The Galapagos Rift at 86° W: 4. Structure and morphology of hydrothermal fields and their relationship to the volcanic and tectonic processes of the Rift Valley

Abstract

The Angus camera system is used to investigate the detailed structure and morphology of the active hydrothermal vent fields of the Galapagos Rift. Precision navigational data are combined with microtopographic information and detailed geological and biological observations obtained from an analysis of the color bottom pictures to create a series of three‐dimensional models for each vent field. The five isolated vents are distributed along the southern boundary of a fissure wedge that cuts along the active axial high. All but one vent occur in pillow terrain. The fifth vent is at the contact between a pillow flow and sheet flow. Vents range in size from 400 m2 to 1600 m2 and are spread out along 2.8 km of a lineated fissure system that trends 095°. They are spaced at intervals ranging from 250 m to 1600 m with an average separation of 700 m. Vents are bound by tectonic features such as fissures or small throw faults. The egress of water is localized in small pockets within the fields. These zones of egress are distinctly unfissured. Instead, the water emanates out of rock to rock brecciated contacts. These thermal pockets range from 15 m to 40 m in diameter and are characterized by large concentrations of benthic fauna. On the basis of animal communities the fields may be divided into two different types, the western and the eastern groups. The two western fields are predominantly populated by clams (Vesicomyidae) and mussels (Mytilidae). The eastern three fields are dominated by tube worms (Vestimentifera), mussels, serpulid worms, and the spherical, colonial animal dandelion. The positioning of the vents on the fissure wedge implies a tectonic propagation and age gradation up and down the rift. At any one time, adjacent segments of the rift valley may be in different stages of a volcanic and tectonic cycle. As cooling of a central magma intrusion occurs, fissures propagate toward the remaining heated core, opening up pathways for the egress of heated bottom water. Cooling continues and the outermost vents die out. Precipitates block passages, capping the terrain. As extension continues, the very oldest sector of the valley will fracture. The incipient stress drop generates the eruption of a new magma center down strike from the former center, thus ensuring a volcanic and tectonic periodicity in space and time up and down the rift valley.