Evolution of depositional and slope instability processes on Bryant Canyon area, Northwest Gulf of Mexico

Bryant and Eastern Canyon systems are located on the northwest Gulf of Mexico, and they are characterized by a very complex sedimentological history related to glacioeustatic cycles, river discharges, and interactions of depositional and halokinetic processes. Both canyon systems were active during the low sea-level stand of Oxygen Isotope Stage 6, and provided the pathways for the transport of enormous amounts of sediments on the continental slope and abyssal plain of the northwest Gulf of Mexico. Right after their abandonment, at the beginning of Stage 5, salt diapirs encroached into the canyons, and resulted in their transformation into a network of intraslope basins. The transformation of the canyons resulted in the generation of massive sediment failures. The mid-shelf (Stages 4 and 3) to shelf edge (Stage 2) lowering of the sea-level during the last glacial episode resulted in: 1) extensive river-sourced deposits on the outer shelf and/or upper continental slope that contributed in a seaward mobilization of the underlying salt masses, and 2) the generation of numerous gravity flows and turbidity currents on the outer shelf/upper continental slope. The seaward mobilization of the salt masses resulted in the oversteepening of the flanks of the basins, and consequently in the generation of numerous and massive sediment failures. The turbidity currents were confined on the intraslope basins of the upper continental slope, depositing their coarsest material. However, their most diluted upper and end members were able to continue their downslope propagation depositing characteristic fine-grained turbidites. The frequency of the turbidity currents was highly increased during the last glacial maximum (Stage 2), and three short melt-water pulses centered at 30.5, 36, and 52 ky B.P. The last deglaciation event is characterized by the development of a major melt water event that resulted in the deposition of distinct organic rich sediments, similar to the sapropels of the Eastern Mediterranean. At about 11 ky B.P. the melt water discharges of the North America switched from Mississippi River to St Lawrence Seaway, causing the domination of hemipelagic sedimentation on the continental slope of the northwest Gulf of Mexico

Influence of bottom currents on the sedimentary processes at the western tip of the Gulf of Corinth, Greece

We investigated the sedimentary processes that were active during the Holocene in the Gulf of Corinth, using high-resolution seismic reflection profiles and gravity cores. Seismic reflection data clearly show the presence of shallow-water sediment drifts at the western end of the Gulf, close to the Rion Sill that links the gulf to the Ionian Sea. Short cores indicate that drifts are composed of homogenous bioturbated mud in their upper part. The drift deposits flank a wide central area where the sea floor is eroded and where pre-Holocene deposits locally outcrop. The sea floor morphology in this area is marked by furrows oriented in different directions and by a depression attributed to the action of bottom-currents. The magnetic fabric of sediment samples from the drift, shelves, sub-basins and from the basin floor show a significant anisotropy and a similar orientation of Kmax axes along core. The largest anisotropy (P = 1.043 ± 0.007) is observed in the drift and is interpreted as resulting from the action of bottom currents. The similar orientation of Kmax axes in the other cores, collected from areas East of the drifts, suggests that bottom currents also affect sediment deposition in the rest of the study area, even if seismic profiles and core analyses demonstrate that gravitational processes such as submarine landslides and turbidity currents exert the main control on sediment transport and deposition. Average Kmax axes for four cores were reoriented using the declination of the characteristic remanent magnetization. Kmax axes show variable orientations relatively to the slope of the sea floor, between along-slope and roughly parallel to the contour lines.SISCO

Basement depth and sedimentary infill from deep seismic reflection data at the western tip of the offshore Corinth Rift

