19. THE CORK EXPERIMENT IN HOLE 949C: LONG-TERM OBSERVATIONS OF PRESSURE AND TEMPERATURE IN THE BARBADOS ACCRETIONARY PRISM 1K. Becker,2 A.T. Fisher,3 and E.E. Davis 4 |
ABSTRACTAt the end of Leg 156, Hole 949C was sealed and a CORK (circulation obviation retrofit kit) was installed for long-term monitoring of temperatures and pressures to study the hydrology of the Barbados accretionary prism. The hole was completely cased except for a 53-m perforated and screened section spanning the décollement, so that CORK pressures should reflect hydrologic conditions in the décollement. Nearly a year and a half after beginning the experiment, a 512-day record was obtained by using the submersible Nautile. The temperature data define a linear gradient consistent with the 82°C/km gradient determined during Leg 156, and they show no evidence of transients caused by fluid flow. Pressures in the décollement stabilized after 250 days at a value of 1.02 MPa above hydrostatic, less than one-third of the overburden pressure. At this fluid pressure value, there should be little enhancement of permeability along the décollement, and the pressure data show little evidence for the transient flow processes thought to dominate the hydrology of fine-grained accretionary prisms like those off Barbados. A step-wise increase of pressure from 0.9 MPa to the stable value of 1.02 MPa was observed from 170 to 250 days, and this might be interpreted as a minor fluid-flow event. The tidal loading signal was recorded in the sealed hole attenuated to about 85% of the seafloor level, with a phase lag of about 15 min. The attenuation value is consistent with the high porosity of the muddy sediments (~60%). The phase lag requires a contrast in the bulk moduli of the sediments in the décollement as opposed to those above or below the décollement, consistent with its seismic signature and its nature as a plate boundary fault. The CORK experiment continues to function in Hole 949C and should provide further data through 1997. |
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