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The Sheahan Diamond Literature Reference Compilation - Scientific and Media Articles based on Major Keyword - Tomography
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.
Tomography refers to the method of imaging a section of something through the data generated by some sort of penetrating wave. It is relevant to diamonds because it enables mapping of the crust and upper mantle where diamonds are formed and magmas are generated which entrain the diamonds during their ascent. Articles tagged with the keyword "tomography" are exclusively of a scientific nature; no editor of a media publication would ever allow this topic to be visible enough to justify the 'tomography" keyword.
Upper mantle tomographic VP and VS images of the Middle Rocky Mountains in Wyoming, Colorado and New Mexico: evidence for a thick heterogeneous chemical lithosphere
Geological Society of America Annual Meeting Oct. 27-30, Abstract p. 473.
P wave tomography of eastern North America: evidence for mantle evolution from Archean to Phanerozoic, and modification during subsequent hotspot tectonism.
Journal of Geophysical Research, Vol. 117, B12302, 15p.
Upper mantle structure underlying the diamondiferous Slave craton from teleseismic body-wave tomography.
2018 Yellowknife Geoscience Forum , p.104-105. abstract
Canada, Northwest Territories
tomography
Abstract: Cratons are, by definition, the most tectonically stable and oldest parts of the continental lithosphere on Earth. The Archean Slave craton is located in the northwestern part of the Canadian Shield. The propensity of diamondiferous kimberlite pipes in the central Slave craton raises many questions regarding their structural environment and source. Here, we provide the most robust teleseismic P and S body wave tomography models over the Slave craton region based on 20,547 P-wave delay times, 6,140 direct S-wave delay times and 3,381 SKS delay times. The P-wave model reveals an alternating pattern of relative positive and negative anomalies over a fine broad scale region within the central Slave craton. Furthermore, the P-wave model revealed two fine structures located in the lithosphere beneath the Lac de Gras kimberlite cluster, with relatively slow anomalies (B - C) that extend from 75 km to 350 km depths with an apparent dip to the north. These relatively slow P- and S-wave anomalies are associated with metasomatised regions within the lithosphere. The S-wave model displays a slow S-wave anomaly lying from 300 km depth to the transition zone beneath the central Slave craton. This anomaly is located beneath the Lac de Gras kimberlite cluster. We suggest that this anomaly is not the cause of the actual kimberlites at the surface since last eruption occurred 75-45 Ma ago but may be related to a potential kimberlite magma ascent in the asthenosphere.
Abstract: Dynamic topography is a well-established consequence of global geodynamic models of mantle convection with horizontal dimensions of >1000 km and amplitudes up to 2 km. Such physical models guide the interpretation of geological records on equal dimensions. Continent-scale geological maps therefore serve as reference frames of choice to visualize erosion/non-deposition as a proxy for long-wavelength, low-amplitude vertical surface motion. At a resolution of systems or series, such maps display conformable and unconformable time boundaries traceable over hundreds to thousands of kilometres. Unconformable contact surfaces define the shape and size of time gap (hiatus) in millions of years based on the duration of time represented by the missing systems or series. Hiatus for a single system or series base datum diminishes laterally to locations (anchor points) where it is conformable at the mapped resolution; it is highly dependent upon scale. A comparison of hiatus area between two successive system or series boundaries yields changes in location, shape, size and duration, indicative of the transient nature of vertical surface motion. As a single-step technique, it serves as a quantitative proxy for palaeotopography that can be calibrated using other geological data. The tool magnifies the need for geological mapping at the temporal resolution of stages, matching process rates. The method has no resolving power within conformable regions (basins) but connects around them. When applied to marine seismic sections that relate to rock record, not to time, biostratigraphic and radiometric data from deep wells are needed before hiatus areas - that relate to time - can be mapped.
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.
Geochemistry, Geophysics, Geosystems, Vol. 20, 1, pp. 120-147.
