Interpretation Of Fission-track Data Research Articles

Convective heat transfer in a steeply dipping fault zone and its impact on the interpretation of fission-track data - a modelling study

AbstractThe effects of convective heat transfer by hydrothermal fluid flow on fission-track (FT) thermochronology are studied using numerical modelling techniques. Parameter studies are carried out on two-dimensional crustal segments with a steeply dipping fault zone exposed to constant denudation to evaluate the relative importance of different variables, including denudation rate as well as hydraulic and material properties. Time–temperature histories of particle points are calculated in the vicinity and also a few kilometres away of the fault zone. These time–temperature paths are then used in a forward-modelling approach to determine the expected FT cooling ages and track-length distributions.Modelling results indicate that hydrothermal fluid flow can significantly disturb the background conductive thermal state of the upper crust, and the interpretation of FT data using a steady-state geothermal gradient can result in erroneous denudation rates that overestimate the true erosion rates by more than 80%. A pattern of highly varied FT cooling ages from samples at the same elevation does not necessarily ask for differential tectonic movements, instead it can be generated by deep circulation of groundwater within a few million years (Ma). Denudation rates inferred from FT cooling age–elevation plots are likewise inaccurate in a hydrothermally active area because the important assumption about closure temperature isotherms being horizontal or at a constant depth below the surface is not met.

Late stages of exhumation constrained by structural, fluid inclusion and fission track analyses (Sesia–Lanzo unit, Western European Alps)

In metamorphic areas, paleogeothermal gradients are difficult to properly estimate, leading to great uncertainty in exhumation rate calculations based on fission track analysis. In this work, a new multidisciplinary approach based on fluid inclusion analysis, structural analysis and fission track dating has proven to be able to constrain the late stages of exhumation of metamorphic rocks. The study area is located in the Sesia–Lanzo unit of the Western European Alps, along the Hone shear zone. Kinematic analysis at different scales unraveled a polyphase postmetamorphic deformation history, constrained in the P–T space thanks to laboratory investigation of kinematically referenced fluid inclusions coupled with the analysis of the rheological behavior of quartz and feldspar inside syn-kinematic veins. The resulting P–T path points to an increasing paleogeothermal gradient from 18 to 30 °C km − 1 after greenschist facies metamorphic conditions, and provides a reference framework for a quantitative interpretation of fission track data in terms of exhumation rates and fault throws. According to fission track data, throws accommodated by the Hone shear zone between 33 and 30 Ma are on the order of 4.5 km, and would be underestimated by ∼25% if a “normal” steady gradient was assumed for the whole post-metamorphic history of the area. The deepest portions of this structure were exhumed thanks to the activity of the E–W fault system that accommodated a throw of ∼4 km between 28 and 20 Ma, and a throw of ∼0.8 km during the last 20 Myr.

Discussion on Mesozoic cover over northern England: interpretation of apatite fission track data

A. A. McCulloch writes: In a recent Special paper, Holliday (1993) attempted to 'reconcile AFTA-derived palaeo-temperatures with observed geological information and the probable Mesozoic burial history of N and E England'. The thermal history interpretation of the apatite fission track data from northern England (Green 1986; Lewis et al. 1992), on which the analysis by Holliday (1993) relies, is based principally on those samples which were totally annealed prior to cooling. The thermal history interpretation of these samples is then extended to understand the remaining results in terms of a common thermal history involving cooling at 65 ± 5 Ma. The primary purpose of this comment is to highlight to readers unfamiliar with apatite fission track analysis, the shortcomings in this interpretation of the fission track data and to stress the absence of an express link, implicit in the analysis of Holliday (1993), between a rigorous thermal history interpretation of the fission track data and Early Tertiary uplift and erosion. An alternative geological interpretation of the thermal history of northern England, described in more detail in McCulloch (1993), is briefly outlined. Thermal history interpretation of N England fission track data. In samples which give mean track lengths between 14 and 15.5 pm and standard deviations around 1 pm, the apatite fission track age may be interpreted as indicating a distinct cooling event from palaeotemperatures greater than 110 ºC to less than 50 ºC (Gleadow et al. 1986). Using this standard criterion, the data reported by Lewis et al. 1992 and