Mid-Atlantic Ridge at 13-14N: Evidence of Unstable Seafloor Spreading Processes From Deep-Towed Magnetic Measurements

During cruise JC007 in March-April 2007 we recorded total magnetic field anomalies over two active and one defunct oceanic core complex (OCC) and the intervening seafloor. Measurements were made by towed magnetometer at the sea surface, and by the TOBI deep-towed vehicle approximately 400 m above seafloor, along 13 E-W lines about 60 km long and spaced 3 to 6 km apart. Sea-surface data show a fairly coherent central anomaly on most lines, though on some it is significantly displaced from the spreading axis as indicated by bathymetry and side-scan sonar data. Modelling in terms of a standard, simple (but probably unrealistic), continuous reversal sequence requires total spreading rates ranging from about 15 to 40 km/Myr with offsets of the axis up to 20 km and highly asymmetric spreading. The deep-towed data were corrected for the heading-dependent magnetic effects of the TOBI vehicle before inversion to crustal magnetisation using the 2D Parker & Huestis (1974) procedure. These results were checked by comparing with inversions of the sea-surface field, which shows similar features at reduced resolution. The deep-towed inversion results show a rather incoherent magnetisation pattern. The central magnetisation high is nowhere more than 13 km wide, only 70% of the expected width of the Brunhes here, and several profiles yield apparently negative magnetisation over areas we expect to be of Brunhes age based on sonar and bathymetry data. This may due to a combination of destruction of magnetisation by faulting (Hussenoeder at al., 1996), departure from the 2D geometry assumed for the inversions, and departure (via tectonic rotation) from the assumed constant magnetisation direction. We are now carrying out fully 3D inversions and forward modelling guided by the structural evidence provided by sidescan and bathymetry. These results will be presented and discussed in relation to the seafloor spreading history and structure of the region.