Heat Flow from Depth Variations in Bottom Simulating Reflector

In 1996, several grids of short-offset multichannel seismic data were acquired in the continental slope region. Piston coring was also carried out at 18 sites, and physical property analyses (including P-wave velocity, density, resistivity and porosity) allowed ground-truth calibration of the seismically-derived seafloor reflection coefficients (Mi, 1998). Over a grid of seismic lines southeast of Site 889 (Area3 in Fig. 1), heat flow was calculated from the depth of the BSR. The regional variation of heat flow, decreasing landwards from about 80 to 65 mW/m2, was consistent with tectonic thickening of accretionary wedge sediments (Ganguly et al., 2000). Significant local variations in heat flow

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£ 35380 35400 35420 35440 S.P.

Figure 6. Part of a migrated seismic section from a short-offset multichannel line in area A3 (Fig. 1). Heat flow calculated from the BSR shows a minimum over the topographic high and a maximum on the flanks of the high. A portion of the heat flow behavior is explained by static topographic effects (solid line in a). Fluid flow associated with a possible thrust fault may explain the remainder.

Figure 6. Part of a migrated seismic section from a short-offset multichannel line in area A3 (Fig. 1). Heat flow calculated from the BSR shows a minimum over the topographic high and a maximum on the flanks of the high. A portion of the heat flow behavior is explained by static topographic effects (solid line in a). Fluid flow associated with a possible thrust fault may explain the remainder.

were also observed, notably low heat flow values over topographic highs and high heat flow values over the flanks of the highs. As shown in Fig. 6, heat flow at some localities increased by as much as 50% (from 65 to 100 mW/m ) over a horizontal distance of 1-2 km. Much of this variation may be due to the focussing and defocussing effects of topography alone; applying an analytic solution by Lachenbruch (1968) for the topographic effect, we see that about half of the landward increase in heat flow at SP 35400 (Fig. 6) can be explained by the static effects of topography. The remaining component of this heat flow variation may be produced by dynamic effects. For example, the topographic ridge may be associated with thrust faulting, as identified in Fig. 6. The fault may provide a pathway for the upward migration of warm fluids to produce localized increases in heat flow. Furthermore, regions of reduced heat flow may occur as the thrust fault brings colder near-surface sediments to greater depths.

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