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The Sheahan Diamond Literature Reference Compilation - Scientific and Media Articles based on Major Keyword - Geophysics
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.
In the resource sector the term Geophysics is treated as short-hand for the various methods used to measure physical aspects of the earth, collected under the umbrella term "geophysical surveys". But as the Wikipedia entry reveals, "geophysics" is a scientific term that refers to all the physical processes and nature of the earth as well as the methods used to measure them. Articles tagged with the key word "geophysics" are always of a scientific or technical nature. Sometimes they are about the measurement technology itself, but most of the time they are about what geophysical methods have revealed about the earth ranging from the large scale structure of the earth's mantle to the small scale structure of a kimberlite pipe. Some of the articles are about the process that resulted in whatever physical state exists at different scales, which seems hard to imagine as possible. Yet one of the most fascinating goals of academic geology is to conjure a transformational history of the earth. Many other obscure key words such as "geochronology", "rheology" and "tomography" show up in articles. They are relevant to diamonds when they describe the conditions conducive to diamond formation, and when they describe the pipe or dyke like intrusions that can host diamonds.
Experiments in Interpretation of the Regional Magnetic Field and the Relation of Magnetized Bodies to Tectonics in the Diamond Fields of Western Yakutia.
In: Symposium Primeneniye Aeromoetodoy Pri Poiskakh Korennykh Me, PP. 101-114.
Some Information on the Resolution Capacity of Aeromagnetic surveys As Applied to the Search for Anomalies Created by Bodies of Tubular Shape Under Complex Geologic Conditions.
Soviet Geology GEOPHYSICS, Vol. 15, No. 7, PP. 103-107.
Use of Dipole Electric-magnetic Sounding in Determining The thickness of Alluvium During Exploration for Beach Diamond Placers in the Region of Anabar Bay.
Methods for the Study, Processing and Interpretation of Magnetic and Gravimetric Fields in Prospecting for Kimberlite Bodies in Traps. Methodological Recommendations.
The Relationship of Geophysical and Remote Sensing Lineaments to Regional Structure and Kimberlite Intrusions in the Appalachian Plateau of Pennsylvania.
Geological Society of America (GSA), Vol. 11, No. 1, P. 48, (abstract.).
The Aeromagnetic Delineation of the Distribution Patterns Of Karroo Volcanic in Botswana and Consequent Implications For the Tectonics of the Sub Continent.
Botswana Geological Survey, Bulletin. No. 22, PP. 93-140.
Geothermal Studies in the San Juan Basin and the Four Corners Area of the Colorado Plateau. Ii Steady State Models of The Thermal Source San Juan Volcanic Field.
Latest Age Dating Results in South Central Kansas: Incompatible with History of Precambrian Events As Reconstructed From Newmag* Basement Mapping Studies.
6th. International Conference Basement Tectonics, Held Sante Fe, Septem, P. 17. (abstract.).
Constraints on lithospheric structure from satellite potential field at:Africa and Asia. Analysis and interpretation of Magsat anomalies over NorthAfrica
National Technical Information Service, No. N86-21968/0 104p
The nature of faulting along the margins of the Fitzroy trough, CanningBasin, and implications for the tectonic development of the trough
Australian Society of Exploration Geophysicists and Geological Society of Australia, 8th. Exploration Conference in the Bulletin., Vol. 22, No. 1, March pp. 111-116
Tectonic imbrication and foredeep development in the Penokean Orogen, east-central Minnesota-interpret. on regional geophysics and the results of testdrilling
United States Geological Survey (USGS) Bulletin, No. 1904 C, 15p
The Canadian Mining and Metallurgical Bulletin (CIM Bulletin) , Annual Meeting Abstracts approximately 10 lines, Vol. 86, No. 968, March ABSTRACT p. 70
CAT scanning the earth... supercomputers are enabling geophysicists to visualize the earth's churning interior - and gain new insights into how itworks
The Canadian Mining and Metallurgical Bulletin (CIM Bulletin) , Annual Meeting Abstracts approximately 10 lines, Vol. 86, No. 968, March POSTER ABSTRACT p. 69
Release of multiparameter profile line dat a for an electromagnetic/magnetic survey in the Blake River Syncline area, Cochrane and Timiscaming Districts
Exploration for kimberlites: a review of geophysical exploration methodswith examples from Point Lake, northwest Territories using an optimized GEOTEM.
Geoterrex Handout PDA Conference March 30, 1993, 23p
Discovery of kimberlites Kirkland Lake area(Part III) - a decisive contribution by aeromagnetic dat a analysis in conditions of deep overburden.
Mid-continent diamonds Geological Association of Canada (GAC)-Mineralogical Association of Canada (MAC) Symposium ABSTRACT volume, held Edmonton May, pp. 31-36.
The application of geophysics to exploration for diamonds
Mid-continent diamonds Geological Association of Canada (GAC)-Mineralogical Association of Canada (MAC) Symposium ABSTRACT volume, held Edmonton May, pp. 21-26.
Prospectors and Developers Diamond Workshop, held March 27th, Toronto, 14p. Inc, article by Reed The Canadian Institute of Mining, Metallurgy and Petroleum (CIM) Vol. 84, 947, March91
The aeromagnetic survey program of the Geological Society of Canada (GSC)- implications for kimberliteexploration.
Mid-continent diamonds Geological Association of Canada (GAC)-Mineralogical Association of Canada (MAC) Symposium ABSTRACT volume, held Edmonton May, pp. 27-30.
High resolution maps of the shaded residual total magnetic field and shaded magnetic first derivative with Keating coefficients Boothia Peninsula, Nunavut.
Geological Survey of Canada Open File, OF 4897-4918 $ 15.00 22 maps @ 15.00 each
Upper mantle temperature determined from combining mineral composition, electrical conductivity laboratory studies and magnetotelluric field observations:
Earth and Planetary Science Letters, Vol. 236, 1-2, pp. 258-268.
Canada, British Columbia, Alberta, Yukon, Cordillera
Upper mantle temperature determined from combining mineral composition, electrical conductivity laboratory studies and magnetotelluric field observations.
Earth and Planetary Science Letters, Advanced in press,
Paleomagnetism and U-Pb geochronology of the Lac de Gras diabase dyke swarm, Slave Province Canada: implications for relative drift of Slave and Superior provinces in the Paleoproterozoic.
Canadian Journal of Earth Sciences, Vol. 46, pp. 361-379.
The application of airborne, ground and borehole geophysics to the exploration for rare earth elements associated with the Lofdal carbonatite complex, Namibia
Peralk-Carb 2011... workshop June 16-18, Tubingen, Germany, Abstract p.69. (very brief no info)
The application of airborne, ground and borehole geophysics to the exploration for rare earth elements associated with the Lofdal carbonatite complex, Namibia
Peralk-Carb 2011... workshop June 16-18, Tubingen, Germany, Abstract p.69. (very brief no info)
A structural analysis of the geophysical signature relationship between linear features and plug like bodies of sheets 2229AB and 2229AD in the Limpopo province, South Africa.
Integration of structural, gravity and magnetic dat a using the weights of evidence method as a tool for kimberlite exploration in the Buffalo Head Hills, northern central Alberta Canada
Devriese, S.G.R., Corcoran, N., Cowan, D., Davis, K., Bild-Enkin, D., Fournier, D., Heagy, L., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three papers. In the first, we find a 3D magnetic susceptibility model for the area; in the second, we find a 3D conductivity model; and in the third paper, we find a 3D chargeability model. Our goal is to explain all the geophysical results within a geologic framework. In this first paper, we invert three independent airborne magnetic data sets flown over the Tli Kwi Cho kimberlite complex located in the Lac de Gras kimberlite field in Northwest Territories, Canada. The complex consists of two kimberlites known as DO-27 and DO-18. An initial airborne DIGHEM survey was flown in 1992 and AeroTEM and VTEM data subsequently acquired in 2003 and 2004, respectively. In this paper, we invert each magnetic data set in three dimensions. Both kimberlites are recovered in each model, with DO-27 as a more susceptible body than DO-18. Our goal is to simultaneously invert the three data sets to generate a single susceptibility model for Tli Kwi Cho. This project is part of a larger, on-going investigation by UBC-GIF on inverting magnetic, electromagnetic, and induced polarization data from the Tli Kwi Cho area.
Fournier, D., Heagy, L., Corcoran, N., Cowan, D., Devriese, S.G.R., Bild-Enkin, D., Davis, K., Kang, S., Marchant, D., McMillan, M.S., Mitchell, M., Rosenkjar, G., Yang, D., Oldenburg, D.W.
Abstract: The magnetic and electromagnetic responses from airborne systems at Tli Kwi Cho, a kimberlite complex in the Northwest Territories, Canada, have received considerable attention over the last two decades but a complete understanding of the causative physical properties is not yet at hand. Our analysis is distributed among three posters. In the first we find a 3D magnetic susceptibility model for the area; in the second we find a 3D conductivity model; and in the third we find a 3D chargeability model that can explain the negative transient responses measured over the kimberlite pipes. In this second paper we focus upon the task of finding a conductivity model that is compatible with three airborne data sets flown between 1992 and 2004: one frequency-domain data set (DIGHEM) and two time-domain systems (AeroTEM and VTEM). The goal is to obtain a 3D model from which geologic questions can be answered, but even more importantly, to provide a background conductivity needed to complete the 3D IP inversion of airborne EM data. We begin by modifying our pre-existing 1D frequency and time domain inversion codes to produce models that have more lateral continuity. The results are useful in their own right but we have also found that 1D analysis is often very effective in bringing to light erroneous data, assisting in estimating noise floors, and providing some starting information for developing a background model for the 3D EM inversion. Here we show some results from our Laterally Constrained Inversion (LCI) framework. The recovered conductivity models seem to agree on the general location of the kimberlite pipes but disagree on the geometry and conductivity values at depth. The complete 3D inversions in time and frequency, needed to resolved these issues, are currently in progress.
Journal of Volcanology and Geothermal Research, Vol. 310, pp. 1-11.
China
LiDAR
Abstract: Dyke geometries are useful indicators of the palaeostress field during magma emplacement. In this paper, we present a multi-scale extraction method of dyke geometries by integrating WorldView-2 (WV2) imagery and terrestrial light detection and ranging (LiDAR) data. Color composite and fusion WV2 images with 0.5-m resolution were generated by using the Gramm-Schmidt Spectral Sharpening approach, which facilitates the discrimination of dyke swarms and provides the ability to measure the orientation, exposed length, and thickness of dykes in sub-horizontal topographic exposures. A terrestrial laser scanning survey was performed on a sub-vertical exposure of dykes to obtain LiDAR data with point spacing of ~ 0.02 m at 30 m. The LiDAR data were transformed to images for extracting dyke margins based on image segmentation, then the dyke attitudes, thicknesses, and irregularity of dyke margins were measured according to the points on dyke margins. This method was applied at Sijiao Island, Zhejiang, China where late Cretaceous mafic dyke swarms are widespread. The results show that integrating WV2 imagery and terrestrial LiDAR improves the accuracy, efficiency, and objectivity in determining dyke geometries in two and three dimensions. The ENE striking dykes are dominant, and intruded the host rock (mainly granite) with sub-vertical dips. Based on the aspect ratios of the dykes, the magmatic overpressure was estimated to be less than 11.5 MPa, corresponding to a magma chamber within 6.6 km in the lithosphere.
