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We question the timing of continental breakup and early oceanization in Baffin Bay, North-East Atlantic. North of the Ungava fault zone, the breakup was syn-magmatic and led to the development of conjugate volcanic passive margins (VPMs). We investigated the innermost part of the W-Greenland VPM where a remarkable inner-SDR is fully exposed in the Svartenhuk area. Our new radiometric ages and paleomagnetic data from syn-tectonic basaltic lavas indicate that continental stretching and thinning spanned the C26r to, at least, the C24r period, giving an Eocene lower boundary age for continental breakup in Baffin Bay. These results contradict the proposed flooring of Baffin Bay by a Paleocene oceanic crust older than C24n and also question the accretion of oceanic crust before C22. We confront our results to the dynamics of the northward oceanic-rift propagation across the Ungava transform fault system, and we suggest that plate breakup in Baffin Bay occurred ~8 m.y. later than in N-Labrador Sea as a result of the thermal and mechanical barrier effect induced by the Ungava transform zone.

The structure and tectonostratigraphic development of the mid-Norwegian volcanic passive margin have been extensively studied over last 30 years. However, an understanding of its crustal architecture and basin evolution remains incomplete and debated. A main point of a debate concerns the crustal and basin structure of the yet underexplored outer parts of the Møre and Vøring basins which are significantly covered by breakup-related volcanics. This discussion generally resides on the origin of the high-velocity (7+km/s) lower crustal body which alternatively interpreted either as a wide zone of exhumed/serpentinized mantle assuming direct structural similarities with the magma-poor Iberian margin or instead inherited high-grade Caledonian crust later intruded by breakup-related magmatic intrusions. Another important point of contention is whether the Møre and Vøring basins developed through either several discrete extensional events, or alternatively a single phase of continuous extension from Late Jurassic-Early Cretaceous necking to lithospheric breakup in the late Paleocene-early Eocene.Recently, a new generation of high-quality 2D and 3D seismic data acquired in the outer parts of the mid-Norwegian margin allowed a better imaging of deep Vøring and Møre basins and sub-basalt domains. Also new well data allowed a better regional seismostratigraphic control. An integrated 3D/4D interpretation of new seismic data calibrated with published refraction data and tested by potential field and forward basin modelling helped to better reveal the crustal and basin architecture of the Møre and Vøring basins.Our results support the crustal nature of the controversial high-velocity and high-density lower crustal body and associated deep reflections, which we interpret as an old exhumed high-grade Caledonian crust later mixed with breakup-related mafic and ultra-mafic magmatic material. Our seismic interpretation shows that the basins were subjected to discrete and localized Cretaceous-Paleocene rifting events which sequentially migrated laterally and towards the future breakup axis and were separated by intermediate cooling/subsidence phases. We explain this migration of the rift axes by a strain hardening due to lithospheric cooling with possible enhancement from lateral lower crustal flow.We suggest that the outer portion of the Vøring and Møre basins represents distal “marginal plateaus” that likely formed an elevated crustal domain bounded to the east by a failed and cooling inner rift system and to the west by Cenozoic volcanic margins. The presence of such a marginal plateau may better explain (1) the observed structural styles and 3D geometries of the sedimentary successions in the outer basins (e.g. shallowing of the Base Cretaceous Unconformity), (2) the long-time lag (˃80-100 Myr) between the mid-Mesozoic necking and the final (off axis) lithospheric breakup, (3) the subaerial and shallow marine emplacement of breakup-related lavas, (4) the signatures of upper crustal contamination in breakup-related flows, and (5) the relatively low magnetization of the basement in the outer basins. Our interpretations do not support the magma-poor Iberian margin model which were recently extrapolated and applied to the pre-breakup development and structural environment of the mid-Norwegian volcanic passive margin.

Extension rate and crustal strength are two important factors that influence the shape of initial rift basins and final passive margin architecture. The extension rate will also affect crustal strength as the passive margin evolves, therefore adding more complexity to the system. We used thermomechanical numerical experiments to systematically explore the relationship of extension rate and initial crustal strength on final passive margin structure. Varying the initial amount of ductile crust from 33% to 70% and extension rates from 2 to 50 mm yr −1 results in passive margins of different widths, shapes, and evolution histories. We find four distinct margin types: Type I–narrow symmetric margins, Type II–wide symmetric margins, Type III–asymmetric margins, and Type IV–rift jumping. Experiments with strong initial crust (33% ductile crust) have crust-mantle coupling of strain and produce narrow, symmetric margins with high rift flank uplift (Type I margins). Ultraslow extension experiments (2 mm yr −1) undergo thermal cooling and produce wide, symmetric margins with hyperextended crust (Type II margins). Experiments with intermediate to fast extension rates (10–50 mm yr −1) and thick layers of ductile crust will produce initial rift basins that are symmetric, but later rifting stages yield asymmetric conjugate margins with hyperextended domains 100–250 km wide (Type III margins). In special cases with intermediate extension rates and thick ductile crustal layers, we observe a jump in rift location over 50 km and abandoned rifts with isolated crustal blocks (Type IV margins). These results highlight how ductile crustal layers and extension rates influence passive margin formation during different stages of rifting.

International audience; The stunningly increased resolution of the deep crustal levels in recent industrial seismic profiles acquired along most of the world's rifted margins leads to the unraveling of an unexpected variety of structures. It provides unprecedented access to the processes occurring in the middle and lower continental crust. We present a series of so far unreleased profiles that allows the identification of various rift-related geological processes such as crustal boudinage, ductile shear and low-angle detachment faulting, and a rifting history that differs from the classical models of oceanward-dipping normal faults. The lower crust in rifted margins appears much more intensely deformed than usually represented. At the foot of both magma-rich and magma-poor margins, we observe clear indications of ductile deformation of the deep continental crust along large-scale shallow dipping shear zones. These shear zones generally show a top-to-the-continent sense of shear consistent with the activity of Continentward Dipping Normal Faults (CDNF) observed in the upper crust. This pattern is responsible for a migration of the deformation and associated sedimentation and/or volcanic activity toward the ocean. We discuss the origin of these CDNF and investigate their implications and the effect of sediment thermal blanketing on crustal rheology. In some cases, low-angle shear zones define an anastomosed pattern that delineates boudin-like structures. The maximum deformation is localized in the inter-boudin areas. The upper crust is intensely boudinaged and the highly deformed lower crust fills the inter-boudins underneath. The boudinage pattern controls the position and dip of upper crustal normal faults. We present some of the most striking examples from the margins of Uruguay, West Africa, South China Sea and Barents Sea, and discuss their implications for the time-temperature history of the margins.

Despite many years of study, the processes involved in the post-breakup development of passive margins remain poorly understood. Integration of apatite fission track analysis (AFTA) and stratigraph ...

