Roderick Brown

Quantifying passive margin denudation and landscape development using a combined fission-track thermochronology and cosmogenic isotope analysis approach

Abstract A new approach to empirically constraining numerical models of denudation and landscape development across passive continental margins is presented involving a combination of apatite fission-track thermochronology and cosmogenic isotope analysis. This integrated strategy is applied to the Namibian sector of the south-west African margin where the conventional landscape evolution model of inland retreat of an escarpment initiated at the coast at break-up implies a mean rate of retreat of ∼1 km Ma −1 . The thermochronological and cosmogenic isotope data are incompatible with a simple escarpment retreat model since they show that denudation rates oceanward of the present escarpment position have been low ( −1 ) since the end of the Eocene, and the estimated rate of escarpment retreat has been only ∼10 m Ma −1 . This low rate of retreat is, however, consistent with numerical landscape evolution models where the escarpment is pinned at an inland drainage divide.

Shaping the Australian crust over the last 300 million years: insights from fission track thermotectonic imaging and denudation studies of key terranes

Apatite fission track thermochronology is a well‐established tool for reconstructing the low‐temperature thermal and tectonic evolution of continental crust. The variation of fission track ages and distribution of fission track lengths are primarily controlled by cooling, which may be initiated by earth movements and consequent denudation at the Earths surface and/or by changes in the thermal regime. Using numerical forward‐modelling procedures these parameters can be matched with time‐temperature paths that enable thermal and tectonic processes to be mapped out in considerable detail. This study describes extensive Australian regional fission track datasets that have been modelled sequentially and inverted into time‐temperature solutions for visualisation as a series of time‐slice images depicting the cooling history of present‐day surface rocks during their passage through the upper crust. The data have also been combined with other datasets, including digital elevation and heat flow, to image the denudation history and the evolution of palaeotopography. These images provide an important new perspective on crustal processes and landscape evolution and show how important tectonic and denudation events over the last 300 million years can be visualised in time and space. The application of spatially integrated denudation‐rate chronology is also demonstrated for some key Australian terranes including the Lachlan and southern New England Orogens of southeastern Australia, Tasmania, the Gawler Craton, the Mt Isa Inlier, southwestern Australian crystalline terranes (including the Yilgarn Craton) and the Kimberley Block. This approach provides a readily accessible framework for quantifying the otherwise undetectable, timing and magnitude of long‐term crustal denudation in these terranes, for a part of the geological record previously largely unconstrained. Discrete episodes of enhanced denudation occurred principally in response to changes in drainage, base‐level changes and/or uplift/denudation related to far‐field effects resulting from intraplate stress or tectonism at plate margins. The tectonism was mainly associated with the history of continental breakup of the Gondwana Supercontinent from Late Palaeozoic time, although effects related to compression are also recorded in eastern Australia. The results also suggest that the magnitude of denudation of cratonic blocks has been significantly underestimated in previous studies, and that burial and exhumation are significant factors in the preservation of apparent ‘ancient’ features in the Australian landscape.

Quantitative resolution of the debate over antiquity of the central Australian landscape: implications for the tectonic and geomorphic stability of cratonic interiors

We report the first measure of long- (∼100 Myr) and short- (∼1 Myr) term denudation rates from key geologically stable landforms in the Davenport Range, central Australia. These landforms have previously been assigned a Cambrian age, which arguably places them amongst the oldest persistent landforms on the continent, if not on Earth. Our results from combined apatite fission track thermochronology and in situ cosmogenic radionuclide analyses using ^(10)Be and ^(26)Al show that while average exhumation rates are low, the denudation history for this cratonic region is incompatible with extreme, sub-aerial longevity and long-term tectonic and geomorphic stability. Our revised model for the landscape evolution of this region is consistent with one of maximum burial prior to and during the Mesozoic, followed by a phase of kilometre-scale exhumation that was largely complete by the beginning of the Cainozoic. We suggest that a similar process of burial and exhumation has probably been responsible for the sub-aerial preservation of seemingly ancient landforms elsewhere in Australia.