Enigmatic relationship between silicic and volcanic and plutonic rocks: geophysical evidence for silicic crustal melt in the continents: where. What kind, and how much?
Geophysical Prospecting, Vol. 64, 5, pp. 1275-1304.
Geophysics
Abstract: Igneous intrusions, notably carbonatitic-alkalic intrusions, peralkaline intrusions, and pegmatites, represent significant sources of rare-earth metals. Geophysical exploration for and of such intrusions has met with considerable success. Examples of the application of the gravity, magnetic, and radiometric methods in the search for rare metals are presented and described. Ground gravity surveys defining small positive gravity anomalies helped outline the shape and depth of the Nechalacho (formerly Lake) deposit within the Blatchford Lake alkaline complex, Northwest Territories, and of spodumene-rich mineralization associated with the Tanco deposit, Manitoba, within the hosting Tanco pegmatite. Based on density considerations, the bastnaesite-bearing main ore body within the Mountain Pass carbonatite, California, should produce a gravity high similar in amplitude to those associated with the Nechalacho and Tanco deposits. Gravity also has utility in modelling hosting carbonatite intrusions, such as the Mount Weld intrusion, Western Australia, and Elk Creek intrusion, Nebraska. The magnetic method is probably the most successful geophysical technique for locating carbonatitic-alkalic host intrusions, which are typically characterized by intense positive, circular to sub-circular, crescentic, or annular anomalies. Intrusions found by this technique include the Mount Weld carbonatite and the Misery Lake alkali complex, Quebec. Two potential carbonatitic-alkalic intrusions are proposed in the Grenville Province of Eastern Quebec, where application of an automatic technique to locate circular magnetic anomalies identified several examples. Two in particular displayed strong similarities in magnetic pattern to anomalies accompanying known carbonatitic or alkalic intrusions hosting rare-metal mineralization and are proposed to have a similar origin. Discovery of carbonatitic-alkalic hosts of rare metals has also been achieved by the radiometric method. The Thor Lake group of rare-earth metal deposits, which includes the Nechalacho deposit, were found by follow-up investigations of strong equivalent thorium and uranium peaks defined by an airborne survey. Prominent linear radiometric anomalies associated with glacial till in the Canadian Shield have provided vectors based on ice flow directions to source intrusions. The Allan Lake carbonatite in the Grenville Province of Ontario is one such intrusion found by this method. Although not discovered by its radiometric characteristics, the Strange Lake alkali intrusion on the Quebec-Labrador border is associated with prominent linear thorium and uranium anomalies extending at least 50 km down ice from the intrusion. Radiometric exploration of rare metals hosted by pegmatites is evaluated through examination of radiometric signatures of peraluminous pegmatitic granites in the area of the Tanco pegmatite.
Abstract: Current models for unconformity-associated uranium deposits predict fluid flow and ore deposition along reactivated faults in >1.76 Ga basement beneath Mesoproterozoic siliciclastic basins. In frontier regions such as the Thelon Basin in the Kivalliq region of Nunavut, little is known about the sub-basin distribution of units and structures, making exploration targeting very tenuous. We constructed a geological map of the basement beneath the unconformity by extrapolating exposed features into the subsurface. The new map is constrained by detailed geological, geophysical, and rock property observations of outcrops adjacent to the basin and by aeromagnetic and gravity data over the geophysically transparent sedimentary basin. From rock property measurements, it is clear that the diverse magnetic and density characteristics of major rock packages provide quantitative three-dimensional constraints. Gravity profiles forward modelled in four cross sections define broad synforms of the Amer Belt and Archean volcanic rocks that are consistent with the structural style outside the basin. Major lithotectonic entities beneath the unconformity include: supracrustal rocks of the Archean Woodburn Lake group and Marjorie Hills meta sedimentary gneiss and associated mixed granitoid and amphibolitic gneiss; the Amer Mylonite Zone and inferred mafic intrusions oriented parallel and sub-parallel; other igneous intrusions of 2.6 Ga, 1.83 Ga, and 1.75 Ga vintage; and the <2.3 Ga to >1.84 Ga Amer Group. Four main brittle regional fault arrays (040°-060°, 075°-90°, 120°, and 150°) controlled development and preservation of the basin. The reactivated intersections of such faults along fertile basement units such as the Rumble assemblage, Marjorie Hills assemblage, Nueltin igneous rocks, and Pitz formation are the best targets for uranium exploration.
Exploration Geophysics , http://dx.doi.org/10.1071/EG16015 10p. Available
Russia
Deposit Amakinskaya, Geophysics
Abstract: There have been multiple occurrences in the literature in the past several years of what has been referred to as the induced polarisation (IP) effect in airborne time domain electromagnetic (TDEM) data. This phenomenon is known to be responsible for incorrect inversion modelling of electrical resistivity, lower interpreted depth of investigation (DOI) and lost information about chargeability of the subsurface and other valuable parameters. Historically, there have been many suggestions to account for the IP effect using the Cole-Cole model. It has been previously demonstrated that the Cole-Cole model can be effective in modelling synthetic TDEM transients. In the current paper we show the possibility of extracting IP information from airborne TDEM data using this same concept, including inverse modelling of chargeability from TDEM data collected by VTEM, with field examples from Canada (Mt Milligan deposit) and Russia (Amakinskaya kimberlite pipe).
Abstract: The concept of a mineral deposit forming via a mineral system that operates across areas of perhaps 1000s of squares kilometres and to mantle depths has important implications for greenfields mineral exploration. Geographically widespread datasets and deep penetrating geophysical methods are required to map key mineral system elements such as fluid/metal source zones and migration paths. Developed primarily for academic studies of the deep crust, there are several established geophysical techniques that can potentially be used to identify elements of mineral systems in the deep crust and upper mantle. Although the seismic reflection method produces the highest quality images, it is prohibitively expensive and the recommended approach is a combination of MT surveys and receiver function recordings with CCP stacking. Mineral system elements that can be detected in this fashion include major structures and geological boundaries which are potential controls on fluid flow and also areas of crust and mantle that have been altered by one or both of fluid creation and migration.
Journal of Geophysical Research, Vol. 121, 9, pp. 6675-6697.
United States, Canada
Geophysics
Abstract: The observation of widespread seismic discontinuities within Archean and Proterozoic lithosphere is intriguing, as their presence may shed light on the formation and early evolution of cratons. A clear explanation for the discontinuities, which generally manifest as a sharp decrease in seismic velocity with depth, remains elusive. Recent work has suggested that midlithospheric discontinuities (MLDs) may correspond to a sharp gradient in seismic anisotropy, produced via deformation associated with craton formation. Here we test this hypothesis beneath the Archean Superior and Wyoming Provinces using anisotropic Ps receiver function (RF) analysis to characterize the relationship between MLDs and seismic anisotropy. We computed radial and transverse component RFs for 13 long-running seismic stations. Of these, six stations with particularly clear signals were analyzed using a harmonic regression technique. In agreement with previous studies, we find evidence for multiple MLDs within the cratonic lithosphere of the Wyoming and Superior Provinces. Our harmonic regression results reveal that (1) MLDs can be primarily explained by an isotropic negative velocity gradient, (2) multiple anisotropic boundaries exist within the lithospheric mantle, (3) the isotropic MLD and the anisotropic boundaries do not necessarily occur at the same depths, and (4) the depth and geometry of the anisotropic boundaries vary among stations. We infer that the MLD does not directly correspond to a change in anisotropy within the mantle lithosphere. Furthermore, our results reveal a surprising level of complexity within the cratonic lithospheric mantle, suggesting that the processes responsible for shaping surface geology produce similar structural complexity at depth.
International Journal of Remote Sensing, Vol. 38, 3, pp. 827-849.
Technology
LIDAR
Abstract: In recent years lidar technology has experienced a noticeable increase in its relevance and usage in a number of scientific fields. Therefore, software capable of handling lidar data becomes a key point in those fields. In this article, we present GPU-based viewer lidar (GVLiDAR), a novel web framework for visualization and geospatial measurement of lidar data point sets. The design of the framework is focused on achieving three key objectives: performance in terms of real-time interaction, functionality, and online availability for the lidar datasets. All lidar files are pre-processed and stored in a lossless data structure, which minimizes transfer requirements and offers an on-demand lidar data web framework.
Abstract: Induced polarization (IP) effects are becoming more evident in time-domain helicopter airborne electromagnetic (AEM) data thanks to advances in instrumentation, mainly due to improvements in the signal-to-noise ratio and hence better data quality. Although the IP effects are often manifested as negative receiver voltage values, which are easy to detect, in some cases, IP effects can distort recovered transients in other ways so they may be less obvious and require careful data analysis and processing. These effects represent a challenge for modeling and inversion of the AEM data. For proper modeling of electromagnetic transients, the chargeability of the subsurface and other parameters describing the dispersion also need to be taken into consideration. We use the Cole-Cole model to characterize the dispersion and for modeling of the IP effects in field AEM data, collected by different airborne systems over different geologies and exploration targets, including examples from diamond, gold, and base metal exploration.
Abstract: Current models for unconformity-associated uranium deposits predict fluid flow and ore deposition along reactivated faults in >1.76 Ga basement beneath Mesoproterozoic siliciclastic basins. In frontier regions such as the Thelon Basin in the Kivalliq region of Nunavut, little is known about the sub-basin distribution of units and structures, making exploration targeting very tenuous. We constructed a geological map of the basement beneath the unconformity by extrapolating exposed features into the subsurface. The new map is constrained by detailed geological, geophysical, and rock property observations of outcrops adjacent to the basin and by aeromagnetic and gravity data over the geophysically transparent sedimentary basin. From rock property measurements, it is clear that the diverse magnetic and density characteristics of major rock packages provide quantitative three-dimensional constraints. Gravity profiles forward modelled in four cross sections define broad synforms of the Amer Belt and Archean volcanic rocks that are consistent with the structural style outside the basin. Major lithotectonic entities beneath the unconformity include: supracrustal rocks of the Archean Woodburn Lake group and Marjorie Hills meta sedimentary gneiss and associated mixed granitoid and amphibolitic gneiss; the Amer Mylonite Zone and inferred mafic intrusions oriented parallel and sub-parallel; other igneous intrusions of 2.6 Ga, 1.83 Ga, and 1.75 Ga vintage; and the <2.3 Ga to >1.84 Ga Amer Group. Four main brittle regional fault arrays (040°-060°, 075°-90°, 120°, and 150°) controlled development and preservation of the basin. The reactivated intersections of such faults along fertile basement units such as the Rumble assemblage, Marjorie Hills assemblage, Nueltin igneous rocks, and Pitz formation are the best targets for uranium exploration.
