Breaking new ground: Estimates of crack depth along the axial zone of the East Pacific Rise (9°12′–54′N)

Using simple fracture mechanics models, the depths of fissures that were observed along the axial zone of the East Pacific Rise (EPR) from 9°12′ to 54′N with the deep-towed Argo I vehicle are estimated. The widest cracks (∼ 4–8 m) along this fast-spreading portion of the EPR are the deepest, and are spatially correlated with the broadest, youngest, and most hydrothermally active portions of the ridge crest. If the widest (deepest) cracks are not primarily eruptive, they should be most abundant in areas of older lava flows, having increased in width with time. This is not observed. Some of the widest cracks are located where the 1991 eruption of the ridge crest occurred at ∼ 9°45′–52′N and may be deep enough to reach the sheeted dikes of Layer 2B. These cracks may have tapped melt during the eruption and facilitated the flux of vapor-rich hydrothermal fluids through overlying lava flows. The narrowest, presumably shallowest, cracks correspond to the narrowest, oldest, and least hydrothermally active portions of the ridge crest. We interpret the wide, deep cracks as primarily eruptive in origin, and suggest that they may be associated with the inflation of an axial magma chamber, whereas the narrow, shallow cracks are interpreted as primarily tectonic and are thought to be associated more with far-field plate stresses.