Africa
tomography
Abstract: We use advanced seismic imaging techniques (full?waveform tomography), constrained by data from background (ambient) seismic noise to image the upper mantle beneath the African continent and search for low?velocity structures (hot spots) that might coincide with regions of volcanism, surface uplift, and continental rifting, particularly along the East African Rift. We also searched for high?velocity structures (old, rigid blocks) that could influence how warm, buoyant material flows within the Earth's upper mantle. Our seismic tomography method allowed us to obtain a clear image of structure beneath parts of Africa where no or very few seismometers are located (such as the Sahara Desert and the Congo Basin). Our results provide indications for segmented secondary (or shallow) upwellings in the upper mantle beneath East Africa, as opposed to earlier models suggesting one large, continuous plume within the upper mantle. Our results also suggest that the one large, rigid, cratonic block previously imaged beneath the Congo region may instead be composed of smaller, distinct blocks. These results provide insight into the factors that control continental rifting along East Africa and provide new testable models that help us to understand the relationships between upper mantle flow, rifting, volcanism, surface uplift, and sedimentation records.
Abstract: Researchers peering into Earth’s interior found two continent-sized structures that upend our picture of the mantle. What could their existence mean for us back on Earth’s surface?
Earth and Planetary Science letters, Vol. 524, 115723 11p.
United States
tomography
Abstract: The leading hypothesis to explain 410 and 660 km discontinuity topography and coincident velocity variations is the thermal hypothesis stated as: temperature variations are the primary modulator of discontinuity topography and seismic velocity variations. To test the thermal hypothesis, discontinuity topography maps are correlated with coincident P- and S-velocity variations for the eastern half of the United States sampled by IRIS-EarthScope USArray seismic data. The discontinuity topography maps were made via common-conversion point migration of P-wave receiver functions. The receiver functions were made using a multi-event and multi-station deconvolution method. Fundamental to our results is the choice of three-dimensional P- and S-velocity models, which are used as migration velocity models and for correlation analysis. Two three-dimensional velocity models are used in our analysis: the MITS-model of Golos et al. (2018) and the SL-model of Schmandt and Lin (2014). The Pearson correlation coefficient is used to estimate the degree of linearity between the discontinuity topography and coincident velocity variations. A bivariate regression of discontinuity topography versus coincident velocity variations (termed the mineral physics slope) is performed and compared to a range of slopes constrained by published velocity-temperature derivatives and Clapeyron slopes. Using spatially binning, the discontinuity topography and coincident velocity variations, spatial maps of the correlation coefficient and mineral physics slope are made. Most of the discontinuity sampling area has reasonable correlation values (?0.4) and plausible mineral physics slope values. The veracity of the thermal hypothesis is assessed by integrating the probability density functions of the mineral physics slopes over a domain defined by the published range of 410 and 660 Clapeyron slopes. At the 410, the MITS-model and SL-model thermal hypothesis probabilities are 52% and 51%, respectively, and the seismic Clapeyron slope estimates are 2.7 and 1.3 MPa/K, respectively. At the 660, the MITS-model and SL-model thermal hypothesis probabilities are 54% and 75%, respectively and the seismic Clapeyron slope estimates are ?1.1 and ?1.7 MPa/K, respectively. These Clapeyron slopes estimates are in the middle of plausible Clapeyron slope ranges. Using these Clapeyron slopes, temperature maps show a ±300 K range at the 410 and a ±600 K range at the 660. For regions that are inconsistent with the thermal hypothesis, we suggest that the leading explanations are uncertainties in the velocity models used and secondarily, hydration effects.
Abstract: Based on SEMUCB?WM1 tomographic model, validated by other recent models, and fluid mechanics constraints, we show that the large low shear velocity provinces (LLSVPs) present at the base of the Earth's mantle beneath the Pacific and Africa do not extend as compact, uniform structures very high above the core?mantle boundary. In contrast, they contain a number of well?separated, low?velocity conduits that extend vertically throughout most of the lower mantle. The conceptual model of compact piles, continuously covering the areal extent of the LLSVPs, is therefore not correct. Instead, each LLSVP is composed of a bundle of thermochemical upwellings probably enriched in denser than average material. It is only when the tomographic model is filtered to long wavelengths that the two bundles of plumes appear as uniform provinces. Furthermore, the overall shape of the LLSVPs is probably controlled by the distribution of subducted slabs, and due to their thermochemical nature, the position of both LLSVPs and individual upwelling dynamics should be time dependent. There is also evidence for smaller plumes originating near the CMB in the faster than average regions of the voting map of Lekic et al. (2012, https://doi.org/10.1016/j.epsl.2012.09.014) as well as other, barely resolved, weaker plumes within the LLSVPs. These finer?scale features are starting to be resolved tomographically owing to improvements in full waveform modeling of body waves, including diffracted S waves (Sdiff) and waves multiply reflected on the core?mantle boundary (ScS) and their codas.
