Hervé Nouzé

Tectonic history of northern New Caledonia Basin from deep offshore seismic reflection: Relation to late Eocene obduction in New Caledonia, southwest Pacific

New, high-quality multichannel seismic reflection data from the western New Caledonia offshore domain allow for the first time the direct, continuous connection of seismic reflectors between the Deep Sea Drilling Project 208 drill hole on the Lord Howe Rise and the New Caledonia Basin. A novel seismic interpretation is hence proposed for the northern New Caledonia Basin stratigraphy, which places the Eocene/Oligocene unconformity deeper than previously thought and revisits the actual thickness of the pre-Oligocene sequences. A causal link is proposed between the obduction of the South Loyalty Basin over New Caledonia (NC) and the tectonic history of the northern New Caledonia Basin. Here it is suggested that as the South Loyalty Basin was being obducted during early Oligocene times, the NC Basin subsided under the effect of the overloading and underthrusted to accommodate the compressional deformation, which resulted in (1) the uplift of the northern Fairway Ridge and (2) the sinking of the western flank of New Caledonia. This event also had repercussions farther west with the incipient subsidence of the Lord Howe Rise.

Observation and tentative interpretation of a double BSR on the Nankai slope

Seismic data collected during the French–Japanese KAIKO-Tokai cruise of R/V L’Atalante on the upper slope of the eastern Nankai margin reveal the simultaneous presence at two distinct depths below the seafloor of two bottom simulating reflector (BSR)-type reflectors. The upper BSR is traced as a continuous reflector over about 10 km. As water depth decreases from 850 m to 550 m, its depth below seafloor decreases from 200 m to 40 m. The lower BSR is traced at 50–100 m below the upper one. The two BSRs end abruptly near the summit of the Daichii-Tenryu Knoll into an area where the 3.5-kHz record suggests active gas expulsion through the seabed. The observed depth of the upper BSR fits the predicted one for the base of the methane gas hydrate stability zone as estimated from present temperature and pressure conditions at the seafloor and in the slope sediments. Thus, we interpret the upper BSR as an active methane hydrate BSR. We further suggest that the lower BSR is a residual hydrate-related BSR. This could have followed a recent migration of the base of the methane hydrate stability zone from the lower BSR to the upper one. As possible causes for this migration we discuss sea bottom warming and tectonic uplift. The BSR migration could have occurred as a response to a 1–2°C sea bottom warming or, with an equivalent effect, an event of fast uplift of the seafloor by about 90 m. We do not discard other interpretations of the lower BSR, such as an active hydrate-related BSR formed from a mixture of gases.

The crustal structure of the Moroccan continental margin from wide-angle and reflection seismic data

SUMMARY The Atlantic margin off Morocco with its neighbouring Jurassic oceanic crust is one of the oldest on earth. It is conjugate to the Nova Scotia margin of North America. The SISMAR marine seismic survey acquired deep reflection seismic data as well as wide-angle seismic profiles in order to image the deep structure of the margin, characterize the nature of the crust in the transitional domain and define the geometry of the synrift basins. We present results from the combined interpretation of the reflection seismic, wide-angle seismic and gravity data along a 440-km-long profile perpendicular to the margin at 33‐34 ◦ N, extending from nearly normal oceanic crust in the vicinity of Coral Patch seamount to the coast at El Jadida and approximately 130 km inland. The shallow structure is well imaged by the reflection seismic data and shows a thick sedimentary cover that is locally perturbed by salt tectonics and reverse faulting. The sedimentary basin thickens from 1.5 km on the normal oceanic crust to a maximum thickness of 6 km at the base of the continental slope. Multichannel seismic (MCS) data image basement structures including a few tilted fault blocks and a transition zone to a thin crust. A strong discontinuous reflection at 12 s two-way travel-time (TWT) is interpreted as the Moho discontinuity. As a result of the good data quality, the deep crustal structure (depth and velocity field) is well constrained through the wide-angle seismic modelling. The crust thins from 35 km underneath the continent to approximately 7 km at the western end of the profile. The transitional region has a width of 150 km. Crustal velocities are lowest at the continental slope, probably as a result of faulting and fracturing of the upper crust. Uppermantle velocities could be well defined from the ocean bottom seismometer (OBS) and land station data throughout the model.

Multiple bottom-simulating reflections in the Black Sea: Potential proxies of past climate conditions

Abstract A previously unknown pattern of multiple bottom-simulating reflections (BSRs) occurs on high-resolution reflection seismic data in the Danube deep-sea fan, associated with acoustic features indicating free gas. Our study provides evidence that this pattern is developed in relation with the architecture of distinct channel–levee systems of the Danube fan. Channel–levee systems hosting multiple BSRs act as relatively sealed gas-bearing systems whose top is situated above the base of the gas hydrate stability zone (BGHSZ). Inside these systems, free gas accumulates below the BGHSZ under a combined lithological, structural and stratigraphical control. The uppermost BSR marks the current equilibrium BGHSZ, for a gas composition of more than 99% methane. Model-derived depths of the BGHSZ for different gas compositions and pressure–temperature conditions show that multiple BSRs would correspond to the BGHSZ either for (1) layers of gas hydrates with high contents of heavy hydrocarbons or hydrogen sulphide, or (2) stable climatic episodes with temperatures between glacial values and the present-day conditions. As the gas hydrate compositions required by hypothesis (1) are in sharp contradiction with the general background of the gas composition in the study area, we suggest that multiple BSRs are most probably relics of former positions of the BGHSZ, corresponding to successive steps of climate warming. In this case, they can provide sea-bottom paleotemperature values for these episodes, and hence they are potential new proxies for deciphering past climate conditions.