Abstract: We have developed a synthetic multiparametric modeling and inversion exercise undertaken to study the robustness of inverting airborne time-domain electromagnetic (TDEM) data to extract Cole-Cole parameters. The following issues were addressed: nonuniqueness, ill posedness, dependency on manual processing and the effect of constraints, and a priori information. We have used a 1D layered earth model approximation and lateral constraints. Synthetic simulations were performed for several models and the corresponding Cole-Cole parameters. The possibility to recover these models by means of laterally constrained multiparametric inversion was evaluated, including recovery of chargeability distributions from shallow and deep targets based on analysis of induced polarization (IP) effects, simulated in airborne TDEM data. Different scenarios were studied, including chargeable targets associated with the conductive and resistive environments. In particular, four generic models were considered for the exercise: a sulfide model, a kimberlite model, and two generic models focusing on the depth of investigation.
Abstract: The James Bay Lowlands is a large remote area of Northern Ontario with very limited access. The Archean basement rocks lie beneath a layer of Paleozoic limestone, up to 300 m thick, that is topped by glacial till and bog. This setting, without outcrop or hard geological knowledge, presented a blank slate well suited to airborne geophysical exploration. This paper presents the aeromagnetic survey methodology and analysis techniques that evolved from the initial kimberlite aeromagnetic program carried out by Selco in 1979 through the 1989 DeBeers discovery of the Victor kimberlite and the 1993 Spider/KWG discovery of the older sub-Paleozoic Kyle series kimberlites and eventually the Ring of Fire. Without property constraints the exploration methodology was a cycle of survey-interpret-drill then move on and repeat as discoveries and finances permitted. After 3 cycles of kimberlite discovery a Spider/KWG/DeBeers partnership encountered VMS copper mineralization in 2001. An airborne EM survey in 2003 identified a number of excellent prospects and the most technically promising became the Noront Eagles Nest MMS nickel deposit that began the Ring of Fire saga. These greenfield discoveries, in a blind geological environment beneath limestone cover, illustrate the potential effectiveness of geophysically directed exploration.
Abstract: The magnetotelluric component of the EarthScope USArray program has covered over 35% of the continental United States. Resistivity tomography models derived from these data image lithospheric structure and provide constraints on the distribution of fluids and melt within the lithosphere. We present a three-dimensional resistivity model of the northwestern United States which provides new insight into the tectonic assembly of western North America from the Archean to present. Comparison with seismic tomography models reveals regions of correlated and anti-correlated resistivity and velocity that help identify thermal and compositional variations within the lithosphere. Recent (Neogene) tectonic features reflected in the model include the subducting Juan de Fuca–Gorda plate which can be traced beneath the forearc to more than 100 km depth, high lithospheric conductivity along the Snake River Plain, and pronounced lower-crustal and upper-mantle conductivity beneath the Basin and Range. The latter is abruptly terminated to the northwest by the Klamath–Blue Mountains Lineament, which we interpret as an important structure during and since the Mesozoic assembly of the region. This boundary is interpreted to separate hot extended lithosphere from colder, less extended lithosphere. The western edge of Proterozoic North America, as indicated by the Cretaceous initial 87Sr/86Sr = 0.706 contour, is clearly reflected in the resistivity model. We further image an Archean crustal block (“Pend Oreille block”) straddling the Washington/Idaho border, which we speculate separated from the Archean Medicine Hat block in the Proterozoic. Finally, in the modern Cascades forearc, the geometry and internal structure of the Eocene Siletz terrane is reflected in the resistivity model. The apparent eastern edge of the Siletz terrane under the Cascades arc suggests that pre-Tertiary rocks fill the Washington and Oregon back-arc.
Earth and Planetary Science Letters, Vol. 472, pp. 164-173.
Mantle
geophysics, seismic
Abstract: Subduction zones are likely a major source of compositional heterogeneities in the mantle, which may preserve a record of the subduction history and mantle convection processes. The fine-scale structure associated with mantle heterogeneities can be studied using the scattered seismic wavefield that arrives as coda to or as energy preceding many body wave arrivals. In this study we analyse precursors to PP by creating stacks recorded at globally distributed stations. We create stacks aligned on the PP arrival in 5° distance bins (with range 70–120°) from 600 earthquakes recorded at 193 stations stacking a total of 7320 seismic records. As the energy trailing the direct P arrival, the P coda, interferes with the PP precursors, we suppress the P coda by subtracting a best fitting exponential curve to this energy. The resultant stacks show that PP precursors related to scattering from heterogeneities in the mantle are present for all distances. Lateral variations are explored by producing two regional stacks across the Atlantic and Pacific hemispheres, but we find only negligible differences in the precursory signature between these two regions. The similarity of these two regions suggests that well mixed subducted material can survive at upper and mid-mantle depth. To describe the scattered wavefield in the mantle, we compare the global stacks to synthetic seismograms generated using a Monte Carlo phonon scattering technique. We propose a best-fitting layered heterogeneity model, BRT2017, characterised by a three layer mantle with a background heterogeneity strength (?=0.8%) and a depth-interval of increased heterogeneity strength (?=1%) between 1000 km and 1800 km. The scalelength of heterogeneity is found to be 8 km throughout the mantle. Since mantle heterogeneity of 8 km scale may be linked to subducted oceanic crust, the detection of increased heterogeneity at mid-mantle depths could be associated with stalled slabs due to increases in viscosity, supporting recent observations of mantle viscosity increases due to the iron spin transition at depths of ?1000 km.
Geophysical Research Letters, Vol. 42, 10. pp. 3829-3846.
Australia
geophysics, seismic
Abstract: A new 3-D S wave speed model for the Australian region derived from multimode surface waves allows us to examine the nature of the lithosphere-asthenosphere transition (LAT) and its relation to radial anisotropy. In eastern Phanerozoic Australia the estimated depths of the LAT tie well with those from receiver functions. However, in the Archean and Proterozoic lithosphere in western and central Australia, the LAT derived from the surface wave model is generally much deeper than the discontinuities recognized from receiver functions and shows a smooth transition. There is significant radial anisotropy (SH?>?SV) in the upper lithosphere as well as in the LAT and the underlying asthenosphere. Strong anisotropy in the asthenosphere reflects the effects of present shear flow in the mantle beneath the continent. The lateral variation of lithospheric anisotropy correlates well with the suture zones between cratonic blocks, representing frozen anisotropy associated with the ancient tectonics of Australia.
Abstract: The Precambrian geology of eastern Zambia and Malawi is highly complex due to multiple episodes of rifting and collision, particularly during the formation of Greater Gondwana as a product of the Neoproterozoic Pan-African Orogeny. The lithospheric structure and extent of known Precambrian tectonic entities of the region are poorly known as there have been to date few detailed geophysical studies to probe them. Herein, we present results from electromagnetic lithospheric imaging across Zambia into southern Malawi using the magnetotelluric method complemented by high-resolution aeromagnetic data of the upper crust in order to explore the extent and geometry of Precambrian structures in the region. We focus particularly on determining the extent of subcontinental lithospheric mantle (SCLM) beneath the Archean-Paleoproterozoic cratonic Bangweulu Block and the Mesoproterozoic-Neoproterozoic Irumide and Southern Irumide Orogenic Belts. We also focus on imaging the boundaries between these tectonic entities, particularly the boundary between the Irumide and Southern Irumide Belts. The thickest and most resistive lithosphere is found beneath the Bangweulu Block, as anticipated for stable cratonic lithosphere. Whereas the lithospheric thickness estimates beneath the Irumide Belt match those determined for other orogenic belts, the Southern Irumide Belt lithosphere is substantially thicker similar to that of the Bangweulu Block to the north. We interpret the thicker lithosphere beneath the Southern Irumide Belt as due to preservation of a cratonic nucleus (the pre-Mesoproterozoic Niassa Craton). A conductive mantle discontinuity is observed between the Irumide and Southern Irumide Belts directly beneath the Mwembeshi Shear Zone. We interpret this discontinuity as modified SCLM relating to a major suture zone. The lithospheric geometries determined from our study reveal tectonic features inferred from surficial studies and provide important details for the tectonothermal history of the region.
Abstract: The physical nature of two regions called large low-shear-velocity provinces at the base of Earth's mantle is uncertain. A measurement of their density has implications for our understanding of mantle dynamics.
45th. Annual Yellowknife Geoscience Forum, p. 82 abstract
Canada, Northwest Territories
geophysics
Abstract: As part of the Slave Province Geophysical, Surficial Materials and Permafrost Study, the Northwest Territories Geological Survey (NTGS) commissioned high resolution geophysical surveys in the Slave Geological Province (SGP). The high resolution aeromagnetic survey was flown from February to April 2017 and comprise 87,600 line-km of data flown at 100 m line spacing and nominal aircraft terrain clearance was 60 m with drape flying over the Central Slave craton block. The horizontal gradient magnetic and frequency domain EM (FDEM) survey was flown from February to March 2017 acquired at 75 m line spacing over 6 other blocks with nominal terrain clearance of 60 m to maintain bird height of 25 m, covering 4,580 line-km (Munn Lake, Margaret Lake, Zyena Lake, Lac de Gras West, Big Blue and Mackay Lake). The objective of this work is to develop multi-parameter models to help mineral exploration and mining companies better understand the range of geophysical signatures associated with kimberlites in the SGP. A regular geophysical-based approach for kimberlite exploration usually involves inverting geophysical data with limited geological input. In this contribution we present different ways of looking at the geophysical data and try to obtain a more thorough geological understanding out of it. The workflow starts with a complete GIS compilation of all the ancillary data available in the area: previous industry reports, geology, remote sensing, topographic layers. Secondly, we compute a number of interpretation sub-products from the total magnetic intensity data (tilt derivatives, analytic signal, and other edge detection routines). The next stage involves the computation of a susceptibility distribution from the FDEM data (Tschirhart et al, 2015). With this we are able to generate a magnetic model of the near surface susceptibility distributions, which are then subtracted from the observed data. The resultant map shows anomalous sources that could be associated to either remanent magnetization and/or deeper sources. Following the work of Sterritt (2006), post-emplacement alteration is ubiquitous in kimberlite pipes. Alteration results in production of secondary oxide minerals and alteration of primary oxide minerals to phases with different magnetic susceptibilities (e.g. non-magnetic iron oxides). This can lead to a dramatic increase of magnetic susceptibility due to serpentinization (Clark, 1997). On the other hand, remanent magnetization can change the polarity of the observed magnetic anomalies or even completely remove the expected signature due to an equal but opposite combination of remanent and induced magnetic components. Therefore, a thorough compilation of petrophysical and mineralogical data over kimberlites and altered rocks in the vicinity of known occurrences is critical for the geological understanding of the existing geophysical data. This contribution will show some preliminary processing and compilation work completed over the Slave province kimberlites using the newly acquired geophysical data.
Abstract: Constructing palaeogeographical maps is best achieved through the integration of data from hotspotting (since the Cretaceous), palaeomagnetism (including ocean-floor magnetic anomalies since the Jurassic), and the analysis of fossils and identification of their faunal and floral provinces; as well as a host of other geological information, not least the characters of the rocks themselves. Recently developed techniques now also allow us to determine more objectively the palaeolongitude of continents from the time of Pangaea onwards, which palaeomagnetism alone does not reveal. This together with new methods to estimate true polar wander have led to hybrid mantle plate motion frames that demonstrate that TUZO and JASON, two antipodal thermochemical piles in the deep mantle, have been stable for at least 300 Ma, and where deep plumes sourcing large igneous provinces and kimberlites are mostly derived from their margins. This remarkable observation has led to the plume generation zone reconstruction method which exploits the fundamental link between surface and deep mantle processes to allow determination of palaeolongitudes, unlocking a way forward in modelling absolute plate motions prior to the assembly of Pangaea. The plume generation zone method is a novel way to derive ‘absolute’ plate motions in a mantle reference frame before Pangaea, but the technique assumes that the margins of TUZO and JASON did not move much and that Earth was a degree-2 planet, as today.
