On apatite fission-track analysis and heat transfer processes in the upper crust
Fission-track ages and confined track-length measurements from detrital apatites recovered from Ocean Drilling Program (ODP) Leg 116 Site 717 and 718 cores of distal Bengal Fan sediments provide information on both the thermal history of sediments recovered at these two sites and the cooling rates of apatite source areas. Site 717 was drilled to a depth of 820 meters below the seafloor (mbsf) bottoming out in upper Miocene strata. Site 718, located approximately 7 km to the south of Site 717 on an adjacent fault block, was drilled to a depth of 960 mbsf bottoming out in lower Miocene strata. Fourteen composite apatite samples were derived by combining fine-grained sand samples from Site 717 and 718 cores over 70- to 120-m intervals. Thirty apatite grains per composite sample from ten samples (at least every other sampled interval) were dated and track length measurements (20 - 50 per sample) were obtained for all samples. Mean track lengths from Site 717 samples are statistically identical, ranging from 14.4 ± 0.4 to 14.8 ± 0.3 μm (all uncertainties are the standard error of the mean at the 95% confidence interval), and mean fission-track ages increase monotonically down-section from 4.8 ± 1.1 to 14.3 ± 2.2 Ma. At Site 718, mean track lengths to 560 mbsf are equivalent to those measured from Site 717 samples. A monotonic decrease in mean track length (14.6 ± 0.3 to 13.2 ± 0.4 μm) and a corresponding decrease in mean fission-track age (21.1 ± 2.9 to 15.8 ± 2.4 Ma) with depth for samples between 560 and 960 mbsf at Site 718 indicates that shortening of fission-tracks in these samples occurred at elevated temperatures after deposition. Track length shortening, relative to an unannealed mean track length of 16.3 μm, is approximately 10% for all Site 717 samples and for samples from the upper 560 m at Site 718. Mean track length reduction of the lowermost sample at Site 718 is approximately 20%. Assuming that elevated temperatures responsible for this reduction are related to the onset of deformation at approximately 7 Ma, temperatures of about 55 ± 5 °C at 860- to 960-mbsf are inferred based on extrapolation of laboratory annealing experiments. These temperature estimates imply significant advective heat transfer through the sediment pile near Site 718 because temperatures at these depths are expected to be on the order of 30-35 °C for conductive heat transfer predicted by cooling plate models for oceanic lithosphere. Dated samples have mean apatite fission-track ages that are less than 10 m.y. older than sample depositional ages. These young ages indicate that rocks in the upper crust of Bengal Fan source areas cooled at rates greater than 15 °C/m.y. These cooling rates imply source area denudation rates exceeded 300 m/m.y. These estimates indicate that source areas similar to the present Himalayas have supplied sediment to the distal Bengal Fan since at least 17 Ma.