The Leg 190/196 drilling area shows high heat flow because the Muroto Transect is located near the extension of a ridge on the Philippine Sea plate (represented by the Kinan Seamounts) that ceased spreading at only 15 Ma. Conductive heat flow values range from 180 mW/m2 at Sites 1173 and 1174 (Shipboard Scientific Party, 2001a, 2001b) to 130 mW/m2 at Site 808 (Shipboard Scientific Party, 1991). Heat flow values decrease rapidly upslope from the Nankai Trough, verifying the anomalously warm nature of the trench area (Yamano et al., 1992). The simple extrapolation of conductive heat flow with depth may overestimate temperature because of the potential of heat advection associated with fluid expulsion from the deforming sedimentary sequence. The temperatures at the top of oceanic crust at Sites 1173 and 808 were estimated at 110° and 120°C, respectively, by a conductive extrapolation of shallower measurements (Shipboard Scientific Party, 1991, 2001a). A less reliable extrapolation of only two very shallow data points at Site 1174 suggests a comparable basement temperature there (Shipboard Scientific Party, 2001b). Thus, the evidence indicates that basement temperatures exceed 100°C at Sites 1173, 1174, and 808. These high temperatures drive diagenetic reactions that significantly influence log response.
Both thermal and geochemical measurements suggest fluid flow through the sediments in the Leg 196 area, although its magnitude is controversial. The temperature of the updip limit of the seismogenic zone is estimated to be ~150°C and is predicted to occur ~50 km northwest of Site 1174 (Wang et al., 1995). This estimated temperature is not much more than the basement temperatures estimated at Sites 808 and 1174. The apparently limited change in temperature between the seismogenic zone and the trench may be caused by flow of fluid upward along the décollement zone or through the permeable upper basaltic basement (Fisher, 1998).
Landward of the deformation front, the décollement zone steps down through a water-rich massive hemipelagite below, suggesting that the décollement zone is being progressively dewatered with the fluid possibly being expelled seaward up the décollement zone (Park et al., 2000). The broad minimum in the pore water chloride profile obtained during Legs 131 and 190 at Sites 808 and 1174, extending from right above to well below the décollement, was probably produced by a combination of in situ clay dehydration and fluid migration from depth (Kastner et al., 1993; Shipboard Scientific Party, 2001b).
Overall, the magnitude and location of active fluid flow in this accretionary prism and the potential linkage to the Nankai seismogenic zone are not clearly defined. Our desire to investigate this system motivated the deployment of long-term hydrogeological and geochemical monitoring systems or advanced circulation obviation retrofit kits (ACORKs) during Leg 196. By sampling particular stratigraphic and structural intervals, the ACORKs will constrain fluid pressures and permeability and provide a time series of the fluid flow regime at the toe of the Nankai accretionary prism. Thus, our results may provide a better understanding of the linkage of near-surface conditions to those in the seismogenic zone, especially in the case of an earthquake rupturing the deeper levels of the décollement zone.
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