Hélène Ondréas

Architecture of an active mud-rich turbidite system: The Zaire Fan (Congo-Angola margin southeast Atlantic): Results from ZaïAngo 1 and 2 cruises

Multichannel seismic data newly acquired during two ZaAngo surveys now provide an almost complete view of the Quaternary architecture of the Zaire Fan. Extending laterally from the southern Gabon margin to the Angola margin and longitudinally more than 800 km, the overall fan consists of three main individual fans that were deposited successively as overlapping depocenters. The individual fans are composed of channel/levee systems exhibiting similar seismic facies, external configurations, and organization to those described in other large mud-rich systems (e.g., the Amazon Fan). In particular, high-amplitude reflection units with a high oil-reservoir potential are recognized almost systematically as a basal sole for channel/levee systems. They possibly include true high-amplitude reflection packets related to avulsion processes below the avulsion points and coarse-grained basal levees related to the initial stages of levee aggradation subsequent to the avulsion. Correlations with Ocean Drilling Program Leg 175 Site 1077 indicate that the studied part of the Zaire Fan began to build in the late Pleistocene (780 ka). During the upper Quaternary, a great number of channel/levee systems (more than 80) were developed, possibly explained either by its permanent activity even during high sea level conditions or by the low Zaire River inputs. The frequent occurrence of channel entrenchment of either old or recent channels is another characteristic specific to the fan. Overdeepening of channels is probably partly caused by regressive erosion inside the parent channel in response to an avulsion and also in part because of other causes that are not fully understood.

Detailed Study of Three Contiguous Segments of the Mid-Atlantic Ridge, South of the Azores (37° N to 38°30′ N), Using Acoustic Imaging Coupled with Submersible Observations

Using a new tool of seafloor characterisation (sonar images from FARA-SIGMA cruise; Needham et al., 1992), coupled with submersible observations (DIVA1 cruise) we compare, at different scales of observation, three contiguous segments of the Mid-Atlantic Ridge, South of the Azores Triple Junction, between 37° N and 38°30′ N.The two northernmost segments (‘38°20′ N’ and Menez-Gwen) show unusual morphological features for the MAR; the rift valley is absent and the present-day magmatism is focused on shallow axial volcanoes. On the third segment (Lucky Strike), the morphology is the one usually found on the MAR. On the Menez-Gwen and ‘38°20′ N’ segments, volcanic constructional activity can obliterate, during periods of high magmatic supply, the morphology inherited from tectonic activity. The dive results constrain the recent evolution of each segment and show that a temporal variability in volcanic dynamics exists. On the three segments, outcrops of eruptive lavas alternate with large areas of explosive volcanic ejecta. This cycle in volcanic activity is influenced by changes in water depth, both spatially (i.e. between segments) and temporally (i.e. for the same segment through time).Each segment has known a specific history in its accretionary processes with a succession of tectonic and volcanic predominance and changes in its volcanic phases between volcanic ejecta and effusive dynamics.The hydrothermal activity is focused at the central part of each segment and is controlled by the presence of fresh lava and major tectonic features.

Geodiversity of Hydrothermal Processes Along the Mid‐Atlantic Ridge and Ultramafic‐Hosted Mineralization: a New Type Of Oceanic Cu‐Zn‐Co‐Au Volcanogenic Massive Sulfide Deposit

