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The Sheahan Diamond Literature Reference Compilation - Scientific and Media Articles based on Major Keyword - Geodynamics
The Sheahan Diamond Literature Reference Compilation is compiled by Patricia Sheahan who publishes on a monthly basis a list of new scientific articles related to diamonds as well as media coverage and corporate announcements called the Sheahan Diamond Literature Service that is distributed as a free pdf to a list of followers. Pat has kindly agreed to allow her work to be made available as an online digital resource at Kaiser Research Online so that a broader community interested in diamonds and related geology can benefit. The references are for personal use information purposes only; when available a link is provided to an online location where the full article can be accessed or purchased directly. Reproduction of this compilation in part or in whole without permission from the Sheahan Diamond Literature Service is strictly prohibited. Return to Diamond Keyword Index
Sheahan Diamond Literature Reference Compilation - Scientific Articles by Author for all years
Each article reference in the SDLRC is tagged with one or more key words assigned by Pat Sheahan to highlight the main topics of the article. In an effort to make it easier for users to track down articles related to a specific topic, KRO has extracted these key words and developed a list of major key words presented in this Key Word Index to which individual key words used in the article reference have been assigned. In most of the individual Key Word Reports the references are in crhonological order, though in some such as Deposits the order is first by key word and then chronological. Only articles classified as "technical" (mainly scientific journal articles) and "media" (independent media articles) are included in the Key Word Index. References that were added in the most recent monthly update are highlighted in yellow.
Geodynamics is about the processes that move material within the earth, such as convection cells and plate tectonics, as well as mountain chain building, described in geophysical terms, as opposed to geochemistry which describes these mechanisms in terms of chemsitry. The closest these scientific articles come to being relevant to diamonds is when they discuss cratons, the thick stable chunks of continental crust where diamonds form.
Products of 2.65 - 2.58 Ga orogenesis in the Slave Province correlated with Slave - Northern Cordillera Lithospheric Evolution ( SNORCLE) seismic reflection
Canadian Journal of Earth Science, Vol. 39,8,August pp. 1189-1200.
Northwest Territories
Geophysics - seismic, Slave Province - tectonics, geodynamics
The Precambrian Earth, tempos and events, editors Eriksson, P.G., Altermann, W., Nelson, D.R., Mueller, W.U., Elsevier, Developments in Precambrian Geology No. 12, C
Electrical signatures due to thermal anomalies along mobile belts reactivated by the trail and outburst of mantle plume: evidences from the Indian subcontinent.
Methane rich fluid inclusions from ophiolitic dunite and post collisional mafic/ultramafic intrusion: the mantle dynamics underneath the Paleo-Asian Ocean floor
Earth and Planetary Science Letters, Vol. 242, 3-4, pp. 286-301.
GPS constraints on continental deformation in the Africa Arabia Eurasia continental collision zone and implications for the dynamics of plate interactions.
Journal of Geophysical Research, Vol. 111,B5 B05411.
Electrical signatures due to thermal anomalies along mobile belts reactivated by the trail and outburst of mantle plume: evidence from the Indian subcontinent.
Journal of Applied Geophysics, Vol. 58, 4, April, pp. 313-320.
Phanerozoic burial and unroofing of the western Slave Craton and Wopmay Orogen from apatite ( U Th/He thermochronometry, assessing links between surface/deep
American Geological Union, Fall meeting Dec. 15-19, Eos Trans. Vol. 89, no. 53, meeting supplement, 1p. abstract
The equivalent elastic thickness (Te) seismicity and the long term rheology of continental lithosphere: time to burn out 'creme brulee?' insights from large scale geodynamic modeling.
An improved and extended internally consistent thermodynamic data set for phases of petrological interest, involving a new equation of state for solids.
Journal of Metamorphic Geology, in print available
Strain heating as a mechanism for partial melting and ultrahigh temperature metamorphism in convergent orogens: implications of temperature dependent thermal
Journal of Geophysical Research, Vol. 115, B 12 B12417
Martirosyan, N., Yoshino, T., Shatskiy, A., Chanyshev, A., Litasov, K.
Kenetic study of Ca- carbonate - iron interaction. ( global geodynamic processes - diamond formation)
V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch Russian Academy of Sciences International Symposium Advances in high pressure research: breaking scales and horizons ( Courtesy of N. Poikilenko), Held Sept 22-26, 2p. Abstract
Abstract: Knowing when the geodynamo started is important for understanding the evolution of the core, the atmosphere, and life on Earth. We report full-vector paleointensity measurements of Archean to Hadean zircons bearing magnetic inclusions from the Jack Hills conglomerate (Western Australia) to reconstruct the early geodynamo history. Data from zircons between 3.3 billion and 4.2 billion years old record magnetic fields varying between 1.0 and 0.12 times recent equatorial field strengths. A Hadean geomagnetic field requires a core-mantle heat flow exceeding the adiabatic value and is suggestive of plate tectonics and/or advective magmatic heat transport. The existence of a terrestrial magnetic field before the Late Heavy Bombardment is supported by terrestrial nitrogen isotopic evidence and implies that early atmospheric evolution on both Earth and Mars was regulated by dynamo behavior.
Abstract: The bulk composition of the silicate part of Earth has long been linked to chondritic meteorites. Ordinary chondrites — the most abundant meteorite class — are thought to represent planetary building materials. However, a landmark discovery showed that the 142Nd/144Nd ratio of the accessible parts of the modern terrestrial mantle on Earth is greater than that of ordinary chondrites. If Earth was derived from these precursors, mass balance requires that a missing reservoir with 142Nd/144Nd lower than ordinary chondrites was isolated from the accessible mantle within 20 to 30 million years of accretion. This reservoir would host the equivalent of the modern continents' budget of radioactive heat-producing elements (uranium, thorium and potassium), yet has not been discovered. We argue that this reservoir could have been lost to space by ablation from early impactors. If so, Earth's radiogenic heat generation is between 18 and 45% lower than estimates based on a chondritic composition. Calculations of Earth's thermal history that incorporate such reduced radiogenic heating are consistent with a transition to the current plate tectonic mode in the past 2.5 billion years or so, a late onset of the dynamo and an evolving rate of volcanic outgassing consistent with Earth's long-term habitable climate. Reduced heat production compared with Venus and Mars could also explain aspects of the differences between the current climatic regimes of these planets and Earth.
Abstract: The driving force for transient vertical motions of Earth's surface remains an outstanding question. A main difficulty lies in the uncertain role of the underlying mantle, especially during the geological past. Here I review previous studies on both observational constraints and physical mechanisms of North American topographic evolution since the Mesozoic. I first summarize the North American vertical motion history using proxies from structural geology, geochronology, sedimentary stratigraphy, and geomorphology, based on which I then discuss the published physical models. Overall, there is a progressive consensus on the contribution of mantle dynamic topography due to buoyancy structures associated with the past subduction. At the continental scale, a largely west-to-east migrating deformation pattern suggests an eastward translation of mantle dynamic effects, consistent with models involving an eastward subduction and sinking of former Farallon slabs since the Cretaceous. Among the existing models, the inverse model based on an adjoint algorithm and time-dependent data constraints provides the most extensive explanations for the temporal changes of North American topography since the Mesozoic. At regional scales, debates still exist on the predicted surface subsidence and uplift within both the western and eastern United States, where discrepancies are likely due to differences in model setup (e.g., mantle dynamic properties and boundary conditions) and the amount of time-dependent observational constraints. Toward the development of the next-generation predictive geodynamic models, new research directions may include (1) development of enhanced data assimilation capabilities, (2) exploration of multiscale and multiphysics processes, and (3) cross-disciplinary code coupling.
Journal of Geophysical Research,, Vol. 120, 12, pp. 8633-8655.
Mantle
Geodynamics
Abstract: With the advances in technology, seismological theory, and data acquisition, a number of high-resolution seismic tomography models have been published. However, discrepancies between tomography models often arise from different theoretical treatments of seismic wave propagation, different inversion strategies, and different data sets. Using a fixed velocity-to-density scaling and a fixed radial viscosity profile, we compute global mantle flow models associated with the different tomography models and test the impact of these for explaining surface geophysical observations (geoid, dynamic topography, stress, and strain rates). We use the joint modeling of lithosphere and mantle dynamics approach of Ghosh and Holt (2012) to compute the full lithosphere stresses, except that we use HC for the mantle circulation model, which accounts for the primary flow-coupling features associated with density-driven mantle flow. Our results show that the seismic tomography models of S40RTS and SAW642AN provide a better match with surface observables on a global scale than other models tested. Both of these tomography models have important similarities, including upwellings located in Pacific, Eastern Africa, Iceland, and mid-ocean ridges in the Atlantic and Indian Ocean and downwelling flows mainly located beneath the Andes, the Middle East, and central and Southeast Asia.
Abstract: Convective circulation of the Earth’s mantle maintains some fraction of surface topography that varies with space and time. Most predictive models show that this dynamic topography has peak amplitudes of about ±2?km, dominated by wavelengths of 104?km. Here, we test these models against our comprehensive observational database of 2,120 spot measurements of dynamic topography that were determined by analysing oceanic seismic surveys. These accurate measurements have typical peak amplitudes of ±1?km and wavelengths of approximately 103?km, and are combined with limited continental constraints to generate a global spherical harmonic model, the robustness of which has been carefully tested and benchmarked. Our power spectral analysis reveals significant discrepancies between observed and predicted dynamic topography. At longer wavelengths (such as 104?km), observed dynamic topography has peak amplitudes of about ±500?m. At shorter wavelengths (such as 103?km), significant dynamic topography is still observed. We show that these discrepancies can be explained if short-wavelength dynamic topography is generated by temperature-driven density anomalies within a sub-plate asthenospheric channel. Stratigraphic observations from adjacent continental margins show that these dynamic topographic signals evolve quickly with time. More rapid temporal and spatial changes in vertical displacement of the Earth’s surface have direct consequences for fields as diverse as mantle flow, oceanic circulation and long-term climate change.
Abstract: The nature of crustal and lithospheric mantle evolution of the Archean shields as well as their subsequent deformation due to recent plate motions and sustained intraplate geodynamic activity, has been a subject of considerable interest. In view of this, about three decades ago, a new idea was put forward suggesting that out of all shield terrains, the Indian shield has an extremely thin lithosphere (?100 km, compared to 250-350 km, elsewhere), apart from being warm, non-rigid, sheared and deformed. As expected, it met with scepticism by heat flow and the emerging seismic tomographic study groups, who on the contrary suggested that the Indian shield has a cool crust, besides a coherent and thick lithosphere (as much as 300-400 km) like any other shield. However, recently obtained integrated geological and geophysical findings from deep scientific drillings in 1993 Killari (Mw: 6.3) and 1967 Koyna (Mw: 6.3) earthquake zones, as well as newly acquired geophysical data over other parts of Indian shield terrain, have provided a totally new insight to this debate. Beneath Killari, the basement was found consisting of high density, high velocity mid crustal amphibolite to granulite facies rocks due to exhumation of the deeper crustal layers and sustained granitic upper crustal erosion. Similar type of basement appears to be present in Koyna region too, which is characterized by considerably high upper crustal temperatures. Since, such type of crust is depleted in radiogenic elements, it resulted into lowering of heat flow at the surface, increase in heat flow contribution from the mantle, and upwarping of the lithosphere-asthenosphere boundary. Consequently, the Indian shield lithosphere has become unusually thin and warm. This study highlights the need of an integrated geological, geochemical and geophysical approach in order to accurately determine deep crust-mantle thermal regime in continental areas.
