Marine magnetic anomalies and plate motions in the central Atlantic region during the Oligocene and early Miocene

Although the plate kinematics associated with the opening of the central Atlantic ocean after the break-up of Pangaea has been the subject of several studies since the late 1960s, there are still open problems and debated solutions to the tectonic evolution of this area. In particular, the initial fit of Pangaea, the spreading directions during the early stages of opening, the existence of ridge jumps, and the entity of intraplate deformation processes in northwest Africa are still subject to different interpretations by different research groups. The objective of this study is a reassessment of the central Atlantic plate kinematics since the early Jurassic through a re-examination of marine magnetic anomalies, fracture zone trends and geologic data. A total of 215 magnetic ship-tracks from the NGDC GEODAS database for the time interval from 1964 through 1994, in the area comprised between the Atlantis FZ and the Azores triple junction, were analyzed. The data quality was assessed through an examination of Kp indices. Magnetic data collected during disturbed days were filtered away. Thus, 29 reliable magnetic profiles were extracted having an azimuth that differed from the fracture zones trend by less than 30° and did not cross any fracture zone. The ship-track segments were projected onto flow lines that parallel existing fracture zones in order to avoid shape distortion and possible misinterpretation of the magnetic anomalies. Finally, the magnetic data were high-pass filtered to remove trends. A new advanced software tool for the analysis and interpretation of the anomalies was used improving the reliability of magnetic anomaly identifications. The main result of this work has been the assessment that an independent Moroccan plate existed during the Oligocene and early Miocene, which moved eastward relative to northwest Africa. This event has significant implications for the fit of central Pangea during the early Mesozoic. A new map of the magnetic lineations in the central Atlantic is proposed, which is based on an analysis of magnetic anomaly profiles and fracture zone geometry in the zone north of the Atlantis FZ. This map overcomes the flaws of previous compilations, even if it retains much of the classic works. The structural pattern that results from this study evidences that: 1) the whole set of North American isochrons are shifted to the west in the area north of the Atlantis FZ with respect to the classical compilations; 2) a unique spreading direction existed during the early and middle Jurassic, and until the M25 M21 time interval in the late Jurassic. Such a spreading direction is compatible with that proposed in a recent model of opening of the proto-Atlantic, but extends the trend well beyond the early Jurassic.