The Eisenhower Range, Transantarctic Mountains: Evaluation of qualitative interpretation concepts of thermochronological data

Abstract

The Transantarctic Mountains (TAM) were one of the first regions where apatite fission track (AFT) thermochronology was applied routinely to study exhumation processes and long term landscape evolution. Pioneering publications from the region introduced or refined interpretation concepts of thermochronological data such as the break in slope in vertical age profiles as qualitative marker for the onset of accelerated rock cooling.New AFT data were compiled from vertical profiles in the Eisenhower Range, northern TAM, and compared with published data. Samples originally examined by the population technique were re-analyzed via the external detector technique. AFT ages increase from 32±2Ma at an elevation of 220m to 175±14Ma at 2380m. Geological evidence and thermal history modeling of the AFT data require Jurassic to Late Eocene reheating of the samples and an onset of cooling at ~35–30Ma. This requires the deposition of an ~3 to 3.5km thick sedimentary sequence on the granitic basement subsequent to Jurassic Ferrar magmatism at ~180Ma. The regression of paleotemperatures against sample altitudes infers a high Jurassic geothermal gradient of ~60°C/km related to rifting processes and Ferrar magmatism, and a moderate Cretaceous/Eocene geothermal gradient of ~30°C/km.Comparison of ages generated with population and external detector technique shows the importance of determining single-grain ages for each sample, even from granitic rocks of the same intrusion, and thus strongly supports previous cases made for the determination of annealing kinetics and grain-age evaluation. Age comparison additionally illustrates that samples above a break in slope record larger deviations between population and external detector ages than samples below a break in slope.We demonstrate that the position and shape of a break in slope result from various factors, such as the thermal history prior to final cooling, maximum paleotemperatures, cooling rate, and geothermal gradient. A break in slope does not straightly date the onset of final cooling and cannot substitute thermal history modeling. Therefore, earlier studies from the TAM and similar settings elsewhere need to be validated by combining thermal history modeling of thermochronological data and supplementary geological information.