Late Cretaceous reactivation of major crustal shear zones in northern Namibia: constraints from apatite fission track analysis
Introduction
The Pan-African mobile belts in southern Tankard et al., 1982, Porada, 1989 and west central Africa typically follow the margins of the Kalahari and Congo cratons and strongly influenced the subsequent morphotectonic evolution of the continent by imposing a regional structural framework across Africa. The reactivation of pre-existing structures within these belts has long been known as an important factor in the subsequent geological evolution of these regions Rosendahl, 1987, Unternehr et al., 1988, Versfelt and Rosendahl, 1989. It is well known that old lineaments respond sensitively to later tectonic activity (e.g., Donath, 1961, Handin, 1969), but it is much more difficult to resolve and quantify multiple, discrete phases of reactivation, particularly where the amplitude of reactivation is relatively subtle.
As a temperature-sensitive thermochronological technique, apatite fission track analysis is a powerful tool for constraining the low-temperature history of rocks over a range of ∼60–110 °C. These temperatures, depending on the geothermal gradient, equate to a burial depth of 3–5 km. So the method can reconstruct the cooling history of rocks as they approached the surface in response to erosion and tectonic processes. Apatite fission track analysis is therefore an ideal method for studying the morphotectonic effects of intracontinental deformation.
In order to constrain and quantify the tectonically driven reactivation and the subsequent denudation history of the major structural entities in northern Namibia, 66 apatite fission track samples were collected over a ca. 60-km-wide and 550-km-long coast-parallel transect across the Great Escarpment, the Okahandja Lineament (OKL), the Omaruru Lineament–Waterberg Thrust (OML–WT) and the Autseib Fault–Otjohorongo Thrust (AF–OT) to the Kamanjab Inlier (see Fig. 1). The area covers the Central and Northern Zone of the Damara Orogen. Stratigraphic ages of samples range from Proterozoic (Kamanjab Inlier) over Pan-African igneous and metasedimentary rocks to Early Cretaceous igneous intrusive rocks.
The relative vertical displacement between different tectonic blocks can be qualitatively assessed by comparing the mean apparent apatite fission track age for the block with the regional age–elevation pattern. Foster and Gleadow (1992) elucidated the existence and reactivation of important structures of the Kanmantoo Fold Belt and the lachland Fold Belt in southeastern Australia, based on significant differences in the apparent apatite fission track age. Alternatively, relative vertical displacements can be estimated for any given time by comparing the modelled palaeotemperatures for the different blocks at that time. O'Sullivan et al. (1998) successfully applied forward modelling techniques to samples taken across structures along the Philip Smith Mountain front, Alaska, to reveal complex deformation sequences where exposure is poor and the topography subdued. The latter approach has been applied in this paper and the result of this study shows that the area of the Damara Orogen experienced a kilometer-scale reactivation of an inherited basement structure along the Omaruru Lineament (Fig. 1) with associated accelerated denudation during the Late Cretaceous.
Section snippets
Thermochronology
Apatite fission track thermochronology is a powerful tool over a temperature range from ambient surface temperatures up to about 110 °C, which characterises the upper few kilometers of the Earth's crust. Denudation rates are most likely to control the cooling pattern of rocks at these relatively shallow depths for tectonically quiet settings (e.g. Gleadow and Brown, 2000). A detailed overview of fission track thermochronology is provided by, e.g. Brown et al. (1994), Gallagher et al. (1998),
Geological setting
The regional basement structure in northern Namibia is dominated by the NE–SW striking intracontinental branch of the Pan-African Damara mobile belt (Tankard et al., 1982). The Damara Orogen separates the Congo and Kalahari cratonic terranes and is divided into several tectonostratigraphic zones. The main units are subdivided by lineaments forming deep, steeply dipping ductile shear zones Miller, 1983, Daly, 1986, Daly, 1989. These regional lineaments form southwest to northeast striking
Previous fission track work
The first fission track studies in Namibia were presented by Haack (1983). As the potential of confined track length information was not known at that time, only apparent apatite cooling ages are available from this earliest work. Haack interpreted the young (<130 Ma) cooling ages north of the OKL as a very slow cooling of the Damara intrusives due to elevated heat production caused by the radioactive decay of U-enriched granites. However, these ages are not significantly different from our
Fission track results
A total number of almost 200 apatite fission track ages are now available for northern Namibia (see Fig. 1, Fig. 2). To show the regional trend of cooling ages, we contoured all available ages in Fig. 2. The youngest apatite fission track ages occur within a ca. 150-km-wide coastal zone and generally decrease systematically inland, a common trend in passive margins (Gallagher and Brown, 1997). However, this regional pattern is modified by a well-defined NE–SW trending zone of cooling ages
Thermal modelling
We used the maximum likelihood approach outlined in Gallagher (1995) to test and quantify possible thermal histories using single grain age distributions and track length distributions. We used the annealing algorithm of Laslett et al. (1987) for Durango apatite (0.4 wt.% Cl). To find a thermal history which maximises the probability of obtaining the measured data, the maximum likelihood approach has been applied to constrain the time–temperature path. For the maximum likelihood approach we
Constraining reactivation and denudation
As mentioned earlier, samples with the youngest age and the longest mean track length provide some constraints on the time of cooling. We therefore ran the thermal history models to test whether samples show a distinct cooling signal in the Late Cretaceous. Twenty-six representative samples were chosen to present a equidistant distribution along the transect (Table 2). After significant differences in maximum palaeotemperatures became apparent, we modelled more samples in the vicinity of the
Discussion
Apparent AFT ages are shown plotted against distance along the transect in Fig. 7a. Owing to a strong relationship between AFT age and elevation (Fig. 5), ages are highly variable (<100 to >500 Ma) in the southeast of the profile, as the local relief and sample elevation vary significantly in this sector. In contrast, only minor variations in age are observed within the vicinity of the shear zones and the observed ages are generally less than ca. 100 Ma (between the Okahandja Lineament and the
Conclusions
Although the measured apatite fission track ages across the Damara sector of the Namibian margin range between about 62 and 549 Ma, the apatite fission track data for all samples are consistent with a discrete period of accelerated cooling beginning at about 70 Ma. The variation of modelled Late Cretaceous palaeotemperatures along the study transect shows a distinct discontinuity across the Omaruru Lineament, with the maximum palaeotemperatures changing abruptly from approximately 120 to 80 °C
Acknowledgements
We do thank R. Watkins and J. Jacobs for their careful reviews of the manuscript. This research was partly funded by the Deutsche Forschungsgemeinschaft (Grant WE488-48/1), the Deutscher Akademischer Austauschdienst (DAAD) as part of the third higher education special program (HSP III), and the Natural Environment Research Council (Grant GR9/1573). Fission track research at The University of Melbourne is supported by grants from the Australian Institute of Nuclear Science and Engineering and
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