Although the chaotic fabric of melange (from the French, mélange, or mixture) is characteristic of rocks formed within the accretion prisms of subduction zones, geologists disagree about the details of melange formation (Raymond and Terranova, 1984), as reflected in the many aliases of melange (e.g., chaotic formations, wildflysch, mega-breccia, argille scagliose and friction carpets). For engineering purposes, Medley (1994) considers melange to be a bimrock (block-in-matrix rock) which he defines as "a mixture of relatively large, competent blocks within a bonded matrix of finer and weaker texture", a definition that ignores rock genesis. Bimrocks similar to melange that are formed from cataclasis and fragmentation include breccias, coarse pyroclastics, lahars and tillites. Other bimrocks form from weathering (decomposed granite) or sedimentation (boulder conglomerates). Analagous soils, such as colluvium and till are termed bimsoils.

Melange bodies have been identified in the mountains of over 60 countries (Medley, 1994) and are exemplified by the Franciscan Assemblage (the Franciscan) a regional-scale jumble in northern California. Typically, the Franciscan contains chaotic zones of relatively strong blocks of greywacke sandstone, chert, basalts, limestone and exotic metamorphic rocks embedded within a matrix of pervasively sheared shale and argillite. The blocks range between sand particles and mountain masses, are irregularly shaped, and generally trend NNW-SSE (Figure 1). Shears are confined to the weak shale matrix, which flows around the closely jointed blocks. The sheared shale, and the sheared serpentinite bodies common in the Franciscan, are both responsible for the myriad earth-flow landslides of the Fransciscan. Also, encounters with unpredictably distributed blocks in melanges cause expensive surprises during earthwork excavations and foundation preparations.

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