taymyr). Pangea breakup (rifting since 250 Ma and oceanic spreading since the Cretaceous) led to the emplacement of Cretaceous and Paleogene flood basalts, new oceanic crust in the Labrador Sea, North Atlantic Ocean, and Arctic Ocean, and orogens characterized by relatively small but far-traveled accreted terranes with provenance in Laurentia, Baltica, and Siberia. Paleogeographic similarities and geological correlations among Laurentia, Baltica, Siberia, and the North China craton suggest that Rodinia formed following incomplete breakup of Nuna (Columbia) and/or by introversion, whereas unique paleogeographic traits for Pangea within the Arctic region point to supercontinent formation by extroversion.

Reply to Comment by A.P. Nutman et al. on “Tectonics of the Isua Supracrustal Belt 1: P‐T‐X‐d Constraints of a Poly‐Metamorphic Terrane” by A. Ramírez‐Salazar et al. and “Tectonics of the Isua Supracrustal Belt 2: Microstructures Reveal Distributed Strain in the Absence of Major Fault Structures” by J. Zuo et al.

AbstractStructural and metamorphic analyses from the works under discussion (Ramírez‐Salazar et al., 2021, https://doi.org/10.1029/2020tc006516; Zuo et al., 2021, https://doi.org/10.1029/2020tc006514) show that the Isua supracrustal rocks can be interpreted to record one single deformation and metamorphic event featuring quasi‐homogeneous deformation and amphibolite facies metamorphism, followed by late static retrogression or thermal event(s). Observed deformation and metamorphic records are consistent with three hypotheses: (a) they represent Neoarchean plate tectonic overprints following Eoarchean plate tectonic evolution (e.g., Nutman et al., 2022, https://doi.org/10.1029/2021TC007036); (b) they represent Eoarchean heat‐pipe and/or plate tectonic deformation that survived later tectonic event(s) (e.g., Ramírez‐Salazar et al., 2021, https://doi.org/10.1029/2020tc006516; Zuo et al., 2021, https://doi.org/10.1029/2020tc006514), and; (c) they represent one major Neoarchean tectonic event, such that the Isua supracrustal belt (ISB) records Eoarchean protolith‐related processes but does not record Eoarchean metamorphism nor deformation. While a heat‐pipe model for crustal formation is central to hypothesis 2, it is also a viable crustal formation mechanism for hypothesis 3 where the ISB would still form in a heat‐pipe setting in Eoarchean time, but the major deformation of the heat‐pipe lithosphere happened during Neoarchean time, probably by (proto‐)plate tectonic processes. If the data presented in Zuo et al. (2021), https://doi.org/10.1029/2020tc006514 and Ramírez‐Salazar et al. (2021), https://doi.org/10.1029/2020tc006516 only reflect Neoarchean histories, then these cannot be used to refute or support any Eoarchean geodynamic background for the formation of the ISB.

Destabilization of Long‐Lived Hadean Protocrust and the Onset of Pervasive Hydrous Melting at 3.8 Ga

AbstractThe nature of Earth's earliest crust and crustal processes remain unresolved questions in Precambrian geology. While some hypotheses suggest that plate tectonics began in the Hadean, others suggest that the Hadean was characterized by long‐lived protocrust and an absence of significant plate tectonic processes. Recently proposed trace‐element proxies for the tectono‐magmatic settings in which zircons formed are a relatively novel tool to understand crustal processes in the past. Here, we present high‐spatial resolution zircon trace and rare earth element geochemical data along with Hf and O isotope data of a new location with Hadean materials, 4.1–3.3 Ga detrital zircons from the 3.31 Ga Green Sandstone Bed, Barberton Greenstone Belt. Together, the hafnium isotope and trace element geochemistry of the detrital zircons record a major transition in crustal processes. Zircons older than 3.8 Ga show evidence for isolated, long‐lived protocrust derived by reworking of relatively undepleted mantle sources with limited remelting of surface‐altered material. After 3.8 Ga, Hf isotopic evidence for this protocrust is muted while relatively juvenile source components for the zircon's parental magmas and flux‐like melting signatures become more prominent. This shift mirrors changes in Hf isotopes and trace element geochemistry in other Archean terranes between ∼3.8 and 3.6 Ga and supports the notion that the global onset of pervasive crustal instability and recycling—A possible sign for mobile‐lid tectonics—Occurred in that time period.

