SYN-THICKENING TO POST-THICKENING EXTENSI ON - MODE AND CONSEQUENCES

Studies in Asia and the Basin and Range Province have been central in proving that thickened continental crusts may collapse, producing syn- to post-thickening extension tectonics. In the Western European Varis cides, Late-Visean-Westphalian extension is nearly parallel to the bel t and took place during escape tectonics controlled by still-active co mpression forces. Late-Stephanian to Early-Permian extension implies c omplex changes in extension direction and intensity. These variations are accommodated by crustal-scale transfer faults. A quantitative and theoretical analysis, supported by geological examples, permits an est imate of the amount of horizontal stretching across the Eastern Massif Central. It shows that extension exaggerated considerably the displac ement of collision-related thrusts and implies that a large part of th e lower orogenic crust is transferred to upper crustal levels during l ate-orogenic processes without producing sediments.