Hotspot origin for asymmetrical conjugate volcanic margins of the austral South Atlantic Ocean as imaged on deeply penetrating seismic reflection lines

2019 ◽  
Vol 7 (4) ◽  
pp. SH71-SH97 ◽  
Author(s):  
Kyle Reuber ◽  
Paul Mann ◽  
Jim Pindell
Keyword(s):  
Oceanic Crust ◽  
Seismic Reflection ◽  
Seismic Refraction ◽  
South Atlantic ◽  
Plate Motion ◽  
Basin Evolution ◽  
Spreading Ridge ◽  
The South ◽  
Volcanic Material ◽  
Conjugate Margins

We have interpreted 27,550 km of deep-penetrating, 2D-seismic reflection profiles across the South Atlantic conjugate margins of Uruguay/Southern Brazil and Namibia. These reflection profiles reveal in unprecedented detail the lateral and cross-sectional, asymmetrical distribution of voluminous, postrift volcanic material erupted during the Barremian-Aptian (129–125 Ma) period of early seafloor spreading in the southernmost South Atlantic. Using this seismic grid, we mapped the 10–200 km wide, continental margin-parallel limits of seaward-dipping reflector (SDR) complexes — that are coincident with interpretations from previous workers using seismic refraction data from the South American and West African conjugate margins. Subaerially emplaced and tabular SDRs have rotated downward 20° in the direction of the mid-Atlantic spreading ridge and are up to 22 km thick near the limit of continental crust. The SDR package is wedge shaped and thins abruptly basinward toward the limit of oceanic crust where it transitions to normal, 6–8 km thick oceanic crust. We have developed a model for the conjugate rifted margins that combine diverging tectonic plates and northwesterly plate motion relative to a fixed mantle position of the mantle plume. Our model explains an approximately 30% higher volume of SDRs/igneous crust on the trailing Namibian margin than on the leading Brazilian margin during the syn- and postrift phases. Our model for volcanic margin asymmetry in the South Atlantic does not require a simple shear mechanism to produce the asymmetrical volcanic material distribution observed from our data and from previously published seismic refraction studies. Determining the basinward extent of the extended continental basement is crucial for understanding basin evolution and for hydrocarbon exploration. Although these conjugate margins have evolved asymmetrically, their proximity during the early postrift stage suggests a near-equivalent, early basin evolution and similar hydrocarbon potential. Understanding the tectonic and magnetic processes that produce these observed asymmetries is critical for understanding volcanic passive margin evolution.

Crustal Thickness and Composition of the São Paulo Plateau and Florianópolis Ridge, SE Brazilian Margin

2020 ◽  
Author(s):  
Michelle Graça ◽  
Leanne Cowie ◽  
Nick Kusznir ◽  
Natasha Stanton
Keyword(s):  
Oceanic Crust ◽  
Seismic Reflection ◽  
Joint Inversion ◽  
Gravity Data ◽  
South Atlantic ◽  
Gravity Inversion ◽  
The South ◽  
Subsidence Analysis ◽  
Base Salt ◽  
Thermal Subsidence

<p>The São Paulo Plateau (SPP) and the Florianópolis Ridge (FR), located on the Santos segment of the SE Brazilian margin in the South Atlantic, are large positive bathymetric features with a combined lateral dimension of approximately 500 km. An important question is whether they are underlain by thinned continental crust or by anomalously thick magmatic crust. Each hypothesis has implications for the breakup of the South Atlantic and the evolution of the overlying saline Santos basin.</p><p>Integrated quantitative analysis consisting of gravity inversion, RDA (residual depth anomaly) analysis and flexural subsidence analysis has been applied to a deep long-offset seismic reflection line running NW-SE across the SPP and FR. Gravity inversion predicts crustal basement thicknesses in the range of 12 to 15 km for the SPP and FR, deceasing to 7-8 km thickness at the extreme SE end of the profile. The SPP and FR are separated by a region of thinner crust approximately 80 km wide. Thinning factors from subsidence analysis for SPP and FR are typically between 0.6 and 0.7.</p><p>RDA values close to zero and a thinning factor of 1 were obtained for the region with 7-8 km thick crust at the SE end of the profile which are all consistent with normal oceanic crust rather than previously interpreted exhumed mantle. This oceanic crust is highly tectonised and corresponds to the location of the Florianópolis Fracture Zone.</p><p>Flexural backstripping and reverse thermal subsidence modelling were performed to calculate palaeo-bathymetry at breakup and give 2.5 km below sea level at the SE end of the profile consistent with this region being oceanic crust. Flexural subsidence analysis applied to base salt shows that the observed base salt subsidence requires a component of syn-tectonic subsidence as well as post-rift thermal subsidence, and that the salt was deposited while the crust was still thinning.</p><p>Joint inversion of time seismic reflection and gravity data to determine the lateral variation in basement density by comparing seismic and gravity Moho in the time domain gives a basement density under the SPP and FR of between 2600 and 2700 kg/m<sup>3</sup>. The same method gives a basement density of 900kg/m<sup>3</sup> for the oceanic crust at the SE end of the profile. The FR basement in the NW shows a basement density similar to that of the SPP while in its SE the basement density is much higher approaching 2950 kg/m3.  We interpret the relatively low basement densities of the SPP with respect to that of oceanic crust as indicating a continental rather than magmatic composition. A similar analysis to determine basement density applied to the Evain et al. (2015) seismic refraction profile in the same location also gives a SPP basement density that supports a continental composition.</p>