Dating blueschist-facies metamorphism within the Naga ophiolite, Northeast India, using sheared carbonate veins

ABSTRACT The tectonic significance of blueschist-facies rocks associated with the Indo-Myanmar ophiolite belt is uncertain, given the lack of detailed petrological study and the paucity of reliable age data for different stages in their geological evolution. Here, we present new integrated petrological and geochronological data for samples from the Nagaland complex of the Indo-Myanmar ophiolite belt, northeastern India, which constrains the pressure–temperature conditions and absolute ages of peak and retrograde metamorphism. Several samples of blueschist were collected from the region, which have been variably deformed and subjected to shear recrystallization. Based on microstructural constraints and mineral geochemistry, garnet, omphacite, barroisite, chlorite and muscovite are interpreted to represent a high-pressure prograde-to-peak metamorphic assemblage, and omphacite, actinolite, hornblende and albite represent a lower-pressure retrograde metamorphic assemblage that formed during shear-related exhumation. Petrological modelling and thermobarometry indicates that unsheared samples equilibrated at ~1.9 GPa and ~480–520°C at peak metamorphism, indicating subduction to ~60 km depth, whereas sheared and recrystallised samples re-equilibrated at ~0.6 GPa and ~470°C during retrograde metamorphism associated with obduction of the Naga ophiolite onto the Indian foreland. U–Pb in-situ analysis of carbonate grains (aragonite–calcite) and associated silicate phases (epidote, prehnite, amphibole, etc.) at different microstructural positions, including within dynamically recrystallised shear bands that cross-cut older metamorphic fabrics and cogenetic silicate phases, constrains the age of peak metamorphism to be c. 95 Ma and retrograde metamorphism to be c. 90 Ma. Based on the overall progression of ages in the sheared and unsheared samples, we interpret that the area experienced exhumation at a time-averaged rate of ~1 cm/year in the order of Phanerozoic period plate tectonic rate, which is in the order of rates of plate tectonic processes on the Phanerozoic Earth

Modeling principal plate tectonic processes by sandbox experiments: tools for teaching at universities

Analogue sand models have been used to reproduce the principle plate tectonic processes, providing the insight of plate tectonics in nature. A simple and versatile sandbox was built and used for the extension and compression experiments modelling principle plate tectonic such as plate divergence or convergence by stretching or compressing sand in a sandbox model. The glass sidewalls allow observing and analyzing the sequential development of faults and structures like normal fault, thrust fault, graben structure, fold and thrust belts, so on. The results can be correlated with geological structures in nature. In an educational context, students can perform the experiments and also vary experimental parameters such as sand layer thickness, geometry of the sandbody or use of different experimental materials. The obtained results are then compared with natural geological examples to improve the students’ receptivity. This indicates that sandbox experiments provide an effective means for teaching geology courses and also soil mechanics courses at universities.

Archean versus Phanerozoic oceanic crust formation and tectonics: Ophiolites through time

• Global compilation of the salient features of ophiolites. • Lithological differences between Archean and Proterozoic/Phanerozoic ophiolites. • Subduction-dominant Archean oceanic crust with accretionary cycle plate tectonics. • For Proterozoic and Phanerozoic a combination of both accretionary cycle and Wilson cycles. A global compilation of structural, lithological, and geochemical data on a selection of Archean, Proterozoic and Phanerozoic magmatic complexes, interpreted as ophiolites, is presented. Ophiolites, based on Phanerozoic examples, can be classified into subduction-related and subduction unrelated categories. These categories can be further subdivided into several subtypes depending on the proximity to subduction zones, and on sequential development from rifting – drifting to seafloor spreading for the subduction-unrelated category. From bottom to top ophiolites exhibit a magmatic sequence of ultramafic rocks (upper mantle units), gabbros (layered and isotropic), basaltic dikes and lavas, as well as boninites and felsic dikes and lavas in the subduction-related types. Archean greenstone belts show large variations in their construction, but Eoarchean examples display identical structure and lithology to the Phanerozoic ophiolites, attesting to the operation of seafloor spreading and subduction zone processes in the early Earth's history. Lithological differences between the Archean and Proterozoic/Phanerozoic ophiolites are demonstrated in the common occurrence of komatiites and felsic rocks, and scarcity of sheeted dike complexes in the former, compared to the opposite situation in those ophiolites that are younger than ca. 2 Ga. Geochemically there is a concomitant decrease in the content of incompatible elements (e.g., Sr, Zr, Y, Nb) and an increase in the content of compatible elements (e.g., Mg, Cr, Ni). In terms of tectonic environment analyzes, the Archean ophiolites are more abundantly subduction-related than those of Proterozoic and Phanerozoic ages. The subduction-dominant Archean oceanic crust (ophiolites) was characterized by accretionary cycle plate tectonics, whereas for those of Proterozoic and Phanerozoic ages, a combination of both accretionary cycle and Wilson cycles plate tectonic processes was operative.

