Geological and biological diversity of seeps in the Sea of Marmara

The Sea of Marmara hosts part of the North Anatolian Fault as an active submarine strike-slip fault. This area has suffered numerous earthquakes and presents a major seismic risk. Although the Sea of Marmara has been studied for many years, the link between geological morphostructures, the nature of fluids and biological communities is still rarely described. During the Marsite cruise (November 2014), dives with Remotely Operated Vehicle (ROV) VICTOR 6000 focused on detailed seafloor explorations of four different areas: the Central and Western highs and the Tekirdağ and Çinarcik basins. Based on 130 h of in situ videos, high-resolution seafloor mapping of seeps was conducted, emphasizing their significant geological and biological diversity from one seeping site to another, from one basin/high to another. Gas bubbles (CH4, CO2), shimmering water (brine, marine and fresh water) and oil, escape from the seafloor into the water column with low to strong fluxes. Black patches of reduced sediments, authigenic carbonate crusts and chimneys compose the seep environments with various types of bacterial mats and chemosynthetic fauna. Several venting sites discovered during previous cruises are still active 7–12 years later. The seeps are mostly, but not only, focalized along the Main Marmara Fault (MMF), at the southern border of the Tekirdağ Basin and along the Western High. Fluid emission is also occurring at secondary faults and at their intersection with the MMF. Our study emphasizes the location of seeps at the foot of slopes, gully outlets and crossroads. Sedimentary features, such as mass wastings, stratigraphic discontinuities or canyons, also interact with fluid emissions. The observed fauna is dominated by Bathymodiolinae, Vesicomyidae, Lucinidae-like empty shells and tubiculous worms resembling Ampharetidae polychatea. Most of the symbiont-bearing taxa encountered and previously sampled in the Marmara Sea, are characterized by thiotrophic symbioses. Vesicomyids and Idas sp. mussels are present at gas seeps, but also in areas where crude oil escapes from the seafloor. Moreover, other taxa unusually encountered at cold seeps such as large-sized amphipod and vagile worms were observed in the Çinarcik Basin. Idas-like mussels were observed in the western part of the Sea of Marmara, in the Tekirdağ Basin and possibly on the Western High active seep sites. There, the sampled fluids had high methane content (reaching 65 μmol/l) but not as high as on the Central High (363 μmol/l) and Çinarcik Basin (228 μmol/l) where no mussels were observed in the video records. Bottom waters oxygen levels in the Sea of Marmara showed a west to east decreasing gradient (57–8.5 μmol/l). These oxygen conditions, which fall under the limit of Oxygen Minimum Zones (OMZ <20 μm/l) in the eastern part, may impact benthic fauna and explain the absence of symbiotrophic bivalves at cold seep sites of the Çinarcik Basin, whereas densely aggregated amphipods, likely more tolerant to oxygen stress were observed in the seepage area. Finally, no specific fauna was observed near the CO2-rich seep sites. First observations suggest that seep fauna composition in the Sea of Marmara does not seem to be strongly influenced by the nature (e.g., oil, gas bubbling, brines) of fluid venting through seeps. The seep environments are highly variable and characterized by distinctive geological morphostructures. They sustain typical Mediterranean cold seep fauna, but also unusual communities likely related to the interaction of seeps with hypoxic conditions

Deep structure of the Grenada Basin from wide-angle seismic, bathymetric and gravity data

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Caribbean plate boundaries control on the tectonic duality in the back-arc of the Lesser Antilles subduction zone during the Eocene

International audienceThe Eocene tectonic evolution of the easternmost Caribbean Plate (CP) boundary, i.e. the Lesser Antilles subduction zone (LASZ), is debated. Recents works shed light on a peculiar period of tectonic duality in the arc/back-arc regions. A compressional-to-transpressional regime occurred in the north, while rifting and seafloor spreading occurred in Grenada basin to the south. The mechanism for this strong spatial variation and its evolution through time has yet to be established. Here, using 3-D subduction mechanical models, we evaluate whether the change in the trench-curvature radius at the northeast corner of the CP could have modulated the duality. We assume asymmetrical CP boundaries at the north (from east to west: oblique subduction to strike-slip) and at the south (subduction-transform edge propagator-like behavior). Regardless of the imposed trench curvature, the southern half of our modeled CP undergoes a NW-to-W-oriented extension due to the tendency of the southernmost part of the South-America oceanic slab to rollback. In contrast, the tectonic regime in the northeast corner of the CP depends on the trench-curvature radius. A low radius promotes transtension-to-transpression, with a NE-oriented compressive component of the principal stress. A high radius largely reduces the compressive component and promotes an extensional regime similar to that in the south. We thus propose that an initially low-curvature radius of the NE-LASZ triggered the tectonic N-S duality in the Eocene and led to an ephemeral period of transpression/compression at the north. However, an additional mechanism might have been required to locally enhance compression

The control of the northern and southern Caribbean plate boundaries on the tectonic duality in the back-arc of the Lesser Antilles subduction zone during the Eocene

