Rifting continental : Causes, effets, évolution - Exemple du rift Baïkal

This report for ‘Habilitation à Diriger des Recherches’ presents a summary of my research activity about the Baikal rift which has began ten years ago, when I obtained my present position as Assistant Professor at Villefranche-sur-Mer Observatory (UPMC). Activities concerning the geodynamics of the Ligurian Sea and previous studies in theAndes (Ph. Thesis) are not developped in this report. What I have tried to perform, together with several colleagues, is a multidisciplinary study of the Baikal rift system. This was done in the framework of a collaboration between the Institute of the Earth’s Crust (Irkutsk) and Géosciences Azur (CNRS-UPMC, Nice, France). We attempt to gather several geophysical and geological data (either revised or new) and to interpret them with some caution, in order to determine a realistic picture of the on-going deformation pattern in the Baikal area, and more widely, to better understand the dynamics of the lithospheric deformation in an intracontinental setting. Our main objectives were the following : (1) describe the geometry and rate of active faulting using satellite imagery, field investigations, and seismicity, (2) directly measure crustal deformation and fault motion using GPS geodesy, (3) determine the stress field using earthquake source data, and (4) understand the physical mechanisms at work through numerical modeling. A number of funding agencies made this project possible : INTAS, NATO, the Siberian Branch of the Russian Academy of Science, the Mongolian Academy of Science, the FrenchMinistry for Foreign Affairs, the French National Center for Scientific Research (CNRS), and University Pierre and MarieCurie.Chapter I gives a brief review of the problems that are currently debated about continental rifts and the way I have used and complemented ‘seismo -geology’ for the analysis of causes and effects of continental deformation under extension. Chapter II is a critical review of the deformation pattern in Asia during Tertiary, which aims at situating the Baikal rift in this important tectonic context. Chapter III is an overview of the main results obtained concerning thestructure, evolution and dynamics of the Baikal rift, and puts in perspective the 11 publications reproduced in Annex 2.Finally, Chapter IV briefly opens some perspectives of work.Ce rapport d’Habilitation à Diriger des Recherches présente, outre un compte-rendu détaillé d’activité, un résumé de mon activité de recherche sur le rift Baïkal qui a commencé il y a 10 ans, lors de ma nomination comme Maître de Conférence de l’Université Pierre et Marie Curie à l’Observatoire de Villefranche-sur-Mer. Comme pendant ma thèse, un fil directeur thématique a relié mes différentes activités de recherches : l’extension continentale et ses effets, qu’elle se produise au sein d’une chaîne de montagne liminaire (la Cordillère Blanche au Pérou), dans un paléorift maintenant immergé, le bassin Ligure et ses marges, en contexte d’arrière-arc, ou au cœur d’un continent, le rift Baïkal. Si j’ai choisi de ne présenter que ce dernier, c’est à la fois pour des raisons de temps, et pour éviter une ‘gymnastique’ intellectuelle un peu artificielle : la comparaison entre ces deux derniers objets, par la différence de contexte tectonique et de méthodes d’investigation qu’elle implique, ne me paraît pour l’instant pas assez fructueuse. J’ai voulu plutôt tirer parti du fait qu’un cycle d’étude s’achevait en domaine intracontinental asiatique, avec une confrontation utile entre plusieurs résultats récents (modélisations gravimétriques, mesures GPS et modélisations numériques) et l’analyse plus spécifiquement ‘sismo-géologique’ que j’ai menée. Enfin une autre circonstance m’a incité à agir de la sorte : la soutenance proche de la thèse de doctorat de Nadège Rollet sur le bassin Ligure.Le chapitre I fournit un bref aperçu des problèmes couramment débattus à propos des rifts continentaux actuels, et justifie la démarche générale que j’ai choisie afin de mieux comprendre les causes et les effets de la déformation active en extension en domaine intracontinental : l’utilisation de la « sismo-géologie », au cœur de mon domaine de compétence, dans un cadre élargi par l’apport d’autres approches complémentaires (gravimétrie, géodésie, modélisations numériques). Le Chapitre II est un examen critique du comportement de la lithosphère asiatique au cours du Tertaire, d’après la bibliographie. Cette partie peut surprendre le lecteur dans le cadre de cette Habilitation : elle m’a paru nécessaire pour renforcer la compréhension des processus de déformation observée à l’échelle du rift Baïkal, car le contexte de la collision Inde-Asie et des subductions le long de l’océan Pacifique y joue à l’évidence un rôle primordial. Le Chapitre III donne et discute les principaux résultats obtenus concernant la structure, le comportement thermo-mécanique, l’évolution et la dynamique actuelle du rift Baïkal, en se référant aux 11 publications reproduites en Annexe 2. Finalement, le Chapitre IV ouvre brièvement quelques perspectives d’étude à la suite de ces travaux

