Compressional deformation north of the Easter microplate: a manned submersible and seafloor gravity investigation

We have investigated an E-W compressional ridge in 0.9 Ma lithosphere of the Nazca Plate, just north of the Easter microplate on the East Pacific Rise, using a combination of visual observation and seabed gravity measurements from four manned submersible dives. Three of the dives made a 26-km-long traverse across the ridge during which 14 gravity measurements were made. The observations confirm the presence of active thrust faulting on the 1.1-km-high south face of the ridge. However, the gravity precludes the existence of a simple low-angle thrust, and is only compatible with significant crustal shortening and thickening if the block to the south of the thrust has a lower density than the overriding block. Visual and sonar observations of pervasive fine-scale tectonism in this area lend some support to this idea. Alternatively, there could be oblique thrusting on a high-angle fault with relatively little crustal shortening. In addition, a small degree of crustal shortening may take place in the form of buckling of an elastic or plastic lithosphere, though this cannot explain the strong asymmetry of the ridge, which we believe must imply faulting.

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