Study of the deep structure of conjugate passive continental margins combined with detailed plate kinematic reconstructions can provide constraints on the mechanisms of rifting and formation of initial oceanic crust. In this study the central Atlantic conjugate margins are compared based on compilation of wide-angle seismic profiles from NW Africa Nova Scotian and U.S. passive margins. The patterns of volcanism, crustal thickness, geometry, and seismic velocities in the transition zone suggest symmetric rifting followed by asymmetric oceanic crustal accretion. Conjugate profiles in the southern central Atlantic image differences in the continental crustal thickness. While profiles on the eastern U.S. margin are characterized by thick layers of magmatic underplating, no such underplate was imaged along the African continental margin. In the north, two wide-angle seismic profiles acquired in exactly conjugate positions show that the crustal geometry of the unthinned continental crust and the necking zone are nearly symmetric. A region including seismic velocities too high to be explained by either continental or oceanic crust is imaged along the Canadian side, corresponding on the African side to an oceanic crust with slightly elevated velocities. These might result from asymmetric spreading creating seafloor by faulting the existing lithosphere on the Canadian side and the emplacement of magmatic oceanic crust including pockets of serpentinite on the Moroccan margin. After isochron M25, a large-scale plate reorganization might then have led to an increase in spreading velocity and the production of thin magmatic crust on both sides.

Comparative tectonic analysis of passive margins of the Atlantic Ocean has been performed. Tectonotypes of both volcanic and nonvolcanic margins are described, and their comparison with other passive Atlantic margins is given. The structural features of margins, peculiarities of magmatism, its sources and reasons for geochemical enrichment of melts are discussed. The important role of melting of the continental lithosphere in the development of magmatism is demonstrated. Enriched EM I and EM II sources are determined for the lower parts of the volcanic section, and a depleted or poorly enriched source is determined for the upper parts of the volcanic section based on isotope data. The conclusions of the paper relate to tectonic settings of the initial occurrence of magmatism and rifting and breakup during the period of opening of the Mesozoic Ocean. It was found out that breakup and magmatism at proximal margins led only to insignificant structural transformations and reduction of the thickness of the ancient continental crust, while very important magmatic events happened later in the distal zone. New growth of magmatic crust at the stage of continental breakup is determined as a typical feature of distal zones of the margins under study. The relationship of development of margins with the impact of deep plumes as the source of magmatic material or a heat source only is discussed. Progradation of the zone of extension and breakup into the areas of cold lithosphere of the Atlantic and the formation of a single tectonomagmatic system of the ocean are under consideration.

Seismic reflection profiles of Volcanic Passive Margins show a two-domain reflective lower crust. The particular reflectors are interpreted as mafic intrusions associated to the margin evolution.From those geophysical observations it is not possible to understand the consequences of mafic magma emplacement on the rheology and effective deformation of volcanic margins lower crust. This thesis explores two geologic examples of outcropping lower crusts affected by important volumes of magma during their extensional story: the Ivrea Zone (Italian Alps) and Seiland (Finmarkian Caledonides). An important field study associated to geochemical and isotopic analyses, and to Patersonapparatus deformation experiences led to (1) define a synthetic structure of an extensional lower crust affected by one or more major magmatic events and to indentify a ductile middle crust in the Ivrea Zone where roots the upper crust faults, (2) show the rheologic and deformation evolution of the lower crust in relation to magma emplacement timing, (3) characterize, from Seiland observations, the crustmantle interface in an extensional System. The whole study shows the complexity of the temporal, spatial and structural relationships between mafic rocks (as magma and as crystallised rock) and extensional deformation in the lower crust., Les profils de sismique réflexion de Marges Passives Volcaniques imagent une croûte inférieure réflective formée de deux domaines distincts. Ces réflecteurs sont interprétés comme des intrusions mafiques associées à la formation de la marge. Les observations géophysiques ne permettent pas de comprendre les conséquences associées à la mise en place de ces magmas sur la rhéologie et la déformation effective de la croûte inférieure des marges volcaniques. Cette thèse explore deux exemples géologiques de croûte inférieure à l’affleurement qui ont été affectés par d’importants volumes magmatiques pendant leur histoire extensive : la Zone d’Ivrée (Alpes italiennes) et la province de Seiland (Calédonides finmarkiennes). Une étude approfondie de terrain, associée à des analyses géochimiques et isotopiques ainsi qu’à des expériences de déformation en Presse Paterson permet, (1) de définir la structure synthétique finie d’une croûte inférieure extensive affectée par un ou plusieurs épisodes magmatiques intenses et d'identifier dans la Zone d’Ivrée une croûte moyenne ductile, dans laquelle s’enracinent les failles de la croûte supérieure, (2) de montrer l’évolution de la rhéologie et de la déformation ductile de la croûte inférieure relativement à la mise en place des magmas (3) de caractériser à partir de Seiland l’interface croûte manteau dans un contexte d’extension synmagmatisme mafique. L’ensemble des résultats montre la complexité des relations, à la fois structurales, spatiales et temporelles, entre les magmas et roches mafiques et la déformation en extension dans la croûte inférieure.

The continental margins of the southwest subbasin in the South China Sea mark a unique transition from multi-stages magma-poor continental rifting to seafloor spreading. We used reflection and refraction profiles across the margins to investigate the rifting process of the crust. Combining with the other seismic profiles acquired earlier, we focused on the comparative geological interpretation from the result of multichannel seismic analysis and wide-angle seismic tomography. Our result provides the evidence of upper crustal layer with abundant fractures below the acoustic basement with a P-wave velocity from 4.0 to 5.5 km s−1. It indicates extensive deformation of the brittle crust during the continental rifting and can make a good explanation for the observed extension discrepancy in the rift margins of the South China Sea. The seismic chronostratigraphic result shows the possibility of the intra-continental extension center stayed focused for quite a long time in Eocene. Additionally, our evidence suggested that continental margin of the southwest subbasin had experienced at least three rifting stages and the existence of the rigid blocks is an appropriate explanation to the asymmetric rifting of the South China Sea.

