Mélanges are defined as mappable bodies of deformed rocks characterized by the inclusion of native and exotic blocks, which may range up to several miles long, in a pervasively sheared, commonly pelitic matrix. Fragmentation and mixing of mélanges result from tectonic deformation under an overburden pressure.
Mélanges are not rock-stratigraphic units. Structural and stratigraphic interpretations of mélange geology cannot be based upon presumptions of superposed and lateral stratal continuity. Assignment of an age of deposition to a mélange on the basis of fossil occurrences alone can be incorrect. The interpretation that a formation and a partly coeval mélange must be separated by an overthrust contact has also been proved invalid for certain instances. These considerations led to a formulation of five rules of mélanges.
The confusion and controversy about the stratigraphic and structural relations between the Franciscan rocks and the Knoxville Formation of the California Coast Ranges have been traced to the fact that the former have been interpreted, and are still being interpreted by some, as a rock-stratigraphic unit. Recognition that those rocks are mélanges and application of the five mélange rules have helped resolve the apparently contradictory stratigraphic and structural relations known as the Franciscan-Knoxville paradox.
A large body of mélanges can be subdivided into several tectono-stratigraphic units, based upon the different natures and origins of the mélange blocks and the matrix. Mapping of such mélange units may reveal tectonic superpositions within an apparently chaotic mass.