Strike-Slip Versus Extensional Tectonics at the Oceanographer Transform Fault, Mid-Atlantic Ridge at 35°N

Oceanic transform faults and their fracture zones are among the most striking features of ocean basins. Plate tectonics describes them as strike-slip zones connecting mid-ocean ridge segments. Still, no generally accepted theory exists for the lateral strain partitioning resulting in the deep and wide transform valleys and extensively tectonized inside corners. Here, we present results from multibeam bathymetry and a micro-seismicity survey from the slow-slipping Oceanographer transform on the Mid-Atlantic Ridge near 35°N. Swath-mapping echosounder data reveal a segmented transform fault. Away from the ridge-transform intersections (RTI) and in the eastern half of the transform valley, micro-earthquakes recorded on ocean-bottom-seismometers focus along the observed fault strands. Approaching the RTI, however, many micro-earthquakes cut across the inside corner, while the active faults step toward the inside corner, paralleling the trend of the transform valley. Focal mechanisms point to extension in the inside corner region, while strike-slip deformation is only recorded at distances larger than 15 km and away from the RTIs. These observations support a scenario in which deformation beneath a right-angular ridge-transform boundary at the seafloor develops into an oblique shear zone at depth, causing crustal thinning and consequently forming transform valleys. Away from RTIs, seismicity is focused on a narrow and segmented strike-slip fault as predicted by plate tectonics. Oceanic transform faults are consequently not only strike-slip but are also shaped by extensional processes, arguing for a revision of the concept of conservative plate boundaries to account for their morphology, segmentation, and significant lateral differences in seismic behavior.