Long-term hydrogeochemical records in the oceanic basement and forearc prism at the Costa Rica subduction zone

作者：Solomon Evan A^*; Kastner Miriam; Wheat C Geoffrey; Jannasch Hans; Robertson Gretchen; Davis Earl E; Morris Julie D

来源：Earth and Planetary Science Letters, 2009, 282(1-4): 240-251.

DOI：10.1016/j.epsl.2009.03.022

• subduction zone
• fluid flow
• CORK
• Ocean Drilling Program
• pore fluids

Two sealed borehole hydrologic observatories (CORKs) were installed in two active hydrogeochemical systems at the Costa Rica subduction zone to investigate the relationship between tectonics, fluid flow, and fluid composition. The observatories were deployed during Ocean Drilling Program (ODP) Leg 205 at Site 1253, similar to 0.2 km seaward of the trench, in the upper igneous basement, and at Site 1255, similar to 0.5 km landward of the trench, in the decollement. Downhole instrumentation was designed to monitor formation fluid flow rates, composition, pressure, and temperature. The two-year records collected by this interdisciplinary effort constitute the first co-registered hydrological, chemical, and physical dataset from a subduction zone, providing critical information on the average and transient state of the subduction thrust and upper igneous basement. The continuous records at ODP Site 1253 show that the uppermost igneous basement is highly permeable hosting an average fluid flow rate of 0.3 m/yr, and indicate that the fluid sampled in the basement is a mixture between seawater (similar to 50%) and a subduction zone fluid originating within the forearc (similar to 50%). These results suggest that the uppermost basement serves as an efficient pathway for fluid expelled from the forearc that should be considered in models of subduction zone hydrogeology and deformation. Three transients in fluid flow rates were observed along the decollement at ODP Site 1255, two of which coincided with stepwise increases in formation pressure. These two transients are the result of aseismic slip dislocations that propagated up-dip from the seismogenic zone over the course of similar to 2 weeks terminating before reaching ODP Site 1255 and the trench. The nature and temporal behavior of strain and the associated hydrological response during these slow slip events may be an analog for the response of the seaward part of the subduction prism during or soon after large subduction zone earthquakes.

• 出版日期2009-5-30