Seismic volcanostratigraphy of the western Indian rifted margin: The pre-Deccan igneous 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 Runion 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. Copyright 2011 by the American Geophysical Union

Seismic evidence of gas hydrates, multiple BSRs and fluid flow offshore Tumbes Basin, Peru

International audienceIdentification of a previously undocumented hydrate system in the Tumbes Basin, localized off the north Peruvian margin at latitude of 3°20′—4°10′S, allows us to better understand gas hydrates of convergent margins, and complement the 36 hydrate sites already identified around the Pacific Ocean. Using a combined 2D–3D seismic dataset, we present a detailed analysis of seismic amplitude anomalies related to the presence of gas hydrates and/or free gas in sediments. Our observations identify the occurrence of a widespread bottom simulating reflector (BSR), under which we observed, at several sites, the succession of one or two BSR-type reflections of variable amplitude, and vertical acoustic discontinuities associated with fluid flow and gas chimneys. We conclude that the uppermost BSR marks the current base of the hydrate stability field, for a gas composition comprised between 96% methane and 4% of ethane, propane and pure methane. Three hypotheses are developed to explain the nature of the multiple BSRs. They may refer to the base of hydrates of different gas composition, a remnant of an older BSR in the process of dispersion/dissociation or a diagenetically induced permeability barrier formed when the active BSR existed stably at that level for an extended period. The multiple BSRs have been interpreted as three events of steady state in the pressure and temperature conditions. They might be produced by climatic episodes since the last glaciation associated with tectonic activity, essentially tectonic subsidence, one of the main parameters that control the evolution of the Tumbes Basin

Tectonics of the Deccan Large Igneous Province: an introduction

International audienc

New evidence for the origin of the Porcupine Median Volcanic Ridge: Early Cretaceous volcanism in the Porcupine Basin, Atlantic margin of Ireland

International audienceTwo-dimensional pre-stack depth migrated seismic reflection data, gravity and velocity models are used to assess the nature and origin of a prominent, buried ridge, the Porcupine Median Volcanic Ridge (PMVR) within the Porcupine Basin, offshore Ireland. The debate on the origin of the PMVR during the past 30 years has followed the evolution of the concept of continental margin genesis. In this paper, the origin of the ridge is evaluated on the basis of the internal geometry and velocity structure, revealed by the seismic data. Implication of the presence of these type of ridges in hyper-extensional rifted margins is discussed and compared with other margins. The analysis indicates that the ridge is an extrusive volcanic ridge, probably tholeiitic in composition, constructed by stacked hyaloclastite deltas and topped by carbonate platforms. The results invalidate previously proposed models involving highly rotated fault blocks and the serpentinite mud volcanism. The extension magnitude analysis suggests a highly stretched setting where limited mantle serpentinization may have occurred, but the architecture and velocity of the PMVR demonstrates that it is made of lower velocity materials than serpentinite. During the opening of the North Atlantic, the PMVR represents the northern time-equivalent magmatic event expressed along the Newfoundland-Iberia-Galicia, recorded by the J anomaly that originate from Cretaceous volcanic deposits

MID TO LATE MIOCENE UPLIFT AND DOMING OF MADAGASCAR:CONSTRAINTS FROM TOPOGRAPHY, CENOZOIC STRATIGRAPHYAND PALEOGEOGRAPHY

International audienceMadagascar is an Archean to Neoproterozoic continental crust surrounded by transform, oblique anddivergent margins: the oblique Morondava Basin to the west, pounded by the Davie Fracture Zone, and to thenorth, the divergent Mahajanga (Majunga) Basin connected to the Somali Oceanic Basin. This 1600 km longisland is a high axial plateau with elevations from 1200 to 1800 m. The top of the plateau corresponds toweathered planation surfaces (etchplains), bounded by more or less high scarps.We here present geological arguments for the age and the timing of the Madagascar Plateau. Thisanalysis is based on a double, coupled analysis of the onshore geomorphology (stepped planation surfaces)and the offshore margin stratigraphy (seismic stratigraphy, and paleogeography). The geomorphologicalanalysis is based on a characterization, mapping and dating of stepped planation surfaces (mantled to strippedetchplains, pediments and pediplains). The dating is based on their geometrical relationships with datedmagmatic rocks and sediments. The difference of elevation between two planation surfaces (corresponding tolocal base level) provides a proxy of the uplift. The sequence stratigraphic analysis is based on abiostratigraphic reevaluation of 4 industrial wells (foraminifers and nannofossils on cuttings) and theinterpretation of several industrial and academic seimic datasets.Uplift periods are characterized by (1) seaward tilting of the margins overlain by planar, onlappingreflectors, (2) forced regression wedges and (3) stepping planation surfaces.(1) During Paleocene to Early Miocene times (66 to16 Ma), Madagascar is a quite flat low elevationdomain with remnants of an oldest pre-Madagascar Trap (90 Ma) surface. This low relief is highly weatheredwith growth of numerous lateritic profiles and surrounded by large carbonate platforms with very fewsiliciclastic sands influx.(2) Late Miocene is the uplift paroxysm with (i) margin tilting (Morondava), (ii) increase of siliciclasticsand flux since middle Miocene and (iii) major stepping of dated planation surfaces.(3) This result in a convex up shape pattern for the weathered upper Cretaceous surface, giving the islandits present-day dome morphology (with a central plateau).(4) Uplift amplitude can be estimated based on present-day elevation of Paleocene to Eocene marinesediments located 100 km north-east of Toliara and now at an elevation of 900 m. If the absolute sea level wasaround 50-100 m above present-day sea level during Late Eocene times, this means a surface uplift of around800-850 m.(5) Uplift mechanism has to explain a very long wavelength deformation (x1000 km) necessary due tomantle dynamics. Relationships with other East African domes (Ethiopia, East Africa, South Africa) arediscussed.Acknowledgments: This study was founded by TOTAL and IFREMER in the frame of the researchproject PAMELA (Passive Margin Exploration Laboratories)