Progress in Earth and Planetary Science, Open access
Mantle
Convection, geodynamics
Abstract: Subduction initiation is a key in understanding the dynamic evolution of the Earth and its fundamental difference to all other rocky planetary bodies in our solar system. Despite recent progress, the question about how a stiff, mostly stagnant planetary lid can break and become part in the global overturn of the mantle is still unresolved. Many mechanisms, externally or internally driven, are proposed in previous studies. Here, we present the results on subduction initiation obtained by dynamically self-consistent, time-dependent numerical modelling of mantle convection. We show that the stress distribution and resulting deformation of the lithosphere are strongly controlled by the top boundary formulation: A free surface enables surface topography and plate bending, increases gravitational sliding of the plates and leads to more realistic, lithosphere-scale shear zones. As a consequence, subduction initiation induced by regional mantle flow is demonstrably favoured by a free surface compared to the commonly applied, vertically fixed (i.e. free-slip) surface. In addition, we present global, three-dimensional mantle convection experiments that employ basal heating that leads to narrow mantle plumes. Narrow mantle plumes impinging on the base of the plate cause locally weak plate segments and a large topography at the lithosphere-asthenosphere boundary. Both are shown to be key to induce subduction initiation. Finally, our model self-consistently reproduces an episodic lid with a fast global overturn due to the hotter mantle developed below a former stagnant lid. We conclude that once in a stagnant-lid mode, a planet (like Venus) might preferentially evolve by temporally discrete, global overturn events rather than by a continuous recycling of lid and that this is something worth testing more rigorously in future studies.
Abstract: The Kumta and Mercara suture zones welding together Archean crustal blocks in western peninsular India offer critical insights into Precambrian continental juxtapositions and the crustal evolution of eastern Gondwana. Here we present the results from an integrated study of the structure, geology, petrology, mineral chemistry, metamorphic P-T conditions, zircon U-Pb ages and Lu-Hf isotopes of metasedimentary rocks from the two sutures. The dominant rocks in the Kumta suture are greenschist- to amphibolite-facies quartz-phengite schist, garnet-biotite schist, chlorite schist, fuchsite schist and marble. The textural relations, mineral chemistry and thermodynamic modelling of garnet-biotite schist from the Kumta suture indicate peak metamorphic P-T conditions of ca. 11 kbar at 790 °C, with detrital SHRIMP U-Pb zircon ages ranging from 3420 to 2547 Ma, ?Hf (t) values from ? 9.2 to 5.6, and TDMc model ages from 3747 to 2792 Ma. The K-Ar age of phengite from quartz-phengite schist is ca. 1326 Ma and that of biotite from garnet-biotite schist is ca. 1385 Ma, which are interpreted to broadly constrain the timing of metamorphism related to the suturing event. The Mercara suture contains amphibolite- to granulite-facies mylonitic quartzo-feldspathic gneiss, garnet-kyanite-sillimanite gneiss, garnet-biotite-kyanite-gedrite-cordierite gneiss, garnet-biotite-hornblende gneiss, calc-silicate granulite and metagabbro. The textural relations, mineral chemistry and thermodynamic modelling of garnet-biotite-kyanite-gedrite-cordierite gneiss from the Mercara suture indicate peak metamorphic P-T conditions of ca. 13 kbar at 825 °C, followed by isothermal decompression and cooling. For pelitic gneisses from the Mercara suture, LA-ICP-MS U-Pb zircon ages vary from 3249 to 3045 Ma, ?Hf (t) values range from ? 18.9 to 4.2, and TDMc model ages vary from 4094 to 3314 Ma. The lower intercept age of detrital zircons in the pelitic gneisses from the Mercara suture ranges from 1464 to 1106 Ma, indicating the approximate timing of a major lead-loss event, possibly corresponding to metamorphism, and is broadly coeval with events in the Kumta suture. Synthesis of the above results indicates that the Kumta and Mercara suture zones incorporated sediments from Palaeoarchean to Mesoproterozoic sources and underwent high-pressure metamorphism in the late Mesoproterozoic. The protolith sediments were derived from regions containing juvenile Palaeoarchean crust, together with detritus from the recycling of older continental crust. Integration of the above results with published data suggests that the Mesoproterozoic (1460-1100 Ma) Kumta and Mercara suture zones separate the Archean (3400-2500 Ma) Karwar-Coorg block and Dharwar Craton in western peninsular India. Based on regional structural and other geological data we interpret the Kumta and Mercara suture zones as extensions of the Betsimisaraka suture of eastern Madagascar into western India.
Abstract: Apatite fission track (AFT) and (U-Th)/He (AHe) thermochronometry data are reported and used to unravel the exhumation history of crystalline basement rocks from the elevated (>1000?m above sea level) but low-relief Tanzanian Craton. Coeval episodes of sedimentation documented within adjacent Paleozoic to Mesozoic basins of southern Tanzania and the Congo basin of the Democratic Republic of Congo indicate that most of the cooling in the basement rocks in Tanzania was linked to erosion. Basement samples were from an exploration borehole located within the craton and up to 2200?m below surface. Surface samples were also analyzed. AFT dates range between 317?±?33?Ma and 188?±?44?Ma. Alpha (Ft)-corrected AHe dates are between 433?±?24?Ma and 154?±?20?Ma. Modeling of the data reveals two important periods of cooling within the craton: one during the Carboniferous-Triassic (340-220?Ma) and a later, less well constrained episode, during the late Cretaceous. The later exhumation is well detected proximal to the East African Rift (70?Ma). Thermal histories combined with the estimated geothermal gradient of 9°C/km constrained by the AFT and AHe data from the craton and a mean surface temperature of 20°C indicate removal of up to 9?±?2?km of overburden since the end of Paleozoic. The correlation of erosion of the craton and sedimentation and subsidence within the Congo basin in the Paleozoic may indicate regional flexural geodynamics of the lithosphere due to lithosphere buckling induced by far-field compressional tectonic processes and thereafter through deep mantle upwelling and epeirogeny tectonic processes.
Journal of Geophysical Research, Vol. 121, 9, pp. 6966-6987.
Mantle
Geodynamics
Abstract: Crustal inheritance is often considered important in the tectonic evolution of the Wilson Cycle. However, the role of the mantle lithosphere is usually overlooked due to its difficulty to image and uncertainty in rheological makeup. Recently, increased resolution in lithosphere imaging has shown potential scarring in continental mantle lithosphere to be ubiquitous. In our study, we analyze intraplate deformation driven by mantle lithosphere heterogeneities from ancient Wilson Cycle processes and compare this to crustal inheritance deformation. We present 2-D numerical experiments of continental convergence to generate intraplate deformation, exploring the limits of continental rheology to understand the dominant lithosphere layer across a broad range of geological settings. By implementing a "jelly sandwich" rheology, common in stable continental lithosphere, we find that during compression the strength of the mantle lithosphere is integral in generating deformation from a structural anomaly. We posit that if the continental mantle is the strongest layer within the lithosphere, then such inheritance may have important implications for the Wilson Cycle. Furthermore, our models show that deformation driven by mantle lithosphere scarring can produce tectonic patterns related to intraplate orogenesis originating from crustal sources, highlighting the need for a more formal discussion of the role of the mantle lithosphere in plate tectonics.
Abstract: The geodynamic environment in which Earth’s first continents formed and were stabilized remains controversial1. Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 2.5 billion years ago) comprises tonalite-trondhjemite-granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium basaltic rocks2; notably, these TTGs have ‘arc-like’ signatures of trace elements and thus resemble the continental crust produced in modern subduction settings3. In the East Pilbara Terrane, Western Australia, low-magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup have trace-element compositions that are consistent with these being source rocks for TTGs. These basalts may be the remnants of a thick (more than 35?kilometres thick), ancient (more than 3.5 billion years old) basaltic crust4, 5 that is predicted to have existed if Archaean mantle temperatures were much hotter than today’s6, 7, 8. Here, using phase equilibria modelling of the Coucal basalts, we confirm their suitability as TTG ‘parents’, and suggest that TTGs were produced by around 20 per cent to 30 per cent melting of the Coucal basalts along high geothermal gradients (of more than 700 degrees Celsius per gigapascal). We also analyse the trace-element composition of the Coucal basalts, and propose that these rocks were themselves derived from an earlier generation of high-magnesium basaltic rocks, suggesting that the arc-like signature in Archaean TTGs was inherited from an ancestral source lineage. This protracted, multistage process for the production and stabilization of the first continents—coupled with the high geothermal gradients—is incompatible with modern-style plate tectonics, and favours instead the formation of TTGs near the base of thick, plateau-like basaltic crust9. Thus subduction was not required to produce TTGs in the early Archaean eon.
Abstract: Neoproterozoic tectonic geography was dominated by the formation of the supercontinent Rodinia, its break-up and the subsequent amalgamation of Gondwana. The Neoproterozoic was a tumultuous time of Earth history, with large climatic variations, the emergence of complex life and a series of continent-building orogenies of a scale not repeated until the Cenozoic. Here we synthesise available geological and palaeomagnetic data and build the first full-plate, topological model of the Neoproterozoic that maps the evolution of the tectonic plate configurations during this time. Topological models trace evolving plate boundaries and facilitate the evaluation of “plate tectonic rules” such as subduction zone migration through time when building plate models. There is a rich history of subduction zone proxies preserved in the Neoproterozoic geological record, providing good evidence for the existence of continent-margin and intra-oceanic subduction zones through time. These are preserved either as volcanic arc protoliths accreted in continent-continent, or continent-arc collisions, or as the detritus of these volcanic arcs preserved in successor basins. Despite this, we find that the model presented here still predicts less subduction (ca. 90%) than on the modern earth, suggesting that we have produced a conservative model and are likely underestimating the amount of subduction, either due to a simplification of tectonically complex areas, or because of the absence of preservation in the geological record (e.g. ocean-ocean convergence). Furthermore, the reconstruction of plate boundary geometries provides constraints for global-scale earth system parameters, such as the role of volcanism or ridge production on the planet's icehouse climatic excursion during the Cryogenian. Besides modelling plate boundaries, our model presents some notable departures from previous Rodinia models. We omit India and South China from Rodinia completely, due to long-lived subduction preserved on margins of India and conflicting palaeomagnetic data for the Cryogenian, such that these two cratons act as ‘lonely wanderers’ for much of the Neoproterozoic. We also introduce a Tonian-Cryogenian aged rotation of the Congo-São Francisco Craton relative to Rodinia to better fit palaeomagnetic data and account for thick passive margin sediments along its southern margin during the Tonian. The GPlates files of the model are released to the public and it is our expectation that this model can act as a foundation for future model refinements, the testing of alternative models, as well as providing constraints for both geodynamic and palaeoclimate models.
Abstract: The global geodynamic regime of early Earth, which operated before the onset of plate tectonics, remains contentious. As geological and geochemical data suggest hotter Archean mantle temperature1, 2 and more intense juvenile magmatism than in the present-day Earth3, 4, two crust-mantle interaction modes differing in melt eruption efficiency have been proposed: the Io-like heat-pipe tectonics regime dominated by volcanism5, 6 and the “Plutonic squishy lid” tectonics regime governed by intrusive magmatism, which is thought to apply to the dynamics of Venus7, 8, 9. Both tectonics regimes are capable of producing primordial tonalite-trondhjemite-granodiorite (TTG) continental crust5, 10 but lithospheric geotherms and crust production rates as well as proportions of various TTG compositions differ greatly9, 10, which implies that the heat-pipe and Plutonic squishy lid hypotheses can be tested using natural data11. Here we investigate the creation of primordial TTG-like continental crust using self-consistent numerical models of global thermochemical convection associated with magmatic processes. We show that the volcanism-dominated heat-pipe tectonics model results in cold crustal geotherms and is not able to produce Earth-like primordial continental crust. In contrast, the Plutonic squishy lid tectonics regime dominated by intrusive magmatism results in hotter crustal geotherms and is capable of reproducing the observed proportions of various TTG rocks. Using a systematic parameter study, we show that the typical modern eruption efficiency of less than 40 per cent12 leads to the production of the expected amounts of the three main primordial crustal compositions previously reported from field data4, 11 (low-, medium- and high-pressure TTG). Our study thus suggests that the pre-plate-tectonics Archean Earth operated globally in the Plutonic squishy lid regime rather than in an Io-like heat-pipe regime.
