The significance of the partial annealing zone in apatite fission-track analysis: Projected track length measurements and uplift chronology of the transantarctic mountains

Abstract

Apatite fission-track analysis yields information on the thermal history of rocks for temperatures < 150°C. The effective closure temperature for fission tracks in apatite is ∼ 100°C, but recent developments in projected track length studies also allow us the dating of the ∼ 60°C cooling temperature. Projected track length measurements were carried out on apatites from the basement granites of the Transantarctic Mountains (TAM), southern Victoria Land, Antarctica, where a previous study had revealed an uplifted, fossil partial annealing zone (PAZ). These measurements confirm the original conclusions reached, namely, that the break in slope in the apatite age profile represents the base of an uplifted PAZ. Furthermore this study provides a test case for not only the PAZ concept in interpretations of apatite age profiles, but also for the use of projected track lengths in determining the ∼ 60°C cooling temperature. It is therefore possible to locate the-position of the base and top of a fossil PAZ by use of the ∼ 100 and ∼ 60°C cooling ages. Model predictions of the PAZ concept for tectonics indicate that the break in slope, such as seen in the TAM, only approximates the onset of uplift. The true time of initiation of uplift was somewhat earlier because samples uplifted from near the base of the fossil PAZ are further annealed as they pass through the PAZ to the zone of full track stability.