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

The Bunce Fault and Strain Partitioning in the Northern Lesser Antilles

International audienceStrain partitioning related to oblique plate convergence has long been debated in Northern Lesser Antilles. Geophysical data acquired during the ANTITHESIS cruises highlight that the sinistral strike‐slip Bunce Fault develops along the vertical, long, and linear discontinuity between the sedimentary wedge and a more rigid backstop. The narrowness of the 20‐ to 30‐km‐wide accretionary wedge and its continuity over ~850 km is remarkable. The Bunce Fault extends as far south as 18.5°N where it anastomoses within the accretionary prism where the sharp increase in convergence obliquity possibly acts as a mechanical threshold. Surface traces related to subducting seamounts suggest that 80% of the lateral component of the convergent motion is taken up by internal deformation within the accretionary prism and by the Bunce Fault. The absence of crustal‐scale, long‐term tectonic system south of the Anegada Passage casts doubt upon the degree of strain partitioning in the Northern Lesser Antilles

Seismic imaging across a slow slip event area, along the Ecuadorian subduction zone

International audienceThe Mw 8.8 1906 earthquake that occurred at the Ecuador-Colombia subduction zone is the 7th largest event ever recorded worldwide, and one of several large earthquakes that have affected the region since then.At its southern border, episodes of aseismic slip have been recorded at the shallow updip part of the subduction interplate fault, in the form of both slow slip events (Vaca et al., 2018) and afterslip in the aftermath of the 2016 Mw 7.8 Pedernales earthquake (Rolandone et al., 2018). During the geophysical experiment HIPER, in march 2020, 47 Ocean Bottom Seismometers (OBS) have been densely deployed along a 93-km-long trench-normal profile, crossing the supposed area of the SSE event below the margin. These OBSs recorded the shots produced by 4990 cu.in. airguns, with the R/V Atalante, in order to obtain a high-resolution P-velocity 2D profile. A coincident Multi-Channel Seismic (MCS) profile was acquired, providing information on the structure of the oceanic crust (see companion abstract by Schenini et al.). The combined inversion of the first-arrival and oceanic Moho reflection PmP phases, using the traveltime tomography Tomo2D code (Korenaga et al., 2000), provides both the P-velocity structure of the subducting and upper plates as well as the oceanic Moho geometry down to 20 km depth. The obtained velocity model displays a low-velocity anomaly within the Nazca plate’s oceanic crust at the trench together with a strong increase of the oceanic crust thickness over a distance of only 50 km (from 5.5 km to 16 km thick). The thickened crust in our profile shares a clear seismic signature affinity with the nearby aseismic Carnegie Ridge, resulting from the interaction between the Galapagos hotspot and the Cocos-Nazca Spreading Center (CNSC), 20 Myr ago. Taking this into account, our results nevertheless show a low-velocity anomaly of 11% at the trench which is higher than Vp anomalies observed before the trench in other subduction zones, that have been related to oceanic crust hydration, favored by bending faults. This may thus be an indication of excessive fracturing and hydration of this thicker oceanic crust portion of the Carnegie aseismic ridge.Next steps will be to better understand the influence of this feature on the slipping behavior, by observing the coincident and adjacent MCS profiles

From long‐ to short‐term inter‐plate coupling at the subducted Carnegie Ridge crest, offshore Central Ecuador

We investigate the relationship between the long-term (Quaternary) interplate coupling and the short-term geodetically derived interseismic coupling at the Central Ecuador subduction zone. At this nonaccretionary margin, the Cabo Pasado shelf promontory and coastal area are associated with two inter-plate geodetically locked patches. The deepest patch ruptured co-seismically during the Mw7.8-2016 Pedernales earthquake, while the shallowest underwent dominantly after-slip. Marine geophysical and chronostratigraphic data allow reconstructing the Quaternary tectonic evolution of the shelf promontory and substantiating variation of the long-term inter-plate coupling that led to the geodetically locked patches. Prior to ∼1.8 Ma, the outer-wedge inter-plate coupling was strong enough to activate trench-subparallel strike-slip faults. Then, between ∼1.8-0.79 Ma, shortening and uplift affected the shelf promontory, implying a locally increased inter-plate coupling. After a short, post-0.79 Ma period of subsidence, shortening and uplift resumed denoting a high inter-plate coupling that endured up to the present. The synchronicity of the structural evolution of the shelf promontory with the subduction chronology of two reliefs of the Carnegie Ridge crest suggests that the locked patches are caused by a geometrical resistance to subduction that propagates landward causing permanent deformation. In 2016, the deepest subducted relief localized stress accumulation and high seismic slip, while the shallowest relief, which is associated with a weakened outer-wedge, prevented updip rupture propagation. Thus, at nonaccretionary margins, active outer-wedge strike-slip faults might be considered a proxi of near-trench coupling, and subducted relief a cause of plate coupling but an obstacle to the tsunami genesis when the relief is shallow. Key Points A trench-parallel strike-slip fault and its earthquake-controlled fault scarps substantiate a pre-1.8 Ma, outer-wedge inter-plate coupling From 1.8 Ma, a robust shelf uplift caused by subducted reliefs highlights a long-term coupling that led to geodetically locked patches The shallowest subducted relief likely impeded the generation of a major tsunami during the Mw7.8, 2016 event Plain Language Summary The 2016-Ecuador earthquake ruptured a subduction fault segment previously locked for decades beneath the coastline. The rupture was arrested updip by another locked fault segment called locked patch, which instead slipped slowly. To understand the cause of the locked patches, their rupture behaviors, and whether the decadal fault locking and long-term subduction processes are related, we reconstructed the Quaternary tectonic evolution of the margin offshore Central Ecuador using geophysical data. We consider that tectonic deformation reflects the long-term inter-plate coupling, which is the ability of the fault to transfer long-term stress and strain to the margin. Prior to ∼1.8 Ma, a trench-subparallel fault accommodating lateral displacement indicates a shallow plate coupling, which increased locally between ∼1.8-0.79 Ma as shown by margin shortening. After a brief subsidence, shortening resumed, denoting a strong coupling that persisted until today in the form of the locked patches. Although many physical factors have been proposed to control plate coupling, here we find that the locked patches are caused by the subduction of two reliefs of a submarine ridge. Remarkably, in 2016, the deepest relief released high elastic strain, while the shallower relief, thrust under a weakened outer-margin, damped updip rupture propagation, impeding a significant tsunami

