Abstract:
The evolution of oceanic hydrothermal systems depends heavily upon the origin, timing and spatial distribution of permeability near the magmatic heat source. At fast-spreading ridges, where permanent magma chambers are inferred, permeability in the plutonic portion of the crust is generated during cooling by time, strain-rate- and temperature-dependent fracturing, which propagates downwards through the crust as the rocks cool and move off-axis. At major axial discontinuities, however, where magmatism may be intermittent and tectonism plays an important role, the controls on hydrothermal alteration are likely to be more complex. Gabbros exposed at Hess Deep and sampled by the Ocean Drilling Program during Leg 147 provide the first insights from direct observation into the structural and metamorphic evolution of the lower crust near axial discontinuities. Early amphibolite-facies deformation and alteration are related to cooling, fracturing, and hydrothermal fluid flow within several kilometres of the East Pacific Rise axis. Later, dense fracturing and limited alteration occurred in the greenschist facies as the gabbros were transported into the deformation field at the "tectonic tip" of the westward-propagating Cocos-Nazca ridge. Although this is only one example of the possible influences of axial segmentation on hydrothermal circulation, and may represent a comparatively simplified view of other natural systems, given the potential variations in strain and strain rate coupled with the possibility of reheating events associated with renewed magmatism, we suggest that the multi-stage structural and metamorphic evolution of the Hess Deep plutonics may be a good analogue for the cooling history of the lower crust at other fast-spreading ridge axial discontinuities.