Interplay between sedimentation and volcanism at a young and highly segmented oceanic ridge

Seismic reflection profiles from the Liberia basin (Equatorial Atlantic) image characteristic structures of lower oceanic crust, down to Moho. However, instead of the characteristic strong reflection topping oceanic crust, a transitional seismic facies neither typically magmatic nor sedimentary in nature is observed. This transitional package is interpreted as a volcano-sedimentary sequence. Within this package, reflections decrease in intensity continentward, which may correspond with a decrease of the volume of extrusive material, and upward, which may correspond with a migration of the magmatic source away from the continent. These observations support the idea that magma was emplaced during sedimentation at an active spreading center. Thus, in contrast to classical sediment-starved mid-oceanic ridges, where the ratio between sedimentation rate and volcanic eruption rate is low, in the Liberian example it is high. Such high sedimentation rates require a sediment dispersal system that can directly supply a spreading center. Such a situation is likely to occur in an en-echelon segmented, young spreading system bounded by transform margins. There, seafloor spreading occurs within pull-apart basins that acted as sinks for sedimentary gravity flows. In this setting, as is the case for the Equatorial Atlantic at Albian/Cenomanian times, sediments are transported mostly from the hinterland toward the center of the basin, i.e., onto the young oceanic ridge.