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It is well known that the optically measured lengths of fission tracks in apatite crystals are a function of etching conditions, crystallographic orientation of the track, composition of the crystal, and the state of thermal annealing. In this study we standardize etching conditions and optimize track length measurability by etching until etch pits formed at the surface of each apatite crystal reached widths of about 0.74 μm. Etching times using 5M HNO3 at 21°C were 31 s for Otter Lake, Quebec, fluorapatite; 47 s for Durango, Mexico, apatite; 33 s for Portland, Connecticut, manganoanapatite; and 11 s for Bamle, Norway, chlorapatite. An etching experiment using two etchant strengths (5M and 1.6M HNO3) revealed that, despite significant differences in etch pit shape, fission-track length anisotropy with respect to crystallographic orientation of the tracks is not a chemical etching effect. A series of 227 constant-temperature annealing experiments were carried out on nuclear reactor induced tracks in oriented slices of the apatites. After etching, crystallographic orientations of tracks were measured along with their lengths. The 200–300 track lengths measured for each slice were ellipse-fitted to give the major (c crystallographic direction) and minor (a crystallographic direction) semi-axes used to calculate equivalent isotropic lengths. The equivalent isotropic length is more useful than mean length for thermal history analysis because the variation caused by anisotropy has been removed. Using normalized etching procedures and equivalent isotropic length data, we found that the fluorapatite anneals most readily, followed by Durango apatite, manganoanapatite, and lastly chlorapatite.

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Apatite separated from 47 Cretaceous and Tertiary drill core and surface samples from the foreland basin strata of the Western Canada Sedimentary Basin (WCSB) have been analyzed for their apatite fission track (AFT) age and confined track length distribution. Thermal histories of the fission track data were then estimated using a constrained random search inversion technique based on the Durango apatite annealing model. Most importantly, this technique provides an estimate of the peak postdepositional temperature experienced by each sample with error bounds determined by the precision of the fission track data. Most apatite samples retain at least some tracks that formed prior to the time of maximum Cenozoic burial, assumed to be coincident with maximum temperature. Seven apatite samples were fully annealed during burial, during or following the Laramide Orogeny, and provide a minimum age of 42 Ma for heating. Lower Cretaceous samples from near the deformation front were fully annealed and attained temperatures of at least 119°–138°C during maximum burial. In contrast, stratigraphically equivalent samples at the northeastern end of the transect near the Cold Lake heavy oil and tar sand deposits did not exceed 80°C. Paleogeothermal gradients calculated using bounded estimates of the peak temperatures experienced by the samples, together with maximum burial estimates from coal moisture studies, range from ∼20°C/km near the deformation front to as high as 60°C/km near the cratonic edge of the basin. This variation has the same trend as present geothermal gradients, which range from ∼30°C/km to 45°C/km, but indicates a greater contrast at the end of the Laramide. The results are consistent with the concept of heat transport by basinal scale fluid flow.