Abstract: This review paper focuses on joint modelling and interpretation of electromagnetic data and other geophysical and petrological observables. In particular, integrated geophysical-petrological modelling approaches, where the electrical conductivity and other physical properties of rocks are required to be linked by the common subsurface thermochemical conditions within a self-consistent thermodynamic framework, are reviewed. The paper gives an overview of the main geophysical electromagnetic techniques/data sets employed in lithospheric and mantle imaging including recent advances using satellite data, and an up-to-date summary of the most relevant laboratory experiments regarding the electrical conductivity of upper mantle minerals for various temperature-pressure-water conditions. The sensitivity of electrical conductivity and other geophysical parameters (density, seismic velocities) of mantle rocks to changes in temperature and composition are presented based on a Monte Carlo method parameter exploration. Finally, a case study in Central Tibet is presented where both seismological (long-period surface wave phase velocities) and electromagnetic (magnetotelluric) data—simultaneously including the constraints offered by topography, surface heat flow and mantle xenoliths—have been integrated. The modelling is based on a self-consistent petrological-geophysical thermodynamic framework where mantle properties are calculated as a function of temperature, pressure, and composition. The Tibetan case study offers an excellent opportunity to illustrate the different and complementary sensitivities of the various data sets used and to show how integrated thermochemical models of the lithosphere can help understand settings with a complex tectonic evolution.
Abstract: The Precambrian geology of eastern Zambia and Malawi is highly complex due to multiple episodes of rifting and collision, particularly during the formation of Greater Gondwana as a product of the Neoproterozoic Pan-African Orogeny. The lithospheric structure and extent of known Precambrian tectonic entities of the region are poorly known as there have been to date few detailed geophysical studies to probe them. Herein, we present results from electromagnetic lithospheric imaging across Zambia into southern Malawi using the magnetotelluric method complemented by high-resolution aeromagnetic data of the upper crust in order to explore the extent and geometry of Precambrian structures in the region. We focus particularly on determining the extent of subcontinental lithospheric mantle (SCLM) beneath the Archean-Paleoproterozoic cratonic Bangweulu Block and the Mesoproterozoic-Neoproterozoic Irumide and Southern Irumide Orogenic Belts. We also focus on imaging the boundaries between these tectonic entities, particularly the boundary between the Irumide and Southern Irumide Belts. The thickest and most resistive lithosphere is found beneath the Bangweulu Block, as anticipated for stable cratonic lithosphere. Whereas the lithospheric thickness estimates beneath the Irumide Belt match those determined for other orogenic belts, the Southern Irumide Belt lithosphere is substantially thicker similar to that of the Bangweulu Block to the north. We interpret the thicker lithosphere beneath the Southern Irumide Belt as due to preservation of a cratonic nucleus (the pre-Mesoproterozoic Niassa Craton). A conductive mantle discontinuity is observed between the Irumide and Southern Irumide Belts directly beneath the Mwembeshi Shear Zone. We interpret this discontinuity as modified SCLM relating to a major suture zone. The lithospheric geometries determined from our study reveal tectonic features inferred from surficial studies and provide important details for the tectonothermal history of the region.
Geochemistry, Geophysics, Geosystems G3, pp. 4326-
Mantle
geophysics
Abstract: Regional characterization of the continental crust has classically been performed through either geologic mapping, geochemical sampling, or geophysical surveys. Rarely are these techniques fully integrated, due to limits of data coverage, quality, and/or incompatible data sets. We combine geologic observations, geochemical sampling, and geophysical surveys to create a coherent 3-D geologic model of a 50 × 50 km upper crustal region surrounding the SNOLAB underground physics laboratory in Canada, which includes the Southern Province, the Superior Province, the Sudbury Structure, and the Grenville Front Tectonic Zone. Nine representative aggregate units of exposed lithologies are geologically characterized, geophysically constrained, and probed with 109 rock samples supported by compiled geochemical databases. A detailed study of the lognormal distributions of U and Th abundances and of their correlation permits a bivariate analysis for a robust treatment of the uncertainties. A downloadable 3-D numerical model of U and Th distribution defines an average heat production of math formula µW/m3, and predicts a contribution of math formula TNU (a Terrestrial Neutrino Unit is one geoneutrino event per 1032 target protons per year) out of a crustal geoneutrino signal of math formula TNU. The relatively high local crust geoneutrino signal together with its large variability strongly restrict the SNO+ capability of experimentally discriminating among BSE compositional models of the mantle. Future work to constrain the crustal heat production and the geoneutrino signal at SNO+ will be inefficient without more detailed geophysical characterization of the 3-D structure of the heterogeneous Huronian Supergroup, which contributes the largest uncertainty to the calculation.
Abstract: Small pockets of water exist deep beneath Earth’s surface, according to an analysis of diamonds belched from hundreds of kilometers within our planet. The work, which also identifies a weird form of crystallized water known as ice VII, suggests that material may circulate more freely at some depths within Earth than previously thought. Geophysical models of that flow, which ultimately influences the frequency of earthquakes driven by the scraping of tectonic plates at Earth’s surface, may need to be substantially tweaked, scientists say. Such models also help scientists estimate the long-term rates of heat flow through Earth’s surface and into space.
Abstract: During the early Archaean, the Earth was too hot to sustain rigid lithospheric plates subject to Wilson Cycle-style plate tectonics. Yet by that time, up to 50% of the present-day continental crust was generated. Preserved continental fragments from the early Archaean have distinct granite-dome/greenstone-keel crust that is interpreted to be the result of a gravitationally unstable stratification of felsic proto-crust overlain by denser mafic volcanic rocks, subject to reorganization by Rayleigh-Taylor flow. Here we provide age constraints on the duration of gravitational overturn in the East Pilbara Terrane. Our U-Pb ages indicate the emplacement of ~3,600-3,460-million-year-old granitoid rocks, and their uplift during an overturn event ceasing about 3,413?million years ago. Exhumation and erosion of this felsic proto-crust accompanied crustal reorganization. Petrology and thermodynamic modelling suggest that the early felsic magmas were derived from the base of thick (~43?km) basaltic proto-crust. Combining our data with regional geochronological studies unveils characteristic growth cycles on the order of 100?million years. We propose that maturation of the early crust over three of these cycles was required before a stable, differentiated continent emerged with sufficient rigidity for plate-like behaviour.
Finland Geological Survey, http://tupa.gtk.fi /julkaisu/ specialpaper/ sp_058.pdf
Finland
geophysics
Abstract: In this Special Paper volume, the review of physical properties of ore deposit types or mineralization styles is mainly based on published information, in particular on the key note speeches and presentations that were given at two geosciences conferences: Exploration07 held in Toronto in 2007 (proceedings by Milkereit (ed.) 2007), and the SGA meeting held in Uppsala in 2013 (proceedings by Johnsson et al. (eds.) 2013). Diamonds referred to on p. 13.
Abstract: A geometrically quantitative plate-kinematic model, based on paleomagnetism, for the initial assembly of Laurentia has taken form in the past few decades. Within this framework, there remains but one problematic interval of data predominantly from the Slave craton, which is the 1.96-1.87?Ga Coronation apparent polar wander path (APWP). The Coronation APWP shows large (?110°) back-and-forth oscillations that are difficult to explain in terms of plate motion. Nonetheless, poles from the Coronation APWP have been incorporated in various paleogeographic reconstructions of Laurentia and the supercontinent Nuna, pointing to the importance of testing its veracity. In this study, we conducted a detailed paleomagnetic and rock magnetic study of the ca. 1.87?Ga Pearson Formation, East Arm of Great Slave Lake, Northwest Territories, Canada. Our results show that Pearson Formation yields a characteristic remanent magnetization carried by single-domain or small pseudo-single-domain magnetite. The age of the magnetization is constrained to be older than Paleoproterozoic deformation and is interpreted as primary. Paleomagnetic declinations reveal a one-to-one correlation with local structural attitudes, indicating that some small blocks in the fold belt likely experienced significant (?60°) vertical-axis rotations, presumably related to large dextral displacements along the McDonald Fault system. Alternative explanations, such as true polar wander or a non-dipole magnetic field, are considered less parsimonious for the data presented here. It is suspected that some existing Christie Bay Group poles (the Stark and Tochatwi Formations), which were sampled in areas with anomalous structural attitudes and differ from time-equivalent poles obtained from areas of the Slave craton far from major transcurrent faults, may similarly suffer from vertical-axis rotation. We suggest further study before using possibly rotated Christie Bay Group poles for paleogeographic reconstructions.
Abstract: A weak asthenosphere, or low viscosity zone (LVZ) underlying Earth’s lithosphere has historically played an important role in interpreting isostasy, post-glacial rebound (PGR), and the seismic low velocity zone, as well as proposed mechanisms for continental drift, plate tectonics, and post-seismic relaxation (PSR). Careful consideration of the resolving power of PGR, PSR, and geoid modeling studies suggests a sublithospheric LVZ perhaps ~100-200 km thick with a viscosity contrast of ~100-1000. Ab initio numerical models of plate-like boundary layer motions in mantle convection also suggest a key role for the LVZ. Paradoxically, a thinner LVZ with a strong viscosity contrast is most effective in promoting plate-like surface motions. These numerical results are explained in terms of the reduction in horizontal shear dissipation due to an LVZ, and a simple scaling theory leads to somewhat non-intuitive model predictions. For example, an LVZ causes stress magnification at the base of the lithosphere, enhancing plate boundary formation. Also, flow within the LVZ may be driven by the plates (Couette flow), or pressure-driven from within the mantle (Poiseuille flow), depending upon the degree to which plates locally inhibit or drive underlying mantle convection. For studies of the long-wavelength geoid, PGR, and mantle convection, a simple dimensionless parameter controls the effect of the LVZ. This “Cathles parameter” is given by the expression Ct=v*(D/L)3, where v* is the viscosity contrast and D is the thickness of the LVZ, and L is the flow wavelength, emphasizing the tightly-coupled, largely un-resolvable tradeoff between LVZ thickness and viscosity contrast.
Mineralogy and Petrology, doi.org/10.1007/ s00710-018-0619-2. 15p.
