Processus gravitaires sous-marins le long de la zone de subduction Nord Equateur – Sud Colombie : Apports à la connaissance de l’érosion tectonique et de la déformation d’une marge active, et implications sur l’aléa tsunamis

The aim of this study is to constrain recent deformation of an active margin, using superficial structures deposits on the medium (Quaternary) to short (kyr to a century) term.The Ecuadorian margin is characterised by a tectonic erosion regime evolving northward, in Colombia, to a tectonic accretion. The margin undergoes strong seismicity, with 4 great earthquakes (Mw 7,7–8,8) occurred during the XXth century. Areas of preferential destabilisations are located on the margin and on canyon walls. An analysis of canyons Mira and Patia, transverse to the accretion margin allowed establishing the incision modalities of a canyon on an active margin, and use the system as active deformation marker. Antecedence figures, overincision, and convex-up incision profiles shows a distributed deformation across the margin during the last ~150kyr, accommodated by out-of-sequence reactivated structures. In the trench facing the tectonic erosion segment, geophysical and sedimentological data allowed to study MTDs, which return time varies between 1.4 and 12kyr contrasts with the one of turbidites (50 to 250yr). The latter is consistent with the recurrence of Mw>8.2 earthquakes (>73yr) implying probably a seismic trigger. The large return time of MTDs is attributed to a weakening of the margin during many seismic cycles that ultimately leads to its collapse. We also numerically simulated the tsunamigenic potential of submarine landslides selected on the margin toe. We show that major submarine structures play a key-role of maximum wave energy concentrator, from where the radiative energy tends to dissipate, thus naturally protecting some coastal segments from the worst tsunami effects.L’objectif de ce travail est de contraindre les déformations récentes d’une marge active à partir de l’analyse des structures et dépôts superficiels à moyen (Quaternaire) et court (ka à siècles) terme. La marge Equatorienne a un régime tectonique en érosion, passant au Nord, en Colombie, en accrétion. Elle subit une forte sismicité, avec 4 séismes (Mw 7,7–8,8) produits au XXe siècle. Les zones préférentielles de déstabilisation de pente se situent en front de marge et sur les flancs de canyons. L’étude des canyons Mira - Patia, transverses à la marge en accrétion, a mis en évidence les modalités du développement de canyons en marge active et permet d’utiliser ceux-ci comme marqueurs de la déformation. Des figures d’antécédence, surincision, et des profils d’érosion convexes montrent une surrection distribuée sur la marge au cours des ~150ka au travers de structures crustales hors-séquence réactivées. Dans la fosse du segment de marge en érosion, des données géophysiques et sédimentaires ont permis d’étudier des MTDs dont le temps de retour variant entre 1,4 et ~12ka contraste avec celui des turbidites (50 à 250a). Ce dernier est cohérent avec la récurrence des séismes de Mw>8,2 (>73a) impliquant un probable déclenchement sismique. Le grand temps de retour des MTDs serait attribué à un affaiblissement de la marge sur plusieurs cycles sismiques menant à son effondrement. Nous avons aussi simulé le potentiel tsunamigène de glissements en front de marge. Nous montrons que des structures sous-marines majeures jouent un rôle clé de concentrateur de l’énergie maximale des vagues, d’où l’énergie radiative se dissipe, protégeant ainsi naturellement certains segments de côte des effets les plus néfastes du tsunami

Processus gravitaires sous-marins le long de la zone de subduction Nord Équateur-Sud Colombie (apports à la connaissance de l'érosion tectonique et de la déformation d'une marge active)

