THERMAL STRUCTURE OF SOUTH AMERICAN CONTINENTAL LITHOSPHERE DURING ARCHEAN AND PROTEROZOIC

VALIYA M. HANZA

Resumo


A review of South American terrestrial heat flow data is presented. Heat flow density variations across the Andean subduction zone show a general trend of increasing heat flow from the coastal regions to the eastern cordillera and altiplano. This trend is attributed to progressive partial fusion of the descending oceanic slab beneath the South American continental lithosphere. The spatial distribution of major mineral deposits shows a pattern that appears closely related to the variations in the geothermal regime. Heat flow density variations in the stable eastern parts of the continent are found to be related to the sequence of thermotectonic events in the Proterozoic and Archean. A method of estimating the thickness of the eroded crust is presented that could be of help in determining the evolution of geotherrnal regimes during Archean and Proterozoic. This method is based on making use of a relation for radiogenic heat productivity in terms of heat flow parameters whose time dependences are known. The variation of heat productivity with age suggests that most of the archean crust had an initial heat generation of about 7μ/W/m3. Radioactive decay and erosion reduced the surface heat productivity to the present day values, but it is possible to separate the decay component so that effects of erosion can be examined. The available data suggest erosion rates compatible with present day values for Phanerozoic times but extremely low erosion rates are found for Proterozoic and Archean. Extension or stretching followed by subsidence is pointed out as the dorrunant tectonic process for Precambrian times as it is the most logical mechanism capable of guaranteeing low erosion rates.


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