Abstract: The majority of this paper is a transcription from the video of the AusIMM Webinar at the Western Australian, South West Branch on the 30th July 2020. To view the video go to https://vimeo.com/464013825/1ed4a0c752 . Start listening at about 5 minutes in. The language in this paper is thus vernacular and not geologese. This should make it more easily read and understood by the majority of readers. Africa is a rich source of minerals. The main mining fields in Africa are located on the ring structures and linears from the surface right to the limits of detailed data at 400 km depth. The mechanism and source of the fluid for most mineralisation may have been discovered by this research. This Part 6 section describes the relationship of the metal and diamond mineralisation to the linear and ring structures observed in African seismic tomography at 170-250 km depth.
Abstract: The oval-shaped basin of Hudson Bay occurs near the center of the round-oval Archaean crustal domain of the North American continent. This paper presents models of the geological structure and evolution of the subcontinental lithospheric mantle underlying Hudson Bay and surrounding tectonic provinces based on geological interpretations of regional geological and geophysical data and results of seismic tomography investigations that have been conducted under the Hudson Bay Lithospheric Experiment. The experiment was aimed at lithospheric processes directly related to the origin of the North American craton and the Hudson Bay basin. Hudson Bay is located directly above the lithospheric keel of North America. The geological history demonstrates systematic "renovation" of the basin: (1) origin and evolution of the Neoarchaean Lake Minto basin (~2.75 Ga); (2) accumulation of the Palaeoproterozoic volcanic-sedimentary filling of the epicontinental basin, relics of which is preserved on its passive margins (2.03-1.87 Ga); (3) origin of Ordovician-Late Devonian sedimentary sequence whose maximum thickness reaches 2.5 km; and (4) the development of Late Jurassic-Miocene sediment-filled ring-shaped trough immediately above the lithospheric keel. The Hudson Bay basin occurs above the lithospheric keel in compliance with thermomechanical model of ascending plume. Tomography studies have not detected evidence of either production or transformation of the lithosphere in the Palaeoproterozoic, which are implied by the model of the United Plates of America. Interpretations of tomography data reveal a vertical axial zone in the lithosphere beneath Hudson Bay, which extends from the lithosphere-asthenosphere boundary to the base of the crust or, perhaps, even to the present day surface. The zone is made up of relatively light low-velocity igneous rocks, probably a swarm of kimberlite dikes or pipes. At 2.75 Ga, the North American continent was a single continental mass with Hudson Bay at its center.
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: EarthScope's USArray seismic component provided unprecedented coverage of the contiguous United States and has therefore spurred significant advances in tomographic imaging and geodynamic modelling. Here, we present a new global, radially anisotropic shear wave velocity tomography model to investigate upper mantle structure and North American Plate dynamics, with a focus on the contiguous United States. The model uses a data-adaptive mesh and traveltimes of both surface waves and body waves to constrain structure in the crust and mantle in order to arrive at a more consistent representation of the subsurface compared to what is provided by existing models. The resulting model is broadly consistent with previous global models at the largest scales, but there are substantial differences under the contiguous United States where we can achieve higher resolution. On these regional scales, the new model contains short wavelength anomalies consistent with regional models derived from USArray data alone. We use the model to explore the geometry of the subducting Farallon Slab, the presence of upper mantle high velocity anomalies, low velocity zones in the central and eastern United States and evaluate models of dynamic topography in the Cordillera. Our models indicate a single, shallowly dipping, discontinuous slab associated with the Farallon Plate, but there are remaining imaging challenges. Inferring dynamic topography from the new model captures both the long-wavelength anomalies common in global models and the short-wavelength anomalies apparent in regional models. Our model thus bridges the gap between high-resolution regional models within the proper uppermost mantle context provided by global models, which is crucial for understanding many of the fundamental questions in continental dynamics.
