MID-OCEAN RIDGE AXIAL FAULTING AND FLUID FLOW IN THE UPPER OCEANIC CRUST

Faulting can enhance the permeability for hydrothermal fluid flow along mid-ocean ridges, and specifically across the sheeted dike complex. Near-bottom studies of rift walls provide images and samples of relict axial faults and related hydrothermal alteration. Fault rocks include cataclasites with structures and textures indicative of high shear strain, granular gouges with no clear strain indicators, and low-strain damage zones with fracture and vein networks. Chlorite-quartz-epidote veins, fault-rock matrix minerals, and sharp increases in MgO in transects across faults are some signatures of focused fluid flow. Focusing of fluid flow bears some similarity to the localization of deformation described here in terms of a displacement:width ratio (D/W). Along the Mid-Atlantic Ridge south of the Kane transform (the SMARK area), the axial graben is bound to the east by a fault with kilometers of displacement. The fault contains cataclasites and breccias with textural relationships showing coeval mineralization and brittle deformation. The SMARK fault has a D/W >>100. In contrast, the northern rift wall of Hess Deep provides a window into the intermediate-spread East Pacific Rise (EPR) crust. There are few mappable axial faults in the Hess Deep escarpment but brittle deformation and alteration is widespread. Dike margins are intensely brecciated with associated crack-seal textured veins. In places there are granular gouges, but these have little evidence of high shear strain. Hess Deep has D/W <1, inconsistent with proposed scaling laws. Lastly, a study of superfast spread EPR crust exposed in Pito Deep has a mostly undeformed upper crustal section. Two mapped faults contain crude cataclastic and gouge foliations and multiple crosscutting vuggy and crack-seal vein sets. The Pito Deep faults do not have large displacements (tens of meters), but are highly localized with D/W >10. The contrasts between Pito Deep, Hess Deep, and SMARK demonstrate that it is not just spreading rate but a range of processes that control faulting and fluid flow across the sheeted dike complex of mid-ocean ridge axial systems. Two specific controls worth exploring are the frequency of dike emplacement relative to spreading rate and fault localization through slip weakening.