SeaMARC II side scan and bathymetric data, combined with magnetic anomaly and Sea Beam bathymetric data, define a large (~90--120 km) nontransform offset of the East Pacific Rise (EPR) between the Easter and Juan Fernandez microplates near 29 ¿S, where one or more transforms had been suggested by the seismicity pattern. On the basis of morphology, structural, and tectonic features, three major regions are delineated: an eastern spreading axis, a western spreading axis, and a central transfer zone. These two spreading axes overlap by at least 35 km and possibly as much as 100 km. The eastern axis is characterized by an axial ridge that narrows and deepens toward the tip (28¿38'S, 112¿08'W). Approaching the tip, the spreading axis azimuth changes from approximately N5¿E to N25¿W, and axial magnetic anomaly values increase to over 700 nT km south of the rift tip. A pair of deep (>600 m) bathymetric troughs on either side of the ridge axis, trending N11 ¿W and N44 ¿W, converge on the ridge tip and define the outermost boundaries of ridge-parallel structures. These structures are interpreted as pseudofaults resulting from northward rift propagation. The western spreading axis is approximately 2400 m deep and corresponds to a young volcanic area.

The central lithospheric transfer zone, west of the inner pseudofault of the eastern spreading axis, is characterized by oblique abyssal hill and magnetic fabric interpreted to result from shear between overlapping spreading centers and a dispersed pattern of teleseismic earthquake epicenters. The eastern spreading axis is propagating rapidly northward at up to 265 km/m.y., based on a 158 km/m.y. spreading rate and the 33¿ angle between the pseudofaults. However, complex patterns of seafloor fabric, which differ from that predicted by the simple propagating rift model, suggest that multiple propagation events may have occurred. Furthermore, southward propagation of the western limb may be dominant in the long term. The absence of transform faults along this superfast spreading EPR segment documents that transforms do not presently exist at such superfast spreading rates. ¿ American Geophysical Union 1991