Mantle
Geophysics
Abstract: As three-dimensional (3-D) modelling of the subcontinental mantle lithosphere is increasingly performed with ever more data and better methods, the robustness of such models is increasingly questioned. Resolution thresholds and uncertainty within deep multidisciplinary 3-D models based on geophysical observations exist at a minimum of three levels. Seismic waves and potential field measurements have inherent limitations in resolution related to their dominant wavelengths. Formal uncertainties can be assigned to grid-search type forward or inverse models of observable parameter sets. Both of these uncertainties are typically minor when compared to resolution limitations related to the density and shape of a specific observation array used in seismological or potential field surveys. Seismic wave source distribution additionally applies in seismology. A fourth, more complex level of uncertainty relates to joint inversions of multiple data sets. Using independent seismic wave phases or combining diverse methods provides another measure of uncertainty of particular physical properties. Extremely sparse xenolith suites provide the only direct correlation of rock type with observed or modelled physical properties at depths greater than a few kilometers. Here we present one case study of the Canadian Mohorovi?i? (Moho) discontinuity using only two data sets. Refracted and converted seismic waves form the primary determinations of the Moho depth, gravity field modeling provide a secondary constraint on lateral variations, the slope of the Moho, between the sparse seismic estimates. Although statistically marginal, the resulting co-kriged Moho surface correlates better with surface geology and is thus deemed superior.
Abstract: Thorough information on geological structure of the Ukrainian shield (USh) allows to verify opinions existing nowadays that plume and plate tectonic activities are possible only after conclusive formation of sublithospheric mantle 2,8—2,55 Ga and plume episodes of Precambrian occurred 2,75—2,7, 2,45—2,4, 1,9, 1.8—1,75, 1,65 Ga ago. It can be proved nowadays that formation of the crust occurred before 4.0 Ga and plume events result in later transformation of not only crust but also of mantle. Isotope dating of the USh allows allocating of plume episodes in close-range time. Nevertheless these limits for the USh in many cases have connection with geological processes, because not detrite zircons are dated but those ones included in rocks. Individual episodes are united into lasting plume events (activizations) up to 3,2, 3,2—2,8, 2,7—2,3, 2,3—1,65 Ga. Special features of the structure of granite-greenstone area of the Middle Dnieper megablock of the USh support the opinion that before 2,7 Ga thick sub-continental lithosphere consisted of refractory peridotite did not exist. Crystallizable layer consists of relatively low-melt minerals and is enriched by basaltoid component. The thickness of crystallized layer reduces up to 100 and 50 km in case of addition of reheated deep matter and increases during crystallization up to 150—200 km. Asthenosphere compulsory contains the melt diluted by silicate-aqueous-carbonate fluide. After 2,7 Ga subdivision of the area into two big blocks occurred. At the mark of the Archean and Proterozoic (2,5 Ga ago) the territory of Ukraine was subdivided into blocks and territories which spatially were drawn towards suture zones. The composition of the terrains allows supposing that at the mark of the Archean and Proterozoic exarticulation of tectonic units where plume processes went in different ways occurred. The following plume event (2,3—2,65 Ga ago) was manifested as a stage of transformation of mantle and crust on all the cratons. In the structure of the USh obvious features of plate tectonic processes have been found. Movements maximal by amplitude occurred 2,0—1,8 Ga ago. During the process of this plume event abrupt change of composition of fluids and melts removed from the mantle occur. Granitization takes place with participation of aqueous chloride-potassium fluids. After granitization active dry carbonate-fluoride-sodium fluids became active. Removal of melts, fluids occurs by small portions and the regime of pulsating plume considerably specifies the composition of magmatic rocks and hydrothermal solutions.
Abstract: Obtaining an accurate image of the subsurface still remains a great challenge for the seismic method. Migration algorithms aim mainly on positioning seismic events in complex geological contexts. Multiple reflections are typically not accounted for in this process, which can lead to the emergence of artefacts. In Marchenko imaging, we retrieve the complete up? and downgoing wavefields in the subsurface to construct an image without such artefacts. The quality of this image depends on the type of imaging condition that is applied. In this paper, we propose an imaging condition that is based on stabilized unidimensional deconvolution. This condition is computationally much cheaper than multidimensional deconvolution, which has been proposed for Marchenko imaging earlier. Two specific approaches are considered. In the first approach, we use the full up? and downgoing wavefields for deconvolution. Although this leads to balanced and relatively accurate amplitudes, the crosstalk is not completely removed. The second approach is to incorporate the initial focussing function in the deconvolution process, in such a way that the retrieval of crosstalk is avoided. We compare images with the results of the classical cross?correlation imaging condition, which we apply to reverse?time migrated wavefields and to the up? and downgoing wavefields that are retrieved by the Marchenko method.
Abstract: Something strange is going on at the top of the world. Earth’s north magnetic pole has been skittering away from Canada and towards Siberia, driven by liquid iron sloshing within the planet’s core. The magnetic pole is moving so quickly that it has forced the world’s geomagnetism experts into a rare move. On 15 January, they are set to update the World Magnetic Model, which describes the planet’s magnetic field and underlies all modern navigation, from the systems that steer ships at sea to Google Maps on smartphones. The most recent version of the model came out in 2015 and was supposed to last until 2020 — but the magnetic field is changing so rapidly that researchers have to fix the model now. “The error is increasing all the time,” says Arnaud Chulliat, a geomagnetist at the University of Colorado Boulder and the National Oceanic and Atmospheric Administration’s (NOAA’s) National Centers for Environmental Information. The problem lies partly with the moving pole and partly with other shifts deep within the planet. Liquid churning in Earth’s core generates most of the magnetic field, which varies over time as the deep flows change. In 2016, for instance, part of the magnetic field temporarily accelerated deep under northern South America and the eastern Pacific Ocean. Satellites such as the European Space Agency’s Swarm mission tracked the shift. By early 2018, the World Magnetic Model was in trouble. Researchers from NOAA and the British Geological Survey in Edinburgh had been doing their annual check of how well the model was capturing all the variations in Earth’s magnetic field. They realized that it was so inaccurate that it was about to exceed the acceptable limit for navigational errors.
Abstract: Since the commercialisation of ground penetrating radar (GPR) in the 1970s, radar technology has been employed for niche applications in the mining industry. Although reliant on electrically resistive environments, GPR has gained acceptance in recent years as a standard exploration method for a number of deposit types, ranging from paleochannel delineation to iron ore mapping and kimberlite imaging. Numerous case studies have been published on GPR's applications to specific mineral exploration projects. Provided herein is an overview of commercialised GPR applications for surface mineral resource evaluations, covering examples of alluvial channels, nickel and bauxitic laterites, iron ore deposits, mineral sands, coal, kimberlite and massive sulphide examples.
Abstract: A magnetotelluric survey in the Barotse Basin of western Zambia shows clear evidence for thinned lithosphere beneath an orogenic belt. The uppermost asthenosphere, at a depth of 60-70 km, is highly conductive, suggestive of the presence of a small amount of partial melt, despite the fact that there is no surface expression of volcanism in the region. Although the data support the presence of thicker cratonic lithosphere to the southeast of the basin, the lithospheric thickness is not well resolved and models show variations ranging from ~80 to 150 km in this region. Similarly variable is the conductivity of the mantle beneath the basin and immediately beneath the cratonic lithosphere to the southeast, although the conductivity is required to be elevated compared to normal lithospheric mantle. In a general sense, two classes of model are compatible with the magnetotelluric data: one with a moderately conductive mantle and one with more elevated conductivities. This latter class would be consistent with the impingement of a stringer of plume?fed melt beneath the cratonic lithosphere, with the melt migrating upslope to thermally erode lithosphere beneath the orogenic belt that is overlain by the Barotse Basin. Such processes are potentially important for intraplate volcanism and also for development or propagation of rifting as lithosphere is thinned and weakened by melt. Both models show clear evidence for thinning of the lithosphere beneath the orogenic belt, consistent with elevated heat flow data in the region.
Geochemistry International, Vol. 56, 13, pp. 1289-1321.
Mantle
geophysics
Abstract: The generation of crustal material and the formation of continental crust with a thickness of ?40 km involve different physical mechanisms operating over different time-scales and length-scales. This review focusses on the building of a thick crustal assemblage and on the vertical dimension where the consequences of gravity-driven processes are expressed most clearly. Continental crustal material is produced by a sequence of crust and mantle mlelting, fractionation of basaltic melts and sinking of dense mafic cumulates. The repeated operation of these mechanisms over tens of million years leads to a thick stably stratified crust. We evaluate the main mechanisms involved from a physics perspective and identify the key controls and constraints, with special attention to thermal requirements. To form magma reservoirs able to process significant magma volumes and to allow the foundering of mafic cumulates, melt must be fed locally at rates that are larger than that of average crustal growth. This requires the temporary focussing of magmatic activity in a few centers. In some cases, foundering of dense cumulates does not go to completion, leaving a deformed residual body bearing tell-tale traces of the process. Crust must be thicker than a threshold value in a 30-45 km range for mafic cumulates to sink into the mantle below the crust. Once that threshold thickness has been reached, further additions lead to increase the proportion of felsic material in the crust at the expense of mafic lithologies which disappear from the crust. This acts to enhance radiogenic heat production in the crust. One consequence is that crustal temperatures can be kept at high values in times of diminished melt input and also when magmatic activity stops altogether, which may lead to post-orogenic intracrustal melting and differentiation. Another consequence is that the crust becomes too weak mechanically to withstand the elevation difference with neighbouring terranes, which sets a limit on crustal thickening. The thermal structure of the evolving crust is a key constraint on the overall process and depends strongly on radiogenic heat production, which is surely one of the properties that make continental crust very distinctive. In the Archean Superior Province, Canada, the formation of juvenile continental crust and its thermal maturation 2.7 Gy ago can be tracked quite accurately and reproduced by calculations relying on the wealth of heat flow and heat production data available there. Physical models of magma ascent and storage favour the formation of magma reservoirs at shallow levels. This suggests that crustal growth proceeds mostly from the top down, with material that gets buried to increasingly large depths. Vertical growth is accompanied by lateral spreading in two different places. Within the crust, magma intrusions are bound to extend in the horizontal direction. Deeper down, lateral variations of Moho depth that develop due to the focussing of magmatic activity get relaxed by lower crustal flow. This review has not dealt with processes at the interface between the growing crust and the mantle, which may well be where dikes get initiated by mechanisms that have so far defied theoretical analyses. Research in this particular area is required to further our understanding of continental crust formation.
Abstract: Geomagnetic jerks are abrupt changes in the second time derivative "the secular acceleration" of Earth’s magnetic field that punctuate ground observatory records. As their dynamical origin has not yet been established, they represent a major obstacle to the prediction of geomagnetic field behaviour for years to decades ahead. Recent jerks have been linked to short-lived, temporally alternating and equatorially localized pulses of secular acceleration observed in satellite data, associated with rapidly alternating flows at Earth’s core surface. Here we show that these signatures can be reproduced in numerical simulations of the geodynamo that realistically account for the interaction between slow core convection and rapid hydromagnetic waves. In these simulations, jerks are caused by the arrival of localized Alfvén wave packets radiated from sudden buoyancy releases inside the core. As they reach the core surface, the waves focus their energy towards the equatorial plane and along lines of strong magnetic flux, creating sharp interannual changes in core flow and producing geomagnetic jerks through the induced variations in magnetic field acceleration. The ability to numerically reproduce jerks offers a new way to probe the physical properties of Earth’s deep interior.
Geochemistry, Geophysics, Geosystems, Vol. 20, 4, pp. 2091-2109.
