Megamullion

Megamullion, called also oceanic core complex (OCC), is an abyssal geologic structure showing ellipsoidal shield-like positive morphologic relief. It is constituted mainly by serpentinized ultramafic rocks of abyssal mantle and secondary by gabbroic rocks. The positive relief is variable in size, 10 to 150 km long, 5 to 15 km wide, and 500 to 1500 m high.

The megamullion structure is formed in divergent oceanic plate boundary by means of tectonic expansion, that is, plate divergence with limited magma generation. Volcanic eruptions and intrusions are greatly reduced in comparison to normal expansion along mid-ocean ridges. In addition to abyssal mantle, newly accreted gabbro bodies are exposed directly on the ocean bottom. At the expansion axis, a rift valley is not present. The expansion occurs by means of low-angle fault, called “detachment fault”. The megamullion takes place on the upper block of the fault.

The plate tectonic expansion and consequent megamullion formation occur in the plate boundary of low-expansion rate and of low upper mantle temperature. In such regions, so-called "cold spot" or "cold zone", long transform faults are present. Many megamullions take place along transform fault. The Parece Vela Megamullion, northwest Pacific Ocean, between Japan and Philippines, is 150 km long, being the longest megamullion of the world. Because of the length, it is called "Godzilla Megamullion". The Saint Peter Saint Paul Megamullion, Equatorial Atlantic Ocean, is 90 km long and 4000 m high. Because of the form, it is called "Brachiosaurus Megamullion". The top of this megamullion is exposed forming the Saint Peter Saint Paul Islets, being the only one abyssal mantle exposure of the world above the sea level.