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

Oceanic transform faults 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. In this study, we report the presence of a 10-15-km-thick in-depth band of microseismicity in 10-34 km depth range associated with a high-temperature (700-900°C) mantle below the brittle lithosphere along the Romanche mega-transform fault in the equatorial Atlantic Ocean. The occurrence of the shallow 2016 Mw 7.1 super-shear rupture earthquake and these deep microearthquakes indicate that although large earthquakes along transform faults occur in the upper brittle lithosphere, a significant amount of deformation is accommodated in the semi-brittle 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 heterogeneity along the transform fault.

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