Tectonic deformation et thermal structure of the North Ecuador - South Colombian Margin (0°-3.5°N) - implication for the seismogenesis

aaL’ensemble des forces impliquées dans la subduction contrôle le transfert de mouvement du panneau plongeant vers la plaque chevauchante et par voie de conséquence le régime tectonique de la marge convergente. Ce régime peut être caractérisé& par une accrétion frontale ou du sous-placage d’une part et une érosion frontale ou basale, d’autre part. Les processus de l’érosion de la base de la plaque chevauchante sont encore mal connus, et pourraient résulter soit d’une abrasion mécanique par les rugosités kilométriques à centimétriques de la plaque plongeante en régime de fort couplage mécanique soit d’une hydrofracturation basale résultant des surpressions de fluides expulsés des sédiments et de la croûte subduits [von Huene and Culotta, 1989]. Cette dernière hypothèse est envisageable en régime de faible couplage mécanique.Á l’échelle de la lithosphère, la nucléation des grands séismes de subduction semble également dépendre des forces impliquées dans les zones de subduction, puisque près de 90% de l’énergie sismique accumulée sur le globe est libérée dans ces zones de subduction. Ces grands séismes se produisent par rupture d’une portion, au comportement fragile, du contact interplaque, appelée zone sismogène, dont les limites semblent être principalement contrôlées thermiquement. Ils peuvent se produire par rupture d’une zone fortement couplée et propagation de la rupture dans une zone de couplage moindre (modèle des aspérités) [Kanamori, 1986; Lay and Kanamori, 1981; Lay et al., 1982] ou par la rupture d’une zone faiblement couplée et la propagation jusqu’à une zone de fort couplage (modèle des barrières) [Aki, 1979; Das and Aki, 1977].L’étude de la marge Nord Équateur – Sud Colombie (0° - 3,5°N) a été abordée selon trois axes. (1) Les caractéristiques morpho-structurales du front de déformation, du bassin avant-arc et de son substratum nous renseignent sur le régime tectonique dominant de la marge et ses variations spatio-temporelles. (2) Les caractéristiques des Bottom Simulating Reflectors permettent de calculer des valeurs du flux de chaleur, points de départ d’une modélisation thermique de la marge qui fournit une estimation des dimensions et de la localisation de la zone sismogène et permet de discuter des variations longitudinales du régime thermique de la marge. (3) Enfin, nous avons abordé le rôle des failles crustales héritées de la marge, sur le contrôle de la propagation de la rupture des grands séismes de subduction du XX° siècle.Cette marge apparaît segmentée par des failles crustales transverses : les failles d’Esmeraldas et de Manglares, ainsi que par le promontoire d’Esmeraldas au Sud et le promontoire de Patia au Nord. Elle a globalement subi une déformation compressive, depuis le saut de subduction, à l’Éocène moyen – supérieur, notamment lors de l’entrée en subduction de la jeune plaque Nazca, ce qui se traduit par une augmentation de la déformation. Cette déformation reste active au nord de la faille de Manglares alors qu’elle est scellée au sud. Le substratum et le front de la marge, d’Esmeraldas à Buenaventura, semblent avoir subit une érosion tectonique apparemment initiée au Miocène moyen – supérieur. Depuis environ 1 Ma, l’accrétion semble se propager du nord vers le sud.La zone sismogène mesurerait 110 à 160 km de large et sa limite supérieure, serait située à ~11 km de profondeur et ~42 km de distance du front de déformation dans le nord de la zone d’étude. Le flux de chaleur, au front de la marge, varie du simple au double mettant en évidence une segmentation thermique de la marge.Enfin la localisation et l’extension des zones de rupture des séismes montrent que les failles transverses héritées d’Esmeraldas et de Manglares limitent la propagation de la rupture sismique et segmentent sismologiquement la marge

Régimes tectoniques et thermiques de la marge Nord Equateur-Sud Colombie (0-3,5N) - (implications sur la sismogenèse)

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Structure of the Malpelo Ridge (Colombia) from seismic and gravity modelling