The Corinth rift is a young continental rift located in central Greece. The active part of the rift forms an E-W striking depression – the Gulf of Corinth – that is the deepest in its central part. Extensive seismic surveys have imaged the basin's basement and allowed to estimate the total extension across most of the Gulf except its western tip. Extension is high in the central part and decreases westward and eastward, as reflected in the present-day bathymetry. Two decades of GPS measurements have shown that the extension rate increases westwards from ~5 to 10-15 mm yr-1, but this is not consistent with the long term pattern. However, no data allowed so far to estimate the basement depth at the western tip of the Gulf, where the geodetic extension rate is the largest. Such data would allow to check the apparent inconsistency between the present rate and the long-term estimates of crustal extension. We present here an unpublished multichannel seismic line dating from 1979 and crossing the western tip of the Gulf of Corinth. The line is 22 km long and strikes WNW-ESE, from the Mornos delta to the West-Channel fault. A Maxipulse source has been used, allowing to image the basement below the synrift sedimentary infill. To the east, a ~1.6 km deep basin is imaged between the southern margin of the Gulf and an inactive south-dipping fault located between the Aigion and the Trizonia faults. The sedimentary infill consists in an alternation between basin-focused bodies made of incoherent reflections and more extensive high-amplitude reflectors. Attributing this alternation to eustatic variations give an age of 300-350 ka to the oldest well imaged deposits. Northwest of the Trizonia fault, the basement is imaged at shallower depth, i.e. ~450 m. The western tip of the seismic line reaches the Mornos delta, close to the northern shoreline. There, the depth to the basement is larger, reaching ~1.2 km. The infill is made of 3 units : on the basement lies a thin unit of incoherent reflections that may corresponds to coarse-grained fluvial deposits. A second unit of parallel, high-amplitude, low-frequency reflections could represent deeper-water deposits. The last seismic unit represents the Mornos delta coarse-grained deposits, from 0 to ~0.7 km deep. The depth of the basement deduced from this seismic line at the western tip of the Gulf of Corinth (1.2-1.6 km) is shallower than the one in the central part of the Gulf (2.5-3 km). This reinforce the inconsistency between long-term and short-term rates of extension in the Corinth Rift, which may be explained by assuming that the Western Corinth Rift initiated much later than the Central Rift. These data also allow to constrain the total displacement on the N-dipping Psathopyrgos fault, one of the major, normal, basin-bounding faults at the western tip of the Rift. The total offset would reach 2.1-2.3 km and the uplift/subsidence ratio would be ~1:1.7, implying a slip rate of 2.2-2.5 mm yr-1 based on footwall uplift rate data

Sedimentary infill at the western tip of the Gulf of Corinth during the last 120 ka: Evidence for an acceleration of the subsidence

The Corinth Rift, Greece, is a young and active continental rift stretching over 150 km between the cities of Patras and Athens, and is partly covered by the sea forming the Gulf of Corinth. The present study is focused on the western tip of the Gulf, west of the town of Aigion, where the extension rate measured by GPS is the highest, reaching 14-16 mm/yr. The sediments were investigated using seismic reflection profiling (600 km) to characterize the evolution over the last 120 ka of the sedimentation, subsidence and faulting activity. We combined two lines of evidence, the position of lowstand deltas and isopach maps. The isopach maps were built using two stratigraphic markers could be traced through the seismic grid, the most recent one corresponds to the last post-glacial transgression and the antecedent one to MIS 6 / MIS 5 transgression, at ca. 130 ka. The related isopach maps evidence a spatial change in sedimentary infill along the rift axis probably related to a decrease in activity of the south-dipping faults (i.e. Trizonia/Mornos Faults) that formed the northern edge of the westernmost Corinth Rift in an early stage of the rifting. The different identified lowstand fluvio-deltaic deposits are related to global sea-level lowstands during which the Gulf of Corinth was a lake, whose last reconnection to the Sea occurred around 11.5 ka. Concerning lowstand deltas formed around 11.5 ka, the subsidence rates exceed 3 mm/yr and are maximal under the apex of the Mornos fan-delta (5.0-6.6 mm/yr) and in the hanging wall of the north-dipping Lambiri fault (5.9-7.5 mm/yr). Regarding the anterior lowstand delta, the subsidence was lower ranging from 1 to 2.7 mm/yr. These changes would arise because of the northward migration of the strain toward the north, e.g. the Marathias fault.Ruptures Sismiques du Rift de Corinth

Active faulting at the western tip of the Gulf of Corinth, Greece, from high-resolution seismic data

