Seaward-dipping Reflectors Research Articles

Mesozoic rifting in SW Gondwana and break-up of the Southern South Atlantic Ocean

Abstract The opening of the South Atlantic Ocean in the Early Cretaceous was only the final stage of the complex rifting process of SW Gondwana. In this contribution, we reassess the chronology of Mesozoic basin formation in southern South America and Africa and integrate it in the long-term rifting and break-up history of SW Gondwana. During the Triassic, after the Gondwanides orogeny, plate-scale instabilities produced intracontinental rifting in Africa, and retro-arc extension on the SW-margin of Gondwana. This process was followed and accentuated by the impingement of the Karoo plume in the Early Jurassic, which triggered rifting in East Africa and ultimately produced the break-up of Eastern from Western Gondwana in the Middle Jurassic. Retro-arc extension continued to affect the palaeo-Pacific margin, with emplacement of the Chon Aike magmatic province in the Patagonian retro-arc during the Early–Middle Jurassic. By the Late Jurassic, retro-arc rifting reached a point of oceanic crust accretion, with the establishment of the Rocas Verdes back-arc basin in southern Patagonia, together with the formation of the Weddell Sea further south, between the South American plate and Antarctica. The core of the Late Paleozoic Gondwanides orogen, between southern South America and Africa, was subjected to oblique rifting at this time and produced the Outeniqua and Rawson/Valdés basins. This area was the locus of extension and oceanization in the Early Cretaceous associated with a rotation of the stress field from NE–SW to east–west extension. The formation of the South Atlantic Ocean resulted from lithospheric extension and was accompanied by extensive intrusive magmatism and extrusive flood basalts identified as seaward dipping reflectors, which were emplaced diachronically from south to north, along different segments along both conjugate margins. These volcanic rocks form the South Atlantic Large Igneous Province. The chronology of the South Atlantic opening and the magmatic sources and processes associated with the formation of seaward dipping reflectors remain interpretative as they have only been studied on seismic data but are still undrilled; hence, scientific drilling will be key to unravel many of these unknowns.

An integrated stratigraphic re-evaluation of key Central Atlantic DSDP sites

Re-sampling and new analysis of DSDP core from key Central Atlantic sites, drilled over 30 years ago, has allowed an updated stratigraphic framework to be developed. This study refines the timing and nature of key unconformities that have previously been documented to occur elsewhere in the oceanic domain of the Central Atlantic during the drilling of many DSDP legs. Biostratigraphic dating using calcareous nannofossils and palynology has been performed on 168 samples. This higher biostratigraphic resolution improves regional correlation and has allowed identification of major hiati previously unrecognised within the oceanic domain stratigraphy.This work builds on the pioneering work of the DSDP cruises, and utilises the valuable data collected for new age dating, supplemented with new organic geochemistry analysis and sedimentological characterisation to improve documentation of the Mesozoic strata. Results are extrapolated along the continental margin of northwest Africa and the conjugate in South America, using regional two-dimensional seismic sections, providing a correlation between the oceanic domain stratigraphy and shelfal depositional systems penetrated by hydrocarbon exploration wells.At DSDP Site 367, a middle Berriasian unconformity is recognised for the first time in Core 33, where middle Berriasian white pelagic limestones (Blake-Bahama Formation) sit unconformably on Early Tithonian red argillaceous limestones (Cat Gap Formation). Faunal dating indicates a ca. 5 Myr hiatus at the Jurassic-Cretaceous boundary. Marl interbeds within the pelagic limestones increase in occurrence through the Barremian, reflecting increased terrigenous input, with associated total organic content (TOC) averaging 5%.A second major lithological break is identified at DSDP Site 367, interpreted as a base Albian unconformity, between the Blake-Bahama and overlying Hatteras Formations. Sediments above are intra late Albian variegated claystones with abundant terrestrial detritus and distal turbidites. Seismic data that intersects the well location show onlapping reflections of early-middle Albian strata landward of DSDP Site 367, indicating paleo-topography prior to Albian deposition. These sediments are contemporaneous with a major progradational phase on the shelf and a sedimentary wedge observed at the base of the carbonate escarpment. Early Albian sediments sit above the interpreted base Albian unconformity at DSDP Site 534 A on the conjugate US Atlantic margin, suggesting the super-regional nature of this event. The refined dating provides age calibration of a basin-wide unconformity, previously known from seismo-stratigraphic studies as Reflector β.In addition to the revised stratigraphic framework, evidence is presented for the occurrence of seaward dipping reflectors (SDRs) northwest of the Guinea Plateau. SDRs have not been reported along this section of the African margin, demonstrating volcanic addition during early seafloor spreading and extending the occurrence of volcanics associated with the African Blake Spur magnetic anomaly further south beyond the Cap Vert fracture zone previously mapped. At DSDP Site 368 we also note that the late Albian to late Cenomanian strata (Hatteras Formation) is composed of a 164 m thick organic-rich interval, containing up to 37% TOC, with facies and organic geochemical heterogeneity. This transgressive sequence is associated with a flooding of the shelf.This revised stratigraphic framework provides a solid foundation for future Central Atlantic studies intending to use these sites as calibration of the oceanic domain stratigraphy.