OS21C-08. Eberhart, G. L., P. A. Rona, and J. Honnorez (1989), Geologic controls of hydrothermal activity in the Mid-Atlantic Ridge rift valley; tectonics and volcanics, Mar. Geophys. Res., 10(3-4), 233–259. Edmond, J. M., A. C. Campbell, M. R. Palmer, G. P. Klinkhammer, C. R. German, H. N. Edmonds, H. Elderfield, G. Thompson, and P. Rona (1995), Time series studies of vent fluids from the TAG and MARK sites (1986, 1990) Mid-Atlantic Ridge: a new solution chemistry model and a mechanism for Cu/Zn zonation in massive sulphide orebodies, in Hydrothemal Vents and Processes, edited by L. M. Parson et al., pp. 77–86, Geol. Soc., London. Edmonds, H. N., P. J. Michael, E. T. Baker, D. P. Connelly, J. E. Snow, C. H. Langmuir, H. J. B. Dick, R. Muhe, C. R. German, and D. W. Graham (2003), Discovery of abundant hydrothermal venting on the ultraslow-spreading Gakkel ridge in the Arctic, Nature, 421(6920), 252–256. Elderfield, H., et al. (1993), Preliminary geochemical results from the Broken Spur hydrothermal field, 29° N, Mid-Atlantic Ridge, Eos Trans. AGU, 74(43), Fall Meet. Suppl., 99. Escartín, J., and M. Cannat (1999), Ultramafic exposures and the gravity signature of the lithosphere near the Fifteen-Twenty fracture zone (Mid-Atlantic Ridge, 14°–16.5° N), Earth Planet. Sci. Lett., 171(3), 411–424. Escartín, J., and J. Lin (1998), Tectonic modification of axial crustal structure; evidence from spectral analyses of residual gravity and bathymetry of the Mid-Atlantic Ridge flanks, Earth Planet. Sci. Lett., 154(1-4), 279–293. Escartín, J., D. K. Smith, J. Cann, H. Schouten, C. H. Langmuir, and S. Escrig (2008), Central role of detachment faults in accretion of slow-spreading oceanic lithosphere, Nature, 455(7214), 790–794. Fouquet, y. (1997), Where are the large hydrothermal sulphide deposits in the oceans?, Philos. Trans. R. Soc. London, Ser. A, 355(1723), 427–440. Fouquet, y., et al. (1993a), Sulfide mineralizations associated with ultramafic rocks on the MAR near 15° 20′N, Terra Nova Abstr., 5, suppl. 1, 444–445. Fouquet, y., U. von Stackelberg, J. L. Charlou, J. Erzinger, P. M. Herzig, R. Muehe, and M. Wiedicke (1993b), Metallogenesis in back-arc environments; the Lau Basin example, Econ. Geol., 88(8), 2150–2177. Fouquet, y., A. Wafik, P. Cambon, C. Mevel, G. Meyer, and P. Gente (1993c), Tectonic setting and mineralogical and geochemical zonation in the Snake Pit sulfide deposit (Mid-Atlantic Ridge at 23° N), Econ. Geol., 88(8), 2014–2032. Fouquet, Y., J. L. Charlou, I. Costa, J. P. Donval, J. Radford-Knoery, H. Pellé, H. Ondréas, N. Lourenço, M. Ségonsac, and M. KingstonTivey (1994), A detailed study of the Lucky Strike hydrothermal site and discovery of a new hydrothermal site: Menez Gwen. Preliminary results of the DIVA1 cruise (5–29 May, 1994), InterRidge News, 3(2), 14–18. Fouquet, Y., H. Ondréas, J. L. Charlou, J. P. Donval, J. RadfordKnoery, I. Costa, N. Lourenço, and M. K. Tivey (1995), Atlantic lava lakes and hot vents, Nature, 377, 201. Fouquet, Y., R. Knott, P. Cambon, A. Fallick, D. Rickard, and D. Desbruyeres (1996), Formation of large sulfide mineral deposits along fast spreading ridges; example from off-axial deposits at 12° 43′N on the East Pacific Rise, Earth Planet. Sci. Lett., 144(1-2), 147–162. Fouquet, y., et al. (1997), Discovery and first submersible investigations