Continent-ocean Boundary Research Articles

3D Process-Oriented Gravity Modelling applied north of 49°S on the Argentine continental margin

The Process-Oriented Gravity Modelling (POGM) technique represents a useful way to distinguish the contribution that different geological processes make to the observed gravity in passive margins. The POGM is an innovative gravity modelling approach that can give us information about the role that processes such as sedimentation and magmatic underplating, together with their loading effects, may play in the evolution of a margin. In this work, the POGM methodology has been applied with in a 2D and 3D approach. 2D profiles spaced every one arc-minute in the area of the Argentine continental margin, between 38.5°S and 49°S latitude and 64°W and 50°W longitude, were used to generate the latter.The 3D POGM was also solved and the result was compared with that obtained from 2D profiles. The comparison with the observed anomaly, using the 3D approach from 2D profiles gave results with enhanced resolution.The best fit between the calculated and observed gravity anomaly is given by an effective elastic thickness of 15 km. A cortical thickness map obtained as a result of the POGM calculations shows basin areas characterized by a thinned crust and a structural variation where the continental-oceanic boundary (COB) could be indicated. Besides, results of POGM allow us to detect an alignment between the Valdés and Rawson basins and possibly a third basin as a probable aulacogen. A stretching factor analysis shows that in these basins a stretching period existed but it did not reach the stage of oceanic crust formation. A strong positive residue in the Colorado basin is shown by the flexural isostatic anomaly, suggesting that the basin may continue in subsidence.

Tectonic evolution and extension at the Møre Margin – Offshore mid-Norway

Lithospheric stretching is the key process in forming extensional sedimentary basins at passive rifted margins. This study explores the stretching factors, resulting extension, and structural evolution of the Møre segment on the Mid-Norwegian continental margin. Based on the interpretation of new and reprocessed high-quality seismic, we present updated structural maps of the Møre margin that show very thick post-rift sediments in the central Møre Basin and extensive sill intrusion into the Cretaceous sediments. A major shift in subsidence and deposition occurred during mid-Cretaceous. One transect across the Møre continental margin from the Slørebotn Subbasin to the continent-ocean boundary is reconstructed using the basin modelling software TecMod. We test different initial crustal configurations and rifting events and compare our structural reconstruction results to stretching factors derived both from crustal thinning and the classical backstripping/decompaction approach. Seismic interpretation in combination with structural reconstruction modelling does not support the lower crustal bodies as exhumed and serpentinised mantle. Our extension estimate along this transect is ~188±28km for initial crustal thickness varying between 30 and 40km.

Public domain satellite gravity inversion offshore Somalia combining layered-Earth and voxel based modelling

At the end of 2016, Spectrum Geo released two long offset seismic reflection profiles across the Somalian rifted margin, in the Juba Lamu and Obbia basins respectively – part of two larger 2D surveys acquired in 2014 and 2015/16 (Stanca et al., 2016; Figure 1). In frontier areas, potential field data interpretation, in particular public domain satellite-derived gravity, can play a key role in the early stages of exploration by identifying basement structure, sediment thickness, and the continental-oceanic crust transition (COT) or continental-oceanic boundary (COB), and hence indirectly contribute to the understanding of thermal history and the hydrocarbon system. This is particularly relevant over the Somalia margin, where geophysical surveys have been extremely sparse until very recent years. A high-gradient in the gravity is often used as a marker of the COB along passive margins (see Pawlowski (2008) for a review), expressing laterally contrasting physical properties between the continental and oceanic crusts. However, at magma-poor rifted margins, such as the Somalia margin, the lithosphere undergoes a progressive thinning/stretching process and the transition between unaltered continental crust and oceanic crust becomes gradual (Manatschal, 2004). Crustal hyper-extension, serpentinization, embrittlement, and exhumation of mantle peridotites can precede accretion of true oceanic crust, which in turn can blur the simple transitional model between a dense and highly magnetized oceanic crust and a weakly magnetized and lighter continental crust. Along a magma-poor margin, analysis of potential field data can distinguish the different rifting domains and quantify crustal thinning (Stanton et al., 2016; Cowie et al., 2015). In offshore Kenya and Somalia, the image is further complicated by thick sedimentary cover.

