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

The lavas–dikes transition in oceanic crust represents a critical boundary between two crustal portions with different lithology, structure, porosity, permeability, and other physical properties. We refined the structural features of the lavas–dikes 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–dikes 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 have triggered the circulation of seawater from above and hydrothermal fluids from below enhancing the crust alteration and the mechanical response of the basaltic crust near the lavas–dikes boundary. The high concentration of permeable structures including cataclasites in the lavas–dikes transition in the two holes are in favour for interpreting this crustal interval as a (large scale?) mechanically weak zone, possibly a fault zone at least for intermediate-spreading ridge of Hole 504B. The lack of clear kinematic indicators prevents a conclusive interpretation. Other processes that may concur to fracturing are fluid overpressure and diking. These processes are not mutually exclusive: the intense fluid circulation concentrated in the transition zone is responsible of alteration inducing a dramatic weakening of the rock; conversely, rock fissuring due to thermal cracks, diking and eventually faulting developed a permeable structure that may channelize the circulation of fluids. Our finding of cataclasites at a shallower depth in Hole 1256D and at deeper depth in Hole 504B with respect to what has been described so far suggests that the extension of the lavas–dike boundary should be revised by taking into consideration not only the change in lithology but also the structural features. As well, our finding of chlorite and amphibole at shallower depths with respect to those described so far suggests that the alteration boundary across the lavas–dikes transition is thicker than previously thought, allowing the upflow of high-temperature hydrothermal fluids towards shallower levels.