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Title: Landscape development in the Southern Drakensberg and Lesotho Highlands, SE Africa : quantifying denudation rates using in situ-produced cosmogenic chlorine-36 data
Author: Fleming, Alastair
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2000
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Over geological timescales, the spatial and temporal pattern of denudation is a primary control on the evolution of landscapes. Hence the ability to quantify denudation is fundamentally important to the comprehension and explanation of long-term landscape development. The recently developed technique of in situ-produced cosmogenic isotope analysis affords unique insights into the exposure history of the Earth's surface. In particular, in denudational terrains, this technique provides useful new, landform-scale, data on denudation rates applicable to timescales of 103 - 106 years. By using the analysis of one such in situ-produced cosmogenic isotope, chlorine-36 (36CI), this research quantifies, directly, rates of bedrock summit denudation and free face retreat in the basaltic terrain of the southern Drakensberg escarpment and Lesotho Highlands, both cardinal components of the SE African passive margin landscape. Cosmogenic 36Cl concentrations in 33 samples collected from flat-lying bedrock summits in the southern Drakensberg and Lesotho Highlands yield a mean denudation rate, integrated over the past ~ 50 ka, of 10.4 ± 1.6 m Ma-1. Individual rates range from 1.2 ± 0.4 m Ma-1 to 43.0 ± 6.6 m Ma-1. Possible site-specific resources of this variability are discussed. The mean summit denudation rate is similar to longer term (107 - 108 a), regional scale, estimates of crustal stripping provided by, for example, zeolite zonation studies and apatite fission track thermochronology (AFTT). Denudation of these summit flats at the measured rates is sufficient to prevent the long-term intact survival of erosion cycle surfaces formed prior to the break-up of Gondwana which were previously inferred for this region. This research confirms that the analysis of in situ-produced cosmogenic 36Cl provides an important new technique for assessing, directly, landscape change at the sale of individual landforms over timescales of 103 - 105 years. When these data are coupled with longer term, regional scale, estimates of denudation, such as provided by AFTT, and the heuristic predictions of numerical modelling of morphological evolution, they provide new insights into the long-term development of landscapes.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available