HR: 0800h
AN: V41C-1461    [Abstracts]
TI: Analysis of Off-axis Volcanic Ridges on the Flanks of the Pacific-Antarctic Ridge. Results From Pacantarctic2 Cruise
AU: Briais, A
EM: Anne.Briais@dtp.obs-mip.fr
AF: CNRS UMR5562, OMP, 14, Ave. E. Belin, Toulouse, 31400 France
AU: Ondreas, H
EM: Helene.Ondreas@ifremer.fr
AF: IFREMER, B.P. 70, Plouzane, 29280 France
AU: * Klingelhoefer, F
EM: Frauke.Klingelhoefer@ifremer.fr
AF: IFREMER, B.P. 70, Plouzane, 29280 France
AU: Dosso, L
EM: Laure.Dosso@ifremer.fr
AF: CNRS UMR6538, B.P. 70, Plouzane, 29280 France
AB: During the Pacantarctic 2 cruise of the R/V L'Atalante in December 2004 and January 2005, we mapped and dredged large off-axis volcanic structures on the flanks of the Pacific-Antarctic ridge: two ridges on either side of the Menard transform fault (TF) near 50°S, and E-W-trending ridges near 42°S. One objective of the cruise was to understand how these structures relate to axial dynamics and to recent kinematics. South of the Menard TF, 9 large off-axis volcanoes, 12 km diameter on average and 500 m to 2000 m high, show two morphologies. One type, located on 2-5 My-old lithosphere, exhibits conical shapes with a summittal caldera, and low backscattering in the EM12 imagery, probably due to the presence of sediments. Dredges recovered altered and porphyric basalts and coarse-grained rocks. The other type, located on 0-3 My-old lithosphere, is marked by structures elongated N70 or EW, associated to narrow, EW-trending ridges, some of them showing strong backscattering. Plagioclase-bearing pillows, as well as aphyric fresh, or altered pillows have been dredged near their tops. North of the Menard TF, the volcanoes have an average diameter of 6 km, and heights of 100 to 1600 m. They are elongated and linked by N100 to EW-trending narrow ridges. The volcanoes closest to the axis show the strongest EM12 backscattering. Dredges brought back altered basalts and pillows. Two EW-trending ridges have been surveyed near 42°S. One ridge, near 41°55'S, appears to be very linear and narrow, and probably did not form by the coalescence of individual volcanoes. Fresh pillows with basaltic glass have been dredged there. Another one, near 41°15'S, is made of elongated volcanoes linked by volcanic ridges. All volcanic ridges appear to have formed on lithosphere younger than 5 Ma. Most are located on the traces of large ridge discontinuities. The volcanic activity, as suggested by the strong backscatter in sonar images, appears to be limited to areas younger than 3 Ma. We suggest that the major volcanic structures on the flanks of the Pacific-Antarctic ridge might result from intraplate deformation near the axis, following recent kinematic changes.
DE: 1021 Composition of the oceanic crust
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1219 Gravity anomalies and Earth structure (0920, 7205, 7240)
DE: 3038 Oceanic plateaus and microcontinents
DE: 3075 Submarine tectonics and volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005