Jacques Boulègue

Nankai Trough and Zenisu Ridge: a deep-sea submersible survey

Abstract Eight submersible dives between 3000 and 4200 m water depth were made off southern Japan in the eastern Nankai subduction zone. Benthic communities associated with chemosynthetic processes were discovered along the 800 m wide active tectonic zone, at the toe of the accretionary prism. A benthic community was also discovered along a zone of active compression, at the foot of Zenisu Ridge, 30 km south of Nankai Trough. Temperature measurements within the sediments below the benthic communities confirm that upward motion of interstitial water occurs there. Studies of water samples indicate advection of methane and light hydrocarbons. Specimens of the benthic community have been shown to have included in their shells carbonate resulting from methane consumption. Thus the benthic communities are related to overpressure-driven fluid advection along tectonic zones with active surface deformation. A 300 m high active scarp at the toe of the accretionary prism is related to relative motion in a 280° direction which is close to the 305° average direction of subduction in this area. The dives establish further that compressive deformation is presently occurring at the foot of Zenisu Ridge. The previous interpretation of the Zenisu Ridge as a zone of recent north-south intraplate shortening, 40 km south of the Nankai Trench, is confirmed. We conclude that tectonic evolution might well lead to future detachment of the Zenisu Ridge and overthrusting of this large piece of oceanic crust over the continental margin. Such a process might be an efficient one to emplace ophiolites over continents.

Mechanism of gold transfer and deposition in a supergene environment

Abstract The possibility of gold mobility in supergene environments is suggested by numerous indirect observations of gold particles. However, presently there are no dissolution or transport data based on the chemistry of actual weathering solutions. We have investigated models of gold transport by studying the water chemistry of a stream where weathering of auriferous minerals is occurring under supergene conditions in a temperate climate. Dissolved gold concentrations in the auriferous zone are forty times higher (200 pmolL−1) than in non-auriferous zones (5 pmolL−1). We have examined the potential gold ligands: thiosulfate, hydroxide, and chloride. Speciation calculations using the field data show that gold solubility is controlled by a gold thiosulfate complex: Au(S2O3)23−. The oxidation of the thiosulfate ligand induces the destabilization of the aurothiosulfate complex. Gold is then reduced to the metallic state as submicronsized suspended particles. This conclusion is supported by the measured increase in gold content of suspended particulates. Mass balance calculations performed with both dissolved and paniculate gold data permit the quantification of gold deposition along the stream.

Permeability changes due to mineral diagenesis in fractured crust: implications for hydrothermal circulation at mid-ocean ridges

Abstract The hydrothermal processes at ridge crests have been extensively studied during the last two decades. Nevertheless, the reasons why hydrothermal fields are only occasionally found along some ridge segments remain a matter of debate. In the present study we relate this observation to the mineral precipitation induced by hydrothermal circulation. Our study is based on numerical models of convection inside a porous slot 1.5 km high, 2.25 km long and 120 m wide, where seawater is free to enter and exit at its top while the bottom is held at a constant temperature of 420°C. Since the fluid circulation is slow and the fissures in which seawater circulates are narrow, the reactions between seawater and the crust achieve local equilibrium. The rate of mineral precipitation or dissolution is proportional to the total derivative of the temperature with respect to time. Precipitation of minerals reduces the width of the fissures and thus percolation. Using conventional permeability versus porosity laws, we evaluate the evolution of the permeability field during the hydrothermal circulation. Our computations begin with a uniform permeability and a conductive thermal profile. After imposing a small random perturbation on the initial thermal field, the circulation adopts a finger-like structure, typical of convection in vertical porous slots thermally influenced by surrounding walls. Due to the strong temperature dependence of the fluid viscosity and thermal expansion, the hot rising fingers are strongly buoyant and collide with the top cold stagnant water layer. At the interface of the cold and hot layers, a horizontal boundary layer develops causing massive precipitation. This precipitation front produces a barrier to the hydrothermal flow. Consequently, the flow becomes layered on both sides of the front. The fluid temperature at the top of the layer remains quite low: it never exceeds a temperature of 80°C, well below the exit temperature of hot vent sites observed at black or white ‘smokers’. We show that the development of this front is independent of the Rayleigh number of the hydrothermal flow, indicating that the mineral precipitation causes cold, diffusive vents. Finally, we present a model suggesting that the development of smokers is possible when successive tectonic/volcanic events produce a network of new permeable fissures that can overcome the permeability decrease caused by mineral precipitation. Such a model is consistent with recent seismic data showing hydrothermal vents located at seismologically active ridge segments.

