Fluid flow from pore pressure measurements off La Palma, Canary Islands

In situ subseafloor pore pressure results from the western flank of the island of La Palma, Canary Islands, are presented. The data obtained with a Pup Up Pore Pressure Instrument (PUPPI) provide constraints on the fluid circulation and its causes in a very special context: The sediment piles near an intraplate oceanic island built on the continental rise of the Northwest African Margin. The ambient pore pressures estimated from 2 to 4 days long record are negative in almost all cases with values, at depths of a few meters below sea floor, usually on the order of - 10 to - 70 Pa. Excess pore pressures develop only in the distal most areas. The permeabilities and compressibilities obtained respectively from the decay of the insertion pressures and the amplitude of the tidally induced pore pressure variations range between 2.5 x 10(-18) and 6.6 x 10(-16) m(2) and 6.2 x 10(-9) and 1.5 x 10(-7) Pa-1. According to these permeabilities fluid flow is estimated to be mostly downward and usually on the range between 0 and similar to 0.3 mm y(-1), However, from the excess pore pressure profile a complex pattern of fluid circulation is inferred where horizontal fluid motion cannot be neglected. Horizontal flow is probably controlled by significant contrasts in the permeability of the different layers. The prevailing downward fluid flow is abnormal for a classical passive margin. We thus interpret these results as the superposition to the loss of fluids by sediment compaction ton the continental rise), of a large-scale flow system stimulated, by thermal buoyancy (100 km wide) related to the volcanic activity on the island of La Palma. (C) 2000 Elsevier Science B.V. All rights reserved.