Les processus gravitaires sur une marge passive sont majoritairement contrôlés par l eustatisme, le taux de sédimentation, la sismicité, et la déformation tectonique. Ces paramètres agissent également sur les marges actives, mais la sismicité et la déformation tectonique y sont prépondérants. L objectif de ce travail est de caractériser les principales zones sujettes à des processus gravitaires sur la marge active Nord Equateur Sud Colombie, afin de contraindre ses modes de déformation à court (plusieurs cycles sismiques) et moyen terme (Quaternaire). La marge convergente Nord Equateur est caractérisée par un régime en érosion tectonique, qui passe vers le Nord, en Colombie, à un régime d accrétion tectonique. Cette marge est le siège d une forte sismicité, puisque 4 forts séismes (Mw 7.7-8.8) s y sont produits au XXe siècle. Un premier travail a permis d identifier les zones préférentielles des déstabilisations. Elles sont situées en front de marge dans la partie en érosion tectonique, et sur les flancs des canyons subissant une surrection. Le long du segment de marge en érosion, la fosse est isolée et reçoit peu de sédiments détritiques, laissant supposer que les MTDs identifiés résultent des séismes. Des données bathymétriques, de sismique réflexion, de CHIRP (3.5 kHz) et des carottes sédimentaires, ont permis d étudier la mise en place des dépôt en masse (MTDs) au cours des derniers ~ka et des turbidites depuis ~ 4.5 ka. L étude de l organisation spatiale des MTDs, de leurs sources, et de leur temps de retour, permet de proposer un modèle de déstabilisation sur une marge en érosion à l échelle d un et de nombreux cycles sismiques. Parallèlement, l étude morpho-structurale d un important système de canyons transverse à la marge (canyon de Mira et Patia) a permis de mettre en évidence les modalités de développement de canyons sur une marge active et de replacer les instabilités gravitaires de bord de canyon dans cette évolution ? Cette étude permet aussi de préciser l évolution tectonique quaternaire de la marge. Nous avons également simulé numériquement le potentiel tsunamigène de glissements sous-marins à partir de cicatrices et du volume de masses glissées sélectionnées en front de marge. Nous montrons que des structures tectoniques sous-marines majeures jouent un rôle clef de concentrateur de l énergie maximale des vagues, à partir desquelles l énergie radiative tend à se dissiper, protégeant ainsi naturellement certains segments de côte des effets les plus néfastes du tsunami.Gravity processes on a passive margin are mainly controlled by eustatism, sedimentation rate, seismicity, and tectonic deformation? These parameters also act on an active margin, however seismicity and tectonic deformation are dominant? The aim of this study is to characterize the main areas undergoing gravity processes along the North Ecuador South Colombia active margin, in order to constrain its deformation modes on the short term (several seismic cycles) and mid term (Quaternary). North Ecuadorian convergent margin is characterized by a tectonic erosion regime, evolving to tectonic accretion northward in South Colombia. The margin is undergoing strong seismicity as 4 great earthquakes (Mw 7.7-8.8) have occurred during the XXth century. A first study allowed identifying destabilization preferential areas. They are located at the margin toe on the erosional margin, and along canyon walls undergoing tectonic uplift. Along the erosional margin segment, the trench is isolated and receives few sediment, suggesting that the MTDs for the last ~ 20 kyr and turbidites for the last ~ 4.5 kyr. MTDs spatial organization, source and return time analysis lead to the establishment of a destabilization model along an erosional margin for one and many seismic cycles. On the other side, the morphostructural analysis of an important canyons system (Mira and Patia canyons) has led to establish canyon development modalities on an active margin. We have also simulated numerically the tsunamigenic potential of submarine landslides using selected slump scars and their volume on the margin toe. We show that submarine tectonic structures tend to play a key-role as maximum wave energy focus from which radiative energy tends to dissipate, thus protecting some coastal segments form tsunami damage.NICE-BU Sciences (060882101) / SudocSudocFranceF

Mass-transport deposits in the northern Ecuador subduction trench : result of frontal erosion over multiple seismic cycles

Investigations of Mass-Transport Deposits (MTDs) and turbidite deposition in the confined North Ecuador subduction trench provide access to paleoseismic information and insights into long-term mechanisms for frontal tectonic erosion at a convergent margin. The studied trench has been the site of four great subduction earthquakes (7.7 <= Mw <= 8.8) during the 20th century. The trench is isolated from major continental sediment input, so that investigated MTDs and turbidites are considered of local origin. Swath bathymetry, seismic reflection and Chirp data, together with sedimentary cores and C-14 dating revealed that seven MTDs were emplaced in distinct trench sub-basins since similar to 23 kyr, and 27 turbidites deposited in the southernmost trench sub-basin since similar to 4.9 kyr. Our analysis shows that six MTDs were derived from the margin, while a single one stemmed from the outer trench wall. Temporal correlations between MTDs emplaced within trench sub-basins separated by a structural saddle, indicate that the seven MTDs were emplaced during five main events. Three were triggered locally and tentatively dated 5.8, 1.6 kyr and Recent, whereas four were emplaced in distinct trench sub-basins as a result of two regional events at 22.6 and 15.4 kyr. None of the MTDs occurred during the fast stage of the last sea-level rise (similar to 13 to 8 kyr). However, dissociation of gas hydrates during the last 8 kyr-stage of slow sea-level rise might have contributed to trigger the three youngest MTDs. The large 1.5-13.5 kyr return time of the MTDs contrasts with that of 189 yr of the turbidites. The later is consistent with the 73 yr return time of two local Mw 8.2 earthquakes, implying that turbidites might have been triggered by large earthquakes. The very large MTDs return time is attributed to long-term deformation processes and mechanical weakening of the margin outer wedge, in response to repeated variations in basal friction, pore pressure and margin extensional/contraction strain over multiple earthquake cycles. This process contributes to short-term frontal erosion, the rate of which is estimated to be 8.6 . 10(-3) km(3)/kyr/km, since at least 15.4 kyr