Wide-angle and multichannel seismic data collected on the Malpelo Ridge provide an image of the deep structure of the ridge and new insights on its emplacement and tectonic history. The crustal structure of the Malpelo Ridge shows a 14 km thick asymmetric crustal root with a smooth transition to the oceanic basin southeastward, whereas the transition is abrupt beneath its northwestern flank. Crustal thickening is mainly related to the thickening of the lower crust, which exhibits velocities from 6.5 to 7.4 km/s. The deep structure is consistent with emplacement at an active spreading axis under a hotspot like the present-day Galapagos Hotspot on the Cocos-Nazca Spreading Centre. Our results favour the hypothesis that the Malpelo Ridge was formerly a continuation of the Cocos Ridge, emplaced simultaneously with the Carnegie Ridge at the Cocos-Nazca Spreading Centre, from which it was separated and subsequently drifted southward relative to the Cocos Ridge due to differential motion along the dextral strike-slip Panama Fracture Zone. The steep faulted northern flank of the Malpelo Ridge and the counterpart steep and faulted southern flank of Regina Ridge are possibly related to a rifting phase that resulted in the Coiba Microplate's separation from the Nazca Plate along the Sandra Rift

By Land or Sea: How Did Mammals Get to the Caribbean Islands?

International audienceA multidisciplinary team is jointly investigating mammal evolution and subduction dynamics to unravel how flightless land mammals migrated to the Greater Antilles and other Caribbean islands

Three-dimensional velocity structure of the outer fore arc of the Colombia-Ecuador subduction zone and implications for the 1958 megathrust earthquake rupture zone

In 2005, an onshore, offshore 3-D refraction and wide-angle reflection seismic experiment was conducted along the convergent margin at the border between Colombia and Ecuador, over the rupture zone of the 1958, M-w 7.6 subduction earthquake. A well-defined Vp velocity model of the plate boundary and upper and lower plates was constructed, down to 25 km depth, using first arrival traveltimes inversion. The model reveals a several kilometers thick, low-velocity zone in the upper plate, located immediately above the interplate contact. This low-velocity zone might be related to alteration and fracturing of the mafic and ultramafic rocks, which composed the upper plate in this area by fluids released by the lower plate with possible contributions from sediment underplating. Near the toe of the margin, the model shows a low-velocity gradient in the outer wedge, which is interpreted as highly faulted and fractured rocks. This low-velocity/low-gradient region appears to limit the oceanward extension of the rupture zones of the 1958 and 1979 earthquakes, possibly because coseismic deformation and uplift of the outer margin wedge dissipates most of the seismic energy

Does regional-scaled vigorous fluid fluxes reconcile thermal segmentation and interplate coupling variations at the Ecuadorian subduction zone?