The Japan Trench and its juncture with the Kuril Trench: cruise results of the Kaiko project, Leg 3

This paper presents the results of a detailed survey combining Seabeam mapping, gravity and geomagnetic measurements as well as single-channel seismic reflection observations in the Japan Trench and the juncture with the Kuril Trench during the French-Japanese Kaiko project (northern sector of the Leg 3) on the R/V “Jean Charcot”. The main data acquired during the cruise, such as the Seabeam maps, magnetic anomalies pattern, and preliminary interpretations are discussed. These new data cover an area of 18,000 km2 and provide for the first time a detailed three-dimensional image of the Japan Trench. Combined with the previous results, the data indicate new structural interpretations. A comparative study of Seabeam morphology, single-channel and reprocessed multichannel records lead to the conclusion that along the northern Japan Trench there is little evidence of accretion but, instead, a tectonic erosion of the overriding plate. The tectonic pattern on the oceanic side of the trench is controlled by the creation of new normal faults parallel to the Japan Trench axis, which is a direct consequence of the downward flexure of the Pacific plate. In addition to these new faults, ancient normal faults trending parallel to the N65° oceanic magnetic anomalies and oblique to the Japan trench axis are reactivated, so that two directions of normal faulting are observed seaward of the Japan Trench. Only one direction of faulting is observed seaward of the Kuril Trench because of the parallelism between the trench axis and the magnetic anomalies. The convergent front of the Kuril Trench is offset left-laterally by 20 km relative to those of the Japan Trench. This transform fault and the lower slope of the southernmost Kuril Trench are represented by very steep scarps more than 2 km high. Slightly south of the juncture, the Erimo Seamount riding on the Pacific plate, is now entering the subduction zone. It has been preceded by at least another seamount as revealed by magnetic anomalies across the landward slope of the trench. Deeper future studies will be necessary to discriminate between the two following hypothesis about the origin of the curvature between both trenches: Is it due to the collision of an already subducted chain of seamounts? or does it correspond to one of the failure lines of the America/Eurasia plate boundary?

Identification of some sulphur species in a high organic sulphur coal

Abstract In this paper, a preliminary study is described concerning the characterization of sulphur forms in a subbituminous coal rich with organic sulphur deposited in a lacustrine carbonate environment (Upper Cretaceous, Provence, France). Two American high organic sulphur coals were studied for comparison with the Provence coal. Optical microscopy, scanning electron microscopy and electron microprobe approaches offered a global view of the relations between sulphur, metals and coal petrography. Sulphur occurs in all the macerals and most minerals. Vitrinite contains the major part of organic sulphur and metals. This common occurrence of organic sulphur and metals in vitrinite suggests the presence of organosulphur-metallic species. X-Ray photoelectron spectrometry showed that mineral and organic sulphur is very much reduced (divalent). It would mainly include pyrite, sulphides and thiophenes. Programmed temperature oxidation and programmed temperature reduction (Attars test) revealed fragile sulphur compounds such as aliphatic thiols and sulphides and thermal stable sulphur compounds such as aromatic and high molecular weight compounds. Curie-point pyrolysis in combination with mass spectrometry, gas chromatography, and gas chromatography-mass spectrometry indicated the absence of free organic sulphur compounds and of elemental sulphur. Pyrolysis yielded large amounts of low molecular weight products (H 2 S, COS, etc.) and smaller amounts of thiopenes, benzothiophenes, dibenzothiophenes and their alkylated homologues. These sulphur compounds could result from the thermal degradation of organic sulphur moieties of the coal as well as from secondary reactions.

