Journal of Conference Abstracts

Along-Axis Variations in Lithospheric Stress at Slow-Spreading Mid-Ocean Ridges

Donna K. Blackman

Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK

d.blackman@earth.leeds.ac.uk

Three-dimensional numerical models of asthenospheric flow and deformation in the oceanic lithosphere predict along-axis variability in the stress field that reflects the geometry of the ridge-transform boundary. Comparison of the predicted lithospheric deformation and the patterns of faulting and morphology observed along the Mid-Atlantic Ridge can provide constraints on the relative importance of mantle vs. crustal processes in creating seafloor relief.

The series of 3-D Boundary Element calculations performed in this study show how spreading rate, transform offset and segment length each influence the flow/stress fields that develop during plate driven asthenospheric flow at a ridge-transform boundary. The predicted patterns of stress-supported seafloor relief generally follow those observed: median valleys are predicted at slow-spreading ridges vs. small axial highs at fast spreading ridges; nodal deeps occur at ridge-transform intersections for offsets greater than 25 km; longer segments have more along-axis deepening than short segments which do not shoal much in their center. The predicted amplitude of the stress supported topography is about one third to one half that observed for an assumed asthenospheric viscosity of 5 x 10¹⁹ Pa s and a weak lithosphere (local compensation).

Deformation of the lithosphere is modelled by applying the stresses calculated from the asthenospheric flow model as boundary conditions on a variable thickness elastic plate model. In this case, all edges of the lithosphere are discretized into Boundary Elements. The seafloor is stress-free, the transform plate edge has no or low stress, and at large distance from the ridge axis a tensional stress is applied in the plate spreading direction. Stresses within the plate are then calculated and the associated seafloor relief and fault orientation patterns can be predicted. Preliminary results show that the variable plate thickness, in combination with the free-surface effect near the ridge-transform intersection, may be quite important in the development of inside corner highs such as are often observed at slow-spreading centers. Rather than a simple pattern of seafloor deepening with some asymmetry with respect to the ridge-transform intersection (as predicted for local compensation of the flow stresses), a pattern of axial tension switching to compression just off-axis and then back to vertical tension further away is predicted for the elastic plate model in the vicinity of the intersection. Thus, even though vertical normal stresses at the base of the plate would not directly result in the local formation of inside corner highs or transverse ridges, the response of a variable thickness plate with strength to these loads is more complex and may be conducive to formation of such features.

The sensitivity of the model results to assumed parameters will be discussed and maps of the principal stress directions within the lithosphere will be presented. Comparison of the predicted fault orientations with observed patterns will be based on both pre-existing and newly collected data. Initial results from a Spring 1996 sidescan/bathymetry experiment using TOBI along the Mid-Atlantic Ridge 29°-30°N are expected to be available in a preliminary format for the meeting. These data should cover a portion of the ridge-transform intersection and the axial section of a complete ridge segment from the Atlantis transform south to a non-transform offset. Quantitative assessment of the similarities and differences in observed morphology, gravity and fault orientations vs. that predicted to develop due to viscous flow stresses can help delimit the role that crustal structure must play in seafloor deformation.