TY - JOUR
T1 - Fluid expulsion and overpressure development during initial subduction at the Costa Rica convergent margin
AU - Screaton, Elizabeth J.
AU - Saffer, Demian M.
N1 - Funding Information:
Although the sediments directly beneath the decollement zone show partial drainage, the lower part of the underthrust section, which consists of pelagic carbonates, appears essentially undrained since its subduction [6] . The nearly undrained pore pressures inferred from the consolidation testing [6] are supported by the observed lack of thinning within the carbonates [7] . The inferred higher pore pressures and lack of significant consolidation within the lower, carbonate-rich part of the underthrust section are surprising, because laboratory permeability measurements suggest that the pelagic carbonates are one to two orders of magnitude more permeable than the overlying hemipelagic sediments [6,8] .
Copyright:
Copyright 2011 Elsevier B.V., All rights reserved.
PY - 2005/5/15
Y1 - 2005/5/15
N2 - A high convergence rate and almost complete subduction of incoming sediments make the Costa Rica convergent margin an extremely dynamic environment in which to examine the interrelationship of tectonically driven loading, compaction, and fluid expulsion. In this study, we investigate overpressure development and fluid expulsion using a numerical modeling method that allows fluid pressures, porosities, and permeabilities to evolve during subduction. Incoming sediments at the Costa Rica margin consist of ∼180 m of hemipelagic sediments overlying 200 m of pelagic carbonates. Results of the modeling suggest that a permeability-porosity relationship for the hemipelagic sediments derived from laboratory tests (log(k) = - 22 + 7.8 × porosity) is compatible with pore pressures inferred from consolidation tests and compaction rates estimated from sediment porosity and thickness changes, in contrast to results of previous simplified calculations. Comparison of one and two-dimensional modeling indicates that lateral fluid flow modifies the pore pressure profile only moderately, unless the permeability of the pelagic carbonate sediment was an order of magnitude or more higher than indicated by laboratory tests (5 × 10-16 m2). Due to complete subduction of the incoming sediment column, high porosity, and thus high permeability sediments are directly below the decollement zone. Rapid dewatering of these sediments can support high rates of fluid flow along the decollement or upward through the wedge. However, within the first 1.5 km of subduction, drainage leads to a rapid decrease in simulated porosity (0.8 to 0.5) of the uppermost sediments, which would decrease permeabilities to 0.5% of initial values and inhibit further dewatering.
AB - A high convergence rate and almost complete subduction of incoming sediments make the Costa Rica convergent margin an extremely dynamic environment in which to examine the interrelationship of tectonically driven loading, compaction, and fluid expulsion. In this study, we investigate overpressure development and fluid expulsion using a numerical modeling method that allows fluid pressures, porosities, and permeabilities to evolve during subduction. Incoming sediments at the Costa Rica margin consist of ∼180 m of hemipelagic sediments overlying 200 m of pelagic carbonates. Results of the modeling suggest that a permeability-porosity relationship for the hemipelagic sediments derived from laboratory tests (log(k) = - 22 + 7.8 × porosity) is compatible with pore pressures inferred from consolidation tests and compaction rates estimated from sediment porosity and thickness changes, in contrast to results of previous simplified calculations. Comparison of one and two-dimensional modeling indicates that lateral fluid flow modifies the pore pressure profile only moderately, unless the permeability of the pelagic carbonate sediment was an order of magnitude or more higher than indicated by laboratory tests (5 × 10-16 m2). Due to complete subduction of the incoming sediment column, high porosity, and thus high permeability sediments are directly below the decollement zone. Rapid dewatering of these sediments can support high rates of fluid flow along the decollement or upward through the wedge. However, within the first 1.5 km of subduction, drainage leads to a rapid decrease in simulated porosity (0.8 to 0.5) of the uppermost sediments, which would decrease permeabilities to 0.5% of initial values and inhibit further dewatering.
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U2 - 10.1016/j.epsl.2005.02.017
DO - 10.1016/j.epsl.2005.02.017
M3 - Article
AN - SCOPUS:18344384958
SN - 0012-821X
VL - 233
SP - 361
EP - 374
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
IS - 3-4
ER -