Permeability of underthrust sediments at the Costa Rican subduction zone: Scale dependence and implications for dewatering

Abstract

[1] Sediment permeability is a key parameter controlling overpressure development and fluid egress at subduction zones. Existing data compilations for argillaceous sediments illustrate that scale effects on permeability are generally minor, although this is a topic of active debate. Here, we report laboratory permeability measurements from sediments incoming to the Costa Rican subduction zone, at porosities from 26% to 78%. We then apply a method for directly estimating average permeability at the scale of sediment layer thickness, constrained by published estimates of excess pore pressure. Permeability values from both methods are in excellent agreement, and exhibit a trend consistent with permeability-porosity relationships used in numerical models at scales of several to 10's of km. Our results indicate no scale dependence of permeability, and strengthen previous interpretations drawn from more limited data sets. The permeability-porosity relationship we derive is comparable to those inferred at the Nankai and Barbados subduction zones, indicating that the high porosity, and hence high permeability, of incoming sediments at Costa Rica is the most likely explanation for comparatively rapid dewatering there. Citation: Saffer, D. M., and A. W. McKiernan (2005), Permeability of underthrust sediments at the Costa Rican subduction zone: Scale dependence and implications for dewatering, Geophys. Res. Lett., 32, L02302,

Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering

“…Our results for Nankai are slightly higher than those at N. Barbados and considerably lower than at Costa Rica. This is consistent with low permeability expected for the clay rich underthrust section at N. Barbados, and with the fact that initial burial to ∼400 mbsf at Nankai prior to underthrusting results in lower overall porosity (and thus lower permeability) than at nonaccretionary margins like Costa Rica where the entire section is underthrust [e.g., Saffer , 2003; Saffer and McKiernan , 2005; Screaton , 2006]. …”

“…Past studies have indicated that at many subduction zones dewatering of the underthrusting layer is primarily vertical, especially as distance from the trench increases. Thus, a one‐dimensional model is sufficient to simulate the process of interest [ von Huene and Lee , 1982; Screaton and Saffer , 2005; Saffer and McKiernan , 2005; Gamage and Screaton , 2006]. The governing equation for one‐dimensional, transient fluid flow is [ Bear , 1972; Neuzil , 1995] where h is hydraulic head (m), t is time (s), K is hydraulic conductivity (m s −1 ), S S is specific storage (m −1 ), and Γ is a source term representing loading processes (and in a subset of our simulations, mineral dehydration) (m s −1 ).…”

Interpreting in situ pressure and hydraulic properties with borehole penetrometers in ocean drilling: DVTPP and Piezoprobe deployments at southern Hydrate Ridge, offshore Oregon

“…On the basis of our analysis, the Piezoprobe will achieve 90% dissipation 14 times sooner than the DVTPP (Figure 16). Coefficients of consolidation for sediments penetrated in the Ocean Drilling Program range from 4 × 10 −8 to 2 × 10 −6 m 2 /s (Table 5) [ Dugan and Flemings , 2003; Saffer and McKiernan , 2005; Tan , 2004]. For this range of properties it will take from 6 min to 5 hours to achieve 90% dissipation with the Piezoprobe.…”

In situ stress and pore pressure in the Kumano Forearc Basin, offshore SW Honshu from downhole measurements during riser drilling

“…In situ permeability measurements have been conducted in a small number of boreholes using drill stem packer systems [e.g., Screaton et al ., ; Fisher & Zwart , ; Becker & Davis , ]. More commonly, in situ permeability is extrapolated from laboratory measurements on core samples [e.g., Saffer & McKiernan , ; Reece et al ., ; Boutt et al ., ], estimated from the pore pressure response to tidal or other transient forcing measured in instrumented boreholes [e.g., Wang & Davis , ] or inferred from inverse modeling [e.g., Bekins et al ., ; Saffer & Bekins , ; Spinelli et al ., ].…”