The rift to break-up evolution of the Gulf of Aden: Insights from 3D numerical lithospheric-scale modelling

International audienceThe Gulf of Aden provides an ideal setting to study oblique rifting since numerous structural data are available onshore and offshore. Recent surveys showed that the spatio-temporal evolution of the Gulf of Aden rift system is dominated by three fault orientations: displacement-orthogonal (WSW), rift-parallel (WNW) and an intermediate E-W trend. The oldest parts of the rift that are exposed onshore feature displacement-orthogonal and intermediate directions, whereas the subsequently active necking zone involves mainly rift-parallel faults. The final rift phase recorded at the distal margin is characterised by displacement-orthogonal and intermediate fault orientations. We investigate the evolution of the Gulf of Aden from rift initiation to break-up by means of 3D numerical experiments on lithospheric scale. We apply the finite element model SLIM3D which includes realistic, elasto-visco-plastic rheology and a free surface. Despite recent advances, 3D numerical experiments still require relatively coarse resolution so that individual faults are poorly resolved. We address this issue by proposing a simple post-processing method that uses the surface stress-tensor to evaluate stress regime (extensional, strike-slip, compressional) and preferred fault azimuth. The described method is applicable to any geodynamic model and easy to introduce. Our model reproduces the observed fault pattern of the Gulf of Aden and illustrates how multiple fault directions arise from the interaction of local and far-field tectonic stresses in an evolving rift system. The numerical simulations robustly feature intermediate faults during the initial rift phase, followed by rift-parallel normal faulting at the rift flanks and strike-slip faults in the central part of the rift system. Upon break-up, displacement-orthogonal as well as intermediate faults occur. This study corroborates and extends findings from previous analogue experiments of oblique rifting on lithospheric scale and allows new insights in the timing of fault successions of the Gulf of Aden and continental rifts in general

What Can We Learn from Marine Geophysics to Study Rifted Margins?

This chapter describes the main geophysical methods used when studying offshore rifts and rifted margins. Geophysical data are often the only constraints available for interpretation of offshore rifted continental margins. Seismic reflection, refraction and potential fields are all valuable datasets carrying relevant information. In the absence of rock samples, geophysical methods are the only way to study rifted margins. They provide indirect imaging of the crust and sediments, as well as potential field data. There is a large spectrum of interpretations for rifted margins. Sometimes it is linked to the large diversity of rifted margins, sometimes it is linked to different geological scenarios. It is thus important to understand how each geological or geophysical object is recognized and their possible interpretation. In marine geophysics, the free-air gravity anomaly is computed from raw data

Tectonic evolution of the Colorado Basin, offshore Argentina, inferred from seismo-stratigraphy and depositional rates analysis

International audienceBased on a dense 2D seismic reflection dataset and information from 8 exploration wells, we reinterpreted the stratigraphic evolution of the Colorado Basin. The basin is located on the continental shelf and slope within 50 to 2250 m of bathymetry. The total sediment fill can be up to 16,000 m. Seismic-to-well log correlations provide a chrono-stratigraphic framework for the interpreted seismic sequences. We show that the Colorado Basin records the development of a Permian pre-rift period, a Triassic/Jurassic to Early Cretaceous rift phase and a Lower Cretaceous to Tertiary drift phase. This passive margin represents the evolution of lithospheric extension from active rifting to the thermal subsidence/drift stage. Several Cretaceous to Cenozoic slumping episodes were identified and related to progradation of the sequences and sediment build-up in the slope, as well as to the development of seaward dipping extensional faults

Continental break-up history of a deep magma-poor margin based on seismic reflection data (northeastern Gulf of Aden margin, offshore Oman)

International audienceRifting between Arabia and Somalia started around 35 Ma followed by spreading at 17.6 Ma in the eastern part of the Gulf of Aden. The first-order segment between Alula-Fartak and Socotra-Hadbeen fracture zones is divided into three second-order segments with different structure and morphology. Seismic reflection data were collected during the Encens Cruise in 2006 on the northeastern margin. In this study, we present the results of Pre-Stack Depth Migration of the multichannel seismic data from the western segment, which allows us to propose a tectono-stratigraphic model of the evolution of this segment of the margin from rifting to the present day. The chronological interpretation of the sedimentary sequences is mapped out within relation to the onshore observations and existing dating. After a major development of syn-rift grabens and horsts, the deformation localized where the crust is the thinnest. This deformation occurred in the distal margin graben (DIM) at the northern boundary of the ocean-continent transition (OCT) represented by the OCT ridge. At the onset of the OCT formation differential uplift induced a submarine landslide on top of the deepest tilted block and the crustal deformation was restricted to the southern part of the DIM graben, where the continental break-up finally occurred. Initial seafloor spreading was followed by post-rift magmatic events (flows, sills and volcano-sedimentary wedge), whose timing is constrained by the analysis of the sedimentary cover of the OCT ridge, correlated with onshore stratigraphy. The OCT ridge may represent exhumed serpentinized mantle intruded by post-rift magmatic material, which modified the OCT after its emplacement