Abstract: High-resolution P-wave velocity and anisotropy structure of the hitherto elusive uppermost mantle beneath the Indian shield and its surrounding regions are presented to unravel the tectonic imprints in the lithosphere. We inverted high quality 19,500 regional Pn phases from 172 seismological stations for 4780 earthquakes at a distance range of 2° to 15° with a mean apparent Pn velocity of 8.22 km/s. The results suggest that the Pn velocity anomalies with fast anisotropic directions are consistent with the collision environments in the Himalaya, Tibetan Plateau, Tarim Basin, and Burmese arc regions. The higher Pn anomalies along the Himalayan arc explicate the subducting cold Indian lithosphere. The cratonic upper mantle of the Indian shield is characterized by Pn velocity of 8.12-8.42 km/s, while the large part of the central Indian shield has higher mantle-lid velocity of ~8.42 km/s with dominant anisotropic value of 0.2-0.3 km/s (~7.5%) suggesting the presence of mafic ‘lava pillow’ related to the Deccan volcanism. The impressions of the rifts and the mobile belts are conspicuous in the velocity anomaly image indicating their deep seated origin. The Pn anisotropy in the Indian shield exhibits a complex pattern and deviates from the absolute plate motion directions derived from the SKS study, demonstrating the presence of frozen anisotropy in the Indian lithospheric uppermost mantle, due to the large scale tectonic deformation after its breakup from the Gondwanaland. Whereas, Pn and SKS anisotropic observations are well consistent in Tarim basin, Tibetan regions, eastern Himalayan syntaxis and the Burmese arc. The modeled anisotropic Pn clearly manifests a lower velocity anomaly bounded by 85°E and 90°E ridges in the southern Bay of Bengal. Further, 85°E ridge spatially separates the BoB lithosphere into faster and slower regions consistent with the body wave tomography and free-air gravity observation.
Abstract: The two most abundant minerals in the Earth’s lower mantle are bridgmanite and ferropericlase. The bulk modulus of ferropericlase (Fp) softens as iron d-electrons transition from a high-spin to low-spin state, affecting the seismic compressional velocity but not the shear velocity. Here, we identify a seismological expression of the iron spin crossover in fast regions associated with cold Fp-rich subducted oceanic lithosphere: the relative abundance of fast velocities in P- and S-wave tomography models diverges in the?~1,400-2,000 km depth range. This is consistent with a reduced temperature sensitivity of P-waves throughout the iron spin crossover. A similar signal is also found in seismically slow regions below?~1,800 km, consistent with broadening and deepening of the crossover at higher temperatures. The corresponding inflection in P-wave velocity is not yet observed in 1-D seismic profiles, suggesting that the lower mantle is composed of non-uniformly distributed thermochemical heterogeneities which dampen the global signature of the Fp spin crossover.
Abstract: To investigate lateral and depth variations of seismic anisotropy beneath the central-western United States, we determined a detailed 3-D model of P-wave anisotropic tomography by inverting a large number of arrival-time data of local and teleseismic events. Our results reveal significant azimuthal anisotropies in the crust and lithosphere, which are associated with ancient orogenic collisional and magmatic activities. As depth increases, the fast-velocity direction (FVD) pattern becomes gradually trended and small features fade away. There is a boundary in the FVD distribution, which separates the tectonically active region in the west from the stable cratonic region in the east. Frozen-in anisotropy with a NW-SE FVD is preserved in the thick Wyoming cratonic lithosphere that exhibits as a high-velocity (high-V) anomaly to a depth of ~250 km. In the asthenosphere beneath the western thin lithosphere, FVDs are generally parallel with the absolute motion direction of the North American plate due to shearing between the plate and the asthenosphere. In the deeper areas, the subducted and fragmented slab exhibiting as high-V anomalies leads to slab-related mantle flows. These results indicate that seismic anisotropies exist in both the lithosphere and asthenosphere with different geodynamic mechanisms and it is feasible to link the P-wave azimuthal anisotropy to lithospheric deformations, fossil anisotropy in the lithosphere, and flows in the asthenosphere.
