The Bouvet triple junction, 20 to 10 Ma, and extensive transtensional deformation adjacent to the Bouvet and Conrad transforms

The Bouvet triple junction has been proposed to have evolved as a ridge-fault-fault (RFF) type between 20 and 10 Ma, connecting the southernmost Mid-Atlantic Ridge (SMAR) with the Bouvet and Conrad transforms, to the east and west, respectively. We surveyed immediately north of these two transforms with side-scan and multibeam sonars, on seafloor that would have originally been created at the SMAR close to its junction with the two transforms. The sonar data reveal that SMAR fabrics on the Bouvet and Conrad sides, when corrected for plate rotation, are parallel to each other, so they were most likely formed at the same spreading ridge and confirm that the triple junction was indeed RFF. Our second major result is that the SMAR fabrics are extensively crosscut by normal faults, over most of the 400 km surveyed along both transforms and most intensely north of the Bouvet transform. Growth faults and faults affecting the sediment surface in multichannel seismic images show that the deformation has been long-lived and is probably ongoing. Since the orientations of the crosscutting faults are similar to those of shear zone tension fractures, we interpret these areas to be transtensional zones. This extent of deformation adjacent to major oceanic transforms is rare, and we develop a number of ideas to explain its origin. Copyright 2000 by the American Geophysical Union.