VOLCANISM AND TECTONICS OF THE AXIAL ZONE OF THE EAST PACIFIC RISE NEAR 13°N AS RELATED TO ITS HYDROTHERMAL ACTIVITY

Abstract

Recent work has shown that high-temperature hydrothermal activity in the rift structure of the East Pacific Rise (EPR) is concentrated within individual segments and occurs in zones extending along it for as far as several tens of kilometers. Therefore, the question arises: do these hydrothermal zones have a specific structure or contain basalts of any particular type? This question will be considered below on the basis of geologic work (echo sounding and dredging) done in the EPR area near 12° 50′ N. The information obtained here was analyzed together with the published results of a detailed study of the morphologic, textural and compositional features of basalts in a narrow ridge zone. Our data confirm that the model of cyclic evolution of axial volcanoes in oceanic rift structures can be applied in principle to the region studied. They also suggest that a primitive magnesian basalt composition is characteristic of segments of a sea-floor spreading system which are in the phase of active expansion of an axial volcano, and that the presence of magnesian-aluminous basalt in the axial graben in individual parts of the rift structure of the EPR is a further criterion of possible high-temperature hydrothermal activity in such places. Consequently, magnesian-aluminous basalt should be regarded as a prospecting criterion for deep-sea polysulfide mineralization.