Denudation rates and geomorphic evolution of the Cape Mountains, determined by the analysis of the in situ-produced cosmogenic 10BE

• Main Author: Scharf, Taryn E
• Other Authors: De Wit, Maarten
• Format: Dissertation
• Language: English
• Published: University of Cape Town 2014
• Subjects: Geology
• Online Access: http://hdl.handle.net/11427/4220

Includes abstract. === Includes bibliographical references. === Southern Africa is host to a unique mountain system, the Cape Mountains, which includes the coastal Cape Fold Belt (CFB) and an inland Escarpment. Apatite fission track analysis has shown that this mountain system is an erosion feature, exhumed from beneath 2-7 km of overburden by large-scale denudation processes affecting the subcontinent during Gondwana break-up (ca. 140 – 65 Ma). Despite its antiquity and location on a passive continental margin, the ruggedness of the present-day topography of the Cape Mountains compares to that of the world’s active orogens. The coastal Cape Mountains are traversed by deeply-incised, meandering rivers that cut canyons through the most resistant quartzite ridges of these mountains, perpendicular to their structural grain inherited from the CFB. The evolution of this landscape is poorly understood, because little quantitative data exists on the denudation history of the Cape Mountains. This study presents the first in situ-produced cosmogenic 10Be inventories determined for quartz from catchment sediments and bedrock surfaces within the coastal Cape Mountains, with which to quantify denudation rates, exposure ages and the recent geomorphic evolution of these Cape Mountains. River sediments sampled from catchments within the Langeberg and Swartberg Ranges of the Western Cape, as well as bedrock from the Tradouw River traversing the Langeberg Range, were analysed. In addition, charcoal from alluvial material was collected for radiocarbon dating. Catchment-averaged denudation rates reported from these mountains range between 2.1 ± 0.3 and 6.9 ± 1.9 m·Myr-1. These are amongst the lowest reported rates globally, despite the rugged terrain of the mountain system. The spatial consistency between the low denudation rates suggests a landscape approaching geomorphic steady-state. This finding is best attributed to lithological control on denudation rates in a tectonically quiescent environment, and a relatively dry climate.