BREAKING NEW GROUND - ESTIMATES OF CRACK DEPTH ALONG THE AXIAL ZONE OF THE EAST PACIFIC RISE (9-DEGREES-12'-54'N)

Using simple fracture mechanics models, the depths of fissures that we re observed along the axial zone of the East Pacific Rise (EPR) from 9 degrees 12' to 54'N with the deep-towed Argo I vehicle are estimated. The widest cracks (similar to 4-8 m) along this fast-spreading portio n of the EPR are the deepest, and are spatially correlated with the br oadest, youngest, and most hydrothermally active portions of the ridge crest. If the widest (deepest) cracks are not primarily eruptive, the y should be most abundant in areas of older lava flows, having increas ed 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 si milar to 9 degrees 45'-52'N and may be deep enough to reach the sheete d dikes of Layer 2B. These cracks may have tapped melt during the erup tion and facilitated the flux of vapor-rich hydrothermal fluids throug h overlying lava flows. The narrowest, presumably shallowest, cracks c orrespond to the narrowest, oldest, and least hydrothermally active po rtions of the ridge crest. We interpret the wide, deep cracks as prima rily eruptive in origin, and suggest that they may be associated with the inflation of an axial magma chamber, whereas the narrow, shallow c racks are interpreted as primarily tectonic and are thought to be asso ciated more with far-field plate stresses.