Néotectonique et cinématique de la déformation continentale en Equateur

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 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.
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Alexandra Alvarado. Néotectonique et cinématique de la déformation continentale en Equateur. Sciences de la Terre. Université de Grenoble, 2012. Français. ⟨NNT : 2012GRENU026⟩. ⟨tel-00870332⟩



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