Overlapping spreading centres: new accretion geometry on the East Pacific Rise

Abstract

In a detailed Seabeam investigation of the East Pacific Rise (EPR) from 8°N to 18°N, a new kind of volcano-tectonic geometry associated with fast-spreading centres has been discovered (Figs 1, 2). At several locations along the rise axis the neovolcanic zone is discontinuous, and is laterally offset a short distance (1–15 km). In contrast to a classic ridge–transform–ridge plate boundary, however, the offset ridge terminations overlap each other by a distance approximately equal to or greater than the offset. They curve sharply towards each other and often merge into one another along strike. Separating the overlapping spreading centres (OSCs) is a closed contour depression up to several hundred metres deep which is sub-parallel to the trend of the OSCs. The region between the OSCs is a complex zone of both shear and rotational deformation with no obvious transform parallel structures. Based on wax model studies of spreading centres, we suggest here that transform faults fail to develop at fast spreading centres where the lateral offsets are small (<15 km), because the lithosphere is too thin and weak to maintain a classic, rigid plate spreading centre–transform fault pattern. The OSC geometry is unstable and evolves rapidly. One of the two OSCs prevails while the other is abandoned. A significant area of sea floor created at fast spreading rates may bear the imprint of this newly observed process.