Kevin D. Crowley

Thermal significance of fission-track length distributions

Abstract The semi-analytical solution of an equation describing the production and shortening of fission tracks in apatite suggests that certain thermal histories have unique length-distribution “signatures”. Isothermal-heating histories should be characterized by flattened, length-shortened distributions; step-heating histories should be characterized by bimodal track length distributions; and linear-cooling histories should be characterized by negatively skewed, length-shortened distributions. The model formulated here to investigate track length distributions can be used to constrain the thermal histories of natural samples for which unbiased track length data are available-provided that the geologic history of the system of interest can be used to partially constrain one of the unknowns in the model equations, time or temperature.

Neutron dosimetry in fission-track analysis

Abstract This paper reports a series of irradiation experiments performed to illustrate the effects of neutron energy on fission-track age determinations. These experiments reveal that errors of several percent can be introduced into an age determination when fluence monitors which contain U in non-natural isotopic ratios and (or) contain Th are used in the irradiations, or when age standards and samples of unknown age are irradiated at different conditions (i.e. with neutrons having different energy spectra). To avoid errors in age determinations it is recommended that all irradiations be performed at identical conditions, preferably with a highly thermalized neutron flux, or that fluence monitors which do not contain Th and which contain U in natural abundances be used in the age-determination process.

Origin and epeirogenic history of the Williston Basin: Evidence from fission-track analysis of apatite

We have tested the hypothesis that the lithosphere beneath the Williston Basin was heated, uplifted, and eroded prior to basin subsidence; this was accomplished by fission-track dating of apatite from samples of basement rock beneath and adjacent to the basin. Apatite fission-track ages of basement rocks beneath the basin range from about 4 Ma near the center to 554 Ma near the eastern edge. Combined with stratigraphic evidence, the ages suggest that at least 3 km of basement were eroded prior to subsidence of the Williston Basin. Rates of apparent uplift and erosion were on the order of 0.10 to 0.15 km/m.y. Apatite fission-track ages of basement outcrop samples on the eastern flank of the basin range from about 572 Ma to 441 Ma. These ages suggest that erosion of the flank occurred from the Early Cambrian to the Late Ordovician. We conclude that timing of uplift of the basement prior to subsidence of the Williston Basin is consistent with the thermal-origin hypothesis of basin formation.

Tectonic significance of precambrian apatite fission-track ages from the midcontinent United States

Abstract Apparent apatite fission-track ages from drill core penetrating basement on the flank of the Transcontinental Arch in northwestern Iowa range from 934 ± 86 to 641 ± 90 Ma. These ages, the oldest reported in North America, record at least two thermal events. The 934 Ma age, which is synchronous with K-Ar ages in the Grenville Province and many K-Ar whole-rock and Rb-Sr isochron ages from the Lake Superior region, may document basement cooling caused by regional uplift and erosion of the crust. The remaining fission-track ages are products of a more recent thermal event, relative to the age of the samples, which raised temperatures into the zone of partial annealing. Heating may have occurred between the Middle Ordovician and Middle Cretaceous by burial of the basement with additional sediment. It is estimated that burial raised temperatures in the part of the basement sampled by the core to between 50 and 75°C. These temperature estimates imply paleogeothermal gradients of about 20°C/km, approximately two and one-half times present-day values, and burial of the basement by an additional 2–3 km of sediment.