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

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

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

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

The tsunami signature on a submerged promontory: the case study of the Atacames Promontory, Ecuador

P>Shelf promontories exhibit very specific bathymetric features with regards to tsunamis. Because of their submerged cape morphology, a potential tsunami generated seawards of the promontory will exhibit a specific mode of propagation and coastal impact. To identify this peculiar tsunami signature, the Atacames Promontory, Ecuador, was chosen as a case study (another example is the shelf of the Nile delta, Egypt). The area is tectonically very active, hosts earthquakes among the most powerful recorded, as well as areas of slope instabilities that have triggered significant submarine landslides in the past (several cubic kilometres of volume). Both types of events are likely to be tsunamigenic. To examine the tsunami behaviour at the coastal area of the promontory and at its vicinity, we have considered two examples of tsunamigenic landslides of which scars were identified near the base of the continental slope. We also took into consideration two earthquake scenarios that are likely to represent most classes of earthquakes possibly occurring in this area depending on their locations and subsequent tsunami directivity, that is, a sensitivity test investigation. We took two distinct earthquake scenarios which are based on the 1942 and 1958 events that stroke the area. Then we computed their derived tsunamis and analysed their coastal impact. We found that significant tsunamis can be generated by either landslides or earthquakes. However, the maxima of wave amplitude occur offshore (but still above the underwater promontory): the concave-type shape of the bathymetric field often yields a refraction/focusing area that is located on the shelf promontory and not at the coast area of the promontory: the wave propagates first through the focusing area before striking the considered coast. This area may be considered as a sheltered zone. Besides, in the vicinity of the promontory (not exactly concerned by the study), the city of Esmeraldas, is relatively sheltered due to the presence of the underwater canyon at its termination and due to diverging waves

Earthquake-triggered deposits in the subduction trench of the north Ecuador/south Colombia margin and their implication for paleoseismology

International audienceThe north Ecuador/south Colombia convergent margin is affected by recurrent subduction earthquakes with magnitudes > 7.5, like the 1906, 1942, 1958, 1979 and 2016 events. The subduction trench is characterized by the construction of the Esmeraldas Turbidite System (ETS) fed by the large Esmeraldas Canyon that deeply incises the continental slope and that connects directly onshore with the Esmeraldas River. The detailed description of cores collected in the left-hand (western) proximal levee of the ETS and in two lobes allowed discriminating two types of coarse-grained deposits: (1) “classical” flood-generated turbidites are normally graded beds with structureless, laminated and cross-laminated intervals and high organic-matter content, while (2) earthquake-induced deposits consist of amalgamated normally-graded laminated/cross-laminated intervals separated by erosive surfaces. These latter are interpreted to be deposited by quasi-synchronous flows generated during a single earthquake. Organic matter is absent in such beds while ferromagnesian minerals and pumices are abundant, suggesting remobilization of the slope deposits. When two amalgamated beds are superimposed, the interbedded clayey interval is not bioturbated, suggesting a short time period between the beds deposition, and thus the impact of a major earthquake shock and following earthquakes on the triggering of landslides.Along the ETS, core-to-core correlation based on 210Pb excess revealed that 20th Century sedimentation occurred mainly in the proximal levee. There, a temporal relationship was established between the 1906, 1942, and 1979 earthquakes, and three coarse-grained beds showing features of earthquake-induced turbidites, suggesting the Esmeraldas Canyon was the main source for sediments to be remobilized during these earthquakes. The fining and thinning observed between the 1906, 1942 and 1979 turbidites correlate with the increasing distance of the rupture zone of each earthquake with the Esmeraldas Canyon. Earthquakes with magnitudes lower than 7 also affected the margin during the 20th Century but were not recorded in the trench sedimentation, suggesting that the turbidite levee acts as a natural filter so that potentially the highest the levee the strongest the earthquake magnitude recorded. At least ten earthquakes with the highest magnitudes were recorded on the turbidite levee within the last 800 years with a recurrence time ranging from about 268 years to 42–82 years, or less for the 20th Century earthquakes. The comparison of the main features of the 1906 turbidite with older earthquake-triggered turbidites identified in a core collected in the trench suggests that one or two earthquakes similar to the 1906 event might have occurred ~ 600 years ago