When subduction rates are several cm/yr or more, various geologic and heat flow considerations indicate the portion of the accretionary wedge beneath the overriding plate will consist of two structural “layers.” The upper layer is composed of schistose rocks directly in contact with, and heated by the base of the overriding plate. The lower layer is composed of fluid-rich, undercompacted, less heated mud and sand. With continued convergence, large shear strains are concentrated in the lower layer and tectonic flow melange develops. The portion of the accretionary wedge that accumulates in front of the overriding plate can be, as in the Franciscan of California, largely composed of subduction-generated melange blanketed by bedded slope sediments. Mappable units within an accreted melange wedge are defined by variations in the type and relative numbers of clasts. The population of clasts in flowing melange can vary where high-pressure metamorphic rocks are only locally plucked from the base of the overriding plate, where upflow ceases, and where features such as seamounts are dismembered as they locally impinge the accretionary complex. Melange intrusions, in the form of dikes and sills, can also form mappable melange units within an accretionary wedge.