Igneous seismic geomorphology of buried lava fields and coastal escarpments on the Vøring volcanic rifted margin

Voluminous igneous complexes are commonly present in sedimentary basins on volcanic rifted margins, and they represent a challenge for petroleum explorationists. A [Formula: see text] industry-standard 3D seismic cube has recently been acquired on the Vøring Marginal High offshore mid-Norway to image subbasalt sedimentary rocks. This cube also provides a unique opportunity for imaging top- and intrabasalt structures. Detailed seismic geomorphological interpretation of the top-basalt horizon, locally calibrated with high-resolution P-Cable wide-azimuth data, reveals new insight into the late-stage development of the volcanic flow fields and the kilometer-high coastal Vøring Escarpment. Subaerial lava flows with compressional ridges and inflated lava lobes cover the marginal high, with a comparable structure and size to modern subaerial lava fields. Pitted surfaces, likely formed by lava emplaced in a wet environment, are present in the western part of the study area near the continent-ocean boundary. The prominent Vøring Escarpment formed when eastward-flowing lava reached the coastline. The escarpment morphology is influenced by preexisting structural highs, and these highs are locally bypassed by the lava. Volcanogenic debris flows are well-imaged on the escarpment horizon, along with large-scale large slump blocks. Similar features exist in active volcanic environments, e.g., on the south coast of Hawaii. Numerous postvolcanic extensional faults and incised channels cut into the marginal high and the escarpment, and we found that the area was geologically active after the volcanism ceased. In summary, igneous seismic geomorphology and seismic volcanostratigraphy are two very powerful methods to understand the volcanic deposits and development of rifted margins. Our study demonstrates great promise for further understanding the igneous development of offshore basins as more high-quality 3D seismic data become available.

Mt Bambouto Volcano, Cameroon Line: Mantle Source and Differentiation of Within-plate Alkaline Rocks

The Late Cretaceous–Quaternary Cameroon Volcanic Line (CVL) is a 1600 km long chain of volcanoes that straddles the continent–ocean boundary and extends from the Gulf of Guinea to the interior of t ...

Geophysical evidence for the crustal variation and distribution of magmatism along the central coast of Mozambique

For our understanding of the timing and geometry of the initial Gondwana break-up, still a consistent image of the crustal composition of the conjugated margins of central Mozambique and Antarctica and the location of their continent-ocean boundaries is missing. In this regard, a main objective is the explanation for the source of the different magnetic signature of the conjugate margins. Based on a revised investigation of wide-angle seismic data along two profiles across the Mozambican margin by means of an amplitude modelling, this study presents the crustal composition across and along the continental margin of central Mozambique. Supported by 2D magnetic modelling, the results are compared to the conjugate margin in Antarctica and allow new conclusions about their joined tectonic evolution. An observed crustal diversity between the north-eastern and south-western parts of the central Mozambican margin, testifies to the complex break-up history of this area. Conspicuous is the equal spatial extent of the HVLCB along the margin of 190–215km. The onset of oceanic crust at the central Mozambican margin is refined to chron M38n.2n (164.1Ma). Magnetic modelling supports the presence of reversed polarized SDRs in the continent-ocean transition that were mainly emplaced between 168.5 and 166.8Ma (M42–M40). Inferred SDRs in the Riiser-Larsen Sea might be emplaced sometime between 166.8 and 164.1Ma (M39–M38), but got overprinted by normal polarized intrusions of a late stage of rift volcanism, causing the opposite magnetic signature of the conjugate margins. The distribution of the magmatic material along the central coast of Mozambique clearly indicates the eastern extension of the north-eastern branch of the Karoo triple rift along the entire margin. The main magmatic phase affecting this area lasted for at least 12Myr between 169 and 157Ma, followed by the cease of the magmatism, perhaps due to the relative southwards motion of the magmatic centre.

