A Deep Dive Into a Ridge-Transform Fault Intersection: Volcano-Tectonic Relationships in an Enhanced Cold-Edge Effect at the Romanche Fracture Zone

At mid-ocean ridges, the mantle temperature and composition, and the lithosphere thickness control the melting conditions, which influence the feeding and structure of the neo-volcanic areas. We present a study of the eastern intersection between the Romanche transform fault (TF) and the adjacent southward segments of the Mid-Atlantic Ridge (MAR). A strong thermal gradient is expected along the axis. We focus on the construction of the axial domain and on the spatial and temporal volcano-tectonic interplays. The geological mapping of the intersection shows a progressive variation in the orientation of the normal faults from about 30° to 90° oblique to the spreading direction with increasing distance from the TF. This attests to the decrease of the transform-related shear stress component and the thinning of the axial lithosphere southward. Contextually, the coupled thermal edge effect influences the effusion rate and magma supply which in this area are low compared to other MAR segments. However, recent volcanic activity along the axis starts about 10 km south of the ridge-transform intersection. Rather than a progressive increase of the magma supply with the distance from the TF, the volcanic activity seems to increase abruptly about 30 km south of the intersection. The segment RC2, from ∼30 to 55 km from the Romanche TF, displays more numerous and larger seamounts than the segment RC3 located ∼80 km from the TF, an observation which we interpret to result from the increased thickness of the lithosphere under RC2, due to its proximity with the ridge-transform intersection.