Marine Science Faculty Publications

Recent Pacific-Easter-Nazca Plate Motions

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Book Chapter

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Sea, floor spreading, Congresses, Mid-ocean ridges, Congresses

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Instantaneous relative plate motions have been calculated for the Pacific, Easter and Nazca plates by inverting spreading rates since the Brunhes/Matuyama reversal boundary (obtained from modeling 39 magnetic anomaly profiles across the divergent boundaries of all three plates), along with 10 transform azimuths (obtained from recent SeaMARC II, GLORIA and Sea Beam data) and 20 published seismic slip vectors. The rates along the Nazca-Pacific and Nazca-Easter spreading axes increase to the south. The rates along the Pacific-Easter spreading axis decrease to the south. Along ∼2400 km of the southern Nazca-Pacific plate boundary where spreading rates range from 145 to 160 km there are no Nazca-Pacific transform faults where spreading axes are offset. Instead, the offsets are accommodated by microplates, propagating rifts, or overlapping spreading centers. The origin of the Easter microplate cannot be attributed solely to fast spreading rates along the preexisting Nazca-Pacific boundary because the fastest seafloor spreading is to the south of the microplate. The Nazca-Pacific Euler vector (0–0.73 Ma) from this study has a slower angular velocity and lies outside the confidence ellipse of the Minster and Jordan RM2 Euler vector (0–3.0 Ma). It also lies outside of the confidence ellipse of the DeMets et al. NUVEL-1 Euler vector (0–3.0 Ma) but has approximately the same angular velocity. Our preferred Euler vector describing the absolute motion of the Easter microplate is near the center of the microplate with an angular velocity of about 15°m.y., making it a fast ‘spinning' plate. Oblique convergence is predicted along the proposed Nazca-Easter and Pacific-Easter transform segments of the proposed northern and southern triple junctions, respectively. The similarity between the best fitting Euler vector for all three plate pairs and the Euler vectors derived by the three-plate closure condition suggests the microplate interior is behaving mostly rigidly. Reduced chi-squared values and F-ratio tests support this finding. However, comparison of the predicted motion vectors with the observed structures interpreted to be microplate boundaries indicates that deformation must be occurring over a broad area along the northern microplate boundary. This deformation is suspected to be a direct consequence of the large-scale rift propagation and rapid microplate rotation.

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Citation / Publisher Attribution

Recent Pacific-Easter-Nazca Plate Motions, in J. M. Sinton (Ed.), Evolution of Mid Ocean Ridges, American Geophysical Union, p. 9-30