The determination of melt distribution in the crust and the nature of the crust–mantle boundary (the ‘Moho’) is fundamental to the understanding of crustal accretion processes at oceanic spreading centres. Upper-crustal magma chambers have been imaged beneath fast- and intermediate-spreading centres1,2,3,4 but it has been difficult to image structures beneath these magma sills. Using three-dimensional seismic reflection images, here we report the presence of Moho reflections beneath a crustal magma chamber at the 9° 03′ N overlapping spreading centre, East Pacific Rise. Our observations highlight the formation of the Moho at zero-aged crust. Over a distance of less than 7 km along the ridge crest, a rapid increase in two-way travel time of seismic waves between the magma chamber and Moho reflections is observed, which we suggest is due to a melt anomaly in the lower crust. The amplitude versus offset variation of reflections from the magma chamber shows a coincident region of higher melt fraction overlying this anomalous region, supporting the conclusion of additional melt at depth.
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The 3D seismic reflection data were acquired onboard RV Maurice Ewing, processed by Robertson Research Ltd, and interpreted using the 3D VoxelGeo software of Paradigm Geophysical. The ARAD seismic experiment was an international collaborative project between investigators from the University of Cambridge and Scripps Institution of Oceanography and was funded by the UK Natural Environment Research Council, British Institutions Reflection Profiling Syndicate, and the United States National Science Foundation. This is a Department of Earth Sciences (Cambridge) contribution and an Institut de Physique du Globe de Paris contribution.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
Two-dimensional seismic sections through the 3D cube. Seismic sections along cross-line distance of 2.25 km (a) and in-line distance of 14.55 km (b). Moho reflections are clearly observed beneath the western limb of the OSC, beneath the OSC basin and beneath the AMC. On the eastern limb a continuous AMC reflection is observed at about 4500 ms two-way travel time (b). The two-way travel time between the AMC reflection and the Moho reflection increases from 1.30 sec in the south to 2.0 sec in the north. (PDF 1823 kb)
Three-dimensional view of reflection images of Moho and melt sill. (a) Colour version of Figure 2 without any interpretation and (b) black and white version of Figure 2 without any interpretation. See Figure caption of Figure 2 for detailed description. (PDF 3060 kb)
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Singh, S., Harding, A., Kent, G. et al. Seismic reflection images of the Moho underlying melt sills at the East Pacific Rise. Nature 442, 287–290 (2006). https://doi.org/10.1038/nature04939
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