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Seismic Imaging of the Structure of the Central Ecuador Convergent Margin : Relationship with the Inter-Seismic Coupling Variations Eddy Sanclemente Ordońez

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Seismic Imaging of the Structure of the Central Ecuador Convergent Margin : Relationship with the Inter-Seismic Coupling Variations Eddy Sanclemente Ordońez Seismic imaging of the structure of the central Ecuador convergent margin : relationship with the inter-seismic coupling variations Eddy Sanclemente Ordońez To cite this version: Eddy Sanclemente Ordońez. Seismic imaging of the structure of the central Ecuador convergent margin : relationship with the inter-seismic coupling variations. Earth Sciences. Université Nice Sophia Antipolis, 2014. English. NNT : 2014NICE4030. tel-01005320 HAL Id: tel-01005320 https://tel.archives-ouvertes.fr/tel-01005320 Submitted on 12 Jun 2014 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. UNIVERSITE DE NICE-SOPHIA ANTIPOLIS - UFR Sciences École Doctorale de Sciences Fondamentales et Appliquées T H E S E pour obtenir le titre de : Docteur en Sciences de l'UNIVERSITÉ de Nice-Sophia Antipolis Discipline : Sciences de la Planète et de l’Univers présentée et soutenue par Eddy SANCLEMENTE IMAGERIE SISMIQUE DE LA STRUCTURE DE LA MARGE CONVERGENTE D’EQUATEUR CENTRAL : RELATIONS AVEC LES VARIATIONS DE COUPLAGE INTERSISMIQUE SEISMIC IMAGING OF THE STRUCTURE OF THE CENTRAL ECUADOR CONVERGENT MARGIN: RELATIONSHIP WITH THE INTER-SEISMIC COUPLING VARIATIONS Thèse dirigée par : Jean-Yves COLLOT et Alessandra RIBODETTI soutenue le 28 Mai 2014 Jury : M. Bertrand Delouis, Professeur, Examinateur M. Jacques Deverchère, Professeur, Rapporteur M. Serge Lallemand, Directeur de Recherche CNRS, Rapporteur M. Valenti Sallares, Chargé de Recherche, Examinateur M. David Graindorge, Maitre de conference, Examinateur Mme. Alessandra Ribodetti, Chargé de Recherche IRD, Co-Directrice de Thèse M. Jean-Yves Collot, Directeur de Recherche émérite IRD, Co-Directeur de Thèse i ii Résumé L’interprétation structurale de sections de Sismique Réflexion Multitrace-2D acquises pendant la campagne SISTEUR sur la marge convergente de l’Equateur Central et migrées en profondeur avant sommation (PSDM) a été combinée avec la bathymétrie multifaisceaux, des modèles tomographiques de sismique grand-angle OBS, un modèle d’inversion GPS, et 13 années de sismicité relocalisée, afin de déchiffrer les causes de la variabilité de la sismicité et du Couplage Inter Sismique (CIS) le long de la subduction. Cette partie de la marge, dont le socle est formé de roches océanique crétacé, est étroite (50-80 km) et érosive. Elle chevauche vers l’Ouest, à 4.7 cm/an, la Ride de Carnégie épaisse de ~15 km avec une surface irrégulière. Ce segment de subduction est découplé, à l’exception d’un patch bloqué de 50 X50 km et centré sur la région de l’Ile La Plata. Notre étude montre que la marge de l’Equateur Central comprend deux segments caractérisés par des propriétés physiques long termes différentes qui peuvent rendre compte de leur CIS et de leur sismicité. Le segment nord « Manta-Ile La Plata » de la marge, qui est bloqué, est caractérisé par une pente lisse entaillée par un réentrant de 50 km de large à morphologie douce. Aucun chenal de subduction n’est identifié le long de ce segment qui révèle la présence d’un important (50 X 40+ km) massif océanique (MO) subduit, haut de ~2.5 km, et dont le flanc arrière irrégulier plonge vers le continent de 2-4°, et coïncide avec la zone de CIS bloquée. Cette concordance suggère que le MO, qui est en contact avec le socle océanique résistant (Vp= 5 km/s) de la marge, concentre suffisamment d’énergie élastique pour créer une barrière sismique, et peut- être rompre comme une aspérité sismique. Le flanc avant du MO déduit de notre étude coïncide avec une zone de CIS partiel et des essaims de séismes chevauchant déformant le socle de la marge. Sous l’Ile de La Plata, la sismicité associé à un séisme lent (SSE) de 2010 se corrèle avec une zone de faible vitesse sismique et des réflecteurs sismique de forte amplitude (DR) à l’intérieur du MO subduit, traduisant un cisaillement intra plaque plongeante sous l’effet de fortes contraintes. A l’inverse, le segment sud « Puerto-Lopez-Salinas » de la marge, qui est découplé, affiche une pente sous-marine très perturbée avec des escarpements abrupts, de profond réentrants et d’importants dépôts en masse. Le contact inter plaque plonge de 6-7° sous le continent et porte des monts sous- marins isolés séparés par un chenal de subduction de ~1km d’épaisseur qui agit comme lubrifiant, facilitant le découplage inter-plaques. Les monts sous-marins sont en contact avec une marge à faible vitesse (Vp = 3.5 km/s) interprétée comme un complexe tectonique écaillé se déformant le long de failles normales listriques et incapable d’accumuler suffisamment d’énergie élastique pour permettre la propagation de la rupture. Un scénario en 3 étapes est proposé pour la subduction d’un MO de forme émoussée sous la marge résistante de l’Ile La Plata. 