Geologic and structural interpretations based primarily on photographic data collected by the Marine Physical Laboratory deep-tow instrument, the Woods Hole Oceanographic Institution ANGUS, and the French manned submersible CYANA show that extensional tectonic activity modifies and locally replaces extrusive volcanic processes along a 6-km segment of the East Pacific Rise axial zone near lat. 21°N. Relatively young pillow flows dominate the spreading axis; however, fissures are present throughout the axial zone, and there is a light dusting of pelagic sediment on even the youngest flows. In contrast, the axial zone immediately southwest in the RISE Project study area is free of fissures and sediment. Photographic evidence for the existence of undiscovered active hydrothermal vents or narrow zones of diffuse hydrothermal emissions includes anomalously high concentrations of common deep-sea organisms (not the exotic hydrothermal vent communities). Some of these are coincident with high 3He values and positive temperature anomalies discovered in deep-tow water samples (Lupton and others, 1980). Estimation of the absolute age of pillow flows based on the sedimentation rate and the volume of interpillow sediment ponds suggests that the average pillow flow with a sediment cover of 70% is approximately 1,000 yr old, and that a pillow flow will be completely covered with sediment in about 40,000 yr. This relation indicates that the extrusive terrane of the inner axial zone in the study area, although undergoing extensional modification, is considerably less than 1,000 yr old.

Visual geologic data collected by the several deep-sea photographic systems are complementary in nature, and the best geologic interpretations are made through use of more than one system to provide various photographic perspectives and scales of observation.

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