The remarkable parallels between the North East Atlantic and Arctic regions

Geological understanding of the NE Atlantic and Arctic regions has increased greatly over the last two decades, revealing remarkable similarities. Continental extension in both regions onset during pausing or cessation of adjacent orogenies – the Verkhoyansk-Chukotka and Alpine orogenies – which relaxed compressional stresses and permitted extension. Severe extension accompanied by high-volume magmatism did not onset abruptly but was the culmination of long-term, regional tectonic and magmatic unrest. It was complex and piecemeal, with extension and volcanism occurring along multiple axes simultaneously, a process still ongoing in Iceland today. Large Igneous Province emplacement did not drive breakup but occurred as a consequence of continental extension. The High Arctic Large Igneous Province did not progress to breakup. As a result, much of the Arctic Ocean is floored by hyper-extended continental crust overlain by seaward-dipping reflectors. This crust resembles the passive margins of the NE Atlantic. The Alpha-Mendeleev Rise in the Arctic Ocean is a double-sided passive margin underlain by hyper-extended continental crust draped with seaward-dipping reflectors. A similar structure has been proposed for the Greenland-Iceland-Faroe Ridge. If correct, present-day volcanism in Iceland is building SDRs today. Up to ∼50% of the NE Atlantic Ocean oceanward of the continental shelves is likely underlain by at least some continental crust. For the Arctic Ocean this figure is ∼80%. Similar structures are found elsewhere, including the Madagascar Channel, the NW Indian Ocean, and the South Atlantic. These new ideas suggest promising directions of future study including assessing the true extent of continental crust in the oceans globally.

CRUSTAL STRUCTURE OF THE MENDELEEV RISE IN THE ARCTIC OCEAN: A SYNTHESIS OF SEISMIC PROFILES AND ROCK SAMPLING DATA

The Mendeleev Rise is located in the Amerasia Basin of the Arctic Ocean. The work is based on a synthesis of interpretation of regional seismic profiles of the OGT 2D DOM and data from rock sampling using special underwater vehicles on the slopes of seamounts and scarps. The uplift is represented by alternation of highs (horsts) and half-grabens. At the base of the horst sections, bright reflectors are distinguished, which are interpreted as volcanics. Half-graben sections are wedge-shaped in section and are similar in geometry to seaward-dipping reflectors (SDRs) of continental passive volcanic margins. Rock sampling has shown that the horsts are composed of sedimentary rocks of Palaeozoic age, penetrated by intrusions. Aptian-Albian sections with volcanics (basalts, trachybasalts, trachyandesites) were identified on the horsts. U/Pb dating of igneous rocks showed that typical age of rocks is 110-114 Ma. Magmatic Cretaceous rocks contain zircons with ages ranging from pre-Barremian Mesozoic to Palaeozoic and Precambrian. These zircons were captured by basaltic magma during its upward movement. The presence of these ancient zircons indicates that the Mendeleev Rise is composed of continental crust. A model of the crustal structure of the Mendeleev Rise is proposed. The base of the section visible on seismic profiles is dominated by volcanics (on horsts from basalts to trachyandesites, in half-grabens mainly basalts). The upper and lower crust is approximately 20-30% saturated with intrusions of basic composition. At the base of the crust, a high-velocity layer up to 5 km thick is distinguished. It is assumed that its lower part is entirely represented by gabbro-type intrusions, and the upper part is the lowest part of the lower crust, maximally saturated with intrusions.

