The results of a detailed field research on the Jurassic terraines of the centrai Marches Apennines are here presented. The paleotectonic evolution of this region during the Jurassic rifting stage, when the new Ligurian ocean basin began to open far­ther west, appears to be quite atypical and considerably different from what we know of other ltalian areas belonging to former Jurassic continental margins (Southern Alps, Tuscany, Southern Apennines, Sicily; etc.). The irregular orientation of the

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... paleofaults, the lack of a precise structural trend, and the extreme fragmentary character of the various Lower Liassic carbonate blocks suggest a possible strong contro! of the underlying Triassic evaporites. The Jurassic structural behaviour of the Umbria-Marches region could be, in large part, the result of a «latent diapirism». The very thièk (1.500-2.000 m) Triassic evaporites underlying the thick (> 500) Liassic carbonate platform (Calcare Massiccio), were under confining pressure unti! the overlying huge carbonate bank, subjected to an encreasing crustal extension, started to break up and collapsed. The mobility of the Triassic evaporites, triggered by the Liassic rifting, is therefore considered the cause of the slow and unpredictable di­smembering of the overlying, more rigid and dense, Calcare Mas­siccio.