Mantle Rock Exhumation at Saint Peter and Saint Paul Islets, Equatorial Atlantic Ocean, and its Implication to Possible Generation of Abiogenic Hydrocarbons by Serpentinization

Abstract

This paper presents the mantle rock exhumation at Saint Peter and Saint Paul Islets, Equatorial Atlantic Ocean, and its implication to possible generation of abiogenic hydrocarbons by serpentinization. In Saint Paul transform fault zone, there are two contrasting tectonic processes for mantle exhumation: Extension at amagmatic spreading centre and compression along a pressure ridge. On the ocean bottom, the ultramafic rocks of exhumed mantle react chemically with sea-water generating heat and abiogenic hydrocarbons, the phenomenon called serpentinisation. The amagmatic spreading with megamullion formation occurs along inter-transform ridge segments. The ultramafic rocks are originated from uppermost mantle and have advanced serpentinisation and little expressive plastic deformation. The olivine porphyroblasts show brittle fracturing. On the other hand, the pressure-ridge occurs along the transform fault span where the fault directions and relative plate motion are oblique. Due to the direction discordance, the strike-slip movement of the transform fault generates compression stress perpendicular to the fault. This stress squeezes out the underlying deep mantle from the ductile deformation depth up to the Earth's surface. The pressure ridge at Saint Peter and Saint Paulo Islet, namely Brachiosaurus Ridge, is the only confirmed example of mantle exhumation above sea level in Atlantic Ocean. The mantle rocks have little expressive serpentinisation and extremely developed plastic deformation, showing mylonitic texture. The porphyroclasts and matrix show either brittle fracturing or plastic deformation.