The structure of the upper mantle beneath southern Africa
Published:January 01, 2002
Keith Priestley, Dan McKenzie, 2002. "The structure of the upper mantle beneath southern Africa", The Early Earth: Physical, Chemical and Biological Development, C. M. R. Fowler, C. J. Ebinger, C. J. Hawkesworth
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A large number of velocity models derived from a variety of seismic data and using different seismic techniques have been published for the Archaean and Proterozoic shields. Here, we focus on the structure beneath southern Africa, where velocity models derived from most regional seismic data find the thickness of the seismic lithosphere to be less than 200 km. In contrast, velocity models derived from teleseismic body-wave and long-period surface-wave data determine the seismic lithosphere to be as much as 400 km thick. We believe that this disagreement is due to the ways in which the various datasets average the velocity structure. Our analysis of regional seismograms from propagation paths largely confined to the stable region shows that the average thickness of the seismic lithosphere beneath southern Africa does not exceed 160 km. We compare the vertical S-wave travel time of our velocity model derived from regional seismic data and those models derived from teleseismic data and find no significant difference. We determine the in situ velocities and densities from nodules from beneath southern Africa using a recently derived geobarometer and geothermometer; these are in excellent agreement with the velocity found in the high-velocity lid from the analysis of the regional seismic data. The lithospheric model that best fits the nodule data has a mechanical boundary layer thickness of 156 km and a lithosphere thickness of 176 km. However, the shear-wave velocity decrease at the base of the lid does not correspond to a change in mineralogy. Recent experimental studies of the shearwave velocity in olivine as a function of temperature and period of oscillation demonstrate that this decrease can result from grain boundary relaxation at high temperatures at the period of seismic waves. This decrease in velocity occurs where the mantle temperature is closest to the melting temperature.
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The Early Earth: Physical, Chemical and Biological Development
This volume contains a series of papers that cover a wide range of aspects, including geophysics, structure and tectonics, atmosphere, origin of life, biosphere, deep mantle geochemistry, early oceans, microbial ecology, on the development of the Earth in the first 2000 Ma of its history.The aim of this publication is to facilitate future discussions and understanding of this area of research.
This book is divided into three parts:
Geophysical and petrological constraints on Archaean lithosphere
Models of cratonic evolution and modification
Constraints on the Archaean environment
Subjects covered include the chemical and biological controls on the atmosphere and oceans, early controls on the carbon cycle and photosynthesis, petrologic, isotopic, tectonic and seismic evidence for the composition and structure of Archaean lithosphere.
This volume should be of interest to geologists and geophysicists who work on the Archaean, and students at all levels.