Recent debris-flows and megaturbidite in a confined basin of the North Ecuador subduction trench

International audienc

Nature and distribution of recent sedimentsin the trench of the subduction margin ofEcuador

National audienceRecent deep marine sedimentation in subduction trenches ischaracterized by the inter-stratification of hemipelagic and turbiditesediments locally interbedded with debris flows, which canresult from continental slope shaking by large earthquakes. Theobjective of this study is to describe the morphologic complexityon the Ecuadorian border of the Nazca tectonic plate where a setof deep marine asperities is subducting at different scales, and itsconsequences on the lateral distribution of the sediments in thedifferent sub-basins.The Ecuadorian active margin comprises three geomorphologicalsegments: The northern segment characterized by a wide (5-10km) and deep trench (3800 – 4000 m), a rough gullied continentalslope and a shelf (10-40 km wide) with active subsidence.The central segment strongly influenced by the subduction of theCarnegie ridge which induced a narrow (0–5 km wide) and relativelyshallow trench (3100 – 3700 m depth), a smooth andgullied slope with no canyons and a 15–40 km wide shelf characterizedby areas with active subsidence and uplift. Finally thesouthern segment presents a wide (5–10 km) and deep (4000–4700 m) trench, an irregular and starved continental slope withcanyon systems and a wide subsiding shelf (20–50 km).Bathymetry, high-resolution seismic profiles and sedimentary corescollected during the scientific campaign ”ATACAMES” along theactive margin of Ecuador show that since the last glacial maximum,the trench is filled by turbidites, hemipelagites, volcaniclasticdeposits, homogenites as well as slumps, debris flow andother mass transport deposits. Hemipelagites, turbidites and homogenitesare found in all segments. Hemipelagite beds range inthickness 5cm to 1m reaching over 3m in the north. Turbiditesare coarser and sandier showing a higher frequency in the centralsegment. Mass transport deposits are mainly found in thenorthern and southern segments when volcaniclastic deposits aremainly present in the central segment.The distribution of sedimentary facies along the trench could berelated to the subduction of the Carnegie Ridge. The ridge exerts astrong influence and control over the individualization of the threesegments as well as on the nature and the lateral distribution of thesediments in the trench. Due to shallower deposition conditions,the central segment facing the Carnegie Ridge is full of sandy andsilty turbidites in a proportion of 2:1 with respect to the other segmentsand its location on the main ash pathways might explainthe concentration of the volcaniclastic deposits. The southernand northern segments, which are fed by large canyons and affectedby isolated seamounts, show more slump, debris flows andmass-transport deposits

Nature and Architecture of the Sedimentary Deposits in the Trench of the Ecuadorian Subduction Margin

