Hotspot signatures at the North American passive margin

Geology ◽  
2020 ◽  
Author(s):  
Zhongmin Tao ◽  
Aibing Li ◽  
Karen M. Fischer
Keyword(s):  
North America ◽  
New England ◽  
Velocity Model ◽  
Passive Margin ◽  
Small Scale ◽  
Low Velocity ◽  
Velocity Anomaly ◽  
The North ◽  
New Information ◽  
Velocity Anomalies

The presence of localized low-velocity anomalies in the upper mantle beneath the passive Atlantic margin in North America is a puzzling geophysical observation. Whether the anomalies are caused by the remnant heat from past hotspots or ongoing asthenospheric upwelling is still debated. We addressed the formation of the anomalies based on a recent velocity model for eastern North America, which reveals new information on their shapes and anisotropic signatures. The low-velocity anomaly in New England appears as a narrow column above 90 km depth and broadens westward at depths of 120–200 km. Two slow anomalies are imaged under the central Appalachian Mountains between 140 km and 240 km. These low velocities correspond to pronounced positive radial anisotropy (Vsh > Vsv), indicating a dominantly horizontal asthenospheric flow. They also coincide with the tracks of the Great Meteor hotspot (140–115 Ma) and an inferred hidden hotspot (60–50 Ma). The anomalies in the central Appalachians could be due to lithospheric interaction with the second hotspot and subsequent lithospheric instabilities. The complex shape of the New England anomaly is consistent with interaction with both hotspots. The first hotspot could have eroded the base of the lithosphere, forming a channel, and the second hotspot could have further thinned the lithosphere and produced a localized cavity at shallow depths. Consequently, the indented lithosphere base would have filled with channelized asthenospheric flow or produced small-scale convection, helping to sustain the slow anomaly. Low-velocity anomalies at the North America passive margin are likely the consequences of prior hotspot interactions.

Resolving near-seabed velocity anomalies: Deep water offshore eastern India

Geophysics ◽  
2008 ◽  
Vol 73 (5) ◽  
pp. VE235-VE241 ◽  
Author(s):  
Juergen Fruehn ◽  
Ian F. Jones ◽  
Victoria Valler ◽  
Pranaya Sangvai ◽  
Ajoy Biswal ◽  
...  
Keyword(s):  
Velocity Field ◽  
Deep Water ◽  
Model Building ◽  
Velocity Model ◽  
Narrow Channel ◽  
Small Scale ◽  
Low Velocity ◽  
Near Surface ◽  
Velocity Model Building ◽  
Velocity Anomalies

Imaging in deep-water environments poses a specific set of challenges, both in data preconditioning and velocity model building. These challenges include scattered, complex 3D multiples, aliased noise, and low-velocity shallow anomalies associated with channel fills and gas hydrates. We describe an approach to tackling such problems for data from deep water off the east coast of India, concentrating our attention on iterative velocity model building, and more specifically the resolution of near-surface and other velocity anomalies. In the region under investigation, the velocity field is complicated by narrow buried canyons ([Formula: see text] wide) filled with low-velocity sediments, which give rise to severe pull-down effects; possible free-gas accumulation below an extensive gas-hydrate cap, causing dimming of the image below (perhaps as a result of absorption); and thin-channel bodies with low-velocity fill. Hybrid gridded tomography using a conjugate gradient solver (with [Formula: see text] vertical cell size) was applied to resolve small-scale velocity anomalies (with thicknesses of about [Formula: see text]). Manual picking of narrow-channel features was used to define bodies too small for the tomography to resolve. Prestack depth migration, using a velocity model built with a combination of these techniques, could resolve pull-down and other image distortion effects in the final image. The resulting velocity field shows high-resolution detail useful in identifying anomalous geobodies of potential exploration interest.

scholarly journals Seismic Imaging and Seismicity Analysis in Beijing-Tianjin-Tangshan Region

2011 ◽  
Vol 2011 ◽  
pp. 1-13 ◽  
Author(s):  
Xiangwei Yu ◽  
Wenbo Zhang ◽  
Yun-tai Chen
Keyword(s):  
Focal Depth ◽  
Velocity Model ◽  
Double Difference ◽  
Low Velocity ◽  
Arrival Times ◽  
Velocity Anomaly ◽  
The North ◽  
Tomographic Method ◽  
Vertical Slice ◽  
Tomography Method