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Les failles polygonales affleurantes du bassin de Grenade : un marqueur de la subsidence différentielle dans le bassin d'arrière–arc des Caraïbes

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Relations génétiques entre la Ride d'Aves et le bassin arrière-arc de Grenade, Caraïbe orientale

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Geodynamic of the Lesser Antilles Back arc Domain; contributions of the GARANTI cruise, in the Grenada Basin

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Paleogeographic reconstructions of the northern Lesser Antilles during the Neo-gene (24-2 Ma)

Extended abstract : Caribbean science and Innovation Meeting 2019International audienc

Giant seabed polygons and underlying polygonal faults in the Caribbean Sea as markers of the sedimentary cover extension in the Grenada Basin

Based on an extensive seismic and multibeam dataset, 1-5 km wide giant polygons were identified at the bottom of the Grenada basin, covering a total area of ~55000 km². They represent the top part of an active underlying polygonal fault system due to the volumetric contraction of clay- and smectite-rich sediments during burial. To date, this is the widest area of outcropping polygonal faults ever found on Earth. The seabed polygons are bounded by rectilinear ~1000-1500 m wide and ~10-60 m deep furrows, depending on the ESSOAr | https://doi.org/10.1002/essoar.10506638.1 | CC_BY_NC_4.0 | First posted online: Thu, 1 Apr 2021 00:40:54 | This content has not been peer reviewed location in the basin. They are relatively regular in the north Grenada Basin, whereas they are getting longer and more elongated in the south Grenada Basin. The polygonal faults consist in a set of discrete normal faults affecting a 700 to 1200 m thick interval, initiated in the shallow sub-surface at the transition between Early to Middle Pliocene and then having propagated both upward and downward during sedimentation. The centre43 to-centre method has been applied to determine the local ellipse of strains, providing a major orientation for extension needed for polygons to initiate. In the north, the minor axes are oriented N40°, indicating a general NE-SW extension of the upper part of the sedimentary cover consistent with the forearc/backarc regional extension. In the south Grenada Basin, minor axes are progressively turning towards the south, pointing out the actual maximum subsidence point. This implies that seabed polygonal faults could thus be indicative of the present-day (or recent) strain state within the upper sedimentary column

Unravelling the genetic relations between the Grenada Basin, the Aves Ridge, and the Lesser Antilles: a structural and stratigraphic analysis

International audienceLocated in the southeastern Caribbean, the Grenada Basin is bounded to the east by the modern Lesser Antillesisland arc, to the west by the Aves Ridge, commonly interpreted as a Cretaceous-Paleocene extinct volcanic arc,and to the south by the transpressive plate boundary with South America. The Grenada Basin has long beenregarded as a classical back-arc basin until recent studies suggest alternative models, such as forearc openingor Wernicke-type simple shear. The genetic relations between the Grenada Basin and its adjacent arcs remaintherefore controversial. Our analysis of seismic reflection and refraction data acquired during the GARANTIcruise (May-June 2017 onboard R/V L’Atalante) sheds light on basement nature and topography, depositionalhistory and deformation of the sedimentary infill, including vertical motions, of the Lesser Antilles back-arc area.Correlations with well logs located on the northern Venezuelan shelf, DSDP sites on the Aves Ridge, and IODPsites off the west coast of Martinique Island, also provide chronostratigraphic constraints.Seismic lines across the Grenada Basin reveal a significant asymmetry: the basement deepens from 5 to 10km southeastwards while flat-lying sediment units thicken from 2 to 7 km. A 6.5 to 7 km thick oceanic crustunderlies the southeastern half of the basin over a width of about 80 km. The Grenada Basin comprises threemajor depositional sequences defined by unconformities and/or changes in the seismic facies, from bottom to top:- Sequence 1: undifferentiated Eocene sediments represented by strong reflectors that drape the acoustic basement,as well as syntectonic deposits.- Sequence 2: Oligocene to Middle Miocene distal turbidites, probably originating from the Orinoco River thatflowed from the south into the Grenada Basin at that time. Sequence 2 lies unconformably over Sequence 1.- Sequence 3: Late Miocene to Recent arc-derived turbidites and pelagic sedimentation, with little detri-tal input from South America, due to the emplacement of the eastward drainage of the Orinoco River southof the northern Venezuelan coastal range during Middle Miocene. Sequence 3 lies unconformably over Sequence 2.Since the oldest syntectonic sediments date from the Eocene, the last tectonic event that shaped the present-daybasement topography dates back from the Eocene. Given the horizontality of Sequences 2 and 3 within the basinand along the Aves Ridge, no differential vertical motions occurred between the Grenada Basin and the AvesRidge since then. This raises questions about the subsidence mechanisms that led to the current depth of theoceanic crust in the southeastern Grenada Basin. By contrast, reflectors in Sequences 2 and 3 are bent upwardsalong the Lesser Antilles slope, reflecting the uplift of the Neogene Lesser Antilles arc. These observations will beintegrated in a future model for the evolution of the Lesser Antilles back-arc area, as part of the ANR GAARAntiproject