Bathymetric and multichannel seismic data collected during the Multipso cruise aboard the R/V Jean-Charcot show a compressive area on the eastern flank of the North Aoba intra-arc basin, located in the central part of the New Hebrides island arc zone. N170 trending faults and folds and NllO left lateral strike-slip faults are consistent with a N86 tectonic stress. This compression is the result of the' d'Entrecasteaux Ridge indenter which collides andlor subducts the New Hebrides central arc. Abridged English Version - Seabeam bathymetric, 24-channel seismic reflection ( 11, gravi- metric and magnetic data were collected during the Multipso cruise aboard the R. V. Jean- Charcot. The objective of the cruise was to survey the ODP sites on the New Hebrides central arc.

Singlechannel seismic-reflection profiles collected acros the convergent plate boundary in the central New Hebrides Arc display structures that can account for the observed absence of a deep geotectonic trench. These structures include (1) the protrusion of the Sanbhlalakula block at the leading edge of the overriding plate, (2) collision of ridges on the downgoing plate with this block, (3) slumping of sediment from the insular wall of the trench, and/or the tectonic displacement of material outward to compensate for adjacent indentation by collid- ing features, (4) compressive doming of the downgoing plate adjacent to the zone of plate contact, and (5) sedi- ment filling of incipient trenches. The superficial structures indicate that there are three major zones of dominant stress affecting the top of the downgoing plate in this region. North of the North D'Entrecasteaux Ridge and south of latitude 17 S, ten- sional stress regimes are related to the passive bending of the plate under the weight of the downgoing slab. There is weak coupling between the plates and subduction is mainly controlled by creep. Between the North D'Entrecasteaux Ridge and latitude 17 o S, a compressional stress regime indicates relatively strong coupling between the plates. The highest compressional stresses occur at the Bougainville Spur and probably at the North D'Entrecasteaux Ridge-collision points that can be related to localized shallow asperities in the Benioff zone. The transitions between these regions of contrasting stress regime that occur at latitude 17s and along the northern flank of the D'Entrecasteaux zone are probably areas of shearing transverse to the trench.

The west African continentrd mclrgin between Abidion (Ivory Coost) and Walvis scy (Southw-st Africa) wos surveyed in 1971 by the R/V Jeon Charcot. Fifty. eight seismic-reflection (flexotir source), bcthymetric (3.5 and 12 kc), grcvimetric, and magn-tic profi'cs wcre cb- toined. The seowird limit of cn evopxitic zon~ o>Jtiined during the survey is qt th:: boundcry b-.:vrxn continental s:ope and contin-xkl risc. Variolion oi depth of this lim't os o funztion of Iat:tude shows the presence of a large offset ot 11"s which sccms to be relalecl :o on cist-east.so.J:heost line of mcgnetic se2ma-Jnts cutting into thz ccntinen'cl slop?. The Annobon-Cr-mcroun volccnk axis sep2rat.s the solt zoccs ci the N'g,xicn basin and Congo-Ango'a bcsin. T!io n-:.thorn lim:t of solt deposi. tion in thc Congo Angolo basin is marked new 1"N by a s?ronq siuthwes!-north?cst mctgnctic I-cod. On lhe south it extends to new 14"s. The Walvis Ridgz, located forthor south, does nst seem to hwre cn'rffect on evoporite deposition.