Evolution of seaward-dipping reflectors at the onset of oceanic crust formation at volcanic passive margins: Insights from the South Atlantic

In this contribution, we examine the evolution of the South Atlantic passive margins, based on a new thermal lithosphere-asthenosphere-boundary (LAB) model. Our model is calculated by 1D advection and diffusion with rifting time, crustal thickness and stretching factors as input parameters. The initial lithospheric thickness is defined by isostatic equilibrium with laterally variable crustal and mantle density. We simulate the different rifting stages that caused the opening of the South Atlantic Ocean and pick the LAB as the T=1330&#176; C isotherm. The modelled LAB shows a heterogeneous structure with deeper values at equatorial latitudes, as well as a more variable lithosphere along the southern part. This division reflects different stages of the South Atlantic opening: Initial opening of the southern South Atlantic caused substantial lithospheric thinning, followed by the rather oblique-oriented opening of the equatorial South Atlantic accompanied by severe thinning. Compared to global models, our LAB reflects a higher variability associated with tectonic features on a smaller scale. As an example, we identify anomalously high lithospheric thickness in the South American Santos Basin that is only poorly observed in global LAB models. Comparing the LAB of the conjugate South American and African passive margins in a Gondwana framework reveals a variable lithospheric architecture for the southern parts. Strong differences up to 80 km for selected margin segments correlate with strong gradients in margin width for conjugate pairs. This mutual asymmetry suggests highly asymmetric melting and lithospheric thinning prior to rifting.

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Inferences regarding initiation of oceanic crust formation from the U.S. East Coast margin and conjugate South Atlantic margins

Atlantic Rifts and Continental Margins - Geophysical Monograph Series ◽

10.1029/gm115p0211 ◽

2000 ◽

pp. 211-233 ◽

Cited By ~ 31

Author(s):

Manik Talwani ◽

Vitor Abreu

Keyword(s):

Oceanic Crust ◽

East Coast ◽

South Atlantic ◽

Crust Formation ◽

The U.S

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

10.5194/egusphere-egu2020-10862 ◽

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

The S&#227;o Paulo Plateau (SPP) and the Florian&#243;polis 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.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.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 Florian&#243;polis Fracture Zone.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.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/m3. The same method gives a basement density of 900kg/m3 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.&#160; 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.

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Insights from recent gravity satellite missions in the density structure of continental margins – With focus on the passive margins of the South Atlantic

Gondwana Research ◽

10.1016/j.gr.2017.04.015 ◽

2018 ◽

Vol 53 ◽

pp. 285-308 ◽

Cited By ~ 4

Author(s):

Hans-Jürgen Götze ◽

Roland Pail

Keyword(s):

South Atlantic ◽

Continental Margins ◽

Density Structure ◽

The South ◽

Passive Margins ◽

Satellite Missions

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

Energy Exploration & Exploitation ◽

10.1177/0144598719840751 ◽

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.

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Conjugate volcanic passive margins in the austral segment of the South Atlantic – Architecture and development

Earth-Science Reviews ◽

10.1016/j.earscirev.2020.103461 ◽

2021 ◽

Vol 212 ◽

pp. 103461

Author(s):

François Chauvet ◽

François Sapin ◽

Laurent Geoffroy ◽

Jean-Claude Ringenbach ◽

Jean-Noël Ferry

Keyword(s):

South Atlantic ◽

The South ◽

Passive Margins

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

Interpretation ◽

10.1190/int-2018-0256.1 ◽

2019 ◽

Vol 7 (4) ◽

pp. SH71-SH97 ◽

Cited By ~ 3

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.

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