16.6 Uplift and subsidence rates

Wagner and Reimer (1972) demonstrated the usefulness of apatite fission track ages for tectonic studies by applying them to Alpine uplift rates. Subsequently, Wagner et al. (1977) developed this technique by measuring apatite fission track ages over a 3000 m range of vertical relief in the Central Alps. Fission track ages on Alpine apatites do not conform to metamorphic isograds or terrane boundaries, but display a strong correlation with topographic relief (Fig. 16.15). They clearly represent cooling ages from Alpine metamorphism due to tectonic uplift, and can be used directly to calculate apparent uplift rates over the last few million years.

Fig. 16.15. Plot of fission track ages against topographic altitude for Alpine apatites, and deduced apparent uplift rates. After Wagner et al. (1977).

If a ‘freezing-in’ or ‘blocking’ temperature could be calculated for the Alpine apatites then the uplift rates in Fig. 16.14 could be converted into cooling rates. Two problems are faced in this task. The first is that the laboratory experiments show that blocking occurs over a range of temperatures. The second is that this range is itself dependent on the cooling rate. Hence, the argument is to some extent circular. Wagner et al. (1977) estimated a temperature for 50% track retention of 100 ) 120 ^oC, half-way between 0% and 100% annealing temperatures of 60 and 180 ^oC, at a cooling rate of ca. 20 ^oC/Myr estimated from Rb)Sr biotite ages.

By combining apatite fission track data with biotite K)Ar and Rb)Sr, muscovite K)Ar, muscovite Rb)Sr and monazite U)Pb data, Wagner et al. were able to calculate cooling rates for different regions of the Alps over the last 35 Myr (Fig. 16.16). These results suggest that cooling in the Central Alps (Ticino and Gotthard areas) has been relatively uniform, while that in the East (Bergell) has slowed and the West (Simplon and Monte Rosa) has speeded up in the last few Myr. These conclusions are consistent with Fig. 16.15, and suggest that the Alps have undergone differential geographic uplift through time.

Fig. 16.16. Proposed cooling history for different regions of the Alps, based on time since closure of different radiogenic mineral systems (whose blocking temperatures are shown on right side) . After Wagner et al. (1977).

The idea of using a vertical traverse of apatite fission track ages to deduce tectonic histories was applied by Naeser (1979b) to bore-hole studies of sedimentary basins. Naeser proposed that in sedimentary sequences which are at their maximum burial temperature, apparent fission track ages would show a relationship with burial depth similar to Fig. 16.17. At shallow depths, burial heating is insignificant and fission track ages reflect the sediment source (detrital ages). As depth of burial increases, apatites undergo increased thermal annealing, and display decreasing apparent fission track ages, until they finally reach a total annealing zone with zero apparent age. The interval between zero and total annealing is called the Partial Annealing Zone (PAZ).

Fig. 16.17. Schematic illustration of the variation of apparent fission track age with depth in bore-hole samples from a sedimentary basin. After Naeser (1979b); Naeser et al. (1989).

The upper- and lower-temperature bounds of the PAZ will depend on the age of the sedimentary basin. Naeser (1981) collected fission track age data from sedimentary basins with different burial rates. By making geological estimates of the effective burial (annealing) time in each basin, Naeser was able to make geological determinations of the Boltzmann lines for thermal annealing in apatite. These were confirmed by Gleadow and Duddy (1981) in a study of bore-hole data from the Otway Basin of Victoria, SE Australia. This basin is particularly suitable for fission track studies of burial rates because the basinal sediments were derived from an active volcanic province. Hence, the sediments entering the basin were essentially of zero age, with very little older provenance.

The effective annealing time at present-day down-hole temperatures was estimated from the burial history of the basin, suggesting that peak temperatures have been maintained for ca. 30 Myr. Using these estimates, annealing properties were determined for the Otway Basin apatites (Fig. 16.18), which were consistent with other bore-hole data (Naeser, 1981). In addition, the Boltzmann line for 50% annealing was consistent with laboratory annealing experiments. However, the temperature interval between 0 and 100% annealing was narrower than that predicted by the divergence of Arrhenius relation annealing lines from the laboratory data of Naeser and Faul (1969).

