18. LONG-TERM OBSERVATIONS OF PRESSURE AND TEMPERATURE IN HOLE 948D, BARBADOS ACCRETIONARY PRISM ¹

J.-P. Foucher,² P. Henry, ³ and F. Harmegnies ²

ABSTRACT

In June–July 1994, during Ocean Drilling Program (ODP) Leg 156, Holes 948D and 949C were drilled 4 and 2 km land-ward, respectively, of the deformation front of the Barbados accretionary prism. The two holes were equipped with subseafloor monitoring instrumentation to determine the fluid pressure in the décollement, and the thermal structure of the sediments after drilling disturbances had dissipated, and to monitor any natural temporal variations, if present. Strings of temperature and pressure sensors were suspended from circulation obviation retrofit kit (CORK) boreseals in both holes. In December 1995, data collected for 18 months were recovered by American and French scientists onboard Nautile during the joint expedition ODP-Naut. In this paper, we present the results from Hole 948D. The temperature data indicate conductive heat transfer with a constant heat flow of 85 mW/m² at this hole. There is no thermal indication of fluid movement along the décollement or vertically through the prism sediments. The pressure record demonstrates that Hole 948D was not properly sealed from the sea bottom. One interpretation of the pressure record is that the weight of the mud column in the borehole gradually equilibrated with the fluid pressure in the décollement. According to this interpretation, the décollement is inferred to be moderately overpressured at 2.2 MPa above the hydrostatic pressure.

¹ Shipley, T.H., Ogawa, Y., Blum, P., and Bahr, J.M. (Eds.), 1997. Proc. ODP, Sci. Results, 156: College Station, TX (Ocean Drilling Program).
² Département de Géosciences Marines, Centre IFREMER de Brest, BP 70, 29280 Plouzané Cédex, France. jfoucher@ifremer.fr
³ Laboratoire de Géologie, Ecole Normale Supérieure, 24 rue Lhomond, 75231 Paris Cédex 05, France.