Deep scientific dives in the Japan and Kuril Trenches

Abstract In the summer of 1985, during the French-Japanese Kaiko program, ten dives to depths of 6000 m in the Japan and Kuril Trenches were made in the newly launched submersible “Nautile”. The sites of the dives were selected on the basis of surface geophysical surveys made during the preceding summer involving Seabeam mapping, geomagnetic and gravimetric measurements, and single-channel seismic profiling. The results of the dives provide new constraints on the geodynamics of these subduction zones. In the Japan and Kuril Trenches huge slump scars were observed on the landward slopes of the trenches. Slumps produce a typical active erosional morphology with vertical or even overhanging cliffs in poorly consolidated material. The slump scars allowed us to observe the internal structure of the margin; the monoclinal structure on the northern Japan Trench margin deduced from the seismic profiles and DSDP drilling was confirmed. Several dives on Kashima Seamount confirmed that this volcano has recently been split into two parts by a normal fault system. Comparisons of lithology and paleontology on the two separated parts of the seamount were made. Deep-sea clams colonies were observed from nearly 6000 m up to 5000 m on the landward slopes of the trenches. It can be concluded that the whole margin is venting fluids from depths of 2–3 km which is consistent with the indications of overpressure observed in drill sites on the Japan Trench margin. The fluids probably originate by dewatering of the subducting sediments and then migrate to the seafloor.

Trinidad mud volcanoes: where do the expelled fluids come from?

Abstract In order to establish the origin of fluids expelled from mud volcanoes in Trinidad, we analyzed their major-element, trace-element, and isotopic (H, O, Sr) compositions. The mineralogical, chemical, and Sr isotope compositions of associated expelled muds were also determined. On the basis of their chemical and Sr isotope compositions, the fluids were divided into two groups—those southwest and northeast of a major right lateral wrench fault (the Los Bajos fault) that both controls the chemical quality of the fluids and acts as a drain. Strontium isotopes were derived via mixing between a radiogenic source (0.71135) and a nonradiogenic source (≤0.70671) for both southwest and northeast groups. However, the nonradiogenic source possibly feeding the northeast group showed a lower Sr concentration than that of the southwest group. H and O isotope data show that the fluids became enriched in δ 18 O through interaction with wall rocks. The fluids were originally oceanic, but their properties were subsequently modified by diagenesis as evidenced by chemical data, and mixing between fluids issued from two deep-seated reservoirs and surface aquifer end-members. The gas phase expelled with the mud and the fluid through the mud volcanoes is composed mainly of methane with minor carbon dioxide. The amount of expelled gas seems the same in samples from both sides of the Los Bajos fault. The almost unique methane content of the gas phase and the large positive δ 18 O shifts of the reservoir end-member, as well as B, Li, and Ba contents, reveal that the fluids from the deep-seated reservoirs inherited their chemical compositions at high-temperature fluid–rock interactions. Estimates of equilibrium temperatures of the two deep-seated reservoirs suggest that, to acquire the estimated higher temperature (150°C), the mud volcanoes must have been fed partly by a reservoir located at a depth of more than 3 km. This depth corresponds to previous geologic information, which located the deep-seated reservoir in Miocene sediments at such a comparable depth. The mixing of the original fluids with a less deep-seated reservoir and meteoric waters on the ascending path suggests the presence of a recharge mechanism and contributes to the dynamics of the expulsion itself.

