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- The polycyclic lithosphere: an attempt to assess its orogenic memory
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The polycyclic lithosphere: an attempt to assess its orogenic memory
Abstract
The orogenic memory of the continental lithosphere is evaluated. Lithostratigraphic, tectonic, metamorphic, plutonic and geochemical criteria for polycyclicity are reviewed. Their mutual relations, significance and resolving power are discussed, and the possible causes of fading of the orogenic memory are explored. Emphasis is placed on criteria such as metamorphosed molasse deposits, interference patterns of superimposed folds, basement upthrusts and basement nappes, mylonitized mylonites, diaphthoretic features, contrasting metamorphic facies series, polymetamorphic marker formations, thermal domes, diapirs and the convecting basement effect. The geochemical polarity of granites and granulites is reviewed in the light of thermal basement fractionation, and its possible reflections on the evolution patterns and dating powers of some radiogenic isotopes are considered. It is concluded that the memory of rocks for orogenic cycles, although finite, can be stretched considerably when judicious use is made of carefully weighted parameters. No single criterion for polycyclicity can as yet be applied unequivocally.