Rifts and passive margins often develop along old suture zones where colliding continents merged during earlier phases of the Wilson cycle. For example, the North Atlantic formed after continental break-up along sutures formed during the Caledonian and Variscan orogenies. Even though such tectonic inheritance is generally appreciated, causative physical mechanisms that affect the localization and evolution of rifts and passive margins are not well understood. We use thermo-mechanical modeling to assess the role of orogenic structures during rifting and continental breakup. Such inherited structures include: 1) Thickened crust, 2) eclogitized oceanic crust emplaced in the mantle lithosphere, and 3) mantle wedge of hydrated peridotite (serpentinite). Our models indicate that the presence of inherited structures not only defines the location of rifting upon extension, but also imposes a control on their structural and magmatic evolution. For example, rifts developing in thin initial crust can preserve large amounts of orogenic serpentinite. This facilitates rapid continental breakup, exhumation of hydrated mantle prior to the onset of magmatism. On the contrary, rifts in thicker crust develop more focused thinning in the mantle lithosphere rather than in the crust, and continental breakup is therefore preceded by magmatism. This implies that whether passive margins become magma-poor or magma-rich, respectively, is a function of pre-rift orogenic properties. The models show that structures of orogenic eclogite and hydrated mantle are partially preserved during rifting and are emplaced either at the base of the thinned crust or within the lithospheric mantle as dipping structures. The former provides an alternative interpretation of numerous observations of ‘lower crustal bodies’ which are often regarded as igneous bodies. The latter is consistent with dipping sub-Moho reflectors often observed in passive margins.

The NE Atlantic region evolved through several rift episodes, leading to break-up in the Eocene that was associated with voluminous magmatism along the conjugate margins of East Greenland and NW Europe. Existing seismic refraction data provide good constraints on the overall tectonic development of the margins, despite data gaps at the NE Greenland shear margin and the southern Jan Mayen microcontinent. The maximum thickness of the initial oceanic crust is 40 km at the Greenland–Iceland–Faroe Ridge, but decreases with increasing distance to the Iceland plume. High-velocity lower crust interpreted as magmatic underplating or sill intrusions is observed along most margins but disappears north of the East Greenland Ridge and the Lofoten margin, with the exception of the Vestbakken Volcanic Province at the SW Barents Sea margin. South of the narrow Lofoten margin, the European side is characterized by wide margins. The opposite trend is seen in Greenland, with a wide margin in the NE and narrow margins elsewhere. The thin crust beneath the basins is generally underlain by rocks with velocities of >7 km s−1 interpreted as serpentinized mantle in the Porcupine and southern Rockall basins; while off Norway, alternative interpretations such as eclogite bodies and underplating are also discussed.

The relationship between plate- and plume-tectonics is considered in view of the growth and breakdown of supercontinents, active rifting, the formation of passive volcanic-type continental margins, and the origin of time-progressive volcanic chains on oceanic and continental plates. The mantle wind phenomenon is described, as well as its effect on plume morphology and anisotropy of the ambient mantle. The interaction of plumes and mid-ocean ridges is discussed. The principles and problems of plume activity analysis in subduction- and collision-related foldbelts are considered and illustrated with examples.

The 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.

Crustal-scale forward models of marine satellite potential field data suggest that the Red Sea comprises a single segment of ocean crust, which extends along ~60% of the Red Sea. The segment “tips” are bounded by continental crust, and there is no evidence for transforms at the segment terminations at the south. These observations indicate that ocean crust formation does not necessarily occur in response to wholesale tearing or “unzipping” of continental lithosphere nor is it necessarily controlled by preexisting transform faults. Ocean crust initiation occurs as a series of isolated segments that coalesce as the basin evolves. The recognition of this process in an orthogonal extension setting is comparable to spreading segmentation in modern ocean systems generated at a highly oblique convergent margin, suggesting that oceanic crust segmentation is not controlled by kinematic boundary conditions. The Red Sea may represent a combination of incipient type I and type II passive margins development.

Recent research in Afar (northern Ethiopia) has largely focused on the formation of the present-day ocean-continent transition at active segments (e.g., Manda Hararo). However, the Oligo-Miocene history of extension, from the onset of rifting at ~25 Ma to the eruption of the massive Stratoid flood basalts at ~4 Ma, remains poorly constrained. Here we present new structural data and radiometric dating from Central Afar, obtained along a zone stretching from the undeformed Oligocene Ethiopian plateau to the Manda Hararo and Tat'Ale active volcanic segments. Basaltic and rhyolitic formations were mapped in two key areas corresponding to the proximal and distal parts of a half-rift. We present a balanced composite cross section of Central Afar, reconstructed using our new data and previously published geophysical data on the crustal structure. Our main findings are as follows: (1) Extension during the Mio-Pliocene corresponds to a “wide rift” style of rifting. (2) The lower crust has been underplated/intruded and rethickened during rifting by magmatic injection. (3) Our restoration points to the existence of midcrustal shear zones that have helped to distribute extension in the upper crust and to localize extension at depth in a necking zone. Moreover, we suggest that there is a close relationship between the location of a shear zone and the underplated/intruded material. In magma-rich environments such as Central Afar, breakup should be achieved once the initial continental crust has been completely replaced by the newly, magmatically accreted crust. Consequently, and particularly in Afar, crustal thickness is not necessarily indicative of breakup but instead reflects differences in tectono-magmatic regimes.

The rifting and breakup of continents, and subsequent seafloor spreading, give rise to passive continental margins, many of which are underlain by enormous volumes of igneous rocks and termed volcanic passive margins. The relationships between the igneous rocks, rifting, and mantle plumes remain unresolved despite extensive studies and proposed alternative models. To support such studies, we use published, previously interpreted, seismic reflection and refraction data to compile a dataset of igneous rock volumes and geometries at volcanic passive continental margins. The VOLMIR (VOLcanic passive Margin Igneous Rocks) dataset is based on margin-crossing profiles on which the volumes and geometries of both shallow seaward dipping reflector (SDR) and deeper high velocity lower crustal (HVLC) units can be measured. It includes information about the ages of continental breakup, and profile distances from the associated Euler pole and from hotspots that may have played roles in the breakup process. Despite local variations, the dataset shows general patterns. The average ratio of SDR to HVLC volumes, ~0.3–0.4, is relatively consistent between profiles, implying that formation of these units is related during continental breakup. The volumes of igneous rocks display a moderate positive correlation with distances from the Euler pole, but only a weak negative correlation with distances from the nearest hotspot (at the time of margin formation), suggesting that in continental breakup lithospheric processes (passive rifting) have greater effects than hotspots (active rifting). These results suggest that this dataset will be useful for exploring aspects of the rifting process, relationships between volcanic rifted margins, and potential explanations for their similarities and differences.

