ODP Legs 170 and 205 helped define and address objectives for subduction factory and seismogenic zone studies in Central America. Between the active monitoring of fluid flow across the Middle America Trench in Costa Rica and the postcruise science of sedimentary and igneous samples the legs provide the basis for an integrated study of the margin. This includes the sedimentary and igneous character of the incoming plate, its thermal structure, and evidence for multiple episodes of fluid flow, including recent to contemporary flow in a cold setting far off-axis. It also includes the structure, sedimentology, geochemistry, and permeability characteristics of the forearc sediment wedge, décollement, and underthrust sediments. Interesting in their own right, these studies also provide context for the monitoring of active flow of deeply sourced fluids along the décollement and the link between fluid flow and the seismic cycle.
Studies of the incoming plate speak to modification of the crust generated at the East Pacific Rise and to multiple episodes of fluid flow. Coring, petrology, and geochemistry show extensive addition (>180 m) of enriched basaltic material, with characteristics of ocean island basalt, to the upper part of the incoming igneous section. The sills and intrusions cored during Leg 170 and 205 form part of a large region of off-axis magmatism, with ages of ~14 and perhaps 8–10 Ma; seamounts and Cocos track tholeiites have similar chemical characteristics to those reported here. Speculation is that fractures and stresses associated with plate rearrangement allowed distal emplacement of enriched mantle (from the Galapagos hotspot?) into the depleted upper mantle, with later tectonic events or magmatic heating triggering melting. Although the rocks are little altered overall, Sr isotope studies reveal localized intervals (<10–20 m thick) of major isotopic shifts (from ~0.7033 to 0.705), requiring water:rock ratios >6. Two fluids, one of which is significantly altered from seawater composition, best satisfy modeling. One alteration horizon is monitored by our downhole instruments, where results to date indicate a vigorous regime of active flow in permeable layers that are slightly subhydrostatic. Chemical data from the collected fluids imply mixing between bottom seawater and a second fluid, which is greatly enriched in Ca and depleted in Mg relative to seawater. Fluid flow rates estimated from various methods (heat flow, pore fluid gradients in overlying sediments, and recovery of formation fluids from seawater composition after closing the borehole) are all on the order of meters per year. Ash deposits on the incoming plate record siliceous explosive volcanic activity in the CAVA over the last 2.5 m.y. Based on various chemical tracers, arc lavas recorded a peak in subducted input (fluids and sediments) at ~1.45 Ma, with lesser contributions before and since.
Studies from the prism sites focus on the origin of the sediments above the décollement, the permeability structure, and biogeochemistry. Sediments from Leg 205 cored just above the décollement are early Pliocene in age, but no younger than 3.75 Ma, as are those between 219 and 222 mbsf. Prism sediments between 150 and 161 mbsf are of early Pleistocene age, and those between 161 and 216 mbsf are late Pliocene. Sediments from Subunit P1B have C and N systematics more like those measured upslope at Site 1041 than the incoming sediment section, consistent with Leg 205 shipboard analyses suggesting that the prism sediments are derived from slumping rather than paleoaccretion. Biomarker indicators of microbial activity range from very low to absent. DNA was identified in half of the samples analyzed; methanogen-specific genes were detected in four samples from the sedimentary prism. Highest concentrations of quinones, sometimes treated as a proxy for total biomass (Hiraishi et al., 1998), are associated with the décollement zone. Measurements of permeability and porosity confirm the low permeabilities of the hemipelagic mud sediments on the order of 10–16 to 10–20 m2 with on average slightly higher value of up to 10–15 m2 for pelagic carbonate samples. Porosities range between 26% and 71% and show a clear inverse correlation with permeability. Long-term monitoring in the décollement zone at Site 1255 shows that flow in the prism sediments and décollement is slightly superhydrostatic. Excursions in the pressure record are observed in late May–early June 2003 and in mid-September to late October 2003; these occur 2–3 weeks after deformational events recorded geodetically. Chemical excursions and changes in flow rate are also associated with these events.