none 10.1144/gsjgs.150.5.0965 Geological Society of London Geological Society of London 1881 30780051 44798 2024111903223300646 10.1144 2024-11-19T12:17:00Z 2007-12-21T18:40:02Z 61 Journal of the Geological Society JGS 0016-7649 2041-479X 05 16 2007 09 1993 150 5 R. C. SEARLE Department of Geological Sciences, University of Durham DH1 3LE, UK R. T. BIRD Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, USA R. I. RUSBY Department of Geological Sciences, University of Durham DH1 3LE, UK Institute of Oceanographic Sciences, Wormley, Godalming GU8 5UB, UK D. F. NAAR University of South Florida, St. Petersburg, FL 33701-5016, USA In this paper we synthesize and discuss the results of recent detailed geophysical surveys and reconstructions of two active mid-ocean ridge microplates on the East Pacific Rise. We quantify their development in terms of the principal dimensions of the microplates and their boundaries, and the temporal changes in these dimensions. The results are discussed in the framework of the ‘roller-bearing’ microplate model of Schouten and others, which postulates that microplates are driven by shear couples imposed on them by the motions of their bounding major plates. We test the specific predictions of this model for the geometry of the ‘pseudofaults’ (traces of the propagating tips of the microplates’ spreading boundaries) and the histories of rotation of the microplates. Both microplates are currently rotating at the rates predicted by the model on the assumption of complete coupling (no slip or deformation) between the microplates and the major plates at instantaneous points of contact near the propagating tips of the spreading boundaries. Juan Fernandez microplate was also rotating at the predicted rate just before 2 Ma when it was driven by the Nazca–Pacific couple rather than the Nazca–Antarctic couple that currently applies. However, earlier rotations were slower than predicted, implying some degree of decoupling or deformation at earlier times. The microplates appear to have grown from moderate-sized ( c . 100 km), probably left-stepping offsets of the East Pacific Rise since about 5 Ma. The nature of these offsets is currently uncertain, but they may have been propagating rifts, which changed from shearing by ‘bookshelf faulting’ to rigid block (microplate) rotation on reaching a critical size. Once rigid block rotation began, the microplates grew by seafloor spreading, accompanied by propagation of their bounding spreading centres. The effects of the neighbouring Easter mantle plume and Pacific–Nazca–Antarctic triple junction can be discerned in the bathymetry and ridge propagation rates, but do not appear to have had a major influence on the microplates’ development. 06 06 2022 09 1993 965 976 10.1144/gsjgs.150.5.0965 1 10.1144/crossmark-policy www.lyellcollection.org true 1992-07-27 1993-02-19 2022-06-06 https://doi.org/10.15223/policy-002 10.1144/gsjgs.150.5.0965 https://www.lyellcollection.org/doi/10.1144/gsjgs.150.5.0965 https://www.lyellcollection.org/doi/pdf/10.1144/gsjgs.150.5.0965 https://geoscienceworld.org/jgs/article-lookup?doi=10.1144/gsjgs.150.5.0965 10.1144/gsjgs.150.5.0941 10.1038/313472a0 10.1016/0012-821X(84)90158-4 10.1029/JB093iB04p02839 10.1016/0012-821X(87)90225-1 10.1016/0012-821X(88)90123-9 10.1016/0040-1951(78)90136-1 EOS, Transactions of American Geophysical Union Gallo D. 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R., Smith D. K.Variations in deformation style within migrating ridge axis discontinuities: insights from slope distributions and strain patterns (abstract)EOS, Transactions of the American Geophysical Union199172466 10.1038/356571a0 10.1029/JB088iB11p09393 10.1029/JB094iB09p12197 10.1038/302055a0 10.1130/0091-7613(1987)15<993:TEORDB>2.0.CO;2 10.1130/0016-7606(1980)91<263:EFTFSR>2.0.CO;2 Marchig V. et al. Geometep 19864. Sonne 40 Cruise Report Bundesanstalt für Geowissenschaften und Rohstoffe 1989 Marchig V. et al. 1989 Geometep 5. Sonne 62 Cruise Report Bundesanstalt für Geowissenschaften und Rohstoffe 1989. 10.1007/BF00366279 10.1016/0012-821X(83)90198-X 10.1029/GM057p0009 10.1029/90JB02398 10.1007/BF00366280 10.1029/JB090iB10p08603 10.1038/320738a0 Rusby R. I. 1992 a. Tectonic Pattern and History of the Easter Microplate Based on GLORIA and Other Geophysical Data. PhD Thesis University of Durham. 10.1144/GSL.SP.1992.060.01.05 Rusby R. I. 1992 b . 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