In this study a new tomographic method is applied to over 43,400 high-quality absolute direct P arrival times and 200,660 relative P arrival times to determine detailed 3D crustal velocity structures as well as the absolute and relative hypocenter parameters of 2809 seismic events under the Beijing-Tianjin-Tangshan region. The inferred velocity model of the upper crust correlates well with the surface geological and topographic features in the BTT region. In the North China Basin, the depression and uplift areas are imaged as slow and fast velocities, respectively. After relocation, the double-difference tomography method provides a sharp picture of the seismicity in the BTT region, which is concentrated along with the major faults. A broad low-velocity anomaly exists in Tangshan and surrounding area from 20 km down to 30 km depth. Our results suggest that the top boundary of low-velocity anomalies is at about 25.4 km depth. The event relocations inverted from double-difference tomography are clusted tightly along the Tangshan-Dacheng Fault and form three clusters on the vertical slice. The maximum focal depth after relocation is about 25 km depth in the Tangshan area.

scholarly journals Imaging of the Upper Mantle Beneath Southeast Asia: Constrained by Teleseismic P-Wave Tomography

2020 ◽  
Vol 12 (18) ◽  
pp. 2975
Author(s):  
Huiyan Shi ◽  
Tonglin Li ◽  
Rongzhe Zhang ◽  
Gongcheng Zhang ◽  
Hetian Yang
Keyword(s):  
South China Sea ◽  
Southeast Asia ◽  
Travel Time ◽  
South China ◽  
High Velocity ◽  
Velocity Model ◽  
P Wave ◽  
Deep Part ◽  
Low Velocity ◽  
Velocity Anomalies

It is of great significance to construct a three-dimensional underground velocity model for the study of geodynamics and tectonic evolution. Southeast Asia has attracted much attention due to its complex structural features. In this paper, we collected relative travel time residuals data for 394 stations distributed in Southeast Asia from 2006 to 2019, and 14,011 seismic events were obtained. Then, teleseismic tomography was applied by using relative travel time residuals data to invert the velocity where the fast marching method (FMM) and subspace method were used for every iteration. A novel 3D P-wave velocity model beneath Southeast Asia down to 720 km was obtained using this approach. The tomographic results suggest that the southeastern Tibetan Plateau, the Philippines, Sumatra, and Java, and the deep part of Borneo exhibit high velocity anomalies, while low velocity anomalies were found in the deep part of the South China Sea (SCS) basin and in the shallow part of Borneo and areas near the subduction zone. High velocity anomalies can be correlated to subduction plates and stable land masses, while low velocity anomalies can be correlated to island arcs and upwelling of mantle material caused by subduction plates. We found a southward subducting high velocity body in the Nansha Trough, which was presumed to be a remnant of the subduction of the Dangerous Grounds into Borneo. It is further inferred that the Nansha Trough and the Dangerous Grounds belong to the same tectonic unit. According to the tomographic images, a high velocity body is located in the deep underground of Indochina–Natuna Island–Borneo–Palawan, depth range from 240 km to 660 km. The location of the high velocity body is consistent with the distribution range of the ophiolite belt, so we speculate that the high velocity body is the remnant of thee Proto-South China Sea (PSCS) and Paleo-Tethys. This paper conjectures that the PSCS was the southern branch of Paleo-Tethys and the gateway between Paleo-Tethys and the Paleo-Pacific Ocean. Due to the squeeze of the Australian plate, PSCS closed from west to east in a scissor style, and was eventually extinct under Borneo.

The upper mantle beneath the South Atlantic Ocean, South America and Africa from waveform tomography with massive data sets

2020 ◽  
Vol 221 (1) ◽  
pp. 178-204 ◽  
Author(s):  
N L Celli ◽  
S Lebedev ◽  
A J Schaeffer ◽  
M Ravenna ◽  
C Gaina
Keyword(s):  
South America ◽  
Upper Mantle ◽  
High Velocity ◽  
South Atlantic ◽  
The South ◽  
Low Velocity ◽  
The West ◽  
Velocity Anomaly ◽  
Archean Crust ◽  
Velocity Anomalies