Geoid roughness and long-wavelength segmentation of the South Atlantic spreading ridge

Nature ◽  
1988 ◽  
Vol 333 (6170) ◽  
pp. 255-258 ◽  
Author(s):  
Dominique Gibert ◽  
Vincent Courtillot
Keyword(s):  
South Atlantic ◽  
Spreading Ridge ◽  
The South ◽  
Long Wavelength

Geophysical Studies in Baffin Bay and some Tectonic Implications

1972 ◽  
Vol 9 (3) ◽  
pp. 239-256 ◽  
Author(s):  
C. E. Keen ◽  
D. L. Barrett ◽  
K. S. Manchester ◽  
D. I. Ross
Keyword(s):  
Oceanic Crust ◽  
Seismic Reflection ◽  
Seismic Refraction ◽  
The Arctic ◽  
Spreading Center ◽  
Labrador Sea ◽  
Baffin Bay ◽  
Definitive Evidence ◽  
Nares Strait ◽  
The North

A recent seismic refraction experiment in the deep central region of Baffin Bay showed that it is underlain by oceanic crust. This paper describes the results of gravity, magnetic, and seismic reflection profiling measurements in the bay. There is no definitive evidence for a buried ridge or for magnetic lineations in the center of the area. The magnetic and gravity anomaly fields have been used to define the boundary between the oceanic and continental crust around the bay and therefore the extent of oceanic crust presumed to have been formed by sea-floor spreading. Some of the characteristics of the seismic reflection lines across the continental margins, perhaps typical of this area, are also discussed. The results have been used to reconstruct the history of opening of Baffin Bay in conjuction with geophysical measurements in the Labrador Sea to the south and over the Alpha Ridge in the Arctic Ocean to the north. An attempt has been made to reconcile the geometry of opening with continental geology. Two phases of spreading are suggested. The first involves openings, in both the Labrador Sea and in Baffin Bay, about a pole in the Canadian Arctic Islands. The second, most recent stage of opening, requires that the Nares Strait was once a transform fault, perhaps connecting a Baffin Bay spreading center to the Alpha Ridge to the north.

scholarly journals Basin evolution, configuration styles, and hydrocarbon accumulation of the South Atlantic conjugate margins

2019 ◽  
Vol 37 (3) ◽  
pp. 992-1008
Author(s):  
Zhixin Wen ◽  
Shu Jiang ◽  
Chengpeng Song ◽  
Zhaoming Wang ◽  
Zhengjun He
Keyword(s):  
Marine Sediments ◽  
Fracture Zone ◽  
South Atlantic ◽  
Source Rocks ◽  
Basin Evolution ◽  
The South ◽  
Passive Margins ◽  
Marine Shale ◽  
Lacustrine Shale ◽  
Northern Segment

The basins of the South Atlantic passive margins are filled with early rifting stage lacustrine sediments (Barremian, 129–125 Ma), transitional lacustrine and marine sediments (Aptian, 125–113 Ma), and drift stage marine sediments since early Cretaceous (Albian, 113 Ma). The South Atlantic margins can be divided into three segments by the Rio Grande Fracture Zone and the Ascension Fracture Zone according to variations in the basin evolution history and configuration style. The lacustrine shale and marine shale source rocks are developed in the rift stage and drift (post-rift) stage in the South Atlantic passive margins, respectively. The southern segment of the margins is dominated by the lacustrine sedimentary filling in the rifted stage overlain by a thin marine sag system as a regional seal, where the hydrocarbons are mainly accumulated in the structural-stratigraphic lacustrine reservoirs formed in the rift stage. The middle segment developed salty rift-sag-type basins with rift and sag systems and with salt deposited in the transitional intercontinental rift stage, where the lacustrine shale in the lower part of the rifted lacustrine sequence and the marine shale in the lower part of the sag sequence formed in the marine post-rift stage are high-quality source rocks. This segment in the middle is mainly dominated by pre-salt lacustrine carbonate and post-salt marine turbidite plays. The northern segment is characterized by sag-type basins with a narrowly and locally distributed rifted lacustrine system and its overlying widely distributed thick marine sag systems. Gravity-flow (mostly turbidite) marine sandstones as good reservoirs were extensively developed in the sag stage due to the narrow shelf and steep slope. The post-rift marine shales in the lower part of the sag sequence are the main source rocks in the northern segment and the hydrocarbons generated from these source rocks directly migrated to and accumulated in the deep marine turbidite sandstones in the same sag sequence formed in the drift stage. From southern segment to northern segment, source rocks and hydrocarbon accumulations tend to occur in the stratigraphically higher formations. The hydrocarbon accumulations in the southern segment are mainly distributed in the rifted lacustrine sequence while that in the northern segment primarily occur in the post-rift marine sequence.

