Studies of the variation in permeability with deformation have been carried out by Saffer et al. (2000) and Bolton et al. (Chap. 3, this volume), the latter using significant structural input from Vannucchi and Tobin (2000). Vannucchi and Tobin (2000) documented deformation patterns in the deformed sediment wedge, the décollement zone, and the underthrust sediment package. Deformation in the deformed wedge occurs through the development of kink bands and shear bands, with both bedding-parallel and bedding-oblique bands cutting across each other. The kink and shear bands illustrate deformation associated with loss of porosity. The alternation of bedding-parallel and bedding-oblique structures implies the possibility of more than one separate deformation mechanism; overburden pressure, gravity sliding, and tectonic stress may all have been factors. A finding of a major shear zone at Site 1040 and a regular progression of strain indicating a continuum from vertical to horizontal maximum principal stress suggest the presence of active tectonic compression (Vannucchi and Tobin, 2000).
Deformation and dewatering in the décollement zone appear less systematic than in the wedge, with the presence of alternating low- and high-porosity zones. Silty high-porosity zones alternate with silt and clay low-porosity regions, with smectite alteration of earlier flow channels. These channels suffered porosity collapse and mineral alteration, with the presence of both flattening and shearing structures. The strain and fluid flow regime was episodic, with periodic buildup of overpressures and structural collapse. Deformation bands in the décollement produced hydraulic brecciation. Both a brittle and ductile regime exists in the décollement zone, and the boundary between them has high potential to transmit fluid (Vannucchi and Tobin, 2000).
Faults are present in the underthrust section as well, but these are narrow, discrete features or braided arrays, with no ductile structures. Most faults show reverse movement. Fault density decreases toward the base of the underthrust section. Deformation bands focused in the upper part of the underthrust section may be dewatering pathways, explaining the mechanism for the much higher porosity loss in the upper hemipelagic layer. Incipient stylolite development and recrystallization, which indicate pressure solutions, characterize the pelagic limestone layer. The basal sediments have a very different structure, with normal faulting dominating, probably a result of spreading ridge processes. At Site 1039, both quartz and calcite precipitate as close as 20 m from the base of the sediments but do not precipitate nearer to the gabbros, suggesting a separate fluid circulation system associated with the gabbros (Vannucchi and Tobin, 2000).
Bolton et al. (Chap. 3, this volume) and Saffer et al. (2000) have each shown a linear relationship between permeability and porosity. Bolton et al. demonstrated this for the deformed wedge and Saffer et al. for the underthrust sediment. Paths of permeability vs. effective stress were quite different for the different structural regimes (Bolton et al., Chap. 3, this volume). In the deformed wedge, increasing effective stress led to decreased permeability until fracture occurred. Release of effective stress resulted in a permeability drop nearly 5 orders of magnitude. For hard chalk in the underthrust section, very little difference was noted during the stress cycle. For underthrust diatomite and soft clay near the wedge surface, increasing effective stress by 1 MPa led to a decrease in permeability of roughly a factor of 3-5, recovering with stress release (Bolton et al., Chap. 3, this volume).
Saffer et al. (2000) found that the in situ excess pore fluid pressures in the underthrust sediment increased from 1.3 MPa at the top to 3.1 MPa near the base of the section. They inferred from this increase that the uppermost sediments drain most easily, whereas the lower sediments remain undrained. The latter implies that the sedimentary system is uncoupled from the underlying hydrologic system in the upper oceanic basement. Saffer et al. further discovered that the measured permeabilities are a factor of about 100 too low to explain the dewatering rates implied by the rate of change of thickness of the underthrust hemipelagic layer. Barring experimental error, they suggest that a combination of narrow, high-permeability horizons within the hemipelagic (probably ash) layers and local vertical dewatering conduits can explain the measurement vs. modeling discrepancy.