on the Rainbow Hydrothermal Field on the MAR (36°14N), Eos Trans. AGU, 78(46), Fall Meet. Suppl., F832. Fouquet, y., et al. (1998a), FLORES diving cruise with the Nautile near the Azores. First dives on the Rainbow field: Hydrothermal seawater/mantle interaction, InterRidge News, 7(1), 24–28. Fouquet, y., K. Henry, R. Knott, and P. Cambon (1998b), Geochemical section of the TAG hydrothermal mound, in TAG: Drilling an Active Hydrothermal System on a Sediment-Free 362 ULTRAMAFIC-HOSTED SULFIDE MINERALIZATION ALONG THE MAR Slow-Spreading Ridge, edited by P. M. Herzig et al., Proc. Ocean Drill. Program Sci. Results, 158, 363–388. Fouquet, y., et al. (2000), Hydrothermal processes in oceanic ultramafic environments; the Rainbow hydrothermal sulfide deposit, paper presented at 31st International Geological Congress, Int. Union of Geol. Sci., Rio de Janeiro, Brazil. Fouquet, Y., G. Cherkashov, J. L. Charlou, H. Ondréas, M. Cannat, N. Bortnikov, S. Silantyev, J. Etoubleau, and P. Serpentine (2007), Diversity of ultramafic hosted hydrothermal deposits on the Mid Atlantic Ridge; first submersible studies on Ashadze, Logatchev 2 and Krasnov vent fields during the Serpentine cruise, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract T51F-03. Fouquet, y., et al. (2008), Serpentine cruise–ultramafic hosted hydrothermal deposits on the Mid Atlantic Ridge: First submersible studies on Ashadze 1 and 2, Logatchev 2 and Krasnov vent fields, InterRidge News, 18, 15–19. Fournier, R. O., R. J. Rosenbauer, and J. L. Bischoff (1982), The Solubility of quartz in aqueous sodium chloride solution, Geochim. Cosmochim. Acta, 46, 1975–1978. Francheteau, J., et al. (1979), Massive deep-sea sulfide ore deposits discovered on the East Pacific Rise, Nature, 277, 523–528. Fruh Green, G. L., D. S. Kelley, S. M. Bernasconi, J. A. Karson, K. A. Ludwig, D. A. Butterfield, C. Boschi, and G. Proskurowski (2003), 30,000 years of hydrothermal activity at the Lost City vent field, Science, 301(5632), 495–498. Gaal, G., and J. Parkkinen (1993), Early Proterozoic ophiolitehosted copper-zinc-cobalt deposits of the Outokumpu type, in Mineral Deposit Modeling, edited by R. V. Kirkham et al., pp. 335–341, Geol. Assoc. of Canada, Toronto, Ont., Canada. Gablina, I. F., N. N. Mozgova, y. S. Borodaev, T. V. Stepanova, G. A. Cherkashev, and M. I. Il’in (2000), Copper sulfide associations in recent oceanic ores of the Logatchev hydrothermal field (Mid-Atlantic Ridge, 14° 45′ N), Geol. Ore Deposits, 42(4), 296–316. Gallant, R. M., and K. L. Von Damm (2006), Geochemcial controls on hydrothermal fluids from the Kairei and Edmond vent fields, 23°–25° S, Central Indian Ridge, Geochem., Geophys., Geosyst., 7, Q06018, doi:10.1029/2005GC001067. German, C. R., and J. Lin (2004), The thermal structure of the oceanic crust, ridge-spreading and hydrothermal circulation: How well do we understand their inter-connections?, in Mid-Ocean Ridges: Hydrothermal Interactions Between the Lithosphere and Oceans, Geophys. Monogr. Ser., vol. 148, edited by C. R. German, J. Lin, and L. M. Parson, pp. 1–18, AGU, Washington, D. C. German, C. R., and L. M. Parson (1998), Distributions of hydrothermal activity along the Mid-Atlantic Ridge; interplay of magmatic and tectonic controls, Earth Planet. Sci. Lett., 160(3-4), 327–341. German, C. R., et