Abstract: Geological, geochronological and isotopic data are integrated in order to present a revised model for the Neoproterozoic evolution of Western Gondwana. Although the classical geodynamic scenario assumed for the period 800–700 Ma is related to Rodinia break-up and the consequent opening of major oceanic basins, a significantly different tectonic evolution can be inferred for most Western Gondwana cratons. These cratons occupied a marginal position in the southern hemisphere with respect to Rodinia and recorded subduction with back-arc extension, island arc development and limited formation of oceanic crust in internal oceans. This period was thus characterized by increased crustal growth in Western Gondwana, resulting from addition of juvenile continental crust along convergent margins. In contrast, crustal reworking and metacratonization were dominant during the subsequent assembly of Gondwana. The Río de la Plata, Congo-São Francisco, West African and Amazonian cratons collided at ca. 630–600 Ma along the West Gondwana Orogen. These events overlap in time with the onset of the opening of the Iapetus Ocean at ca. 610–600 Ma, which gave rise to the separation of Baltica, Laurentia and Amazonia and resulted from the final Rodinia break-up. The East African/Antarctic Orogen recorded the subsequent amalgamation of Western and Eastern Gondwana after ca. 580 Ma, contemporaneously with the beginning of subduction in the Terra Australis Orogen along the southern Gondwana margin. However, the Kalahari Craton was lately incorporated during the Late Ediacaran–Early Cambrian. The proposed Gondwana evolution rules out the existence of Pannotia, as the final Gondwana amalgamation postdates latest connections between Laurentia and Amazonia. Additionally, a combination of introversion and extroversion is proposed for the assembly of Gondwana. The contemporaneous record of final Rodinia break-up and Gondwana assembly has major implications for the supercontinent cycle, as supercontinent amalgamation and break-up do not necessarily represent alternating episodic processes but overlap in time.
Abstract: From the scant Hadean records of the Jack Hills to Cenozoic supervolcanoes, the continental crust provides a synoptic view deep into Earth history. However, the information is fragmented, as large volumes of continental crust have been recycled back into the mantle by a variety of processes. The preserved crustal record is the balance between the volume of crust generated by magmatic processes and the volume destroyed through return to the mantle by tectonic erosion and lower crustal delamination. At present-day, the Earth has reached near-equilibrium between the amount of crust being generated and that being returned to the mantle at subduction zones. However, multiple lines of evidence support secular change in crustal processes through time, including magma compositions, mantle temperatures, and metamorphic gradients. Though a variety of isotopic proxies are used to estimate crustal growth through time, none of those currently utilized are able to quantify the volumes of crust recycled back into the mantle. This implies the estimates of preserved continental crust and growth curves derived therefrom represent only a minimum of total crustal growth. We posit that from the Neoarchean, the probable onset of modern-day style plate tectonics (i.e. steep subduction), there has been no net crustal growth (and perhaps even a net loss) of the continental crust. Deciphering changes from this equilibrium state through geologic time remains a continual pursuit of crustal evolution studies.
Journal of Asian Earth Sciences, in press available, 17p.
India
geodynamics
Abstract: The Pitepani volcanic suite of the Dongargarh Supergroup, central India comprises of a calc-alkaline suite and a tholeiitic suite, respectively. The rare earth element (REE) patterns, mantle normalized plots and relict clinopyroxene chemistry of the Pitepani calc-alkaline suite are akin to high-Mg andesites (HMA) and reveal remarkable similarity to the Cenozoic Setouchi HMA from Japan. The Pitepani HMAs are geochemically correlated with similar rocks in the Kotri-Dongargarh mobile belt (KDMB) and in the mafic dykes of the Bastar Craton. The rationale behind lithogeochemical correlations are that sanukitic HMAs represent fore-arc volcanism over a very limited period of time, under abnormally high temperature conditions and are excellent regional and tectonic time markers. Furthermore, the tholeiitic suites that are temporally and spatially associated with the HMAs in the KDMB and in the mafic dykes of the Bastar Craton are classified into: (a) a continental back-arc suite that are depleted in incompatible elements, and (b) a continental arc suite that are more depleted in incompatible elements, respectively. The HMA suite, the continental back-arc and continental arc suites are lithogeochemically correlated in the KDMB and in the mafic dykes of the Bastar Craton. The three geochemically distinct Neoarchaean magmatic suites are temporally and spatially related to each other and to an active continental margin. The identification of three active continental margin magmatic suites for the first time, provides a robust conceptual framework to unravel the Neoarchaean geodynamic evolution of the Bastar Craton. We propose an active continental margin along the Neoarchaen KDMB with eastward subduction coupled with slab roll back or preferably, ridge-subduction along the Central Indian Tectonic Zone (CITZ) to account for the three distinct magmatic suites and the Neoarchean geodynamic evolution of the Bastar Craton.
Geochemistry, Geophysics, Geosystems, Vol. 18, 10, pp. 3556-3575.
Africa, South Africa
geodynamics
Abstract: The deep mantle African Superswell is considered to contribute to the topographic uplift of the Southern African Plateau, but dynamic support estimates vary wildly depending on the approach and data used. One reason for these large disparities is that the role of lithospheric structure, key in modulating deep dynamic contributions to elevation, is commonly ignored or oversimplified in convection studies. We use multiple high-quality geophysical data coupled with xenolith-based geochemical constraints to compute the isostatic lithospheric contribution to the elevation of the Plateau, facilitating isolation of the current dynamic component from the total observed elevation. We employ a multiobservable stochastic algorithm to invert geoid anomaly, surface-wave dispersion data, magnetotelluric data, and surface heat flow to predict elevation in a fully thermodynamically and internally-consistent manner. We find that a compositionally layered 230?±?7 km thick lithosphere is required to simultaneously fit all four data types, in agreement with abundant independent xenolith evidence. Our stochastic modeling indicates a lithospheric contribution to elevation of the order of 670 m, which implies dynamic support arising from the convecting sublithospheric mantle of ?650 m. Our results have important implications for the understanding of lithospheric-deep mantle feedback mechanisms and for calibrating dynamic topography estimates from global convection studies.
Abstract: There are two types of emerged relief on the Earth: high elevation areas (mountain belts and rift shoulders) in active tectonic settings and low elevation domains (anorogenic plateaus and plains) characteristic of the interior of the continents i.e. 70% of the Earth emerged relief. Both plateaus and plains are characterized by large erosional surfaces, called planation surfaces that display undulations with middle (several tens of kilometres) to very long (several thousands of kilometres) wavelengths, i.e. characteristic of lithospheric and mantle deformations respectively. Our objective is here (1) to present a new method of characterization of the very long and long wavelength deformations using planation surfaces with an application to Central Africa and (2) to reconstruct the growth of the very long wavelength relief since 40 Ma, as a record of past mantle dynamics below Central Africa. (i) The African relief results from two major types of planation surfaces, etchplains (weathering surfaces by laterites) and pediplains/pediments. These planation surfaces are stepped along plateaus with different elevations. This stepping of landforms records a local base level fall due to a local tectonic uplift. (ii) Central Africa is an extensive etchplain-type weathering surface - called the African Surface - from the uppermost Cretaceous (70 Ma) to the Middle Eocene (45 Ma) with a paroxysm around the Early Eocene Climatic Optimum. Restoration of this surface in Central Africa suggests very low-elevation planation surfaces adjusted to the Atlantic Ocean and Indian Ocean with a divide located around the present-day eastern branch of the East African Rift. (iii) The present-day topography of Central Africa is younger than 40 -30 Ma and records very long wavelength deformations (1000 -2000 km) with (1) the growth of the Cameroon Dome and East African Dome since 34 Ma, (2) the Angola Mountains since 15 -12 Ma increasing up to Pleistocene times and (3) the uplift of the low-elevation (300 m) Congo Basin since 10 -3 Ma. Some long wavelength deformations (several 100 km) also occurred with (1) the low-elevation Central African Rise since 34 Ma and (2) the Atlantic Bulge since 20 -16 Ma. These very long wavelength deformations record mantle dynamics, with a sharp increase of mantle upwelling around 34 Ma and an increase of the wavelength of the deformation and then of mantle convection around 10 -3 Ma.
Abstract: Experimental data reveal that Earth’s mantle melts more readily than previously thought, and may have remained mushy until two to three billion years ago.
Abstract: The generation, preservation and destruction of continental crust on Earth is of wide interest in understanding the formation of continents, cratons and supercontinents as well as related mineral deposits. In this contribution, we integrate the available field, petrographic, geochronologic, elemental Nd-Hf-Pb isotope data for greenstones, TTG gneisses, sanukitoids and anatectic granites from the Dharwar Craton (southern India). This review allows us to evaluate the accretionary processes of juvenile crust, mechanisms of continental growth, and secular evolution of geodynamic processes through the 3600-2500?Ma window, hence providing important insights into building of continents in the Early Earth. The Dharwar Craton formed by assembly of micro-blocks with independent thermal records and accretionary histories. The craton can be divided into three crustal blocks (western, central and eastern) separated by major shear zones. The western block contains some of the oldest basement rocks with two generations of volcano-sedimentary greenstone sequences and discrete potassic plutons whereas the central block consist of older migmatitic TTGs, abundant younger transitional TTGs, remnants of ancient high grade supracrustal rocks, linear volcanic-dominated greenstone belts, voluminous calc-alkaline granitoids of sanukitoid affinity and anatectic granites. In contrast, the eastern block comprises younger transitional TTGs, abundant diatexites, thin volcanic-sedimentary greenstone belts and calc-alkaline plutons. Published geochronologic data show five major periods of felsic crust formation at ca. 3450-3300?Ma, 3230-3150?Ma, 3000-2960?Ma, 2700-2600?Ma, and 2560-2520?Ma which are sub-contemporaneous with the episodes of greenstone volcanism. U-Pb ages of inherited zircons in TTGs, as well as detrital zircons together with Nd-Pb-Hf isotope data, reveal continental records of 3800-3600?Ma. The U-Pb zircon data suggest at least four major reworking events during ca. 3200?Ma, 3000?Ma, 2620-2600?Ma, and 2530-2500?Ma corresponding to lower crustal melting and spatially linked high grade metamorphic events. The TTGs are sub-divided into the older (3450-3000?Ma) TTGs and the younger (2700-2600?Ma) transitional TTGs. The older TTGs can be further sub-divided into low-Al and high-Al groups. Elemental and isotopic data suggest that the low-Al type formed by melting of oceanic island arc crust within plagioclase stability field. In contrast, the elemental and isotopic features for the high-Al group suggest derivation of their magmatic precursor by melting of oceanic arc crust at deeper levels (55-65?km) with variable garnet and ilmenite in residue. The transitional TTGs likely formed by melting of composite sources involving both enriched oceanic arc crust and sub-arc mantle with minor contamination of ancient crustal components. The geochemical and isotopic compositions of granitoids with sanukitoid affinity suggest derivation from enriched mantle reservoirs. Finally, anatectic granites were produced by reworking of crustal sources with different histories. In the light of the data reviewed in this contribution, we propose the following scenario for the tectonic evolution of the Dharwar Craton. During 3450-3000?Ma, TTGs sources (oceanic arc crust) formed by melting of down going slabs and subsequent melting of such newly formed crust at different depths resulted in TTG magmas. On the contrary, by 2700?Ma the depth of slab melting increased. Melting of slab at greater depth alongside the detritus results in enriched melts partly modified the overlying mantle wedge. Subsequent melting of such newly formed enriched oceanic arc crust and surrounding arc-mantle generated the magmatic precursor to transitional TTGs. Finally at ca. 2600-2560?Ma, eventual breakoff of down going slab caused mantle upwelling which induced low degree (10-15%) melting of overlying enriched mantle at different depths, thereby, generating the sanukitoid magmas which upon emplacement into the crust caused high temperature metamorphism, reworking and final cratonization.
