Morphological reorganization within the Pacific-Antarctic Discordance

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. 00 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.