THERMAL ANOMALIES IN THE CENTRAL INDIAN-OCEAN - EVIDENCE FOR DEWATERING OF THE BENGAL FAN

Seafloor heat flow measurements in the central Indian Ocean are locally higher than predicted by lithospheric cooling models. These anomalously high heat flow values occur in a region affected by folding and faulting of oceanic crust and overlying distal Bengal Fan sediments. Three closely spaced sites drilled during Ocean Drilling Program Leg 116 dated the onset of deformation in this area at about 7-8 Ma. Here apatite fission track thermochronometric data from sites 717 and 718 and numerical simulations are used to (1) quantify the effects of Bengal Fan sedimentation on seafloor heat flow, (2) evaluate the thermal history of sediments penetrated at these two sites and (3) constrain mechanisms responsible for anomalous seafloor heat flow in this area. At the rates sediments were deposited along the distal fan (less-than-or-equal-to 350 m/m.y.), numerical simulations predict a 6% to 9% reduction in present-day heat flow at the seafloor relative to heat flow through the base of the lithosphere. At site 717, thermal history simulations assuming heat transfer dominated by conduction are consistent with available down-hole temperature measurements and apatite fission track data. Site 718 is located 8 km south of Site 717 and 2 km south of a high-angle fault that offsets oceanic crust by about 300 m. A previously published detailed heat flow survey conducted around site 718 documented seafloor heat flow ranging from 44 to 166 mW/m2 over kilometer-scale distances. In addition, a monotonic 10 to 20% reduction in the mean length of fission tracks in apatite from 560 to 960 m below the seafloor at site 718 implies bottom-hole temperatures of the order of 50-degrees-60-degrees-C for a time period greater than a few million years. Both the seafloor heat flow and apatite fission track data argue for localized, possibly intermittent, advection of heat by lateral, updip migration of pore fluids in the vicinity of site 718. Taken together, the heat flow data and temperature range inferred from the apatite fission track data at site 718 imply a fluid source in the upper crust. Upward movement of pore fluids at velocities of less than 1 cm/yr through a basement-sediment boundary maintained at temperatures ranging from 65-degrees to 80-degrees-C is capable of explaining both sets of observations. As a hypothesis to reconcile existing observations, I suggest that the anomalous thermal structure in the central Indian Ocean represents a joint response to compaction-driven dewatering of the Bengal Fan and local enhancement of the vertical permeability in the upper crust and sediment pile due to intraplate deformation.