|
Deformation of Weak Sediments |
Department of Geological Sciences, 2124 Snee Hall, Cornell University, Ithaca, NY 14853, USA
The mechanical behaviour of porous sediments in the toes of accretionary prisms can be analyzed by combining the principles of soil mechanics and the results of experimental sediment deformation with the in-situ physical properties observed from deep sea drilling. Porosity is the only physical property that has been extensively collected and even these data have generally been inadequately analysed. After correction for porosity rebound, variations due to lithology, and lack of representative sampling, porosity data must be converted from a spatial or Eulerian description to a Lagrangian description, which follows the behaviour of a given sediment element as it moves through the prism toe. Such an approach shows that sediment elements compact during deformation in prism toes; strongly in the Nankai prism and much less so in the Lesser Antilles. Compactive stress paths are ductile, and there is evidence that much of the deformation, at least in the Nankai toe, is of this type. Nevertheless, there is also a very strong component of brittle deformation, both on diffuse structures and along the major faults. This brittle overprint is explained in the light of experiments as showing that sediments already at a state of ductile failure will shear brittlely if post-failure deformation is associated with a decrease in
m'. Such reduction of m' probably occurs in prism toes during fluctuations of pore pressure. Persistent brittle faults are modelled as zones in which porosity and pore pressure are higher than in the surrounding sediments, a condition maintained by fluid channelled up the faults from deeper in the prism. Outward diffusion of fluid from the faults may be responsible for broad zones of brittle failure, characterized by scaly clay.
