HR: 1340h
AN: T13B-0470    [Abstracts]
TI: Influence of along-strike temperature differences on 3-D fluid flow patterns in the Nicoya margin subduction zone, Costa Rica
AU: * Spinelli, G A
EM: spinelli@nmt.edu
AF: Department of Earth and Environmental Science, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801 United States
AU: Saffer, D M
EM: dsaffer@geosc.psu.edu
AF: Department of Geosciences, Penn State Univeristy, Deike Building, Univeristy Park, PA 16802 United States
AB: Along the subduction zone off Nicoya Peninsula, Costa Rica, a <5 km wide transition separates warm crust (seafloor heat flow ~110 mW/m²) generated at the Cocos-Nazca Spreading Center (CNS) from cool crust (surface heat flow ~30 mW/m²) formed at the East Pacific Rise (EPR). The EPR crust (northwest of the CNS crust) is cooled by hydrothermal circulation in the basaltic basement. Along the decollement, estimated temperatures are ~30 °C warmer on the CNS side than those on the EPR side. As a result, fluid viscosity is lower (therefore hydraulic conductivity is higher) on the CNS side. Additionally, thermally-driven opal-to-quartz and smectite-to-illite diagenetic reactions proceed more rapidly on the warm CNS side of the system. Therefore, fluids from these dewatering reactions are released ~10 km closer to the trench on the warmer CNS side of the system than on the colder EPR side. We have modeled 3-D coupled fluid and solute transport through the Nicoya margin subduction zone. Compaction and diagenetic dewatering reactions in the underthrust sediment drive fluid flow primarily out the decollement to the trench. The enhanced hydraulic conductivity on the warm CNS side of the system results in along-strike fluid flow from the EPR side to the CNS side of the system. At the same distance into the subduction zone, fluid pressures are lower on the CNS side than on the EPR side. These along-strike differences in fluid pressure control effective stress and may affect coupling on the plate interface. The offset in the position of the diagenetic dewatering reactions has a much smaller effect on along-strike fluid flow than the along-strike difference in fluid viscosity. Understanding along-strike fluid flow in this subduction zone may have implications for interpretations of along-strike variations in locking on the plate interface and microseismicity. In addition, this provides an estimate of the path taken by fluid sampled at drill sites near the margin toe. Along strike differences in fluid viscosity may be important in driving along-strike fluid flow in other subduction zones with fairly cool (<80 °C) decollements.
DE: 3021 Marine hydrogeology
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 8104 Continental margins: convergent
SC: Tectonophysics [T]
MN: Fall Meeting 2005