International audienceThe active margin of Ecuador is characterized by strong tectonic erosion that contributes to the formation of adeep trench filled by a complex suite of sedimentary facies. Gravity flow sedimentation is ubiquitous along themargin and facies range from laterally continuous m-thick mass transport deposits to isolated cm-thick turbiditesintercalated with hemipelagite and ash layers. However, the nature and architecture of those deposits remainequivocal.This study presents the interpretation of detailed bathymetry, high-resolution seismic profiles and sedimentcores recently acquired along the 600 km-long Ecuadorian margin (ATACAMES campaign onboard the R/VL’Atalante, 2012). The margin comprises three morphological segments: (1) the central segment marked bythe subduction of the Carnegie Ridge, which induced a narrow (10-30 km wide) and relatively shallow trench(3100-3700 m deep), a steep and gullied continental slope with no canyon and a 20-60 km wide shelf characterizedby active subsidence, (2) the northern segment characterized by a wider (100 km) and deeper (3800-4000 m)trench, a gentler gullied continental slope and similar shelf settings (10-50 km wide), (3) the southern segmentpresents a wide (20-60 km) and deep (4000-4700 m) trench, a starved continental slope with well-defined canyonsystems and a wide subsiding shelf (50-100 km wide). The sedimentary dynamics along the margin is evaluatedby the analysis of 15 sediment cores. High-resolution visual description of the cores, X-Ray imagery and themeasurement of petrophysical properties (gamma density, magnetic susceptibility, P-wave velocity) led to theidentification of 6 sedimentary facies that characterize 6 sedimentary processes: Turbidite beds (turbidity currents),Hemipelagites (continuous marine sedimentation), Tephras (airfall ash layers consecutive to volcanic eruptions),Debris flow deposits (cohesive debris flows), Megaturbidite/Homogenite (large-scale and/or hybrid gravity flows),Mass Transport Deposits (mass wasting). The chronostratigraphy of the deposits is defined by radiocarbon datingof well-identified hemipelagite sediments. Ages range from 500 to 48,000 years BP over the topmost 10m of thetrench deposits.Sedimentation patterns along the Ecuadorian trench are highly heterogeneous. Holocene sedimentation isrecorded almost everywhere apart from some locations within the central segment possibly due to the presenceof contour currents. However, sediment supply to the trench varies greatly between the starved central segment,where sedimentation rate and gravity flow (Turbidites, homogenites) frequency are low, and the northern andsouthern segments characterized by higher gravity flow frequencies (Turbidites, MTDs) and sedimentation rates.Although the northern and southern segments share similar characteristics, historic seismicity is four times higherin the North. Future work will include the identification of the mechanisms that trigger the gravity flows in orderto evaluate the potentiality of the Ecuadorian margin for submarine paleoseismology studies

Nature and distribution of recent sediments in the trench of the subduction margin of Ecuador.

International audienc

Formation of a giant honeycomb seafloor morphology on the Carnegie ridge: potential geodynamic significance

International audienc

Seafloor giant polygons associated with underlying polygonal faults in the Caribbean Sea, west of Grenada Basin

International audienceThe initial sediment lithification reactions start with complex interactions involving all components of the sedimentary material (minerals, surface water, decomposing organic matter and living organisms). This is the eogenesis domain (0 to 2000 m below seafloor), covering a burial interval ranging from the interface with the biosphere down to depths where physical compaction processes become predominant. Compared to studies performed on sedimentation (and sedimentary dynamics) and on deep diagenesis (mesogenesis), there is a true lack of data concerning diagenetic processes occurring during eogenesis, in particular concerning siliciclastic diagenesis. However, shallow sediments within the eogenesis domain undergo intense deformation and fracturing. In clay-rich sediments the created faults are organized in polygons due to the volumetric contraction leading to a volume loss during burial. The polygonal fault systems (PFS) have been identified in many basins worldwide, such as in the China Sea, in the Australian Eromanga Basin, in the Lower Congo Basin, in the Danish Central Trough, in the Canadian Atlantic margin and in the Irish Sea. These area are all located in petroleum provinces, either onshore or at water depths ranging from 200 to 1500 m. During the Garanti Cruise in May-June 2017, giant polygons have been identified on the slope of the Caribbean sea, west of Grenada Basin, between 1800 and 2500 m water depth. On seismic profiles the polygonal faults are characterized by an intense dimming of reflections on both edges of the fault planes suggesting that fluids are currently migrating upward. They affect a 700 to 900 m thick interval and they can locally reach the modern seafloor where they form polygons visible on multibeam data. On chirp profiles, the polygons have very steep flanks, defining rectilinear depressions (or furrows) that are 40 m deep compared to the regional slope. Various mechanisms have been referenced in the literature as responsible for polygonal fault initiation and propagation, such as diagenetic transformations or reactivation by sediment loading for instance. Four hypotheses are actually proposed to explain the formation of these polygonal faults: i) syneresis related to colloidal properties of such fine-grained sediments, ii) density inversions and associated hydrofracturing, iii) smectite-rich clays causing residual friction at low burial depth and iv) grain dissolution in incemented media inducing a decrease in horizontal stress that leads to shear failure and shear strain localization. In the Grenada basin it seems that the volumetrical contraction starts very early after deposition suggesting that the smectite-rich clays play a key role in the formation of polygons. This is compatible with the volcano-clastic context in the area where clays may come from the in-situ alteration of volcanic material