Statistics of the Earth’s Topography

The so-called “typical” values for the Earth’s topography are often used in the literature, such as the mean continental altitude (MCA), the Moho depth for “normal” continental crust, or “typical” depth of mid-oceanic ridges. However, the statistical relevance of those values is hardly discussed. Focussed on data for the global topography, this paper presents statistical analyses regarding various environments. It is shown in particular that the definition of the mid-oceanic ridge is not straightforward, and varies considerably according to what is actually considered: the ridge “inner-rift”, the ridge “crest”, or the “virtual ridge” at spreading centre. This definition is also a function of the spreading rate and has strong implications for the rationale on the age-depth relationship of the sea-floor. In addition, the latter relationship is highly dependent on how the topographic data are corrected from sediment load. The correction itself implies numerous aspects that relies on the precision and associated uncertainties of, in particular, the sediment thickness, sediment porosity, and the mantle, water and sediment densities. In this respect, the analysis carried out here favours a plate cooling model (PCM) for the age-depth dependence of the sea-floor. The topographic elevation at trench proves also to be related to the age of the sea-floor through a different PCM equation. Away from the trench, the oceanic lithosphere is affected by flexuration, for which equations can be defined assuming that the end-load position is not located at trench. On the other hand, the elevation of magmatic arc does not appear to be related to sea-floor age or spreading rate. However, the correlation between the arc-trench distance and the topographic elevation of arc for continental crust seems to be an indicator of slab dip and therefore the existence of slab roll-back processes. Along intra-oceanic magmatic arc, a periodicity in topographic elevation suggests a periodicity in the occurrence of magma chambers, and therefore magmatic processes that need to be further studied. At passive margin, the transition between continental and oceanic crust seems to be relatively sharp in average. Subdivision of the datasets according to the age of the continent-ocean boundary (COB) indicates that rift and passive margin shoulders are found within a couple of degree away from the COB and for ages younger than ca. 20 Ma. Finally, the statistical analysis of continental data assumed to be free of thinning or thickening effects suggests that the MCA should rather be considered in terms of “lowlands” and “highlands”. Relying on model of Moho depth, the “normal” crustal depth might be thinner than commonly accepted. In any case, the filtering of reduced topography can help to determine the impact of dynamic topography.

Isostasy as a tool to validate interpretations of regional geophysical datasets – application to the mid-Norwegian continental margin

Abstract Isostasy is a well understood concept, yet rarely applied to its full capacity in regional interpretations of crustal structures. In this study, we utilize a recent density model for the entire NE Atlantic, based on refraction seismic data and gravity inversion, to calculate isostatically balanced bathymetry along the mid-Norwegian margin. Since gravity and isostatically balanced elevation are independent observables but both depend on the underlying density model, consistencies and discrepancies point towards model deficits, erroneously interpreted or poorly understood areas. Four areas of large isostatic residuals are identified. Along the outer Vøring Margin, a mass deficit points to more extensive high-density bodies or a shallower Moho than currently mapped. Farther seaward, along the Vøring Marginal High, a mass excess indicates inaccurate mapping of the continent–ocean boundary and surrounding structures. A number of eclogitic bodies along the proximal mid-Norwegian margin have been described in recent publications and their presence is now also confirmed by isostatic calculations. Major elevation and gravity residuals along the transition between the Vøring and Møre margins signify that the structure of this region is poorly understood and modifications to the mapped continent–ocean transition may be required.

Break-up and seafloor spreading domains in the NE Atlantic

Abstract An updated magnetic anomaly grid of the NE Atlantic and an improved database of magnetic anomaly and fracture zone identifications allow the kinematic history of this region to be revisited. At break-up time, continental rupture occurred parallel to the Mesozoic rift axes in the south, but obliquely to the previous rifting trend in the north, probably due to the proximity of the Iceland plume at 57–54 Ma. The new oceanic lithosphere age grid is based on 30 isochrons (C) from C24n old (53.93 Ma) to C1n old (0.78 Ma), and documents ridge reorganizations in the SE Lofoten Basin, the Jan Mayen Fracture Zone region, in Iceland and offshore Faroe Islands. Updated continent–ocean boundaries, including the Jan Mayen microcontinent, and detailed kinematics of the Eocene–Present Greenland–Eurasia relative motions are included in this model. Variations in the subduction regime in the NE Pacific could have caused the sudden northwards motion of Greenland and subsequent Eurekan deformation. These events caused seafloor spreading changes in the neighbouring Labrador Sea and a decrease in spreading rates in the NE Atlantic. Boundaries between major oceanic crustal domains were formed when the European Plate changed its absolute motion direction, probably caused by successive adjustments along its southern boundary. Supplementary material: Figures showing the long wavelength of the NAG-TEC magnetic anomaly grid, detailed magnetic anomalies and isochrons, and a Table documenting aeromagnetic surveys for NAG-TEC magnetic compilation are available at https://doi.org/10.6084/m9.figshare.c.3661925

