Continuous Chemical and Fluid Flux Monitoring in Two Distinct Fluid Flow Systems at the Costa Rica Subduction Zone

HR: 10:35h
AN: T12B-02 [Abstracts]
TI: Continuous Chemical and Fluid Flux Monitoring in Two Distinct Fluid Flow Systems at the Costa Rica Subduction Zone
AU: Kastner, M
T12B-02 AF: Scripps Institution of Oceanography, La Jolla, CA, 92037
AU: * Solomon, E
EM: esolomon@ucsd.edu
AF: Scripps Institution of Oceanography, La Jolla, CA, 92037
AU: Wheat, G
T12B-02 AF: MBARI, Moss Landing, CA, 95039
AU: Jannasch, H
T12B-02 AF: MBARI, Moss Landing, CA, 95039
AU: Davis, E
T12B-02 AF: Pacific Geoscience Center, Sidney, B.C., Canada
AU: Villinger, H
T12B-02 AF: University of Bremen, Bremen, Germany, 28334
AU: Heesemann, M
T12B-02 AF: University of Bremen, Bremen, Germany, 28334
AU: Robertson, G
T12B-02 AF: Scripps Institution of Oceanography, La Jolla, CA, 92037
AU: Morris, J
T12B-02 AF: Washington University, St. Louis, MO, 63130
AB: Two fluid flow systems were identified during ODP Legs 170 & 205, at the Costa Rica Subduction Zone (SZ). The first was inferred to be in the uppermost oceanic basement on the incoming plate, where lateral flow of seawater (SW) cools the oceanic crust. The second, sampled in the décollement and a prism thrust fault, taps a deeply sourced fluid originating arcward. Continuous water samplers (OsmoSamplers [OS]) with temperature (T) loggers were deployed from Oct. 2002 to Sept. 2004 in CORKed borehole observatories near the SZ toe. Site 1253 is situated ~0.2 km seaward of the trench in the upper igneous section, and Site 1255 is ~0.4 km arcward of the trench within the décollement. Fluid flux meters were also deployed at Site 1255. The OS fluids were sampled at weekly resolution. The chemical and fluid flux sampling is part of a larger coordinated effort involving long-term T and pressure (P) monitoring. Within the uppermost igneous basement at Site 1253, chlorinity (Cl) increases from SW to a steady-state value of 650 mM, 62 days after installation. The time to reach steady-state provides an estimate for fluid flux within the basement. When the borehole was briefly opened during an Alvin dive in March 2004, Cl dropped to SW value. The borehole then remained closed for 6 months until the recovery of the OS in Sept. 2004 on Leg 301T. During this period Cl did not reach the previous steady-state value; T, however, recovered within ~3 months. The less than hydrostatic P in the upper basement, by 6 kPa, indicates that it is highly permeable and hydrologically connected to distant ventilation points. The steady-state Cl value of 650 mM (16% >SW) indicates hydration of the upper basement. Ca and Mg concentrations are ~3 times and ~1/2 SW value, respectively, indicating advection of an altered fluid in the interval sampled. The chemistry of this fluid suggests mixing of SW with a basement fluid sampled ~30 km seaward of Site 1253 during the TicoFlux expeditions, as well as with a similar fluid sampled in a thrust fault at Site 1042, Leg 170, ~6 km inboard of the trench. The former would suggest the permeable basement at Site 1253 is hydrologically connected to points of SW recharge at distant igneous outcrops, and the latter would suggest that it acts as a fluid flow pathway for deeper sourced fluids being expelled from the SZ. At Site 1255, Cl steadily increases from SW value to 636 mM over the deployment period; the high values likely reflect active in situ gas hydrate formation in the prism. A steady decline in P was coupled with a decrease in fluid flow rate over the first 5 months of the deployment; it may reflect relaxation of the outer prism after a contractional strain event. Overprinted on these trends are two secondary excursions manifested by Cl increases that correlate with P and T increases in the décollement and with fluid flow rate changes. The first occurred in May to June 2003, and the second in mid-Sept. to late Oct.-2003 and may be related to an aseismic slip event on the subduction thrust beneath the Costa Rica coast documented by GPS in mid-Sept. 2003. After the first event, both P and fluid flow rates increase appreciably and remain constant to April 2004. This is the first high-resolution time series data set of chemistry and fluid flow at a SZ, with direct implications for seismogenic zone activity, and chemical fluxes to the ocean and to the arc volcanoes.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 3021 Marine hydrogeology
DE: 3050 Ocean observatories and experiments
DE: 3075 Submarine tectonics and volcanism
SC: Tectonophysics [T]
MN: Fall Meeting 2005