Geochemical analyses of pore fluids in the décollement and overlying deformed sedimentary wedge indicate fluid origins from deep within the subduction system. Potassium and lithium concentrations and polymerized hydrocarbons indicate formation temperatures of between 100° and 150°C (Kimura, Silver, Blum et al., 1997; Chan and Kastner, 1998). Using the measured heat flow at Site 1041 and a conservative value for deep crustal thermal conductivity, Silver et al. (2000) suggest maximum depths of origin of 10-15 km, or locations along the décollement 40-60 km from the trench. Profiles of chlorinity and salinity through the deformed wedge at Site 1040 and the slope apron at Site 1041 both show significantly lower values than that of seawater, indicating freshening by several sources. One source is flow from listric thrust faults that root in the décollement. Significant local excursions of freshening are seen in such conduits. Another source of local freshening is gas hydrates (Kimura, Silver, Blum, et al., 1997; Kopf et al., 2000), which are abundant at both sites. A third alternative that was not evaluated is influx of meteoric water through the apron from land sources.
From surface heat flow measurements, Langseth and Silver (1996) documented very low heat flow on the incoming Cocos plate and only slightly increased heat flow over the middle to lower slope region. Their findings were corroborated by downhole temperature measurements during Leg 170, at Sites 1039, 1040, and 1041 (Kimura, Silver, Blum et al., 1997). In fact, heat flow values measured downhole were uniformly lower than those measured at the surface. Ruppel and Kinoshita (2000) carried out a careful analysis of the Leg 170 heat flow data, finding similar results, though values are slightly adjusted from those reported by Kimura, Silver, Blum, et al. (1997). They explained the difference between surface and borehole heat flow by nonlinear perturbation of the thermal regime by advective flux, which was strongest at Site 1040 and weakest at Site 1041. In the gas hydrate zone, above the décollement zone, Ruppel and Kinoshita (2000) find a vertical advective flux rate of 5-7 mm/yr at Site 1040, less than 1 mm/yr at Site 1041, and 19 mm/yr in the hemipelagic sediments at Site 1039. The high vertical flux rates for Site 1040 may be facilitated by fracture permeability.
Langseth and Silver (1996), following suggestions by Langseth and Herman (1981) and Yamano and Uyeda (1990), proposed that flow of seawater into the uppermost oceanic crust was the most likely explanation for the drastic lowering of heat flow from expected equilibrium values. During Leg 170, geochemical profiles at Site 1039 showed marked changes in gradient for numerous chemical species, including Ca, Mg, Sr, Li (Chan and Kastner, 1998) and other elements, moving rapidly toward seawater values near the base of the sediment pile (Kimura, Silver, Blum, et al., 1997). Using the gradients for Sr and Li concentration and isotope ratio gradients, the formation of the basement water is found to be younger than 15 k.y. (Li) or 20 k.y. (Sr).
To estimate the possible driving mechanism for fluid flow in the crust, Silver et al. (2000) used an analytical thermal model from Fisher and Becker (1999), in which temperature differences between sites of recharge and discharge of fluids are responsible for driving the flow. Unknowns included the permeability and thickness of the zone of crustal flow and the distance between sites of discharge and recharge. By assuming a temperature difference of 52°, permeability ranging between 10-9 and 10-14 m2, and pressure differences of 80 to 230 kPa between recharge and discharge sites, they found that specific discharge is in the 1 to 5 m/yr range and ages of basement water are in the 2- to 16-k.y. range, generally consistent with the geochemical model. Lower pressure difference between sites, caused by loss of head during vertical flow or lower temperature differences between recharge and discharge sites, requires the permeabilities to be higher for sustained flow. Alternatively, an additional driving force could be active along with temperature differences, which would allow lower permeabilities (indeed, the permeabilities measured in the sediment section were lower).
One potential source of additional driving force is cyclic seismic flexure and associated crack dilation and contraction, as suggested by Sibson (1994) and Muir-Wood (1994). Crack dilation is expected to be associated with interseismic strain on the trench outer wall, allowing entrance of water into the uppermost crust. During coseismic periods, fluids would be forced to flow, and a component of flow might be along the trench axis (Silver et al., 2000). Whether such a mechanism has any significance remains to be tested through additional drilling and a much more extensive network of heat-flow observations.