Serpentinites and peridotites are the predominant lithological components of Oceanic Core Complexes (OCCs), located commonly at triple junctions of slow-spreading oceanic accreting ridges, fracture zones and transform faults. These lithologies differ from the surrounding oceanic brittle lithosphere, built of basalt and gabbro, and the structural evolution of these OCCs is enigmatic. The present investigation suggests that the tectonics of OCCs is derived from the ascent of serpentinic diapirs generated by the unique proximity of shallow asthenosphere, faulted lithosphere and the juxtaposition of oceanic crusts of contrasting densities. Such setting initiated two structural stages in the evolution of the OCC, the first is spontaneous subduction of old and dense oceanic crust under the fresh and lighter basalt at the edge of the MOR across its intersection with transform fault - fracture zone. The sub ducted slab would be affected by the combined effect of the steep thermal gradient and the availability of volatiles there to enhance the alteration of pyroxene into serpentine. Analog and numeric experiments show that spontaneous subduction can initiate spontaneously if the density contrast between the juxtaposed slabs is significant, as is the case between fresh basalt (ρ~2.7 g/cm3 ) and older basalt (ρ~2.9 g/cm3 ). Since the average thermal gradient under the MOR is ca. 130o /km, then at depth of 4-5 km the pyroxene and plagioclase would start their alteration, mostly to serpentine. The abundant faults of MOR normal rifting and strike-slip transform faulting could enable the light and malleable serpentinite to form diapirs, which would ascent to the seafloor at the rift-fracture zone intersections. The friction at the top of the diapirs during their ascent would probably generate breccia at their tops.

Highlights

a) OCCs are built mostly of serpentinite diapirs that carry peridotite inclusions.

b) Serpentinization occurs under low pressure-high temperature conditions.

c) OCC source rock is slab of cold oceanic crust sub ducted under fresh crust.

d) Such subduction occurs mostly where large density contrast between slabs exists.

e) The link between OCCs and detachment normal faults requires support.