The Gulf of Corinth is one of the fastest-spreading intra-continental rifts on Earth. GPS data indicate that the rift is currently opening in a NNE-SSW direction, with a rate of extension reaching up to 16 mm yr-1 in its westernmost part. Although the rest of the offshore rift has been well studied, the western tip of the rift is still poorly explored. We present an accurate map of submarine faults in this area based on two high-resolution seismic reflection surveys (single channel sparker). In the eastern part of the studied area, the sedimentary infill is affected by the known North Eratini, South Eratini and West Channel faults. Further to the west, the seafloor is mostly flat, and is bounded to the north by the normal, south-dipping, Trizonia fault. To the north, the shallower part of the Gulf shows to the east a diffuse pattern of normal and strike-slip deformation which is replaced to the west by a 7.5 km long SE striking strike-slip fault zone, called the Managouli fault zone. To the westernmost tip of the Gulf, in the Nafpaktos Basin, two fault sets with different strikes are encountered; the one with aNE-SW strike exhibits a clear strike-slip component. The western tip of the Gulf of Corinth is the only part of the Corinth Rift where convincing evidence for strike-slip movement has been found. This fault pattern is likely related to the complex deformation occurring at the diffuse junction at the western tip of the Rift between three crustal blocks: Continental Greece, Peloponnese, and the Ionian Island-Akarnia block

Late Quaternary paleoseismic sedimentary archive from deep central Gulf of Corinth: time distribution of inferred earthquake-induced layers

A sedimentary archive corresponding to the last 17 cal kyr BP has been studied by means of a giant piston core retrieved on board R/V MARION-DUFRESNE in the North Central Gulf of Corinth. Based on previous methodological improvements, grain-size distribution and Magnetic Susceptibility Anisotropy (MSA) have been analysed in order to detect earthquake-induced deposits. We indentified 36 specific layers -Homogenites+Turbidites (HmTu) - intercalated within continuous hemipelagictype sediments (biogenic or bio-induced fraction and fine-grained siliciclastic fraction). The whole succession is divided into a non-marine lower half and a marine upper half. The “events” are distributed through the entire core and they are composed of two terms: a coarse-grained lower term and an upper homogeneous fine-grained term, sharply separated. Their average time recurrence interval could be estimated for the entire MD01-2477 core. The non-marine and the marine sections yielded close estimated values for event recurrence times of around 400 yrs to 500 yrs.</p

Sédimentation et géométrie des failles à l'extrémité ouest du golfe de Corinthe, Grèce

The Gulf of Corinth is one of the fastest-spreading intracontinental rift on Earth, a 120km long E-W structure propagating westward toward the Aegean subduction zone. Present day kinematics (GPS data) indicates an opening direction oriented NNE-SSW and an opening rate increasing westward from 11 mm y-1 in the central part to 16 mm y-1 in the westernmost part. The high extension rate in the western part of the rift would imply a high seismic hazard if faults are not creeping. Our work concerns this western extremity of the Gulf of Corinth, for which we propose an accurate map of submarine faults as well as first chronostratigraphic interpretations. The map is based on two high-resolution seismic reflection surveys (single channel sparker) performed aboard HCMR’s R/V ALKYON, within the frame of SISCOR ANR Project. About 600 km of seismic lines were acquired, with a 200 msTWTT maximum penetration. We identified last glacial maximum (LGM) lowstand erosion surfaces along the northern coast. They made possible the mapping of post-LGM sediment thickness as well as estimates of subsidence rates. Depocenters location is controlled by river deltas where up to 75m of post-LGM sediments are stored. Numerous, up to 15m thick, mass transport deposits fill the central and eastern parts. Seafloor erosion is observed on 7.5 km2 in the western part, involving action of marine currents. The northern coast is subsiding between 1.7 and 2.2 mm y-1. We also mapped the following fault network described from east to west. In the eastern part, the sedimentary infill is faulted by the known North Eratini, South Eratini and West Channel faults. At the longitude of the Trizonia Island, the seafloor is mainly horizontal and the only fault is the south dipping Trizonia fault. Between the Trizonia Island and the Mornos Delta, the shallower northern part of the gulf shows a diffuse pattern of deformation with faults striking mainly E-W and ESE-WNW. In the southern part of the rift, no fault has been observed between the Psatopyrgos fault bounding the southern side of the Gulf and the Mornos Delta. To the West, between the Mornos Delta and the Rion Straits, three main south dipping, normal and oblique faults have been identified. This NE-SW striking fault system could be part of a local transfer zone linking the Patras and the Corinth Basins, or of the NE-SW right-lateral slip fault system interconnecting the Gulf of Corinth to the Kephalonia transform Fault and the Hellenic subduction