SUMMARY We present a tomographic model of the crust, upper mantle and transition zone beneath the South Atlantic, South America and Africa. Taking advantage of the recent growth in broadband data sampling, we compute the model using waveform fits of over 1.2 million vertical-component seismograms, obtained with the automated multimode inversion of surface, S and multiple S waves. Each waveform provides a set of linear equations constraining perturbations with respect to a 3-D reference model within an approximate sensitivity volume. We then combine all equations into a large linear system and solve it for a 3-D model of S- and P-wave speeds and azimuthal anisotropy within the crust, upper mantle and uppermost lower mantle. In South America and Africa, our new model SA2019 reveals detailed structure of the lithosphere, with structure of the cratons within the continents much more complex than seen previously. In South America, lower seismic velocities underneath the transbrasilian lineament (TBL) separate the high-velocity anomalies beneath the Amazon Craton from those beneath the São Francisco and Paraná Cratons. We image the buried portions of the Amazon Craton, the thick cratonic lithosphere of the Paraná and Parnaíba Basins and an apparently cratonic block wedged between western Guyana and the slab to the west of it, unexposed at the surface. Thick cratonic lithosphere is absent under the Archean crust of the São Luis, Luis Álves and Rio de La Plata Cratons, next to the continental margin. The Guyana Highlands are underlain by low velocities, indicating hot asthenosphere. In the transition zone, we map the subduction of the Nazca Plate and the Chile Rise under Patagonia. Cratonic lithosphere beneath Africa is more fragmented than seen previously, with separate cratonic units observed within the West African and Congo Cratons, and with cratonic lithosphere absent beneath large portions of Archean crust. We image the lateral extent of the Niassa Craton, hypothesized previously and identify a new unit, the Cubango Craton, near the southeast boundary of the grater Congo Craton, with both of these smaller cratons unexposed at the surface. In the South Atlantic, the model reveals the patterns of interaction between the Mid-Atlantic Ridge (MAR) and the nearby hotspots. Low-velocity anomalies beneath major hotspots extend substantially deeper than those beneath the MAR. The Vema Hotspot, in particular, displays a pronounced low-velocity anomaly under the thick, high-velocity lithosphere of the Cape Basin. A strong low velocity anomaly also underlies the Cameroon Volcanic Line and its offshore extension, between Africa and the MAR. Subtracting the global, age-dependent VS averages from those in the South Atlantic Basins, we observe areas where the cooling lithosphere is locally hotter than average, corresponding to the location of the Tristan da Cunha, Vema and Trindade hotspots. Beneath the anomalously deep Argentine Basin, we image unusually thick, high-velocity lithosphere, which suggests that its anomalously great depth can be explained, at least to a large extent, by isostatic, negative lithospheric buoyancy.

Velocity estimation by beam stack

Geophysics ◽  
1992 ◽  
Vol 57 (8) ◽  
pp. 1034-1047 ◽  
Author(s):  
Biondo Biondi
Keyword(s):  
Estimation Method ◽  
Velocity Model ◽  
Velocity Estimation ◽  
Gradient Algorithm ◽  
Stratified Medium ◽  
Detailed Knowledge ◽  
Low Velocity ◽  
Velocity Anomaly ◽  
Interval Velocity ◽  
Lateral Variations

Imaging seismic data requires detailed knowledge of the propagation velocity of compressional waves in the subsurface. In conventional seismic processing, the interval velocity model is usually derived from stacking velocities. Stacking velocities are determined by measuring the coherency of the reflections along hyperbolic moveout trajectories in offset. This conventional method becomes inaccurate in geologically complex areas because the conversion of stacking velocities to interval velocities assumes a horizontally stratified medium and mild lateral variations in velocity. The tomographic velocity estimation proposed in this paper can be applied when there are dipping reflectors and strong lateral variations. The method is based on the measurements of moveouts by beam stacks. A beam stack measures local coherency of reflections along hyperbolic trajectories. Because it is a local operator, the beam stack can provide information on nonhyperbolic moveouts in the data. This information is more reliable than traveltimes of reflections picked directly from the data because many seismic traces are used for computing beam stacks. To estimate interval velocity, I iteratively search for the velocity model that best predicts the events in beam‐stacked data. My estimation method does not require a preliminary picking of the data because it directly maximizes the beam‐stack’s energy at the traveltimes and surface locations predicted by ray tracing. The advantage of this formulation is that detection of the events in the beam‐stacked data can be guided by the imposition of smoothness constraints on the velocity model. The optimization problem of maximizing beam‐stack energy is solved by a gradient algorithm. To compute the derivatives of the objective function with respect to the velocity model, I derive a linear operator that relates perturbations in velocity to the observed changes in the beam‐stack kinematics. The method has been successfully applied to a marine survey for estimating a low‐velocity anomaly. The estimated velocity function correctly predicts the nonhyperbolic moveouts in the data caused by the velocity anomaly.