Has the tectonic regime of the Baltic Shield always remained the same?

ABSTRACT Recent theories on the growth of continental crust stress its formation in pre-Archean and Archean times with minor additions at later times. The Earth has gone through a unidirectional evolution including surface changes, the genesis of life and an important loss of energy. The energy loss drives tectonic processes, but at a rate declining with available energy and thus with time. Semiquantitative modelling of energy-changes and transport indicates important differences between today’s Earth and the Paleoproterozoic Earth. Actualistic but non-uniformitarian arguments suggest that principal tectonic processes have changed over time. The Baltic Shield displays a fundamental difference in conditions between the c. 0.9 Ga high-pressure metamorphism including the occurrence of eclogite in the western Sveconorwegian Orogen and c. 2 Ga low-pressure metamorphism in the eastern Svecokarelian Orogen. This suggests that modern platetectonic processes were operative at c. 0.9 Ga but leaves the question whether it was active during the Svecokarelian orogeny open. The lack of eclogite in the Svecokarelian Orogen is explained with a change with time of the geotherm slope. The occurrence of two generations of granitic rocks of different compositions in the Svecokarelian is easily explained by an increased mantle heat flow in a stratified continental crust. In contrast to calc-alkaline platetectonic compositions, alkali-calcic rocks occur in a quarter-circle around the Svecokarelian core. The question arises whether modern plate-processes successively replaced proto-plate processes during Paleo- and Mesoproterozoic times.

Archean crustal growth and reworking revealed by combined U-Pb-Hf-O isotope and trace element data of detrital zircons from ancient and modern river sediments of the eastern Kaapvaal Craton

It is intriguing to speculate on how continental crust grew and differentiated during the Archean and which tectonic regimes and magmatic mechanisms were responsible for such a process. The continental nucleus of the Kaapvaal Craton, which includes the Barberton granitoid-greenstone terrane (BGGT) in South Africa and the Ancient Gneiss Complex (AGC) in Swaziland, exhibits the best preserved geological record from 3.6 to 3.0 Ga and thus provides the optimal location to address this issue. In this study, combined U-Pb, O and Lu-Hf isotope and trace element analyses were carried out on detrital zircon grains from two Moodies Group sandstones and three modern river sand samples in the Barberton and Swaziland regions, southern Africa. The results, in conjunction with previously published data, show that the detrital zircons in the Moodies sandstones on both sides of the Inyoka Fault, had two similar U-Pb age clusters of 3.25–3.30 Ga and 3.40–3.47 Ga and the oldest age up to 3.57 Ga. This suggests that they were derived from older felsic volcanic rocks of the greenstone belt and the granitoid gneisses in the BGGT and AGC. Using the approach created for Earth’s oldest zircons (i.e., Jack Hills detrital zircons), we inferred that the source magmas of the studied detrital zircons had average arc-like andesitic compositions (SiO2 = 58.0 ± 5.6 wt.%; Th/Nb = 5.0 ± 3.2), similar to those of the Jack Hills detrital zircons. However, these inferred compositions conflict with the source rocks of the detrital zircons, namely, the felsic volcanic rocks and the granitoid gneisses in the BGGT and AGC, which have high SiO2 contents (average = 70 ± 4.3 wt.%) and moderate Th/Nb ratios (average = 0.76 ± 0.44). These findings suggest that the inferred average andesitic compositions for the Earth’s oldest zircons should be reevaluated. The U-Pb age spectra of detrital zircons in modern river sands suggest that the exposed rocks in southwestern Swaziland are dominantly ca. 3.45 Ga tonalitic, trondhjemitic and granodioritic (TTG) gneisses without Eoarchean crust. The concordant zircon U-Pb age data reveal four major magmatic episodes for the eastern Kaapvaal Craton at 3.52, 3.46, 3.26 and 3.10 Ga. The first magmatic episode is suggested to dictate crustal growth, whereas the other three magmatic episodes are predominated by crustal reworking. This process is manifested by major positive changes in εHf(t) and εNd(t) values starting at ∼3.5 Ga, gradual decreases in εHf(t) and εNd(t) values afterwards, and increases in δ18O values at ∼3.25 Ga. Combined with other geological observations, we suggest that modern-style plate tectonic processes, such as subduction, may not have been a requirement for crustal growth and reworking in the eastern Kaapvaal Craton during the Paleoarchean (3.6–3.2 Ga). Instead, they could be ascribed to episodic partial convective overturns at different crustal depths.

