Faults and damage zones in fast‐spread crust exposed on the north wall of the Hess Deep Rift: Conduits and seals in seafloor hydrothermal systems

The northern escarpments of the Hess Deep Rift provide cross‐sectional views of in situ, ∼1‐Ma‐old, upper oceanic crust that underwent extensive, spreading‐related brittle deformation. Most of the deformation and associated alteration occurred within the locus of magmatic construction of the East Pacific Rise, in the presence of high‐temperature hydrothermal fluids. Passing laterally from undeformed host rocks, brittle deformation zones are classified as (1) damage zones where densely spaced fractures overprint the primary structure of dikes and lavas, (2) cataclastic zones where interconnected fractures, comminuted grains, and matrix minerals define deformational fabrics, and (3) very fine‐grained, gouge‐filled fault cores. Relative to the host rock, damage and cataclastic zones are rich in veins of chlorite and/or actinolite, and lesser amounts of titanite, epidote, and quartz. These phases mark relict hydrothermal fluid pathways. Trace and major element compositions of representative samples also indicate fault‐localized hydrothermal alteration, including an increase in MgO by several weight percent within cataclastic and damage zones. In contrast, the fault cores are composed of very finely comminuted basaltic material and have MgO concentrations similar to the damage zones. Integrated compositional, textural, and outcrop‐scale structural data inform an evolutionary model for fault growth from the early, widespread dilational phases of damage‐zone development to more restricted noncoaxial strain in the cataclastic zones. With continued fault development, gouge develops and seals the fault cores. While the fault cores are sealed by gouge, surrounding zones remain conduits to hydrothermal fluid flow, except where sealed by secondary minerals. Sealed faults can later be reactivated as conduits with additional increments of fault slip. The dual behavior of faults as conduits and seals inevitably leads to compartmentalization of the flow regime in subaxial and ridge‐flank areas.