Structure and evolution of the Baikal rift: A synthesis

Active continental rifts are spectacular manifestations of the deformation of continents but are not very numerous at the surface of the Earth. Among them, the Baikal rift has been extensively studied during the last decades. Yet no simple scenario explains its origin and development because the style of rifting has changed throughout its ∼30 Myr history. In this paper, we use forward and inverse models of gravity data to map the Moho and lithosphere-asthenosphere boundary in three dimensions. We then integrate these new results with existing geophysical and geological data on the Baikal rift structure and dynamics, and propose a scenario of its evolution. Earthquake depths, mantle xenoliths, heat flow, and seismic and gravity models advocate for a normal to moderately thinned continental lithosphere and crust, except beneath the Siberian craton, which exhibits a >100-km-thick lithosphere. Relatively thin lithosphere (70–80 km) is found east and south of the rift system and is in spatial connection with the Hangai-Hövsgöl region of anomalous mantle in Mongolia. From top to bottom, the rift structure is asymmetric and appears strongly controlled by the geometry of the suture zone bounding the Siberian craton. Moreover, the mode of topography support changes significantly along the length of the rift: mountain ranges south and north of the rift are underlain by negative Bouguer anomalies, suggesting deep crustal roots and/or anomalous mantle; rift shoulders in the center of the rift seem to result from flexural uplift. The commonly assumed “two-stage” rift evolution is not corroborated by all stratigraphic and seismic data; however, it seems clear that during the Oligocene, an “early stage,” which might be dominated by strike-slip tectonics instead of pure extension, created primitive basins much different from the present ones. Most of the “true” rift basins seem to have initiated later, during the Late Miocene or Pliocene. This kinematic change from strike-slip to extensional tectonics in the Baikal rift is part of a more general kinematic reorganization of Asia and can be associated with the rapid growth of the Tibetan plateau and the end of marginal basins opening along the Pacific boundary

A broken plate beneath the North Baikal rift zone revealed by gravity modelling

International audienceWe modelled a 1200 km long gravimetric profile in the North Baikal rift to assess the mechanical behaviour of the lithosphere, using a numerical model that accounts for realistic brittle-elasto-ductile rheology. We use published seismicity and re-fraction data, a new 5'x7.5' free-air/Bouguer gravity and topography data set, and a detailed map of faults obtained from high resolution SPOT imagery. Analysis of the gravity field over the North Baikal rift zone indicates significant asymmetry of the mechanical processes governing the deformation of the diverging sides of the rift. These anomalies cmmot be explained by a conventional continuous plate undergoing extension beneath the rift zone, whereas a strong mechanical discontinuity (wedge shaped detachment zone beneath the rift axis) is able to reproduce observations. Such a discontinuous model provides a good fit to the gravity and crustal thickness data and explains the deep seismicity reported there

Mass-transport deposits on the Algerian margin (Algiers area) : morphology, lithology and sedimentary processes

