Ocean Drilling Program, United States

We present new magnetic measurements from deep-towed magnetometers, operated within a few hundred meters of the seabed of the Iberia Abyssal Plain, over oceanic crust and within the ocean-continent transition itself. In December 1991, the Charles Darwin towed a three-component fluxgate magnetometer, mounted on the Institute of Oceanographic Sciences' towed ocean-bottom instrument (TOBI), along 40 degrees 40'N in the Iberia Abyssal Plain. In July 1993, Le Suroit towed a similar magnetometer fitted to Ifremer's Systeme Acoustique Remorque (SAR) deep-tow vehicle over Ocean Drilling Program (ODP) Sites 897, 898, and 899. The observed TOBI profile shows not only a steep gradient on the east flank of the J anomaly, but also anomalies of hundreds of nT over the oceanic crust. Immediately east of the peridotite ridge sampled at Site 897 there are two strong peaks, but further east the anomalies become more subdued. The SAR profile similarly shows a large broad anomaly of hundreds of nT over the oceanic crust and a very strong peak over Site 899 within the ocean-continent transition. Both profiles exhibit the broad trough identified on surface anomalies immediately west of the peridotite ridge. We present two-dimensional models constrained by seismic refraction and reflection profiles and loosely constrained by the magnetic properties of the ODP cores. The common mismatch in amplitude and phase, between the observations and the anomalies resulting from basement relief alone, indicates that magnetization contrasts must exist within the ocean-continent transition crust itself. The results presented here do not at present help us to distinguish definitively between a number of alternative hypotheses for the formation of the ocean-continent transition. However, the unexpectedly low amplitude of some of the relatively short (a few kilometers) wavelength anomalies in the observations may indicate that no significant magnetization contrasts exist on scales of less than 10 km along the profiles near the top of basement. Nevertheless, such contrasts may still exist a few kilometers below the top of basement.