MORPHOLOGICAL REORGANIZATION WITHIN THE PACIFIC-ANTARCTIC DISCORDANCE

Two domains with different satellite gravity signatures [I] appear alo ng the axis and on the adjacent basins of the Pacific-Antarctic Ridge (PAR) between Udintsev FZ and 180 degrees W. One of these signatures i s of rough and faulted seafloor, with a high density of apparent, well -marked fracture zones; the other is of smooth seafloor that is compar able with that of oceanic basins that are generally formed at fast-spr eading centres. Between Udintsev FZ and 157 degrees W these two domain s are separated by a V-shaped structure that extends for more than 100 0 km along the rise axis, whereas west of 157 degrees W the boundary i s 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 fractu re zone (about 60 mm/yr, full rate), To interpret these features, we p ostulate that the domains with an apparently rough seafloor morphology have been created at a spreading centre with an axial valley, and tha t smooth morphology testifies to a spreading centre which was with or evolving into an axial high at the time the crust was formed. With thi s hypothesis, we show that, since An 21o time (ca. 48 Ma), the ridge m orphology changed from an axial valley to an axial high wherever and w henever the spreading rate exceeded a given threshold value. We also s how that there is no unique threshold value. Geophysical evidence sugg ests that the differences in spreading rate threshold values that we o bserve are probably related to upper mantle temperature heterogeneitie s below the axis of the PAR. Therefore, we conclude that changes in sp reading rates, combined with changes in the upper mantle temperature, constitute the key process that has governed the morphological reorgan ization of the PAR between Udintsev FZ and 180 degrees 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 re flects a change in axial morphology that progressively propagated sout hwards 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 asthenospher e propagated below the PAR axis for the last 30 m.y., from a relativel y 'hot' mantle province north of Udintsev FZ to a relatively 'cold' pr ovince south of the fracture zone. This flow model (originally propose d by Marks and Stock [3]) needs to be tested through further investiga tion.