Geological overview of the Oceanographer Transform Fault

Recent studies on oceanic transform faults, one of the three fundamental types of plate boundaries, has suggested that they may not be purely conservative features and that the crust formed adjacent to them (on the "inside corners" of the ridge-transform intersection) may differ in structure and composition significantly from outside-corner crust. Here we present a geological map of the Oceanographer Transform (Atlantic Ocean, southwest of the Azores) created by combining an interpretation of multibeam bathymetry, rock sampling and seafloor visual observations. We find that outside- and inside-corner crust at the ridge transform intersection have distinctive morphologies and petrography: the outside corner shows rough seafloor, from which only pillow basalts are recovered, extending all the way to the fracture zone. The inside corners, in contrast, are characterized by both rough, basaltic seafloor and regions that are much smoother, from which serpentinized peridotite are often recovered. The width of the inside-corner region showing this variable morphology, bathymetry and petrography seems to vary over time from 10 to 25 km. In two places, oceanic core complex crust is recognized close to the transform in this inside-corner region. We emphasize that plate production at the inside corner appears to occur via a variety of magmatic and amagmatic processes.