Passive Margins

The passive continental margins of India have evolved as India broke and drifted away from East Antarctica, Madagascar and Seychelles at various geological times. In this study, we have attempted to collate and re-examine gravity and topographic/bathymetry data over India and the adjoining oceans to understand the structure and tectonic evolution of these margins, including processes such as crustal/lithosphere extension, subsidence due to sedimentation, magmatic underplating and so on. The Eastern Continental Margin of India (ECMI) seems to have evolved in a complex rift and shear tectonic settings in its northern and southern segments, respectively, and bears similarities with its conjugate in East Antarctica. Crustal extension rates are uniform along the stretch of the ECMI in spite of the presence or absence of crustal underplated material, variability in lithospheric strength and tectonic style of evolution ranging from rifting to shearing. The Krishna-Godavari basin is underlain by a strong (∼ 30 km) elastic lithosphere, while the Cauvery basin is underlain by a thin elastic lithosphere (∼3 km). The coupling between the ocean and continent lithosphere along the rifted segment of the ECMI is across a stretched continental crust, while it is direct beneath the Cauvery basin. The Western Continental Margin of India (WCMI) seems to have developed in an oblique rift setting with a strike-slip component. Unlike the ECMI, the WCMI is in striking contrast with its conjugate in the eastern margin of Madagascar in respect of sedimentation processes and alignment of magnetic lineations and fracture zones. The break up between eastern India and East Antarctica seems to have been accommodated along a Proterozoic mobile belt, while that between western India and Madagascar is along a combination of both mobile belt and cratonic blocks.

Seismic profiles across the transform continental margin off the Ivory Coast and Ghana (Western Africa) illustrate the structural style resulting from the early Cretaceous phase of shear stress which leads to the final separation between the African and Brazilian continental margins in this area. Most of the characteristic tectonic features observed along this portion of margin (asymmetric grabens on the Ghanean platform, folds of the deep Ivory Coast basin, the Ivory Coast-Ghana marginal ridge) are believed to result from progressive transform contacts between the African and Brazilian continents as their margins were created during early Cretaceous time. A major tectonic unconformity inferred to be of upper Albian-lower Cenomanian age, may be a direct conseqence of the final separation of the continental margins. The later evolution of the transform margin is chiefly explained by thermal subsidence.

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 southern Alps were characterized by strong variations,
both in space and time, of heat flow during Mesozoic rifting.
The regional thermal history was reconstructed using organic
matter (OM) maturity data from outcropping sediments.
One-dimensional (1-D) thermal modeling performed on selected
successions suggests thatOMmaturity wasmainly controlled
by high geothermal gradients (heat flow peaks of 85 to
105 mW/m2 in the Middle Jurassic) and differential burial
during Norian–Early Jurassic extensional phases. The results
of 1-D modeling show an eastward increase of heat flow peak
values.
These results were compared with those obtained with
two-dimensional (2-D) thermokinematic models. Themodels
show a time shift (ca. 10 Ma) in the heat-flow peak (Aalenian-
Bajocian for 2-D and Bajocian for 1-D modeling). However,
the Bajocian age was a priori imposed on 1-Dmodels.Available
geochemical data could be fitted assuming Aalenian-Bajocian
peak ages. Consequently, this misfit is not alarming. The eastward
increase in heat-flow peak values is tentatively explained
with an eastward increase of radiogenic heat production in the
crust instead of with differential stretching.
The comparison of paleothermal data and numericalmodeling
was done to gain knowledge on the potentials and limitations
of numerical modeling in frontier areas. Although some differences do exist in the results of geochemical and thermokinematicmodels,
we can conclude that if a reasonable knowledge
of the thermal parameters of both covers and basement is
available, thermokinematic modeling can provide useful firstorder
estimates in frontier areas of heat flow and temperature
evolution through time.

Twelve combined wide-angle refraction and coincident multi-channel seismic profiles were acquired in the Jequitinhonha-Camamu-Almada, Jacuípe, and Sergipe-Alagoas basins, NE Brazil, during the SALSA experiment in 2014. Profiles SL11 and SL12 image the Jequitinhonha basin, perpendicularly to the coast, with 15 and 11 four-channel ocean-bottom seismometers, respectively. Profile SL10 runs parallel to the coast, crossing profiles SL11 and SL12, imaging the proximal Jequitinhonha and Almada Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. basins with 17 ocean-bottom seismometers. Forward modelling, combined with pre-stack depth migration to increase the horizontal resolution of the velocity models, indicates that sediment thickness varies between 3.3 km and 6.2 km in the distal basin. Crustal thickness at the western edge of the profiles is of around 20 km, with velocity gradients indicating a continental origin. It decreases to less than 5 km in the distal basin, with high seismic velocities and gradients, not compatible with normal oceanic crust nor exhumed upper mantle. Typical oceanic crust is never imaged along these about 200 km-long profiles and we propose that the transitional crust in the Jequitinhonha basin is a made of exhumed lower continental crust. Highlights ► The transitional domain in the Jequitinhonha basin is, at least, 150 km wide. ► The transitional crust is composed of exhumed lower continental crust. ► Necking occurs within less than 100 km. ► An anomalous velocity zone is imaged at the base of the crust.