Africa
geophysics
Abstract: In Ethiopia, the African Continent is rifting apart to slowly form a new ocean basin, which will expand the Red Sea and the Gulf of Aden. How and why this rifting is occurring remains an important unanswered question in earth science. We know tectonic forces are partly responsible, but magmatism also seems a key ingredient for breaking up Africa. Here we use seismic images obtained from signals pulled out of noise, to understand the crustal structure of the region; In particular, how and where magma is stored in the crust, and its relationship to the different stages of continental breakup visible in the region. We find evidence for long?term melt storage in places where rifting is just beginning in southern Ethiopia; whereas in regions where the crust is thinner due to extensive rifting, magma erupts more regularly. The long?term storage of magma in unrifted crust may help to heat and weaken it, allowing rifting to accelerate and propagate further south. We are also able to image regions with hydrothermal fluids in the shallow parts of the crust in inactive fault zones. These results provide insight into the breakup process and the role magma plays at different stages of rifting.
American Geophysical Union, Fall Meeting. , V23A-05 1p.
Mantle
geophysics
Abstract: Variations in radiogenic isotopes in mid-ocean ridge basalts (MORB) are interpreted to reflect the presence of enriched and depleted mantle components in their source regions and have been used to infer the abundance and time scales of crustal recycling. However, MORB are homogenized via magma mixing prior to eruption and may not capture the full heterogeneity of melts generated in their upper mantle source. Here we show that primitive cumulate minerals, formed by crystallization of mantle melts in the lower crust, retain the signature of the recycled material. We performed high spatial resolution Nd and Sr isotopic analyses on clinopyroxene and plagioclase of gabbroic cumulates from the Atlantis massif, located on a depleted ridge segment on the northern Mid-Atlantic Ridge, and compared these data with whole rock isotopic compositions of diabase and microgabbros collected on the same core, associated basalts flows, and MORB data from the literature. We find that cumulate minerals: (1) are significantly more isotopically heterogeneous than the associated diabase and lavas, exceeding the range of 143Nd/144Nd in MORB by a factor of seven; and (2) contain the full Nd isotopic heterogeneity of all of North Atlantic MORB. Furthermore, we find that isotopic heterogeneity occurs down to the sample scale, with plagioclase and clinopyroxene from individual samples commonly not in isotopic equilibrium. We further demonstrate that the MORB and cumulate mineral data can be reconciled with constant high magnitude, small length scale heterogeneity through the North Atlantic upper mantle, with limited magma mixing in the mantle and extensive mixing in the oceanic crust.The isotopic heterogeneity revealed in the lower oceanic crust provides strong evidence that MORB is not an accurate representation of the heterogeneity of its mantle source. Hence, the true isotopic variation of the upper mantle requires rigorous further examination, and models of convective thinning and stretching and melt migration must be re-evaluated to account for greater local variation.
Geophysical Research Letters, Vol. 46, 14, pp. 7982-7989.
Mantle
geophysics
Abstract: In a metal, as in Earth's core, the thermal and electrical conductivities are assumed to be correlated. In a planetary dynamo this implies a contradiction: that both electrical conductivity, which makes it easier to induce current and magnetic field, and conductive heat transport, which hinders thermal convection, should increase simultaneously. Here we show that this contradiction implies that the magnetic induction rate peaks at a particular value of electrical and thermal conductivity and derive the low? and high?conductivity limits for thermal dynamo action. A dynamo regime diagram is derived as a function of electrical conductivity and temperature for Earth's core that identifies four distinct dynamo regimes: no dynamo, thermal dynamo, compositional dynamo, and thermocompositional dynamo. Estimates for the temperature?dependent electrical conductivity of the core imply that the geodynamo may have come close to its high?conductivity “no dynamo” limit prior to inner core nucleation, consistent with recent paleomagnetic observations.
Geochemistry, Geophysics, Geosystems, Vol. 20, 7, pp. 3311-3327.
Africa, South Africa
geophysics
Abstract: We conduct a joint inversion of teleseismic receiver functions and Rayleigh wave phase velocity dispersion from both ambient noise and earthquakes using data from 79 seismic stations in southern Africa, which is home to some of the world's oldest cratons and orogenic belts. The area has experienced two of the largest igneous activities in the world (the Okavango dyke swarm and Bushveld mafic intrusion) and thus is an ideal locale for investigating continental formation and evolution. The resulting 3?D shear wave velocities for the depth range of 0-100 km and crustal thickness measurements show a clear spatial correspondence with known geological features observed on the surface. Higher than normal mantle velocities found beneath the southern part of the Kaapvaal craton are consistent with the basalt removal model for the formation of cratonic lithosphere. In contrast, the Bushveld complex situated within the northern part of the craton is characterized by a thicker crust and higher crustal Vp/Vs but lower mantle velocities, which are indicative of crustal underplating of mafic materials and lithospheric refertilization by the world's largest layered mafic igneous intrusion. The thickened crust and relatively low elevation observed in the Limpopo belt, which is a late Archean collisional zone between the Kaapvaal and Zimbabwe cratons, can be explained by eclogitization of the basaltic lower crust. The study also finds evidence for the presence of a stalled segment of oceanic lithosphere beneath the southern margin of the Proterozoic Namaqua?Natal mobile belt.
2019 Twelth International Conference Oct 1-3. Moscow, IEEE DOI 11.09/MLSD .2019.8911014
Africa, Angola, Russia, Yakutia
geophysics
Abstract: We show how to increase the effectiveness of the prognoses of kimberlite bodies by using airborne geophysical technologies. We show the advantages of electromagnetic and magnetic methods for predicting kimberlite pipes. You will see examples of a regional diamond survey in Angola and Siberia.
Abstract: In the last two decades, airborne laser scanning (ALS) has found widespread application and driven fundamental advances in the Earth sciences. With increasing availability and accessibility, multi-temporal ALS data have been used to advance key research topics related to dynamic Earth surface processes. This review presents a comprehensive compilation of existing applications of ALS change detection to the Earth sciences. We cover a wide scope of material pertinent to the broad field of Earth sciences to encourage the cross-pollination between sub-disciplines and discuss the outlook of ALS change detection for advancing scientific discovery. While significant progress has been made in applying repeat ALS data to change detection, numerous approaches make fundamental assumptions that limit the full potential of repeat ALS data. The use of such data for 3D change detection is, therefore, in need of novel, scalable, and computationally efficient processing algorithms that transcend the ever-increasing data density and spatial coverage. Quantification of uncertainty in change detection results also requires further attention, as it is vitally important to understand what 3D differences detected between epochs represent actual change as opposed to limitations in data or methodology. Although ALS has become increasingly integral to change detection across the Earth sciences, the existence of pre- and post-event ALS data is still uncommon for many isolated hazard events, such as earthquakes, volcanic eruptions, wildfires, and landslides. Consequently, data availability is still a major limitation for many ALS change detection applications.
Abstract: New high-resolution airborne magnetic data of Uruguay allowed constructing new maps concerning the spatial distribution of dyke swarms, main faults and other magnetic bodies, which compose the Uruguayan Shield. We combined geophysical analyses (vertical derivatives, upward continuation, Euler deconvolution), structural analyses of the magnetic maps and previous geological data in order to discriminate the main structural features of the Uruguayan Shield and contribute to a better understanding of its tectonic evolution. The magnetic maps revealed several outstanding features in the Uruguayan Shield. The Paleoproterozoic dyke swarm is larger, denser, more widespread and complex than originally thought, suggesting a possible plume origin. In addition, a new Mesozoic dyke swarm, as complex as the previous one, was identified crosscutting the Paleoproterozoic dyke swarm and the Neoproterozoic orogenic structures. Moreover, this swarm is connected to volcanic calderas in the Merín basin, and shows displacements along Neoproterozoic shear zones, in the magnetic maps, revealing its brittle reactivation during Mesozoic times. The new observations clarify how Proterozoic basement structures controlled the development of the Mesozoic rift. Paleoproterozoic dyke swarms were reactivated as normal faults and Neoproterozoic structures hindered the rift growth, deflecting the deformation in transcurrent movements. Meanwhile, the Mesozoic dyke swarm was developed in a perpendicular direction to the Neoproterozoic structures. Moreover, these findings contradict the current rift model for Uruguay and rise a new model in which the Mesozoic rift developed as two rift basins connected by a central transfer zone, generated by the reactivation of Dom Feliciano Belt structures, between the Sierra Ballena and Sarandí del Yí Shear Zones.
Abstract: From 2009 to 2016, a drastic increase in seismic activity occurred in the Central and Eastern US (CEUS), particularly in the Oklahoma-Kansas region. The majority of hypocenters were focused in the crystalline basement rock. Information regarding the physical properties (elastic wave velocity, peak strength, etc.) of rocks in the CEUS basement to date is sparse. Forecasting future seismic hazard and predicting the in situ response of the crystalline basement requires their geomechanical parameters be adequately constrained. This work assesses the mechanical and petrophysical properties of several sets of basement rocks from Oklahoma to provide a better framework for understanding intraplate seismicity and overall basement deformation in the continental United States. Laboratory experiments were conducted with granite, rhyolite and diabase basement rock samples collected from southern Oklahoma. Evolution of compressional and shear wave velocity with increasing confinement was measured through a series of ultrasonic velocity tests. A suite of uniaxial and triaxial tests were conducted to measure the elastic and inelastic deformation behavior of the basement rocks. Deformation data was evaluated using the Mohr-Coulomb criterion and compared with additional preexisting deformation data of igneous basement rocks. Dynamic and static elastic properties compare favorably with available field measurements and demonstrate the role physical properties can play in varying mechanical behavior. Granitic samples demonstrate moderate variation of intrinsic physical properties can alter elastic properties and failure behavior significantly. Water-weakening in the basement rocks may indicate fluid-assisted processes such as stress corrosion cracking enhance deformation in the crystalline basement.
Abstract: LitMod2D integrates geophysical and petrological data sets to produce the thermal, density, and seismic velocity structure of the lithosphere and upper mantle. We present a new LitMod2D_2.0 package with improvements focused on (i) updated anelastic attenuation correction for anharmonic seismic velocities, (ii) chemical composition in the sublithospheric mantle, and (iii) incorporation of sublithospheric mantle anomalies. Sublithospheric mantle anomalies can be defined with different chemical composition, temperature, seismic velocities, and a combination of them, allowing the application of LitMod2D_2.0 to regions affected by mantle upwelling, subduction, delamination, and metasomatism. We demonstrate the potential application of LitMod2D_2.0 to such regions and the sensitivity of thermal and compositional anomalies on density and seismic velocities through synthetic models. Results show nonlinearity between the sign of thermal and seismic velocity anomalies, and that S wave velocities are more sensitive to temperature whereas P wave velocities are to composition. In a synthetic example of subduction, we show the sensitivity of sublithospheric mantle anomalies associated with the slab and the corner flow on surface observables (elevation, geoid height, and gravity anomalies). A new open?source graphic user interface is incorporated in the new package. The output of the code is simplified by writing only the relevant physical parameters (temperature, pressure, material type, density, and seismic velocities) to allow the user using predefined post?processing codes from a toolbox (flexure, mineral assemblages, synthetic passive seismological data, and tomography) or designing new ones. We demonstrate a post?processing example calculating synthetic seismic tomography, Rayleigh surface?wave dispersion curves, and P wave receiver functions from the output file of LitMod2D_2.0.
