Paleotemperature history of two transects across the Western Canada Sedimentary Basin: Constraints from apatite fission track analysis

The results of Apatite Fission Track (AFT) analysis for Cretaceous, Paleozo ic and Precambrian borehole samples along two major transects through the W estern Canada Sedimentary Basin (WCSB) are reviewed here. The northern tran sect crosses the Peace River Arch (PRA) and extends approximately 700 km fr om northeast British Columbia to the Alberta-Saskatchewan border northeast of Fort McMurray; the southern transect through central Alberta extends fro m the edge of the disturbed belt near Rocky Mountain House to the Alberta-S askatchewan border north of Cold Lake. Sample AFT age and length data show a wide range in the degree of annealing due to the complicated thermal hist ory of the basin and require interpretation through use of a quantitative m odel. A temperature-dependent fission track annealing model and a controlle d random search technique were used to determine sample thermal histories f rom measured AFT parameters, Many of the Cretaceous samples along the PRA transect appear to have been d eposited with effectively zero fission track age, implying a contemporaneou s volcanic source for the apatite or rapid exhumation of the source area, F ully annealed samples yield an estimate of 60 Ma for the time of maximum te mperature and inferred maximum burial. Maximum AFT paleotemperatures and ma ximum paleoburial depths (estimated from coal moisture data) were used to e stimate paleogeothermal gradient distributions at 60 Ma. For the central Al berta transect, paleogeothermal gradients increase systematically from appr oximately 20 degrees C/km near the deformation front in the southwest to as high as 60 degrees C/km in the up-dip northeastern portion of the WCSB nea r the cratonic edge of the basin. Present geothermal gradients show a simil ar overall pattern but with a narrower range of values (similar to 30 degre es-45 degrees C/km). For the PRA transect, paleogeothermal gradients are el evated in the oil sands region to the east (35 degrees-60 degrees C/km) and in the deep basin to the west (35 degrees-40 degrees C/km), The temporal a nd spatial variations in geothermal gradient are probably best explained by thermal disturbances caused by regional paleofluid flow across the WCSB ne ar the end of the Laramide Orogeny.