Sub-basalt hydrocarbon prospect assessment in Peninsular India using seaward dipping reflectors

Conventionally, volcanic margins have been considered devoid of hydrocarbon, but many discoveries and research in recent past have proved the presence of hydrocarbon prospects within them. However, hydrocarbon exploration within volcanic margin is constrained by seismic imaging. Further, identification of continent to oceanic boundary (COB) is critical to hydrocarbon search as hydrocarbon is found mostly over continental crust. Seaward dipping reflectors, associated with volcano-sedimentary sequences, located along rifted continental margins and represented by highly dipping strong amplitude seismic reflectors, play an important role to study volcanic margins. Keeping in view the problems of hydrocarbon exploration within volcanics, seaward dipping reflectors (SDRs) in Indian context have been studied in detail in this paper. It facilitates mapping of continent-oceanic boundary (COB). It has impact on hydrocarbon prospectivity in volcanic margins as it can provide formidable seal and secondary induced maturity. In India, several discoveries have been made in volcanic margins along west coast, from weathered, fractured basalt and sub-basalt sediments. In the present work, long offset regional seismic data (18 dip lines and 1 cross line), along east coast, have been interpreted. SDRs help in hydrocarbon prospectivity assessment for both discrimination of interbedded sediments and preferential accumulation of hydrocarbon. Mapping of linear features not only helped to demarcate COB but also their presence at 6–10 km depth. For the first time, such comprehensive study on SDRs has been done along Indian peninsular region with indications for hydrocarbon prospectivity at deeper levels along Indian peninsular region.

Detrital zircon and apatite U-Pb provenance and drainage evolution of the Newark Basin during progressive rifting and continental breakup along the Eastern North American Margin, USA

Abstract Mesozoic rift basins of the Eastern North American Margin (ENAM) span from Florida in the United States to the Grand Banks of Canada and formed during progressive extension prior to continental breakup and the opening of the north-central Atlantic. The syn-rift strata from all the individual basins, lumped along the entire margin into the Newark Supergroup, are dominated by fluvial conglomerate and sandstone, lacustrine siltstone, mudstone, and abundant alluvial conglomerate and sandstone lithofacies. Deposition of these syn-rift sedimentary rocks was accommodated in a series of half grabens and subsidiary full grabens situated within the Permo-Carboniferous Appalachian orogen. The Mesozoic ENAM is commonly depicted as a magma-rich continental rift margin, with magmatism (Central Atlantic magmatic province [CAMP]) driving continental breakup. However, the southern portion of the ENAM shows evidence of magmatic breakup (e.g., seaward-dipping reflectors), and rifting and crustal thinning appeared to start ~30 m.y. prior to CAMP emplacement in the Jurassic. This study provides extensive new detrital zircon and apatite U-Pb provenance data to determine the provenance and reconstruct the paleodrainages of the Newark Basin during progressive rifting and magmatic breakup and the implications for the overall rift configuration and asymmetry during progressive rifting along the ENAM rift margin. Detailed new detrital zircon (N = 21; n = 3093) and apatite (N = 4; n = 559) U-Pb results from sandstone outcrop and core samples from the Newark Basin indicate a distinct provenance shift, with relatively older Carnian syn-rift strata predominately sourced from the hanging wall of the basin bounding fault in the east while relatively younger Norian strata were regionally sourced from both the hanging wall and footwall. The syn-rift strata at the Triassic-Jurassic boundary were sourced from the hanging wall before a transition to local footwall terranes. These results suggest two major provenance changes during progressive rifting—the first occurring during Carnian crustal necking and rift flank uplift as predicted by recent numerical models and the second occurring at the onset of the Jurassic due to regional and local thermal uplift during CAMP magmatism as seen along other magma-rich margins, such as the North Atlantic and the southern portion of the South Atlantic margin.