Abstract: Microcontinents occur outboard of passive margins and stranded in ocean basins. Three-dimensional analogue laboratory experiments of continental rifting demonstrate that microcontinent formation at passive margins requires a combination of preexisting linear weaknesses in the lithosphere and rotational extension. Our results suggest that separation of microcontinents from passive margins occurs during the latest stages of continental breakup, before the onset of seafloor spreading, and that preexisting lithospheric weaknesses are a first-order control on where they form. These findings suggest that microcontinent formation may be restricted to localized regions along passive margins associated with zones of lithospheric weakness, providing a new structural and tectonic framework for the interpretation of microcontinents in the geological record.
Earth and Planetary Science Letters, Vol. 494, pp. 42-49.
Mantle
geodynamics
Abstract: The geodynamic regime of the early Earth remains elusive, with so far proposed hypotheses ranging from stagnant lid convection to rapid plate tectonics. Available geological data are severely limited for the first two billion years of the Earth's history, and this scarcity of relevant data is often compounded by the nonuniqueness of interpretation. Here we propose that the samarium-neodymium isotope evolution, which has been suggested to be consistent with stagnant lid convection in the early Earth, may be better understood as the result of rapid crustal growth and extensive crustal recycling. We delineate the permissible scenario of crustal evolution through geochemical box modeling with a Monte Carlo sampling of the model parameter space, and our results suggest that the net growth of continental crust was complete by the end of the Hadean and that the rate of crustal recycling could have been as high as kg Gyr?1 at that time and has gradually decreased since then. Such crustal evolution yields a specific prediction for the present-day distribution of crustal formation ages, which is shown to be in remarkable agreement with a recent estimate based on the global compilation of zircon age data. The mode of subsolidus mantle convection after the putative magma ocean is probably plate tectonics, but its style could have been very different from that of contemporary plate tectonics, characterized by more voluminous magmatism and more destructive subduction.
Geological Society, London Magmatic Rifting and Active Volcanism, Eds. Wright, T.J., Ayele, A., Ferguson, D.J., Kidane, T., Vye-Brown, X. publ. 2016, SP 420, pp. 11-30. from http://sp.lyell collection.org
Africa
tectonics, geodynamics
Abstract: Continental rifting has a fundamental role in the tectonic behaviour of the Earth, shaping the surface we live on. Although there is not yet a consensus about the dominant mechanism for rifting, there is a general agreement that the stresses required to rift the continental lithosphere are not readily available. Here we use a global finite element model of the lithosphere to calculate the stresses acting on Africa. We consider the stresses induced by mantle flow, crustal structure and topography in two types of models: one in which flow is exclusively driven by the subducting slabs and one in which it is derived from a shear wave tomographic model. The latter predicts much larger stresses and a more realistic dynamic topography. It is therefore clear that the mantle structure beneath Africa plays a key part in providing the radial and horizontal tractions, dynamic topography and gravitational potential energy necessary for rifting. Nevertheless, the total available stress (c. 100 MPa) is much less than that needed to break thick, cold continental lithosphere. Instead, we appeal to a model of magma-assisted rifting along pre-existing weaknesses, where the strain is localized in a narrow axial region and the strength of the plate is reduced significantly. Mounting geological and geophysical observations support such a model.
Abstract: Knowledge of the state of stress in Earth’s crust is key to understanding the forces and processes responsible for earthquakes. Historically, low rates of natural seismicity in the central and eastern United States have complicated efforts to understand intraplate stress, but recent improvements in seismic networks and the spread of human-induced seismicity have greatly improved data coverage. Here, we compile a nationwide stress map based on formal inversions of focal mechanisms that challenges the idea that deformation in continental interiors is driven primarily by broad, uniform stress fields derived from distant plate boundaries. Despite plate-boundary compression, extension dominates roughly half of the continent, and second-order forces related to lithospheric structure appear to control extension directions. We also show that the states of stress in several active eastern United States seismic zones differ significantly from those of surrounding areas and that these anomalies cannot be explained by transient processes, suggesting that earthquakes are focused by persistent, locally derived sources of stress. Such spatially variable intraplate stress appears to justify the current, spatially variable estimates of seismic hazard. Future work to quantify sources of stress, stressing-rate magnitudes and their relationship with strain and earthquake rates could allow prospective mapping of intraplate hazard.
Global and Planetary Change, doi.org/j.gloplacha.2017.10.011 24p.
Africa
geodynamics
Abstract: Modern satellite gravimetry is now considered one of the most powerful and effective instrument for regional tectono-geodynamic zonation. Satellite gravity observations clearly fit the definition of 'big data' because of their volume and variety. The Arabian - NE African region discussed in this article has intricate geodynamic features including active rift zones, high seismic activity and collision processes, a rich structural pattern made up of the mosaic block system of continental and oceanic crusts of different ages, as well as several of the greatest gravity anomalies and complex magnetic anomaly mosaics. This region also has the world's main hydrocarbon resources and a vast number of other economic deposits. A comprehensive analysis of these satellite derived gravity data were used to construct a series of new maps that localize the key properties of the lithosphere of the region. A careful examination of numerous geological sources and their combined inspection with satellite derived gravity and other geophysical data resulted in this new integrated tectonic map of the Arabian-African region. An analysis of the series of gravity map transformations and certain geological indicators document the significant geodynamic features of the region.
Abstract: Dynamic topography is a well-established consequence of global geodynamic models of mantle convection with horizontal dimensions of >1000 km and amplitudes up to 2 km. Such physical models guide the interpretation of geological records on equal dimensions. Continent-scale geological maps therefore serve as reference frames of choice to visualize erosion/non-deposition as a proxy for long-wavelength, low-amplitude vertical surface motion. At a resolution of systems or series, such maps display conformable and unconformable time boundaries traceable over hundreds to thousands of kilometres. Unconformable contact surfaces define the shape and size of time gap (hiatus) in millions of years based on the duration of time represented by the missing systems or series. Hiatus for a single system or series base datum diminishes laterally to locations (anchor points) where it is conformable at the mapped resolution; it is highly dependent upon scale. A comparison of hiatus area between two successive system or series boundaries yields changes in location, shape, size and duration, indicative of the transient nature of vertical surface motion. As a single-step technique, it serves as a quantitative proxy for palaeotopography that can be calibrated using other geological data. The tool magnifies the need for geological mapping at the temporal resolution of stages, matching process rates. The method has no resolving power within conformable regions (basins) but connects around them. When applied to marine seismic sections that relate to rock record, not to time, biostratigraphic and radiometric data from deep wells are needed before hiatus areas - that relate to time - can be mapped.
Earth and Planetary Science Letters, Vol. 500, pp. 156-167.
United States
geodynamics
Abstract: The origin of the complex pattern of SKS splitting over the western United States (U.S.) remains a long-lasting debate, where a model that simultaneously matches the various SKS features is still lacking. Here we present a series of quantitative geodynamic models with data assimilation that systematically evaluate the influence of different lithospheric and mantle structures on mantle flow and seismic anisotropy. These tests reveal a configuration of mantle deformation more complex than ever envisioned before. In particular, we find that both lithospheric thickness variations and toroidal flows around the Juan de Fuca slab modulate flow locally, but their co-existence enhances large-scale mantle deformation below the western U.S. The ancient Farallon slab below the east coast pulls the western U.S. upper mantle eastward, spanning the regionally extensive circular pattern of SKS splitting. The prominent E-W oriented anisotropy pattern within the Pacific Northwest reflects the existence of sustaining eastward intrusion of the hot Pacific oceanic mantle to beneath the continental interior, from within slab tears below Oregon to under the Snake River Plain and the Yellowstone caldera. This work provides an independent support to the formation of intra-plate volcanism due to intruding shallow hot mantle instead of a rising mantle plume.
Abstract: Models of the volume of continental crust through Earth history vary significantly due to a range of assumptions and data sets; estimates for 3?Ga range from <10% to >120% of present day volume. We argue that continental area and thickness varied independently and increased at different rates and over different periods, in response to different tectonic processes, through Earth history. Crustal area increased steadily on a pre-plate tectonic Earth, prior to ca. 3?Ga. By 3?Ga the area of continental crust appears to have reached a dynamic equilibrium of around 40% of the Earth's surface, and this was maintained in the plate tectonic world throughout the last 3?billion?years. New continental crust was relatively thin and mafic from ca. 4-3?Ga but started to increase substantially with the inferred onset of plate tectonics at ca. 3?Ga, which also led to the sustained development of Earth's bimodal hypsometry. Integration of thickness and area data suggests continental volume increased from 4.5?Ga to 1.8?Ga, and that it remained relatively constant through Earth's middle age (1.8-0.8?Ga). Since the Neoproterozoic, the estimated crustal thickness, and by implication the volume of the continental crust, appears to have decreased by as much as 15%. This decrease indicates that crust was destroyed more rapidly than it was generated. This is perhaps associated with the commencement of cold subduction, represented by low dT/dP metamorphic assemblages, resulting in higher rates of destruction of the continental crust through increased sediment subduction and subduction erosion.
Abstract: Earth's global magnetic field shields us from the dangers of space, including harmful levels of Solar radiation. This shield is one of the major reasons why life has survived on Earth, and it all starts in the core, where the geomagnetic field is generated by the turbulent motion of liquid iron deep inside the planet. It's the so-called geodynamo. Scientists like DTM's Peter Driscoll devote their career to understanding the nature of such planetary dynamics. In 2016, Driscoll's research suggested that in ancient times Earth may have been significantly different, with prolonged periods of weak field intensity and strange multipolar geometry with many poles in contrast to the modern north-and-south pole orientation. Now, in a recent paper co-authored with DTM's Cian Wilson, Driscoll finds that geologic evidence for such a complex magnetic field will be very difficult to identify. In preparation for his Neighborhood Lecture on November 8, 2018, Driscoll answers some questions about his latest research and why studying the geodynamo is critical to understanding the history of our planet.
Abstract: The Earth's global system of tectonic plates move over a thin, weak channel ("low viscosity zone") in the mantle immediately underlying the plates. This weak channel is commonly referred to as the asthenosphere, and its presence accounts for a number of important Earth observations, including isostasy (e.g., support for the uplift of large mountain ranges), the shape of the Earth's gravity field, the response of the Earth's surface to the removal of large ice sheets ("postglacial rebound"), and the relationship between plate motions and underlying thermal convection in the mantle. In this paper, we show that these phenomena can be understood in terms of a single unifying parameter consisting of the viscosity contrast between the asthenosphere and the underlying mantle, and the cube of the thickness of the asthenosphere. We propose to call this the "Cathles parameter" in recognition of the author who first recognized its importance in postglacial rebound studies.
Abstract: Deep crustal and mantle rocks are exhumed in core complex mode of extension in three types of structures: metamorphic core complexes, oceanic core complexes and magma poor passive margins. Using available analogue and numerical models and their comparison with natural examples, the present paper reviews the mechanical processes involved in these different types of extensional setting. Three main aspects are considered: i) the primary role of lithosphere rheology, ii) the lithosphere-scale patterns of progressive deformation that lead to the exhumation of deep metamorphic or mantle rocks and iii) the initiation and development of detachment zones. Crustal core complexes develop in continental lithospheres whose Moho temperature is higher than 750 °C with “upper crust-dominated” strength profiles. Contrary to what is commonly believed, it is argued from analogue and numerical models that detachments that accommodate exhumation of core complexes do not initiate at the onset of extension but in the course of progressive extension when the exhuming ductile crust reaches the surface. In models, convex upward detachments result from a rolling hinge process. Mantle core complexes develop in either the oceanic lithosphere, at slow and ultra-slow spreading ridges, or in continental lithospheres, whose initial Moho temperature is lower than 750 °C, with “sub-Moho mantle-dominated” strength profiles. It is argued that the mechanism of mantle exhumation at passive margins is a nearly symmetrical necking process at lithosphere scale without major and permanent detachment, except if strong strain localization could occur in the lithosphere mantle. Distributed crustal extension, by upper crust faulting above a décollement along the ductile crust increases toward the rift axis up to crustal breakup. Mantle rocks exhume in the zone of crustal breakup accommodated by conjugate mantle shear zones that migrate with the rift axis, during increasing extension.
