Crustal accretion at intermediate spreading rates: Pacific-Antarctic Ridge at 65 degrees south latitude

The objective of this morpho-tectonic study is to understand the processes associated with crustal accretion along two adjacent ridge segments of the Pacific-Antarctic ridge (full rate: $\approx$56 mm/yr). The studied corridor is centered on the Pitman Fracture Zone and extends to 12 Ma on both flanks. The segment north of the fracture zone is characterized by a shallower ridge crest, larger off-axis seamount volume and abundance, thicker crust (inferred from mantle Bouguer gravity) and smaller abyssal hill spacing, all indications of a hotter axial thermal regime and/or larger magma supply. The spreading history of this portion of the ridge is marked by a $\sim$40% acceleration in spreading rate and a $\sim$5$\sp\circ$ clockwise rotation in spreading direction near Chron 3A ($\sim$6 Ma). This sharp rotation is superimposed on a gradual rotation of $\sim$1$\sp\circ$/Ma which implies that the transform has been in extension for the past 12 Ma. The remarkable agreement between the azimuths of the digitized ridge-parallel abyssal hill lineations and trends predicted by the finite rotation poles suggests that the zone of accretion is, on average, orthofonal to the spreading direction. A statistical characterization study of abyssal hill size indicates that abyssal hills formed during the slower spreading periods are taller and wider than the hills created during the faster spreading periods. Bathymetric slopes dipping toward the ridge crest are interpreted as fault-controlled due to their more linear and steeper character relative to the ones dipping away from the ridge crest. Spreading rates for conjugate plates show wide flow line variations in both magnitude and sense for adjacent ridge segments, an indication that crustal accretion rates are controlled by processes happening near the ridge crest. The remarkable symmetry between sequences of inward facing fault scarps of the same age on opposite plates requires that abyssal hills are formed as conjugate pairs. Based on abyssal hill lineaments and magnetic anomaly widths, I determined that most of the spreading rate asymmetries can be explained by ridge axis jump