Structure and evolution of the Gulf of Lions: The Sardinia seismic experiment and the GOLD (Gulf of Lions Drilling) project

International audienceThe study of the deep structure and evolution of passive continental margins is important for the understanding of rifting processes and the formation of associated sedimentary basins. Since the classical models of McKenzie (1978) and Wernicke (1985), understanding how passive continental margins form, that is to say mainly the way that continental lithosphere is thinned leading to subsidence, remains one of the main challenges in the Earth sciences. Many recent observations and discoveries have modified our basic views of margin formation. The conservational models paradigm (i.e., simple shear, pure shear, or polyphase models), which exclude exchanges between lower continental crust and upper mantle and which are usually proposed to explain lithospheric stretching and consequent crustal thinning of passive continental margins, fail to completely explain all these observations. Furthermore, these models imply a large amount of horizontal movement, movements not observed in the field. In consequence, new concepts need to be built and tested

Eocene intra-plate shortening responsible for the rise of a faunal pathway in the northeastern Caribbean realm

Intriguing latest Eocene land-faunal dispersals between South America and the Greater Antilles (northern Caribbean) has inspired the hypothesis of the GAARlandia (Greater Antilles Aves Ridge) land bridge. This landbridge, however, should have crossed the Caribbean oceanic plate, and the geological evolution of its rise and demise, or its geodynamic forcing, remain unknown. Here we present the results of a land-sea survey from the northeast Caribbean plate, combined with chronostratigraphic data, revealing a regional episode of mid to late Eocene, trench-normal, E-W shortening and crustal thickening by ∼25%. This shortening led to a regional late Eocene–early Oligocene hiatus in the sedimentary record revealing the location of an emerged land (the Greater Antilles-Northern Lesser Antilles, or GrANoLA, landmass), consistent with the GAARlandia hypothesis. Subsequent submergence is explained by combined trench-parallel extension and thermal relaxation following a shift of arc magmatism, expressed by a regional early Miocene transgression. We tentatively link the NE Caribbean intra-plate shortening to a well-known absolute and relative North American and Caribbean plate motion change, which may provide focus for the search of the remaining connection between ‘GrANoLA’ land and South America, through the Aves Ridge or Lesser Antilles island arc. Our study highlights the how regional geodynamic evolution may have driven paleogeographic change that is still reflected in current biology

Structure of the Lesser Antilles subduction forearc and backstop from 3D seismic tomography

In 2007 the Sismantilles II experiment was conducted to constrain structure and seismicity in the central Lesser Antilles subduction zone. The seismic refraction data recorded by a network of 27 OBSs over an area of 65 km×95 km provide new insights on the crustal structure of the forearc offshore Martinique and Dominica islands. The tomographic inversion of first arrival travel times provides a 3D P-wave velocity model down to 15 km. Basement velocity gradients depict that the forearc is made up of two distinct units: A high velocity gradient domain named the inner forearc in comparison to a lower velocity gradient domain located further trenchward named the outer forearc. Whereas the inner forearc appears as a rigid block uplifted and possibly tilted as a whole to the south, short wavelength deformations of the outer forearc basement are observed, beneath a 3 to 6 km thick sedimentary pile, in relation with the subduction of the Tiburon Ridge and associated sea floor reliefs. North, offshore Dominica Island, the outer forearc is 70 km wide. It extends as far as 180 km to the east of the volcanic front where it acts as a backstop on which the accretionary wedge developed. Its width decreases strongly to the south to terminate offshore Martinique where the inner forearc acts as the backstop. The inner forearc is likely the extension at depth of the Mesozoic magmatic crust outcropping to the north in La Désirade Island and along the scarp of the Karukera Spur. The outer forearc could be either the eastern prolongation of the inner forearc, but the crust was thinned and fractured during the past tectonic history of the area or by recent subduction processes, or an oceanic terrane more recently accreted to the island arc.Peer Reviewe