scholarly journals Asymmetry of high-velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental break-up

2014 ◽  
Vol 6 (1) ◽  
pp. 1335-1370 ◽  
Author(s):  
K. Becker ◽  
D. Franke ◽  
R. B. Trumbull ◽  
M. Schnabel ◽  
I. Heyde ◽  
...  
Keyword(s):  
High Velocity ◽  
Lower Crust ◽  
Small Body ◽  
Seismic Refraction ◽  
South Atlantic ◽  
The South ◽  
Cross Sectional ◽  
Velocity Models ◽  
Rifted Margins ◽  
Break Up

Abstract. High-velocity lower crust (HVLC) and seaward dipping reflector sequences (SDRs) are typical features of volcanic rifted margins. However, the nature and origin of HVLC is under discussion. Here we provide a comprehensive analysis of deep crustal structures in the southern segment of the South Atlantic and an assessment of HVLC along the margins. Two new seismic refraction lines off South America fill a gap in the data coverage and together with five existing velocity models allow a detailed investigation of the lower crustal properties on both margins. An important finding is the major asymmetry in volumes of HVLC on the conjugate margins. The seismic refraction lines across the South African margin reveal four times larger cross sectional areas of HVLC than at the South American margin, a finding that is in sharp contrast to the distribution of the flood basalts in the Paraná-Etendeka Large Igneous Provinces (LIP). Also, the position of the HVLC with respect to the seaward dipping reflector sequences varies consistently along both margins. Close to the Falkland-Agulhas Fracture Zone a small body of HVLC is not accompanied by seaward dipping reflectors. In the central portion of both margins, the HVLC is below the inner seaward dipping reflector wedges while in the northern area, closer to the Rio Grande Rise/Walvis Ridge, large volumes of HVLC extend far seawards of the inner seaward dipping reflectors. This challenges the concept of a simple extrusive/intrusive relationship between seaward dipping reflector sequences and HVLC, and it provides evidence for formation of the HVLC at different times during the rifting and break-up process. We suggest that the drastically different HVLC volumes are caused by asymmetric rifting in a simple shear dominated extension.

scholarly journals Coexistence of Hainan Plume and Stagnant Slab in the Mantle Transition Zone beneath the South China Sea Spreading Ridge: Constraints from Volcanic Glasses and Seismic Tomography

Lithosphere ◽  
2021 ◽  
Vol 2021 (Special 2) ◽  
Author(s):  
Shuang-Shuang Chen ◽  
Rui Gao ◽  
Zewei Wang ◽  
Tong Hou ◽  
Jie Liao ◽  
...  
Keyword(s):  
South China Sea ◽  
Oceanic Crust ◽  
Mantle Plume ◽  
South China ◽  
The South China Sea ◽  
Spreading Ridge ◽  
The South ◽  
China Sea ◽  
Volcanic Glasses ◽  
Hainan Mantle Plume

Abstract The influence of Hainan mantle plume and subducting recycled oceanic crust beneath the spreading ridge of the South China Sea (SCS) have been widely proposed recently, but still controversial and ambiguous. Here, we present seismic tomographic evidence, new major and trace element, and Pb isotopic compositions of volcanic glasses from one International Ocean Drilling Program drill core (Site U1434) in the SCS spreading ridge. The volcanic glasses are relatively enriched in alkalis and light rare earth elements (LREEs) and depleted in heavy REEs (HREEs), exhibit slightly positive anomalies in Nb, Ta, Zr, and Hf as well as a positive Nb relative to La and Th, and show relatively high 207Pb/206Pb and 208Pb/206Pb isotopic ratios, suggesting ocean island basalt- (OIB-) type and enriched mantle 2- (EM2-) type geochemical features likely related to a mantle plume. These geochemical features are consistent with those of late Cenozoic volcanic rocks in Hainan and surrounding areas associated with a mantle plume, likely providing the influence of Hainan mantle plume beneath the spreading ridge of the SCS. The SCS primary-melt and volcanic glasses indicate that the source mantle involved 18.5% eclogite (dense, recycled oceanic crust from the stagnant subducted slab) and 46.1% garnet pyroxenite (produced by the reaction between the peridotite melt and recycled oceanic crust). The existence of Hainan mantle plume and stagnant subducted slab is further supported by geophysical evidence from a recent three-dimensional P-wave seismic tomographic model.

