Placing bounds on lithospheric deformation in the central Pacific Ocean

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Lineations in the gravity field in the central Pacific Ocean have been explained by models of both small-scale convection and lithospheric boudinage. The boudinage model requires deformation of the plate and we have investigated bounds on the amount of deformation. We calculate separations between a pair of fracture zones on the Pacific plate which occur in an area where the lineations are well developed and compare these values to the separation of the corresponding fracture zone pair on the Nazca plate where no lineations have been observed. Satellite and surface-ship data was used to locate fracture zones and small circles were fitted to sections of picks which were chosen around their points of intersection with magnetic isochrons. Great circle distances from these small circles to stage poles were calculated and the difference between these distances gave the separations between the fracture zone pairs. We found that between anomalies 13 and 7 the difference in fracture zone spacing on the Pacific plate, compared to the Nazca plate, appears to increase as the age of oceanic lithosphere decreases. The maximum difference calculated was 97 km, about 4% of the distance between the fracture zones, and we attribute this difference to the re-orientation of the ancient Pacific-Farallon spreading centre between anomalies 7 and 5. The main lithospheric stretching models require the fracture zone separation to increase with age but this is not seen in our results.