Paleogene marine deposition records of rifting and breakup of the South China Sea: An overview

A compilation of available marine deposition data from offshore S-SE China reveals evidence of rifting and breakup of the South China Sea (SCS) during the Paleogene. Marine deposition started earlier in the Paleocene in the East China Sea (ECS)-Taiwan region before expanding southwestward into the SCS region in the middle Eocene. Our data indicate the existence of an elongated Paleogene China Sea in these areas stretching along the northeasterly structural belts, probably as part of the marginal western paleo-Pacific. The southwestward shift of marine influence in the middle Eocene was responding to a period of intensive rifting and subsidence in the SCS region, while the sea in the ECS-Taiwan region started to shrink and shoal after the late Eocene, likely associated with local breakup and initial spreading in the Taiwan-Taixinan Basin area. The accumulation of hemipelagic sediments at ODP 1148 and IODP U1435 from near the continent-ocean boundary and at many other shelf-slope sites was in response to a large-scale breakup 34 to 33 Ma ago, subsequently leading to the birth of the SCS in the Oligocene.

The stratigraphic record of prebreakup geodynamics: Evidence from the Barrow Delta, offshore Northwest Australia

The structural and stratigraphic evolution of rift basins and passive margins has been widely studied, with many analyses demonstrating that delta systems can provide important records of postrift geodynamic processes. However, the apparent lack of ancient synbreakup delta systems and the paucity of seismic imaging across continent-ocean boundaries mean that the transition from continental rifting to oceanic spreading remains poorly understood. The Early Cretaceous Barrow Group of the North Carnarvon Basin, offshore NW Australia, was a major deltaic system that formed during the latter stages of continental rifting and represents a rich sedimentary archive, documenting uplift, subsidence, and erosion of the margin. We use a regional database of 2-D and 3-D seismic and well data to constrain the internal architecture of the Barrow Group. Our results highlight three major depocenters: the Exmouth and Barrow subbasins and southern Exmouth Plateau. Overcompaction of pre-Cretaceous sedimentary rocks in the South Carnarvon Basin, and pervasive reworking of Permian and Triassic palynomorphs in the offshore Barrow Group, suggests that the onshore South Carnarvon Basin originally contained a thicker sedimentary succession, which was uplifted and eroded prior to breakup. Backstripping of sedimentary successions encountered in wells in the Exmouth Plateau depocenter indicates that anomalously rapid tectonic subsidence (≤0.24 mm yr−1) accommodated Barrow Group deposition, despite evidence for minimal, contemporaneous upper crustal extension. Our results suggest that classic models of uniform extension cannot account for the observations of uplift and subsidence in the North Carnarvon Basin and may indicate a period of depth-dependent extension or dynamic topography preceding breakup.

Assessing discrepancies between previous plate kinematic models of Mesozoic Iberia and their constraints

Currently, there are several published end-member plate models that describe the evolution of Iberia during the Mesozoic. We review key geological and geophysical data sets previously used as constraints on these models including (1) geological interpretations of Pyrenean geology; (2) end-member interpretations of magnetic anomalies along the West Iberian and Newfoundland margins and Bay of Biscay; (3) the paleomagnetic data set of Iberia; and (4) seismic tomography models, which have previously been used to support Cretaceous subduction between Iberia and Eurasia. From this review we identify key constraints and argue that a reasonable plate kinematic model of Iberia should satisfy all of these. Instead, we determine inconsistencies between these key constraints and several published end-member plate models through a kinematic analysis using the GPlates software. We also analyze published seismic tomography models, not previously considered, across northern Africa and Iberia, and identify no slab preserved within the mantle supposedly linked to Cretaceous subduction between Iberia and Eurasia. A lack of published geological evidence along the Pyrenees supporting this subduction history also casts doubt on this scenario. Our kinematic analysis highlights that, first, the cessation in transtensional motion between Iberia and Eurasia by the Albian cannot be kinematically reconciled with the concurrent breakup between Iberia and Newfoundland in the Atlantic when reconstructing existing continent-ocean boundary interpretations along their respective margins. Second, either fit of the contentious end-member M0 interpretations (~120.6 Ma) between Iberia and Newfoundland implies plate velocities of Iberia that results in its undocumented transpressional or compressional motion relative to Eurasia until C34.