Water Mixing and Circulation within the South Atlantic Basin Constrained by Seismic Reflection Images

2020 ◽  
Author(s):  
Jingxuan Wei ◽  
Robert Reece ◽  
Will Fortin ◽  
Tanner Acquisto
Keyword(s):  
Seismic Reflection ◽  
Water Mass ◽  
South Atlantic ◽  
Intermediate Water ◽  
The South ◽  
Antarctic Intermediate Water ◽  
Low Salinity ◽  
Mid Atlantic Ridge ◽  
The Antarctic ◽  
Seismic Images

<p>South Atlantic water masses and circulation significantly influence the dynamics and water mass structure of the Atlantic Meridional Overturning Circulation (AMOC). Previous research in the South Atlantic has mostly focused on energetic regions such as the Brazil/Malvinas Confluence Zone along the western boundary and the Agulhas retroflection to the east. However, it is also important to understand water circulation and diapycnal mixing within the South Atlantic Basin (SAB). Previous studies have observed low salinity patches of the Antarctic Intermediate Water within the western side of the SAB at 30<sup>o</sup> S, but the temporal variability of the scales, locations and structures of these low salinity patches are still uncertain. Former studies also show an increased level of mixing within the SAB above the Mid-Atlantic Ridge, but did not evaluate mixing on smaller scales such as mesoscale and sub-mesoscale.</p><p>Here we present a water mass structure analysis at 30<sup>o</sup> S from Rio Grande Rise to the Mid-Atlantic Ridge by using Seismic Oceanography (SO). SO is being applied around the world to image mesoscale water mass structures using the seismic reflection method. Reflections in the seismic images are essentially temperature gradients that are proxies for isopycnal surfaces. We paid particular attention in seismic processing to imaging of structures that characterize the boundary between water masses. We imaged the upper South Atlantic Central Water, and identified discontinuous water boundaries (about 150 km long) between the Antarctic Intermediate Water and the North Atlantic Deep Water that could correspond to the intermittent appearance of low salinity patches. We combine seismic images with previous hydrographic measurements to investigate the temporal change of these low salinity patches. We use a horizontal slope spectra to quantify mixing rate from tracked seismic horizons to evaluate mesoscale and sub-mesoscale mixing events such as internal waves and eddies. Through SO, we hope to better constrain South Atlantic circulation and contribute to the understanding of AMOC as a whole.</p>

scholarly journals Asymmetry of high-velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental breakup

Solid Earth ◽  
2014 ◽  
Vol 5 (2) ◽  
pp. 1011-1026 ◽  
Author(s):  
K. Becker ◽  
D. Franke ◽  
R. Trumbull ◽  
M. Schnabel ◽  
I. Heyde ◽  
...  
Keyword(s):  
High Velocity ◽  
Lower Crust ◽  
Small Body ◽  
Seismic Refraction ◽  
South Atlantic ◽  
Large Igneous Province ◽  
Asymmetric Distribution ◽  
The South ◽  
Velocity Models ◽  
Rifted Margins

Abstract. High-velocity lower crust (HVLC) and seaward-dipping reflector (SDR) sequences are typical features of volcanic rifted margins. However, the nature and origin of HVLC is under discussion. Here we provide a comprehensive analysis of deep crustal structures in the southern segment of the South Atlantic and an assessment of HVLC along the margins. Two new seismic refraction lines off South America fill a gap in the data coverage and together with five existing velocity models allow for a detailed investigation of the lower crustal properties on both margins. An important finding is the major asymmetry in volumes of HVLC on the conjugate margins. The seismic refraction lines across the South African margin reveal cross-sectional areas of HVLC 4 times larger than at the South American margin, a finding that is opposite to the asymmetric distribution of the flood basalts in the Paraná–Etendeka Large Igneous Province. Also, the position of the HVLC with respect to the SDR sequences varies consistently along both margins. Close to the Falkland–Agulhas Fracture Zone in the south, a small body of HVLC is not accompanied by SDRs. In the central portion of both margins, the HVLC is below the inner SDR wedges while in the northern area, closer to the Rio Grande Rise-Walvis Ridge, large volumes of HVLC extend far seaward of the inner SDRs. This challenges the concept of a simple extrusive/intrusive relationship between SDR sequences and HVLC, and it provides evidence for formation of the HVLC at different times during the rifting and breakup process. We suggest that the drastically different HVLC volumes are caused by asymmetric rifting in a simple-shear-dominated extension.

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