High Resolution Statistical Estimation of Seafloor Morphology: Oblique and Orthogonal Fabric on the Flanks of the Mid-Atlantic Ridge, 34°-35.5 ° S GREGORY A. NEUMANN ~ and DONALD W. FORSYTH 2 1Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218, U.S.A. 2Department of Geological Sciences, Brown University, Providence, RI 02912, U.S.A. (Received 13 October, 1993; accepted 19 June, 1994) Key words: mid-ocean ridges, oblique extension, abyssal hills Introduction Abstract. On the Mid-Atlantic Ridge (MAR) from 34°-35.5 ° S, three ridge segments span the 108 km distance between the Meteor Fracture Zone (FZ) and the Montevideo FZ. Each of these segments is perpendicular to the adjoining transforms. Magnetic isochrons in the southern half of the region are oblique to the spreading direction and are offset from the morphological expression of the plate boundary, revealing a transition from oblique to orthogonal spreading within the last 750,000 years. Changes in orientation and cross-sectional form of the rift valley, as modified by tectonic processes, are preserved in the off-axis abyssal-hill fabric. We present a new statistical method for describing size and orientation of abyssal hills based on local slopes. For a given offset, the range of sorted slopes from the first to third quartile provides a robust estimate of topographic variability. The variability can be parametrized by azimuthal direction, plan-view aspect ratio, characteristic height and width. We resolve lineation azimuth within 6°, and characteris- tic height, width and aspect ratio within 20-30%, using 18 by 21 km sample boxes crossed by multiple Sea Beam swaths covering approximately 30% of the box. In the northern portion of the survey, the azimuth is mainly ridge parallel, while in the southern portion, the azimuth rotates 23° clockwise from ridge strike. Characteristic height and width are greater in the south- ern half than in the northern half, while aspect ratios are lower. The asymmetry of quartiles about the median slope provides evidence that inward-facing normal faults bounding the rift valley are a significant source of topography. Fabric disrupted by migration of small-offset discontinuitieshas higher than average characteristic height. Characteristic height and width correlate positively with residual gravity, an indicator of crustal thinning. A residual gravity low, possibly the current focus of upwelling, coincides with a newly formed spreading axis. These correla- tions suggest that evolution of ridge geometry can be controlled by crust and mantle thermal structure. Either variation in magma supply, resulting in changes in stress normal to the ridge axis, or a major realignment of the Montevideo Transform, temporarily resulting in increased shear stress across newly activated faults, may have been responsible for changes in orientation and morphology of the spreading center. Marine Geophysical Researches 17: 221-250, 1995. © 1995 Kluwer Academic Publishers. Printed in the Netherlands. A geophysical survey over the southern Mid-Atlan- tic Ridge (MAR) has revealed a recent transition in geometry of the plate boundary from what appears to have been oblique spreading to a present-day, orthogonal, ridge-transform geometry. Magnetic anomalies have been interpreted as showing ob- lique spreading over the past 7 Ma (Weiland et al., 1991; 1995) along a single ridge segment immedi- ately adjacent to the Montevideo FZ. Sea Beam bathymetry (Neumann and Forsyth, 1983), with sparse (30-40%) coverage, shows many off-axis lineations oriented oblique to the present-day rift valley, while the morphological location of the ridge axis is currently displaced 5 km or more from the midpoint of the central magnetic anomaly. Thus a ridge jump may have recently altered the location and geometry of accretion. At slow-spreading ridges, seafloor created in a narrow neovolcanic zone undergoes repeated de- formation through normal faulting as it is lifted out of the deep median valley (Atwater and Mu- die, 1968, 1973; Needham and Francheteau, 1974; Harrison and Stieltjes, 1977; Searle, and Laughton, 1977). While volcanic ridges (Kong et aL, 1988; Pockalny et aL, 1988) contribute to the surrounding abyssal hills, we argue that in our survey the primary source of abyssal hill relief is tectonic. Shaw and Lin (1993) observe that most of the abyssal hill lineations along the MAR de- velop at the median valley walls, and conclude that "the growth of normal faults at the edges of the rift valley is the primary mechanism responsi- ble for Atlantic abyssal hill topography..." Our