Tectonic pattern and evolution of the Easter microplate, based on GLORIA and other geophysical data

Abstract

The GLORIA and other geophysical data obtained during this study have enabled the entire present-day tectonic pattern of the Easter Microplate to be determined. The complex nature of all of the plate boundaries has been revealed, and there is evidence of rapid plate boundary evolution. Earthquake focal mechanisms together with GLORIA data have provided the first unequivocal evidence that thrust faulting is taking place along the northern boundary of the microplate. On the Nazca plate to the north of the Easter Microplate, the GLORIA images have disclosed a series of prominent east-west trending ridges which earthquake focal mechanisms and plate tectonics of the region imply are the first known case of intraplate thrusting in young oceanic lithosphere. Slightly younger Nazca plate lithosphere to the north is overthrust over marginally older lithosphere to the south, in order to accommodate the tectonic shortening predicted in the region. Interpretation of magnetic anomaly data and the complex structural information offered by the GLORIA images have permitted a detailed history of the microplate to be determined. Three-plate Nazca-Pacific, Nazca-Easter and Pacific-Easter Euler poles have been calculated and used to rotate the Easter Microplate and surrounding major plates back in time since the East Rift of the microplate first started to propagate northwards at around 5.25 Ma. The microplate has evolved in two stages. The first is from 5.25 Ma to around 2.5 Ma, and involved continuous northward propagation of the East Rift, while the West Rift became segmented along new transform faults as it adjusted to keep the overall Nazca-Pacific velocities constant. The transpressive northern boundary migrated with the East Rift tip, while the transtensional southern boundary remained more or less still. At sometime after 2.5 Ma, the East Rift ceased northward propagation and the Southwest Rift began to open up along the transtensional Pacific-Easter transform. A convergent northern boundary formed involving southward overthrusting of the Nazca plate to the north over the microplate. No more Nazca plate lithosphere from the north of the microplate was transferred to the microplate interior, and the Pito Deep region was caused to open up by pervasively rifting apart pre-existing Nazca plate lithosphere. This study predicts that the Easter Microplate will be transferred onto the Pacific plate within the next 2-5 myr, when the Southwest Rift has propagated through to the southern EPR, and the shear couple on the microplate has ceased.