BURIAL METAMORPHISM AND EVOLUTION OF RIFT TROUGHS - A MODEL APPROACH

The well-known McKenzie's model of rift formation was investigated. Th e model includes stretching, expansion, and thinning of the lithospher e; ascent of hot asthenosphere; isostatic reequilibration; subsequent cooling; and thermic contraction. Mathematical simulation was used to analyze three scenarios of the final expansion of the lithosphere: ins tantaneous, accelerated, and with a constant speed. It was found that the model involving the instantaneous finite expansion of the lithosph ere is valid for rough estimates of trough subsidence and the evolutio n of thermal flow. The validity of such a rough model is confirmed by a good agreement between the model results and the observed dynamics o f tectonic subsidence inferred from the structure of sedimentary secti ons in rift basins. The modeling of evolution is demonstrated by four rift troughs: central trough of the North Sea, Danish basin, Reikenhag en basin, and Dnieper-Donets aulakogen. It was concluded that the stre tching/expansion of the lithosphere during rift formation normally amo unted to 30 - 50%, the troughs were as deep as 6 - 9 km, and the tempe ratures of rocks at the base of troughs attained levels of prehnite-pu mpellyite and, sometimes, greenschist facies. However, much higher roc k temperatures were probably attained in ancient rifts.