Oceanic transform faults and fracture zones (FZs) represent major bathymetric features that keep the records of past and present strike-slip motion along conservative plate boundaries. Although they play an important role in ridge segmentation and evolution of the lithosphere, their structural characteristics, and their variation in space and time, are poorly understood. To address some of the unknowns, we conducted interdisciplinary geophysical studies in the equatorial Atlantic Ocean, the region where some of the most prominent transform discontinuities have been developing. Here we present the results of the data analysis in the vicinity of the Chain FZ, on the South American Plate. The crustal structure across the Chain FZ, at the contact between ∼10 and 24 Ma oceanic lithosphere, is sampled along seismic reflection and refraction profiles. We observe that the crustal thickness within and across the Chain FZ ranges from ∼4.6-5.9 km, which compares with the observations reported for slow-slipping transform discontinuities globally. We attribute this presence of close to normal oceanic crustal thickness within FZs to the mechanism of lateral dike propagation, previously considered to be valid only in fast-slipping environments. Furthermore, the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale. Our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower-order Mid-Atlantic Ridge segmentation around the equator.

The oceanic lithosphere (crust and upper mantle) is formed along the mid-ocean ridges (MOR) in a 5-20 km wide zone above a mantle melting zone extending down to ∼60-100 km depth (Langmuir & Forsyth, 2007). The newly formed lithosphere subsides as it moves away from these axes and is cooled by conductive and convective processes as it interacts with ocean water (e.g., Hasterok, 2013;Lister, 1972;Stein & Stein, 1994). Although the composition and structure of crust and upper mantle is set at the MOR axis, the thermal and chemical properties likely evolve with age (Stein & Stein, 1994). Except in the regions of seamounts, which account for 5%-10% of the seafloor, the remaining 90% of ocean floor is void of intra-plate magmatism (McClain & Orcutt, 1989). This implies that hydrothermal processes are the predominant factor for crustal evolution and conductive cooling is the main factor for lithospheric mantle evolution. We can thus identify two major types of processes affecting the properties of lithosphere, the processes that contribute to its formation at the ridge axis (e.g., magmatic, tectonic, hydrothermal phenomena) and the processes that contribute to its evolution with age (e.g., conductive cooling and hydrothermal circulation).

Plate tectonics separates Earth' surface into rigid plates (McKenzie & Parker, 1967;Morgan, 1968), and deformation or relative motion between plates reveals three different types of oceanic plate boundaries: (a) constructive plate boundaries at mid-ocean ridges (MOR) where new seafloor is created, (b) destructive plate boundaries at subduction zones where the oceanic lithosphere is transferred into the mantle and recycled, and (c) conservative plate boundaries and hence transform faults (TF) where the lithosphere is neither created nor destroyed as plates move past each other (Morgan, 1968). In ocean basins, transform faults