The location of each crossing of a fracture zone is determined from satellite-derived gravity data. Along the CIR, the locations are crossings along individual processed profiles and are identical to those used by (12
). Along the other two MOR systems, the crossings are interpreted from a gridded gravity map (19
). The center of the fracture zone is assumed to lie at the center of the gravity trough for all fracture zones flanking the SWIR and CIR, which are spreading slowly and presumably resemble the morphology of the better studied fracture zones along the slowly spreading Mid-Atlantic Ridge. The signature of fracture zones flanking the SEIR is more complex. Royer and Sandwell (45
) estimated the locations of fracture-zone crossings assuming that the maximum slope in the calculated gravity (that is, roughly midway between an adjacent gravity peak and trough) lies over the center of the fracture zone, as is expected at a fast-spreading MOR such as the East Pacific Rise. The gravity grid from declassified Geosat data (19
) indicates that the signature of fracture zones flanking the SEIR varies considerably from fracture zone to fracture zone. Some zones appear highly antisymmetric with respect to the SEIR with a fracture-zone trough on one side of the MOR correlating with a fracture-zone ridge on the other side; this indicates that the maximum slope between fracture-zone ridge and trough on one side of the MOR should be correlated with that on the other. Other fracture zones, however, resemble those along slowly spreading MORs, with a fracture-zone trough on one side of the MOR correlating with a trough on the other side. We selected for analysis three fracture zones south of Australia resembling those on slowly spreading ridges (Fig. 3A). Also needed were sets of fracture-zone crossings along the westernmost SEIR, which lacks clear, straight fracture zones that are continuous between anomaly 5 and the SEIR. For this region, we correlated the midpoint between fracture-zone ridge and trough on one side of the MOR with the midpoint on the other side of the MOR for three fracture zones that clearly offset anomaly 5 by about 30, 50, and 70 km from the westernmost to the easternmost fracture zone, respectively (Fig. 3A). We investigated the self-consistency of the interpretation of fracture-zone crossings and the resulting reconstructions by examining the distance between the nearest magnetic-anomaly crossing to a given fracture zone and the distance of the same crossing when reconstructed across the MOR from what was interpreted as the correlative feature. The interpreted crossings are consistent with our best-fitting rotations. The self-consistency was further investigated by matching up conjugate wandering offsets lying just NW of a fracture zone that intersects the SEIR near 32°S, 77°E (Fig. 3A). The three offsets match up within ∼10 km, consistent with our interpretation.