Tsunami hazards in the Catalan Coast, a low-intensity seismic activity area

The final publication is available at Springer via http://dx.doi.org/10.1007/s11069-017-2918-zThe potential impacts of tsunamis along the Catalan Coast (NW Mediterranean) are analysed using numerical modelling. The region is characterized by moderate to low seismic activity and by moderate- to low-magnitude earthquakes. However, the occurrence of historical strong earthquakes and the location of several active offshore faults in front of the coast suggest that the possibility of an earthquake-triggered tsunami is not negligible although of low probability. Up to five faults have been identified to generate tsunamis, being the highest associated possible seismic magnitudes of up to 7.6. Coastal flooding and port agitation are characterized using the Worst-case Credible Tsunami Scenario Analysis approach. The results show a multiple fault source contribution to tsunami hazard. The shelf dimensions and the existence of submerged canyons control the tsunami propagation. In wide shelves, waves travelling offshore may become trapped by refraction causing the wave energy to reach the coastline at some distance from the origin. The free surface water elevation increases at the head of the canyons due to the sharp depth gradients. The effects of potential tsunamis would be very harmful in low-lying coastal stretches, such as deltas, with a high population concentration, assets and infrastructures. The Ebro delta appears to be the most exposed coast, and about the 20% of the delta surface is prone to flooding due to its extremely low-lying nature. The activity at Barcelona port will be severely affected by inflow backflow current at the entrance of up to 2 m/s.Peer ReviewedPostprint (author's final draft

Limits of the seismogenic zone in the epicentral region of the 26 December 2004 great Sumatra-Andaman earthquake: Results from seismic refraction and wide-angle reflection surveys and thermal modeling

The 26 December 2004 Sumatra earthquake (Mw = 9.1) initiated around 30 km depth and ruptured 1300 km of the Indo-Australian Sunda plate boundary. During the Sumatra OBS (ocean bottom seismometer) survey, a wide angle seismic profile was acquired across the epicentral region. A seismic velocity model was obtained from combined travel time tomography and forward modeling. Together with reflection seismic data from the SeaCause II cruise, the deep structure of the source region of the great earthquake is revealed. Four to five kilometers of sediments overlie the oceanic crust at the trench, and the subducting slab can be imaged down to a depth of 35 km. We find a crystalline backstop 120 km from the trench axis, below the fore arc basin. A high velocity zone at the lower landward limit of the raycovered domain, at 22 km depth, marks a shallow continental Moho, 170 km from the trench. The deep structure obtained from the seismic data was used to construct a thermal model of the fore arc in order to predict the limits of the seismogenic zone along the plate boundary fault. Assuming 100C-150C as its updip limit, the seismogenic zone is predicted to begin 530 km from the trench. The downdip limit of the 2004 rupture as inferred from aftershocks is within the 350C 450C temperature range, but this limit is 210-250 km from the trench axis and is much deeper than the fore arc Moho. The deeper part of the rupture occurred along the contact between the mantle wedge and the downgoing plate

High-resolution 3D seismic investigations of hydrate-bearing fluid escape chimneys in the Nyegga region of the Voring Plateau, Norway

Hundreds of pockmarks and mounds, which seismic reflection sections show to be underlain by chimney-like structures, exist in southeast part of the Vøring plateau, Norwegian continental margin. These chimneys may be representative of a class of feature of global importance for the escape of methane from beneath continental margins and for the provision of a habitat for the communities of chemosynthetic biota. Thinning of the time intervals between reflectors in the flanks of chimneys, observed on several high-resolution seismic sections, could be caused by the presence of higher velocity material such as hydrate or authigenic carbonate, which is abundant at the seabed in pockmarks in this area. Evidence for the presence of hydrate was obtained from cores at five locations visited by the Professor Logachev during TTR Cruise 16, Leg 3 in 2006. Two of these pockmarks, each about 300-m wide with active seeps within them, were the sites of high-resolution seismic experiments employing arrays of 4-component OBS (Ocean-Bottom Seismic recorders) with approximately 100-m separation to investigate the 3D variation in their structure and properties. Shot lines at 50-m spacing, run with mini-GI guns fired at 8-m intervals, provided dense seismic coverage of the sub-seabed structure. These were supplemented by MAK deep-tow 5-kHz profiles to provide very high-resolution detail of features within the top 1-40 m sub-seabed. Travel-time tomography has been used to detail the variation in Vp and Vs within and around the chimneys. Locally high-amplitude reflectors of negative polarity in the flanks of chimneys and scattering and attenuation within the interiors of the chimneys may be caused by the presence of free gas within the hydrate stability field. A large zone of free gas beneath the hydrate stability field, apparently feeding several pockmarks, is indicated by attenuation and velocity pull-down of reflectors

Segmentation of the Nazca and South American plates along the Ecuador subduction zone from wide angle seismic profiles

