Estimates have been made of the thickness of the lithosphere in Brazil, based on an empirical relation between surface heat flow and crustal to lithospheric thickness ratio in continental regions and making use of available geothermal data. The results show that cratonic areas such as Sao Fransisco, Amazon and Guiana are characterized by lithopshere with thickness in excess of 150 kilometers while in the northeastern and southern part~ of Brazil the lithosphere may have thickness of less than 150 kilometers. There are indications that the frequency of seismic events is high in areas where the lithosphere is thin. Rheological models, compatible with regional geothermal data, show that onset of brittle-ductile transition in the lower crust and upper mantle may have a significant role in the occurrance and distribution of seismic events in intraplate regions like Brazil.


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