Discontinuous Igneous Addition Along the Eastern North American Margin Beneath the East Coast Magnetic Anomaly

AbstractDetailed models of crustal structure at volcanic passive margins offer insight into the role of magmatism and the distribution of igneous addition during continental rifting. The Eastern North American Margin (ENAM) is a volcanic passive margin that formed during the breakup of Pangea ∼200 Myr ago. The offshore, margin‐parallel East Coast Magnetic Anomaly (ECMA) is thought to mark the locus of syn‐rift magmatism. Previous widely spaced margin‐perpendicular studies seismically imaged igneous addition as seaward dipping reflectors (SDRs) and high velocity lower crust (HVLC; >7.2 km/s) beneath the ECMA. Along‐strike imaging is necessary to more accurately determine the distribution and volume of igneous addition during continental breakup. We use wide‐angle, marine active‐source seismic data from the 2014–2015 ENAM Community Seismic Experiment to determine crustal structure beneath a ∼370‐km‐long section of the ECMA. P‐wave velocity models based on data from short‐period ocean bottom seismometers reveal a ∼21‐km‐thick crust with laterally variable lower crust velocities ranging from 6.9 to 7.5 km/s. Sections with HVLC (>7.2 km/s) alternate with two ∼30‐km‐wide areas where the average velocities are less than 7.0 km/s. This variable structure indicates that HVLC is discontinuous along the margin, reflecting variable amounts of intrusion along‐strike. Our results suggest that magmatism during rifting was segmented. The HVLC discontinuities roughly align with locations of Mid‐Atlantic Ridge fracture zones, which may suggest that rift segmentation influenced later segmentation of the Mid‐Atlantic Ridge.

Evolution of an oblique volcanic passive margin: The case of Nuussuaq in West Greenland

Most rifted margins involve a certain degree of obliquity. Oblique extension has been well-studied in a magma-poor continental rift setting. However, few studies have addressed the structure and evolution of oblique rifting in a magma-rich continental rift setting. In this study, we used remote sensing and field measurements combined with geophysical data in order to study the tectono-magmatic evolution of seaward-dipping basalt sequences, known also as seaward dipping reflectors (SDRs), along the western Nuussuaq oblique volcanic passive margin. We calculated the directions and dips of lavas within onshore SDRs based on remote sensing and field data and analyzed the offshore SDRs through seismic reflection profiles to obtain a precise 3D structure of the inner SDRs along this margin. Our results show that the development of the inner SDRs can be divided into two stages. During the first stage (the late Paleocene), the oblique extension was partitioned into strike-slip and dip-slip components, and the basalt sequences were mainly bounded by N-S-trending continentward-dipping faults. During the second stage (the early Eocene), a local stress reorientation occurred in the western Nuussuaq and the basalt sequences were mainly bounded by NE-SW-trending continentward-dipping faults. The obliquity of the margin originated from both the reactivation of the inherited Itilli Fault and the magma intrusion into the crust, which weakened the crust and produced a pressure-induced extension orthogonal to the margin.

Formation and geophysical character of transitional crust at the passive continental margin around Walvis Ridge, Namibia

Abstract. When interpreting geophysical models, we need to establish a link between the models' physical parameters and geological units. To define these connections, it is crucial to consider and compare geophysical models with multiple, independent parameters. Particularly in complex geological scenarios, such as the rifted passive margin offshore Namibia, multi-parameter analysis and joint inversion are key techniques for comprehensive geological inferences. The models resulting from joint inversion enable the definition of specific parameter combinations, which can then be ascribed to geological units. Here we perform a user-unbiased clustering analysis of the two parameters electrical resistivity and density from two models derived in a joint inversion along the Namibian passive margin. We link the resulting parameter combinations to breakup-related lithology and infer the history of margin formation. This analysis enables us to clearly differentiate two types of sediment cover. The first type of sediment cover occurs near the shore and consists of thick, clastic sediments, while the second type of sediment cover occurs further offshore and consists of more biogenic, marine sediments. Furthermore, we clearly identify areas of interlayered massive, and weathered volcanic flows, which are usually only identified in reflection seismic studies as seaward-dipping reflectors. Lastly, we find a distinct difference in the signature of the transitional crust south of and along the supposed hotspot track Walvis Ridge. We ascribe this contrast to an increase in magmatic activity above the volcanic centre along Walvis Ridge and potentially a change in the melt sources or depth of melting. This change of the predominant volcanic signature characterizes a rift-related southern complex and a plume-driven Walvis Ridge regime. All of these observations demonstrate the importance of multi-parameter geophysical analysis for large-scale geological interpretations. Additionally, our results may improve future joint inversions using direct parameter coupling, by providing a guideline for the complex passive margin's parameter correlations.