The wide-angle seismic image of a complex rifted margin, offshore North Namibia: Implications for the tectonics of continental breakup

Voluminous magmatism during the South Atlantic opening has been considered as a classical example for plume related continental breakup. We present a study of the crustal structure around Walvis Ridge, near the intersection with the African margin. Two wide-angle seismic profiles were acquired. One is oriented NNW–SSE, following the continent–ocean transition and crossing Walvis Ridge. A second amphibious profile runs NW–SE from the Angola Basin into continental Namibia. At the continent–ocean boundary (COB) the mafic crust beneath Walvis Ridge is up to 33km thick, with a pronounced high-velocity lower crustal body. Towards the south there is a smooth transition to 20–25km thick crust underlying the COB in the Walvis Basin, with a similar velocity structure, indicating a gabbroic lower crust with associated cumulates at the base. The northern boundary of Walvis Ridge towards the Angola Basin shows a sudden change to oceanic crust only 4–6km thick, coincident with the projection of the Florianopolis Fracture Zone, one of the most prominent tectonic features of the South Atlantic ocean basin. In the amphibious profile the COB is defined by a sharp transition from oceanic to rifted continental crust, with a magmatic overprint landward of the intersection of Walvis Ridge with the Namibian margin. The continental crust beneath the Congo Craton is 40km thick, shoaling to 35km further SE. The velocity models show that massive high-velocity gabbroic intrusives are restricted to a narrow zone directly underneath Walvis Ridge and the COB in the south. This distribution of rift-related magmatism is not easily reconciled with models of continental breakup following the establishment of a large, axially symmetric plume in the Earth's mantle. Rift-related lithospheric stretching and associated transform faulting play an overriding role in locating magmatism, dividing the margin in a magma-dominated southern and an essentially amagmatic northern segment.

The crustal structure of Beira High, central Mozambique—Combined investigation of wide-angle seismic and potential field data

The timing and geometry of the initial Gondwana break-up between Africa and East Antarctica is still poorly known due to missing information about the continent-ocean boundaries along the rifted margins. In this context, the Beira High off central Mozambique forms a critical geological feature of uncertain crustal fabric. Based on new wide-angle seismic and potential field data across Beira High a P-wave velocity model, supported by amplitude and gravity modelling, provides constraints on the crustal composition of this area. In the Mozambique Basin mainly normal oceanic crust of 5.5–7km thickness with velocities of 6.5–7.0km/s in the lower crust is present. A sharp transition towards Beira High marks the continent-ocean boundary. Here the crust thickens to 23km at maximum. A small velocity-depth gradient and a constant increase in velocity with basal velocities of maximum 7.0km/s are in good agreement with typical velocities of continental crust and continental fragments. The density model indicates the existence of felsic material in greater depths and supports a fabric of stretched, but highly intruded continental crust below Beira High. A gradual decrease in crustal thickness characterizes the transition towards the Mozambican shelf area. Here, in the Zambezi Delta Depression 12km of sediments cover the underlying 7km thick crust. The presence of a high-velocity lower crustal body with velocities of 7.1–7.4km/s indicates underplated, magmatic material in this part of the profile. However, the velocity structure in the shelf area allows no definite interpretation because of the experimental setup. Thus, the crustal nature below the Zambezi Delta and consequently the landward position of the continent-ocean boundary remains unknown. The difference in stretching below the margins of Beira High suggests the presence of different thinning directions and a rift jump during the early rifting stage.