al. (1994), Hydrothermal activity on the Reykjanes Ridge: The Steinahóll vent-field at 63°06′N, Earth Planet. Sci. Lett., 121, 647–654. German, C. R., et al. (1999), A segment scale study of fluxes through the Rainbow hydrothermal plume, 36°N Mid-Atlantic Ridge, Eos Trans. AGU, 80(46), Fall Meet. Suppl., F957– F958. Gibson, H. L., R. L. Morton, and G. J. Hudak (1999), Submarine volcanic processes, deposits, and environments favorable for the location of volcanic-associated massive sulfide deposits, in Volcanic-Associated Massive Sulfide Deposits; Processes and Examples in Modern and Ancient Settings, edited by C. T. Barrie and M. D. Hannington, Rev. Econ. Geol., 8, 13–51. Goodfellow, W. D., and J. M. Franklin (1993), Geology, mineralogy, and chemistry of sediment-hosted clastic massive sulfides in shallow cores, Middle Valley, northern Juan de Fuca Ridge, Econ. Geol., 88(8), 2037–2068. Gracia, E., D. Bideau, R. Hekinian, Y. Lagabrielle, and L. M. Parson (1997), Along-axis magmatic oscillations and exposure of ultramafic rocks in a second-order segment of the Mid-Atlantic Ridge (33° 43′N to 34° 07′N), Geology, 25(12), 1059–1062. Gracia, E., J. L. Charlou, J. Radford Knoery, and L. M. Parson (2000), Non-transform offsets along the Mid-Atlantic Ridge south of the Azores (38° N–34° N): Ultramafic exposures and hosting of hydrothermal vents, Earth Planet. Sci. Lett., 177(1-2), 89–103. Haase, K. M., et al. (2007), young volcanism and related hydrothermal activity at 5° S on the slow-spreading southern Mid-Atlantic Ridge, Geochem., Geophys., Geosyst., 8, Q11002, doi:10.1029/ 2006GC001509. Halbach, P., et al. (1989), Probable modern analogue of Kurokotype massive sulphide deposits in the Okinawa Trough back-arc basin, Nature, 338(6215), 496–499. Halbach, P., B. Pracejus, and A. Maerten (1993), Geology and mineralogy of massive sulfide ores from the central Okinawa Trough, Japan, Econ. Geol., 88(8), 2210–2225. Halls, C., and R. Zhao (1995), Listvenite and related rocks: Perspectives on terminology and mineralogy with reference to an occurrence at Cregganbaun, Co. Mayo, Republic of Ireland, Mineral. Dep., 30, 303–313. Hannington, M., P. Herzig, S. Scott, G. Thompson, and P. Rona (1991), Comparative mineralogy and geochemistry of goldbearing sulfide deposits on the mid-ocean ridges, Mar. Geol., 101(1-4), 217–248. Hannington, M., et al. (2001), First observations of high-temperature submarine hydrothermal vents and massive anhydrite deposits off the north coast of Iceland, Mar. Geol., 177(3-4), 199–220. Hannington, M. D., and S. D. Scott (1988), Gold mineralisation in volcanogenic massive sulphides; modern and ancient, in Bicentennial Gold; 1988; Extended Abstracts; Oral Programme, edited by A. D. T. Goode and L. I. Bosma, pp. 353–358, Geol. Soc. of Aust., Sydney, N.S.W., Australia. Hannington, M. D., and S. D. Scott (1989), Sulfidation equilibria as guides to gold mineralization in volcanogenic massive sulfides; evidence from sulfide mineralogy and the composition of sphalerite, Econ. Geol., 84(7), 1978–1995. Hannington, M. D., G. Thompson, P. A. Rona, and S. D. Scott (1988), Gold and native copper in supergene sulphides from the Mid-Atlantic Ridge, Nature, 333(6168), 64–66. Hannington, M. D., I. R. Jonasson, P. M. Herzig, and S. Petersen (1995), Physical and chemical processes of se

