Evidence for reflectors in the lower continental crust before rifting in the Valencia trough

THE past decade has seen a rapid advance in our understanding of the deep structure of the continental crust, especially in Europe1-3, Australia4 and North America5, where seismic profiling has revealed a highly reflective layer in the lower crust. On many profiles, the reflective layer is associated with regions of extension6,7, suggesting that the reflectors originate by some form of ductile flow or magmatic intrusion during rifting8,9. To test this hypothesis, we have examined geophysical and geological data from the Valencia trough, a rift basin10-12 of Late Oligocene/Early Miocene age. From a comparison of profiles in different parts of the trough, and independent constraints on the initial crustal thickness, we argue that the reflective layer in this basin was probably already in the crust before rifting, and that a mid-Tertiary extensional event is not required to explain it