Past, present, and future controlled source Ocean Bottom Seismometer (OBS) surveys for exploring the oceanic transform discontinuities in the Atlantic Ocean

About 70% of the Earth’s surface is covered by the ocean, hiding some of the fundamental processes behind the inner workings of our planet. Over a century ago, in the unknown seafloor panorama, the maps of Marie Tharp and colleagues revealed the presence of mid-ocean ridges, wrapping around the globe along which the plates have been forming at rates varying from slow to fast. Even in these early bathymetry maps, it was clear that the ridges are not continuous features but interrupted by the transform faults along which the plates slide past each other, leaving the record in the seafloor that can be followed on the flanks for hundreds of millions of years. The lithospheric structure of these tectonic discontinuities has remained one of the big unknowns since the early days of deep-sea exploration.

One way to scan the subsurface of the oceanic crust is by using a controlled source to produce seismic waves that propagate below the seafloor, which are finally captured by Ocean Bottom Seismometers (OBS) laid on its surface. In the 1980/90s, such surveys focused on exploring transform discontinuities offsetting the Mid-Atlantic Ridge: Kane, Vema, Oceanographer, Charlie Gibbs, and Tydeman Transform Faults (TF). Although limited in number of instruments, these studies established the common view that transform faults in slow-slipping environments are typically represented by significantly thinner than average oceanic crust (~3km vs. 6-7km), comprised of a mafic layer.

In the past several years, a wealth of modern OBS data has been acquired, providing new insights into the morphotectonic characteristics of the transforms. Here, I focus on the main findings from the data collected in the equatorial Atlantic¹. First, along the profile crossing the Romanche TF, the most extended tectonic structure on Earth, close to normal depth to Moho (~5 km below seafloor), is found, proposing that the crustal structure along the TF strike can vary significantly and, therefore influence seismogenic behavior along the transform plate boundary, which is poorly understood. In addition, lower velocities in the upper mantle suggest extensive serpentinization and water infiltration down to ~16km. In contrast to the transform fault domains, their fossilized trace consistently shows crustal thicknesses close to the average igneous crust, reported in legacy and modern data (Chain and St. Paul). This intriguing observation is explained by the mechanism of lateral dike propagation, supported by the presence of globally observed J-shaped structures in the seafloor bathymetry. A global compilation of bathymetry data further supports this view, proposing a new framework to be established behind the formation of oceanic crust at the ridge transform intersection. In fact, little is known about the formation of oceanic crust in slow-spreading environments globally. To shed light on this aspect, new dedicated OBS surveys are necessary. One such collaborative project that will employ active and passive seismic, in concert with interdisciplinary data sampling, is in preparation for the Mohn’s Ridge² in the Arctic and will be presented in more detail during the talk.

¹ & ² contributions from the ILAB-SPARC and MoKA-Pot teams, respectively.