The internal structure of fault zones in basaltic sequences

Normal faults in basalt along mid-ocean ridges and continental rifts play a major role in the formation of topography, advection of fluids, and the dynamics of biosphere. However, almost nothing is known of the internal structure of volcanic growth faults at depth, where many key processes take place. In this paper we present the results of scaled model experiments validated against outcrop studies of a volcanic growth fault system on Hawaiʻi. The analogue model uses cohesive hemihydrate powder, with carefully characterized physical properties. A cohesion of 62 Pa, friction angle of 0.71 and tensile strength of 33 Pa imply a length scaling ratio of 1 : 5000–50,000 between the model and the upper part of extending oceanic crust with a basalt column above a normal fault. Time lapse imagery, high resolution particle image velocimetry and field observations show ground flexure in an initial elastic stage before subvertical mode I fissures propagate from the surface downwards. Some of the mode I fractures become inactive, while others develop into massively dilatant normal faults. The dominant processes within the faults are brecciation, block rotation and gravitational transport of fragments along the fault, maintaining meter-sized cavities to a depth of several hundred meters. Even when these volcanic growth faults, are resurfaced, a pervasive dilatancy persist within the system. If circulating seawater provides a sufficient oxygen supply, the large cavities in normal faults at mid-ocean ridges could harbour large ocean floor life forms to several hundred meters depth.