New seismic re¯ection pro®les of the Italian deep crust project CROP provide new insights on the structure of the Ionian sea. In spite of the Apennines and Hellenides Neogene subduction zones, two conjugate passive continental margins are preserved at the margins of the Ionian sea, along the Malta escarpment to the southwest and the Apulian escarpment to the northeast. The Ionian sea is likely to be a remnant of the Mesozoic Tethys Ocean, con®ned by these two conjugate passive continental margins. The transition from continental to oceanic crust appears sharper to the northeast than to the southwest. The basin between southeast Sicily and southwest Puglia was about 330 km wide and suggests a low spreading rate. The inferred oceanic ridge should have been¯attened by thermal cooling and buried by later sediments.

Formation of new oceans by continental break-up is understood as a continuous evolution from rifting to ocean spreading. The Red Sea is one of few locations on Earth where a new plate boundary presently forms. Its evolution provides key information on how the plate tectonics operates and how the plate boundaries form and evolve in time. While the new plate boundary has already been formed in the southern Red Sea where ocean spreading is active, the north-central segment still experiences continental rifting. The region also has west-east asymmetry: in the north-central Red Sea the rift-related magmatism is not located beneath the rift axis, as conventional models predict, but instead is offset by ca 300 km into Arabia. We propose a new geodynamic model which explains the enigmatic asymmetry of the Red Sea region and is fully consistent with various types of geological and geophysical observations. We demonstrate that the north-central rift is a transient feature that will not develop into coincident ocean spreading. Instead, the new plate boundary forms across Arabia. Our numerical experiments, supported by geological, seismic and gravity observations, predict that in 1-5 Myr the north-central extensional axis will jump ~300 km eastward into Arabia. The Ad Damm strike-slip fault, normal to the central Red Sea rift axis, will evolve into a transform fault between the ongoing ocean spreading in the southern Red Sea and the future spreading in north-central Arabia. We demonstrate that crustal-scale weakness zones control lithosphere extension and lead to long-distance jumps of extensional axes in continental lithosphere not affected by hotspots. Therefore, our model also provides theoretical basis for understanding dynamics and mechanisms of the transition from rifting to continental break-up at passive continental margins not affected by hotspots.

Seismo-stratigraphic analysis of the Abda Basin (West Morocco): a case of reverse structures during the Atlantic rifting. The review of the seismic reflection and well data from the coastal Abda Basin (western Morocco) shows that its Triassic and Jurassic sequences were deposited in a submeridean sag basin, whose eastern margin is characterised by progressive truncations and pinching out of these sequences against a prominent Palaeozoic high. The uplift of this latter is interpreted as a response to an Upper Triassic-Middle Jurassic local compressional event that controlled Triassic-Jurassic sedimentation within the Abda Basin. The present day 'West Meseta Flexure' is a surface expression of this uplift. To cite this article: H. Echarfaoui et al., C. R. Geoscience 334 (2002) 371-377.  2002 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS Morocco / passive margins / Triassic / Jurassic / Atlantic / rifting Résumé -Dans le bassin côtier d'Abda (Maroc atlantique), l'interprétation des données de sismique réflexion et des données de forages montre qu'un soulèvement a affecté le flanc oriental de ce bassin au Trias supérieur-Lias. L'analyse des séquences sismiques met en évidence un épisode compressif local, qui a réactivé d'anciennes failles du socle et déterminé la genèse d'une zone de dépôt en flexion de direction subméridienne, compartimentée au cours du Jurassique. La « flexure de la Meseta » est l'expression actuelle en surface de ce soulèvement syn-rift de la marge atlantique marocaine. Pour citer cet article : H. Echarfaoui et al., C. R. Geoscience 334 (2002) 371-377.  2002 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS Maroc / marges passives / Trias / Jurassique / Atlantique / rifting Abridged version * Correspondance et tirés à part. Adresse