Surrection Miocene et bombement deMadagascar

National audienceMadagascar est un lambeau de croute continentale archéenneà néoproterozoïque entouré de marges transformantes, obliques etdivergentes : la marge oblique du bassin de Morondava à l’Ouest,bornée par la ride de Davie, et au Nord, la marge divergente dubassin de Majunga connectée au bassin de Somalie.Nous présentons des arguments géologiques pour l’âge de laformation du plateau malgache à travers une double analyse,couplant la géomorphologie à terre (emboitement de surfacesd’aplanissements) et la stratigraphie des marges à mer (stratigraphiesismique et puits).L’analyse géomorphologique est basée sur la caractérisation, lacartographie et la datation de surfaces d’aplanissement emboitées(pédiments ou pédiplaines). La datation s’appuie sur les relationsgéométriques des surfaces entre elles, de l’intersection deces surfaces avec le volcanisme et enfin de leur relation avec lesformations sédimentaires. La différence d’altitude entre deux surfacesemboitées (correspondant chacune à un niveau de base àun moment donné) constitue un bon indicateur de l’ampleur dela surrection. L’analyse de stratigraphie séquentielle est-elle baséesur la réévaluation biostratigraphique de quatre puits industriels(foraminifères et nannofossiles dans les cuttings). Les phases desurrection sont caractérisées par (1) le basculement vers l’Ouest dela marge, scellé par des réflexions non déformées, (2) des prismesde régression forcée et (3) la présence de sédiments anciens dansdes formations plus récentes enregistrée par le remaniement defaune et flore.(1) Durant le Paléocène jusqu’au Miocène inférieur (66 à 20 Ma),Madagascar est une île relativement plate, de faible altitude, avecdes lambeaux d’une surface plus ancienne que le trapp malgache(90Ma). Ce relief est largement altéré avec la croissance de nombreuxprofils latéritiques, entourés de larges plateformes carbonatées.Les influx silico-clastique dans les bassins sont alors relativementfaibles.(2) Le Miocène moyen à supérieur est le paroxysme de la surrectionavec (1), le basculement de la marge ouest (Morondava),(2) une augmentation du flux de sédimentation silicoclastiquedepuis le Miocène moyen et (3) la mise en place d’une surfaced’aplanissement marquée.(3) Le résultat de cette surrection est la morphologie en dômede l’île de Madagascar (avec un plateau central) marquée par laforme concave de la surface crétacé supérieure altérée à l’Eocène.(4) L’amplitude de la surrection peut être estimée grâce à laprésence de carbonates lagunaires, cent kilomètres au Nord-estde Tuléar, à une altitude de 900m environ. Si l’on considère quele niveau marin absolu à l’Eocène était de 50m au-dessus du niveauactuel (Miller et al., 2005), cela implique une surrection d’à peuprès 850m.(5) Le mécanisme de la surrection doit prendre en compte une déformationde très grande longueur d’onde (x1000 km), forcémentliée à la dynamique mantellique. Les relations avec les dômesd’Afrique de l’Est (Ethiopie, Afrique du Sud) sont discutées