Colorado Basin 3D structure and evolution, Argentine passive margin

International audienceThis 3D structural model of the Colorado Basin provides new insights into the crustal geometry of the basin and its evolution in relation with the Argentine passive margin. Three NW-SE segments (oblique to the N30°E-trending margin) structure the basin. The oldest infill is generally thought to be coeval with the rifting of the South Atlantic margins in Late Jurassic-Early Cretaceous. This coeval development of the Colorado Basin and of the passive margin is still under debate and gives rise to several hypotheses that we investigate in the light of our observations. We propose that reactivation of inherited structures is predominant in the evolution of the Colorado Basin: (1) the Western segment follows the continental continuation of the Colorado transfer zone; (2) the Central segment consists in the continental continuation of the Tona deformation zone; (3) the Eastern segment is superimposed over the Palaeozoic Claromecó Basin. In addition to the 3 segments, the Central High, separating the Central segment to the Eastern segment, corresponds to the Palaeozoic Sierras Australes Fold Belt. The direction of extension responsible for the South Atlantic opening cannot explain the syn-rift infill and thinning of the basin. The structural analysis shows two phases of syn-rift deformation with different directions. Thus, we suggest that the Colorado Basin and the South Atlantic margin are not coeval but that a first extensional event, probably oblique, predates the extension responsible for the South Atlantic opening. This event is then followed by the formation of the N30°-trending distal margin and the reactivation of Palaeozoic N70°-trending faults occurs under the NW-SE opening of the South Atlantic. This two-phase evolution is consistent with the fault chronology and the two directions of thinned crust observed in the distal margin

Reappraisal of the magma-rich versus magma-poor rifted margin archetypes

Abstract Rifted margins are commonly defined as magma-poor or magma-rich archetypes based on their morphology. We re-examine the prevailing model inferred from this classification that magma-rich margins have excess decompression melting at lithospheric breakup compared with steady-state seafloor spreading, while magma-poor margins have inhibited melting. We investigate the magmatic budget related to lithospheric breakup along two high-resolution long-offset deep reflection seismic profiles across the SE Indian (magma-poor) and Uruguayan (magma-rich) rifted margins. Resolving the magmatic budget is difficult and several interpretations can explain our seismic observations, implying different mechanisms to achieve lithospheric breakup and melt production for each archetype. We show that the Uruguayan and other magma-rich margins may indeed involve excess decompression melting compared with steady-state seafloor spreading but could also be explained by a gradual increase with an early onset relative to crustal breakup. A late onset of decompression melting relative to crustal breakup enables mantle exhumation characteristic of magma-poor margin archetypes (e.g. SE India). Despite different volumes of magmatism, the mechanisms suggested at lithospheric breakup are comparable between both archetypes. Considerations on the timing of decompression melting onset relative to crustal thinning may be more important than the magmatic budget to understand the evolution and variability of rifted margins.</jats:p

Déchirure continentale et segmentation du Golfe d'Aden Oriental en contexte de rifting oblique