scholarly journals GROWTH AND FRUITING OF `DELICIOUS' APPLE ON CLONAL ROOTSTOCKS IN THE 1984 NC-140 PLANTING IN MAINE

HortScience ◽  
1991 ◽  
Vol 26 (5) ◽  
pp. 481e-481
Author(s):  
James R. Schupp
Keyword(s):  
North America ◽  
New England ◽  
North Central ◽  
Malus Domestica Borkh ◽  
The Third ◽  
The North ◽  
Cooperative Testing ◽  
North Central Regional ◽  
Root Suckering ◽  
Cooperative Project

In 1984 trees of `Starkspur Supreme Delicious' apple (Malus domestica Borkh) on 16 rootstocks were planted at 30 sites in North America according to guidelines established for cooperative testing by the North Central Regional Cooperative Project (NC-140). Tree loss and root suckering in the Maine planting have been low, similar to that of other sites. Tree size in Maine is smallest amoung all sites after seven seasons. Trees on Budagovsky 9 (B.9) rootstock were the most precocious, producing significantly higher flower numbers and yield in the third year. Other precocious root-stocks in this planting included C.6, M.26EMLA, M.7EMLA and P.1. After seven years, B.9, C.6 and M.26EMLA were the most productive amoung the dwarf trees, and consequently are the most efficient. P.1 and M.7EMLA were the most productive amoung the more vigorous stocks. This trial will be conducted for 3 more seasons, however it appears that B.9, C.6 and P.1 may have potential as rootstocks for commercial apple orchards in New England.

scholarly journals Adjoint tomography of the Hikurangi subduction zone and the North Island of New Zealand

2021 ◽  
Author(s):  
Bryant Chow
Keyword(s):  
New Zealand ◽  
Subduction Zone ◽  
East Coast ◽  
Plate Boundary ◽  
Velocity Model ◽  
Seismic Velocities ◽  
Low Velocity ◽  
Velocity Models ◽  
The North ◽  
Adjoint Tomography