South African Journal of Geology, Vol. 121, pp. 271-286.
Africa, Mozambique
geodynamics
Abstract: Major, trace, radiogenic isotope and stable isotope data from lavas along the northeastern coast of Mozambique are described. The whole rock composition data demonstrate that the rocks are dominantly andesitic with compositions typical of calc-alkaline volcanic rocks from arc environments. SHRIMP U/Pb data from zircons indicate that the zircons are xenocrystic, having ages of between 500 Ma and 660 Ma, with the age of the lava constrained by Rb/Sr data at ~184 Ma. Strontium, Nd and Pb radiogenic isotope data support an interpretation of extensive mixing between a Karoo age basaltic magma (dolerite) from Antarctica and continental crust similar in composition to the Mozambique basement. Oxygen isotope data also imply a significant crustal contribution to the lavas. Possible tectonic settings for the lavas are at the margin of a plume or from a locally restricted compressional setting during Gondwana breakup processes.
Earth and Planetary Science Letters, Vol. 506, pp. 1-7.
Mantle
geodynamics
Abstract: We present a theoretical investigation, based on ab initio calculations and the quasi-harmonic approximation, on the stability properties of magnesium (MgCO3) and calcium (CaCO3) carbonates at high temperatures and pressures. The results indicate that those carbonates should be stable in the Earth's lower mantle, instead of dissociating into other minerals, in chemical environments with excess of SiO2, MgO, or MgSiO3. Therefore, considering the lower mantle chemical composition, consisting mostly of the MgSiO3 and MgO minerals, calcium and magnesium carbonates are the primary candidates as carbon hosts in that region. For the thermodynamic conditions of the mantle, the results also indicate that carbon should be primarily hosted on MgCO3, contrasting with what was found by other theoretical studies, which neglected temperature effects. Finally, the results indicate that carbon, in the form of free CO2, is unlikely in the lower mantle.
Abstract: Cratons are strong and their preservation demonstrates that they resist deformation and fragmentation. Yet several cratons are rifting now, or have rifted in the past. We suggest that cratons need to be weakened before they can rift. Specifically, metasomatism of the depleted dehydrated craton mantle lithosphere is a potential weakening mechanism. We use 2D numerical models to test the efficiency of simulated melt metasomatism and coeval rehydration to weaken craton mantle lithosphere roots. These processes effectively increase root density through a parameterized melt-peridotite reaction, and reduce root viscosity by increasing the temperature and rehydrating the cratonic mantle lithosphere. The models are designed to investigate when a craton is sufficiently weakened to undergo rifting and is no longer protected by adjacent standard Phanerozoic lithosphere. We find that cratons only become vulnerable to rifting following large-volume melt metasomatism (~ 30% by volume) and thinning of the gravitationally unstable cratonic lithosphere from > 250 km to ~ 100 km; at which point its residual crustal strength is important. Furthermore, our results indicate that rifting of cratons depends on the timing of extension with respect to metasomatism. An important effect in the large-volume melt models is the melt-induced increase in temperature which must have time to reach peak values in the uppermost mantle lithosphere before rifting. Release of water stored in the transition zone at the base of a big mantle wedge may provide a suitable natural setting for both rehydration and refertilization of an overlying craton and is consistent with evidence from the eastern North China Craton. An additional effect is that cratons subside isostatically to balance the increasing density of craton mantle lithosphere where it is moderately metasomatized. We suggest that this forms intracratonic basins and that their subsidence and subsequent uplift, and cratonic rifting constitute evidence of progressive metasomatism of cratonic mantle lithosphere.
Abstract: Besides the intrinsic rheological layering of the lithosphere and its thermal structure, inherited heterogeneities may play an important role in strain localization during continental extension. This is similar to the role that defects play in the failure and necking of other materials. Here, we consider both inherited small-scale weak zones and the effects of lateral juxtaposition of two lithospheres with differing properties as mechanisms to localize deformation and initiate necking instabilities. Using 2D finite-element models that contain lateral lithospheric boundaries, in combination with smaller scale heterogeneities, we illustrate that two controls determine how necking instabilities grow and thus lead to varying styles of rifting: Control 1, the stiff/pliable nature of the lithosphere and Control 2, the background strain rate in the lithosphere. Control 1 depends on the lithospheric rheology, such that necking instabilities grow faster in materials with high power-law creeping flow exponents (stiff, brittle lithosphere) than in those with low power-law creeping flow exponents (pliable, viscous lithosphere). Control 2 prevails in lithosphere where background strain rates are highest. This happens because necking amplifies the background strain rate in power-law materials, leading to faster necking where strain rates are highest. The model results show that Control 2 determines the location of localization, unless the background strain rate is equal or near equal in both lithospheres, in which case Control 1 wins. These results explain why rifting does not localize in cratons even though they contain heterogeneities. The results also provide a mechanism for the formation of asymmetric rifted margins.
Geophysical Research Letters, Vol. 45, 24, pp. 13,240-13,248.
Mantle
geodynamics
Abstract: We measure the incorporation of magnesium oxide (one of the main components of Earth's mantle) into iron (the main constituent Earth's core), using extremely high pressure and temperature experiments that mimic the conditions of Earth's mantle and core. We find that magnesium oxide dissolution depends on temperature but not on pressure, and on metal (i.e., core) composition but not silicate (i.e., mantle) composition. Our findings support the idea that magnesium oxide dissolved in the core during its formation will precipitate out during subsequent core cooling. The precipitation should stir the entire core to produce a magnetic field in Earth's distant past, at least as intense as the present?day field.
Abstract: The invention of a robust and accurate sea-going chronometer transformed navigation in the mid-eighteenth century. The calibration of longitude against the prime meridian at Greenwich, in combination with latitude derived from the positions of celestial bodies gave mariners for the first time confidence that they could calculate their position on the Earth's surface. Until recently, Earth scientists have been in a comparable position of having no way of calculating the longitudes of continents before the Cretaceous. Here I discuss Phanerozoic polar wander and paleogeographies and describe ways of quantitatively establishing ancient longitudes which also establish how the Earth's interior can be linked to its surface in geological time. The first method makes use of the fact that longitudinal uncertainty of continents that were assembled in Pangea can, for subsequent times, be eliminated, if longitude motion is known for only one of these continents. The best assumption is zero-longitude motion for Africa and with this assumption we can show that large igneous provinces (LIPs) and kimberlites almost exclusively erupted above the margins of TUZO and JASON in the lower mantle. This remarkable observation, also considering the effect of true polar wander, has led to a second method the plume generation zone reconstruction method unlocking a way forward in modelling absolute plate motions before Pangea and exploring links between plate tectonics, intra-plate volcanism and Deep Earth dynamics. Conceptually, that link can be viewed as a simple mass-balance: subducted lithosphere slabs restore mass to the mantle and trigger the return flow toward the surface including mantle plumes rising from the margins of TUZO and JASON. The surface manifestations of plumes are hotspot lavas, kimberlites and LIPs.
Abstract: A number of NE-SW to ENE-WSW trending Palaeoproterozoic mafic dykes, intruded within the Archean basement rocks and more conspicuous in the southern parts of the western Dharwar Craton (WDC), was studied for their whole?rock geochemistry to understand their petrogenetic and geodynamic aspects. Observed mineralogical and textural characteristics classify them either as meta?dolerites or dolerites/olivine?dolerites. They show basaltic to basaltic-andesitic compositions and bear sub?alkaline tholeiitic nature. Three geochemically distinct groups of mafic dykes have been identified. Group 1 samples show flat REE patterns (LaN/LuN = ~1), whereas the other two groups have LaN/LuN = ~2-3 (Group 2; enriched LREE and flat HREE patterns) and LaN/LuN = ~4 (Group 3; inclined REE patterns). Chemistry is not straightforward to support any significant role of crustal contamination and probably reflect their source characteristics. However, their derivation from melts originated from a previously modified metasomatized lithospheric mantle due to some ancient subduction event cannot be ignored. Most likely different mantle melts were responsible for derivation of these distinct sets of mafic dykes. The Group 2 dykes are derived from a melt generated within spinel stability field by ~10% batch melting of a lithospheric mantle source, whereas the Group 3 dykes have their derivation from a melt originated within the spinel-garnet transition zone and were fed from slightly higher (~12-15%) batch melting of a similar source. The Group 1 samples were also crystallized from a melt generated at the transition zone of spinel-garnet stability field by higher degrees (~20%) of melting of a primitive mantle source. Geochemistry of the studied samples is typical of Palaeoproterozoic mafic dykes emplaced within the intracratonic setting, reported elsewhere globally as well as neighbouring cratons. Geochemistry of the studied mafic dyke samples is also compared with the mafic dykes of the eastern Dharwar Craton (EDC). Except the Group 3 samples, which have good correlation with the 1.88-1.89 Ga Hampi swarm, no other group shows similarity with the EDC mafic dykes. There is an ample possibility to have some different mafic magmatic events in the WDC, which could be different from the EDC. However, it can only be confirmed after precise age determinations.
Abstract: Great advances were achieved during the last several decades in our knowledge of the mineralogy, petrology and geochemistry of the Earth’s lower mantle [1]. Most geological observations agree and confirm earlier suggestions and conclusions made; however, some disagreements or discrepancies appeared between the various geological and experimental data. One of the first, unexpected occurrences was the presence of free silica in the ‘ultramafic’ lowermantle association (“stishovite paradox” [2]). Another unexpected discrepancy is wide variation in the composition of natural periclase-wüstite phase: mg = Mg/(Mg + Fe)at varies from 0.90 to 0.16, while, according to experimental and theoretical data it should, in the pyrolytic system, be with mg at c. 0.90. Iron contents in both ferropericlase and bridgmanite increase with depth, indicating the increasing Fe content in the lower part of the lower mantle. In addition to major lower-mantle minerals (bridgmanite, ferropericlase, CaSi-perovskite and stishovite), some other minerals were identified in association with these, such as Mg-Cr-Fe, Ca- Cr and other orthorhombic oxides, iron carbides and nitrides, and others. In contrast to earlier suggestions on the increase of the oxygen fugacity values in the mantle with depth, the ??logfO2 values for the lower mantle, calculated from the compositions of natural ferropericlase inclusions in diamond, are similar to those of the upper mantle: they lie, in general, between the IW and FMQ buffers. In addition to ‘ultramafic’ and mafic mineral associations, a primary natrocarbonatitic association occurs in the lower mantle. Such and other features observed in lower-mantle samples indicate that the bulk composition of the lower mantle may differ from that of a ‘pyrolite model’, as well as heterogeneity in the lower mantle being as the result of extensive geodynamic processes occurring in the Deep Earth.
Abstract: Cratons are the ancient landmasses that remain stable for billions of years on Earth but also have experienced episodic events of modification and rejuvenation throughout their history [1]. These alteration processes have modified the cratonic lithospheric mantle roots to different extents, e.g., ubiquitous cryptic/modal metasomatism, partial to entire loss of the mantle roots, to rifting apart of the craton. It remains unclear to what extent a cratonic mantle root can withstand modification and retain its integrity. We attempt to discuss this issue from the perspective of the Slave craton that has experienced the multiple impacts of major circum-cratonic Paleoproterozoic (1.93-1.84 Ga) orogenies and the intrusion of several 2.23-1.67 Proterozoic diabase dyke swarms. We use kimberlite-borne peridotite xenoliths to construct a N-S transect across the craton with an aim of probing the effects of these post-Archean events on the composition, age and depth of the lithospheric root. Chemically, all of these rocks are of typical cratonic refractory composition. P-T calculations and paleogeotherms show that they were derived from thick lithospheric mantle roots (>180 km), consistent with their diamondiferous nature. However, these peridotites exhibit variable N-S variation of modes in their Re-depletion Os model ages (TRD). Neoarchean TRD ages dominate in the Central and Southern Slave mantle. Progressing North there is a decreasing proportion of Archean TRD ages through Jericho to Artemisa in the Northern Slave craton. About 70% of the peridotites at Artemisia give TRD ages within error of the ~1.27 Ga Mackenzie LIP event, with the remaining (~ 30%) close to the Paleoproterozoic orogenic events. Combined with new data from regions to the N and NW of the Slave craton [2], the observed age spectrum in the far North of the craton indicates the likelihood of major new generation of lithospheric roots in both the Paleoproterozoic and Mesoproterozoic. Despite its complex history, the Northern Slave craton retains a ‘cratonic-like’ lithospheric root that allowed diamond mineralization.