Paleogene V‐Shaped Basins and Neogene Subsidence of the Northern Lesser Antilles Forearc

International audienceWorldwide, forearc trench-perpendicular basins are interpreted as the result of trench-parallel extension possibly due to either strain partitioning as at the Aleutians (Ryan & Scholl, 1989) and Ryukyu (Nakamura, 2004) Subduction Zones, and/or to increasing margin curvature as at the Marianas (Heeszel et al., 2008) and Hellenic trenches (Angelier, 1978; Mascle & Martin, 1990). In more extreme cases, widespread deformation of forearc domains results from the collision of buoyant crustal features (e.g., oceanic plateaus, seamount chains, or continental fragments) which is prone to generate bending and rotation of subduction zones (e.g., Vogt et al., 1976). Strongly curved convergent plate boundaries are subject to alongstrike variations in subduction obliquity and thus commonly associated with large-scale rigid body rotatio

Quaternary sedimentation and active faulting along the Ecuadorian shelf : preliminary results of the ATACAMES Cruise (2012)

Selected high-resolution seismic-reflection profiles and multibeam bathymetry acquired along the convergent Ecuador margin during the ATACAMES cruise on onboard the R/V L'Atalante (Jan.15-Feb.18, 2012) allow a preliminary evaluation of the neotectonic development and stratigraphic evolution of the margin based on the sismo-stratigraphic analysis of Quaternary sediment preserved on the margin shelf and upper slope. We present three major preliminary results. (1) The evolution of the Esmeraldas, Guayaquil and Santa Elena canyons. The head of the Esmeraldas canyon is the location of a continuous significant sediment transport. The Guayaquil canyon shows several episodes of deposition and incision. Aggrading sedimentation pattern in the canyon records several changes in relative sea-level. The subsidence of the Gulf of Guayaquil probably contributes to the good preservation of the canyon filling stages. The Santa Elena canyon is controlled by a SW-NE trending normal fault. (2) Variations of sediment accumulation and relative vertical motions are shown along-strike the shelf edge. Offshore the uplifted Manta peninsula, a pronounced subsidence of the shelf edge is documented by sedimentary clinoforms that have deposited in a morphological reentrant, and have migrated upslope testifying of a local subsidence meanwhile the adjacent La Plata Island area underwent uplift. In the Esmeraldas canyon area, a local uplift of the shelf is documented. (3) Two neotectonic fault systems with a possible transcurrent component are imaged across the shelf edge and upper margin slope offshore Jama, and Cape Galera. This possible transcurrent motion could be related to the reactivation of ancient faults of the upper plate by the subduction. These preliminary results indicate that the ATACAMES data set has a strong potential to evaluate the spatial and temporal contribution of tectonic and climate changes on the structural development and stratigraphic evolution of the Ecuador continental margin

Elongated giant seabed polygons and underlying polygonal faults as indicators of the creep deformation of Pliocene to recent sediments in the Grenada Basin, Caribbean Sea

Based on 2D seismic profiles, multibeam and seabed grab cores acquired during the Garanti cruise in 2017, 1-5 km wide seabed giant polygons were identified in the Grenada basin, covering a total area of ∼55000 km2, which is the largest area of outcropping polygonal faults (PF) ever found on Earth so far. They represent the top part of an active 700-1200 m thick underlying polygonal fault system (PFS) formed due to the volumetric contraction of clay- and smectite-rich sediments, initiated in the sub-surface at the transition between the Early to Middle Pliocene. The short axes of the best-fit ellipses obtained from a graphical centre-to-centre method were interpreted as the local orientation of a preferential contraction perpendicular to the creep deformation of slope sediments. In the North Grenada Basin, the polygons are relatively regular, but their short axes seem to be parallel to a N40°E extension recently evidenced in the forearc, possibly extending in the backarc, but not shown in the study area. They are most probably related to a progressive burial due to a homogeneous subsidence. In the South Grenada Basin, the polygons are more elongated and their axes are progressively rotating southeastward towards the depocenter, indicating a creep deformation towards the center of the basin created by a differential subsidence. Seabed polygons and underlying PF could thus be indicative of the deformation regime of shallow sediments related to main slopes controlled by two different basin architectures

Seismic exploration of the deep structure and seismogenic faults in the Ligurian Sea by joint MCS and OBS acquisition: preliminary results of the SEFASILS cruise

International audienceThe north Ligurian margin is a complex geological area on many accounts. It has witnessed several phases of highly contrasting deformation styles, at crustal scale and through shallow cover tectonics, simultaneously or in quick succession, and with significant spatial variability. This complex interplay is mirrored in intricate structures that make it hard to identify active faults responsible for both, the significant seismicity observed and the tectonic inversion undergone by the margin, identified on morphostructural grounds. We present here the first preliminary results of the leg 1 of SEFASILS cruise, conducted in 2018 offshore Monaco, in an effort to answer these questions by means of modern deep seismic acquisitions, using multichannel reflection and wide-angle sea-bottom records. Some first interpretations are provided and point towards an active basement deformation that focuses at the limits between main crustal domains