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Néotectonique Et Cinématique De La Déformation Continentale En Equateur Alexandra Alvarado

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Néotectonique Et Cinématique De La Déformation Continentale En Equateur Alexandra Alvarado Néotectonique et cinématique de la déformation continentale en Equateur Alexandra Alvarado To cite this version: Alexandra Alvarado. Néotectonique et cinématique de la déformation continentale en Equateur. Sci- ences de la Terre. Université de Grenoble, 2012. Français. NNT : 2012GRENU026. tel-00870332 HAL Id: tel-00870332 https://tel.archives-ouvertes.fr/tel-00870332 Submitted on 7 Oct 2013 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. 1 Remerciements Je voudrais remercier tout d’abord Laurence, d’avoir accepté de m’encadrer, d‘être aussi disponible tout le temps pour m’aider à résoudre mes problèmes, pour sa générosité dans le partage de ses connaissances, toujours avec sa bonne humeur, qui a fait que ces trois années furent les plus agréables et réconfortantes de ma vie d’étudiante. Je voudrai aussi remercie a Jean-Mathieu pour m’aide avec la compression de la géodésie. Je remercie de plus le jury d’avoir accepté de lire mon manuscrit malgré la période de vacances. L’IRD pour la bourse qui m’a permis de réaliser ce travail et son précédent soutien financier au cours de mon master. L’ l’Instituto Geofísico de m’avoir laissé poursuivre mes études, à savoir notre ‘Jefe Hugo ‘, mais surtout mes collègues qui ont dû assurer mes responsabilités pendant mon absence; tout particulièrement mon amie Monica pour être disponible, m’aider et partager nos découvertes, à Sandro, Lili, Mario et Meche. De même, le groupe de volcanologues, et particulièrement le Dr. Minard Hall de m’encourager et m’aider toujours grâce à ses conseils sans jamais compter son temps. A Daniel Andrade et Pablo Samaniego pour les discussions scientifiques et leur ouverture au travail en équipe. Au groupe informatique, et à Jorge, pour son inestimable soutien avec les logiciels et les MNT. A Sandra, Grace et Margot pour leur soutien pendant ses trois années et leurs conseils administratifs. ISTerre pour l’accueil dans le laboratoire et à toutes les personnes qui ont rendu mon séjour ici très agréable. Je dois remercier tout spécialement Etienne pour nos très intéressantes discussions sur la géologie de l’Equateur, Peter Van der Beek pour ses conseils sur l’analyse morphologique et Xavier pour son aide à la programmation des codes d’analyses des rivières et pour partager avec moi ses connaissances sur la thermochronologie. Des remerciements spéciaux à Fabrice Cotton et sa famille ainsi qu’à Céline Beauval, pour son amitié et son aide dans les problèmes logistiques. A toutes les personnes ici à Grenoble avec lesquelles on a partagé ces dernières années et de très bons souvenirs. Notre famille en Equateur qui nous aide énormément avec son soutien, en essayant de partager avec nous cette expérience, mes beaux-parents pour leur soutien et confiance, finalement particulièrement à ma maman qui est venue nous aider dans les derniers moments de ce travail. Finalement, je voudrais remercier énormément Pancho pour sa générosité, qui a tout laissé derrière lui pour venir avec moi, pour être un soutien toujours précieux pendant ces années, et aussi à mes deux filles qui rendent ma vie heureuse et qui ont supporté tous ces changements de vie avec courage. 2 Abstract Located in the Northern Andes along the active subduction zone of the Nazca plate beneath the South American continent, Ecuador is highly exposed to seismic risk. Moreover, the upper plate is actively deforming and faults responsible for crustal earthquakes are poorly known, showing the need to take them into account in modern assessments of seismic hazard. Our first objective is then the study of regional active faults and their kinematics, in Ecuador. Systematic analysis of air photos and satellite imagery, as well as geomorphic evidences gathered at different scales along these structures permitted us to obtain a regional tectonic map. Each observation was taken in account if also confirmed on digital terrain models (DTM) and field data. Finally this first step permitted to establish a first up-to-date and homogeneous catalog of major tectonic structures, active at the Quaternary time scale, consistent over the entire territory. We also determined the sense of motion of these active faults in Ecuador. Additionally, we integrate the seismic (instrumental and historical seismicity together with focal mechanisms for the higher magnitude events) and geodetic data available from national, global networks and field work to derive consistent kinematics models. Finally, the kinematics of each segment is compared to the instantaneous and cumulated Quaternary deformation. We are thus able to document a major tectonic system in Ecuador: the Chingual- Cosanga-Pallatanga-Puná fault system, showing a relative velocity of 