Searching for the seafloor signature of the 21 May 2003 Boumerdès earthquake offshore central Algeria

Shaking by moderate to large earthquakes in the Mediterranean Sea has proved in the past to potentially trigger catastrophic sediment collapse and flow. On 21 May 2003, a magnitude 6.8 earthquake located near Boumerdès (central Algerian coast) triggered large turbidity currents responsible for 29 submarine cable breaks at the foot of the continental slope over ~150 km from west to east. Seafloor bathymetry and backscatter imagery show the potential imprints of the 2003 event and of previous events. Large slope scarps resulting from active deformation may locally enhance sediment instabilities, although faults are not directly visible at the seafloor. Erosion is evident at the foot of the margin and along the paths of the numerous canyons and valleys. Cable breaks are located at the outlets of submarine valleys and in areas of turbiditic levee overspilling and demonstrate the multi-source and multi-path character of the 2003 turbiditic event. Rough estimates of turbidity flow velocity are not straightforward because of the multiple breaks along the same cable, but seem compatible with those measured in other submarine cable break studies elsewhere. <br><br> While the signature of the turbidity currents is mostly erosional on the continental slope, turbidite beds alternating with hemipelagites accumulate in the distal reaches of sediment dispersal systems. In perspective, more chronological work on distal turbidite successions offshore Algeria offers promising perspectives for paleoseismology reconstructions based on turbidite dating, if synchronous turbidites along independent sedimentary dispersal systems are found to support triggering by major earthquakes. Preliminary results on sediment core PSM-KS23 off Boumerdès typically show a 800-yr interval between turbidites during the Holocene, in accordance with the estimated mean seismic cycle on land, even if at this stage it is not yet possible to prove the earthquake origin of all the turbidites

Submarine slope failures along the convergent continental margin of the Middle America Trench

We present the first comprehensive study of mass wasting processes in the continental slope of a convergent margin of a subduction zone where tectonic processes are dominated by subduction erosion. We have used multibeam bathymetry along ∼1300 km of the Middle America Trench of the Central America Subduction Zone and deep-towed side-scan sonar data. We found abundant evidence of large-scale slope failures that were mostly previously unmapped. The features are classified into a variety of slope failure types, creating an inventory of 147 slope failure structures. Their type distribution and abundance define a segmentation of the continental slope in six sectors. The segmentation in slope stability processes does not appear to be related to slope preconditioning due to changes in physical properties of sediment, presence/absence of gas hydrates, or apparent changes in the hydrogeological system. The segmentation appears to be better explained by changes in slope preconditioning due to variations in tectonic processes. The region is an optimal setting to study how tectonic processes related to variations in intensity of subduction erosion and changes in relief of the underthrusting plate affect mass wasting processes of the continental slope. The largest slope failures occur offshore Costa Rica. There, subducting ridges and seamounts produce failures with up to hundreds of meters high headwalls, with detachment planes that penetrate deep into the continental margin, in some cases reaching the plate boundary. Offshore northern Costa Rica a smooth oceanic seafloor underthrusts the least disturbed continental slope. Offshore Nicaragua, the ocean plate is ornamented with smaller seamounts and horst and graben topography of variable intensity. Here mass wasting structures are numerous and comparatively smaller, but when combined, they affect a large part of the margin segment. Farther north, offshore El Salvador and Guatemala the downgoing plate has no large seamounts but well-defined horst and graben topography. Off El Salvador slope failure is least developed and mainly occurs in the uppermost continental slope at canyon walls. Off Guatemala mass wasting is abundant and possibly related to normal faulting across the slope. Collapse in the wake of subducting ocean plate topography is a likely failure trigger of slumps. Rapid oversteepening above subducting relief may trigger translational slides in the middle Nicaraguan upper Costa Rican slope. Earthquake shaking may be a trigger, but we interpret that slope failure rate is lower than recurrence time of large earthquakes in the region. Generally, our analysis indicates that the importance of mass wasting processes in the evolution of margins dominated by subduction erosion and its role in sediment dynamics may have been previously underestimated

Genetic Relations Between the Aves Ridge and the Grenada Back-Arc Basin, East Caribbean Sea

