Comparison of measured and BSR-derived heat flow values, Makran accretionary prism, Pakistan

Abstract

During the German research cruise SO-124 on RV Sonne (fall 1997) on the Makran accretionary wedge off Pakistan, geophysical investigations were carried out to study the thermal regime at a gas hydrate bearing sediment in a tectonically deformed accretionary wedge. On a transect perpendicular to the strike of the deformation front 42 heat flow measurements were carried out, accompanied by seismic reflection experiments. The investigations start in the south in the abyssal plain and cover the continental slope up to 2300 m water depth. The aim of this study is to compare the BSR derived heat flow (denoted as estimated heat flow) with the values from measurements at the seafloor. This requires the calculation of sediment physical properties at depth using empirical relationships between velocity and porosity. The value measured and corrected for sedimentation of 47 mW/m 2 south of the deformation front is slightly higher than values reported by Hutchison et al. (Earth Planetary Sci. Lett. 56 (1981) 252–262). In all basins the estimated heatflow is significantly higher than the measured values. As a result, temperatures at the BSR extrapolated from seafloor measurements are 5–6 K lower than those taken from Gas hydrate stability considerations. As an overall trend the estimated as well as the measured heat flow show a small decrease from the deformation front to the northward thickening prism. A similar observation was made at other accretionary wedges and described by Wang et al. (J. Geophys. Res. 98 (B3) (1993) 4121–4142) and Ferguson et al. (J. Geophys. Res. 98 (B6) (1993) 9975–9984). Within the slope basins heat flow values show little variation, indicating predominantly conductive heat transport. Fluid flow might occur at the bounding faults where we have little control. The effect of rapid sedimentation on the dynamic behavior of the BSR might also have a significant influence on the estimated heat flow values. Our data set shows clearly that detailed seismic surveys and good control of the subsurface velocity are absolute necessities for the comparison of measured and BSR-derived heat flow values. However, the uncertainty of the velocity–porosity relationship together with a high and only approximately established sedimentation rate represent crucial but missing constraints which can be gained only by drilling.