Along the Ecuadorian margin, oblique subduction induces deformation of the overriding continental plate. For the last 15 Ma, both exhumation and tectonic history of Ecuador suggest that the northeastward motion of the North Andean Sliver (NAS) was accompanied by an eastward migration of its eastern boundary and successive progressively narrowing restraining bends. Here we present geologic data, earthquake epicenters, focal mechanisms, GPS results, and a revised active fault map consistent with this new kinematic model. All data sets concur to demonstrate that active continental deformation is presently localized along a single major fault system, connecting fault segments from the Gulf of Guayaquil to the eastern Andean Cordillera. Although secondary faults are recognized within the Cordillera, they accommodate a negligible fraction of relative motion compared to the main fault system. The eastern limit is then concentrated rather than distributed as first proposed, marking a sharp boundary between the NAS, the Inca sliver, and the Subandean domain overthrusting the South American craton. The NAS limit follows a northeast striking right-lateral transpressional strike-slip system from the Gulf of Guayaquil (Isla Puna) to the Andean Cordillera and with the north-south striking transpressive faults along the eastern Andes. Eastward migration of the restraining belt since the Pliocene, abandonment of the sutures and reactivation of north-south striking ancient fault zones lead to the final development of a major tectonic boundary south and east of the NAS, favoring its extrusion as a continental sliver, accommodating the oblique convergence of the Nazca oceanic plate toward South America.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2016TC004117

Partitioning of oblique convergence in the Northern Andes subduction zone: Migration history and the present-day boundary of the North Andean Sliver in Ecuador

Paleoseismology and tectonic geomorphology of the Pallatanga fault (Central Ecuador), a major structure of the South-American crust

Situe dans le contexte d’une subduction rapide, (vitesse de convergence de 6-8 cm/an), l’Equateur est particulierement expose au risque sismique. La composante de la sismicite qui est directement attribuee au processus de subduction est depuis quelques annees l’objet de nombreuses etudes, mais la sismicite “indirecte” due aux processus de deformation dans la plaque superieure reste peu etudiee. Pourtant, les populations sont directement exposees a cette seismicite intracontinentale, qui reste dangereuse en raison de sa faible profondeur et distance aux zones densement peuplees.Le premier objectif de cette these est l’etude de la cinematique regionale, dominee par la presence d’un grand systeme de failles continentales decrochantes qui s’etend depuis la marge active (Golfe de Guayaquil) jusqu’a la frontiere Colombienne. De nombreuses failles associees au processus d’extrusion du Bloc Nord Andin montrent une trace geomorphologique tres nette mais ne sont pourtant pas cartees car les acces terrain ne permettent pas de conclure sur leur activite. L’analyse des photos aeriennes, d’images satellite et de MNT a differentes echelles nous a permis d’etablir tout d’abord un catalogue tectonique des structures majeures, homogene et systematique, puis de determiner la cinematique de la plupart des structures actives en Equateur a l’echelle Quaternaire. De plus, nous avons integre l’information sismologique (sismicite et mecanismes au foyer) et geodesique disponible a partir des reseaux nationaux de l’Equateur. Ceci nous a permis d’apporter des contraintes cinematiques sur le fonctionnement de ces structures et comparer les processus de deformation instantanes et cumules a l’echelle du Quaternaire. Le systeme tectonique majeur d’Equateur Chingual-Cosanga-Pallatanga-Puna (CCPP) montre une vitesse moyenne de 7 a 8 mm/an par rapport a Sud Amerique. Les structures qui le definissent sont principalement transpressives dextres, avec une distribution nette de la composante decrochant sur les failles de direction NE-SW, et de la composante inverse sur les structures plutot NS. Ce systeme tectonique majeur permet l’extrusion vers le nord du Bloc Nord-Andin. Notre interpretation implique l’existence et la definition d’un nouveau microbloc, isole sur le flanc Ouest de la structure CCPP, qui est limite a l’ouest par les systemes chevauchants de Quito et Latacunga, et a l’est par les failles du systeme Chingual Cosanga, depuis environ 3 Ma.Dans un deuxieme temps, l’analyse des marqueurs morphologiques de la deformation et de l’evolution des reseaux de drainage associee a un travail de detail sur le terrain et de datations nous ont permis d’etudier cette deuxieme region particuliere des Andes d’Equateur. Le systeme de chevauchements de Quito est forme par des failles inverses aveugles qui produisent en surface une serie de plis en echelon a vergence Ouest dans les depots volcaniques Quaternaire. Cette cinematique est confirmee par l’analyse de la sismicite superficielle et locale, et les mecanismes au foyer issus du reseau national RENSIG. Les donnees GPS montrent un taux de raccourcissement EW de 4 mm/an, accommode sur cette structure compressive particulierement active a l’echelle regionale. En profondeur le systeme de faille de Quito se connecte probablement vers l’ouest a l’ancienne suture, qui marque la limite des terrains oceaniques accretes au continent. L’analyse du reseau de drainage souligne de plus la propagation du systeme de failles de Quito vers le nord et vers l’Est, depuis le Quaternaire recent.Pour finir, a partir de ces nouveaux modeles locaux et regionaux de deformation Quaternaire pour l’Equateur, nous avons caracterise et defini 19 nouvelles sources sismogeniques pour la croute continentale. Cet apport permettra d’integrer la tectonique active dans les futurs calculs d’alea sismique pour aider a la definition du risque sur le territoire Equatorien.