CLUSTERS OF MINERAL DEPOSITS OF THE SOUTHERN EAST SIBERIA AND PROSPECTS FOR THEIR DEVELOPMENT: AN OVERVIEW OF THE PROBLEM

This study aims to identify the groups of closely spaced mineral deposits located in the Southern East Siberia and to describe these clusters. The mineral resource base of this region includes the deposits of lead, zinc, tungsten, tin, molybdenum, uranium, copper, gold, lithium, tantalum, niobium, silver, fluorite, zirconium, rare-earth metals, iron, mica, precious and semi-precious stones, oil, gas, and coal. Many of these deposits contain a significant part of the explored mineral reserves of Russia. The deposits are located in the tectonic structures of the Siberian platform (Tunguska syncline, Nepa arch, Cheremkhovo and Priangarie depressions), as well as in the fold belts framing the platform from the south and southeast (Baikal-Patom, Dzhida-Vitim, etc.). These structures and belts formed under the influence of plate tectonic and plume tectonic processes. This article describes the clusters of mineral deposits of the Angara, Sayan, Baikal and Transbaikalia regions. Currently, the most developed are the clusters located in the southern part of the study area (Shilka, Argun, Yeravnino, etc.). In the northern part (Mama-Bodaibo, etc.), the clusters have been either partially developed within the existing mining areas or are at the initial stage of development. In these territories, the road infrastructure, power supply and other facilities required for mining industry are either underdeveloped or lacking. Prospects for the regional development are related to the economic development of the territories adjacent to the Baikal-Amur railroad (BAM) and hydrocarbons production in the zone near the East Siberia – Far East oil pipeline. On the Siberian platform, mineral deposits formed during the stages of formation of its basement (Aldan, Anabar, and Sharyzhalgai shields) and platform cover (Tunguska syncline, and Nepa-Botuoba anticline). Within the fold frame, ore formation was associated with subduction, spreading, collision, and plume tectonic intraplate settings. To start the development of small-size deposits and mining of technogenic raw materials, it is advisable to establish exploration and production enterprises that can operate and manage expeditions and use compact ore-dressing plants.

Using Conceptual Metaphor Theory within the Model of Educational Reconstruction to identify students’ alternative conceptions and improve instruction: A plate tectonics example

Many geoscience phenomena cannot be perceived at human temporal or spatial scales. As a consequence, we can only understand many processes that drive geological phenomena through analogical reasoning. Building deep conceptual understanding requires instruction that activates the appropriate source analogs and allows students to build useful conceptual metaphors. In this paper we illustrate, using plate tectonics as an example, how educational guidelines for teaching geosciences can be developed by linking the Model of Educational Reconstruction with Conceptual Metaphor Theory. Qualitative content analysis in combination with systematic metaphor analysis of scientific textbooks and semi-structured interviews conducted with students (n = 36) provided insight into the image schemas used by scientists and learners. Difficulties in understanding plate tectonics structures and processes, e.g. subduction, arise because many students understand tectonic plates as single bodies, rather than composites. Similarly, students’ difficulties explaining plate movement result from activation of a push rather than a pull schema. Through exploring schemas for plate tectonics, we illustrate the value of combining Conceptual Metaphor Theory and the Model of Educational Reconstruction for (1) understanding alternative conceptions as grounded in image schemas and (2) developing educational guidelines for improving instruction.