International audienceIn subduction zones, questioning the causes for variations in interplate coupling and interplate slip behavior implies deciphering how much deep fluid content and flux influence thermo-mechanical features along the plate interface. A key-question in fluid-rich subduction zones is: how ventilated and insulated hydrothermal systems in basaltic aquifer influence interplate frictional properties and seismogenesis? This requires, in the first place, identifying zones of intense fluid flux at depth; challenging task!The oceanic aquifer is an uppermost basaltic layer, where interconnected porosity is many orders of magnitude greater than in overlying trench sediment, subduction channel (Spinelli et al., 2004) and underlying dike complex (Becker and Davis, 2004). This basaltic aquifer thus proved to be a very efficient pathway for fluids (Fisher and Becker, 2000) with thermal influence depending on the fluid exchange efficiency with surroundings. In ventilated aquifer, widespread fluid pathways favor heat advection from the ocean to the basement triggering hydrothermal cooling that results in unexpectedly low heat flow at the margin outer slope (Harris and Wang, 2002). In contrast, in insulated aquifer, less permeable sedimentary trench fill restrict heat advection and deep fluids flowing updip along the plate interface (Moreno et al., 2014) generate heat-flow higher than expected at the deformation front (Spinelli and Wang, 2008). Thus, dense heat-flow and seismic data may provide constraints on fluid flux at depth by deciphering the heat convection influence onto the margin thermal structure.In Ecuador, 104 Multichannel seismic lines show Bottom Simulating Reflectors (BSRs) along segments that all together extend over more than 2200 km. BSR-derived heat-flow (Yamano et al., 1982) provide an unprecedentedly detailed heat-flow map from south Ecuador to central Colombia. This map reveal the margin thermal segmentation. 50-70-km-large (along strike) margin segments show heat-flow values of 140-200 mW/m², thus 160-200% higher than expected at the margin front. In contrast 20-30-km-large margin segments show heat-flow values of 60-80 mW/m², 50-60% lower than expected. These “anomalously” high and low heat flow are typical of ventilated (Harris and Wang, 2002) and insulated (Spinelli and Wang, 2008) hydrothermal circulation respectively.These thermal variations provide evidences about major questions in subduction zones and in Ecuador in particular.1- The Ecuadorian subduction zone undergoes a fluid-rich hydrothermal circulation, which is the first example to be documented, worldwide, at such a regional scale.2- Insulated hydrothermal circulation fronts the rupture zones for the Pedernales (2016) and, possibly, for the 1942 earthquakes, while, to the south, the poorly coupling interplate zone corresponds with conductive heat-flow. Fluids flowing updip of the rupture zones along the interplate contact may thus favor interseismic coupling and co-seismic stick-slip behavior at greater depth.3- The subducting Atacames seamounts correspond with a poorly coupling interplate patch that shows unexpectedly low heat-flow at the deformation front. The subducting seamounts and the related deep pervasive margin fracturing are likely to provide efficient fluid pathways within the upper plate, interrupting the insulated circulation and favoring interplate decoupling.Becker, K., Davis, E.E., 2004. In situ determinations of the permeability of igneous ocean crust, in: Davies, E.E., Elderfield, H. (Eds.), Hydrogeology of the Ocean Lithosphere:. Cambridge University Press,, New York, pp. 189-224.Fisher, A.T., Becker, K., 2000. Channelized fluid flow in oceanic crust reconciles heat-flow and permeability data. Nature 403, 71-74. doi:10.1038/47463.Harris, R.N., Wang, K., 2002. Thermal models of the Middle America Trench at the Nicoya Peninsula, Costa Rica. Geophysical Research Letters 29, 21, 2010. doi:10.1029/2002GL015406.Moreno, M., Haberland, C., Oncken, O., Rietbrock, A., Angiboust, S., Heidbach, O., 2014. Locking of the Chile subduction zone controlled by fluid pressure before the 2010 earthquake. Nature Geoscience 7, 292-296. doi:10.1038/NGEO2102.Spinelli, G.A., Giambalvo, E.R., Fisher, A.T., 2004. Sediment permeability, distribution, and influence on fluxes in oceanic basement, in: Davies, E.E., Elderfield, H. (Eds.), Hydrogeology of the Oceanic Lithosphere. Cambridge University Press, New York, pp. 151–188.Spinelli, G.A., Wang, K., 2008. Effects of fluid circulation in subducting crust on Nankai margin seismogenic zone temperatures. Geology 36, 11, 887-890. doi:10.1130/G25145A.1.Yamano, M., Uyeda, S., Aoki, A.Y., Shipley, T.H., 1982. Estimates of heat flow derived from gas hydrates. Geology 10, 339-343

Tectonic partitioning at oblique convergence poorly-coupled subduction zones: the example of the Northern Lesser Antilles margin

International audienc

Seamount subduction at the North-Ecuadorian convergent margin : effects on structures, inter-seismic coupling and seismogenesis

At the North-Ecuadorian convergent margin (1 degrees S-1.5 degrees N), the subduction of the rough Nazca oceanic plate leads to tectonic erosion of the upper plate and complex seismogenic behavior of the megathrust. We used three selected pre-stack depth migrated, multi-channel seismic reflection lines collected during the SISTEUR cruise to investigate the margin structure and decipher the impact of the subducted Atacames seamounts on tectonic erosion, interseismic coupling, and seismogenesis in the region of the 1942 Mw7.8 earthquake. This dataset highlights a subducted similar to 30 x 40 km, double-peak seamount that belongs to the Atacames seamount chain and that is associated with a deep morphologic re-entrant containing mass transport deposits. The seamount subduction uplifted the margin basement by similar to 1.6 km and pervasively broke the margin by deep and intense reverse faulting ahead of the seamount, a process that is likely to weaken considerably the margin. In the seamount wake, the basement reverse fault system rotated counter-clockwise. This faulted basement is overlain with slope sediment sliding along listric normal faults that sole out onto the BSR. This superposition of deep tectonic contraction within the basement and shallow gravitational extension deformation within the sediment highlights the key role of gas hydrate on outer slope erosion. In addition to long-term regional basal erosion, the margin basement has thinned locally by an extra 0.8-1 km in response to the subduction of the Atacames seamount chain and hydrofracturing by overpressured fluids at the margin toe. This pervasively and deeply fractured margin segment is associated with a seismically quiet and GPS-modeled low interseismic coupling corridor that terminates downdip near the 1942 epicenter and locked zone. We suggest that the deeply buried double-peak Atacames seamount triggered the 1942 earthquake ahead of its leading flank. This result supports previous studies proposing that subducted seamounts provide unfavorable conditions for locking the updip segment of the plate boundary limiting the updip extent of seismogenic zones, but may favor large subduction earthquakes at greater depths

How wide is the seismogenic zone of the Lesser Antilles forearc?