/pdf/neotectonique-et-cinematique-de-la-deformation-continentale-xytt5h4ue4.pdf

Néotectonique et cinématique de la déformation continentale en Equateur

Abstract The NNW-trending Las Peñas Thrust System is one of the key structures along the Andean orogenic front between 32°15′ and 32°40′S in the Southern Precordillera of Argentina. This east-verging structure crops out over a distance of c. 40 km and provides one of the best opportunities for a detailed field survey of Quaternary thrusting in the Andean frontal deformation zone. We present a systematic description of the geometry and geomorphic signatures of the main thrust deformation zone, which emplaces Neogene rocks over Quaternary alluvium, and usually behaves as a blind propagating thrust into the youngest (Late Pleistocene–Holocene) alluvial deposits. The Las Peñas Thrust System is understood to represent the latest stage of the eastward migration of an imbricated fan structure, which has driven the neotectonic uplift of the Las Peñas–Las Higueras range. Excellent outcrops provided by well-incised creek outlets reveal that the thrust system is made up either by a single fault surface or by two or more frontal splays. Several sections along its length can be differentiated on the basis of thrust geometries and/or morphotectonic features. The northern sections are characterized by isolated outcrops of Neogene rocks in the hanging wall, surrounded by alluvial bajadas. Remnants of fold limbs scarps depict the geomorphic signature of the thrust propagation into the Quaternary layers, although the preserved topographic relief always underestimates the cumulated thrust slip during the Quaternary. The southern part of this thrust system is defined by a frontal range, cored by a transposed south-plunging anticline in bedrock. Our observations suggest a dynamic and unsteady interaction between thrust propagation and sedimentation/erosion processes along the thrust trace during deposition of the Quaternary alluvial layers.

Quaternary shortening at the orogenic front of the Central Andes of Argentina: the Las Peñas Thrust System

Rupture area analysis of the Ecuador (Musine) Mw = 7.8 thrust earthquake on April 16, 2016, using GOCE derived gradients

Latin America and the Caribbean region have experienced numerous destructive earthquakes. Earthquakes represent one of the main causes of massive casualties and economic loss due mainly to the proximity of populated areas to plate boundaries and other dynamic parts of the Earth’s crust. Seismic ruptures related to shallow earthquakes ( Earth–surface deformation during an earthquake is concentrated at the epicentral region where the most important damage usually takes place. This coseismic deformation is related to the faults and folds linked at depth with the primary seismic sources. Part of these fault lines lie offshore, related to subduction zones and other major plate interaction features. However, many earthquake-related structures are located in continental areas. The surface deformation related to historic and prehistoric earthquakes allows their recognition and analysis in terms of the significance and characteristics of past earthquakes and the probability for future ones. These faults and folds can be regarded as potential seismogenic sources, highlighted sometimes by active seismicity and/or historic records of destructive earthquakes. However, many potentially active structures show no evidence of historic or instrumental seismicity. For those cases, gathering data with a time frame larger than the one provided by seismic catalogues is crucial for a proper seismic hazard assessment. Therefore terrain analysis and geomorphologic techniques are considered to be a primary tool for providing basic information in the seismic hazard evaluation of a specific structure or region. This chapter examines, through some examples, the contribution of the geomorphologic analysis to the evaluation of geological structures considered to be hazardous seismogenic sources in Latin America and the Caribbean.

Geomorphology as a Tool for Analysis of Seismogenic Sources in Latin America and the Caribbean

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Geomorphology as a Tool for Analysis of Seismogenic Sources in Latin America and the Caribbean