A near-bottom investigation of the crestal region of the East Pacific Rise at 21°N shows a tectonic zonation symmetrical about the axis that includes a narrow central zone of volcanic extrusion 2.5–3 km wide flanked by regions of crustal extension, primarily by faulting within 2.5–3.5 km from the axis. Farther than 5 or 10 km from the axis, uplift along faults paralleling the rise axis produces typical abyssal hill relief. Near-bottom water temperatures show a relation to the two inner zones. Within the central (main) zone of extrusion, which is dominated by steep local volcanic relief, are numerous irregular temperature anomalies generally located over small enclosed depressions. Bordering the main extrusion zone, locally rough topography grades to symmetric zones of smoother bottom relief where steep linear scarps indicate the onset of normal faulting. Systems of open fissures exceeding 200 m in length and several meters in width border these scarps and are the apparent sources for some of the linear water temperature anomalies. Average temperature increases over the fissures were 3–5×10−2 °C at 50 m above the sea floor. The paired zones of extension are succeeded laterally by regions in which no temperature anomalies were recorded. Heat loss analyses using the equations of Rouse et al.  yield nearly equivalent values for the symmetric zones of extension that flank the main extrusion zone. Analyses of individual anomalies within the central 2.5–3 km, however, indicate a lower heat loss within this region than occurs to either side. Results indicate that hydrothermal emanations may be controlled by the tectonic regime of the ridge crest, where major zones of stress release allow for the increased advection of water.
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