OCEANIC CORE COMPLEXES: A POORLY UNDERSTOOD MODE OF EXTENSION AT SLOW SPREAD MID-OCEAN RIDGES (Invited Presentation)

Slow- and ultralow spreading mid-ocean ridges host large offset normal or detachment faults that denude crust and mantle to the seafloor. These ‘oceanic core complexes’ (OCCs) are common, accommodate plate separation at ridges with reduced magmatism, and occur in lithosphere with no pre-existing structural history.

Oceanic core complexes and their associated detachment fault systems form in thin lithosphere (<20 km), with a high geothermal gradient, dominated by olivine and plagioclase rheology. Like many continental core complexes, they are characterized by corrugated, domal topography; exposures of the fault surface extend tens of kilometers in the slip direction, with dips ≤20°. They form at strain rates ~10⁻¹² to 10⁻¹⁴ s⁻¹, accommodate asymmetric extension (migrating relative to the ridge axis), and are non-conservative, with footwalls more extensive than their hanging walls. The architecture of the faults/shear zones associated with oceanic detachment systems is highly variable, but typically comprise a network of 1-10m thick anastomosing zones of mylonite, cataclasite, and gouge totaling 10 to 500 m in thickness, exhibiting a down-temperature continuum in deformation. The presence or absence of thick zones of high-strain mylonitic rocks is a function of position relative to a breakaway (initial structural depth), slip magnitude, footwall rock type/rheology, and the involvement of water during evolution of the fault zone.

ODP/IODP boreholes in the footwalls of three OCCs (Atlantis Bank, SW Indian Ridge; Atlantis Massif at 30°N, and at 15°20’ on the mid-Atlantic Ridge) provide insight into the footwall deformation history. Downhole paleomagnetic remanence data indicate the oceanic detachment faults (and their footwalls) have undergone significant flexural rotation (i.e. ‘rolling hinge model’) from initial steep dips of 40-70°, consistent with recorded microseismicity, to their subhorizontal orientation at the seafloor. This extreme flexural rotation exhibited by OCCs requires elastic thicknesses of <1km, and deformation of the footwall, including high temperature (≤950°C) reverse-sense ductile shear zones, attributed to bending of the lithospheric beam. Evidence for coaxial, lower crustal flow beneath OCCs is limited, and restricted to the environs of any ephemeral magma chamber.