International audienceOn 21st May 2003, a damaging earthquake of Mw: 6.9 struck the region of Boumerdès 40 km east of Algiers in northern Algeria. The seismic shocks had devastating effects in the offshore area between the cities of Algiers and Dellys, where numerous cable breaks were observed. The submarine area encompassing the 2003 Boumerdès epicenter (central Algerian margin) was recently investigated using geophysical and sampling tools. It has been observed that small-size slides and mass-transport deposits occur across this area. Thus, a detailed analysis was performed on 25 submarine slides identified in water depth from 500 m to 2700 m, by measuring different morphological parameters (i.e. surface, head-scarp height, slope in the source area and in adjacent areas).Various deposits consisting of matrix supported mud-clast, distorted stratified sediments and sandturbidite beds were observed in gravity cores near and within the most significant-size slide. Based on this work and previous studies it can be presumed that the recurrent seismic activity can be considered as the main triggering factor for failure in the central Algerian margin

Structural styles and Neogene petroleum system around the Yusuf-Habibas Ridge (Alboran Basin, Mediterranean Sea)

International audienceThe Algerian offshore is part of the southern margin of the western Mediterranean Sea. The western part of this offshore area represents the transitional margin between the South Algero-Balearic Basin and the Alboran Basin. The study area includes the southern and eastern parts of the Alboran Basin and the northwestern part of the Algerian margin and is in the western part of the plate boundary between Eurasia and Africa (Figure 1). The Yusuf-Habibas Ridge is a major EW-striking structure of this complex plate boundary, separating the eastern and southern parts of the Alboran Basin from the South Algero-Balearic Basin (Martinez-Garcia et al., 2011, and references therein). The ridge played an important role during the Neogene Alboran westward block migration between the Africa and Iberia plates, while the Kabylies blocks migrated southward and accreted to Africa. Furthermore, the ongoing NW-SE convergence between Africa and Iberia has induced a new stress field, since 7 Ma ago, replacing an earlier stress field (Fernandez-Ibañez et al., 2007) and leading to reactivation and polyphased deformation on the main structures in the basin, including the Yusuf-Habibas Ridge

Aftershock sequence of the 1994, Mw 6.8, Liwa earthquake (Indonesia): seismic rupture process in a volcanic arc

International audienceWe present the aftershock activity following the February 15, 1994, Mw 6.8 earthquake which was strongly felt in southern Sumatra, Indonesia, near the Great Sumatran Fault (GSF). At this place, the slip rate is supposed to be low; neverthless, three M>6 events occurred along this segment during this century. No significant instrumental microseismi-city has ever been recorded there. We use data from both the regional Indonesian network and a local seismic array operating 11 days after the mainshock during one month. Aftershocks mostly locate in a broad zone of 55x20 km 2 near two active NW-trending strike-slip segments of the GSF separated by a recent caldera, Suwoh. During the experiment, the NW segment (from Suwoh up to 15 km SE of the Ranau lake caldera, an old right-stepover pull-apart) was very active. As first suggested by the aftershock distribution and the lack of coseismic rupture at the surface, the 20 focal mechanisms determined provide evidence for various post-seismic stress adjustments on secondary faults located in the Ranau-Suwoh paleo-pull-apart graben. Less than 20% of the aftershocks are directly linked to the main rupture, a nearly pure right-lateral strike-slip faulting reaching 25 km depth. A narrow seismic gap underlines the active volcanic area of Suwoh. We conclude that the rupture process along the GSF is controlled both by volcanism and structures, and that the volcanic activity affects the mechanical properties of the crust only in a narrow zone

Deep structure of the Baikal rift zone revealed by joint inversion of gravity and seismology

