Magnetic pattern of the famous zone

2 Surface magnetic anomalies in the Famous area define a typical zebra pattern related to sea floor spreading. This pattern is symmetrical and corresponds to a 1.1 cmlyr half spreading rate up to anomaly 3 ' . The axis of the central block is off-set from the topographic axis of the rift and indicates either asymmetric spreading in the central zone or a ridge axis jump. These two hypotheses are discussed. Carsctbristiquer magnbtiquer de la zone FAMOUS Résurni. --A partir des donnees magnétiques de surface, un réseau d'anomalies positives et négatives paralléle au rift médio-atlantique est mis h jour. Ce réseau est symétrique par rapport A l'anomalie centrale. Il correspond h un modèle d'épanchement de croûte océanique au demi-taux de 1.1 cmlan et ce jusqu'h l'anomalie 3' . L'axe du bloc central du modèle est décalé par rapport h l'axe topographique du Rift, suggérant soit un 6panchement asymétrique, soit un saut de dorsale. Ces deux hypothèses sont discutées. Introduction. Thirdly, the relative amplitude of the anomalies from the most external to the central ones, does not Total field magnetic data has been gathered in but stay rather uniform. the FAMOUS area during several cruises from 1972 to 1974 aboard N. 0. d7~ntrecasteaux, N. O. Jean Charcot. and N. O. Le Noroît. Data cover the norInterpretation. thern half of the FAMOUS ridge segment located between fracture zones A and B [Needham and Francheteau, 19741. Magnetic data have been corrected for the regional field using I.G.R.F. 1965. Residual magnetic anomalies have been projected a t right angle to the ship tracks for the ones perpendicular to the rift axis (trend 0200N). Over the central rift itself, more crossings were available and the projected anomalies are shown in the inset in the bottom left corner of figure 1. Several observations can be made from these data. First of all, there exist some strongly lineated anomalies parallel to the rift axis up to 30 nautical miles from the axis with amplitudes of a few hundred gammas. Beyond that distance, the magnetic anomalies are not lineated anymore and cannot be correlated from profile to profile, their wavelength increase sharply to the east (no data available to the west). Using only 1973 cruise data (in order to avoid secular variations), a correlation of positive and negative anomalies has been made and a zehra pattern typical of ocean spreading has been obtained (fig. 2). The pattern is clearly symmetrical with respect to the central rift anomaly. Trend of anomalies is 0200N for the central ones and 0450 for the most external one. In order to model this pattern, a 400 meters thick, 1500 meters deep uniform slab of 0.01 emu/cc susceptibility was used together with the time scale determined for the Heykjanes ridge in a detail survey [Talwani et al, 19711. Small variations seen in the trend of the magnetic anomalies would result in small variations of spreading velocity and in limited asymmetry but tliese were not judged significant. Figure 2 shows the correspondence between one observed magnetic profile and the magnetic anomaly of the model finally choosen. Half-spreading rate used is 1.1 cmlyr. Stacking of ~ e c o n d l ~ , the central magnetic anomaly exhibits a double peak with intensities decreasing rapidly * Centre océanogr. de Bretagne, Brest. Coiitributioii no 512 to the south. On both flanks of the central anomaly, du scientifique. small upward inflexions can be observed on most Note rése entée le 27 novembre 1975 : maiiuscrit remis le profiles. 3 févrief 1976. Bull. Soc. gkol. Fr., 1976, ilo 4.