Abstract: Earth’s surface topography is a direct physical expression of our planet’s dynamics. Most is isostatic, controlled by thickness and density variations within the crust and lithosphere, but a substantial proportion arises from forces exerted by underlying mantle convection. This dynamic topography directly connects the evolution of surface environments to Earth’s deep interior, but predictions from mantle flow simulations are often inconsistent with inferences from the geological record, with little consensus about its spatial pattern, wavelength and amplitude. Here, we demonstrate that previous comparisons between predictive models and observational constraints have been biased by subjective choices. Using measurements of residual topography beneath the oceans, and a hierarchical Bayesian approach to performing spherical harmonic analyses, we generate a robust estimate of Earth’s oceanic residual topography power spectrum. This indicates water-loaded power of 0.5?±?0.35?km2 and peak amplitudes of up to ~0.8?±?0.1?km at long wavelengths (~104?km), decreasing by roughly one order of magnitude at shorter wavelengths (~103?km). We show that geodynamical simulations can be reconciled with observational constraints only if they incorporate lithospheric structure and its impact on mantle flow. This demonstrates that both deep (long-wavelength) and shallow (shorter-wavelength) processes are crucial, and implies that dynamic topography is intimately connected to the structure and evolution of Earth’s lithosphere.
Abstract: In contrast to modern-day plate tectonics, geodynamics of the early Earth presents a unique challenge, as currently there is no consensus on a global paradigm concerning the mantle dynamics and lithosphere tectonics in the Precambrian (Benn et al., 2006; Gerya, 2014). This challenge is mainly due to the severe objective restrictions of obtaining geological and/or geophysical observations constraining Earth’s surface and interior dynamics back in geological time (Fig. 1). The subject of geodynamics can be schematically represented by the time-depth diagram (see Fig. 1) covering the entire Earth’s history and interior. In theory, the entire diagram should be "covered" by data points characterizing the physical-chemical state of Earth at different depths, for different moments in geological time. However, in practice, observations are only available along two axes: (1) geophysical data for Earth’s internal structure at all ranges of depths, but only for the very short present-day time, and (2) the geological record preserved in rocks formed over a broad range of geological times, but only at a very shallow depth range. As a result, the importance of well-constrained geological and geophysical data, and thoroughly studied present-day geodynamic regime (modern-style plate tectonics) is almost unavoidably exaggerated and "stretched" toward the Precambrian Earth. This "plate tectonics trap" can only be avoided by further calibrating our geological intuition on the basis of numerical geodynamic modeling that integrates available geological, geochemical, petrological, and geochronological records (Gerya, 2014).
IN: Deep carbon: past to present, Orcutt, Daniel, Dasgupta eds., pp. 89-128.
Mantle
geodynamics
Abstract: The science of studying diamond inclusions for understanding Earth history has developed significantly over the past decades, with new instrumentation and techniques applied to diamond sample archives revealing the stories contained within diamond inclusions. This chapter reviews what diamonds can tell us about the deep carbon cycle over the course of Earth’s history. It reviews how the geochemistry of diamonds and their inclusions inform us about the deep carbon cycle, the origin of the diamonds in Earth’s mantle, and the evolution of diamonds through time.
Geochemistry, Geophysics, Geosystems, in press available pdf 20p.
Global
geodynamics
Abstract: The relative significance of various geodynamic mechanisms that drive supercontinent breakup is unclear. A previous analysis of extensional stress during supercontinent breakup demonstrated the importance of the plume?push force relative to the dragging force of subduction retreat. Here, we extend the analysis to basal traction (shear stress) and cross?lithosphere integrations of both extensional and shear stresses, aiming to understand more clearly the relevant importance of these mechanisms in supercontinent breakup. More importantly, we evaluate the effect of preexisting orogens (mobile belts) in the lithosphere on supercontinent breakup process. Our analysis suggests that a homogeneous supercontinent has extensional stress of 20-50 MPa in its interior (<40° from the central point). When orogens are introduced, the extensional stress in the continents focuses on the top 80?km of the lithosphere with an average magnitude of ~160 MPa, whereas at the margin of the supercontinent the extensional stress is 5-50 MPa. In both homogeneous and orogeny?embedded cases, the subsupercontinent mantle upwellings act as the controlling factor on the normal stress field in the supercontinent interior. Compared with the extensional stress, shear stress at the bottom of the supercontinent is 1-2 order of magnitude smaller (0-5 MPa). In our two end?member models, the breakup of a supercontinent with orogens can be achieved after the first extensional stress surge, whereas for a hypothetical supercontinent without orogens it starts with more diffused local thinning of the continental lithospheric before the breakup, suggesting that weak orogens play a critical role in the dispersal of supercontinents.
IN: Deep Carbon: past to present. Editors Orcutt, Danielle, Dasgupta, pp. 89-128.
Mantle
geodynamics
Abstract: The science of studying diamond inclusions for understanding Earth history has developed significantly over the past decades, with new instrumentation and techniques applied to diamond sample archives revealing the stories contained within diamond inclusions. This chapter reviews what diamonds can tell us about the deep carbon cycle over the course of Earth’s history. It reviews how the geochemistry of diamonds and their inclusions inform us about the deep carbon cycle, the origin of the diamonds in Earth’s mantle, and the evolution of diamonds through time.
Annual Review of Earth and Planetary Sciences, Vol. 47, pp. 389-417.
Mantle
geodynamics
Abstract: Noble gases have played a key role in our understanding of the origin of Earth's volatiles, mantle structure, and long-term degassing of the mantle. Here we synthesize new insights into these topics gained from high-precision noble gas data. Our analysis reveals new constraints on the origin of the terrestrial atmosphere, the presence of nebular neon but chondritic krypton and xenon in the mantle, and a memory of multiple giant impacts during accretion. Furthermore, the reservoir supplying primordial noble gases to plumes appears to be distinct from the mid-ocean ridge basalt (MORB) reservoir since at least 4.45 Ga. While differences between the MORB mantle and plume mantle cannot be explained solely by recycling of atmospheric volatiles, injection and incorporation of atmospheric-derived noble gases into both mantle reservoirs occurred over Earth history. In the MORB mantle, the atmospheric-derived noble gases are observed to be heterogeneously distributed, reflecting inefficient mixing even within the vigorously convecting MORB mantle.
Abstract: The most dominant features in the present-day lower mantle are the two antipodal African and Pacific large low-shear-velocity provinces (LLSVPs). How and when these two structures formed, and whether they are fixed and long lived through Earth history or dynamic and linked to the supercontinent cycles, remain first-order geodynamic questions. Hotspots and large igneous provinces (LIPs) are mostly generated above LLSVPs, and it is widely accepted that the African LLSVP existed by at least ca. 200 Ma beneath the supercontinent Pangea. Whereas the continental LIP record has been used to decipher the spatial and temporal variations of plume activity under the continents, plume records of the oceanic realm before ca. 170 Ma are mostly missing due to oceanic subduction. Here, we present the first compilation of an Oceanic Large Igneous Provinces database (O-LIPdb), which represents the preserved oceanic LIP and oceanic island basalt occurrences preserved in ophiolites. Using this database, we are able to reconstruct and compare the record of mantle plume activity in both the continental and oceanic realms for the past 2 b.y., spanning three supercontinent cycles. Time-series analysis reveals hints of similar cyclicity of the plume activity in the continent and oceanic realms, both exhibiting a periodicity of ?500 m.y. that is comparable to the supercontinent cycle, albeit with a slight phase delay. Our results argue for dynamic LLSVPs where the supercontinent cycle and global subduction geometry control the formation and locations of the plumes.
Geochemistry, Geophysics, Geosystems, Vol. 21, 2, 66p. Pdf
Mantle
geodynamics
Abstract: Supercontinent assembly and breakup can influence the rate and global extent to which insulated and relatively warm subcontinental mantle is mixed globally, potentially introducing lateral oceanic?continental mantle temperature variations that regulate volcanic and weathering controls on Earth's long?term carbon cycle for a few hundred million years. We propose that the relatively warm and unchanging climate of the Nuna supercontinental epoch (1.81.3 Ga) is characteristic of thorough mantle thermal mixing. By contrast, the extreme cooling?warming climate variability of the Neoproterozoic Rodinia episode (10.63 Ga) and the more modest but similar climate change during the Mesozoic Pangea cycle (0.30.05 Ga) are characteristic features of the effects of subcontinental mantle thermal isolation with differing longevity. A tectonically modulated carbon cycle model coupled to a one?dimensional energy balance climate model predicts the qualitative form of Mesozoic climate evolution expressed in tropical sea?surface temperature and ice sheet proxy data. Applied to the Neoproterozoic, this supercontinental control can drive Earth into, as well as out of, a continuous or intermittently panglacial climate, consistent with aspects of proxy data for the Cryogenian?Ediacaran period. The timing and magnitude of this cooling?warming climate variability depends, however, on the detailed character of mantle thermal mixing, which is incompletely constrained. We show also that the predominant modes of chemical weathering and a tectonically paced abiotic methane production at mid?ocean ridges can modulate the intensity of this climate change. For the Nuna epoch, the model predicts a relatively warm and ice?free climate related to mantle dynamics potentially consistent with the intense anorogenic magmatism of this period.
Abstract: A time-series analysis of thermobaric ratios (temperature/pressure [T/P]) for Paleoarchean to Cenozoic metamorphic rocks identified significant shifts in mean T/P that may be related to secular change in the geodynamics on Earth. Thermobaric ratios showed significant (>95% confidence) change points at 1910, 902, 540, and 515 Ma, recording drops in mean T/P, and at 1830, 604, and 525 Ma, recording rises in mean T/P. Highest mean T/P occurred during the Mesoproterozoic, and lowest mean T/P occurred from the Cambrian to the Oligocene. Correlated changes were seen between T/P and global data sets of time-constrained hafnium (Hf) and oxygen (O) isotope compositions in zircon. The range of correlated variation in T/P, Hf, and O was larger during the formation of Rodinia than Columbia. Large changes and a wide range for these variables continued through the Phanerozoic, during which a statistically significant 83 m.y. frequency of T/P excursions recorded the high tempo of orogenic activity associated with the separation, migration, and accretion of continental terranes during the formation of Pangea. Since the early Tonian, the decreasing mean T/P of metamorphism, widespread appearance of blueschist and ultrahigh-pressure metamorphism, and wide fluctuations in Hf and O isotope compositions document a change to the modern plate-tectonic regime, characterized by widespread continental subduction and deeper slab breakoff than in the Proterozoic.
Journal of Geophysical Research Solid Earth, 10.1029/2019JB0 17442 28p. Pdf
Global
geodynamics
Abstract: The absolute motion of tectonic plates since Pangea can be derived from observations of hotspot trails, paleomagnetism, or seismic tomography. However, fitting observations is typically carried out in isolation without consideration for the fit to unused data or whether the resulting plate motions are geodynamically plausible. Through the joint evaluation of global hotspot track observations (for times <80 Ma), first?order estimates of net lithospheric rotation (NLR), and parameter estimation for paleo-trench migration (TM), we present a suite of geodynamically consistent, data?optimized global absolute reference frames from 220 Ma to the present. Each absolute plate motion (APM) model was evaluated against six published APM models, together incorporating the full range of primary data constraints. Model performance for published and new models was quantified through a standard statistical analyses using three key diagnostic global metrics: root?mean square plate velocities, NLR characteristics, and TM behavior. Additionally, models were assessed for consistency with published global paleomagnetic data and for ages <80 Ma for predicted relative hotspot motion, track geometry, and time dependence. Optimized APM models demonstrated significantly improved global fit with geological and geophysical observations while performing consistently with geodynamic constraints. Critically, APM models derived by limiting average rates of NLR to ~0.05°/Myr and absolute TM velocities to ~27?mm/year fit geological observations including hotspot tracks. This suggests that this range of NLR and TM estimates may be appropriate for Earth over the last 220 Myr, providing a key step toward the practical integration of numerical geodynamics into plate tectonic reconstructions.
