G. A. Wagner’s research while affiliated with Max Planck Institute for Nuclear Physics and other places

As part of the KTB (Kontinentale Tiefbohrung) programme in Germany, titanite (sphene) fission-track data were acquired for samples to a depth of 9000 m, where the host rock temperature is ∼265°C. At this depth, age and confined track length information suggest that the partial annealing zone for the fission tracks in titanite was not reached. By reconstructing the thermal history and `backstacking' of an uplifted partial annealing zone, it is suggested that the in situ partial annealing zone is likely to be found in the region 265–310°C. This value is more precise than previous estimates of the titanite partial annealing zone, and it is also higher than previous estimates. Microprobe analysis indicates that the chemical composition of the titanites is rather homogeneous throughout the whole drilled section.

The post-Carboniferous crustal evolution of the German Continental Deep Drilling Program (KTB) area, as summarized in this paper, could not be predicted from surface observations: deep drilling was essential for its revelation. The most conspicuous and unexpected feature discovered in the drill hole is the absence of marked gradients with respect to the pre-Carboniferous record. There are no depth-related differences in K-Ar cooling ages of hornblende and white mica, in petrology or in lithology. All metamorphic rocks encountered, both at the surface as well as in the drill hole down to 9100 m depth, were below 300°C from the Carboniferous onward. The late to post-Carboniferous deformation is essentially confined to several fault zones. A major fault zone encountered in the drill hole at 7000 m depth is linked by a prominent seismic reflector to the Franconian Lineament, the surface boundary between Variscan basement and Mesozoic cover. This fault zone probably formed in the late Paleozoic and reactivated as a reverse fault in the Mesozoic. Two important episodes of NE-SW directed shortening by movements along reverse faults took place in the early Triassic and in the late Cretaceous, as indicated by the distribution of apatite and titanite fission-track ages, the sericite K-Ar ages of fault rocks, and the sedimentary record in the adjacent basins. Upper crustal slices were detached at a specific level, corresponding to the approximate position of the brittle-ductile transition in post-Variscan times, and form an antiformal stack that was penetrated by the KTB throughout its entire depth range.

The post-Carboniferous crustal evolution of the German Continental Deep Drilling Program (KTB) area, as summarized in this paper, could not be predicted from surface observations: deep drilling was essential for its revelation. The most conspicuous and unexpected feature discovered in the drill hole is the absence of marked gradients with respect to the pre-Carboniferous record. There are no depth-related differences in K-Ar cooling ages of hornblende and white mica, in petrology or in lithology. All metamorphic rocks encountered, both at the surface as well as in the drill hole down to 9100 m depth, were below 300øC from the Carboniferous onward. The late to post-Carboniferous deformation is essentially confined to several fault zones. A major fault zone encountered in the drill hole at 7000 m depth is linked by a prominent seismic reflector to the Franconian Lineament, the surface boundary between Variscan basement and Mesozoic cover. This fault zone probably formed in the late Paleozoic and reactivated as a reverse fault in the Mesozoic. Two important episodes of NE-SW directed shortening by movements along reverse faults took place in the early Triassic and in the late Cretaceous, as indicated by the distribution of apatite and titanite fission-track ages, the sericite K-Ar ages of fault rocks, and the sedimentary record in the adjacent basins. Upper crustal slices were detached at a specific level, corresponding to the approximate position of the brittle-ductile transition in post-Variscan times, and form an antiformal stack that was penetrated by the KTB throughout its entire depth range.

A detailed fission-track analysis was carried out on 41 apatite samples from the 4001-m-deep pilot hole of the German Continental Deep Drilling 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 partial annealing zone cooled from temperatures above 120 °C in the Cretaceous. The top two kilometres of the profile show less variation of age with depth than was expected. Modelling of the profile indicates that the distribution of fission track ages with depth requires that the profile was disturbed during or after cooling through the partial annealing zone. Modelling of individual samples reveals that at least the upper 2 km experienced reverse faulting, effectively thickening 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 2000 and 4000 m (∼60–110 °C). This confirms earlier in situ observations of the position of the partial annealing zone.

 Since 1985, apatite fission-track analysis was applied to more than 70 samples from surface outcrops and shallow boreholes at the western margin of the Bohemian massif. Apatite ages were determined by the grain-population method. Additional information from the frequency distributions of fully confined spontaneous tracks was used for modelling of t–T paths in the low-temperature range (<120 °C). Seven zircon samples were dated by the external detector method. Zircon ages between 283 and 215 Ma indicate unroofing during the Permian molasse stage and the Triassic. Tectonic quiescence and slow subsidence prevailed from the Jurassic until the middle Cretaceous. In the basement area south of Weiden, a Mesozoic partial annealing zone (for apatite fission tracks) is now exposed at the surface. Farther north, the basement was affected by stronger Cretaceous and Palaeogene erosion, which yielded cooling ages between 110 and 49 Ma. This second period of post-Variscan denudation was correlated to reverse faulting along the Franconian Line.

An overview is presented of the apatite fission-track analysis of the 4000 m deep KTB pilot hole drilled in the Variscan crystalline basement of the Oberpfalz, Germany. The apatite fission-track age profile based on 41 samples suggests a simple thermal history, i.e. a single event of uplift-induced cooling during the late Cretaceous-early Tertiary. The length information provided by the confined tracks and the age spectra of the surface tracks add complications to this tectonothermal history and suggest additional young Cenozoic uplift and faulting together with an increase of the geothermal gradient in relatively recent time. The validity of this interpretation is, however, inevitably connected to the model used to describe track stability in apatite under geological conditions.

Projected length analysis of surface tracks enables the calculation of length dispersive spectra of apatite fission track ages determined by the grain-population technique. Different T-t paths result in characteristic apparent age-spectra which reveal the low temperature ( < 140°C) history of rocks. The concept is successfully tested against geologic constraints.