DEFORMATION AND ALTERATION ASSOCIATED WITH OCEANIC AND CONTINENTAL DETACHMENT FAULT SYSTEMS

Detachment faults and their associated core complexes occur both in continental rifts and at mid-oceanic ridges, and represent a fundamental mode of crustal extension. Both continental and oceanic detachment faults are characterized by corrugated, domal topography; exposures of the fault surface extend tens of kilometers in the down dip direction, with dips ≤20°. In each system, the faults comprise a network of anastomosing fault zones, consisting of mylonite, cataclasite, and gouge 1– to >200 m thick, exhibiting a progressive down-temperature continuum in deformation. Both form at strain rates ~10⁻¹² to 10⁻¹⁴ s⁻¹, and accommodate asymmetric extension. Despite these similarities, the two systems exhibit significant differences, controlled by their environments of formation. Oceanic core complexes form in thinner lithosphere, with higher geothermal gradients, dominated by olivine and feldspar rheology. In contrast, deformation in the continents is controlled by the rheology of quartz and feldspar. Mylonitic rocks are predicted to be rare in oceanic core complexes and relatively common in continental core complexes, depending on the magnitude of slip and presence of water at fault initiation. Oceanic detachment faults have a more intimate association with magmatic accretion. Hydrothermal circulation and the consequent alteration are more pronounced in oceanic detachment faults, and dominate the low-temperature fault evolution. Oceanic detachment faults are rolling-hinge-type normal faults, initiating at dips of 40-60°. In contrast, some continental detachment faults initiated at a low angle (dips ≤20°). Detachment faults cutting oceanic lithosphere are non-conservative; footwalls are much more extensive than the hanging wall of the fault. Beneath continental core complexes, lower crustal flow maintains crustal thickness despite significant extension. Lower crustal flow beneath oceanic core complexes is limited and restricted to the environs of the magma chamber.