In many highly extended rifts on the Earth, tectonic removal of the upper crust exhumes mid-crustal rocks, producing metamorphic core complexes. These structures allow the upper continental crust to accommodate tens of kilometres of extension1, but it is not clear how the lower crust and underlying mantle respond. Also, despite removal of the upper crust, such core complexes remain both topographically high and in isostatic equilibrium. Because many core complexes in the western United States are underlain by a flat Moho discontinuity2,3, it has been widely assumed that their elevation is supported by flow in the lower crust4,5,6 or by magmatic underplating7. These processes should decouple upper-crust extension from that in the mantle. In contrast, here we present seismic observations of metamorphic core complexes of the western Woodlark rift that show the overall crust to be thinned beneath regions of greatest surface extension. These core complexes are actively being exhumed8 at a rate of 5–10 km Myr-1, and the thinning of the underlying crust appears to be compensated by mantle rocks of anomalously low density, as indicated by low seismic velocities. We conclude that, at least in this case, the development of metamorphic core complexes and the accommodation of high extension is not purely a crustal phenomenon, but must involve mantle extension.
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The field effort in PNG would not have been successful without the efforts of S. Sioni, University of Papua New Guinea, J. Dickson, H. Sailasa and others of the Milne Bay Provincial Administration, PNG, and sub-district administrators throughout the province. Instruments and technical assistance were made available through the IRIS PASSCAL Instrument Center. The Geological Survey of Papua New Guinea gave permission to publish the gravity data on Fig. 4. We thank C. Ruppel for comments. This work was supported by the Geosciences Directorate of the National Science Foundation.
The authors declare that they have no competing financial interests.
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Abers, G., Ferris, A., Craig, M. et al. Mantle compensation of active metamorphic core complexes at Woodlark rift in Papua New Guinea. Nature 418, 862–865 (2002). https://doi.org/10.1038/nature00990
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