Dawn J. Wright**, Rachel M. Haymon*, and Daniel J. Fornari+
**Dept. of Geosciences, Oregon State University
The near-bottom imaging capability of the Argo I optical/acoustical system has 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° 12' to 9° 54'N. Argo I video and sonar data were processed in a geographic information system (GIS) from which distribution maps were created for most of a 2nd-order ridge segment, including nine 4th- order ridge segments. Statistical analyses characterize fissure density and width variation on both a 2nd- and 4th-order segment scale. On the 2nd-order scale (the entire 78 km long study area), 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° 49', 9° 35', and 9° 15'N). These may be the areas where most recent voluminous eruptions have occurred above sites of deep-seated magmatic injection from the upper mantle, filling and covering older fissures. On a 4th-order scale (5-15 km long), the mean density of fissuring on a given segment is greater where relative axial lava age is greater, and fissure density also correlates with hydrothermal vent abundance and type. The correlation of fissure density with axial lava age, at both 2nd- and 4th-order scales, reveals the tendency of the crust to accumulate more cracks with time, rather than to widen existing cracks formed early on. Superimposed on this accumulation of cracks are spatially localized, relatively recent eruptions that cover fissures and produce minima in cumulative fissure width. In these regions average fissure widths and depths may increase due to ridge crest inflation and dike propagation, resulting from recent magmatic replenishment of the AMC from upper mantle sources. Increased cracking toward segment tips is observed at the 2nd-order scale, however 4th-order segments tend to be more cracked in the middle rather than at the ends. This suggests that cracking on a 4th-order scale may be driven by the shallow crustal injection of magma and propagation of dikes, rather than by the far-field plate stresses that appear to control 2nd-order crack variations. Active hydrothermal vents are most abundant along 4th-order segments intruded by shallow dikes. The vents are focused along eruptive fissures above the tops of dikes intruding upward from the axial magma lens and migrating laterally along- strike. Where eruptive fissures are partly filled and covered by lava flows, vents may be concentrated at the open tips of these fissures. These relations constrain the model of Haymon et al. [1991] in which individual 4th- order segments are in different phases of a volcanic- hydrothermal-tectonic cycle. The cycle begins with shallow dike intrusion/eruptive fissuring, magmatic drainback, gravitational collapse, and abundant hydrothermal venting, followed by possible development of an axial summit caldera (ASC), cooling of the heat source, onset of tectonic fissuring, and waning of hydrothermal and magmatic activity. The cycle concludes with mass wasting and tectonic cracking that is inferred to be shallow (within Layer 2A) on the basis of fissure widths.
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