Journal of Earth System Science, Vol. 128, 43, 8p. Pdf
India
geophysics, seismics
Abstract: An improved shear wave velocity (Vs) structure of the lithosphere of peninsular India using the surface wave tomography from the ambient noise and earthquake waveforms suggests its bipolar character. While most of the geological domains of India are characterised by a uniform lithospheric mantle of Vs?4.5 km/s, the three cratonic regions, eastern Dharwar, Bastar and Singhbhum, hosting most of the diamondiferous kimberlite fields, show significantly high Vs of 4.7 km/s and above in their lower lithosphere beyond ?90 km depth. The higher velocity could best be explained by the presence of diamond and/or eclogite along with peridotite in mantle. This unique relationship suggests the regional seismic image of lithosphere as a guide for exploration of diamonds.
Abstract: Gravity anomalies across the Indian region depict most of the geological and tectonic domains of the Indian continental lithosphere, which evolved through Archean cratonic nucleation, Proterozoic accretion, Phanerozoic India-Eurasia plate convergence, and modification through many thermal perturbations and rifting. Integrated analysis of gravity and geoid anomalies together with topographic and heat flow data led to deciphering the mechanism of isostatic compensation of topographic and geological loads, lithospheric structure, and composition. This study discusses the nature of gravity (free-air, Bouguer and Isostatic) and geoid anomalies in relation to the topography, geology, and tectonics, and presents a lithospheric density model across the peninsular India and Himalaya. Southern peninsular Indian region shows relatively low Bouguer gravity anomalies compared to the northern region. The mobile belts are generally observed to have relatively higher Bouguer gravity anomalies, e.g., Eastern Ghats Mobile Belt compared to the shield regions. The gravity lows are observed over topographic features like the Western Ghats and Himalaya, while some of the topographic highs like Aravalli show positive gravity anomaly. The Indian Ocean Geoid Low varies from -82 m over Dharwar Craton to -98 m over the Southern Granulite Terrain and finally reaches a significant low of -106 m in the Indian Ocean. Flexural isostatic compensation with variable Effective Elastic Thickness (EET) ~10 km to 50 km prevails over the stable continental region. The lithospheric thickness varies from 80 km along the coastal region to 120-130 km beneath the Saurashtra Plateau, the Southern Granulite Terrain, and the Eastern Indian Shield, and reaches to more than 200 km under the Himalayan orogenic belt in the north. From Dharwar Craton to Bundelkhand Craton in central India, the lithospheric thickness varies between 160 and 180 km.
Abstract: Knowledge of the crust and lithospheric structure of the Indian sub-continent primarily comes from several active and passive seismic experiments. These studies are i) controlled source, ii) surface wave studies, iii) receiver functions and v) tomographic studies. The results from these studies in the Indian shield have emanated several interesting features that were hitherto unknown. The peninsular, central and north-western part of the shields, Himalayan and Andaman-Nicobar regions have shown that continental collision and extension from the Proterozoic to Recent time has played an important role in formation and geodynamics of these features. The granulites, in the southern granulite terrain, are formed primarily due to the release of the carbonic fluids from the supracrustal rocks of the subduction zone and volcanic arc environment. These were later exhumed from the deep crust during the collision process. In the central Indian shield the Narmada-Son lineament and the central Indian suture are the main features of the crust. In the Narmada region, mafic intrusion in the upper crust appears to have played an important role in shaping the present structural trends. The Central Indian suture is a collision zone developed due to the interaction of the Bastar and Bundelkhand cratons. In the northwesternpart of the India, the Aravalli-Delhi trend is the controlling feature for the tectonics of the region. Demarcation of the various boundaries between different crustal units are marked across the trend, by changes in the dip direction and steeply dipping reflections, cutting across the nearly horizontal reflections at various depths in the crust. Plate tectonics appears to be responsible for generation of this belt. In the crustal block between the Delhi-Aravalli system and the Narmada-Son Lineament, which is running to the south of the Saurashtra peninsula the crust up uplifted by as much as 4 to 6 km as compared to the regions outside these trends. Apart from the deep crustal structure, lithospheric and upper mantle studies till 660km depth have also been conducted in the entire Indian plate using seismological tools e.g. P-to-s and S-to-p receiver function, surface waves dispersion and tomographic studies. The Himalayan region shows the architecture of the under thrusting Indian plate beneath the Tibetan plate in the north and north-west, while the subduction beneath the Burmese arc has been mapped in the eastern part. Further, a number of studies have been conducted in the Andaman-Nicobar Islands to image the subduction of Indian oceanic plate in order to understand the genesis of earthquakes in these regions.
Progress in Earth and Planetary Science, doi.org./10.1186/ s40645-020-00374-8 17p. Pdf
Mantle
geophysics, geothermometry
Abstract: In this review, I provide the current status and future prospects for the coupled core-mantle evolution and specifically summarize the constraints arising from geomagnetism and paleomagnetism on the long-term secular variations of the geomagnetic field. The heat flow across the core-mantle boundary (CMB) is essential for determining the best-fit scenario that explains the observational data of geomagnetic secular variations (e.g., onset timing of the inner core growth, geomagnetic polarity reversals, and westward drift) and should include the various origins of the heterogeneous structures in the deep mantle that have affected the heat transfer across the core-mantle boundary for billions of years. The coupled core-mantle evolution model can potentially explain the onset timing of the inner core and its influence on the long-term geomagnetic secular variations, but it is still controversial among modeling approaches on the core energetics because the paleomagnetic data contains various uncertainties. Additionally, with the coupled core-mantle evolution model in geodynamo simulations, the frequency of the geomagnetic polarity reversals can be explained with the time variations of the heat flow across the CMB. Additionally, the effects of the stable region in the outermost outer core to the magnetic evolution are also crucial but there would be still uncertain for their feasibility. However, despite this progress in understanding the observational data for geomagnetic secular variations, there are several unresolved issues that should be addressed in future investigations: (1) initial conditions—starting with the solidification of the global magma ocean with the onset timing of plate tectonics and geodynamo actions and (2) planetary habitability—how the dynamics of the Earth’s deep interior affects the long-term surface environment change that has been maintained in the Earth’s multisphere coupled system.
White-Gaynor, A.L., Nyblade, A.A., Durrheim, R., Raveloson, R., van der Meijde, M., Fadel, I., Paulssen, H., Kwadiba, M., Ntibinyane, O., Titus, N., Sitali, M.
Geochemistry, Geophysics, Geosystems, 10.1029/GC008925 20p. Pdf
Africa, South Africa
Geophysics, seismic
Abstract: We report new P and S wave velocity models of the upper mantle beneath southern Africa using data recorded on seismic stations spanning the entire subcontinent. Beneath most of the Damara Belt, including the Okavango Rift, our models show lower than average velocities (?0.8% Vp; ?1.2% Vs) with an abrupt increase in velocities along the terrane's southern margin. We attribute the lower than average velocities to thinner lithosphere (~130 km thick) compared to thicker lithosphere (~200 km thick) immediately to the south under the Kalahari Craton. Beneath the Etendeka Flood Basalt Province, higher than average velocities (0.25% Vp; 0.75% Vs) indicate thicker and/or compositionally distinct lithosphere compared to other parts of the Damara Belt. In the Rehoboth Province, higher than average velocities (0.3% Vp; 0.5% Vs) suggest the presence of a microcraton, as do higher than average velocities (1.0% Vp; 1.5% Vs) under the Southern Irumide Belt. Lower than average velocities (?0.4% Vp; ?0.7% Vs) beneath the Bushveld Complex and parts of the Mgondi and Okwa terranes are consistent with previous studies, which attributed them to compositionally modified lithosphere resulting from Precambrian magmatic events. There is little evidence for thermally modified upper mantle beneath any of these terranes which could provide a source of uplift for the Southern African Plateau. In contrast, beneath parts of the Irumide Belt in southern and central Zambia and the Mozambique Belt in central Mozambique, deep?seated low velocity anomalies (?0.7% Vp; ?0.8% Vs) can be attributed to upper mantle extensions of the African superplume structure.
Journal of South American Earth Sciences, in press available, 16p. Pdf
South America, Brazil
geophysics
Abstract: Identification of mafic dike swarms and LIPs (Large Igneous Provinces) are of vital importance in geologic history because they provide information on geodynamics, mantle geochemistry, and paleomagnetism. These data provide key information for paleogeographic reconstructions with the aid of barcode matches and precise radiometric ages. Considering such issues, the Brazilian Precambrian shield can be used as a case for refining the cartography of the relevant intraplate activity (e.g., dikes, sills, flood basalts) in space and time. This work presents an updated map of Brazilian mafic dike swarms produced from airborne geophysical maps (Series 1000 - Geological Survey of Brazil). Linear and strong anomalies found on aeromagnetic maps using First Vertical Derivative of the Magnetic Field and Amplitude of the Analytic Signal were mapped on a GIS platform. The obtained data were compared to ternary radiometric maps and geological maps in order to exclude those that do not correspond to mafic dikes. The remaining structures - those believed to represent mafic dikes - were classified based on data compiled from the literature. The updated map exhibits more than 5000 elements, including dikes and magmatic suites, in which about 75% were geologically identified and divided into 60 dike swarms and 10 igneous suites and/or units. The dikes were grouped into sixteen extensional episodes from the Archean to the Cenozoic, although some are related to extension/transtension domains within regional compressive zones akin to orogenic settings. The most frequent records refer to the Proterozoic, representing intraplate episodes, some of them consistent with LIPs. The dataset also includes a large record of the Mesozoic age, which corresponds to major LIP events related to the opening of the Atlantic Ocean and the fragmentation of Gondwana.
Abstract: Most natural diamonds are formed in Earth’s lithospheric mantle; however, the exact mechanisms behind their genesis remain debated. Given the occurrence of electrochemical processes in Earth’s mantle and the high electrical conductivity of mantle melts and fluids, we have developed a model whereby localized electric fields play a central role in diamond formation. Here, we experimentally demonstrate a diamond crystallization mechanism that operates under lithospheric mantle pressure-temperature conditions (6.3 and 7.5 gigapascals; 1300° to 1600°C) through the action of an electric potential applied across carbonate or carbonate-silicate melts. In this process, the carbonate-rich melt acts as both the carbon source and the crystallization medium for diamond, which forms in assemblage with mantle minerals near the cathode. Our results clearly demonstrate that electric fields should be considered a key additional factor influencing diamond crystallization, mantle mineral-forming processes, carbon isotope fractionation, and the global carbon cycle.
Journal of South American Earth Sciences, Vol. 108, 103206, 17p. Pdf
South America, Uruguay
geophysics
Abstract: The integrated observation of geology, gammaspectrometry and magnetometry of southern Uruguay was made possible by the high-resolution aerogeophysical survey carried out during 2014 and 2015. This survey covers nearly the outcropping area of the Uruguayan basement. Previous studies focused on the Proterozoic and Mesozoic dike swarms present in the Uruguayan basement. In this work we address features previously undescribed and unstudied in the Uruguayan basement. Structures previously grouped with the dike swarm are separated (subcircular structures and a lava river) and the basement general patterns were studied. Distinctive magnetic and radiometric features characterize each structural block in the Uruguayan basement, supporting the main tectonic units previously described, but improving the location of their limits.
