THE CRETACEOUS AND YOUNGER THERMAL HISTORY OF THE KTB SITE (GERMANY) - APATITE FISSION-TRACK DATA FROM THE VORBOHRUNG

A detailed fission-track analysis was carried out on 41 apatite sample s from the 4001-m-deep pilot hole of the German Continental Deep Drill ing Program (KTB). The investigations include dating by the population method and length measurements of horizontal confined tracks. Age and length information indicate that all samples above the present-day pa rtial annealing zone cooled from temperatures above 120 degrees C in t he Cretaceous. The top two kilometres of the profile show less Variati on of age with depth than was expected. Modelling of the profile indic ates that the distribution of fission track ages with depth requires t hat the profile was disturbed during or after cooling through the part ial annealing zone. Modelling of individual samples reveals that at le ast the upper 2 km experienced reverse faulting, effectively thickenin g the upper part of the rock column by up to 1000 m in the Tertiary. A present-day partial annealing zone for apatite was observed between 2 000 and 4000 m (similar to 60-110 degrees C). This confirms earlier in situ observations of the position of the partial annealing zone.