Deep hydration and lithospheric thinning at oceanic transform plate boundaries.

Transform faults accommodate the lateral motions between lithospheric plates, producing large earthquakes. Away from active transform boundaries, former oceanic transform faults also form the fracture zones that cover the ocean floor. However, the deep structure of these faults remains enigmatic. Here we present ultra-long offset seismic data from the Romanche transform fault in the equatorial Atlantic Ocean that indicates the presence of a low-velocity anomaly extending down to ~60 km depth below sea level. We performed 3D thermal modelling that suggests the anomaly is likely to be due to extensive serpentinization down to ~16 km depth, overlying a hydrated, shear mylonite zone down to 32 km depth. The water is considered to be sourced from seawater-derived fluids that infiltrate deep into the fault. Below 32 km is interpreted to be a low-temperature, water-induced melting zone that elevates the lithosphere-asthenosphere boundary, causing significant thinning of the lithosphere at the transform fault. The presence of a thinned lithosphere at transform faults could explain observations of volcanism, thickened crust and intra-transform spreading centres at transform faults. It also suggests that migration and mixing of water-induced melt with the high temperature melt may occur beneath the ridge axis.

Semibrittle seismic deformation in high-temperature mantle mylonite shear zone along the Romanche transform fault.

Oceanic transform faults, a key element of plate tectonics, represent the first-order discontinuities along mid-ocean ridges, host large earthquakes, and induce extreme thermal gradients in lithosphere. However, the thermal structure along transform faults and its effects on earthquake generation are poorly understood. Here we report the presence of a 10- to 15-kilometer-thick in-depth band of microseismicity in 10 to 34 kilometer depth range associated with a high-temperature (700° to 900°C) mantle below the brittle lithosphere along the Romanche mega transform fault in the equatorial Atlantic Ocean. The occurrence of the shallow 2016 moment magnitude 7.1 supershear rupture earthquake and these deep microearthquakes indicate that although large earthquakes occur in the upper brittle lithosphere, a substantial amount of deformation is accommodated in the semibrittle mylonitic mantle that resides at depths below the 600°C isotherm. We also observe a rapid westward deepening of this band of seismicity indicating a strong lateral heterogeneity.

Seismic Crustal Structure and Morphotectonic Features Associated With the Chain Fracture Zone and Their Role in the Evolution of the Equatorial Atlantic Region.

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.