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doi:10.2204/iodp.proc.311.109.2006

Thermal regime of the accretionary prism

Thermal data provide one indicator of the upward fluid expulsion, which is thought to carry methane generated at depth into the GHSZ. The thermal regime also controls the depth to the base of the hydrate stability field and the BSR in the region. The geothermal gradient from downhole measurements at Site 889 was determined to be 54°C/km (Westbrook, Carson, Musgrave et al., 1994). The pressure-temperature conditions at the BSR are ~1°C cooler than expected from the theoretical gas hydrate stability for a seawater-methane mixture, but this discrepancy is within the general measurement uncertainty. The discrepancy is, however, larger if the pore water is significantly fresher than seawater.

A regional compilation of heat flow data in the Juan de Fuca plate region was given by Hyndman (1983), and Davis et al. (1990), and they reported heat flow measurements across the Vancouver Island deformation front and lower continental slope. Additional heat-probe measurements in the area of Site 889 were conducted in 2000 along major seismic lines (Spence et al., 2000b; Riedel, 2001) and a cold-vent field with evidence of active fluid flow. Repeat measurements at Site 889 were conducted in 1992 (Hyndman et al., 1994) and at the most prominent cold vent in 2000 (H. Villinger, unpubl. data, 2000). The seismically determined depth to the BSR has been used to map the thermal regime of the continental slope by Hyndman et al. (1993) and Ganguly et al. (2000).

The large-scale subduction thermal regime across the margin has been modeled numerically (Hyndman and Wang, 1993; Wang et al., 1995). The model includes dependence on the age of the subducting plate, the plate convergence rate, the thickness of insulating sediments on the incoming oceanic crust, the angle profile of the subduction fault, and the thermal properties of the overlying accretionary prism and forearc crustal material (Fig. F2). The model also includes radioactive heat generation data from land sites and the continental shelf wells, with values ranging from 0.4 to 0.6 mW/m³ (Lewis et al., 1988; Hyndman and Lewis, 1995). This model does not include the local thermal effects of deformation and thickening or fluid expulsion in the accretionary prism. The measured heat flow generally is in good agreement with the model values. Heat flow is high (~120 mW/m²) in the Cascadia Basin associated with the very young oceanic crust, ~6–7 Ma in age (Fig. F2). The heat flow decreases inland of the trench as a result of the heat sink of the cold upper part of the oceanic plate that is underthrusting the margin (e.g., Hyndman and Wang, 1993; Wang et al., 1995). Heat flow data are available from land boreholes and petroleum exploration wells on the continental shelf (Lewis et al., 1988, 1991), marine probe measurements on the continental slope and Cascadia Basin (Davis et al., 1990; Hyndman et al., 1993; Spence et al., 2000b; Riedel, 2001; Riedel et al., 2006), and from the depth of the BSR on the continental slope (Hyndman et al., 1993, 1992; Ganguly et al., 2000; Riedel, 2001; He et al., 2003).

The regional heat-probe measurements in 1990 and 1992 near Site 889 show values between 80 and 110 mW/m², whereas the repeat measurements carried out in 2000 along almost the same profile show values between 50 and 75 mW/m² (Fig. F5). The reason for this discrepancy is still unresolved, but variations over months to a few years in bottom water temperature are the most likely explanation.

A more reliable estimate for the deeper heat flow may be achieved from BSR depth conversions (e.g., Yamano et al., 1982; Davis et al., 1990; Hyndman et al., 1993; Ganguly et al., 2000; He et al., 2003). The BSR closely approximates the base of the GHSZ, and because of the strong temperature influence on gas hydrate stability, the BSR approximately marks an isotherm and therefore can be used to determine heat flow. Heat flow calculations require three main parameters:

1. The depth to the BSR (from reflection time and average seismic velocity),
2. The temperature at the BSR (from stability field), and
3. The average thermal conductivity to the depth of the BSR (from core measurements and empirical velocity-conductivity relations).

Traveltimes to the seafloor and BSR have to be determined from the seismic sections and are converted into BSR depth using a simplified velocity-depth function. The estimated heat flow is not very sensitive to variations in the velocity profile used because a change in BSR depth also results in a compensating change in BSR temperature without significantly changing the temperature gradient. Thermal conductivity was determined using the empirical regression given by Davis et al. (1990) based on ODP core data. A simple conductive model adapted from Ganguly et al. (2000) was used to convert BSR depth to surface heat flow in the area of Site 889 from regional seismic lines (He et al., 2003). Most of the heat flow variation is due to topographic changes across the two ridges of accreted sediments (Fig. F6). A comparison of heat probe measurements from the 2000 survey along seismic Line PGC9902_ODP2 with BSR-derived heat flow values is shown in Figure F5. The two campaigns of probe measurements are significantly different, as mentioned above, and the BSR-derived estimates are between the two and probably forms the most reliable estimate.

Using the temperature gradient estimate at Site 889 from downhole data of 54°C/km and an estimated average thermal conductivity of 1.15 W/(m·K), the heat flow at this site is 62 ± 8 mW/m² (Westbrook et al., 1994). This value is significantly less than the probe measurements carried in 1990 and 1992 ~3500 and 800 m south of the drill site, respectively. However, measurements carried out in 2000 over the vent field, ~1 km south of Site 889, average ~60 mW/m² and are in excellent agreement with the borehole estimate. The BSR-derived heat flow around Site 889 is slightly higher, with values just below 70 mW/m².

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