<p><b>Seismic tomography is a powerful tool for understanding Earth structure. In New Zealand, velocity models derived using ray-based tomography have been used extensively to characterize the complex plate boundary between the Australian and Pacific plates. Advances in computational capabilities now allow us to improve these velocity models using adjoint tomography, an imaging method which minimizes differences between observed and simulated seismic waveforms. We undertake the first application of adjoint tomography in New Zealand to improve a ray-based New Zealand velocity model containing the Hikurangi subduction zone and the North Island of New Zealand.</b></p> <p>In support of this work we deployed the Broadband East Coast Network (BEACON), a temporary seismic network aimed at improving coverage of the New Zealand permanent network, along the east coast of the North Island. We concurrently develop an automated, open-source workflow for full-waveform inversion using spectral element and adjoint methods. We employ this tool to assess a candidate velocity model’s suitability for adjoint tomography. Using a 3D ray-based traveltime tomography model of New Zealand, we generate synthetic seismic waveforms for more than 10 000 source–receiver pairs and evaluate waveform misfits. We subsequently perform synthetic checkerboard inversions with a realistic New Zealand source–receiver distribution. Reasonable systematic time shifts and satisfactory checkerboard resolution in synthetic inversions indicate that the candidate model is appropriate as an initial model for adjoint tomography. This assessment also demonstrates the relative ease of use and reliability of the automated tools.</p> <p>We then undertake a large-scale adjoint tomography inversion for the North Island of New Zealand using up to 1 800 unique source–receiver pairs to fit waveforms with periods 4–30 s, relating to minimum waveform sensitivities on the order of 5 km. Overall, 60 geographically well-distributed earthquakes and as many as 88 broadband station locations are included. Using a nonlinear optimization algorithm, we undertake 28 model updates of Vp and Vs over six distinct inversion legs which progressively increase resolution. The total inversion incurred a computational cost of approximately 500 000 CPU-hours. The overall time shift between observed and synthetic seismograms is reduced, and updated velocities show as much as ±30% change with respect to initial values. A formal resolution analysis using point spread tests highlights that velocity changes are strongly resolved onland and directly offshore, at depths above 30 km, with low-amplitude changes (> 1%) observed down to 100 km depth. The most striking velocity changes coincide with areas related to the active Hikurangi subduction zone.</p> <p>We interpret the updated velocity model in terms of New Zealand tectonics and geology, and observe good agreement with known basement terranes, and major structural elements such as faults, sedimentary basins, broad-scale subduction related features. We recover increased spatial heterogeneity in seismic velocities along the strike of the Hikurangi subduction zone with respect to the initial model. Below the East Coast, we interpret two localized high-velocity anomalies as previously unidentified subducted seamounts. We corroborate this interpretation with other work, and discuss the implications of deeply subducted seamounts on slip behavior along the Hikurangi margin. In the Cook Strait we observe a low-velocity zone that we interpret as a deep sedimentary basin. Strong velocity gradients bounding this low-velocity zone support hypotheses of a structural boundary here separating the North and South Islands of New Zealand. In the central North Island, low-velocity anomalies are linked to surface geology, and we relate seismic velocities at depth to crustal magmatic activity below the Taupo Volcanic Zone.</p> <p>This new velocity model provides more accurate synthetic seismograms and additional constraints on enigmatic tectonic processes related to the North Island of New Zealand. Both the velocity model itself, and the underpinning methodological contributions, improve our ever-expanding understanding of the North Island of New Zealand, the Hikurangi subduction zone, and the broader Australian-Pacific plate boundary.</p>

An application of induced event interferometry approach at The Geysers Geothermal Field, California, USA

2021 ◽  
Author(s):  
Taghi Shirzad ◽  
Stanisław Lasocki ◽  
Beata Orlecka‐Sikora
Keyword(s):  
Velocity Structure ◽  
General Structure ◽  
Clean Energy ◽  
Velocity Model ◽  
Geothermal Field ◽  
Small Scale ◽  
Low Velocity ◽  
Study Region ◽  
Resultant Velocity ◽  
The Geysers

&lt;p&gt;While the classical tomography approaches, e.g., P-, S-, and/or surface-wave traveltime tomography, provide a general structure of the Earth&amp;#8217;s interior, new developments in signal processing of interferometry approaches are needed to obtain a high-resolution velocity structure. If the number of earthquakes is adequate, the virtual seismometer method may be a solution in regions with sparse instrumental coverage. Theoretically, the empirical Green&amp;#8217;s functions between a pair of events can be retrieved using earthquake&amp;#8217;s cross-correlations. Here, an event interferometry approach was used on a very small scale around Prati-9 and Prati-29 injection wells in the NW of The Geysers Geothermal Field. The study region experienced intense injection-induced seismicity. We selected all events with location uncertainties less than 50 m in a cuboid of the horizontal side ~1 &amp;#215; ~2 km and the vertical edge at depths between 1.0 and 2.0 km. The cuboid was cut into 100m thick layers, and we applied to events from each layer criteria enabling a quasi 2D approach. After calculating the Rayleigh wave group velocity dispersion curves, further processing was performed at a 0.2s period, selected based on the sensitivity kernel criterion. Finally, the relative velocity model of each layer at the depth z was obtained by subtracting the velocity model of the just overlying layer (at the depth z-100m) from the model of this layer. Our resultant velocity model in the study area indicated four low-velocity anomalies. The first one can be linked by the two layers interface topography variation at the top of the cuboid (depth 1000 m). The secondary faults can cause the second low-velocity anomaly. The other two anomalies look to result from fluid injection into Prati-9 and Prati-29 wells.&amp;#160;&lt;br&gt;This work was supported under the S4CE: &quot;Science for Clean Energy&quot; project, which has received funding from the European Union&amp;#8217;s Horizon 2020 research and innovation program, under grant agreement No 764810.&lt;/p&gt;

Role of inherited structures and magmatism in North Tanzania from high resolution teleseismic P and S body-wave tomographies.

