Expedition 301 synthesis: hydrogeologic studies

Abstract

Integrated Ocean Drilling Program Expedition 301 was part of a long term series of operations and experiments focused on quantifying hydrogeologic, lithologic, biogeochemical, and microbiological properties, processes, and linkages in basaltic crust on the eastern flank of the Juan de Fuca Ridge. This paper summarizes peer-reviewed hydrogeologic studies published since the end of Expedition 301. Regional survey data show that patterns of fluid circulation in basement on this ridge flank are complex, including components of fluid flow in the ridge-parallel (along-strike) direction. Numerical models show that hydrothermal circulation can form a hydrothermal siphon between recharging and discharging basement outcrops that penetrate regionally thick sediments, provided basement permeability is ≥10 -12 m 2 . Simulated fluid temperatures in upper basement between outcrops are consistent with observations (60°-65°C) if large-scale basement permeability is ~10 -11 m 2 . Other models show that basement conditions in the Expedition 301 field area may be undergoing thermal rebound following the end of an earlier phase of open hydrothermal circulation that extracted a larger fraction of lithospheric heat than is extracted today. This hypothesis is consistent with trends in basement geophysical properties, as determined with Expedition 301 wireline logs and tests on core samples from upper basement, which suggest that upper basement has not been warmer than it is at present. Drill string packer experiments in upper basement during Expedition 301 indicate a layered crustal structure, with bulk permeabilities from 10 -12 to 10 -11 m 2 . Additional hydrogeologic analyses were completed using the formation pressure response to the long-term flow of cold bottom seawater into basement at Site U1301 during 13 months after drilling, as observed at Site 1027 (2.4 km away). These analyses suggest large-scale permeability at the low end of values indicated by packer testing, 0.7 × 10 -12 m 2 to 2 × 10 -12 m 2 . Results from these two sets of measurements, and the difference between these permeability estimates and others based on modeling and analyses of formation responses to tidal and tectonic perturbations, may be reconciled if the upper crust in this area is anisotropic with respect to basement permeability. This hypothesis will be tested during and after the next drilling expedition in this area, when multidirectional crosshole experiments are run using a network of sealed borehole observatories.