7-8 mm/yr. with respect to South America. The deformation is characterized mainly by a combination of dextral NE-SW transpressive faults and reverse NS faults. This system accommodates the northward tectonic extrusion of the North Andean Block. Our interpretation implies the existence and definition of a new micro block, isolated on the western flank of the CCPP structure, bounded on the west by the thrust fault system of Quito and Latacunga and to the east by the Chingual-Cosanga faults, probably active over the past 3 million years. In a second step, the analysis of the evolution of the drainage system and its interaction with the active tectonic deformation, together with focused sampling and dating was applied to the study of a particular region of the Andes of Ecuador: the Latacunga and Quito micro-block together with the Quito faults system. The Quito reverse faults system 3 consists of blind thrust faults that outcrop at the surface as a series of en-echelon folds, dipping west and affecting Quaternary volcanic deposits. This kinematics is further confirmed by the analysis of surface and local seismicity and focal mechanisms provided mainly by the national network RENSIG. Available GPS data show a EW convergence at a rate of 4 mm/yr, which is accommodated on the Quito fault system, and particularly active at a regional level. At depth the Quito fault system is probably connected to the west to the old suture, which marks the limit of oceanic terranes accreted to the continent. The analysis of the drainage system has allowed us to highlight the propagation of Quito fault system eastward and northward during the Quaternary. To conclude from the new local and regional models of Quaternary deformation in Ecuador, we have characterized and defined new seismic sources for the continental crust. Each source corresponds to a characteristic magnitude value and deformation model. This contribution integrates active tectonics as should be done in future seismic hazard calculations to help better quantity seismic hazard on Ecuadorian territory. 4 Resumen En el contexto de una zona de subducción rápida, (velocidad de convergencia de 6-8 cm/a.), el Ecuador esta altamente expuesto al riesgo sísmico. La componente de sismicidad atribuida al proceso de subducción es desde algunos años el objetivo de numerosos estudios multidisciplinarios, pero la sismicidad ‘indirecta’ debida al proceso de deformación en la placa superior ha sido poco estudiada. Generalmente, se la ha considerado como mas débil en comparación a los sismos generados en la interface de subducción, por lo tanto el estudio de la sismicidad continental ha sido frecuentemente relegada a un segundo plano. Ha pesar de que ella produce sismos de menor magnitud que los de la subducción, su nivel de peligrosidad es alto en razón a que ocurre a poca profundidad y en zonas densamente pobladas, como es el caso de Ecuador, por lo que la población ha sido directamente afectada por ella. Las fallas que producen estos sismos son aun poco estudiadas y algunas incluso no identificas aun. La reciente experiencia con Haití, nos muestra que estas estructuras deben ser tomadas en cuenta dentro de los modelos de peligro sísmico. Como primer objetivo de esta investigación, esta el estudio de la cinemática regional, dominada por la presencia de un gran sistema de fallas continentales transcurrentes que se extienden desde el margen activo (Golfo de Guayaquil) hasta la frontera con Colombia. Numerosas fallas asociadas con el proceso de expulsión del Bloque Nor Andino, que muestran una traza geomorfológica muy clara pero que sin embargo no han sido cartografiadas, debido a que el poder acceder a ellas es muy difícil, no ha permitido concluir sobre su actividad. El análisis de fotos aéreas, de imágenes satelitales y de modelos digitales de terreno a diferentes escalas nos a permitido establecer primero, un catalogo tectónico de estructuras mayores, homogéneo y sistemático, después determinar la cinemática de la mayoría de estructuras activas en el Ecuador a escala del Cuaternario. Además, hemos integrado la información sismológica (sismicidad y mecanismos focales) y geodésica disponible, a partir de las redes nacionales del Ecuador, las redes globales, así como de campañas de campo, que permitieron obtener modelos cinemáticos consistentes. Finalmente, la cinemática de cada segmento fue comparada con la deformación instantánea y acumulada durante el Cuaternario. El sistema tectónico mayor del Ecuador es el Chingual-Cosanga-Pallatanga-Puná (CCPP), que muestra una velocidad relativa de 7-8 mm/año con respecto a Sud América. Las 5 estructuras que lo definen son principalmente sistemas transpresivos dextrales, con una distribución de la componente transcurrente sobre fallas de dirección NE-SW, et la componente inversa sobre las estructuras de dirección NS. Este sistema tectónico mayor permite la expulsión hacia el norte del Bloque Norandino.
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