Volcanic passive margins and break-up processes in the southern Red Sea

We have mapped Oligocene to Quaternary volcanic sequences, emplaced across the Afar magmatic province (AMP: Ethiopia, Eritrea, Djibouti, Yemen), by using 50 cm resolution Pleiades satellite images. Systematic and precise measurements of flow sequences between Djibouti and SW Yemen focus on variations in lava dips and dip directions. From these results, we are able to infer the volcano-tectonic architecture of the upper crust across the southern part of the Afar magmatic province and the evolution of crustal deformation over time. Our results provide evidence that post-trap volcanism within the Afar area was emplaced during the stretching of continental crust, mostly as seaward dipping reflector (SDR) -type volcanic wedges, commonly observed along volcanic passive margins. Compilation of our results with published ages and structures of volcanic sequences in western Afar and Djibouti allows us to map these SDR wedges along a 700 km-long crustal cross-section between the Ethiopia and SW Yemen escarpments. During the Late Oligocene to Lower Miocene, Inner SDR-type wedges developed during the formation of two crustal necking zones on either sides of an uplifted and weakly deformed crustal Danakil C-Block. Following this early extensional event, the C-Block was further stretched and thinned during the development of two transient continental rift axes, the Bab al Mandab to the East and the Afar axis to the West, thought to have been concomitantly active until ∼5 Ma. From Pliocene onwards, rifting focalized W of the Danakil Block. NE-ward diverging wedges of Inner SDRs developed during the Stratoïd volcanic event throughout the whole Afar Depression, along with an onset of continental extension at the tip of a westward propagating Aden rift. A late radical change in tectonics occurred when the Aden Rift propagated into the Afar, immediately before the onset of a slow-spreading oceanic system.

Ignition of the southern Atlantic seafloor spreading machine without hot-mantle booster

The source of massive magma production at volcanic rifted margins remains strongly disputed since the first observations of thick lava piles in the 1980s. However, volumes of extruded and intruded melt products within rifted continental crust are still not accurately resolved using geophysical methods. Here we investigate the magma budget alongside the South Atlantic margins, at the onset of seafloor spreading, using high-quality seismic reflection profiles to accurately estimate the oceanic crustal thickness. We show that, along ~ 75% of the length of the Early-Cretaceous initial spreading centre, the crustal thickness is similar to regular oceanic thickness with an age > 100 Ma away from hot spots. Thus, most of the southernmost Atlantic Ocean opened without anomalously hot mantle, high magma supply being restricted to the Walvis Ridge area. We suggest that alternative explanations other than a hotter mantle should be favoured to explain the thick magmatic layer of seaward dipping reflectors landward of the initial mid-oceanic ridge.

Cretaceous volcanism and intrusive magmatism features in the Mendeleev Rise region (Arctic Ocean) according to seismic data

The current paper is based primarily on the interpretation of 2D seismic lines for the Amerasian Basin. A synrift complex has been identified in half-grabens almost everywhere within the Alpha-Mendeleev Rise and conjugate basins according to the results of seismic data interpretation. Various magmatism features within the synrift complex have been identified on seismic profiles: plateau basalts; sills and dikes; reflections similar to SDRs (Seaward Dipping Reflectors Sequences) and volcanoes. Regional extension and synchronous widespread magmatism are probably associated with the formation of the High Artic Large Igneous Province (HALIP) in the Aptian-Albian. Considering the data on the isotope ages of igneous rocks for the Mendeleev Rise, it is assumed that the top of the synrift complex has an approximate age of 100 Ma and the bottom has an approximate age of 125 Ma. The Alpha-Mendeleev Rise was formed simultaneously with conjugate basins in the Aptian-Albian. An axial line can be drawn along the Alpha-Mendeleev Rise. To the west of the axial line, reflections similar to SDRs dip towards the Podvodnikov basin. To the east of the axial line, reflections dip towards the Toll, Mendeleev, Nautilus and Stefansson basins. The reflections converge in the central parts of the basins. The Alpha Mendeleev Rise is a double-sided volcanic passive continental margin. The Podvodnikov, Toll, Mendeleev, Nautilus, and Stefansson basins are rift basins with thinned continental crust at the base. Their development was interrupted before the start of spreading and the oceanic crust formation.