Lithospheric extension can generate passive margins that bound oceans worldwide. Detailed geological and geophysical studies in present and fossil passive margins have highlighted the complexity of their architecture and their multi-stage deformation history. Previous modeling studies have shown the significant impact of coarse mechanical layering of the lithosphere (2 to 4 layer crust and mantle) on passive margin formation. We built upon these studies and design high-resolution (~100–300 m) thermo-mechanical numerical models that incorporate finer mechanical layering (kilometer scale) mimicking tectonically inherited heterogeneities. During lithospheric extension a variety of extensional structures arises naturally due to (1) structural softening caused by necking of mechanically strong layers and (2) the establishment of a network of weak layers across the deforming multi-layered lithosphere. We argue that structural softening in a multi-layered lithosphere is the main cause for the observed multi-stage evolution and architecture of magma-poor passive margins. Passive continental margins are the focus of intense research activity since decades 1–3 , enhanced by the inten-sification of both industrial and academic geophysical surveys associated with ocean drilling programs (ODP). These research programs brought evidence of hyper-extended domains associated with severely thinned crust (< 10 km thick), exhumed subcontinental mantle and crustal allochthons 4. These observations led to new conceptual and mechanical models for the formation of rift basins and magma-poor margins 5,6. Rifting eventually leading to continental break-up develops in lithospheres that exhibit a complex pre-rift evolution characterized especially by orogenic to post-orogenic processes 7. For example, Mesozoic rifting in Europe occurred in a litho-sphere affected by the Variscan orogeny 8 , while Northern Atlantic rifting took place in a lithosphere affected by the Caledonian orogeny 9 and South China Sea rifting succeeded the Yenshanian orogeny 10. During these pre-rift events, interaction of magmatic, metamorphic and tectonic processes generates a heterogeneous continental lith-osphere (Fig. 1a). The mechanical heterogeneity derived from (i) mechanical anisotropies 11,12 , (ii) a layered crust with a variably metamorphosed upper crust, a migmatitic middle crust and a granulitic lower crust 13 sometimes hosting underplated mafic plutons 14,15 occasionally associated with shallower granitic plutons 16 , and (iii) a layered mantle with structural (e.g. shear zones) or compositional remnants (e.g. pyroxenite) from ancient collisional events 17. Such tectonic inheritance results in an overall layered continental lithosphere 18,19 where mechanical heterogeneities range from a few meters to several kilometers in thickness (Fig. 1a). The extension of small-scale power-law viscous or visco-plastic layered materials can result in shear localiza-tion and asymmetric extension 20. This behavior is the result of structural softening due to necking of strong layers and formation of a network of interconnected weak layers. Structural softening is hence caused by internal reorganizations within heterogeneous materials in order to minimize deformational work 21. Structural softening differs from material softening that requires an a priori defined alteration of material properties with increasing strain and which has been implemented in numerical models to explain the structures of rifted margins 22. The role of coarse mechanical layering of the lithosphere, inherited dipping heterogeneities, and the role of bimineralic crust and mantle on rifting processes has been addressed in previous studies 5,23–31. Here we present high-resolution numerical models that incorporate layering at finer scale, which show that structural softening arises during the extension of lithosphere and controls the development of the key structures observed in passive margins.

We image deep crustal structure using marine seismic refraction data recorded by a linear array of ocean-bottom seismometers in the Gulf of Mexico Basin Opening project (GUMBO Line 3) in order to provide new constraints on the nature of continental and oceanic crust in the northeastern Gulf of Mexico. GUMBO Line 3 extends~524 km from the continental shelf offshore Pensacola, Florida, across the De Soto Canyon and into the central Gulf basin. Travel times from long offset, wide angle reflections and refractions resolve compressional seismic velocities and layer boundaries for sediment, crystalline crust, and upper mantle. We compare our results with coincident multichannel seismic reflection data. Our velocity model recovers shallow seismic velocities (~2.0-4.5 km/s) that we interpret as evaporites and clastic sediments. A Cretaceous carbonate platform is interpreted beneath the De Soto Canyon with seismic velocities >5.0 km/s. Crystalline continental crust thins seaward along GUMBO Line 3 from 23-10 km across the De Soto Canyon. High seismic velocity lower crust (>7.2 km/s) is interpreted as extensive syn-rift magmatism and possibly mafic underplating, common features at volcanic rift margins with high mantle potential temperatures. In the central Gulf basin we interpret thick oceanic crust (>8 km) emplaced at a slow full-spreading rate (~24 mm/yr). We suggest a sustained thermal anomaly during slow seafloor-spreading conditions led to voluminous basalt flows from a spreading ridge that overprinted seafloor magnetic anomalies in the northeastern Gulf of Mexico.