Earth and Planetary Science Letters, Vol. 537, 116171, 12p. Pdf
Mantle
geodynamics
Abstract: A better understanding of the Earth's compositional structure is needed to place the geochemical record of surface rocks into the context of Earth accretion and evolution. Cosmochemical constraints imply that lower-mantle rocks may be enriched in silica relative to upper-mantle pyrolite, whereas geophysical observations support whole-mantle convection and mixing. To resolve this discrepancy, it has been suggested that subducted mid-ocean ridge basalt (MORB) segregates from subducted harzburgite to accumulate in the mantle transition zone (MTZ) and/or the lower mantle. However, the key parameters that control basalt segregation and accumulation remain poorly constrained. Here, we use global-scale 2D thermochemical convection models to investigate the influence of mantle-viscosity profile, planetary-tectonic style and bulk composition on the evolution and distribution of mantle heterogeneity. Our models robustly predict that, for all cases with Earth-like tectonics, a basalt-enriched reservoir is formed in the MTZ, and a harzburgite-enriched reservoir is sustained at 660?800 km depth, despite ongoing whole-mantle circulation. The enhancement of basalt and harzburgite in and beneath the MTZ, respectively, are laterally variable, ranging from ?30% to 50% basalt fraction, and from ?40% to 80% harzburgite enrichment relative to pyrolite. Models also predict an accumulation of basalt near the core mantle boundary (CMB) as thermochemical piles, as well as moderate enhancement of most of the lower mantle by basalt. While the accumulation of basalt in the MTZ does not strongly depend on the mantle-viscosity profile (explained by a balance between basalt delivery by plumes and removal by slabs at the given MTZ capacity), that of the lowermost mantle does: lower-mantle viscosity directly controls the efficiency of basalt segregation (and entrainment) near the CMB; upper-mantle viscosity has an indirect effect through controlling slab thickness. Finally, the composition of the bulk-silicate Earth may be shifted relative to that of upper-mantle pyrolite, if indeed significant reservoirs of basalt exist in the MTZ and lower mantle.
Abstract: Determining whether magma fragments during eruption remains a seminal challenge in volcanology. There is a robust paradigm for fragmentation of high viscosity, silicic magmas, however little is known about the fragmentation behaviour of lower viscosity systems—the most abundant form of volcanism on Earth and on other planetary bodies and satellites. Here we provide a quantitative model, based on experiments, for the non-brittle, fluid dynamic induced fragmentation of low viscosity melts. We define the conditions under which extensional thinning or liquid break-up can be expected. We show that break-up, both in our experiments and natural eruptions, occurs by both viscous and capillary instabilities operating on contrasting timescales. These timescales are used to produce a universal break-up criterion valid for low viscosity melts such as basalt, kimberlite and carbonatite. Lastly, we relate these break-up instabilities to changes in eruptive behaviour, the associated natural hazard and ultimately the deposits formed.
Abstract: Earth is so far the only planet that exhibits plate tectonics, and along with the right heliocentric distance and the presence of surface water, plate tectonics is among necessary conditions for a habitable planet. Yet, the physics of this particular style of mantle convection is poorly understood, creating a substantial bottleneck in developing the general theory of planetary evolution. As plate tectonics is characterized by the subduction of oceanic lithosphere, a better understanding of the oceanic upper mantle could potentially help to break this stalemate. In this review, I summarize available theoretical, observational, and experimental constraints on the evolution of the oceanic upper mantle and its rheology, place the study of the oceanic upper mantle in the big picture of Earth evolution, and provide some suggestions for future research in relevant disciplines, including marine geophysics and computational geodynamics.
Earth and Planetary Science Letters, Vol. 547, 15p. Pdf
Mantle
geodynamics
Abstract: Mantle plumes provide valuable information about whole-mantle convection: they originate at the core-mantle boundary, cross Earth's mantle and interact with the lithosphere. For instance, it has been proposed that the mobility/stability of plumes depends on plume intrinsic properties, on how slabs interact with the basal boundary layer, on mantle flow, or on their proximity to mid-ocean ridges. Here, we use 3D-spherical models of mantle convection generating self-consistent plate-like behaviour to investigate the mechanisms linking tectonics and mantle convection to plume dynamics. Our models produce fully-dynamic mantle plumes that rise vertically with deflection and present excess temperatures, rising speeds, buoyancy and heat fluxes comparable to observations. In the absence of plate tectonics, plumes are stable and their lifetime exceeds hundreds of million years. With plate tectonics, plumes are more mobile, and we identify four physical mechanisms controlling their stability. 1/ Fixed plumes are located at saddle points of basal mantle flow. 2/ Plumes moving at speeds between 0.5-1 cm yr?1 are slowly entrained by passive mantle flow. 3/ Fast plume motions between 2-5 cm yr?1 lasting several tens of million years are caused by slab push. 4/ Plumes occasionally drift at speeds >5 cm yr?1 over <10 Myr through plume merging. We do not observe systematic anchoring of plumes to mid-oceanic ridges. Independent of the presence of a dense basal layer, plate-like regimes decrease the lifetime of plumes compared to stagnant-lid models. Plume age, temperature excess or buoyancy flux are not diagnostic of plume lateral speed. The fraction of plumes moving by less than 0.5 cm yr?1 is >25%, which suggests that fixed hotspot reference frames can be defined from carefully selected hotspot tracks.
ANAS Transactions, Earth Sciences, Vol. 2, pp. 3-32. pdf doi: 10.33677 /ggianas20200200043
Mantle
geodynamics
Abstract: Numerous attempts have been made to understand the rules of Earth’s tectono-geodynamic processes over the past centuries. While no paradigm has offered comprehensive answers to all of the questions, the present review aims to acquaint readers with the modern state of developments in the tectonic insights of Earth's evolution. A number of very interesting and unique processes and features took place during the evolution of early Earth. Most of these, however, were largely erased over the course of Earth’s ensuing evolution; some leaving only traces of their existence and some remnant phenomena, especially those taking place in the Hadean and Early to Late Archean. Among such processes and features are: the planetary accretion of Earth, formation of unique rock complexes, initiation of the plate tectonics phenomenon, main forces driving plate tectonics, significant influence of thermal parameters, role of overpressure under different physical-geological environments, stratification of Earth's crust and lithosphere by density, and various other thermodynamic models. Nearly all of these remain enigmatic, due to considerable uncertainty in the timing and methods of their evolution, and the ambiguity of their secondary processes and tectono-geophysical indicators. At the same time, majority of tectono-geodynamic processes and features are also interrelated, and the simultaneous fluctuation of myriad different factors played a significant role in their influence to the geological medium. Some of these intricate questions are discussed in this paper. For instance, what is the role of the plate tectonics phenomenon and when did this process initiate on Earth? Especial attention is paid in the review to the sophisticated methods of understanding tectonic processes over the course of various generations of geoscientists. In the conducted analyses, certain physical data derived from other planets of the Solar System were utilized as well.
Earth-Science Reviews, Vol. 210, doi.org/10.1016 /j.earscirev.2020. 103356 27p. Pdf
Mantle
UHP, geodynamics
Abstract: Eclogites are witnesses of geodynamic processes that are commonly related to subduction of oceanic crust. Information on the part of these processes that refers to the burial of this rock type is rarely published but stored in the eclogitic garnet core and inclusions therein. To better understand general aspects of the burial process, a literature search on the chemical characteristics of garnet in worldwide occurrences of eclogite was undertaken. In most cases extended garnet cores show either a prograde growth zoning with increasing Mg, starting at a few percent of pyrope component, and decreasing Mn contents (type I eclogite) or a (nearly) constant chemical composition frequently with pyrope contents significantly above 10 percent (eclogites of types II and III). Only in minor cases, it is difficult to assign the reported garnet core to an eclogite type. The growth zoning of garnet was thermodynamically modelled for the chemical composition of a basalt following different burial paths. These paths are characterized either by a trajectory along a low geothermal gradient (type I eclogite), as expected for the subducting upper portion of oceanic crust, or a one characterized by nearly isothermal burial at temperatures above 500 °C reaching peak pressures up to 2.1 GPa (type III eclogite), as possibly due to crustal thickening during continent-continent collision, or more (type II eclogite) when basic rocks are tectonically eroded from the overriding continental plate before deep subduction. In addition, diffusion modelling was undertaken on mm-sized garnet demonstrating that the characteristics of the core zoning are not fully obliterated even during residence at temperatures of 800-850 °C within 10 million years. The scrutiny of more than 200 eclogites reported in the literature led to the following result: about half of them are type II eclogites; a third and a sixth can be related to type I and type III, respectively. Among type III are almost all of the few Proterozoic eclogites considered. To demonstrate the benefit of our study, we link the core zoning of eclogitic garnet from various (ultra)high-pressure terranes in Phanerozoic orogenic belts to the geodynamics shaping corresponding orogens. The eclogites in these belts are dominated by type II. Thus, we propose that some of the material of the lower portion of the overriding continental crust was tectonically eroded by a subducted oceanic plate and brought to great depth. Afterwards, this material was exhumed first in a deep subduction channel and then in an exhumation channel during continent-continent collision where a contact with the upper continental plate was re-established. Furthermore, we suggest that type II eclogite can also occur in extrusion wedges as far as oblique subduction took place.
Abstract: Many Archean terranes are interpreted to have a tectonic and metamorphic evolution that indicates intra-crustal reorganization driven by lithospheric-scale gravitational instabilities. These processes are associated with the production of a significant amount of felsic and mafic crust, and are widely regarded to be a consequence of plume-lithosphere interactions. The juvenile Archean felsic crust is made predominantly of rocks of the tonalite-trondhjemite-granodiorite (TTG) suite, which are the result of partial melting of hydrous metabasalts. The geodynamic processes that have assisted the production of juvenile felsic crust, are still not well understood. Here, we perform 2D and 3D numerical simulations coupled with the state-of-the-art of petrological thermodynamical modelling to study the tectonic evolution of a primitive Archean oceanic plateau with particular regard on the condition of extraction of felsic melts. In our numerical simulations, the continuous emplacement of new, dry mafic intrusions and the extraction of the felsic melts, generate an unstable lower crust which drips into the mantle soon after the plume arrival. The subsequent tectonic evolution depends on the asthenosphere TP. If the TP is high enough (? 1500 ?C) the entire oceanic crust is recycled within 2 Myrs. By contrast at low TP, the thin oceanic plateau slowly propagates generating plate-boundary like features.
Geophysical Research abstracts, EGU, EGU2019-9348, 1p. Pdf
Canada
geodynamics
Abstract: We present the first oxidation state measurements for the subcontinental lithospheric mantle (SCLM) beneath the Rae craton, northern Canada, one of the largest components of the Canadian shield. In combination with major and trace element compositions for garnet and clinopyroxene, we assess the relationship between oxidation state and metasomatic overprinting. The sample suite comprises peridotite xenoliths from the central part (Pelly Bay) and the craton margin (Somerset Island) providing insights into lateral and vertical variations in lithospheric character. Our suite contains spinel, garnet-spinel and garnet peridotites, with most samples originating from 100 to 140 km depth. Within this narrow depth range we observe strong chemical gradients, including variations in oxygen fugacity (ƒO2) of over 4 log units. Both Pelly Bay and Somerset Island peridotites reveal a change in metasomatic type with depth. Observed geochemical systematics and textural evidence support the notion that Rae SCLM developed through amalgamation of different local domains, establishing chemical gradients from the start. These gradients were subsequently modified by migrating melts that drove further development of different types of metasomatic overprinting and variable oxidation at a range of length scales. This oxidation already apparent at ~?100 km depth could have locally destabilised any pre-existing diamond or graphite.