In recent years, the Brazilian Equatorial Margin has drawn attention due to its similarity to areas with new hydrocarbon discoveries in the African conjugated margin, and in French Guiana. However, studies on the tectonic regimes associated with transform margins and their evolution, structures, and petroleum potential are still lacking due to the geological complexity of this region. To address this knowledge gap, research has been done to better understand the geological structures, as well as to identify potential hydrocarbon accumulations in the deepwater Ceara Basin. To achieve this, we performed an integrated interpretation of a large 2D seismic data, new exploratory borehole data, as well as older well data with revised biostratigraphy. This data analysis refines the basin architecture and the Cretaceous-Paleocene tectonic evolution, including implications for hydrocarbon prospectivity in the Ceara Basin deepwater. 2D seismic interpretation was performed using modern concepts of continental break-up. To accomplish this, the transition of continental-oceanic crust was taken into account for restoration of the sediments of the rift stage in the basin. The analysis also identifies potential hydrocarbon accumulations in turbiditic reservoirs and presents new insights about the dimensions of the underlying rift features situated in the continental slope. The results reveal a high potential for drift sequences in deepwater where the Late Albian-Early Cenomanian-Turonian sediments reach thicknesses of approximately 3048–4894 m. Moreover, this research shows evidence of Cretaceous to Paleocene magmatism, indicated by the well-imaged volcanoes and associated sills in the seismic data. This analysis indicates that the Mundau sub-basin can be classified as a volcanic passive margin that was developed during the oblique dextral separation between South America and Africa. The variety of stratigraphic and structural features developed through the Cretaceous history of the Mundau sub-basin offers a variety of potential hydrocarbon traps and plays in a number of rift and post-rift sequences.

The More and Voring basins of the mid-Norwegian volcanic passive margin are characterized by thick accumulations of Cretaceous to Paleocene sedimentary strata. They were formed during a series of Late Mesozoic-Early Cenozoic extensional events and represent vast underexplored areas with a limited number of wells. Recently, a new generation of long-offset 2D seismic reflection lines and 3D seismic data, together with new well data, has permitted a significant improvement in the regional understanding of the More and Voring basins. This has enabled much better imaging of the deep Cretaceous subbasins and sub-basalt structures. In light of this significant data improvement, we performed a regional tectonostratigraphic synthesis of the pre-breakup development of the More and Voring basins. We have interpreted eight regional Cretaceous and Paleocene horizons and constructed a series of structural and thickness maps. The new interpretations allow us to examine the sequential evolution of the Cretaceous to Paleocene sedimentary infill and to discuss its relationship to the deep crustal structures and regional tectonic events. We conclude that the long and polyphased development of the More and Voring basins is partly controlled by deep-seated structural highs. We show that active deposition in the Early Cretaceous was mainly focused in the More Basin, while the main Cenomanian and subsequent Late Cretaceous-Paleocene depocentres developed principally in the Voring Basin and migrated sequentially west towards the present continent-ocean boundary. We argue that the outer More and Voring basins are likely underlain by a relatively thick continental crust compared to the inner part of the regional sag basin. In this setting our observations do not support evidence for a large zone of exhumed upper mantle, which has previously been proposed to have formed before magmatism and breakup.

Even though it is well accepted that the Earth's surface topography has been affected by mantle-convection induced dynamic topography, its magnitude and time-dependence remain controversial. The dynamic influence to topographic change along continental margins is particularly difficult to unravel, because their stratigraphic record is dominated by tectonic subsidence caused by rifting. We follow a three-fold approach to estimate dynamic topographic change along passive margins based on a set of seven global mantle convection models. We first demonstrate that a geodynamic forward model that includes adiabatic and viscous heating in addition to internal heating from radiogenic sources, and a mantle viscosity profile with a gradual increase in viscosity below the mantle transition zone, provides a greatly improved match to the spectral range of residual topography end-members as compared with previous models at very long wavelengths (spherical degrees 2–3). We then combine global sea level estimates with predicted surface dynamic topography to evaluate the match between predicted continental flooding patterns and published paleo-coastlines by comparing predicted versus geologically reconstructed land fractions and spatial overlaps of flooded regions for individual continents since 140 Ma. Modelled versus geologically reconstructed land fractions match within 10% for most models, and the spatial overlaps of inundated regions are mostly between 85% and 100% for the Cenozoic, dropping to about 75–100% in the Cretaceous. Regions that have been strongly affected by mantle plumes are generally not captured well in our models, as plumes are suppressed in most of them, and our models with dynamically evolving plumes do not replicate the location and timing of observed plume products. We categorise the evolution of modelled dynamic topography in both continental interiors and along passive margins using cluster analysis to investigate how clusters of similar dynamic topography time series are distributed spatially. A subdivision of four clusters is found to best reveal end-members of dynamic topography evolution along passive margins and their hinterlands, differentiating topographic stability, long-term pronounced subsidence, initial stability over a dynamic high followed by moderate subsidence and regions that are relatively proximal to subduction zones with varied dynamic topography histories. Along passive continental margins the most commonly observed process is a gradual motion from dynamic highs towards lows during the fragmentation of Pangea, reflecting the location of many passive margins now over slabs sinking in the lower mantle. Our best-fit model results in up to 500 (± 150) m of total dynamic subsidence of continental interiors while along passive margins the maximum predicted dynamic topographic change over 140 million years is about 350 (± 150) m of subsidence. Models with plumes exhibit clusters of transient passive margin uplift of about 200 ± 200 m, but are mainly characterised by long-term subsidence of up to 400 m. The good overall match between predicted dynamic topography to geologically mapped paleo-coastlines makes a convincing case that mantle-driven topographic change is a critical component of relative sea level change, and indeed the main driving force for generating the observed geometries and timings of large-scale continental inundation through time. © 2017 International Association for Gondwana Research This work was supported by Australian Research Council grants ARC grants DP130101946, IH130200012 and DE160101020, and by resources from the National Computational Infrastructure (NCI), which is supported by the Australian Government. MG was supported by the National Science Foundation (EAR-1247022, EAR-1358646, and EAR-1645775) and Statoil ASA.

Continental rifting to seafloor spreading is a continuous process, and rifting history influences the following spreading process. However, the complete process is scarcely simulated. Using 3D thermo-mechanical coupled visco-plastic numerical models, we investigate the complete extension process and the inheritance of continental rifting in oceanic spreading. Our modeling results show that the initial continental lithosphere rheological coupling/decoupling at the Moho affects oceanic spreading in two manners: (1) coupled model (a strong lower crust mechanically couples upper crust and upper mantle lithosphere) generates large lithospheric shear zones and fast rifting, which promotes symmetric oceanic accretion (i.e. oceanic crust growth) and leads to a relatively straight oceanic ridge, while (2) decoupled model (a weak ductile lower crust mechanically decouples upper crust and upper mantle lithosphere) generates separate crustal and mantle shear zones and favors asymmetric oceanic accretion involving development of active detachment faults with 3D features. Complex ridge geometries (e.g. overlapping ridge segments and curved ridges) are generated in the decoupled models. Two types of detachment faults termed continental and oceanic detachment faults are established in the coupled and decoupled models, respectively. Continental detachment faults are generated through rotation of high angle normal faults during rifting, and terminated by magmatism during continental breakup. Oceanic detachment faults form in oceanic crust in the late rifting–early spreading stage, and dominates asymmetric oceanic accretion. The life cycle of oceanic detachment faults has been revealed in this study.