A submersible study in the western Blanco fracture Zone, N.E. Pacific: Structure and evolution during the last 1.6 Ma

During July and August 1991, the French-American Blanconaute dive program used the French submersibleNautile to investigate the West Blanco Depression (WBD), a deep, elongate trough located at the intersection of the Blanco Transform Fault Zone with the southern Juan de Fuca Ridge (JdFR). Twenty dives were carried out along the north wall of the WBD, which exposes the upper oceanic crust over a 65 km distance, from the JdFR axis (to the west) to the oblique trace of an ancient propagator (to the east, crustal age around 2 Ma). Thirteen of these dives were precisely located within a 13 × 7 km zone of the north wall, covered by a high-resolution sonar mapping operation during the Blancotrough cruise in 1987. This series of geological traverses, plus 4 dives across the south wall of the WBD (one dive) and the adjacent Parks Plateau (3 dives), collected 242 rock samples. We report here the main results of the dive program and preliminary laboratory studies:1.Transform-related tectonic activity has recently abandoned the southern margin of Parks Plateau, and is presently located inside the WBD area, mainly along its northern wall. The tectonic features observed are compatible with a right-lateral strike-slip system, with a NE-SW extensional component.2.Three main lithological units are exposed along the north wall of the WBD. From top to bottom, they are: (1) a Volcanic Unit, forming a steep upper cliff, made of massive and pillow flows and basaltic dikes, with an estimated average thickness of 800 m; (2) a less steep Transition Zone, about 150 to 400 m thick, largely masked by rubble but exposing both diabase outcrops and pillow flows; and (3) a massive Diabase Unit, exposed over 700–800 m, with a dike complex structure visible from place to place, and cut by a net of hydrothermal veins. Deep crustal rocks such as gabbros were not observed.3.Spectacular mass-wasting features are visible all along the north wall of the WBD. About 60% of the face of the wall is masked by talus cones, rubble, rock avalanche deposits and slide blocks. Three main landslides, of approximately one km3 in volume each, were tentatively identified. One of them was mapped in detail and consists of an approximately 300 m thick (0.85 km3), coherent slide block detached from a zone where intense hydrothermal alteration and faulting have obviously weakened the bedrock, that is in places entirely altered to blue clays.4.The basaltic lavas of the WBD north wall show a remarkable evolution with time, from east to west. Around the tip of the ancient propagator, they are restricted to primitive, olivine-rich picritic basalts. Proceeding westward, they exhibit a wide range of differentiation, including highly fractionated, FeTi-rich ferrobasalts at about 35–45 km from the JdFR axis. When approaching the JdFR axis, the FeTi enrichment decreases gradually, and the ferrobasalts evolve towards slightly differentiated MORB-type basalts, typical of the southern JdFR. This magmatic evolution marks the transition from the end of a propagating rift regime to a steady-state accretion regime.5.The WBD north wall also permits the study of weathering and hydrothermal alteration processes and their evolution in space and time. Vertically, the alteration products evolve from oceanic weathering and zeolite facies (Volcanic Unit) to the greenschist facies (Transition Zone and Diabase Unit). Horizontally, the evolution with time is mainly a general hydration of the crust that is, however, very irregularly distributed.6.Several vertical magnetic traverses along the north wall of the WBD, using a bottom magnetometer attached to the basket of the submersible, have shown a sharp 5000 to 7000 nT positive anomaly at about 3500 m depth. This anomaly corresponds exactly to the first appearance of extrusive pillow-lava outcrops, and confirms the dramatic decrease in magnetic anomaly amplitude below that depth, detected during the Blancotrough cruise in 1987. The vertical magnetic profiles thus appear to have imaged the base of the magnetic source layer.