The Indian Plate has been the focus of intensive research concerning the flood basalts of the Deccan Traps. Here we document a volcanostratigraphic analysis of the offshore segment of the western Indian volcanic large igneous province, between the shoreline and the first magnetic anomaly (An 28 ∼63 Ma). We have mapped the different crustal domains of the NW Indian Ocean from stretched continental crust through to oceanic crust, using seismic reflection and potential field data. Two volcanic structures, the Somnath Ridge and the Saurashtra High, are identified, extending ∼305 km NE–SW in length and 155 km NW–SE in width. These show the internal structures of buried shield volcanoes and hyaloclastic mounds, surrounded by mass-wasting deposits and volcanic sediments. The structures observed resemble seismic images from the North Atlantic and northwest Australia, as well as volcanic geometries described for Réunion and Hawaii. The geometry and internal seismic facies within the volcanic...

1] We employ a numerical surface processes model to study the controls on postbreakup landscape development and denudational history of the southeast African margin. Apatite fission track data, presented in the companion paper, suggest that the Drakensberg Escarpment formed by rapid postbreakup river incision seaward of a preexisting drainage divide, located close to its present position, and subsequently retreated at rates of only $100 m m.y. À1 . Numerical modeling results support such a scenario and show that the prebreakup topography of the margin has exerted a fundamental control on subsequent margin evolution. The rheology of the lithosphere, lithological variations in the eroding upper crust, and inland base level falls provided secondary controls. A relatively low flexural rigidity of the lithosphere (T e % 10 km) is required to explain the observed pattern of denudation as well as the observed geological structure of the southeast African margin. Lithological variations have contributed to the formation of flat-topped ridges buttressing the main escarpment, as well as major fluvial knickpoints. Both these features have previously been interpreted as supporting significant Cenozoic uplift of the margin. An inland base level fall, possibly related to back-cutting of the Orange River drainage system and occurring 40-50 m.y. after breakup, explains the observed denudation inland of the escarpment as well as the development of inland drainage parallel to the escarpment. Our model results suggest that in contrast to widely accepted inferences from classical geomorphic studies, the southeast African margin has remained tectonically stable since breakup and escarpment retreat has been minimal (<25 km). Modeling postbreakup landscape development and denudational history across the southeast African (Drakensberg Escarpment) margin,

Fission track analysis was applied to the Precambrian suites of Madagascar in order to identify the lower-temperature cooling histories and their relationships to the Phanerozoic events that affected the island. Apatite ages range from 431 to 68 Ma, and zircon ages range from 452 to 238 Ma. Thermochronologically, the island can be divided into a southern, central, and northern region each with a subdivision on an east-west basis. The southern region is sharply separated from the central region by strongly contrasting apparent apatite ages over the northwest-southeast striking Ranotsara Shear Zone (RSZ). The change in apparent ages over the RSZ is indicative of later reactivation along younger brittle faults. The central region has the oldest ages of the island and has a diffuse contact to the third region northward. Along the entire western margin of the Precambrian basement initial Paleozoic exhumation was followed by heating (burial by sediments) during Jurassic and Cretaceous times. A decrease in ages along the eastern margin from 119 to 68 Ma coincides with the predicted positions of the Marion hot spot after effects of erosion are considered. On the other hand, these ages may represent progressive opening of the margin in a southward direction together with associated denudation of the rift shoulder. The eastern part of the central region has remained very stable since at least Devonian times, undergoing only long-term very slow exhumation at rates of 1-5 m/Myr.

The Indian Plate has been the focus of intensive research concerning the flood basalts of the Deccan Traps. Here we document a volcanostratigraphic analysis of the offshore segment of the western Indian volcanic large igneous province, between the shoreline and the first magnetic anomaly (An 28 ∼63 Ma). We have mapped the different crustal domains of the NW Indian Ocean from stretched continental crust through to oceanic crust, using seismic reflection and potential field data. Two volcanic structures, the Somnath Ridge and the Saurashtra High, are identified, extending ∼305 km NE-SW in length and 155 km NW-SE in width. These show the internal structures of buried shield volcanoes and hyaloclastic mounds, surrounded by mass-wasting deposits and volcanic sediments. The structures observed resemble seismic images from the North Atlantic and northwest Australia, as well as volcanic geometries described for Réunion and Hawaii. The geometry and internal seismic facies within the volcanic basement suggest a tholeiitic composition and subaerial to shallow marine emplacement. At the scale of the western Indian Plate, the emplacement of this volcanic platform is constrained by structural lineations associated with rifting. By reviewing the volcanism in the Indian Ocean and plate reconstruction of the area, the timing of the volcanism can be associated with eruption of a pre-Deccan continental flood basalt (∼75-65.5 Ma). The volcanic platform in this study represents an addition of 19-26.5% to the known volume of the West Indian Volcanic Province.