The Gulf of Aden stretches between the Somalia and the Arabia plates. It constitutes an interesting area to study the development of the passive margins. Its rifting originated ca. 35 Ma ago and the spreading began at least 17.6 Ma ago in the studied area. Moreover the Gulf opening direction is strongly oblique to its mean trend. This characteristic is studied in this thesis. On the northeastern margin, the Encens cruise (R/V l'Atalante, 2006) undertook the acquisition of new multichannel seismics data (360 traces), especially on the first-order segment between Alula-Fartak and Socotra fracture zones. A second-order segmentation is observed. It shows different structures and morphologies according to the segment, especially concerning the ocean-continent transition (OCT). From the studied segments, it appears that the western Ashawq-Salalah segment is characterized by an important post-rift magmatic event, whereas the eastern Mirbat segment shows a highly tectonised OCT basement high. The tectono-stratigraphic evolution of the Ashawq-Salalah segment was precisely studied (pre-stack depth migration processing and offshore-onshore correlation of the sedimentary processes). First of all, the margin was structured by syn-rift horsts and grabens. Then a strain localisation appeared on the distal margin. At the start of the OCT formation, a local or regional uplift induced a landslide atop of the more distal horst. Crustal deformation was then located in the OCT, where continental breakup finally occurred. OCT could be constituted of serpentinised underlying mantle. It was finally intruded by magmatic material during post-rift time. Besides, the analysis of the sedimentary cover of the OCT basement high shows that it was subjected to a post-rift surrection related with a volcanic activity (volcano, flows and sills). This evolution may be compared to the latest evolution models of passive margins. The conjugated margin (southeastern Gulf of Aden) shows the same segmentation that the northeastern one (d'Acremont et al., 2005). The microstructural analysis of Socotra Island allows the comparison of the margins and constrains the oblique rifting. As on the northern margin, normal faults directions are classified in three faults families: N110°E normal to the extension, N70°E parallel to the gulf direction and the intermediate N90°E. The stress fields deduced from the micro-structural data inversion are in agreement with the direction of extension indicated by the three major fault directions. Chronologies observed in the field show rotations of the direction of extension from N20°E to N160°E, and reciprocally. This suggests that stress fields alternate during the rifting. Analogue modelling of the oblique rifting allowed to better understand the development of the Gulf of Aden. Horsts and grabens organisation follows an " en-échelons " pattern, with sigmoid shapes, between lateral velocity discontinuities of the model. The three fault families linked to obliquity are observed : N110°E, N90°E and N70°E. Distribution of faults directions during extension shows a N90°E and N110°E faults dominance at the beginning of the extension and the later development of N70°E faults (with or without an oblique-to-extension heterogeneity in the model initial setup). Finally N110°E faults are anew created. Thus, the stress fields chronologies observed on Socotra island can be explained. Bellahsen et al. (2006) conceptual model is applicable to the first stages of the evolution : inherited N110°E basin reactivation and new en-echelon faults formation have occurred according to the plates direction of extension (N20-25°E) since 35 Ma. Later, as the lithosphere carried on thinning along the Gulf direction (N70-75°E), local stresses due to lateral thicknesses variations cause the formation of N70°E faults and reactivation of the N110°E faults. N20°E then N160°E extension chronology observed on the margins is thus reproduced. We can propose a third stage: once the rift thinning is sufficient, local stresses only apply on the rift borders or on major horsts. Anywhere else, N110°E faults are created and N70°E faults are obliquely reactivated. The second extension chronology N160°E then N20°E of micro-structural analysis is also reproduced. Horsts may be subjected to important clockwise rotation inducing left-lateral shear zones. Those may initiate the numerous transfer zones leading to the strong segmentation of the Gulf of Aden.Le Golfe d'Aden sépare la plaque Somalie de la plaque Arabie. Il constitue un objet d'étude intéressant pour la compréhension du développement des marges continentales passives. Le rifting débute vers 35 Ma et l'accrétion se développe à partir de 17,6 Ma dans la zone étudiée. De plus, la direction d'ouverture du Golfe d'Aden est fortement oblique par rapport à sa direction. Sur la marge nord-est, la campagne Encens (N/O l'Atalante, 2006) a permis l'acquisition de nouvelles données de sismique réflexion 360 traces, notamment sur le segment de premier ordre entre les zones de fracture d'Alula-Fartak et de Socotra. À la segmentation de premier ordre (zones de fracture) s'ajoute une segmentation de second ordre qui présente des structures et des morphologies différentes selon les segments, notamment au niveau de la transition océan-continent (TOC). Les segments étudiés suggèrent que la partie ouest de la zone d'étude (segment d'Ashawq-Salalah) est caractérisée par un magmatisme post-rift conséquent tandis que la partie orientale de la zone d'étude (segment de Mirbat) possède une morphologie qui semble être fortement tectonisée. L'évolution tectono-stratigraphique du segment d'Ashawq-Salalah a pu être étudiée en détail (migration avant sommation profondeur et corrélation terre-mer des processus sédimentaires). La marge est tout d'abord structurée par des grabens et des horsts syn-rift. Puis on observe une localisation de la déformation sur la marge distale. Au début de la formation de la TOC, un soulèvement local ou régional induit un glissement de terrain au sommet du horst le plus distal. La déformation crustale est alors localisée dans la TOC, où la rupture continentale va finalement se produire. La nature de la TOC pourrait être du manteau serpentinisé, postérieurement intrudé par du matériel magmatique pendant la période post-rift. La couverture sédimentaire à proximité de la TOC montre qu'elle subit une surrection pendant le post-rift en relation avec la mise en place d'un volcan et des coulées et sills associés. Cette évolution peut être comparée aux modèles d'évolution les plus récents des marges passives. La marge conjuguée (au sud-est du Golfe d'Aden) présente la même segmentation que la marge nord-est (d'Acremont et al., 2005). Une analyse microstructurale de l'île de Socotra (marge sud émergée) permet de comparer les marges et de mieux contraindre le rifting oblique. Comme sur la marge nord, les directions des failles normales sont réparties en trois familles : N110°E perpendiculaire à l'extension, N70°E parallèle à la direction du Golfe, N90°E intermédiaire. L'inversion des données microstructurales montre des directions d'extension en accord avec les trois familles de failles. Des chronologies sont observées depuis une direction d'extension N20°E vers N160°E et réciproquement. Les variations de la direction d'extension suggèrent une alternance des champs de contraintes pendant la phase de rifting du Golfe d'Aden. Des modélisations analogiques dans le Golfe d'Aden ont permis de mieux cerner son développement en rifting oblique. Les horsts et les grabens sont disposés en échelons, avec des formes sigmoïdes. Les trois familles de failles liées à l'obliquité sont observées : N110°E, N90°E et N70°E. L'évolution des directions des failles montre une dominance des failles N90°E et N110°E au début de l'extension puis le développement plus tardif de failles N70°E et ce, avec ou sans la présence initiale d'une hétérogénéité oblique à l'extension. Enfin des failles N110°E sont de nouveau formées. Le modèle conceptuel de Bellahsen et al. (2006) serait donc applicable pour les premiers stades d'évolution : la réactivation de bassins N110°E et la formation de nouvelles failles en échelons s'effectuent sous la direction d'extension des plaques (N20°E) depuis 35 Ma. Puis l'amincissement de la lithosphère se poursuivant le long de la direction du Golfe (N70°E), les contraintes locales dues aux variations latérales d'épaisseur provoquent la formation de failles N70°E et la réactivation de failles N110°E. La chronologie d'extension N20°E puis N160°E observée sur les marges est donc expliquée. Nous proposons une troisième étape : une fois l'amincissement du rift suffisamment important, les contraintes locales ne s'exercent que sur les bords du rift ou sur les horsts majeurs. Partout ailleurs des failles N110°E sont formées et les failles N70°E sont réactivées de manière oblique. La seconde chronologie d'extension N160°E puis N20°E des études microstructurales est aussi expliquée. Les horsts peuvent subir des rotations horaires importantes qui induisent des zones de cisaillement senestres. Elles pourraient initier les nombreuses zones de transfert concordant avec la forte segmentation du le Golfe d'Aden