International audience[1] The question of plate boundary forces and deep versus shallow asthenospheric uplift has long been debated in intracontinental rift areas, particularly in the Baikal rift zone, Asia, which is colder than other continental rifts. As previous gravity and teleseismic studies support the dominance of opposing mechanisms in the Baikal rift, we reconsidered both data sets and jointly inverted them. This more effective approach brings insight into location of the perturbing bodies related to the extension in this region. Our new joint inversion method allows for inverting the velocity-density relationship with independent model parametrization. We obtain velocity and density models that consistently show (1) crustal heterogeneities that coincide with the main tectonic features at the surface, (2) a faster and denser cratonic mantle NW of Lake Baikal that we relate to the thermal contrast between old and depleted Archean (Siberian platform) and Paleozoic orogenic belt (Sayan-Baikal belt), (3) three-dimensional topographic variations of the crust-mantle boundary with well-located upwarpings, and (4) the lithosphere-asthenosphere boundary uplift up to 70 km depth with a NW dip. Our resulting velocity and density models support the idea of a combined influence of lithospheric extension and inherited lithospheric heterogeneities for the origin of the Baikal rift zone. INDEX TERMS: 1234 Geodesy and Gravity: Regional and global gravity anomalies and Earth structure; 7218 Seismology: Lithosphere and upper mantle; 8122 Citation: Tiberi, C., M. Diament, J. Déverchère, C. Petit-Mariani, V. Mikhailov, S. Tikhotsky, and U. Achauer, Deep structure of the Baikal rift zone revealed by joint inversion of gravity and seismology

Comment on “Zemmouri earthquake rupture zone (Mw 6.8, Algeria): Aftershocks sequence relocation and 3D velocity model” by A. Ayadi et al.

International audienceAlthough often difficult to characterize, the relationship between a seismic rupture, its aftershock sequence, and cumulative subsurface or surface faulting or folding is an important challenge to modern seismology and seismotectonics. Among other benefits, it helps document fault length, slip, and magnitude relationships, reconstruct the evolution of the rupture process through historical and prehistorical times and identify the complexity of the deformation in its path toward the surface. This approach is a prerequisite to any seismic hazard assessment but is particularly difficult for faults whose surface trace projects offshore. A specific effort to identify and quantify the source parameters of large earthquakes in coastal areas is therefore needed, not only in subduction zones but also in areas of slow rate and/or diffuse deformation

Preliminary results of a paleoseismological analysis along the Sahel fault (Algeria): New evidence for historical seismic events

International audienceThe ∼60 km-long Sahel ridge west of Algiers (Tell Atlas, north Algeria) is considered as an ENE-WSW fault-propagation fold running along the Mediterranean coast and associated with a north-west dipping thrust. Its proximity with Algiers makes this structure a potential source of destructive earthquakes that could hit the capital city, as occurred in 1365 AD and 1716 AD. The first paleoseismologic investigation on the Sahel ridge was conducted in order to detect paleo-ruptures related to active faulting and to date them. From the first investigations in the area, a first trench was excavated across bending-moment normal faults induced by flexural slip folding in the hanging wall of the Sahel anticline thrust ramp. Paleoseismological analyses recognize eight rupture events affecting colluvial deposits. 14C dating indicates that these events are very young, six of them being younger than 778 AD. The first sedimentary record indicates two ruptures before 1211 AD, i.e. older than the first historical earthquake documented in the region. Three events have age ranges compatible with the 1365, 1673 and 1716 Algiers earthquakes, whereas three other ones depict very recent ages, i.e. younger than 1700 AD. Potential of these secondary extrados faults for determining paleoseismic events and thrust behaviour is discussed

Recent and active deformation pattern off the easternmost Algerian margin, Western Mediterranean Sea: New evidence for contractional tectonic reactivation

International audienceWe describe for the first time a set of large active thrusts and folds near the foot of the easternmost Algerian margin, Western Mediterranean, from swath bathymetry and high-resolution seismic data acquired in 2005 during the Maradja2/Samra cruise. This active system resumes a previous passive margin and creates growth strata deposition on the limbs of large folds, resulting in the development of perched basins at the foot of the margin since less than ~ 1 Ma. They form a set of overlapping fault segments verging toward the Algerian basin, in a way similar to what has been observed off eastern Algiers on the rupture zone of the 2003 Mw 6.8 Boumerdes earthquake. The horizontal shortening rate across large folds is estimated to be of the order of 1 mm/yr. Although no historical earthquakes are reported here, these fault segments could have been responsible for large (M ~ 7.5) events in the past. This young tectonic system further supports the hypothesis of subduction inception of the Neogene oceanic lithosphere in the context of the Africa–Eurasia convergence