FLOW LINE VARIATIONS IN ABYSSAL HILL MORPHOLOGY FOR THE PACIFIC-ANTARCTIC RIDGE AT 65-DEGREES-S

We present the results of a statistical study on the morphological cha racteristics of abyssal hills recently mapped along two adjacent segme nts of the Pacific-Antarctic Ridge at 65 degrees S. The studied area i s a densely surveyed corridor (60 km wide by 600 km long) which is cen tered on the Pitman Fracture Zone (PFZ) and extends to 12 Ma crust on both sides of the ridge. Abyssal hill size parameters (RMS height H an d characteristic width lambda) are estimated using Hydrosweep multibea m data. Variations in abyssal hill characteristics are compared with s preading rate history and crustal structure (as inferred from the mant le Bouguer gravity) in order to indirectly quantify the evolution of t his ridge crest system. The magnetic data document an abrupt accelerat ion in spreading rate from similar to 36 to similar to 63 mm/yr (full rate) at Chron 3a (5.7-6.4 Ma). Our results indicate a statistically s ignificant negative correlation between abyssal hill size parameters a nd full spreading rates. Abyssal hills formed during the slower spread ing period (ages >8 Ma; full rates 36-44 mm/yr) are 31-86% taller and 21- >100% wider than hills created during the faster spreading interva l (ages <4 Ma; full rates 52-63 mm/yr). The well-resolved positive cor relation between H and lambda is interpreted as an indication of tempo ral changes in the flexural rigidity of the lithosphere near the vicin ity of the ridge crest and, by implication, axial thermal structure. H owever, we cannot rule out that such positive trend is due to construc tional volcanism. The lack of correlation between crustal thick-ness a nd abyssal hill size parameters is likely to be caused by the small ma gnitude of crustal thickness variations along flow lines (similar to 0 .4 km in contrast to similar to 2 km reported in previous studies for the Mid-Atlantic Ridge). The most significant variations in crustal th ickness are seen across the PFZ (thinning from north to south by 0.5-0 .7 km), which coincide with a well-resolved increase in the averaged l ambda estimate. The predictions of the detachment surface model in ter ms of morphological and structural inside/outside corner asymmetries a re not supported by our observations. The main variations in H and lam bda that cannot be explained in terms of either the spreading rate or crustal thickness effect include the following: (1) anomalously large abyssal hills north of the PFZ for 4-6 Ma age crust; (2) abyssal hill size estimates for crustal ages greater than 8 Ma show significant asy mmetry for opposite ridge flanks north of the PFZ; and (3) toward the segment ends, H estimates are 27-68% larger, while lambda estimates ei ther do not significantly change (to the north of the PFZ) or are up t o 40% smaller (to the south of the PFZ). We suggest that the H and lam bda changes seen toward the segment ends are related to either an incr ease in the amount of extension (without a corresponding increase in t he strength of the lithosphere) or variations in the relative contribu tion of constructional volcanism to overall abyssal hill morphology.