Melanges were once considered gravity-slide deposits, but many current workers link their genesis to sub- duction along convergent plate margins. Franciscan melanges are the best known examples. They are impressive because of the scale of chaotic deformation. The pervasively sheared mass of ophiolites, cherts, and graywackes in a dominantly pelitic matrix bears witness to an oceanward migration of a late Mesozoic Benioff zone between oceanic and North American plates. Similar melanges have been reported from other circum- Pacific mountains, but their absence in the central Andean province is noteworthy. There the distribution of igneous rocks suggests a landward migration of the Benioff zone. Melanges are present in the Swiss Alps, but the Alpine melanges are dwarfs compared to the Franciscan giants. Andesitic activity, the twin brother of melange, so to speak, was also inconspicuous during the Alpine orogenesis. Those facts might be construed as evidence that the magnitude of plate consumption in the Alps was modest compared to that along the eastern Pacific margin.
Melanges are tectonic units bounded by shear surfaces. In contrast, olistostromes are stratigraphic units, generally separated from overlying and underlying formations by depositional contacts. Regional mapping is one of the tools for distinction. In a mélange terrane, one can observe different degrees of severity of fragmentation and mixing, grading from internally intact allochthonous slabs to broken formations, and to pervasively sheared and intimately mixed melanges. In an olistostrome terrane, one finds gradations from boulder beds to graded turbidites. Melanges are massive and they developed on a regional scale, although thin mélanges occupying certain tectonic horizons are not uncommon. Olistostromes, as sedimentary units, are commonly limited in size and could be placed in a sedimentary basin within a simple paleogeographic framework. Mélanges are deformed under an overburden, so that the pelitic materials flow and other rocks fracture. Partially broken blocks are commonly bounded by shear fractures, although tectonic transport may ultimately lead to rounding. Olistostrome blocks are commonly sedimentary boulders, already rounded prior to sedimentary transport. Aside from the common pelitic matrix, sandy matrix is present locally in some olistostromes.
Pervasively sheared olistostromes are practically indistinguishable from melanges. For example, the argille scagliose of the Apennines include both melanges and olistostromes. The olistostrome aggregates there reached proportions comparable to those of the Franciscan. However local specialists have been able to recognize in those units individual olistostrome beds produced by separate events, and those beds are rarely more than 100 m thick. Pervasively sheared olistostromes are practically indistinguishable from melanges. Where a genetic interpretation of a chaotically deformed deposit cannot be made, I recommend the use of the descriptive term “wildflysch.”