Relief du socle et des failles qui l’affectent dans le Bassin du Colorado, sur la marge Ouest du continent Sud-Américain

Carte réalisée avec le logiciel Petrel représentant le relief du socle et des failles qui l’affectent dans le Bassin du Colorado, sur la marge Ouest du continent Sud-Américain (au large de l’Argentine).Le logiciel Petrel est utilisé pour l’interprétation des profils de sismiques réflexion 2 dimensions et 3 dimensions. Ces images permettent de reconnaître la structure profonde des roches sédimentaires et de la croûte terrestre. La géolocalisation et l’interprétation d’un grand nombre de ces profils permet de reconstituer la structure en 3 dimensions des bassins sédimentaires. Ces études visent à mieux comprendre comment les plaques tectoniques rompent et se séparent, notamment lorsque l’apport en magma est faible

Biases in RCS tree ring chronologies due to sampling heights of trees

The Regional Curve Standardization (RCS) is one of the most employed standardization methods to remove biological signals in long tree ring chronologies. The approach assumes that an overall age-related growth trend typify all tree ring series to be included in a standardized tree ring chronology. Although several potential problems of the method have been examined, the influence of varying the sampling height along tree stems has not been evaluated. Considering that age-related growth trends may vary with stem height, biases may arise when combining samples from unknown or variable sampling heights, a frequent situation with subfossil logs. In this study we perform a detailed stem analysis of 15 lakeshore black spruce (Picea mariana Mill. B.S.P.) trees in the taiga of eastern Canada to describe how the age-related growth trend varies with stem height and evaluate associated biases in RCS chronologies built from living and subfossil trees. Results show that the age-related growth trends vary markedly and systematically along stems, potentially generating large methodological biases in RCS chronologies, especially near the recent chronology end. These biases may lead to erroneous reconstructions of recent climatic trends and cause false divergence between tree ring and climate series. We have developed a correction procedure that appears efficient in removing these biases from chronologies built with the lakeshore trees and associated subfossil logs. -- Keywords : Dendrochronology ; Paleoclimatology ; Tree ring chronology ; Subfossil tree ; Living tree ; Sampling height ; Methodological bias ; Boreal forest ; Picea mariana ; RCS standardization