Active Spreading and Hydrothermalism in North Fiji Basin (SW Pacific). Results of Japanese French Cruise Kaiyo 87

The aim of the Japanese-French Kaiyo 87 cruise was the study of the spreading axis in the North Fiji Basin (SW Pacific). A Seabeam and geophysical survey allowed us to define the detailed structure of the active NS spreading axis between 16° and 22° S and its relationships with the left lateral motion of the North Fiji Fracture Zone. Between 21° S and 18°10′ S, the spreading axis trends NS. From 18°10 S to 16°40 S the orientation of the spreading axis changes from NS to 015°. North of 16°40′ S the spreading axis trends 160°. These two 015° and 160° branches converge with the left lateral North Fiji fracture zone around 16°40′ S to define an RRFZ triple junction. Water sampling, dredging and photo TV deep towing give new information concerning the hydrothermal activity along the spreading axis. The discovery of hydrothermal deposits associated with living communities confirms this activity.

New structural and geochemical observations from the Pacific‐Antarctic Ridge between 52°45′S and 41°15′S

We investigated the morphology and structure of the Pacific-Antarctic Ridge between 52°45′S and 41°15′S during the Pacantarctic2 cruise using multibeam echosounder together with gravity measurements and dredges. Analysis of the bathymetric, gravity and geochemical data reveal three ridge segments separated by overlapping spreading centers south of the Menard transform fault (MTF) and five segments north of it. Calculation of the cross-sectional area allows quantification of the variation in size of the axial bathymetric high. Together with the calculation of the mantle Bouguer anomaly, these data provide information about variations in the temperature of the underlying mantle or in crustal thickness. Areas with hotter mantle are found north and south of the MTF. Geochemical analyses of samples dredged during the survey show a correlation of high cross-sectional area values and negative mantle Bouguer anomalies in the middle of segments with relatively less depleted basalts.

Distribution of dissolved sulfide, methane, and manganese near the seafloor at the Lucky Strike (37°17′N) and Menez Gwen (37°50′N) hydrothermal vent sites on the mid-Atlantic Ridge

During the 1994 DIVA-1 cruise, the distribution of sulfide, methane and total dissolvable manganese (TDM) in the water column was examined near the seafloor at two locations on the mid-Atlantic ridge (MAR) affected by hydrothermal venting. Samples for this study were obtained using a miniature rosette mounted on the Nautile submersible and hydrographic data was collected using a CTD probe. Water samples were analyzed on board ship (sulfide and methane) or at the shore laboratory (TDM). The results presented here show that nanomolar level sulfide measurements can be used to describe hydrothermal plumes. In the first study area, called the Lucky Strike hydrothermal field (MAR at 37°17′N), the distribution of hydrothermal tracers (<5 m above the seafloor) was strongly influenced by the location of known hydrothermal vents. A study of water samples collected above the Tour Eiffel smoker shows that for the buoyant plume, sulfide is rapidly removed, while TDM and methane behave similarly (conservatively) upon mixing with ambient seawater. At the Menez Gwen hydrothermal field, which was the second site and which was discovered during the DIVA-1 cruise, a distribution map (3 km×4 km) of sulfide, TDM, and methane concentrations was completed. The highest tracer concentrations were found in close proximity to the vent field and ca. 1 km to the west of the venting site. These and other data are evidence for possibly additional venting on the western wall of the axial graben of the Menez Gwen segment.

Analysis of propagators along the Pacific–Antarctic Ridge: evidence for triggering by kinematic changes

Abstract We analyze the structure and evolution of two propagators along the Pacific–Antarctic Ridge (PAR) that we surveyed during the Pacantarctic cruise of the N/O L’Atalante. A large propagator at 63°30′S, 167°W shows a N50°E-trending segment of the PAR propagating southwestward, while the adjacent, N45°E-trending segment retreats. The propagating and doomed ridges are offset by about 43 km. They both curve into the offset to define an overlap zone about 25 km long. The inner pseudofault is juxtaposed to a series of E–W-trending ridges inferred to represent the failed axes. Their direction and arrangement suggest an evolution as an overlapping propagator with cyclic rift failure. The pseudofaults are 35±5° oblique to the propagating ridge, which implies a rate of propagation of 44±8 mm/yr, using a 62 mm/yr full spreading rate, comparable to that of other propagators with similar morphology. The age of the initiation of the propagation from the Heirtzler fracture zone is estimated to be 5–6 Ma, which coincides with the age of a clockwise change in spreading direction. A second, smaller, southwestward propagator is observed northeast of the major one, at 63°15′S, 165°10′W, with a morphology very similar to that of the larger one. It is inferred to have started near 1 Ma, again at the time of a clockwise change in spreading direction along the PAR. These two propagators are likely to have evolved from extensional relay zones which developed within the Heirtzler transform fault (TF) valley following clockwise changes in spreading direction. The present-day axial discontinuity is less than 40 km in offset and may not be a TF anymore. The development of propagators in this area of the PAR appears to be triggered by kinematic changes rather than by thermal gradients along the ridge. Other propagators that have left similar signatures on the flanks of the PAR, appear to have developed at similar spreading rates near 50–60 mm/yr full rate, as a result of kinematic changes.