1) La subduction de OM est partiellement accommodée par fluage contre sa proue, alors que sa poupe reste bloquée. 2) Les contraintes accumulées dans le MO et dans le prisme interne de la marge sont partiellement libérées lors de SSE et d’essaims sismiques (Mw ~6.0), tandis que la déformation élastique s’accumule dans le prisme externe. 3) La zone bloquée se rompt de façon exceptionnelle lors un séisme de Mw 6.9-7.1 provoquant un éventuel tsunami. Un modèle cinématique est proposé pour rendre compte du soulèvement de l’île de La Plata en réponse à la subduction du MO au cours des derniers 1.3- 1.4 Ma. iii Abstract The structural interpretation of 2D-Pre-stack Depth Migrated Multichannel Seismic Reflection sections collected during the SISTEUR cruise across the Central Ecuadorian convergent margin was combined with multibeam bathymetry, OBS wide-angle tomographic models, a GPS inversion model, and 13 years of relocated seismicity to decipher the causes of the along-trench variability of the seismicity and Inter-Seismic Coupling (ISC). The margin submarine part, which basement consists of Cretaceous Oceanic terranes, is narrow (50-80 km) and dominated by subduction erosion. It is underthrust eastward at 4.7 cm/yr by the ~15 km-thick, rugged Carnegie Ridge, and figures a decoupled subduction segment with the notable exception of a 50 X 50 km locked patch centered over La Plata Island region. Our study shows that the Central Ecuador margin divides in two contrasting segments with dissimilar long-lived physical properties that may account for their specific ISC and seismicity patterns. The locked northern “Manta-La Plata Island” segment shows a smooth outer-wedge slope scalloped by a gentle, 50 km-wide morphologic re-entrant. No subduction channel is detected across this segment that reveals a broad 50 X 40+ km, ~2.5-km- high subducted Oceanic Massif (OM), which bumpy trailing flank dips landward 2-4° and coincides with the ISC locked zone. This connection suggests that the OM, which is in contact with the strong (Vp= 5 km/s) oceanic margin basement, concentrates sufficient elastic strain energy to form a barrier to seismic slip propagation, and eventually break as a seismic asperity. The inferred OM leading flank concurs with partial ISC and thrust earthquakes swarms deforming the margin basement. Beneath La Plata Island, the seismicity associated with a 2010 Slow Slip Event correlates with a Low Velocity Zone and high-amplitude reflectors DR identified within the subducted OM, thus reflecting OM internal thrusting under high shear stress. In contrast, the decoupled southern Puerto Lopez-Salinas margin segment shows a highly disrupted outer-wedge seafloor with steep scarps, deep re-entrants and large Mass Transport Deposits. The plate interface dips landward ~6- 7°, and is spotted by isolated seamounts separated by a ~1 km-thick subduction channel that may act as a lubricant favoring inter-plate decoupling. The seamounts impact against a low velocity (Vp=3.5 km/s) margin interpreted as a thrust sheet complex prone to deform along listric normal faults and incapable of accumulating sufficient elastic strain energy to support rupture propagation. A 3-step scenario is put forward for the subduction of a low-drag shaped OM beneath the resistant margin wedge of La Plata Island. 1) The OM subduction is partly accommodated by creeping along its leading flank, whereas its trailing flank remains locked; 2) Stress accumulated within the OM and inner margin wedge is partially released during slow slip events and earthquakes swarms (Mw ~6.0), whereas elastic strain keeps building up within the outer margin wedge; 3) Infrequent Mw 6.9-7.1 earthquakes may break the locked zone resulting in a possible tsunami. Moreover, a kinematic model accounting for the uplift history of La Plata Island is proposed as a result of the OM subduction over the last 1.3-1.4 Myr. iv Remerciements Je tiens tout d’abord consacrer ce travail à ma mère Celeste et à toute ma famillie, ainsi, je la remercie de son soutien, son courage et son éternel appui, grâce aux quels j’ai pu réussir ce qu’un jour on avait rêvé, et qui ont vécu cette longue aventure avec moi. Je remercie et exprime mes sentiments de respect et admiration à mes directeurs de these: Jean-Yves COLLOT et Alessandra RIBODETTI, son appui personnel et professionnel, pour son énorme intérêt a ma formation, pour avoir partagé avec moi ses connaissances et pour les avoir mis dans ce manuscrit, et pour avoir suivi de très près toutes les étapes de ma thèse.
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