Nc. Mitchell et al., The Bouvet triple junction, 20 to 10 Ma, and extensive transtensional deformation adjacent to the Bouvet and Conrad transforms, J GEO R-SOL, 105(B4), 2000, pp. 8279-8296
The Bouvet triple junction has been proposed to have evolved as a ridge-fau
lt-fault (RFF) type between 20 and 10 Ma, connecting the southernmost Mid-A
tlantic 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. Th
e 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 junc
tion was indeed RFF. Our second major result is that the SMAR fabrics are e
xtensively crosscut by normal faults, over most of the 400 km surveyed alon
g both transforms and most intensely north of the Bouvet transform. Growth
faults and faults affecting the sediment surface in multichannel seismic im
ages 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.