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 Exupéry 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‐Exupéry FZ, where it occurred for spreading rates >60 to 65 mm/yr. Independent results from melting model calculations of major elements [ Vlastelic et 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‐Exupéry FZ.