The Indian Plate has been the focus of intensive research concerning the flood basalts of the Deccan Traps. Here we document a volcanostratigraphic analysis of the offshore segment of the western Indian volcanic large igneous province, between the shoreline and the first magnetic anomaly (An 28 ∼63 Ma). We have mapped the different crustal domains of the NW Indian Ocean from stretched continental crust through to oceanic crust, using seismic reflection and potential field data. Two volcanic structures, the Somnath Ridge and the Saurashtra High, are identified, extending ∼305 km NE–SW in length and 155 km NW–SE in width. These show the internal structures of buried shield volcanoes and hyaloclastic mounds, surrounded by mass-wasting deposits and volcanic sediments. The structures observed resemble seismic images from the North Atlantic and northwest Australia, as well as volcanic geometries described for Réunion and Hawaii. The geometry and internal seismic facies within the volcanic...

Large-scale asymmetries in structures, and in the uplift, width and volcanism of pairs of conjugate passive margins have been documented in various parts of the world; these asymmetries reflect the dominant mode of extension preceding the break-up of the continental lithosphere. Of the many parameters involved, the pre-rift crustal structure has been shown to play a fundamental role in controlling the process of continental rifting. In this work, we have used analogue models to show how initial asymmetric Moho configurations are prone to produce asymmetric rifting, with prominent asymmetries in the amount of extension accommodated by boundary faults, in the patterns of lithospheric thinning/asthenospheric upwelling, and in the trajectories of magma migration. After continental break-up, these deformative patterns are expected to lead to the development of a pair of asymmetric passive margins, with one narrow, strongly volcanic margin and a wider, less volcanic (or with areas of concentrated volcanic activity) counterpart.

In this paper, we review ideas on the geomorphological history of the southwest African passive margin, focusing on the central Namib sector and presenting new evidence on the late Neogene landscape evolution of this region. The hyperarid central Namib Desert occupies the 100-150-km-wide pediment at the foot of the Great Escarpment and forms part of the southwest African passive margin, which formed after breakup in the South Atlantic at around 118 Ma. Previous apatite Ž. fission track AFT and cosmogenic isotope studies and numerical models of coupled tectonic-surface processes in the same Ž. area suggest that long-term denudation rates of this passive margin after a period of significant post-rift denudation have been very low, ; 5 mrmy. Aridity of the Namib Desert is generally assumed to have started with the onset of Benguela upwelling in the SE Atlantic at 10-15 Ma and to have prevailed ever since. It has been implied that during this period, the landscape has undergone only marginal change. Here, we present new evidence from in situ cosmogenic 21 Ne dating of pediment surfaces, river terraces and river cut bedrock benches. Correlation of terrace sediment bodies defining ancient river courses is supported by a sediment provenance study using heavy mineral analyses. Our results generally confirm previous work, adding new insights in the late Neogene landscape evolution of the central Ž. Namib. They indicate that denudation rates since at least 5 Ma have been even lower of the order of 0.5-1 mrmy than those calculated by previous studies of inselbergs. In the central Namib, inselbergs appear to be located in or near areas of PliorPleistocene incision and badland erosion. These inselbergs therefore are not the best sites to obtain representative passive margin denudation rates. We obtained a 21 Ne age of 5.18 " 0.18 Ma from a resistant quartz vein projecting above pediment unaffected by recent incision, the oldest exposure age from the Namib so far. The pediment surface stretching from the base of the escarpment to the ocean, on average, probably has undergone less than 10 m of surface lowering since the mid Miocene. Our results also indicate that a late Neogene episode of accelerated denudation in the central Namib Desert and incision of deep canyons started around 2.81 " 0.11 Ma ago. We interpret this as a signal of increasingly wet conditions in this region, in response to PliorPleistocene global cooling. As aggradation of the river systems is followed by degradation, their drainage patterns change from braided to meandering. Evidence of, yet undated, earlier incision episodes suggests that the denudation history of this passive margin is one of slow long-term denudation-as other workers concluded on the basis of AFT and cosmogenic nuclide analyses-interrupted by pulses of rapid denudation.

To address the question of secular changes in the speed of the supercontinent cycle, we use two major databases for the last 2.5 Gyr: the timing and locations of collisional and accretionary orogens, and average plate velocities as deduced from paleomagnetic and paleogeographic data. Peaks in craton collision occur at 1850 and 600 Ma with smaller peaks at 1100 and 350 Ma. Distinct minima occur at 1700-1200, 900-700, and 300-200 Ma. There is no simple relationship in craton collision frequency or average plate velocity between supercontinent assemblies and breakups. Assembly of Nuna at 1700-1500 Ma correlates with very low collision rates, whereas assemblies of Rodinia and Gondwana at 1000-850 and 650-350 Ma, respectively correspond to moderate to high rates. Very low collision rates occur at times of supercontinent breakup at 2200-2100, 1300-1100, 800-650, and 150-0 Ma. A peak in plate velocity at 450-350 Ma correlates with early stages of growth of Pangea and another at 1100 Ma with initial stages of Rodinia assembly following breakup of Nuna. A major drop in craton numbers after 1850 Ma corresponds with the collision and suturing of numerous Archean blocks. Orogens and passive margins show the same two cycles of ocean basin closing: an early cycle from Neoarchean to 1900 Ma and a later cycle, which corresponds to the supercontinent cycle, from 1900 Ma to the present. The cause of these cycles is not understood, but may be related to increasing plate speeds during supercontinent assembly and whether or not long-lived accretionary orogens accompany supercontinent assembly. LIP (large igneous province) age peaks

