Abstract

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.

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