The Lesser Antilles subduction zone has produced no recent strong thrust earthquakes, making it difficult to quantify the seismic hazard from such events. The Lesser Antilles arc has a low subduction rate and an accretionary wedge that is very wide at its southern end. To investigate the effect of the wedge on seismogenesis, numerical models of forearc thermal structure were constructed along six transects perpendicular to the arc in order to determine the thermally predicted width of the seismogenic zone. The geometry of each section is constrained by published seismic profiles and crustal models derived from gravity and seismic data and by earthquake hypocenters at depth. A major constraint on the deep part of the model is that mantle temperature beneath the volcanic arc should achieve a temperature of 1,100 degrees C to generate partial melts. Predicted surface heat flow is compared to the available heat flow observations. Thermal modeling results indicate a systematic southward increase in the width of the seismogenic zone, more than doubling in width from north to south and corresponding to a dramatic southward increase in forearc width (distance from the arc to the deformation front of the accretionary wedge). The minimum width of the seismogenic zone (distance between the intersections of the subduction interface with the 150 degrees C and 350 degrees C isotherms) increases from about 80 km, north of 16 degrees N, to 230 km, at 13 degrees N. The maximum width (between the 100 degrees C and 450 degrees C isotherms) ranges from about 150 km in the north to up to 320 km in the south. This large variation in the width of the seismogenic zone is a consequence of the increasing width of the accretionary wedge to the south, caused by the increased thickness of sediment on the subducting plate. There is good agreement between the thermally predicted seismogenic limits and the sparse distribution of recorded thrust earthquakes, which are observed only in the northern portion of the arc. Possible scenarios for mega-thrust earthquakes are discussed. Depending on the segment length (along-strike) of the rupture plane, the occurrence of an event of magnitude 8-9 cannot be excluded.L’absence de grands séismes récents à mécanismes chevauchants dans la zone de subduction des Petites Antilles rend difficile l’évaluation de l’aléa sismique lié à de tels événements. L’arc des Petites Antilles est caractérisé par une faible vitesse de subduction et par la présence d’un prisme d’accrétion très développé à son extrémité méridionale. Afin d’évaluer les effets de la largeur de ce prisme sur la genèse des séismes, nous avons étudié six sections perpendiculaires à l’arc, du nord au sud de celui-ci, pour déterminer la largeur de la zone sismogène prédite par les modèles thermiques appliqués à chacune de ces coupes. La géométrie de ces dernières est contrainte par les profils sismiques publiés, par les modèles de structure crustale déduits des données gravitaires et sismiques, et enfin par la distribution des hypocentres des séismes. Un contrôle important permettant de tester la validité des modèles thermiques en profondeur est qu’une température minimale de 1 100oC, compatible avec la fusion partielle du manteau hydraté, doit être atteinte sous l’arc volcanique actif. Par ailleurs, le flux thermique en surface prédit par ces modèles doit être compatible avec les mesures de flux de chaleur. Les modèles thermiques retenus d’après ces critères montrent une augmentation du simple au double vers le sud de la largeur de la zone sismogène, qui correspond à un élargissement considérable de la taille du domaine avant-arc. En effet, la largeur minimale de la zone sismogène (définie comme la distance entre les intersections de l’interface des plaques avec les isothermes 150o et 350oC) augmente d’environ 80 km au nord de 16oN jusqu’à 230 km à 13oN. La largeur maximale de cette zone (définie par les intersections de l’interface avec les isothermes 100o et 450oC) augmente, quant à elle, d’environ 150 km au nord jusqu’à 320 km au sud de l’arc. Cette variation considérable est la conséquence de l’augmentation de la largeur du prisme d’accrétion, elle-même causée par l’accumulation croissante des sédiments déposés sur la plaque plongeante. Les largeurs de la zone sismogène prédites à l’aide des modèles thermiques sont en bon accord avec les rares données disponibles sur les séismes à mécanismes chevauchants dans la partie nord de l’arc. Les scénarios possibles relatifs à des méga-séismes de ce type n’excluent pas de futurs événements atteignant des magnitudes de 8 à 9