A THERMOCONVECTIVE INTERPRETATION OF HEAT-FLOW DATA IN THE AREA OF OCEAN DRILLING PROGRAM LEG-116 IN A DISTAL PART OF THE BENGAL FAN

We present the results of a numerical simulation study of the thermo-c onvective circulation in a sedimentary pile, in the distal part of the Bengal Fan. The occurrence of faults and the variability of surface h eat flow data led previous workers to suggest hydrothermal circulation in this area. According to the purpose of Ocean Drilling Program surv ey Leg 116, data were collected which are adequate to constrain a mode l for the comprehension and the quantification bf the relationship bet ween ocean basin heat flow and pore fluid circulation. In this paper t he sedimentary pile is modeled as an anisotropic and inclined porous l ayer interrupted by faults with a periodic distribution. An important observation the model tries to reproduce is the shift bf approximately 2-3 km between the surface expression of the heat flow maximum and th e fault zone near site 718. The influence of permeability anisotropy a nd of permeability within those fault zones is analyzed. When the maxi mum vertical permeability in the fault zones is 5 times greater than t he vertical permeability of the porous block, the circulation consists of longitudinal rolls (i.e.,rolls with their axis oriented downslope) , with more vigorous convection in fault zones. When the maximum perme ability of the fault zone is 10 or more times greater than the vertica l permeability of the porous layer, convection is primarily restricted to the fault plane. In all cases maximum heat flow values are located directly above faults. This paper shows that the best agreement betwe en those data suggests a 1 m yr(-1) forced convection and a permeabili ty of the sediments of 0.5 darcy (5 x 10(-13) m(2)) and 5 mdarcy respe ctively,in the horizontal and vertical derictions.