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

Abstract

In situ subseafloor pore pressure results from the western flank of the island of La Palma, Canary lslands, are presented. The data, obtained with a Pop Up Pore Pressure lnstrument (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 ambicnt pore prcssures estimatcd from 2- to 4-days-long records are ncgativc in almost ali cases with values, at depths of a few meters below sea floor, usually on the order of - 1 O to - 70 Pa. Excess pore pressures develop only in the distalmost areas. The permeabilities and compressibilities, obtained respectively from the decay of the insenion pressures and the amplirude of the tidally induced pore pressure variations, range between 2.5 x 10-18 and 6.6 X 10-16 m2, 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 O and -0.3 mm yr-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 signiticant 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 (on the continental rise), of a large-scale flow system stimulated by thermal buoyancy ( 100 km wide) related to the volcanic activity on thc island of La Palma.