Lithos, https://doi.org/ 10.1016/j.lithos. 2021.106335, 4p. Pdf
Mantle
geodynamics
Abstract: The issue is devoted to new data on composition, structure and dynamics of the Earth’s lower mantle. The Earth’s lower mantle is the largest portion of our planet, comprising more than 50 % of its volume, but major questions remain as to chemical composition, thermal regime and global heterogeneity, as well as to its role in controlling mantle dynamic processes. The composition of the lower mantle is now understood to be more complex than had been suggested in theoretic and experimental works based solely on ‘pyrolitic’ compositions. Little is known about lower-mantle mineralogy and phase chemistry, especially at greater depth. Recent studies of inclusions in so-called superdeep diamonds have revealed a range of mineral associations having their specific geochemical features. Aside from a few inclusions found in diamonds, the mineralogy of the lower mantle is, for the most part, inferred from the results of theoretic and experimental data resembling natural environments. This volume presents new results on all aspects of composition of lower mantle including high pressure measurements, petrology and geochemistry, as well as numerical modelling of both the propagation of seismic waves and the geochemical composition and physical state of the deepest mantle rocks.
Geological Society of London Special Publications, doi:10.1144/SP513-2021-60
geodynamics
Abstract: Our pilot study reveals potential fingerprints of Li isotopes recorded in the Mesoproterozoic (?1.4-1.1 Ga) kimberlites, lamproites and lamprophyres from the Eastern Dharwar Craton and Paleocene (62 Ma) orangeite from the Bastar Craton in India. The new data are interpreted in the context of available Li isotope composition of lamproitic to lamprophyric rocks occurring in Variscan (Bohemian Massif) and Alpine-Himalayan (SW Tibet) orogenic belts formed in response to Gondwana-Pangea amalgamation and break-up. As a result of supercontinents development, kimberlites from the Eastern Dharwar Craton and ‘orangeite’ from the Bastar Craton show clear presence of a component with a heavy Li isotope signature (?7Li up to 9.7‰) similar to an ancient altered oceanic crust, whereas the Eastern Dharwar Craton lamproites (2.3-6.3‰) and lamprophyres (3.3-6.7‰) show Li isotope signatures indicative of a dominant contribution from heterogeneous lithospheric mantle. Variscan lamprophyric to lamproitic rocks and post-collisional mantle-derived (ultra)potassic volcanic rocks from SW Tibet, i.e., rocks from the orogenic belts outside the cratonic areas, are characterized by a clear Li isotope shift towards isotopically lighter component (?7Li as low as -9.5‰) comparable with the involvement of an evolved continental crust and high-pressure metamorphic rocks in their orogenic mantle source. Such components with isotopically light Li are strikingly missing in the source of cratonic kimberlites, lamproites and lamprophyres.
Abstract: Many Archean cratons exhibit Paleoproterozoic rifted margins, implying they were pieces of some ancestral landmass(es). The idea that such an ancient continental assembly represents an Archean supercontinent has been proposed but remains to be justified. Starkly contrasting geological records between different clans of cratons have inspired an alternative hypothesis where cratons were clustered in multiple, separate "supercratons". A new ca. 2.62 Ga paleomagnetic pole from the Yilgarn craton of Australia is compatible with either two successive but ephemeral supercontinents or two long-lived supercratons across the Archean-Proterozoic transition. Neither interpretation supports the existence of a single, long-lived supercontinent, suggesting that Archean geodynamics were fundamentally different from subsequent times (Proterozoic to present), which were influenced largely by supercontinent cycles.
Abstract: Plate reconstruction studies show that the Neotethys Ocean was closing due to the convergence of Africa and Eurasia toward the end of the Cretaceous. The period around 75 Ma reflects the onset of continental collision between the two plates as convergence continued to be taken up mostly by subduction of the Neotethys slab beneath Eurasia. The Owen transform plate boundary in the northeast accommodated the fast northward motion of the Indian plate relative to the African plate. The rest of the plate was surrounded by mid-ocean ridges. Africa was experiencing continent-wide rifting related to northeast-southwest extension. We aim to quantify the forces and paleostresses that may have driven this continental extension. We use the latest plate kinematic reconstructions in a grid search to estimate horizontal gravitational stresses (HGSs), plate boundary forces, and the plate's interaction with the asthenosphere. The contribution of dynamic topography to HGSs is based on recent mantle convection studies. We model intraplate stresses and compare them with the strain observations. The fit to observations favors models where dynamic topography amplitudes are smaller than 300 m. The results also indicate that the net pull transmitted from slab to the surface African plate was low. To put this into context, we notice that available tectonic reconstructions show fragmented subduction zones and various colliding micro-continents along the northern margin of the African plate around this time. We therefore interpret a low net pull as resulting from either a small average slab length or from the micro-continents' resistance to subduction.
Earth Science Reviews , Vol. 211, 103402, 23p. Pdf
Mantle
geodynamics
Abstract: In the last few decades, advanced monitoring networks have been extended to the main active volcanoes, providing warnings for variations in volcano dynamics. However, one of the main tasks of modern volcanology is the correct interpretation of surface-monitored signals in terms of magma transfer through the Earth's crust. In this frame, it is crucial to investigate decompression-induced magma degassing as it controls magma ascent towards the surface and, in case of eruption, the eruptive style and the atmospheric dispersal of tephra and gases. Understanding the degassing behaviour is particularly intriguing in the case of poorly explored evolved alkaline magmas. In fact, these melts frequently feed hazardous, highly explosive volcanoes (e.g., Campi Flegrei, Somma-Vesuvius, Colli Albani, Tambora, Azores and Canary Islands), despite their low viscosity that usually promotes effusive and/or weakly explosive eruptions. Decompression experiments, together with numerical models, are powerful tools to examine magma degassing behaviour and constrain field observations from natural eruptive products and monitoring signals. These approaches have been recently applied to evolved alkaline melts, yet numerous open questions remain. To cast new light on the degassing dynamics of evolved alkaline magmas, in this study we present new results from decompression experiments, as well as a critical review of previous experimental works. We achieved a comprehensive dataset of key petrological parameters (i.e., 3D textural data for bubbles and microlites using X-ray computed microtomography, glass volatile contents and nanolite occurrence) from experimental samples obtained through high temperature-high pressure isothermal decompression experiments on trachytic alkaline melts at super-liquidus temperature. We explored systematically a range of final pressures (from 200 to 25 MPa), decompression rates (from 0.01 to 1 MPa s?1), and volatile (H2O and CO2) contents. On these grounds, we integrated coherently literature data from decompression experiments on evolved alkaline (trachytic and phonolitic) melts under various conditions, with the aim to fully constrain the degassing mechanisms and timescales in these magmas. Finally, we simulated numerically the experimental conditions to evaluate strengths and weaknesses in decrypting degassing behaviour from field observations. Our results highlight that bubble formation in evolved alkaline melts is primarily controlled by the initial volatile (H2O and CO2) content during magma storage. In these melts, bubble nucleation needs low supersaturation pressures (? 50-112 MPa for homogeneous nucleation, ? 13-25 MPa for heterogeneous nucleation), resulting in high bubble number density (~ 1012-1016 m?3), efficient volatile exsolution and thus in severe rheological changes. Moreover, the bubble number density is amplified in CO2-rich melts (mole fraction XCO2 ? 0.5), in which continuous bubble nucleation predominates on growth. These conditions typically lead to highly explosive eruptions. However, moving towards slower decompression rates (? 10?1 MPa s?1) and H2O-rich melts, permeable outgassing and inertial fragmentation occur, promoting weakly explosive eruptions. Finally, our findings suggest that the exhaustion of CO2 at deep levels, and the consequent transition to a H2O-dominated degassing, can crucially enhance magma vesiculation and ascent. In a hazard perspective, these constraints allow to postulate that time-depth variations of unrest signals could be significantly weaker/shorter (e.g., minor gas emissions and short-term seismicity) during major eruptions than in small-scale events.
South African Journal of Geology, Vol. 124, pp. 519-536.
Africa, South Africa
geodynamics
Abstract: The delicate interplay of various Earth’s systems processes in the Critical Zone is vital in ensuring an equilibrium across the different spheres of life. The upper crust forms a thin veneer on the Earth’s surface that is defined by an interconnected network of brittle structures. These brittle structures enable various Earth System processes. Increased anthropogenic interactions within the very upper crust have seemingly resulted in a growing number of negative natural effects, including induced seismicity, mine water drainage and land degradation. Brittle structures across South Africa are investigated. These structures include various fractures and dykes of different ages and geodynamic evolutions. The orientation of these structures is compared to the underlying tectonic domains and their bounding suture zones. The orientations corroborate an apparent link between the formation of the brittle structures and the tectonic evolution of the southern African crust. Reactivation and the creation of new structures are also apparent. These are linked to the variability of the surrounding stress field and are shown to have promoted magmatism, e.g., Large Igneous Provinces, and the movement of hydrothermal fluids. These fluids were commonly responsible for the formation of important mineral deposits. The preferred structural orientations and their relationship to underlying tectonic zones are also linked to fractured groundwater aquifers. Subsurface groundwater displays a link to structural orientations. This comparison is extended to surficial water movement. Surface water movement also highlights an apparent link to brittle structures. The apparent correlation between these Earth’s systems processes and the interconnectivity developed by brittle structures are clear. This highlights the importance of high-resolution geological and structural mapping and linking this to further development of the Earth’s Critical Zone.
Abstract: Mg2GeO4 is important as an analog for the ultrahigh-pressure behavior of Mg2SiO4, a major component of planetary interiors. In this study, we have investigated magnesium germanate to 275 GPa and over 2,000 K using a laser-heated diamond anvil cell combined with in situ synchrotron X-ray diffraction and density functional theory (DFT) computations. The experimental results are consistent with the formation of a phase with disordered Mg and Ge, in which germanium adopts eightfold coordination with oxygen: the cubic, Th3P4-type structure. DFT computations suggest partial Mg-Ge order, resulting in a tetragonal I4¯2d structure indistinguishable from I4¯3d Th3P4 in our experiments. If applicable to silicates, the formation of this highly coordinated and intrinsically disordered phase may have important implications for the interior mineralogy of large, rocky extrasolar planets.
Researchgate preprint Chapter from book Elsevier, March 61p. Pdf
Mantle
geodynamics
Abstract: Isolating planetary feedbacks, and feedback analysis, are prevalent aspects of climate and Earth surface process science. An under appreciation of internal planet feedbacks, and feedback analysis for plate tectonics research, motivate this chapter. We review feedbacks that influence the Earth's thermal evolution and expand them to include magmatic history and planetary water budgets. The predictions from feedback models are shown to be consistent with petrological constraints on the Earth's cooling. From there, we isolate feedbacks that connect structural elements within the mantle dynamics and plate tectonics system. The feedbacks allow for a reciprocal causality between plates, plumes, the asthenosphere, and mantle flow patterns, with each element being co-dependent on the others. The linked elements and feedbacks define plate tectonics are part of a self-sustaining flow system that can bootstrap itself into existence. Within that framework, plate tectonics involves the co-arising of critical system factors. No single factor is the cause of another. Rather, they emerge with the links between them and the generation of functional elements coincides, within relatively narrow time windows, with the co-emergence of factors that are critical for the maintenance of the elements themselves. What emerges is not a tectonic state but a process. That is, a set of feedbacks that can transform the tectonics of a planet and/or maintain plate tectonics. The feedback functions are not permanent but can operate over extended time frames such that plate tectonics can remain stable. The nature of the feedbacks, and their stability, can be studied at various levels of detail but questions of origin can become ill-defined. Observational tests of a feedback framework for plate tectonics and mantle dynamics are presented, along with research paths that apply feedback methodology to solid planet dynamics and comparative planetology.