Fig. 16.18. Arrhenius plot for fission track annealing in apatite from Otway Basin bore-holes (blocks), other bore-holes (stars) and laboratory experiments (spots and dashed lines). After Gleadow and Duddy (1981).

A complicating factor in the analysis of track fading in apatite is the discovery that annealing temperature is compositionally dependent (Green et al., 1985). Fission track analyses were performed on individual apatite grains from a single horizon in an Otway drill hole with a present-day temperature of 92 ^oC. These conditions result from progressive burial over the last 120 Myr. Chlor-apatite grains were found to give results near the depositional age, whereas fluor-apatites gave ages as low as zero (Fig. 16.19). Hence, when laboratory and geological annealing processes are compared, it is important that the material in the two types of experiment is as near compositionally identical as possible.

Fig. 16.19. Plot of measured fission track ages in individual apatites from Otway Group sandstones, Australia, to show compositional dependence of track annealing. After Green et al. (1985).

Bearing these findings in mind, Green et al. (1985) argued that Boltzmann lines for different percentages of track annealing did not have a fan-shaped distribution, but were parallel. This would imply that the activation energy for track fading was constant for all tracks in a given sample, and that track fading would not occur over a temperature interval, as assumed in the plateau method. However, close examination by Laslett et al. (1987) of the data set of Green et al. (1985) suggested that Boltzmann lines are divergent, although not to the extent suggested by early experiments (Fig. 16.20). This conclusion is also supported by more recent comparisons between laboratory data for apatite annealing and drill-hole data from the Otway basin, Australia (Green et al., 1989). Hence, using more sophisticated models for the thermal and burial history of basin development, the laboratory and drill-hole data give coherent results.

Fig. 16.20. Interpretation of laboratory annealing data showing gently fanning Boltzmann lines that are consistent with field data from the Otway basin. Each line corresponds to a given percentage of track fading, fitted between each set of solid and open points. After Laslett et al. (1987).

In the above discussion, geologically well-known thermal basin histories were used to calibrate the annealing behaviour of apatite tracks. Given this background, fission tack data can then be used to study geologically unknown basins. This evidence is especially pertinent to oil fields, because fission track annealing occurs over the same temperature range as hydrocarbon maturation (Gleadow et al., 1983). For example, Briggs et al. (1981) used this approach to compare the thermal histories of two sedimentary basins of the Tejon oil field, in the San Joaquin Valley of California. This oil field is divided into two parts by the seismically active White Wolf fault. One part, the Basin Block, was a Late Tertiary depocentre which underwent strong subsidence. The other, Tejon Block, was less depressed. Fission track analysis of apatite from bore-holes reveals the different geological history of the two blocks (Fig. 16.21).

Fig. 16.21. A comparison between fission track ages in bore-holes from the Basin and Tejon blocks of the San Joaquin Valley, California. Apatite data ( O , Q ) give thermal history information while zircon data ( > , Î ) yield provenance ages. After Naeser et al. (1989).

Naeser et al. (1989) used these data, along with Boltzmann annealing lines from other geological locations, to calculate the thermal histories of the two blocks. Given geological evidence that the present down-hole temperatures represent peak values, the temperatures necessary for total annealing can be used to calculate effective heating times of ca. 1 Myr and 10 Myr for the Basin and Tejon block respectively (Fig. 16.22). These results are consistent with geological evidence for much more rapid burial of the Basin block, and do not require a perturbation in geothermal gradient.

Fig. 16.22. Use of the Arrhenius plot to calculate effective heating times for the total annealing horizon in bore-holes from the Tejon and Basin blocks, San Joaquin Valley, California. After Naeser et al. (1989).

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