It is commonly assumed that elevated, passive continental margins have remained uplifted since the time of rifting. In many areas, e.g. Scandinavia, the timing and extent of uplift movements are difficult to determine because the uplifted area consists almost exclusively of ancient metamorphic rocks. However, the preserved Mesozoic-Cenozoic sedimentary and volcanic record of West Greenland makes this a key area for studying the uplift of passive continental margins.

We have reconstructed the kinematic, thermal and rheological evolution of the Triassic-Cretaceous South Alpine passive continental margin (Northern Italy) and estimated lateral strength variations associated with rifting. Rifting initially caused the formation of a roughly symmetrical, ca. 100 km wide graben located 150-200 km away from the future break-up site. 30 Myr after the onset of rifting, the rift axis shifted laterally and eventually led to oceanic crust formation. The thermal evolution of the margin was first controlled by the waning of a thermal anomaly, which was emplaced shortly before rifting and was cooling during the first extensional stages. Numerical modelling of rifting and drifting, constrained by field data, is used to trace thermal and rheological changes affecting the rifted margin. The effects of the thermal anomaly are restricted to the first lo-20 Myr of rifting. Subsequently, isotherms are slightly affected by lithospheric thinning and show very little distortion after break-up. Rheological modelling demonstrates that the South Alpine lithosphere preserved substantial strength even shortly before break-up and became stronger thereafter. The modelling also shows that, at break-up and 100

Palé oformes Palé oalté rites Soulè vement tectonique Taux de dé nudation Morphostratigraphie Marges passives Massif armoricain France A B S T R A C T Like other low-elevation passive margins, the French Atlantic margin is characterized by a gradual topographic transition from the coast to low-altitude interior plains or plateaus. Here we propose a morphostratigraphic analysis to constrain long-term landscape evolution and denudation rates, through the characterization of palaeotopographies and related palaeoweatherings in an area restricted to the southeast Armorican Massif. Two regionalscale palaeosurfaces are recognized: (i) the Infraliassic palaeosurface, the truncated weathering profiles of which are sealed by Liassic marine deposits; (ii) the Eocene palaeosurface, underlain by thick kaolinite-and iron-rich palaeosaprolites and by siliceous duricrusts (silcretes). Quantitative constraints on large-scale tectonic uplift and long-term denudation are obtained from these morphostratigraphic markers. Mean uplift and denudation rates calculated on post-Eocene times range between 0.5 and 2 m.Ma -1 . These low values imply high landscape stability of the inland margin over most of the Cenozoic. ß 2010 Acadé mie des sciences. Published by Elsevier Masson SAS. All rights reserved. R É S U M É Comme d'autres marges passives de faible é nergie de relief, la marge atlantique française est caracté risé e par une transition topographique progressive, passant de la cô te à des plaines et plateaux inté rieurs de basse altitude. Nous proposons ici une analyse morphostratigraphique pour caler l'é volution à long terme et les taux de dé nudation, à travers la caracté risation des palé otopographies et palé oalté rations associé es dans une zone limité e au sud-est du Massif armoricain. Deux palé osurfaces d'extension ré gionale y sont identifié es : (i) la palé osurface infraliasique, dont les profils d'alté ration tronqué s sont scellé s par des dé pô ts marins liasiques ; (ii) la palé osurface é ocè ne, souligné e par d'é paisses palé oalté rations ferrugineuses et des indurations siliceuses (silcrè tes). Des valeurs quantifié es du soulè vement tectonique ré gional et de l'é rosion à long terme sont obtenues à partir de ces surfaces-repè res daté es. Les taux moyens calculé s sur la pé riode post-é ocè ne sont compris entre 0,5 et 2 m.Ma -1 . Ces faibles valeurs impliquent une grande stabilité des paysages de la marge continentale au cours du Cé nozoïque.

