Structure, permeability, and strength of a fault zone in the footwall of an oceanic core complex, the Central Dome of the Atlantis Massif, Mid-Atlantic Ridge, 30°N

Abstract

Fault-zone samples recovered from 159 to 174 m below the seafloor (mbsf) in Hole 1309D from the Integrated Ocean Drilling Program (IODP) at the Atlantis Massif, Mid-Atlantic Ridge, 30°N, include both cataclastic and mylonitic fault rocks, and their gabbroic and ultramafic host rocks. Laboratory experiments determined the strength and permeability of nine fault-zone and host-rock samples with a triaxial apparatus under conditions that simulate present-day, in situ pressure conditions. The permeabilities of cataclasites are ∼10 −18 m 2, while host-rock and mylonite permeabilities are <2 × 10 −19 m 2. When subjected to increasing differential stress, visible fractures increased the permeability of most rock types to >10 −17 m 2. The strength of cataclasites is 260–380 MPa, weaker than that of mylonites of ∼600 MPa. High resolution X-ray computed tomography (HRXCT) and optical microscopy shows that experimentally produced fractures preferentially form interconnected networks within cataclastic matrices. Thus, permeability and strength are a function of the fault-zone microstructure, which evolved during exhumation from upper mantle and lower crustal depths. Localization of cataclastic zones adjacent to altered ultramafic and mylonitic gabbroic rocks likely make the cataclastic portion of this fault a long-lived fluid conduit within the Atlantis Massif Oceanic core complex.