THE EFFECTS OF CHANGES IN PLATE MOTIONS ON THE SHAPE OF THE MARQUESASFRACTURE-ZONE

The Marquesas fracture zone (MFZ), one of the great fracture zones tha t formed on the Pacific-Farallon ridge system, shows dramatic variatio ns in morphology along its strike that cannot be explained by models f or lithospheric flexure in response to thermal stresses and differenti al subsidence. Marine geophysical data collected by the R/V Maurice Ew ing in fall 1991 show that major structures in the MFZ can be correlat ed with changes in plate motions as the fracture zone formed. The west ern MFZ shows two significant changes in trend: at 209 degrees E and a t 224 degrees E. The change in strike at 209 degrees E records a reori entation that put the transform fault under compression and created a 100 km-long sinuous ridge within the fracture zone. The change in plat e motions at 224 degrees E induced extension in the transform fault, c reating three subparallel fracture zones. These results are in accord with observations of morphological changes observed at active transfor m faults and other fracture zones, and suggest that even relatively sm all changes (similar to 6 degrees) in spreading direction can produce significant structures that are preserved in the fracture zone.