Seaward‐Dipping Reflector Influence on Seafloor Magnetostratigraphy—A Pelotas Basin View

AbstractPrevious works used marine magnetic survey data and interpreted magnetic anomalies related to seaward‐dipping reflectors (SDRs) as oceanic crust or SDR age constraints in the South Atlantic Ocean. However, we advise against using SDR‐related magnetic anomalies to constrain ages because of the mostly low‐dipping geometry causing the overlapping of rocks extruded during periods of different polarities. The data are difficult to interpret correctly because of the SDR compositional heterogeneity, sedimentary intercalations, variations in package thickness with different compositions and magnetic polarities, and the absence of a link between the SDR magmatic position and age. SDR‐related magnetic anomalies are mainly caused by magnetic susceptibility contrasts rather than remanent magnetism. Due to this complexity, the SDR geometry and variable composition make SDR‐related magnetic anomalies challenging to use for age constraints. The Pelotas Basin SDRs age cannot be estimated by magnetostratigraphy, and this method likely cannot constrain the age of any SDR wedge.

Hot or Fertile Origin for Continental Break-Up Flood Basalts: Insights from Olivine Systematics

Abstract The break-up of supercontinents is often temporally and spatially associated with large outpourings of basaltic magmas in the form of large igneous provinces (LIPs) and seaward dipping reflectors (SDRs). A widespread view is that the upwelling of hot mantle plumes drives both continental break-up and generation of associated LIPs. This is supported by petrologic estimates of the temperature from olivine-melt thermometers applied to basaltic magmas. These thermometers must be applied to a primary mantle-derived magma, requiring the selection of an appropriate primitive magma and an assumption of how much olivine is to be back-added to correct for fractional crystallization. We evaluated the effects of these assumptions on formation temperatures by compiling and analyzing a database of North Atlantic igneous province (NAIP) and Central Atlantic magmatic province (CAMP) lavas and olivines. Ni and FeOT systematics suggest that many picrite magmas have undergone olivine addition and are not true liquids, requiring careful selection of primitive magmas. The maximum amount of back-added olivine was determined by constraining mantle peridotite melt fractions for a range of possible mantle potential temperatures and continental lithosphere thicknesses. Using an empirical relationship between melting degree and forsterite (Fo) content, we show that the possible maximum olivine forsterite content in equilibrium with NAIP magmas is 90.9, which is lower than the maximum olivine forsterite content observed in the NAIP olivine population. We infer primary magmas that lead to mantle potential temperatures of 1420°C for the NAIP and 1330°C for CAMP. Using a similar approach for consistency, we estimate a mantle potential temperature of 1350°C for mid-ocean ridge basalts (MORB). Our results suggest that LIPs associated with continental break-up are not significantly hotter than MORB, which suggests that continental break-up may not be driven by deep-seated thermal plumes. Instead, we suggest that such voluminous magmatism might be related to preferential melting of fertile components within the lithosphere triggered by far-field extensional stresses.

Alpha-Mendeleev Rise, Arctic Ocean: A double volcanic passive margin

Data collected by several expeditions to the Arctic Ocean have yielded a seismic stratigraphic framework and basin fill history for the Alpha-Mendeleev Rise (AMR) and adjacent Podvodnikov, Makarov, North Chukchi, Toll, Mendeleev, Nautilus and Stefansson basins. The AMR comprises a double-sided volcanic passive margin formed in Aptian-Albian time. The North Chukchi, Podvodnikov, Toll, Mendeleev, Nautilus, and Stefansson basins formed synchronously with the Alpha-Mendeleev Rise as failed micro-oceanic basins. Their formation started with rifting and volcanism at ∼ 125 Ma and ended at 100–90 Ma. Ages are constrained by new isotope magmatic rock ages. The region now comprising the Amerasia Basin was in an intraplate tectonic setting during Aptian-Albian times. On the Mendeleev Rise seismic units interpreted as seaward-dipping reflectors (SDRs) form wedges separated by highs. We propose an axial line along the Mendeleev Rise that separates probable half-grabens of different polarity. On the western slope of the Rise, all bright reflections have westward dips, while on the eastern slope they have eastward dips. SDR-like seismic units are common within the adjacent basins as well. Gravity/magnetic crustal modelling for two regional seismic lines demonstrate that the Mendeleev Rise and adjacent basins are associated with stretched continental crust.

