Drilling reveals transition from continental breakup to early magmatic crust

Abstract

For the first time in an ocean‐continent transition zone (OCT), drilling has sampled rocks above and below what appears to be a synrift tectonic crust‐mantle contact. Research in the West Iberia OCT is assembling important data on the histories and ages of the basement rocks there. The principal result has been to show that evolution of the margin was dominated by extensional tectonics with only slight evidence of possible synrift melting.A significant preliminary finding of the recently completed Leg 173 of the Ocean Drilling Program (ODP) is that many, if not all, fault blocks underlain by a low‐angle reflector on multichannel seismic profiles under the 5000‐m deep Iberia Abyssal Plain consist of continental crust and are quite thin. The drilling has also shown that lower crust and peridotite became exposed in the OCT between the blocks of continental crust and the first products of oceanic accretion over a zone that appears to be tens of kilometers wide. This supports the hypotheses that predict a broad zone of upper mantle basement beneath the sediments in the development of the West Iberia zone. Preliminary observations of textural and compositional variations of the peridotite from east to west suggest that further studies will document the evolution of the mantle from continental breakup to oceanic accretion.