Recent Development of the Large-scale Overlapping Ridge System, EPR 30S Revealed by a new GLORI-B Processing Technique

P D Johnson, R N Hey, F Martinez

In early 1993 we surveyed a segment of the EPR located between the Easter and Juan Fernandez microplates where a large-scale overlapping ridge system was known to exist but little was known of its formation or structures. The survey was conducted using both the SeaBeam 2000 system and the newly modified GLORIA system (GLORI-B). The GLORI-B bathymetry was laden with track parallel artifacts and the swath width of SeaBeam 2000 was only half of GLORI-B. Therefore, we developed a processing technique, done entirely with the GMT software package, to remove the artifacts from the GLORI-B data. The initial step involved fabricating a geographically coordinated GLORI-B grid and subtracting from this a regional grid, made up of the SeaBeam 2000 and centerline bathymetry from other surveys. This created a difference file which contained the unwanted track parallel artifacts as well as the desired seafloor texture. An asymmetric gaussian filter was passed over the difference grid to isolate the long wavelength artifacts from the shorter wavelength seafloor texture. The isolated artifacts were then subtracted from the original GLORI-B data producing a processed grid. However, the new grid still contained high frequency noise which was removed by subtracting a new regional grid, created by strongly filtering the new GLORI-B and SeaBeam 2000 datasets, from the processed GLORI-B creating a noise grid. From the noise grid any points which had values outside a medial range (the noise) were clipped yielding the final GLORI-B grid. Preliminary analysis of the processed data reveals a heavily sheared domain of seafloor within the zone of overlap that is over a degree long and a degree wide. The current configuration of the eastern and western ridges is a recent consequence of propagation in both the northward and southward direction as neither ridge has yet had the opportunity to create much new crust between its axis and pseudofaults. These ridges curve towards each other because of the interaction of their stress fields and thus are characterized by oblique spreading along the entire zone of overlap, > 100 km. As the eastern propagating spreading axis lengthened to the north it cut through an older core of lithosphere that had been previously tectonised from an earlier propagation event. This core is characterized by a bathymetric high, when compared to the rest of the overlap zone, and has a low backscatter return in the sidescan data. The core has multiple cross cutting faults running from east to west and is itself bounded by deep rifts to the east and west. Located to the south of the core there is an apparent transform zone at the southern boundary between the two propagating ridges. However, the northern boundary has not joined in a similar manner. Instead the current situation might be considered a proto-microplate with the northern linkage occurring in the future if the eastern ridge does not fail as it has done in the past.