In order to capture the general tectono-stratigraphic framework of a magma-poor conjugate rifted margin, two examples have been analysed and compared, the seismically imaged and drilled Iberia–Newfoundland rifted margins ( e.g. seismic sections SCREECH1-ISE1) and the fossil distal Tethys rifted margins exposed in the Alps. The Lower Austroalpine and the Ligurian Brianconnais units exposed in SE Switzerland and NW Italy respectively, are here considered and described for the first time as conjugate margins. They form, like the Iberia–Newfoundland rifted margins, conjugate upper and lower plate margin. Both pairs of margins show a similar first-order morpho-tectonic arrangement. Using the Top of Pre-Rift Succession (TPRS) and the Base of Post-Rift Successions (BPRS) as stratigraphic marker levels and two Syn-Tectonic Sequences (STS 1&2 ) three major rift domains can be identified, which experienced different evolutions and preserved different stratigraphic architectures: i) The proximal margin, soled by normal, 30 ± 5 km thick continental crust. It is characterized by widespread fault-bounded graben/half graben basins filled by the STS 1 showing growth structures followed by a succession (time-equivalent unit to STS 2 ) that does not show any evidence for syn depositional tectonic activity. ii) The upper plate distal margin, floored by thinned to ultra-thinned continental crust. It can be subdivided into a distal “high” and a distal “low” separated by a major fault system. The “high” is characterized by the occurrence of a marked syn-rift unconformity leading to the erosion and/or non deposition of the whole syn-tectonic sequence (STS 1 + STS 2 ); the “low” displays complete stratigraphic successions in which the STS 2 becomes very characteristic and exclusive of this part of the margin. iii) The lower plate distal margin, floored by hyper-extended crust, exhumed crust and subcontinental mantle. The pre-tectonic sequence and the STS 1 are only present inside extensional allochthons that are excised from the upper plate and passively transported on top of major exhumation faults. The STS 2 , deposited during the formation of the most distal domain, is mostly formed by clasts derived from the dismantling of the extensional allochthons and of the surrounding exhumed basement. It is overlain by deep marine turbidites and hemi-pelagic sediments. The nature of basement (serpentinized mantle and hydrated upper or lower continental crust) will selectively condition the composition of the STS 2 in the distal lower plate margin. The recognition of a complex rift stratigraphy in the two conjugate pairs of margins shows that simple stratigraphic models are unable to describe the complete evolution of rifted margins from their initial stage, preserved in the proximal margins to the final evolution leading to upper and lower plate margins in the conjugate distal margins. This paper describes for the first time the stratigraphic relationships at distal conjugate rifted margins, which is a key to understand the rift evolution and architecture of these yet little investigated systems.

Pangea results from the progressive amalgamation of continental blocks achieved at 320 Ma. Assuming that the ancient concept of “active” versus “passive” rifting remains pertinent as end-members of more complex processes, we show that the progressive Pangea breakup occurred through a succession of rifting episodes characterized by different tectonic evolutions. A first episode of passive continental rifting during the Upper Carboniferous and Permian led to the formation of the Neo-Tethys Ocean. Then at the beginning of Triassic times, two short episodes of active rifting associated to the Siberian and Emeishan large igneous provinces (LIPs) failed. The true disintegration of Pangea resulted from (1) a Triassic passive rifting leading to the emplacement of the central Atlantic magmatic province (200 Ma) LIP and the subsequent opening of the central Atlantic Ocean during the lowermost Jurassic and from (2) a Lower Jurassic active rifting triggered by the Karoo-Ferrar LIP (183 Ma), which led to the opening of the West Indian Ocean. The same sequence of passive then active rifting is observed during the Lower Cretaceous with, in between, the Parana-Etendeka LIP at 135 Ma. We show that the relationships between the style of rifts and their breakdown or with the type of resulting margins (as magma poor or magma dominated) are not straightforward. Finally, we discuss the respective role of mantle global warming promoted by continental agglomeration and mantle plumes in the weakening of the continental lithosphere and their roles as rifting triggers.

The processes related to hyperextension, exhumed mantle domains, lithospheric breakup, and formation of first unequivocal oceanic crust at magma-poor rifted margins are yet poorly understood. In this paper, we try to bring new constraints and new ideas about these latest deformation stages by studying the most distal Australian-Antarctic rifted margins. We propose a new interpretation, linking the sedimentary architectures to the nature and type of basement units, including hyperextended crust, exhumed mantle, embryonic, and steady state oceanic crusts. One major implication of our study is that terms like prerift, synrift, and postrift cannot be used in such polyphase settings, which also invalidates the concept of breakup unconformity. Integration and correlation of all available data, particular seismic and potential field data, allows us to propose a new model to explain the evolution of magma-poor distal rifted margins involving multiple and complex detachment systems. We propose that lithospheric breakup occurs after a phase of proto-oceanic crust formation, associated with a substantial magma supply. First steady state oceanic crust may therefore not have been emplaced before ~53.3 Ma corresponding to magnetic anomaly C24. Observations of magma amount and its distribution along the margins highlight a close magma-fault relationship during the development of these margins.

Deep seismic profiles and subsidence history of the Gulf of Lion margin reveal an intense stretching of the distal margin and strong post-rift subsidence, despite weak extension of the onshore and shallow offshore portions of the margin. We revisit this evolution from the geological interpretation of an unpublished multi-channel seismic profile and other published geophysical data. We show that an 80 km-wide domain of thin lower continental crust, the “Gulf of Lion metamorphic core complex”, is present in the ocean-continent transition zone and exhumed mantle makes the transition with oceanic crust. The exhumed lower continental crust is bounded upward and downward by shallow north-dipping detachments. The presence of exhumed lower crust in the deep margin explains the discrepancy between the amount of extension deduced from normal faults in the upper crust and total extension. We discuss the mechanism responsible for exhumation and present two scenarios, a first one involving a simple coupling between mantle extension due to slab retreat and crustal extension, and a second one involving extraction of the lower crust and mantle from below the margin by the south-eastward flow of hot asthenosphere in the back-arc region during slab rollback. In both scenarios, the combination of Eocene crustal thickening related to the Pyrenees, the nearby volcanic arc and a shallow lithosphere-asthenosphere boundary weakened the upper mantle and lower crust enough to make them flow south-eastward. The overall hot geodynamic environment also explains the subaerial conditions during most of the rifting stage and the delayed subsidence after breakup.

