The geology of the Oceanographer Transform Fault

A detailed survey of the Oceanographer transform fault and environs at 35º N, 35ºW has yielded detailed information with respect to the generation and evolution of seafloor at a slowly accreting plate margin. From this data detailed bathymetric maps and maps of depth to basement have been constructed for a swath of seafloor 1800 km long and 100-200 km wide centered about the offset region. This data was used to subdivide major phases of seafloor spreading during the Tertiary. The ridge crest and all major topographic features near the transform appear to be affected by their proximity to the transform. The ridge crest widens and deepens towards the transform, and the rift valley walls are higher when they lie on the transform side of the valley than when they lie on the fracture zone side of the rift. Based upon the observed topographic effects the physical properties of the crust and upper mantle must vary markedly near the transform. Constraints provided by a combined ALVIN/ANGUS field program in the summer of 1980 indicate that the present zone of decoupling between the North American and African plates is located in the center of the transform valley; the zone of decoupling is less than 2 km wide, and is defined most often by discontinuous, variable degraded elongate fault or slump-degraded fualt scarps in sediment.-Vertical tectonism and mass-wasting processes dominate near the axial deep whereas deposition and erosion dominate in the terraces and the upper wall province. The petrologic data and the rock distribution data confirm that the crust near the Oceanographer transform is thin and even, perhaps discontinuous and indicates that the mode of crustal formation is likely to change significantly near transforms. Spectacular bedforms including abyssal furrows, dunes, longitudinal and transverse current ripples, and wave ripples are abundant. Except for the furrows these bedforms are concentrated in the very rugged, upper wall province of the transform. Chapter 3 presents a model for the formation of transform topography. This model hinges upon the assumption of a curved zone of decoupling between the plates near each ridge-transform intersection. This curvature gives rise to a geometric peculiarity which requires a gap to open between the plates. This gap conceivably should alter significantly many of the physical properties of the transform such as its depth, the height of the crestal mountains, the presence and width of any transverse ridges, and gradients of the seafloor into the intersection deep. Many of these properties vary in a fashion consistent with the model