Crenulated seafloor: Evidence for spreading-rate dependent structure of mantle upwelling and melting beneath a mid-oceanic spreading center

The availability of dense GEOSAT-GM satellite altimetry data south of 30°S [1] provides the first opportunity to compare at ∼ 25 km resolution seafloor generated at ridges spreading with a wide range of spreading rates. The axis of the slow-spreading Southwest Indian Ridge as well as the seafloor that it has generated has a distinctly crenulated appearance. This crenulated seafloor shows regularly spaced gravity lineations in the spreading direction, crenulations that record stationary centers of mantle upwelling over the life of the Southwest Indian Ridge. A similar pattern of crenulation is seen along sections of the slow-spreading northern Mid-Atlantic Ridge, but along the faster (intermediate) spreading southern Mid-Atlantic Ridge, while regularly spaced highs are apparent along sections of the spreading axis, stationary crenulation does not persist off the axis. This pattern implies that the faster spreading southern Mid-Atlantic Ridge has variable centers of time-dependent upwelling that migrate along the ridge. Crenulations are not visible near the axis of the fast-spreading Pacific-Antarctic Ridge. Crenulation amplitudes decrease strongly with increasing spreading rate. The absence of crenulation at high spreading rate could be due to a 3D/2D transition in flow structure or the effect of mechanisms to redistribute crust along axis. While crenulations are not ubiquitous, even at slowly spreading ridges, where they exist they provide a time record to observe the characteristics of mid-ocean ridge upwelling, melting and melt migration not only beneath active spreading centers but through a much longer spreading history.