DENUDATION AND UPLIFT AT PASSIVE MARGINS: THE RECORD ON THE ATLANTIC MARGIN OF SOUTHERN AFRICA

The onshore region of a passive margin forms an integral part of the geological evolution from continental break–up to later sedimentation in the offshore basins. The dominant surface process in the onshore region is denudation, which acts to remove any direct evidence of surface uplift. However, denudation can be constrained on geological time–scales through low temperature thermochronological data, such as that obtained through apatite fission track analysis. Here, we present a suite of such data from the Atlantic Margin of southern Africa. The data have been modelled in terms of their temperature histories since the Jurassic. These temperature histories have been combined with heat–flow data to estimate the equivalent depth of denudation over these time–scales. Average denudation rates are of the order of a few tens of metres per million years, but show considerable variations both temporally and spatially. These results demonstrate that passive margins experience complex patterns of denudation. Three landscape evolution models are considered. Our results imply the downwarping model is inappropriate. The other two models, scarp retreat and pinned drainage divide, predict trends similar to those observed but the complexities inherent in the data and the evolution of passive margin topography do not allow us to resolve one from the other. In practice, both models probably operate to some extent as a margin evolves. Estimates of palaeotopography have been made, assuming simple isostatic response models to denudational unloading. Flexural models with effective elastic thickness (EET) of 25 km predict elevations 2 km and more above the present day values, while models with EET of 0 km predict elevations up to 750 m higher than the present–day. These models ignore any post–break–up tectonic uplift and need independent constraints on surface elevation to assess their validity.

Late Cretaceous reactivation of major crustal shear zones in northern Namibia: constraints from apatite fission track analysis

Abstract Namibias passive continental margin records a long history of tectonic activity since the Proterozoic. The orogenic belt produced during the collision of the Congo and Kalahari Cratons in the Early Proterozoic led to a zone of crustal weakness, which became the preferred location for tectonism during the Phanerozoic. The Pan-African Damara mobile belt forms this intraplate boundary in Namibia and its tectonostratigraphic zones are defined by ductile shear zones, where the most prominent is described as the Omaruru Lineament–Waterberg Thrust (OML–WT). The prominance of the continental margin escarpment is diminished in the area of the Central and Northern Zone of the Damara belt where the shear zones are located. This area has been targeted with a set of 66 outcrop samples over a 550-km-long, 60-km-broad coast-parallel transect from the top of the escarpment in the south across the Damara sector to the Kamanjab Inlier in the north. Apatite fission track age and length data from all samples reveal a regionally consistent cooling event. Thermal histories derived by forward modelling bracket this phase of accelerated cooling in the Late Cretaceous. Maximum palaeotemperatures immediately prior to the onset of cooling range from ca. 120 to ca. 60 °C with the maximum occurring directly south of the Omaruru Lineament. Because different palaeotemperatures indicate different burial depth at a given time, the amount of denudation can be estimated and used to constrain vertical displacements of the continental crust. We interpret this cooling pattern as the geomorphic response to reactivation of basement structures caused by a change in spreading geometry in the South Atlantic and South West Indian Oceans.

The Mesozoic denudation history of the Atlantic margins of southern Africa and southeast Brazil and the relationship to offshore sedimentation

Abstract A new interpretation of regional apatite fission track data suites from western southern Africa (South Africa, Namibia) and southeast Brazil, in terms of the post-break-up denudation chronologies, is presented. The data reveal that although significant denudation occurs parallel to the present-day margin (associated with the inland escarpment) there are regional variations along-strike and with time. Moreover, the data reveal a significant amount of post-break-up denudation from within the continental interior of southern Africa. The data indicate relatively little post-rift denudation but the rates increase dramatically in both regions in the late Cretaceous-Early Tertiary, more than 50 Ma after initial rifting. The inferred denudation chronologies are broadly consistent with the sedimentary record in the offshore basins, which can show increases in sedimentation rate, contemporaneous unconformities and major changes in depositional character.

Accelerated denudation and tectonic/geomorphic reactivation of the cratons of northeastern Brazil during the Late Cretaceous

Apatite fission track (AFT) data for a suite of 40 outcrop samples from the Sao Francisco and eastern Guapore cratons in northeastern Brazil record between 60° and 100°C of cooling since the separation of South America and Africa at ∼130 Ma. Quantitative thermal histories derived from the data indicate two discrete phases of cooling: the first is coeval with continental breakup at ∼130 Ma and the second is broadly coincident with the change in relative plate motion at ∼60–80 Ma. The thermal histories indicate an increase in the average rate of denudation at ∼60–80 Ma over much of the interior of northeastern Brazil. Estimated total amounts of postbreakup denudation vary between 3 and 7 km, with greater amounts occurring along the eastern and northern margins of the Sao Francisco and Guapore cratons, respectively. Two samples within the Pernambuco shear zone yield anomalously young AFT ages which record cooling caused by locally enhanced denudation rates during the Late Cretaceous. The timing and style of the Late Cretaceous tectonic event in Brazil is similar to the analogous Late Cretaceous history of the central African shear zone system in Africa. This suggests that both events were associated with intracontinental deformation caused by major changes in the relative plate motions between Africa, Antarctica, and South America that began between anomaly C34 (83 Ma) and C31 (67 Ma).