Geological and geophysical data on the Central Atlantic are discussed in order to elucidate the tectonic setting of the initial magmatic activity, rifting, and breakup resulting in the origination of Mesozoic ocean. The structural, magmatic, and historical aspects of the problem are considered. It has been established that the initial dispersed rifting and low-capacity magmatism at proximal margins was followed by the migration of the process toward the central part of region with the formation of distal zones and the development of vigorous magmmatism, further breakup of the lithosphere and ocean opening. Magmatism, its sources, and the features of newly formed magmatic crust at both the rifting and breakup stages of margin development are discussed and compared with subsequent spreading magmatism. Sr, Nd, and Pb isotopic compositions show that the magmatic evolution of the Central Atlantic proximal margins bears the features of two enriched components, one of which is related to the EM-1 source, developing only at the North American margin. Another enriched component typical of the province as a whole is related to the EM-2 source. To a lesser extent, this component is expressed in igneous rocks of Guyana, which also bear the signature of the MORB-type depleted source typical of spreading tholeiites in the Atlantic Ocean. Similar conditions are assumed for subsequent magmatism at the distal margins and for the early spreading basalts in the adjacent Atlantic belt, which also contain a small admixture of enriched material. A comparison of the magmatism at the margins of Central and North Atlantic reveals their specificity distinctly expressed in isotopic compositions of igneous rocks. In contrast to the typical region of the North Atlantic, the immediate melting of the enriched lithospheric source without the participation of plume-related melts is reconstructed for the proximal margins of the Central Atlantic. At the same time, decompression and melting in the lithosphere could have been determined by the thermal effect of the Central Atlantic hot plume. This is testified by the intense melting in a short time interval over a vast area; the generation of rather homogeneous melts and creation of the thick magmatic crust; the radial arrangement of the dikes intruded in the course of plume ascent; as well as by the data of seismic tomography indicating the occurrence of the hot upper mantle material, which has kept the “memory” of the action of a shallow-seated Mesozoic plume. The formation of the Central Atlantic plume can be presumably regarded as a result of upwelling remobilisation adove the ancient African superplume and the rearrangement of the deep structure in Triassic and Jurassic times.

International audience; The similarity between the geometry of the West African and South American coastlines is among one of the strongest natural observations supporting the plate tectonic paradigm. However, using classical plate tectonic approaches to model these conjugate transform margins results in a high degree of variability in palaeogeographic reconstructions. Using state-of-the-art 3D coupled thermo-mechanical numerical models, we simulate for the first time, crustal deformation at the onset of oceanisation along large offset oblique margins. Our models show that obliquity causes oceanic rift propagation to stall, resulting in an apparent polyphased tectonic evolution, and in some circumstances leads to the formation of hyper-extended margins. As a result, conjugate margins located at the edge of future fracture zones are highly asymmetric from rifting to spreading, with their lengths differing by a factor of 5 to 10, before the the final phase of break-up occurs. Accounting for this discrepancy should ameliorate future palaeogeographic reconstructions.

International audience; Here, we focus on the yet unexplored eastern Gulf of Aden, on Socotra Island (Yemen), Southeastern Oman and offshore conjugate passive margins between the Socotra-Hadbeen (SHFZ) and the eastern Gulf of Aden fracture zones. Our interpretation leads to onshore-offshore stratigraphic correlation between the passive margins. We present a new map reflecting the boundaries between the crustal domains (proximal, necking, hyper-extended, exhumed mantle, proto-oceanic and oceanic domains) and structures using bathymetry, magnetic surveys and seismic reflection data. The most striking result is that the magma-poor conjugate margins exhibit asymmetrical architecture since the thinning phase (Upper Rupelian-Burdigalian). Their necking domains are sharp ( 40-10 km wide) and their hyper-extended domains are narrow and asymmetric ( 10-40 km wide on the Socotra margin and 50-80 km wide on the Omani margin). We suggest that this asymmetry is related to the migration of the rift center producing significant lower crustal flow and sequential faulting in the hyper-extended domain. Throughout the Oligo-Miocene rifting, far-field forces dominate and the deformation is accommodated along EW to N110°E northward-dipping low angle normal faults. Convection in the mantle near the SHFZ may be responsible of change in fault dip polarity in the Omani hyper-extended domain. We show the existence of a northward-dipping detachment fault formed at the beginning of the exhumation phase (Burdigalien). It separates the northern upper plate (Oman) from southern lower plate (Socotra Island) and may have generated rift-induced decompression melting and volcanism affecting the upper plate. We highlight multiple generations of detachment faults exhuming serpentinized subcontinental mantle in the ocean-continent transition. Associated to significant decompression melting, final detachment fault may have triggered the formation of a proto-oceanic crust at 17.6 Ma and induced late volcanism up to 10 Ma. Finally, the setting up of a steady-state oceanic spreading center occurs at 17 Ma.

Over the past ten years we have numerically modelled the properties of the magmatism generated at four of the key areas where the ‘mantle plume–volcanic margin hypothesis’ is expected to be valid: the North Atlantic, South Atlantic, India– Seychelles and Afar. Our model incorporates many of the original assumptions in the classic White and McKenzie model, with pure shear of the lithospheric mantle, passive upwelling and decompressional melting. Our model is however two- rather than one-dimensional, can capture the rift history (extension rate changes and axis jumps) and tracks mantle depletion during melting. In all four of our study areas we require the sub-lithospheric mantle to be 100 – 200°C hotter than ‘normal’, nonvolcanic margins to explain the characteristics of the magmatism. In the three passive margin cases we find this excess temperature is limited to a 50 – 100 km thick layer. We require this layer temperature to drop along-strike away from the proposed sites of plume impact at the base of the lithosphere. However, we also find that lithospheric thickness and rift history are as important as temperature for controlling the magmatism. Our work therefore lends support to the hypothesis that the excess magmatism at volcanic margins is due to a thermal anomaly in the asthenosphere, albeit with consideration of extra parameters.

Even though many basic properties of non-volcanic rifted margins are predicted by uniform extension of the lithosphere, uniform extension fails to explain other important characteristics. Particularly significant discrepancies are observed at: 1) the Iberia–Newfoundland conjugate margins (Type I), where large tracts of continental mantle lithosphere are exposed at the seafloor, and at 2) ultra-wide central South Atlantic margins (Type II) where continental crust spans wide regions below which it appears that lower crust and mantle lithosphere were removed. Neither corresponds to uniform extension in which crust and mantle thin by the same factor. Instead, either the crust or mantle lithosphere has been preferentially removed during extension. We show that the Type I and II styles are respectively reproduced by dynamical numerical lithospheric stretching models (Models I-A/C and II-A/C) that undergo depth-dependent extension. In this notation A and C imply underplating of the rift zone during rifting by asthenosphere and lower cratonic lithosphere, respectively. We also present results for models with a weak upper crust and strong lower crust, Models III-A/C, to show that lower crust can also be removed from beneath the rift zone by horizontal advection with the mantle lithosphere. From the model results we infer that these Type I, II, and III margin styles are controlled by the strength of the mid/lower crust, which determines the amount of decoupling between upper and lower lithosphere during extension and the excision of crust or mantle. We also predict the styles of sedimentary basins that form on these margins as a test of the concepts presented.