Normal faulting of the Daiichi-Kashima Seamount in the Japan Trench revealed by the Kaiko I cruise, Leg 3

Abstract A detailed topographic and geophysical survey of the Daiichi-Kashima Seamount area in the southern Japan Trench, northwestern Pacific margin, clearly defines a high-angle normal fault which splits the seamount into two halves. A fan-shaped zone was investigated along 2–4 km spaced, 100 km long subparallel tracks using narrow multi-beam (Seabeam) echo-sounder with simultaneous measurements of gravity, magnetic total field and single-channel seismic reflection records. Vertical displacement of the inboard half was clearly mapped and its normal fault origin was supported. The northern and southern extensions of the normal fault beyond the flank of the seamount were delineated. Materials on the landward trench slope are displaced upward and to sideways away from the colliding seamount. Canyons observed in the upper landward slope terminate at the mid-slope terrace which has been uplifted since start of subduction of the seamount. Most of the landward slope except for the landward walls aside the seamount comprises only a landslide topography in a manner similar to the northern Japan Trench wall. This survey was conducted on R/V “Jean Charcot” as a part of the Kaiko I cruise, Leg 3, in July–August 1984 under the auspices of the French-Japanese scientific cooperative program.

Chemical weathering of basaltic lava flows undergoing extreme climatic conditions: the water geochemistry record

This study was dedicated to the early stage of the weathering of historic basaltic flows located in Mount Cameroon. The combination of high relief (i.e. 0 to 4071 m) and high rainfall range (i.e. 1.8 to 12 m/year) lead to strong climatic contrast. Spring and rivers were sampled all around the volcano. We report here the basic chemistry of the waters as well as strontium and uranium isotopic ratios. The combination of the molar proportions of solute obtained with the modal amounts of the minerals in the basalts gives a prediction of what should be the relative molar concentrations of major compounds in the weathering waters issuing from Mount Cameroon. The measured Alkalinity/Si and Mg/Si ratios are higher than the calculated ones while the measured Ca/Si ratio is equal to the calculated value. We suggest that the Si-poor waters of Mount Cameroon are due to biological pumping, trapping of Si in Fe-silicate minerals such as Si containing ferrihydrite and Si interaction with bacterial cell wall leading to the formation of allophane type minerals which were observed in Mount Cameroon soil profiles. Calcium uptake by plants explains the lower Ca/alkalinity ratios measured in the water samples. The water–rock ratio (R) calculated from the strontium isotopic compositions of the water samples, ranges from 29,452 to 367,450. The calculated weathering rates (WR) range from 1 to 20 mm/ky and from 1 to 103 mm/ky for high and low elevations, respectively, and agree with both the thickness and the age of paleosoils found in the same area and with previously published estimates from coupled reaction-transport models. This difference emphasizes the role of vegetation and rainfall at lower elevations as compared to what happens at high elevations.

Stable isotopic ratios and origins of the carbonates associated with cold seepage at the eastern Nankai Trough

Two types of cold-seeps were located at the eastern Nankai Trough: (1) seeps on sediment-covered seafloor of 3800-3600 m in depth at the toe of the accretionary prism, and (2) those between 1900 and 2100 m, situated on the backthrust of the accretionary prism covered with rough outcrops of calcareous sandstones. Fragments of sandstone pipes or chimneys cemented with calcite were recovered from type-1 seeps, whereas in type-2 seeps, massive carbonate-cemented sandstones and silty sandstones were obtained, many of which contain cemented fragments of different lithologies and textures, including dense aggregates of fossilCalyptogena-type clam shells. The carbon isotopic ratios of the carbonates in these sediments range from −36 to −56‰ PDB. The carbonates formed by mixing between isotopically light carbonate ions derived from biogenic methane carried from depth in advecting fluids and heavy marine sedimentary carbonates. Sharp isotopic gradients in carbon, and sometimes also in oxygen, from the mixing processes are still retained across the walls of the sandstone pipes from type-1 seeps, whereas the isotopic ratios of the samples from type-2 seeps exhibit variation patterns related to lithologies and textures, and confirm the presence of several generations of carbonate cement within a single specimen. These facts and the oxygen isotopic ratios of live and fossil clam shells recovered from the survey areas suggest that the cold-seeps started at least during the last glaciation and have experienced repeated destruction and reformation due to episodic supply of methane-rich fluid of different isotopic ratios and/or temperature.