The Barents Sea region possesses huge economical potential owing to its resources of oil and gas. At the same time, its geological structure and fundamental processes responsible for sedimentary basins formation remain largely ambiguous because of a lack of reliable data. In particular, the northern continent-ocean margin of the Barents Sea is almost not explored by seismic data due to permanent ice cover.
As a part of PETROBAR research project an effort has been undertaken to reveal deep structure of the crust and upper mantle below the northern Barents Sea from new seismic tomography experiments with ocean bottom seismometers. These data are also used to constrain generally more uncertain modelling of gravity anomalies and numerical simulations of lithospheric deformation.
The obtained results agree with the hypothesis that the Arctic Ocean started its formation through breakup of the lithosphere and seafloor spreading in the Early Cretaceous (about 130 million years ago). This process was accompanied by extensive basaltic volcanism known as the High Arctic Large Igneous Province and manifested in the northern Barents Sea as numerous subhorizontal and vertical sheet-like bodies of igneous rocks radiating from a magma source in the central Arctic Ocean. After a period of tectonic uplift (100-70 million years ago) the northern shelf edge of the Barents Sea chopped off and drifted away by magma-poor breakup and sparsely magmatic seafloor spreading. Thus, during the Cretaceous the style of lithospheric deformation at the northern Barents Sea continent-ocean margin has changed from a magma-controlled to tectonically controlled regime which was followed by development of slow cooling passive continental margin.

Twelve combined wide-angle refraction and coincident multi-channel seismic profiles were acquired in the Jequitinhonha-Camamu-Almada, Jacuípe, and Sergipe-Alagoas basins, NE Brazil, during the SALSA experiment in 2014. Profiles SL11 and SL12 image the Jequitinhonha basin, perpendicularly to the coast, with 15 and 11 four-channel ocean-bottom seismometers, respectively. Profile SL10 runs parallel to the coast, crossing profiles SL11 and SL12, imaging the proximal Jequitinhonha and Almada Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. basins with 17 ocean-bottom seismometers. Forward modelling, combined with pre-stack depth migration to increase the horizontal resolution of the velocity models, indicates that sediment thickness varies between 3.3 km and 6.2 km in the distal basin. Crustal thickness at the western edge of the profiles is of around 20 km, with velocity gradients indicating a continental origin. It decreases to less than 5 km in the distal basin, with high seismic velocities and gradients, not compatible with normal oceanic crust nor exhumed upper mantle. Typical oceanic crust is never imaged along these about 200 km-long profiles and we propose that the transitional crust in the Jequitinhonha basin is a made of exhumed lower continental crust. Highlights ► The transitional domain in the Jequitinhonha basin is, at least, 150 km wide. ► The transitional crust is composed of exhumed lower continental crust. ► Necking occurs within less than 100 km. ► An anomalous velocity zone is imaged at the base of the crust.

Twelve combined wide-angle refraction and coincident multi-channel seismic profiles were acquired in the Jequitinhonha-Camamu-Almada, Jacuípe, and Sergipe-Alagoas basins, NE Brazil, during the SALSA experiment in 2014. Profiles SL11 and SL12 image the Jequitinhonha basin, perpendicularly to the coast, with 15 and 11 four-channel ocean-bottom seismometers, respectively. Profile SL10 runs parallel to the coast, crossing profiles SL11 and SL12, imaging the proximal Jequitinhonha and Almada Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. basins with 17 ocean-bottom seismometers. Forward modelling, combined with pre-stack depth migration to increase the horizontal resolution of the velocity models, indicates that sediment thickness varies between 3.3 km and 6.2 km in the distal basin. Crustal thickness at the western edge of the profiles is of around 20 km, with velocity gradients indicating a continental origin. It decreases to less than 5 km in the distal basin, with high seismic velocities and gradients, not compatible with normal oceanic crust nor exhumed upper mantle. Typical oceanic crust is never imaged along these about 200 km-long profiles and we propose that the transitional crust in the Jequitinhonha basin is a made of exhumed lower continental crust. Highlights ► The transitional domain in the Jequitinhonha basin is, at least, 150 km wide. ► The transitional crust is composed of exhumed lower continental crust. ► Necking occurs within less than 100 km. ► An anomalous velocity zone is imaged at the base of the crust.

