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Cinthia Labails, NGU - Geological Survey of Norway (Norway)
Jean-Louis Olivet, Ifremer (France)
Daniel Aslanian, Ifremer (France)
Mohamed Sahabi, University of El Jadida (Morocco)
Walter Roest, Ifremer (France)
Bertrand Sichler, Ifremer (France)
Patrick Unternehr, Total (France)
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Magnetic anomaly maps derived from ship and airborne surveys play a key role in understanding plate tectonics and how our planet works. Gridded magnetic data from the Arctic and North Atlantic oceans and adjacent land areas (Verhoef et al., 1996) provided a basis to better understand the early stages of the evolution of the Central Atlantic Ocean. However, a gridded dataset from the West African margin (South of the Canary Islands) has been lacking. It was therefore decided to initiate a compilation of all digital magnetic data available from this region, which would assist in deciphering the history of the Central Atlantic Ocean that is still debated (ridge jump or not?). The present compilation mostly results from a patchwork of regional surveys from the National Geophysical Data Center (NGDC), Ifremer and others (Van der Linden, 1981 and Roeser et al., 2002), carried out at different epochs and with different instrumentation.
The use of gridded magnetic data improves the understanding of the early stages of the evolution of the Central Atlantic. Our model adopts a break-up at circa 195 Ma (Sinemurian basis) as proposed by Sahabi et al. (2004) - i.e. 20 myr earlier than what has generally been proposed in previous models. Based on this result and on the analysis of geophysical data (including new seismic lines and our interpretation of the newly compiled magnetic data and satellite derived gravimetry), our kinematic study shows evidence for three distinct phases. In contrast to other models, we propose that during the initial 30 myr (195-165 Ma, Lias-Dogger) oceanic accretion was extremely slow (∼0.8 cm/y). At Blake Spur time, (ca. 165 Ma, base Callovian), a drastic change occurred, both in the relative plate motions (from NNW-SSE to NW-SE) and spreading rate (increasing up to ∼ 4.8 cm/y). The BSMA (Blake Spur Magnetic Anomaly) is related to a great basement topographic change. From magnetic chron M22 (150 Ma, base Tithonian ) and onwards, the spreading rate slowed down to about 2.6 cm/y and remained constant until magnetic chron M0 (125 Ma, Barremian-Aptian boundary).
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