Research into the breakup of continents and formation of ocean basins has advanced in recent years, with deep-marine seismic data providing much of the impetus for new concepts. Attention has been focused on nonvolcanic (magma poor) margins; meanwhile, a satisfactory explanation for volcanic (magmatic) margins with their distinctive seaward-dipping reflectors (SDRs) has been missing. SDRs are composed of subaerial lava flows of tholeiitic basalt erupted during the transition from continental rifting to seafloor spreading, when the crust is both stretched and constructed from asthenosphere-derived magma. Here we present remarkable seismic images down to 40 km offshore East Greenland, showing that a newly described type of extensional fault plays an integral part in the ascent of magma and the building of transitional crust. Two such faults define an axial horst block supported by rising magma where the plates separate. Lava collects in seaward-growing half graben on either side of the horst, while new gabbroic crust is accreted along the flanks of the magma chamber in dilational shear zones beneath where the faults detach. Ultimately, rising asthenosphere reaches the base of the magma chamber, causing the horst to split, thereby initiating seafloor spreading. The model we describe shows how extension and magmatism interact at the boundaries of rifting plates.

Based on present knowledge of mantle peridotites from the Ligurian Tethys ophiolites, this paper presents new ideas and a new model for passive rifting to ocean spreading in the slow– ultraslow rifting Europe–Adria realm. Relevant points include: (i) the positive feedback between deformation and melt percolation during passive magmatic rifting; (ii) the positive feedback between natural evidence and experimental data on the behaviour of the mantle lithosphere during passive rifting; (iii) the significance of hidden magmatism and the associated melt thermal advection; (iv) the role of the wedge-shaped weakened and softened axial zone; and (v) the evidence of a transition from passive to active rifting in the Ligurian Tethys. Passive rifting induced passive asthenospheric upwelling and the onset of partial melting. Fractional melts migrated through the mantle lithosphere and stagnated at shallow levels (the hidden magmatism). Melt thermal advection heated the mantle lithosphere to temperatures (T ) of ≥1200 8C and formed a wedge-shaped axial zone of rheological softened/weakened mantle peridotites that served as the future locus of continental break-up. The hotter/deeper asthenosphere ascended within this axial zone, underwent partial melting and formed aggregated mid-ocean ridge basalts (MORBs) that migrated within dunite channels to form olivine gabbro intrusions and basaltic lava flows. Rifting evolved from passive to active, and the actively upwelling asthenosphere established a ridge-type system and thermal regime. Continents, and their ancient cratonic cores, have undergone episodic rejuvenation and metasomatic refertilization related to mafic melt intrusion throughout their history. Piccardo et al. (2014) have summarized those mantle processes (melt thermal advection, thermal erosion, chemical erosion, asthenospherization, rejuvenation) that are considered to destabilize the subcontinental lithosphere, even that of the cold and thick keels of continental cratons, and which have been described in the international literature over the last 15–20 years. Piccardo et al. (2014, 2–3, and references therein) state, in particular, that Foley (2008), Griffin et al. (2009), Roy et al. (2012) and O’Reilly and Griffin (2012) sustained that the sub-continental lithospheric mantle plays an important role in destabilizing continents and tectonic plates, by thermal and chemical modifications during infiltration of melts into the lithospheric mantle column. According to these authors, the melt-infiltration process represents the primary mechanism for weakening and rejuvenating the continental plate through thermal effects. They recalled that the deep lithosphere is weakened and altered by impregnation of magma, producing local density anomalies which have a destabi

Ophiolite massifs (i.e., Lanzo, Voltri, Ligurides, Corsica) of the Alpine–Apennine system represent lithosphere remnants of the Jurassic Ligurian Tethys oceanic basin, which separated the Europe and Adria continental margins. Ophiolitic mantle peridotites record structural and compositional features induced by tectonic and magmatic processes in the sub-continental lithosphere by passive rifting leading to continental breakup and sea-floor spreading in the Jurassic Ligurian Tethys. Field, structural, petrologic and geochemical studies of the lithospheric peridotites provide important tools to unravel the processes that drove extension and rifting of the continental Europe–Adria lithosphere towards breakup and oceanic spreading. Extension in the pre-Triassic Europe–Adria continental domain was a classic example of passive rifting, driven by far field tectonic forces. The early stage of rifting was a-magmatic (a-magmatic passive rifting). The far-field tectonic forces induced lithosphere stretching and thinning by means of melt-free extensional shear zones, that allowed passive upwelling of the asthenosphere until it reached melting conditions on decompression. Silica-undersaturated, isolated single melt increments, strongly depleted in trace elements, were formed by fractional melting. They infiltrated unmixed through the extending mantle lithosphere under spinel-facies conditions by diffuse and focused porous flow (magmatic passive rifting) and induced significant melt/peridotite interactions during upward percolation (i.e., thermo-chemical and mechanical erosion, asthenospherization and rejuvenation of the mantle lithosphere). The percolating liquids became silica-saturated by melt/peridotite interaction (pyroxene dissolution/olivine precipitation) and migrated to shallow lithospheric levels (i.e., plagioclase-peridotite facies conditions). There, increasing heat loss by conduction induced their stagnation, storage and progressive crystallization, that impregnated and refertilized the host peridotite (the hidden, non-extrusive magmatism). Melt thermal advection through the extending lithosphere, above the melting asthenosphere, strongly modified the compositional and rheological characteristics of the percolated mantle lithosphere. A wedge-shaped, softened and weakened zone was formed along the axial mantle lithosphere of the extensional system, between the future continental margins. This axial wedge represented a preferential zone where the underlying hotter and deeper asthenosphere upwelled and “intruded” the extending colder sub-continental mantle lithosphere. Further extension led to continental break-up and splitting, to formation of the extended Europe and Adria margins and to sea-floor exposure of the sub-continental lithospheric mantle. The hot upwelling asthenosphere column was characterized by higher degrees of partial melting on decompression, complete aggregation of the single fractional melt increments, and deepening of the melting sources (i.e., onset of partial melting under garnet-peridotite facies conditions). This partial melting event formed the aggregated MORB liquids (the oceanic magmatism) which migrated from the asthenosphere within high porosity dunite channels through the melt-reacted sub-continental peridotites, without significant interaction with the host peridotites. These aggregated MORBs formed olivine gabbro intrusions in the shallow mantle lithosphere and MOR pillow basalt flows and edifices, above the tectonically denudated and sea-floor exposed, lithospheric mantle peridotites. In this scenario, the divergent forces induced by the active upwelling asthenosphere may compete with far-field tectonic forces and even drive the system causing a change from passive rifting to active rifting and the installation of a ridge-type system.