Formation of SDRs-Ocean transition at magma-rich rifted margins: Significance of a mantle seismic reflector at the western Demerara margin

At distal magma-rich rifted margins, seismic reflections several seconds below the ∼10 s TWT expected base of petrological crust for thermally equilibrated lithosphere, are commonly observed. These deep seismic reflectors are often interpreted as the petrological Moho as they distally rise and merge into the imaged oceanic Moho, however, the origin of these deep seismic reflections beneath the expected base of the petrological crust is not understood.We investigate the example of the Jurassic Demerara Plateau located offshore north-east South America, which shows a strong seismic reflection at ∼15 s TWT, beneath 20 km thick SDRs (Seaward Dipping Reflectors). We use combined inversion of gravity and seismic reflection data, together with seismic interpretation, to investigate the depth of the deep reflector, its origin, formation as well as the composition and structure of the overlying material. Results from quantitative techniques show that the deep reflector is located below the SDRs at least 10 km within the mantle at a depth of ∼42 km. These results, together with seismic interpretation, suggest that the deep reflector corresponds to an intra-mantle detachment limiting semi-ductile and sheared magmatic-infiltrated mantle above from depleted ductile asthenospheric mantle below. Distally, the semi-ductile mantle is removed when the asthenospheric mantle focuses to form a stable mid-oceanic ridge. This lateral evolution results from a progressive coupling between the magmatic SDRs above and the asthenospheric mantle below. Therefore, we propose that the intra-mantle detachment, corresponding to a rheological contrast, plays a key role in the transition from distributed lithosphere extension to focused mid-ocean ridge accretion at distal magma-rich rifted margins. In addition, it may represent an indicator to determine the thermal structure of a magma-rich rifted margin.

The nature of magnetic anomalies in the southern part of the Barents Sea shelf according to the results of an integrated analysis

Research subject. The anomalous magnetic field of the southern part of the Barents Sea Shelf.Materials and methods. The research was based on a digital matrix (grid) of the anomalous magnetic field (AMP) compiled from the materials of magnetic surveys performed in 2002–2007 by a number of research organizations and research and production companies. A model describing the structure and formation of the magneto-active layer of the southern part of the Barentsevomorsk region was developed. An analysis of the radially averaged field spectrum made it possible to establish the confinement of the upper edges of the field sources to several structural horizons. Band filtering in the frequency domain in accordance with the allocated depth ranges allowed anomalies to be distinguished from other sources. To determine the nature of sources of magnetic anomalies at different levels of the earth’s crust, an integrated analysis of gravimagnetic fields, seismic profiling data and ground studies was conducted.Results. At least two levels of magnetic anomaly sources were found: the distribution of effective magnetization for the low-frequency component of AMP, reflecting the depth structure of the region, and the high-frequency component of AMP, reflecting the distribution of local intrusions in the upper part of the foundation and in the sedimentary cover. The lower level is represented by massive blocks of deep laying and corresponds to the SDR (Seaward Dipping Reflectors) complex, which is an alternation of tectonic plates of continental material with ultrabasite basites that were introduced into the crust at the post-rift stage of the continent’s split. The zone of positive linear anomalies of the magnetic field reflects the divergent boundary of the ancient continental plate of the Baltic, which arose during the fragmentation of the supercontinent of Colombia (Paleopangea) in the middle reef and the formation of the Rifean oceanic basin, which was then veiled by subsequent tectonic processes. The upper structural level indicates the introduction of the main composition into the upper layers of the earth’s crust in the zones of rift-forming faults of magma in late Devonian times during the process of continental rifting on the Svalbard Plate. This is confirmed by the presence of manifestations of the main magmatism within the propagation zone of the South Barents riftogenic depression into the body of the Baltic Shield.Conclusions. The conducted integrated analysis of the anomalous magnetic field and other geological and geophysical data allowed the authors to establish the nature of the sources of magnetic anomalies located at different structural levels of the earth’s crust in the southwestern part of the Barents Sea shelf. The magnetoactive layer of this region is characterized by a complex structure, the section of which includes at least two structural levels, each reflecting certain evolutionary stages of the earth’s crust.

