A 9 km length of one of the axial rift valleys of a Gulf of California spreading center was mapped by Deep Tow and examined on ten "Alvin" dives. The Deep Tow CTD towed 10-100 m above the seafloor sensed 20 thermal plumes with temperature anomalies of up to +0.39°C. Most plumes overlie deposits of hydrothermal minerals (mainly anhydrite and sulfides) that form columns and mounds rising up to 25 m above the muddy rift floor. About 130 such sites were mapped by side-scan sonar, and about 20 of them were photographed and sampled with "Alvin". Two of the sites that had been raised tectonically above the trough floor have only inactive residual mounds, but all the others examined by submersible had active hydrothermal vents. Fluids discharge diffusely through the porous deposits, as relatively low-temperature (< 100°C) springs in the surrounding mud, and at high velocity and temperature (270-314°C) through chimneys built on the mounds. Chimneys have orifices 2-20 cm in diameter and are commonly capped with hollow horizontal sheets of anhydrite and pyrrhotite. Conductive heat flow measured by multipenetration probe and from "Alvin" was exceptionally high (> 2 W/m2) adjacent to the thermal springs. The patterns of hydrothermal deposits and measured heat flow, interpreted in light of structural information from the Deep Tow profiler, suggest that shallow sills, some extending laterally from more massive hill-forming intrusions, act as cap rocks for intense, shallow hydrothermal circulations above a magma chamber. Hot discharges from this system occur mainly through tectonic fractures in the sills and around their peripheries; recharge probably occurs on a regional scale, from cooler parts of the rift valley. Some warm discharges, inactive barite deposits, and nonlinear thermal gradients probably mark continuing and past cooling of underlying shallow intrusions, by both recharging and temporary closed-system circulations.
ASJC Scopus subject areas
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science