Structural control on fluid circulation and alteration patterns in the lavas–dykes transition zone of the superfast- and intermediate-spreading oceanic crust, equatorial Pacific Ocean

The lavas–dykes transition in oceanic crust represents a critical boundary between two portions of the crust with different lithology, structure, porosity, permeability and other physical properties. We refined the structural features of the lavas–dykes transition in ODP/IODP Hole 1256D and DSDP/ODP Hole 504B drilled in in situ oceanic crust created at superfast- and intermediate-spreading rate in the Pacific Ocean. Both holes, although showing some differences in the thickness of lithological sections, have a comparable lavas–dykes transition zone characterized by a wide diffusion of veins, breccias, vein network and cataclasites, possibly with development of pseudotachylytes. Fracturing started with fissuring of basalt already in the ridge axis domain due to thermal contraction of lavas. Fissures triggered the circulation of seawater from above and hydrothermal fluids from below, enhancing alteration of the crust and the mechanical response of the basaltic crust near the lavas–dykes boundary. The high concentration of permeable structures including cataclasites in the lavas–dykes transition in the two holes favour interpreting this crustal interval as a (large-scale?) mechanically weak zone, possibly a fault zone at least for the intermediate-spreading ridge of Hole 504B. The lack of clear kinematic indicators prevents a conclusive interpretation. Other processes that may have led to fracturing include fluid overpressure and dyking. These processes are not mutually exclusive: intense fluid circulation concentrated in the transition zone is responsible for alteration inducing a dramatic weakening of the rock; conversely, rock fissuring due to thermal cracks, dyking and eventually faulting developed permeable structures that may channel the circulation of fluids. Cataclasites occurring at a shallower depth in Hole 1256D and at deeper depth in Hole 504B with respect to those described so far suggest that the extension of the lavas–dykes boundary should be revised by taking into consideration not only the change in lithology but also the structural features. In addition, our finding of chlorite and amphibole at shallower depths with respect to those described so far suggests that the alteration boundary across the lavas–dykes transition is thicker than previously thought, allowing the upflow of high-temperature hydrothermal fluids to shallower levels.