Twelve combined wide-angle refraction and coincident multi-channel seismic profiles were acquired in the Jequitinhonha-Camamu-Almada, Jacuípe, and Sergipe-Alagoas basins, NE Brazil, during the SALSA experiment in 2014. Profiles SL11 and SL12 image the Jequitinhonha basin, perpendicularly to the coast, with 15 and 11 four-channel ocean-bottom seismometers, respectively. Profile SL10 runs parallel to the coast, crossing profiles SL11 and SL12, imaging the proximal Jequitinhonha and Almada Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. basins with 17 ocean-bottom seismometers. Forward modelling, combined with pre-stack depth migration to increase the horizontal resolution of the velocity models, indicates that sediment thickness varies between 3.3 km and 6.2 km in the distal basin. Crustal thickness at the western edge of the profiles is of around 20 km, with velocity gradients indicating a continental origin. It decreases to less than 5 km in the distal basin, with high seismic velocities and gradients, not compatible with normal oceanic crust nor exhumed upper mantle. Typical oceanic crust is never imaged along these about 200 km-long profiles and we propose that the transitional crust in the Jequitinhonha basin is a made of exhumed lower continental crust. Highlights ► The transitional domain in the Jequitinhonha basin is, at least, 150 km wide. ► The transitional crust is composed of exhumed lower continental crust. ► Necking occurs within less than 100 km. ► An anomalous velocity zone is imaged at the base of the crust.

In this paper, we review ideas on the geomorphological history of the southwest African passive margin, focusing on the central Namib sector and presenting new evidence on the late Neogene landscape evolution of this region. The hyperarid central Namib Desert occupies the 100–150-km-wide pediment at the foot of the Great Escarpment and forms part of the southwest African passive margin, which formed after breakup in the South Atlantic at around 118 Ma. Previous apatite Ž. fission track AFT and cosmogenic isotope studies and numerical models of coupled tectonic-surface processes in the same Ž. area suggest that long-term denudation rates of this passive margin after a period of significant post-rift denudation have been very low, ; 5 mrmy. Aridity of the Namib Desert is generally assumed to have started with the onset of Benguela upwelling in the SE Atlantic at 10–15 Ma and to have prevailed ever since. It has been implied that during this period, the landscape has undergone only marginal change. Here, we present new evidence from in situ cosmogenic 21 Ne dating of pediment surfaces, river terraces and river cut bedrock benches. Correlation of terrace sediment bodies defining ancient river courses is supported by a sediment provenance study using heavy mineral analyses. Our results generally confirm previous work, adding new insights in the late Neogene landscape evolution of the central Ž. Namib. They indicate that denudation rates since at least 5 Ma have been even lower of the order of 0.5–1 mrmy than those calculated by previous studies of inselbergs. In the central Namib, inselbergs appear to be located in or near areas of PliorPleistocene incision and badland erosion. These inselbergs therefore are not the best sites to obtain representative passive margin denudation rates. We obtained a 21 Ne age of 5.18 " 0.18 Ma from a resistant quartz vein projecting above pediment unaffected by recent incision, the oldest exposure age from the Namib so far. The pediment surface stretching from the base of the escarpment to the ocean, on average, probably has undergone less than 10 m of surface lowering since the mid Miocene. Our results also indicate that a late Neogene episode of accelerated denudation in the central Namib Desert and incision of deep canyons started around 2.81 " 0.11 Ma ago. We interpret this as a signal of increasingly wet conditions in this region, in response to PliorPleistocene global cooling. As aggradation of the river systems is followed by degradation, their drainage patterns change from braided to meandering. Evidence of, yet undated, earlier incision episodes suggests that the denudation history of this passive margin is one of slow long-term denudation—as other workers concluded on the basis of AFT and cosmogenic nuclide analyses—interrupted by pulses of rapid denudation. q

The southern Alps were characterized by strong variations, both in space and time, of heat flow during Mesozoic rifting. The regional thermal history was reconstructed using organic matter (OM) maturity data from outcropping sediments. One-dimensional (1-D) thermal modeling performed on selected successions suggests that OM maturity was mainly controlled by high geothermal gradients (heat flow peaks of 85 to 105 mW/m 2 in the Middle Jurassic) and differential burial during Norian-Early Jurassic extensional phases. The results of 1-D modeling show an eastward increase of heat flow peak values.

We have reconstructed the kinematic, thermal and rheological evolution of the Triassic-Cretaceous South Alpine passive continental margin (Northern Italy) and estimated lateral strength variations associated with rifting. Rifting initially caused the formation of a roughly symmetrical, ca. 100 km wide graben located 150-200 km away from the future break-up site. 30 Myr after the onset of rifting, the rift axis shifted laterally and eventually led to oceanic crust formation. The thermal evolution of the margin was first controlled by the waning of a thermal anomaly, which was emplaced shortly before rifting and was cooling during the first extensional stages. Numerical modelling of rifting and drifting, constrained by field data, is used to trace thermal and rheological changes affecting the rifted margin. The effects of the thermal anomaly are restricted to the first 10-20 Myr of rifting. Subsequently, isotherms are slightly affected by lithospheric thinning and show very little distortion after break-up. Rheological modelling demonstrates that the South Alpine lithosphere preserved substantial strength even shortly before break-up and became stronger thereafter. The modelling also shows that, at break-up and 100 Myr later, thinned domains of the margin are stronger than the less thinned ones.