FAULT AND FRACTURE SYSTEMS IN THE UPPER OCEANIC CRUST: MICROSTRUCTURE AND FIELD OBSERVATIONS

The Sheeted Dike Complex (SDC) of the upper oceanic crust created at the fast spreading East Pacific Rise (EPR) is locally intensely deformed by fault and fracture systems. These systems are important conduits and seals for fluid flow which transfers the majority of the mass and heat between the hydrosphere and the oceanic lithosphere, and thus represent an extreme case of fault-related fluid flow. This study applies an integrated approach, combining microstructural analyses of in situ oceanic crustal fault rocks with field observations of faults and fractures in the SDC of the Troodos Ophiolite in order to better understand the initiation and evolution of brittle structures in the upper oceanic crust. Our primary microstructural tool is particle size distribution (PSD) analysis, which is a useful in determining strain history and deformation mechanism in brittle rocks. We have done PSD analysis on several hand samples of fault rock from various environments of tectonic windows (submarine rift walls that provide cross-sectional views of intact crust, investigated with near-bottom vehicles) Hess and Pito Deep just off the East Pacific Rise. The PSDs are best described as composite log-normal distributions, interpreted to reflect the change from a fluid-pressure induced brecciation to shear-fracture over the faults’ history. Additionally, we conducted a parallel study in the SDC in the Troodos Ophiolite in Cyprus. Several different categories of fracture were identified as a function of orientation relative to dike margins, deformation zone width and fracture density. Every fracture set in the Troodos Ophiolite exhibited a change in fracture distribution as fracture densities reached ~1/cm. Such fracture-distributions have also been observed in images of in situ ocean crust in tectonic windows. Both field observations and microstructural work find a characteristic length scale for fractures created at hydrothermally active fast spreading centers. This indicates that a change in deformational process from hydrothermally controlled fracture to shear faulting occurs at a predictable phase of the system's evolution, and points to an intrinsic mechanical property of the oceanic crust in controlling fracture- and fault-controlled fluid flow.