Abstract

Geologic field relationships observed in the axial valley of the southern Juan de Fuca Ridge suggest that high-effusion-rate sheet-flow volcanism has been active for a maximum of about 16,000 yr. Similar observations in the axial region of the East Pacific Rise near lat 21° N suggest that the duration of the pillow-flow-dominant phase may be 5 × 104 yr. Geochemical evidence from axial and off-axis basalt samples from both the Juan de Fuca Ridge and East Pacific Rise 21° N indicates that surficial geologic cycles of volcanism, tectonism, and hydrothermal activity are an expression of changes in the locus of replenishment within the magma chamber that take place with a periodicity of 104−105+ yr. Anomalously high rates of spreading due to magmatic replenishment and inflation during the first phases of a volcanic cycle may cause the southern Juan de Fuca Ridge to display a morphology more commonly associated with fast-rate spreading centers.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.