Fission-track analyses of detrutal apatites from ODP Sites 116-717 and 116-718

This paper presents fission-track ages and confined track-length measurements from detrital apatites recovered from Ocean Drilling Program Leg 116 Site 717 and 718 cores. We interpret these data in terms of the post-depositional thermal history at these two sites and the thermotectonic history of apatite source areas. Composite apatite samples were derived by combining fine-grained sand samples from Sites 717 and 718 cores over 70- to 120-m intervals over the total depth penetrated at Sites 717 (T.D. = 820 mbsf) and 718 (T.D. = 960 mbsf). 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 ?m (all errors are the 95% confidence interval), and mean fission-track ages increase monotonically downhole from 4.8 ± 1.1 to 14.3 ± 2.3 Ma. For Site 718, located approximately 7 km to the south of Site 717 on an adjacent fault block, mean track lengths to 560 mbsf are equivalent to those measured from Site 717 samples. A 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 from Site 718 indicates that post-depositional downhole shortening of fission tracks at elevated temperatures has taken place.
Track-length shortening, based on mean track lengths 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 of Site 718. The total amount of shortening of the lowermost sample from Site 718 is approximately 20%. Based on extrapolation of published laboratory annealing experiments, maximum isothermal time-temperature condition extremes that could produce this degree of annealing at the base of Site 718 are estimated to range from 50°C for a duration of 17 m.y. (since deposition) to 55°C for a duration of 7.5 m.y. (since the onset of deformation). These estimates argue against regional thermal conduction as the only mechanism for post-depositional heating and support seafloor heat flow and shipboard geochemical evidence for local convective heat transfer in the vicinity of Site 718.
In terms of source-area implications, dated samples have mean apatite fission-track ages that are only 0 to 10 m.y. older than depositional ages. These young ages imply rapid transport of sediment to the distal Bengal Fan and source areas characterized by high denundation rates (<300 m/m.y.). These rates suggest that source areas similar to parts of the present-day Himalayas supplied sediment to the distal Bengal Fan since at least 17 Ma.