Fault Controls on the Mid-Ocean Ridge Hydrothermal System
Nicholas W. Hayman
Earth and Ocean Sciences Division
When: Thursday, January 11, 2007, 11 AM
Where: J.J. Pickle Research Campus, Bldg. 196, Rm 1.603
Host: Nathan Bangs, UTIG
A Ďtruismí in both continental and ocean crustal studies is that faults are conduits to hydrothermal fluid flow. In basalts this is especially important since fractures and faults increase permeability by several orders of magnitude, from <10-18m2 to >10-12m2. However, faults can also be seals to fluids, a concept not widely recognized to date in the mid-ocean ridge systems. Fault sealing is a consequence of the fault zone structure, wherein a damage zone of intensely fractured rock surrounds a fault core. The fault core is where shear strain is localized, and can be filled with gouge, a fine-grained granular material. Gouge continues to deform after its initial development, affecting both the fault zoneís internal stresses, and also the permeability. Submersible investigations of crust accreted at the East Pacific Rise produced images and samples of fault zones that developed in or near the mid-ocean ridge axial center. Geochemical and microstructural data demonstrate that the damage zones localized hydrothermal cycling of seawater, but that the fault cores were seals. The implication is that brittle deformation mechanisms govern the change from diffusive hydrothermal flow to more focused discharge, compartmentalizing hydrothermal systems. This model explains many observed patterns of spatially heterogeneous hydrothermal alteration in oceanic crust.