We describe the deep structure of the south Colombian–northern Ecuador convergent margin using travel time inversion of wide-angle seismic data recently collected offshore. The margin appears segmented into three contrasting zones. In the North Zone, affected by four great subduction earthquakes during the 20th century, normal oceanic crust subducts beneath the oceanic Cretaceous substratum of the margin underlined by seismic velocities as high as 6.0–6.5 km/s. In the Central Zone the subducting oceanic crust is over-thickened beneath the Carnegie Ridge. A steeper slope and a well-developed, high velocity, Cretaceous oceanic basement characterizes the margin wedge. This area coincides with a gap in significant subduction earthquake activity. In the South Zone, the subducting oceanic crust is normal. The fore-arc is characterized by large sedimentary basins suggesting significant subsidence. Velocities in the margin wedge are significantly lower and denote a different nature or a higher degree of fracturing. Even if the distance between the three profiles exceeds 150 km, the structural segmentation obtained along the Ecuadorian margin correlates well with the distribution of seismic activity and the neotectonic zonation

Crustal structure of a young margin pair: New results across the Liguro–Provencal Basin from wide-angle seismic tomography

International audienceTomographic inversion of two wide-angle seismic profiles acquired during the Sardinia cruise (2006) on the conjugate Gulf of Lions–West Sardinia margins pair offers insight into the deep structure of this young basin. It is the first combined conjugate margins study in this area based on deep seismic data. Modelling of the two conjugate profiles reveals that the conjugate margins are symmetric in term of crustal velocity structure, with a partitioning in three similar regions: (1) a zone of low crustal vertical velocity gradients and a crustal thickness of 18 to 10 km, interpreted to be thinned continental crust, (2) a transitional zone characterised by high lower crustal velocities, non-typical of continental or normal oceanic crust, and (3) a 4- to 5-km-thick crust showing vertical velocity gradients and relative velocity ranges typical of oceanic crust. This latter region is interpreted to be a thin oceanic crust resulting from a tectonic heritage and the possible influence of a cool slab back-arc basin. Although the three regions are found on both sides of the margin, they are asymmetric in terms of the width of these domains, wide and smooth along the Gulf of Lions margin, and narrow and abrupt along its conjugate Sardinia side. The width of Region 2 is larger on the Gulf of Lions side (~ 90 km) than on the Sardinia side (~ 40 km). Crustal thickness in Region 3 is about 1 km thinner on the Sardinia side than on its conjugate. In Region 1 the crustal thickness is lower on the Sardinia side at a given distance from the shelf break. The differences in width of Region 2 and in crustal thickness might be due to the origin as a back-arc basin of the NW-Mediterranean basin, with the final breakup located closer to the Western Sardinia margin, also probably linked to the complex tectonic history of the region. Our results do not support a simple shear mechanism of the opening of the basin along a lithospheric detachment fault

Tsunami Hazard in La Reunion Island (SW Indian Ocean): Scenario-Based Numerical Modelling on Vulnerable Coastal Sites

Several major tsunamis have affected the southwest Indian Ocean area since the 2004 Sumatra event, and some of them (2005, 2006, 2007 and 2010) have hit La Réunion Island in the southwest Indian Ocean. However, tsunami hazard is not well defined for La Réunion Island where vulnerable coastlines can be exposed. This study offers a first tsunami hazard assesment for La Réunion Island. We first review the historical tsunami observations made on the coastlines, where high tsunami waves (2–3 m) have been reported on the western coast, especially during the 2004 Indian Ocean tsunami. Numerical models of historical scenarios yield results consistent with available observations on the coastal sites (the harbours of La Pointe des Galets and Saint-Paul). The 1833 Pagai earthquake and tsunami can be considered as the worst-case historical scenario for this area. In a second step, we assess the tsunami exposure by covering the major subduction zones with syntethic events of constant magnitude (8.7, 9.0 and 9.3). The aggregation of magnitude 8.7 scenarios all generate strong currents in the harbours (3–7 m s$^{-1}$) and about 2 m of tsunami maximum height without significant inundation. The analysis of the magnitude 9.0 events confirms that the main commercial harbour (Port Est) is more vulnerable than Port Ouest and that flooding in Saint-Paul is limited to the beach area and the river mouth. Finally, the magnitude 9.3 scenarios show limited inundations close to the beach and in the riverbed in Saint-Paul. More generally, the results confirm that for La Runion, the Sumatra subduction zone is the most threatening non-local source area for tsunami generation. This study also shows that far-field coastal sites should be prepared for tsunami hazard and that further work is needed to improve operational warning procedures. Forecast methods should be developed to provide tools to enable the authorities to anticipate the local effects of tsunamis and to evacuate the harbours in sufficient time when such an earthquake occursThis work was supported by the PREPARTOI project and by the LRC (Laboratoire de Recherche Commun Yves Rocard CEA-ENS Paris)