Analysis of 12,000 electronic still camera images collected with the ARGO II vehicle near the Trans-Atlantic Geotraverse (TAG) active hydrothermal mound, 26 degrees N on the Mid-Atlantic Ridge, has made possible the first quantitative in situ assessment of both fissure orientation and width within the median valley of a slow-spreading ridge. Fissures near the TAG mound are partially rubble-filled extensional fractures that cut lightly sedimented seafloor and in similar to 1% of our observations host pillow lavas. Fissure widths range from 0.15 to 3.5 m, with a mean of 0.7 m, and do not vary systematically within the survey area. First-order estimates of crack depth, based on these width measurements and reasonable elastic moduli, indicate that fissures are restricted to depths <500 m, with a mean depth of similar to 70 m. Fissure-associated eruptives were therefore probably fed by shallow propagating dikes. TAG fissures exhibit a wide range of orientations, with similar to 40% deviating by >45 degrees from the strike of the ridge axis. The formation of obliquely oriented fissures requires that the local least compressive stress direction varies (at least temporarily) from that predicted by the regional tectonic stress field associated with plate separation. This stress field reorientation may be facilitated by variations in the style of magma emplacement within the rift. The close spatial association of long-term hydrothermal activity, fissure-hosted lava Rows, and faults and fissures trending oblique to the spreading axis suggests a causal relationship between the impact of dike intrusion and the maintenance of localized hydrothermal flow. (C) 2000 Elsevier Science B.V. All rights reserved.
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