Geochemical evidence for fluid flow in the upper and subducting plates of the Costa Rica margin; results from CRISP drilling during Exp. 334 and 344

Author(s): Torres, M. E.; Solomon, E. A.; Kastner, M.; Harris, R. N.; Formolo, M.; Choi, J.; Berg, R. D.; Nuzzo, M.
Author Affiliation(s): Primary:
Oregon State University, Corvallis, OR, United States
University of Washington, United States
Scripps Institution of Oceanography, United States
University of Tulsa, United States
Korea Institute of Geoscience and Mineral Resources, South Korea
University of Lisbon, Portugal
Volume Title: AGU 2013 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2013; American Geophysical Union 2013 fall meeting, San Francisco, CA, Dec. 9-13, 2013. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: CRISP (Costa Rica Seismogenesis Project) is designed to investigate the processes that control fault zone behavior during earthquake nucleation and rupture propagation at erosional subduction zones. Fluids and associated diagenetic reactions are key components of this project, as they can have a profound impact on the shallow thermal structure and fluid content of the subducting and upper plates; fault zone stability and seismogenesis; and the transfer of elements and isotopes to the ocean, volcanic arc, and mantle. The pore fluid geochemistry at sites drilled in the upper and middle slope of the Costa Rica margin document fluid advection along fault zones in the upper plate, and demarcate a horizontal fluid transport zone along the discontinuity between the slope apron and underlying upper plate sediments that is continuous between Sites U1378 and U1379. Fluid flow at these sites overprints the general geochemical profiles that are influenced by in situ diagenetic reactions such as ion exchange, microbial metabolic processes, volcanic ash alteration, and carbonate diagenesis. Site U1379, drilled on the upper slope above the locked portion of the plate boundary, intersected a coarser-grained sediment interval with pervasive faulting at ≈600 to 800 mbsf. Here a decrease in the concentration of Cl and of other major elements, and maxima in thermogenic hydrocarbon concentrations are observed. Based on the geothermal gradient at this site the temperature is too low to support the in situ production of thermogenic hydrocarbons or for extensive clay dehydration, thus these geochemical signals indicate a deeper source for the fluid and migration along the permeable horizons. These deep-sourced fluid signatures are even more pronounced at Sites U1378 and 1380, drilled in the middle slope, above the unlocked portion of the plate boundary. Here the horizontal transport zone is well confined to a shear zone that extends from ≈480 to 550 mbsf, at the boundary between lithologic Units I and II that defines the interface between the slope sediment cover and deeper upper plate material. The Cl, Li, and hydrocarbon concentration data indicate that this fluid originated from a source temperature greater than 90°C. Within the well-cemented sediments below the slope cover, there is a more pervasive non-focussed transport of a fluid, fresher than seawater, having a strong signature of ash/basalt interaction (high Ca, low Mg), and a marked increase in C1/C2+ ratio, indicating either a more biogenic signature of the gases or/and an extensive migration effect on the gas composition. Geochemical data at two sites drilled on the incoming plate, reveal fluid flow within the permeable upper oceanic crust. At Site U1381, the sulfate concentration profile shows a reversal below ≈40 mbsf, with a steady increase in concentration with depth. This observation is similar to that previously reported at the nearby incoming sediment section offshore the Nicoya Peninsula, and reflect diffusional communication with a fluid with seawater-like chemistry in the igneous basement. The sulfate concentration profile at Site U1414 is unusual in that it displays a second minimum at 330 mbsf, which corresponds to a sharp minimum in calcium and a maximum in barium concentrations. These data suggest lateral flow of fluid originating landward of Site U1414 where microbial oxidation of methane and/or other organic carbon sources has depleted dissolved sulfate.
Year of Publication: 2013
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; Central America; Costa Rica; Costa Rica Seismogenesis Project; East Pacific; Expedition 334; Expedition 344; IODP Site U1378; IODP Site U1379; Integrated Ocean Drilling Program; Marine sediments; North Pacific; Northeast Pacific; Pacific Ocean; Plate tectonics; Sediments; Subduction; Subduction zones
Coordinates: N084051 N084051 W0840201 W0840202
N083532 N083532 W0840438 W0840438
Record ID: 2015037344
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