Reconstruction of the East Africa and Antarctica continental margins

AbstractThe Early Jurassic separation of Antarctica from Africa plays an important role in our understanding of the dispersal of Gondwana and Pangea. Previous reconstruction models contain overlaps and gaps in the restored margins that reflect difficulties in accurately delineating the continent‐ocean‐boundary (COB) and determining the amount and distribution of extended continental crust. This study focuses on the evolution of the African margin adjacent to the Mozambique Basin and the conjugate Antarctic margin near the Riiser‐Larsen Sea. Satellite‐derived gravity data have been used to trace the orientations and landward limits of fracture zones. A 3‐D gravity inversion has produced a crustal thickness model that reliably quantifies the extent and amount of stretched crust. Crustal thicknesses together with fracture zone terminations reveal COBs that are significantly closer to the African and Antarctic coasts than previously recognized. Correlation of fracture zone azimuths and identified COBs suggests Antarctica began drifting away from Africa at approximately 171 Ma in a roughly SSE direction. An areal‐balancing method has been used to restore the crust to a uniform prerift thickness so as to perform a nonrigid reconstruction for both nonvolcanic and volcanic margins. Both margins reveal a trend of increasing extension from east to west. Our results suggest Africa underwent extension of 60–120 km, while Antarctic crust was stretched by 105–180 km. Various models tested to determine the direction of extension during rifting suggest that Antarctica moved away from Africa in a WNW‐ESE direction during the period between 184 and 171 Ma prior to the onset of seafloor spreading.

3-D magnetotelluric image of offshore magmatism at the Walvis Ridge and rift basin

The Namibian continental margin marks the starting point of the Tristan da Cunha hotspot trail, the Walvis Ridge. This section of the volcanic southwestern African margin is therefore ideal to study the interaction of hotspot volcanism and rifting, which occurred in the late Jurassic/early Cretaceous. Offshore magnetotelluric data image electromagnetically the landfall of Walvis Ridge. Two large-scale high resistivity anomalies in the 3-D resistivity model indicate old magmatic intrusions related to hot-spot volcanism and rifting. The large-scale resistivity anomalies correlate with seismically identified lower crustal high velocity anomalies attributed to magmatic underplating along 2-D offshore seismic profiles. One of the high resistivity anomalies (above 500Ωm) has three arms of approximately 100km width and 300km to 400km length at 120° angles in the lower crust. One of the arms stretches underneath Walvis Ridge. The shape is suggestive of crustal extension due to local uplift. It might indicate the location where the hot-spot impinged on the crust prior to rifting. A second, smaller anomaly of 50km width underneath the continent ocean boundary may be attributed to magma ascent during rifting. We attribute a low resistivity anomaly east of the continent ocean boundary and south of Walvis Ridge to the presence of a rift basin that formed prior to the rifting.

Basement configuration of KG offshore basin from magnetic anomalies

Marine magnetic anomalies along three representative profiles falling between shelf break and continent–ocean boundary in the offshore Krishna–Godavari basin were quantitatively interpreted for understanding the nature and structure of the magnetic basement using inversion technique. The interpretation of the anomalies shows that the magnetic basement lies deeper than the base of the sediments, i.e., acoustic basement identified by the seismic studies. This interpretation also shows that the magnetic basement is faulted along the NW–SE direction with the upthrown side lying to the north of the anomaly trend of this region. The coincidence of magnetizations observed through the present interpretation with that of charnockites of neighbouring EGMB and onshore K–G basin areas indicates that EGMB geology (charnockites, granitic gneiss, etc.) extends up to COB in the offshore K–G basin.

Geodinámica de los márgenes conjugados del Atlántico Sur central: implicaciones para el potencial de hidrocarburos

