CRUSTAL FISSURING AND ITS RELATIONSHIP TO MAGMATIC AND HYDROTHERMAL PROCESSES ON THE EAST PACIFIC RISE CREST (9-DEGREES-12' TO 54'N)

The deep-towed Argo I optical/acoustical vehicle and a geographic information system (GIS) have been used to establish the abundance, widths, and spatial distribution of fissures, as well as the relative age distribution of lavas along the narrow (<500 m wide) axial zone of the East Pacific Rise (EPR) from 9 degrees 12' to 9 degrees 54'N. On a second-order scale (similar to 78 km long), wider but less numerous fissures are found in the northern portion of the survey area; this changes to narrower, more abundant fissures in the south. A profile of the cumulative width added by fissures to the axial zone exhibits minima in three areas along strike (near 9 degrees 49', 9 degrees 35', and 9 degrees 15'N), where the most recent eruptions have occurred above sites of magmatic injection from the upper mantle, filling and covering older fissures. On a fourth-order scale (5-15 km long) the mean density of fissuring on a given segment is greater where relative axial lava age is greater. Fissure density also correlates with hydrothermal vent abundance and type. Increased cracking toward segment tips is observed at the second-order scale, whereas fourth-order segments tend to be more cracked in the middle. Cracking on a fourth-order scale may be driven by the propagation of dikes, rather than by the far field plate stresses. The above relations constrain the model of Haymon ed al. [1991] in which individual fourth-order segments are in different phases of a volcanic-hydrothermal-tectonic cycle.