The Grenada Basin separates the active Lesser Antilles Arc from the Aves Ridge, described as a Cretaceous‐Paleocene remnant of the “Great Arc of the Caribbean.” Although various tectonic models have been proposed for the opening of the Grenada Basin, the data on which they rely are insufficient to reach definitive conclusions. This study presents, a large set of deep‐penetrating multichannel seismic reflection data and dredge samples acquired during the GARANTI cruise in 2017. By combining them with published data including seismic reflection data, wide‐angle seismic data, well data and dredges, we refine the understanding of the basement structure, depositional history, tectonic deformation and vertical motions of the Grenada Basin and its margins as follows: (1) rifting occurred during the late Paleocene‐early Eocene in a NW‐SE direction and led to seafloor spreading during the middle Eocene; (2) this newly formed oceanic crust now extends across the eastern Grenada Basin between the latitude of Grenada and Martinique; (3) asymmetrical pre‐Miocene depocenters support the hypothesis that the southern Grenada Basin originally extended beneath the present‐day southern Lesser Antilles Arc and probably partly into the present‐day forearc before the late Oligocene‐Miocene rise of the Lesser Antilles Arc; and (4) the Aves Ridge has subsided along with the Grenada Basin since at least the middle Eocene, with a general subsidence slowdown or even an uplift during the late Oligocene, and a sharp acceleration on its southeastern flank during the late Miocene. Until this acceleration of subsidence, several bathymetric highs remained shallow enough to develop carbonate platforms

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

Seafloor morphology and sediment transfer in the mixed carbonate-siliciclastic environment of the Lesser Antilles forearc along Barbuda to St. Lucia

International audienceThe Lesser Antilles arc is a mixed siliciclastic and carbonate active margin made of active volcanic and flat Plio-Quaternary carbonate islands. It was built as a result of a complex tectonic history at the slowly converging boundary between the American plates and the Caribbean plate. The sedimentary processes as a consequence of external forcing (earthquakes, volcanism, hurricanes) were rarely documented in such environment and are poorly understood. We exploited an exceptional dataset of high-resolution marine seafloor data acquired during the last 20 years in the northern part of the Lesser Antilles forearc to document the sediment-transport processes. We achieved a detailed morpho-sedimentary study from multi-beam bathymetry, backscattering, and seismic profiles. Two areas could be characterized: 1) the “Rough Area”, along Barbuda to Guadeloupe carbonated islands, characterized by steep (up to 25°) slopes incised by short canyons, and deep basins controlled by major normal faults; 2) the “Channelized Area”, south of Guadeloupe and bordered by active volcanic islands and carbonate platforms, characterized by gentle slopes incised by long canyons. During sea-level high-stands, the sediment seems exported from the carbonate platform by hurricanes or density cascading but appears to settle at the shelf-edge and canyon heads. During sea-level low-stands, a connection may exist between onshore and offshore systems. However, this sediment supply appears not sufficient to generate canyon formation, likely shaped by regressive processes. Shelf breaks of the carbonate banks, platforms and submarine slopes are affected by sediment failures. Some may be associated to voluminous remobilizations and large mass transport deposits. Large earthquakes are likely the main processes in this area to remobilize sediments toward the deep forearc basins by triggering both slope failures and flushing of the canyon heads

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

Late Quaternary geomorphologic evolution of submarine canyons as a marker of active deformation on convergent margins : the example of the South Colombian margin

The morphology of Patia and Mira canyons on the South Colombian convergent margin reflects an interplay between tectonic deformation, sea-level variation and canyon evolution, and provides new insight into the age and location of margin deformation over the last similar to 150 ka. Multibeam bathymetry, seismic, and sedimentary data reveal that tectonically active structural highs control canyon incision and the canyon's course. The canyons developed across the margin in five major stages. First, the upper slope was incised by headward and downward erosion during the Pleistocene, infilling a structurally bounded slope basin. The basin periodically spilled over and breached the accretionary prism at similar to 150 ka, leading to the development of isolated sediment lobes in the trench. The prism was efficiently breached leading to a well-developed trench channel-levee system at 53-67 ka. Today, the system shows limited activity. Antecedent streams, convex-up axial incision profiles, and increasing height/width ratio indicate an active uplift of the structural highs since at least similar to 150 ka and support localized shortening through the margin accommodated by out-of-sequence structures thrusts and folds. An 80 m-high scarp where the canyon crosses a fault on the middle slope further supports active uplift related to a major thrust. Previous seismostratigraphic studies of the margin have demonstrated that active uplift occurred during the Early Pliocene; here we demonstrate that uplift continued throughout the Late Pleistocene. Comparisons with canyons on other convergent margins reveal that features relating to margin deformation and canyon age (tortuous path, convex-up profiles, abandoned canyon paths, overincision, abrupt canyon turns) are generally restricted to accretionary margins. Because of their complex morphology, accretionary margins appear more favorable for the occurrence of such features than margins undergoing tectonic erosion with more simple morphology