2021 ◽  
Author(s):  
Stéphanie Gautier ◽  
Adeline Clutier ◽  
Christel Tiberi ◽  
Fleurice Parat ◽  
Benoit Gibert ◽  
...  
Keyword(s):  
Body Wave ◽  
A Priori ◽  
Regional Scale ◽  
Velocity Model ◽  
S Wave ◽  
Fine Grid ◽  
Local Tomography ◽  
The North ◽  
Velocity Anomalies

&lt;p&gt;The North Tanzanian Divergence (NTD) is a zone of rift initiation. Its surface expression results from interactions between deep-mantle (mantle plume), lithospheric (inherited rheology and stratification, melting...) and crustal (dyke propagation, fault activation...) processes. However, the role of each process on the observed surface activity is still debated, because highly difficult to decorrelate.&lt;/p&gt;&lt;p&gt;We recently carried out a study to obtain enhanced P and S-wave tomography, from the surface down to 150-200 km depth. The particularity of our method consists in its initial velocity model. It is composed of a 1D IASP91 regional velocity model in which we inserted an a priori 3D crustal velocity model with a fine grid. This crustal model was deduced from an independent local tomography inversion.&lt;/p&gt;&lt;p&gt;The P and S images obtained, resulting from the teleseismic inversion of this hybrid method, show strong contrasted velocity anomalies: from 10 % of P (Vp) and S velocity (Vs) variation on the craton, to -17 % below the rift axis. The anomalies locations are consistent with the surface geology (rifting basin, border faults, volcanoes). At a regional scale, the strongest velocity contrasts correspond to the lithospheric inherited structure (Tanzanian craton and Proterozoic belts) boundaries, which control the propagation of the rift. In particular, the Masai cratonic block, south of the NTD, is inferred to have a strong influence in the rift evolution. The transition from the North-South axial valley into three diverging rift arms (Eyasi, Natron-Manyara and Pangani) is likely due to the change in rheology and to the presence of magma along inherited sutures between the craton and the mobile belts.&lt;/p&gt;&lt;p&gt;However, interrogations about the role of the thermal changes, the melt/fluid presence and the mantle composition in the NTD on these velocity anomalies still remain. To distinguish which parameters are acting in the rift, we realize a Vp/Vs ratio map. With this new data, and in the light of parallel petrological studies, we interpret the Vp/Vs anomalies in term of gas and/or melt concentration zones.&lt;/p&gt;

Excavations at Carthage 1977–8. Fourth Interim Report

1979 ◽  
Vol 59 (1) ◽  
pp. 19-49 ◽  
Author(s):  
Henry Hurst
Keyword(s):  
Present Report ◽  
Large Body ◽  
Fourth Century ◽  
Second Century ◽  
Small Scale ◽  
Interim Report ◽  
The North ◽  
New Information ◽  
New Evidence ◽  
Harbour Area

SummaryThe present report covers the fourth and fifth seasons of excavation on the Ilôt de l'Amirauté at Carthage. Evidence from a borehole suggests that the pre-fourth-century B.C. sand previously taken for ‘natural’ may be a 5-m. thick fill above a level containing pottery. The later Punic sequence seems to indicate that the island and circular harbour were not made until the construction of the stone shipsheds in the late third or early second century B.C. The earth and timber ramps of these shipsheds were discovered with barnacles and probable ships’ nails lying on their surface. Much new evidence for the superstructure of the shipsheds was also found and an attempt has been made to reconstruct their appearance. Evidence for the Roman monumental rebuilding of the island in c. A.D. 200 has also been increased to the point where reconstructions can be attempted, and a large body of new information has been obtained for the structural sequence on the island from c. A.D. 200 to 700. Uncertainties remain over the interpretation of its function throughout the Roman period, although the rebuilding c. A.D. 200 might be associated with the creation of the African corn-fleet, the Classis Commodiana, in A.D. 186; and the site was possibly known as forum Karthag(inis) in the late fourth century and ‘the maritime agora’ in the time of Justinian.Apart from further small-scale work, the present excavations are concluded; some further study of the early environmental sequence in the harbour area will be carried out in the next two years. A fifth and final interim report is planned to cover the excavations since 1976 at the north side of the Circular Harbour.

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