Gravity and magnetic anomalies of earthquake-prone areas in the southwestern Ulleung basin margin, East Sea (Sea of Japan)

Submarine earthquakes have increased in the southwestern Ulleung Basin adjacent to the Korean Peninsula. This study analyzed the gravitational and magnetic properties of the three earthquake-prone areas (Hupo Bank and offshore regions near Pohang and Ulsan) in the basin. The basin was affected by tensile and compressive stresses during the formation of the East Sea. The southern Hupo Bank and the Pohang offshore exhibited high gravity anomalies and strong magnetic anomalies. Hupo Bank was separated from the peninsula and earthquakes in this region have been influenced by crustal fractures that facilitated igneous activities during the formation of the basin. Dense volcanic rocks and seaward dipping reflectors along the Pohang coast and continental slope suggest magmatic activities during the formation of the East Sea. Comparatively, the Ulsan offshore, with a thick sedimentary layer, exhibited a slightly higher gravity anomaly than the surrounding area, but no significant differences in the magnetic anomaly. Sequential tensile and compressive stresses related to the creation of the basin produced complex tectonic structures in this region. The magnetic tilt derivative results suggest that earthquakes were located near magnetic source boundaries. The results show that it is important to monitor earthquake-prone areas with gravity and magnetic anomalies.

Plume-lithosphere interaction beneath southwestern Africa – Insights from multi-mode Rayleigh wave tomography

A stationary mantle plume heats the overlying lithosphere from below, generating melts and causing lithospheric erosion. When the lithosphere moves away from the plume, it can be re-established by melt residues that have been since attached, partly due to cooling, at the base of the lithosphere. The Etendeka flood basalt region in northwest Namibia used to overlie the Tristan da Cunha mantle plume in the Early Cretaceous and thus provides an ideal place to examine plume-lithosphere interaction. Here we determine the upper mantle shear wave velocity structure of southern Africa down to 400 km depth by waveform inversion of multi-mode Rayleigh waves in order to find the imprints left by the Tristan da Cunha mantle plume. Thick lithosphere with high shear wave velocities is observed beneath the Congo and Kalahari Cratons, extending down to 200 km depth with the largest thickness beneath the Limpopo Belt. Along the landfall of the Walvis Ridge and Damara Belt, our model reveals a thick lithosphere down to 100–200 km depths. The thick lithosphere seems to constitute the Congo Craton, but the thick Congo cratonic lithosphere is extended farther south than observed in most of the previous models. We propose that the lithosphere along the landward extension of the Walvis Ridge was affected by magmatic processes related to the plume, but has been partially reconstructed since then by melt depletion and successive cooling. A high velocity anomaly beneath the northwest coast of Namibia down to a depth of ∼80 km coincides with distribution of the Etendeka basalt at the surface and the seaward-dipping reflectors at the continental margin. At sub-lithospheric depths, a low-velocity anomaly (at 100–200 km depth) and a high-velocity anomaly (at 250–350 km depth) can be detected. The origin of these anomalies could be related to an ongoing edge-driven convection process.

Nature of the Cuvier Abyssal Plain crust, offshore NW Australia

Magnetic stripes have long been assumed to be indicative of oceanic crust. However, continental crust heavily intruded by magma can also record magnetic stripes. We re-evaluate the nature of the Cuvier Abyssal Plain (CAP), offshore NW Australia, which hosts magnetic stripes and has previously been defined as oceanic crust. We show that chemical data from a basalt within the CAP, previously described as an enriched mid-ocean ridge basalt, could equally be interpreted to contain evidence of contamination by continental material. We also recognize seaward-dipping reflector sequences in seismic reflection data across the CAP. Borehole data from overlying sedimentary rocks suggests that these seaward-dipping reflectors were emplaced in a shallow water (<200 m depth) or subaerial environment. Our results indicate that the CAP may not be unambiguous oceanic crust, but may instead consist of a spectrum of heavily intruded continental crust through to fully oceanic crust. If the CAP represents such a continent–ocean transition zone, then the adjacent unambiguous oceanic crust would be located >500 km further offshore NW Australia than currently thought. This would impact plate tectonic reconstructions, as well as heat flow and basin modelling studies. Our work also supports the growing consensus that magnetic stripes cannot, by themselves, be used to determine crustal affinity. Supplementary material: Enlarged and uninterpreted versions of the magnetic data and seismic reflection lines are available at https://doi.org/10.6084/m9.figshare.c.5332172