Using a new margin restoration model for the central South Atlantic we highlight margin-opposed rift basin subsidence characteristics, reconcile important facies relationships and consider the associated processes responsible for contrasting basinward transitions to oceanic crust. We interpret these parameters as diagnostic of strain evolution during rifting. Pre-salt subsidence patterns and sequence isopachs of late syn-rift and early post-rift sequences are symptomatic of these complex extension behaviours. Inherited basement fault trends partition extensional strain during stretching with resultant rift-related structural styles recording the heterogeneity of the pre-rift crust. Correspondingly the seismic and gravity expression of the transition from continental crust to oceanic crust differs systematically along the length of the ‘conjugate’ basins. The presence of a conspicuous coast parallel positive linear gravity anomaly, referred to as the ‘terminal horst’, defines the basinward extent of attenuated continental crust in the more symmetrical Campos Basin and its interpreted conjugate. In the ‘asymmetrical’ Santos and Benguela Basins the ocean–continent transition at the present-day OCB (ocean–continent boundary) becomes equivocal with little gravity expression. Consequently the relationship of primary salt basin edges to the basinward extent of crustal extension appears more complex. Pre-existing crustal and lithospheric mantle scale heterogeneities are considered to impart a first-order control on whole crust deformation and ultimately ‘sag’ basin development. These structurally defined heterogeneities are believed to partition crustal strain and juxtapose endmember mechanisms of pure-shear and simple shear deformation styles as recorded by the complex distribution of syn-rift subsidence patterns within the pre-salt basins of the central South Atlantic. These conclusions challenge the necessity for invoking dominantly depth-dependent processes as a mechanism to explain apparent pre-salt, syn-rift subsidence anomalies. The complex interplay of structure and stratigraphy with an overlay of palaeoclimatic models provides important insights into hydrocarbon play fairways along the margin. In particular the impact on reservoir and source rock distribution and heat flow are discussed.

Propagated rifting in the Southwest Sub-basin, South China Sea: Insights from analogue modelling

How the South China Sea rifted has long been a puzzling question that is still debated, particularly with reference to the Southwest Sub-basin (SWSB). Analogue modelling remains one of the most useful tools for testing rift models and processes. Here, we present and discuss a series of analogue modelling experiments designed to investigate the rifting process of the SWSB. Convincing geophysical results were compiled to provide realistic constraints to test the experimental results and interpretations. A heterogeneous lithosphere model with a varied lithospheric structure showed tectono-morphological features similar to the natural case of the SWSB, indicating that the initial thermal condition and rheological stratification of the lithosphere should have a dominant effect on the rifting process of the SWSB. Rigid tectonic blocks existed in the continental margin, such as the Macclesfield Bank and the Reed Bank, and they played important roles in both the shaping of the continent–ocean boundary and the coupling between the crust and mantle. The initial thermal condition and rheological stratification of the lithosphere under the South China Sea controlled the propagated rifting process of the SWSB. Extension was centred on the deep troughs between the rigid blocks, and the break-up occurred in these areas between them. The westward rifting propagation is best explained with a heterogeneous lithosphere model characterized by varied lithospheric structure, and it was responsible for producing the V-shaped configuration of the SWSB.

Distribution of crustal types in Canada Basin, Arctic Ocean

Seismic velocities determined from 70 sonobuoys widely distributed in Canada Basin were used to discriminate crustal types. Velocities of oceanic layer 3 (6.7–7.1km/s), transitional (7.2–7.6km/s) and continental crust (5.5–6.6km/s) were used to distinguish crustal types. Potential field data supports the distribution of oceanic crust as a polygon with maximum dimensions of ~340km (east–west) by ~590km (north–south) and identification of the ocean–continent boundary (OCB). Paired magnetic anomalies are associated only with crust that has oceanic velocities. Furthermore, the interpreted top of oceanic crust on seismic reflection profiles is more irregular and sometimes shallower than adjacent transitional crust. The northern segment of the narrow Canada Basin Gravity Low (CBGL), often interpreted as a spreading center, bisects this zone of oceanic crust and coincides with the location of a prominent valley in seismic reflection profiles. Data coverage near the southern segment of CBGL is sparse. Velocities typical of transitional crust are determined east of it. Extension in this region, close to the inferred pole of rotation, may have been amagmatic. Offshore Alaska is a wide zone of thinned continental crust up to 300km across. Published longer offset refraction experiments in the Basin confirm the depth to Moho and the lack of oceanic layer 3 velocities. Further north, toward Alpha Ridge and along Northwind Ridge, transitional crust is interpreted to be underplated or intruded by magmatism related to the emplacement of the High Arctic Large Igneous Province (HALIP). Although a rotational plate tectonic model is consistent with the extent of the conjugate magnetic anomalies that occupy only a portion of Canada Basin, it does not explain the asymmetrical configuration of the oceanic crust in the deep water portion of Canada Basin, and the unequal distribution of transitional and continental crust around the basin.