Variations in axial morphology, segmentation, and seafloor roughness along the Pacific‐Antarctic Ridge between 56°S and 66°S

The spreading rate at the Pacific-Antarctic Ridge (PAR) increases rapidly from 54 mm/yr near Pitman Fracture Zone (FZ) up to 76 mm/yr near Udintsev FZ, resulting in three domains of axial morphology: an axial valley south of Pitman FZ, an axial high north of Saint Exupery FZ, and in between, the transitional domain extends over 650 km. It comprises sections of ridge with an axial valley or an axial high and generally displays a very low cross-sectional relief. It is also characterized by two propagating rifts. Two domains of different seafloor roughness appear south of Udintsev FZ: east of 157 °W these two domains are separated by a 1000-km V-shaped boundary. West of 157 °W, the boundary approximately coincides with Chron 3a or Chron 4. The southward migration of the transitional area during the last 35 Myr explains the V-shaped boundary: (1) increases in spreading rate above a threshold value produced changes in axial morphology; and (2) in the transition zone, rotations of the spreading direction were accommodated by the plate boundary, either by rift propagation or by transitions from fracture zones to non transform discontinuities, leaving trails on the seafloor that presently delineate the V. Seafloor roughness variations are not controlled by exactly the same spreading rate dependence as changes in axial morphology. The transition from rough to smooth seems to have occurred everywhere for spreading rates greater than 50 mm/yr, except in a domain presently centered on Saint-Exupery FZ, where it occurred for spreading rates >60 to 65 mm/yr. Independent results from melting model calculations of major elements [Vlastelicet al., 2000] indicate that the upper mantle temperature is likely to be cooler between Antipodes and La Rose FZs. The combination of these two results reveals the existence of a 700-km-long segmentation of the upper mantle, with a “cool” area centered on Saint-Exupery FZ.

Morphological reorganization within the Pacific-Antarctic Discordance

Abstract Two domains with different satellite gravity signatures [1] appear along the axis and on the adjacent basins of the Pacific-Antarctic Ridge (PAR) between Udintsev FZ and 180°W. One of these signatures is of rough and faulted seafloor, with a high density of apparent, well-marked fracture zones; the other is of smooth seafloor that is comparable with that of oceanic basins that are generally formed at fast-spreading centres. Between Udintsev FZ and 157°W these two domains are separated by a V-shaped structure that extends for more than 1000 km along the rise axis, whereas west of 157°W the boundary is more diffuse. The satellite gravity also reveals an abrupt change in the axial morphology of the PAR across FZ XII, despite the fact that the current spreading rate [2] is the same on both sides of the fracture zone (about 60 mm/yr, full rate). To interpret these features, we postulate that the domains with an apparently rough seafloor morphology have been created at a spreading centre with an axial valley, and that smooth morphology testifies to a spreading centre which was with or evolving into an axial high at the time the crust was formed. With this hypothesis, we show that, since An 21o time (ca. 48 Ma), the ridge morphology changed from an axial valley to an axial high wherever and whenever the spreading rate exceeded a given threshold value. We also show that there is no unique threshold value. Geophysical evidence suggests that the differences in spreading rate threshold values that we observe are probably related to upper mantle temperature heterogeneities below the axis of the PAR. Therefore, we conclude that changes in spreading rates, combined with changes in the upper mantle temperature, constitute the key process that has governed the morphological reorganization of the PAR between Udintsev FZ and 180°W since An 21o time. The cause of upper mantle temperature heterogeneities, however, remains an open question. The 1000 km long ‘V’ south of Udintsev FZ reflects a change in axial morphology that progressively propagated southwards during the last 30 m.y., at a velocity of about 30 mm/yr. Thus, one tentative explanation for mantle heterogeneities which would also help understand the ‘V’ consists in postulating that the asthenosphere propagated below the PAR axis for the last 30 m.y., from a relatively ‘hot’ mantle province north of Udintsev FZ to a relatively ‘cold’ province south of the fracture zone. This flow model (originally proposed by Marks and Stock [3]) needs to be tested through further investigation.