Abstract

The FAMOUS area, which straddles the Mid-Atlantic Ridge on the southwest extension of the Azores Plateau in the Atlantic Ocean, has been studied by means of morphotectonic and magnetic anomaly analysis. Major morphologic elements are (1) the rift valleys and associated fault blocks, (2) the transform faults, and (3) diagonal linear trends intersecting the azimuths of the rift valleys and transform faults. Within the past 6 m.y. the area has been characterized by a reorientation of the spreading axis, from an oblique trend toward an orthogonal pattern relative to the transform faults. The long and nearly continuous early spreading axis, with a strike of N50 °E, broke up into smaller rift segments, which progressively rotated to their present strike (about N23 °E) and now appear to be recombining through asymmetric spreading to form the continuous axis it now has. The process involved complex migrations of the larger rift segments through asymmetric spreading, as well as jumping of shorter rift segments and migration of transform faults, the latter mechanism leading to the diagonal troughs. The presence of parallel spreading axes and the propagation of rift axes may be due to coupling across transform plate boundaries. These boundaries are complex shear zones with tension cracks and en echelon fault scarps deviating from the overall trend of the zone; this implies that the fracture zones might be leaky. The reorientation of the spreading axis in the FAMOUS area seems not to be a unique event but a phenomenon that has been repeated periodically. It has tentatively been correlated with recurrent plume activity below the Azores Plateau.

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