Search

Mapping of seafloor tectonic fabric in the Indian Ocean, using high-resolution satellite-derived vertical gravity gradient data, reveals an extinct Pacific-style oceanic microplate ('Mammerickx Microplate') west of the Ninetyeast Ridge. It is one of the first Pacific-style microplates to be mapped outside the Pacific basin, suggesting that geophysical conditions during formation probably resembled those that have dominated at eastern Pacific ridges. The microplate formed at the Indian-Antarctic ridge and is bordered by an extinct ridge in the north and pseudofault in the south, whose conjugate is located north of the Kerguelen Plateau. Independent microplate rotation is indicated by asymmetric pseudofaults and rotated abyssal hill fabric, also seen in multibeam data. Magnetic anomaly picks and age estimates calculated from published spreading rates suggest formation during chron 21o (~47.3 Ma). Plate reorganizations can trigger ridge propagation and microplate development, and we propose that Mammerickx Microplate formation is linked with the India-Eurasia collision (initial 'soft' collision). The collision altered the stress regime at the Indian-Antarctic ridge, leading to a change in segmentation and ridge propagation from an establishing transform. Fast Indian-Antarctic spreading that preceded microplate formation, and Kerguelen Plume activity, may have facilitated ridge propagation via the production of thin and weak lithosphere; however both factors had been present for tens of millions of years and are therefore unlikely to have triggered the event. Prior to the collision, the combination of fast spreading and plume activity was responsible for the production of a wide region of undulate seafloor to the north of the extinct ridge and 'W' shaped lineations that record back and forth ridge propagation. Microplate formation provides a precise means of dating the onset of the India-Eurasia collision, and is completely independent of and complementary to timing constraints derived from continental geology or convergence histories. © 2015 Elsevier B.V. K.J.M. and R.D.M. were supported by ARC Discovery Project DP130101946 . The CryoSat-2 data were provided by the European Space Agency, and NASA/CNES provided data from the Jason-1 altimeter. This research was supported by the National Science Foundation ( OCE-1128801 ), the Office of Naval Research ( N00014-12-1-0111 ), the National Geospatial-Intelligence Agency ( HM0177-13-1-0008 ) and ConocoPhillips . Version 23 of the global grids of gravity anomaly and VGG can be downloaded from the following ftp site ftp://topex.ucsd.edu/pub/global_grav_1min . All figures were produced using the Generic Mapping Tools ( GMT ) software ( Wessel et al., 2013 ). The open-source plate reconstruction software GPlates ( Boyden et al., 2011 ) was used to compute the distance from the Kerguelen Plume to the initiation point of ridge propagation using different absolute reference frames, and to produce the VGG raster reconstruction in Fig. 3 . Magnetic anomaly picks were accessed from the compilation of Seton et al. (2014) from The Global Seafloor Fabric and Magnetic Lineation Data Base Project website ( http://www.soest.hawaii.edu/PT/GSFML/ ). We thank the editor An Yin and two anonymous reviewers for their thoughtful and constructive comments that improved the manuscript. Appendix A