A HISTORY OF THE EASTER MICROPLATE, 5.25-MA TO PRESENT

GLORIA and other geophysical data are used to interpret the morphology and tectonic pattern of the Easter microplate. We model magnetic anom aly data and calculate spreading rates for use in Euler pole calculati ons. We calculate three-plate closure and best fit poles for Nazca-Pac ific, Nazca-Easter, and Pacific-Easter plate motions and compare the r esults with earthquake focal mechanisms and seismicity. We use the thr ee-plate closure poles to rotate digital GLORIA and magnetic isochron data of the microplate and surrounding major plates back in time since the microplate's East Rift first started to propagate northward at ar ound 5.25 Ma. The microplate has evolved in two stages. The first is f rom 5.25 Ma to around 2.5 Ma, and involved continuous northward propag ation of the East Rift, while the West Rift became segmented along new transform faults as it adjusted to keep the overall Nazca-Pacific vel ocities constant. The transpressive northern boundary migrated with th e East Rift tip, while the transtensional southern boundary remained m ore or less still. At some time after 2.5 Ma the East Rift ceased nort hward propagation and the Southwest Rift began to open up along the tr anstensional Pacific-Easter transform. A convergent northern boundary formed involving southward thrusting of the Nazca plate over the micro plate. No further Nazca plate lithosphere was transferred froth north of the microplate to the microplate interior, and the Pito Deep region opened by pervasively rifting apart preexisting Nazca plate lithosphe re.