Slope instabilities and gravity processes in fluid migration and tectonically active environment in the eastern Nankai accretionary wedge (KAIKO-Tokai'96 cruise)

Abstract One main feature imaged on the Nankai slope by using the SAR/PASISAR system (deep-tow single-channel seismic streamer) during the Franco-Japanese KAIKO-Tokai cruise in April 1996 is the gravitational displacement of large volumes of the sediment. This slope failure is assumed to be mainly related to an increase of the slope angle beyond its threshold value, tentatively linked to the uplift effect of the subduction of a paleo-Zenisu ridge and to the existence of a basal shear plane possibly controlled by altered properties of the sediments at the base of the gas hydrate stability field. The role of pore fluid (or free gas) is discussed but remains unknown without any in situ data, this role may be major if considering the gassy environment in the sedimentary cover where earthquake loading can cause excess pore pressure and act as a major triggering mechanism for sliding.

First sampling of gas hydrate from the Vøring Plateau

Methane hydrate is a clathrate, an ice-like solid formed from methane and water, that is stable under conditions of pressure and temperature found in most of the worlds oceans at depths greater than a few hundred meters. Hydrate occurs beneath the seabed where there is sufficient methane to exceed its solubility in water within the hydrate stability field. It has been speculated that methane released from hydrate by climate-induced changes in pressure and temperature escapes into the ocean and into the atmosphere, where its acts as a greenhouse gas. Further, methane from beneath the seabed is the primary energy source for communities of chemosynthetic biota at the seabed.

HIGH-RESOLUTION 3D SEISMIC INVESTIGATIONS OF HYDRATE- BEARING FLUID-ESCAPE CHIMNEYS IN THE NYEGGA REGION OF THE VØRING 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 Voring 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.

A geophysical study of a pockmark in the Nyegga region, Norwegian Sea

Over the last decade pockmarks have proven to be important seabed features that provide information about fluid flow on continental margins. Their formation and dynamics are still poorly constrained due to the lack of proper three dimensional imaging of their internal structure. Numerous fluid escape features provide evidence for an active fluid-flow system on the Norwegian margin, specifically in the Nyegga region. In June-July 2006 a high-resolution seismic experiment using Ocean Bottom Seismometers (OBS) was carried out to investigate the detailed 3D structure of a pockmark named G11 in the region. An array of 14 OBS was deployed across the pockmark with 1 m location accuracy. Shots fired from surface towed mini GI guns were also recorded on a near surface hydrophone streamer. Several reflectors of high amplitude and reverse polarity are observed on the profiles indicating the presence of gas. Gas hydrates were recovered with gravity cores from less than a meter below the seafloor during the cruise. Indications of gas at shallow depths in the hydrate stability field show that methane is able to escape through the water-saturated sediments in the chimney without being entirely converted into gas hydrate. An initial 2D raytraced forward model of some of the P wave data along a line running NE-SW across the G11 pockmark shows, a gradual increase in velocity between the seafloor and a gas charged zone lying at ~300 m depth below the seabed. The traveltime fit is improved if the pockmark is underlain by velocities higher than in the surrounding layer corresponding to a pipe which ascends from the gas zone, to where it terminates in the pockmark as seen in the reflection profiles. This could be due to the presence of hydrates or carbonates within the sediments.

Shallow bottom-simulating reflectors on the Angola margin, in relation with gas and gas hydrate in the sediments

Abstract Acoustic facies interpretation, high-resolution velocity analysis and amplitude versus offset modelling have been performed on high resolution seismic data acquired on the West African margin offshore Angola, in water depths of about 2000 m. The area has a complex structural, thermal and fluid-flow setting, in which sediments are affected by salt diapirism and faulting associated with sediment compaction. A discontinuous bottom-simulating reflector (BSR) at a depth of about 200 m below sea floor could mark the base of the gas hydrate occurrence zone, which does not always coincide with the top of the free gas zone. Within the gas hydrate stability zone, a shallow bottom-simulating reflector is observed at a depth of about 75 m below seafloor. This shallow bottom simulating reflector, that is termed ‘sheep back reflector’ (SR), correspond to a small amount of gas being trapped in the sediments. It could mark the top of the gas hydrate occurrence zone, where gas hydrate dissociation may occur. A reversed polarity reflector (R1) is also observed about 25 m below the sea floor. This reflector could correspond to a limit between normally compacted and underconsolidated sediments, possibly related to a permeability change in the sediments. Thus, the occurrence of excess pore pressure generated during gas hydrate dissociation could explain some subsurface sediment mobilization processes.

Nested Seismic Stratigraphy Using Profiles of Increasing Resolution

Appropriate type of seismic must be chosen to image geological targets (according to their depth and size). Here, we want to emphasize, that the use of nested seismic data of increasing resolution simultaneously enables a better seismic stratigraphic interpretation. To illustrate our purpose we will present a series of seismic profiles of different resolution but at the same position that enable a comparaison of different vertical scale and variable resolution (from Kilometric conventional seismic, to high resolution, very high resolution and ultra high resolution seismic data) coming from the same area in the Gulf of Lion.