Late Miocene uplift and doming of Madagascar: topographic implications

International audienceMadagascar is an Archean to Neoproterozoic continental crust surrounded by transform, oblique and divergentmargins: the oblique Morondava Basin to the west, pounded by the Davie Fracture Zone, and to the north, thedivergent Mahajanga (Majunga) Basin connected to the Somali Oceanic Basin. This 1600 km long island is ahigh axial plateau with elevations from 1200 to 1800m. The top of the plateau corresponds to weathered planationsurfaces (etchplains), bounded by more or less high scarps.We here present geological arguments for the age and the timing of the Madagascar Plateau. This analysis is basedon a double, coupled analysis of the onshore geomorphology (stepped planation surfaces) and the offshore marginstratigraphy (seismic stratigraphy and wells).The geomorphological analysis is based on a characterization, a mapping and dating of stepped planation surfaces(mantled to stripped etchplains, pediments to pediplains). The dating is based on their geometrical relationshipswith dated magmatic rocks. The difference of elevation between two planation surfaces (corresponding to localbase level) provides a proxy of the uplift. The sequence stratigraphic analysis is based on a biostratigraphicreevaluation of 4 industrial wells (foraminifers and nannofossils on cuttings). Uplift periods are characterized by(1) seaward tiltings of the margins overlain by planar reflectors, (2) forced regression wedges and (3) upstreamerosions of older sediments recorded by fauna/flora reworking.(1) During Paleocene to Middle Miocene times (66 to 13 Ma), Madagascar is a quite flat low elevation domainwith remnants of an oldest pre-Madagascar Trap (90 Ma) surface. This low relief is highly weathered with growthof numerous lateritic profiles and surrounded by large carbonate platforms with no siliciclastic sands influx.(2) The Late Miocene is the paroxysm of uplift with (1) a tilting of the margin (Morondova), (2) an increase of thesiliciclastic sand flux since middle Miocene and (3) a major stepping of dated planation surfaces.(3) The end result of this uplift is a convex up shape pattern for the end Cretaceous surface weathered duringEocene times, creating the present-day dome morphology (with a central plateau) of Madagascar.(4) The amplitude of this uplift can be estimated based on the present-day elevation of Late Eocene lagoonalsediments located 100 km north-east of Toliara and now at an elevation of 900m. If the absolute sea level wasaround 50 m (Miller et al., 2005) above present-day sea level during Late Eocene times, this means a surface upliftof around 850 m.(5) The mechanism of this uplift has to explain a very long wavelength deformation (x1000 km) necessary due tomantle dynamics. The relationships with the other East African domes (Ethiopia, East Africa, South Africa) arediscussed.This study was founded by TOTAL and IFREMER in the frame of the research project PAMELA (Passive MarginExploration Laboratorie

OFFSHORE AND ONSHORE EVIDENCE FORA POLYPHASED UPLIFT HISTORY AND UPPER MIOCENE DOMINGOF MADAGASCAR

International audienceMadagascar is an Archean to Neoproterozoic continental crust surrounded by transform,oblique and divergent margins: the oblique Morondava Basin to the west, pounded by theDavie Fracture Zone, and to the north, the divergent Mahajanga (Majunga) Basin connected tothe Somali Oceanic Basin. This 1600 km long island is a high axial plateau with elevations rangingfrom 1200 to 1800m. The top of the plateau corresponds to weathered planation surfaces(etchplains), bounded by more or less high scarps.We here present geological arguments for the age and the timing of the Madagascar Plateau.This analysis is based on a double, coupled analysis of the onshore geomorphology (steppedplanation surfaces) and the offshore margin stratigraphy (seismic stratigraphy and wells).The geomorphological analysis is based on a characterization, a mapping and dating of steppedplanation surfaces (mantled to stripped etchplains, pediments to pediplains). The datingis based on their geometrical relationships with dated magmatic rocks. The difference of elevationbetween two planation surfaces (corresponding to local base level) providing a proxyof the uplift. The sequence stratigraphic and seismic analysis is based on a biostratigraphicreevaluation of 4 industrial wells (foraminifers and nannofossils on cuttings), several hundredsof kilometers of industrial and oceanographic research seismic surveys. Uplift periods are characterizedby (1) seaward tilting of the margins overlain by planar reflectors, (2) forced regressionwedges, (3) upstream erosions of older sediments recorded by fauna/flora reworking and(4) major paleogeographic changes within the Morondava basin.(1) During Middle Cretaceous (99 to 85 Ma), Madagascar undergoes a major volcanic eventwith the onset of the Madagascar Trap, contemporaneous to Madagascar-India separation.The major mid Cretaceous erosion surface is described onshore as the result of a pre tosyn-trap Uplift.(2) During Paleocene to Lower Miocene times (66 to 20 Ma), Madagascar is a quite flat lowelevation domain with remnants of an oldest pre-Madagascar Trap (90 Ma) surface. This low relief is highly weathered with growth of numerous lateritic profiles and surrounded bylarge carbonate platforms with no siliciclastic sand influx.(3) The Late Miocene is the paroxysm of uplift with (1) a tilting of the margin (Morondova), (2)an increase of the siliciclastic sand flux since middle Miocene and (3) a major stepping ofdated planation surfaces.(4) The end result of this uplift is a convex up shape pattern for the end Cretaceous surfaceweathered during Eocene times, creating the present-day dome morphology (with a centralplateau) of Madagascar.(5) The amplitude of this uplift can be estimated based on the present-day elevation of LateEocene lagoonal sediments located 100 km north-east of Toliara and now at an elevation of900m. If the absolute sea level was around 50 m (Miller et al., 2005) above present-day sealevel during Late Eocene times, this means a surface uplift of around 850 m.(6) The mechanism of this uplift has to explain a very long wavelength deformation (x1000 km)necessary due to mantle dynamics. The relationships with the other East African domes(Ethiopia, East Africa, South Africa) are discussed.This study was funded by TOTAL and IFREMER in the frame of the research project PAMELA (PassiveMargin Exploration Laboratories)