Abstract: Tectonically the considered area of junction of four lithospheric plates (Nubian, Arabian, Aegean- Anatolian and Sinai) belongs to the Eastern Mediterranean, with its Cyprus-Levantine marine and Anatolian-Nubian-Arabian continental framing. The anomalousness of the region is manifested in the tectono-structural features of the mantle, lithosphere, hydrosphere and specifics of atmospheric, biospheric processes, and Hominid evolution.
Abstract: New evidence from seismic tomography reveals a unique mineral fabric restricted to the thick mantle lithosphere beneath ancient continental cratons, providing an important clue to the formation of these prominent and influential features in Earth’s geological history. Olivine, the dominant mineral of Earth’s upper mantle, has elastic properties that differ along its three crystallographic axes, and preferential alignment of individual olivine grains during plastic deformation can affect the bulk nature of seismic-wave propagation. Surface-wave tomography has shown that over most of Earth, deformation of the mantle lithosphere has oriented olivine crystals with the fast axis in the horizontal plane, but at depths centered at ?150 km within cratonic continental-lithosphere roots, the fast crystallographic axis is preferentially aligned vertically. Because of the high viscosity of the cratonic roots, this fabric is likely to be a vestige from craton formation. Geochemical and petrological studies of upper-mantle garnet-peridotite nodules demonstrate that the cratonic mantle roots are stabilized by their reduced density, which was caused by melt removal at much shallower depths than those from which the nodules were subsequently extracted. The mineral fabric inferred from surface-wave tomography suggests that horizontal shortening carried the depleted zone downward after the melt-depletion event to form the thick continental roots, stretching the depleted material in the vertical dimension by pure shear and causing the fast crystallographic axis to be aligned vertically. This seismological fabric at ?150 km is evidence of the shortening event that created the cratonic roots.
Abstract: The West-African craton is defined by a combination of Archean and Palaeoproterozoic rocks that stabilised at ~2 Ga towards the end of the Paleoproterozoic Eburnean Orogeny, and therefore may reflect the transition from Archean to modern tectonic processes. Exploring its present lithospheric architecture aids further understanding of not only the craton’s stability through its history but also its formation. We investigate the lithospheric structure of the craton through analysing and modelling magnetotelluric (MT) data from a 500-km-long east-west profile in northern Ghana and southern Burkina Faso crossing part of the Baoulé-Mossi Domain and reaching the Volta Basin in the south-eastern part of the craton. Although the MT stations are along a 2D profile, due to the complexity of the structures characterising the area, 3D resistivity modelling of the data is performed to obtain insights on the thermal signature and composition of the subcontinental lithosphere beneath the area. The thermal structure and water content estimates from different resistivity models highlight a strong dependence on the starting model in the 3D inversions, but still enable us to put constraints on the deep structure of the craton. The present?day thermal lithosphere?asthenosphere boundary (LAB) depth is estimated to be at least 250 km beneath the Baoulé-Mossi domain. The area likely transitions from a cold and thick lithosphere with relatively low water content into thinner, more fertile lithosphere below the Volta Basin. Although the inferred amount of water could be explained by Paleoproterozoic subduction processes involved in the formation of the Baoulé-Mossi domain, later enrichment of the lithosphere cannot be excluded.
Abstract: Magnetic data are sensitive to both the induced magnetization in rock units caused by the present earth's magnetic field and the remanent magnetization acquired by rock units in past geologic time. Susceptibility is a direct indicator of the magnetic mineral content, whereas remanent magnetization carries information about the formation process and subsequent structural movement of geologic units. The ability to recover and use total magnetization, defined as the vectorial sum of the induced and remanent magnetization, therefore enables us to take full advantage of magnetic data. The exploration geophysics community has achieved significant advances in inverting magnetic data affected by remanent magnetization. It is now feasible to invert any magnetic data set for total magnetization. We provide an overview of the state of the art in magnetization inversion and demonstrate the informational value of inverted magnetization through a set of case studies from mineral exploration problems. We focus on the methods that recover either the magnitude of the total magnetization or the total magnetization vector itself.
Abstract: The Orapa kimberlite field of Botswana is one of the world’s major diamond producing regions. Within this field, there are several small kimberlite pipes that have not been completely explored in terms of their lateral extent, depth, and diamond potential. Two such pipes, BK54 and BK55, were found during a ground gravity and magnetic survey, and subsequent drilling confirmed the presence of kimberlite material. To determine the physical properties of these pipes, their lateral extent, depth, and thickness were estimated using a gravity and magnetic analysis and 2.5D and 3D modeling. Tilt derivatives of the magnetic data indicated that BK54 has a northwest-trending elliptical shape and BK55 has a roughly circular shape. Residual gravity anomaly maps indicate that BK54 does not have a density anomaly whereas BK55 is associated with a gravity maximum. The 3D gravity and magnetic inversion modeling constrained by magnetic susceptibility measurements indicates that BK54 is smaller in volume than BK55 and that neither pipe is thicker than 125 m. The difference in shape and the lack of a gravity anomaly over BK54 implies a different formation mechanism for each kimberlite pipe. Although several mechanisms are suggested, BK54 may have formed by a more explosive eruption producing more tuffistic material in the crater and diatreme facies. The gravity and magnetic analysis also found that the kimberlite pipes, while small, are larger in extent than was determined by drilling and warrant additional drilling to determine their economic potential.
Journal of African Earth Sciences, Vol. 179, 104206, 21p. pdf
Africa, Cameroon, Central African Republic
geophysics
Abstract: The Bangui Magnetic Anomaly (BMA) is one of the largest magnetic anomalies in the world whose origin is still not known. This research investigated the crustal thickness, Curie depths and thermal structures in the Central African sub-regions - Cameroon, Central African Republic and adjacent countries - which are largely characterized by the Bangui Magnetic Anomaly. To achieve a better understanding and clearer idea of the location of the possible sources of the BMA, analyses of geothermal structures were conducted. Two potential methods were used: gravity to evaluate the crustal thickness and magnetics for geothermal analysis. Spectral analysis of gravity data shows that crustal thickness range between 14 and 55 km. The highest depths were found in Central African Republic. The lower values of crustal thickness were obtained in South-Chad basin with a minimum of roughly 14 km. Geothermal analysis is carried out using the Curie point depth, thermal gradient and heat-flow evaluations. The results show that the BMA is related to a thick crust of roughly 40 km. Depth to the bottom of possible sources does not exceed the lower crust. The mean Curie point depth estimated is 38 km with an error of ±2 km. Geothermal results also show the difference in the thermal behaviour between the crust in the Pan African and Precambrian domain. The mobile zone which constitutes the Pan African domain is associated with a thin crust of high heat-flow values of 65 mW/m2. However, the Precambrian domain beneath the BMA is associated with a thick crust with lower heat-flow values (roughly 45 mW/m2). The difference between crustal thickness and Curie point depths shows that all the sources of the BMA are crustal. The present results are in favour of a geological origin for the Bangui Magnetic anomaly.
Abstract: From west to east along the Sunda-Banda arc, convergence of the Indo-Australian plate transitions from subduction of oceanic lithosphere to arc-continent collision. This region of eastern Indonesia and Timor-Leste provides an opportunity for unraveling the processes that occur during collision between a continent and a volcanic arc, and it can be viewed as the temporal transition of this process along strike. We collected a range of complementary geological and geophysical data to place constraints on the geometry and history of arc-continent collision. Utilizing ?4 yr of new broadband seismic data, we imaged the structure of the crust through the uppermost mantle. Ambient noise tomography shows velocity anomalies along strike and across the arc that are attributed to the inherited structure of the incoming and colliding Australian plate. The pattern of anomalies at depth resembles the system of salients and embayments that is present offshore western Australia, which formed during rifting of east Gondwana. Previously identified changes in geochemistry of volcanics from Pb isotope anomalies from the inner arc islands correlate with newly identified velocity structures representing the underthrusted and subducted Indo-Australian plate. Reconstruction of uplift from river profiles from the outer arc islands suggests rapid uplift at the ends of the islands of Timor and western Sumba, which coincide with the edges of the volcanic-margin protrusions as inferred from the tomography. These findings suggest that the tectonic evolution of this region is defined by inherited structure of the Gondwana rifted continental margin of the incoming plate. Therefore, the initial template of plate structure controls orogenesis.
Journal of Geophysical Research: Solid Earth, doi: 10.1029/2021JB023105
Africa
geophysics
Abstract: The present-day composition of Earth's tectonic plates results from past geological processes. We can learn about Earth's composition from deep rock samples that are carried to the surface during volcanic eruptions and by probing its physical properties, like electrical conductivity, with geophysics. In southern Africa, there are extensive deep rock samples, which have been brought to the surface by kimberlite volcanoes that also host diamonds, and also extensive geophysical data. In this article, we compare the rock compositions with electrical conductivity to learn more about Earth's composition. Our results show that the oldest parts of the plates, which retain compositions similar to their initial composition, appear resistive. On the other hand, regions that have been intruded by deep fluids or molten rock can be resistive or conductive, depending on the types of minerals that were formed during the intrusion. The kimberlite volcanoes mostly erupted through the edges of the most resistive parts of the plates and did not erupt through the conductors. These results will help us to make more accurate interpretations about the composition of parts of the Earth where we do not have deep rock samples.
Applied Sciences, , Preprints 2022, 2022040226 (doi: 10.20944/preprints202204.0226.v1) 24p. Pdf
Mantle
geophysics
Abstract: The easternmost Mediterranean is a distinct transition zone from the ocean to the continent located at the junction between the largest Earth's lithospheric segments: Eurasian and African. The methodology of paleomagnetic mapping of such transition zones is based on integrating the mapping techniques for both continental and oceanic platforms: paleomagnetic reconstructions, results of radiometric dating of magnetized rocks, tectonic-structural reconstructions, biogeography, and utilization of the results of various geophysical surveys. The geodynamic-paleomagnetic mapping makes it possible to reveal the multilevel structural heterogeneity and display complex elements of the geodynamics of different ages inherent in this transition zone. Northern Israel is obviously the most complex area in the easternmost Mediterranean. For the combined paleomagnetic mapping, well-studied paleomagnetically and radiometrically areas were selected: (1) the Carmel area, (2) the Atlit area (internal part of the Carmel area), (3) the Sea of Galilee with the adjoining zones (primarily, the Kinnarot Valley), and (4) the area of the Hula Basin with adjacent areas of the Golan Plateau, Hermon Mt., and Galilea uplift. The constructed paleomagnetic profiles for the Carmel area (on the top of the accumulative surface of the Lower Cretaceous traps), and the Kinnarot Valley - Sea of Galilee - Hula Basin, evidently indicate the complex history of the paleogeodynamic evolution of the region. These studies demonstrate the effectiveness of paleomagnetic mapping interated with paleomagnetic profiling, which crosses these geologically complex areas.