Fission track thermotectonic imaging of the Australian continent

Abstract Fission track analyses of apatites from a very large data set across Australia provide a first look at the patterns of low-temperature thermochronology over an entire continent. The rock samples are mostly of granitic rocks, or their metamorphic equivalents, of Palaeozoic or older age from the exposed basement regions. Sample chemical analyses indicate that the apatite suite is overwhelmingly of fluorapatite composition. Approximately 2750 fission track analyses have been completed, of which >1700 are of sufficient quality to form a coherent data set that can be interpolated to show the variation of central fission track age and mean confined track length on a continental scale. The resulting images exhibit some features which are well known, such as the trend towards young apatite ages along the eastern and southeastern rifted continental margins, but others that are more surprising, such as the lack of clear differentiation, in terms of the range of ages and lengths, between the older Precambrian cratonic areas in the west, and younger Phanerozoic mobile belts to the east. The Precambrian rocks from the western two-thirds of the continent do, however, show distinctly different cooling histories to those in the eastern Phanerozoic mobile belts when the relationships between track lengths and fission track ages are considered. The western craton everywhere shows patterns of prolonged slow cooling, whereas all major regions of the eastern part of the continent show discrete episodes of rapid cooling, mostly from the Jurassic to the Palaeogene. Significant areas of unusually young apatite ages (

Sampling strategy is important for producing weed maps: a case study using kriging

Abstract Weed maps are typically produced from data sampled at discrete intervals on a regular grid. Errors are expected to occur as data are sampled at increasingly coarse scales. To demonstrate the potential effect of sampling strategy on the quality of weed maps, we analyzed a data set comprising the counts of capeweed in 225,000 quadrats completely covering a 0.9-ha area. The data were subsampled at different grid spacings, quadrat sizes, and starting points and were then used to produce maps by kriging. Spacings of 10 m were found to overestimate the geostatistical range by 100% and missed details apparently resulting from the spraying equipment. Some evidence was found supporting the rule of thumb that surveys should be conducted at a spacing of about half the scale of interest. Quadrat size had less effect than spacing on the map quality. At wider spacings the starting position of the sample grid had a considerable effect on the qualities of the maps but not on the estimated geostatistical range. Continued use of arbitrary survey designs is likely to miss the information of interest to biologists and may possibly produce maps inappropriate to spray application technology. Nomenclature: Capeweed, Arctotheca calendula (L.) Levyns.

Coincidence mapping - a key strategy for the automatic counting of fission tracks in natural minerals

Abstract We report on new image-analysis techniques that, for the first time, provide a practical solution to the problem of fully automated counting of fission tracks in natural minerals, a long-desired goal in fission-track dating. Specific challenges to be overcome have been the discrimination of fission tracks from non-track defects, polishing scratches, etc.; resolving multiple track overlaps; and reliable identification of small tracks amongst a similarly sized background of surface defects, fluid inclusions, etc. Most previous attempts at automated image analysis have failed in one or more of these tasks. The central component of our system is called ‘coincidence mapping’ and utilizes two images of the same tracks obtained in transmitted and reflected light. The complementary nature of the information in these two images allows a powerful discrimination of true fission tracks from most non-track features. The much smaller average track size in the reflected light image allows the resolution of most track overlaps apparent in transmitted light. The discrimination is achieved by segmenting the two images using a custom-developed thresholding routine and extracting the coincidence of features in the two binary images. The analysis is computationally efficient and takes only a few seconds to complete the processing of images that may contain up to many hundreds of tracks. Preliminary indications are that error rates are about the same as, or better than, those achieved by a human operator using normal counting conditions in transmitted light. The performance is even better at high track densities (>107cm−2) giving the potential for measuring track densities up to an order of magnitude greater than a human operator can count. Automated counting should significantly increase the speed and consistency of analysis and improve data quality in fission-track dating through better counting statistics, increased objectivity and measurement of additional track description parameters that are not currently determined.