The magmatic, tectonic, and hydrothermal processes forming the oceanic crust along mid-ocean spreading centers are strongly modified at sedimented spreading centers but poorly understood owing to the lack of good crustal images. Here we present high-resolution deep seismic reflection images across the sedimented slow-spreading Andaman Sea spreading center. Several sub-horizontal sills are injected within the sedimentary strata, and no surface eruption is observed. On-axis reversed-phase reflections within the igneous crust correspond to axial magma lenses at different depths. The faults within the axial valley are steeply dipping (65°–75°) in a staircase pattern forming the axial graben. Their base coincides with a shallow-dipping (30°) reflection, defining the zone of extension and magmatism. As the sill-sediment sequences are rafted away from the axis, they are rotated and buried due to subsidence and faulting, forming the upper oceanic crust. The gabbroic lower oceanic crust is separated from the mantle by a complex Moho transition zone probably containing dunite lenses.

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