The Wilson Cycle theory that oceans close and reopen along the former suture is a fundamental concept in plate tectonics. It was named after J. Tuzo Wilson who recognised that dissimilar marine palaeo-faunas on both sides of the present-day Atlantic Ocean were best explained by an earlier proto-Atlantic ocean. The Wilson Cycle theory implies that collision zones may localise extensional deformation hundreds of millions of years after collision has waned. We review the passive margins of the Atlantic and Indian Oceans with the aim to evaluate the extent in which oceanic openings used former sutures and analyse the potential role of mantle plumes in continental break-up. We summarise the time of collision, onset of rifting, break-up, and main phase of flood basalt emplacement (if applicable) for eighteen conjugate margins. We find that conjugate margins open along former sutures with the exception of the Madagascar–Seychelles–India system. There is no relationship between suture age and break-up age. Continental break-up occurred on relatively young sutures, such as Morocco–Nova Scotia, and on very old sutures, such as the Greenland–Labrador and East Antarctica–Australia systems. We identified two cases where a suture was reactivated as a transform fault: the Charlie Gibbs Fracture Zone follows the Iapetus suture and the Agulhas–Falkland Fracture Zone possibly follows a Late Palaeozoic–Early Mesozoic suture between Patagonia and Western Gondwana. Continental extension and break-up is not always associated with large amounts of volcanism, as illustrated by the magma-poor margins of Iberia–Newfoundland, the Equatorial Atlantic Ocean, and East Antarctica–Australia. But twelve of the conjugate margins in our review are linked to large igneous provinces, such as, the North Atlantic margins (NAIP), Northwest Africa–Florida (CAMP), Arabia–Northeast Africa (Afar), and South Africa–East Antarctica (Karoo). For these margins we find that break-up occurs concurrent with emplacement of the associated large igneous province. In many margins, rifting began before the main phase of volcanism. This suggests that rifting was initiated by tectonic forces and that plume material flowed to the thinned rifted lithosphere to help trigger final continental break-up.

Interpretation of long-offset 2D depth-imaged seismic data suggests that outer continental margins collapse and tilt basinward rapidly as rifting yields to seafloor spreading and thermal subsidence of the margin. This collapse post-dates rifting and stretching of the crust, but occurs roughly ten times faster than thermal subsidence of young oceanic crust, and thus is tectonic and pre-dates the ‘drift stage’. We term this middle stage of margin development ‘outer margin collapse’, and it accords with the exhumation stage of other authors. Outer continental margins, already thinned by rifting processes, become hanging walls of crustal-scale half grabens associated with landward-dipping shear zones and zones of low-shear strength magma at the base of the thinned crust. The footwalls of the shear zones comprise serpentinized sub-continental mantle that commonly becomes exhumed from beneath the embrittled continental margin. At magma-poor margins, outer continental margins collapse and tilt basinward to depths of about 3 km subsea at the continent–ocean transition, often deeper than the adjacent oceanic crust (accreted later between 2 and 3 km). We use the term ‘collapse’because of the apparent rapidity of deepening (

Continental passive margins are characterized by a wide variety of geometries and widths. Whether these variations have an influence on subsequent dynamics of orogenesis is unresolved. To investigate, a series of upper mantle numerical experiments were performed with systematically varied continental margin widths and geometries. Results show that the vertical geometry of subducting continental margin crust controls both crustal and mantle lithosphere deformation. On both scales, end-members can be identified. Namely, break-off versus delamination of continental mantle lithosphere and double vergence versus single vergence of crustal thrust fronts form as a direct result of passive margin geometry. We find that the subduction of upper crust to depths >100 km promotes lithospheric delamination and is facilitated by an extended passive margin. Modeled orogens show decreasing upper plate deformation with increasing margin width. These results suggest that along-strike deformational variation within orogens may develop due to precollision passive margin geometry.

The Pelotas Basin is the classical example of a volcanic passive margin displaying large wedges of seaward-dipping reflectors (SDR). The SDR fill entirely its rifts throughout the basin, characterizing the abundant syn-rift magmatism (133–113 Ma). The Parana–Etendeka Large Igneous Province (LIP), adjacent to west, constituted the pre-rift magmatism (134–132 Ma). The interpretation of ultra-deep seismic lines showed a very different geology from the adjacent Santos, Campos and Espirito Santo Basins, which constitute examples of magma-poor passive margins. Besides displaying rifts totally filled by volcanic rocks, diverse continental crustal domains were defined in the Pelotas Basin, such as an outer domain, probably constituted by highly stretched and permeated continental igneous crust, and a highly reflective lower crust probably reflecting underplating. The analysis of rifting in this portion of the South Atlantic is based on seismic interpretation and on the distribution of regional linear magnetic anomalies. The lateral accretion of SDR to the east towards the future site of the breakup and the temporal relationship between their rift and sag geometries allows the reconstitution of the evolution of rifting in the basin. Breakup propagated from south to north in three stages (130–127.5; 127.5–125; 125–113 Ma) physically separated by oceanic fracture zones (FZ). The width of the stretched, thinned and heavily intruded continental crust also showed a three-stage increase in the same direction and at the same FZ. Consequently, the Continental-Oceanic Boundary (COB) shows three marked shifts, from west to east, from south to north, resulting into rift to margin segmentation. Rifting also propagated from west to east, in the direction of the final breakup, in each of the three segments defined. The importance of the Parana–Etendeka LIP upon the overall history of rupturing and breakup of Western Gondwanaland seems to have been restricted in time and in space only to the Pelotas Basin.

[1] Passive margins often exhibit uplift, exhumation, and tectonic inversion. We speculate that the compression in the lithosphere gradually increased during the Cenozoic, as seen in the number of mountain belts found at active margins during that period. Less clear is how that compression increase affects passive margins. In order to address this issue, we design a 2-D viscous numerical model wherein a lithospheric plate rests above a weaker mantle. It is driven by a mantle conveyor belt, alternatively excited by a lateral downwelling on one side, an upwelling on the other side, or both simultaneously. The lateral edges of the plate are either free or fixed, representing the cases of free convergence, and collision (or slab anchoring), respectively. This distinction changes the upper mechanical boundary condition for mantle circulation and thus, the stress field. Between these two regimes, the flow pattern transiently evolves from a free-slip convection mode toward a no-slip boundary condition above the upper mantle. In the second case, the lithosphere is highly stressed horizontally and deforms. For a constant total driving force, compression increases drastically at passive margins if upwellings are active. Conversely, if downwellings alone are activated, compression occurs at short distances from the trench and extension prevails elsewhere. These results are supported by Earth-like models that reveal the same pattern, where active upwellings are required to excite passive margins compression. Our results substantiate the idea that compression at passive margins is in response to the underlying mantle flow that is increasingly resisted by the Cenozoic collisions.