The South Yolla Bolly area of northern California contains the intersection of the Franciscan complex, the Klamath Mountains, and the Great Valley sequence. Four distinct lithic units, among them the South Fork Mountain Schist, as well as several subunits, have been discerned in the Franciscan rocks of this area. These units are composed mostly of coherent or semi-coherent layered sedimentary and mafic igneous rocks and their metamorphic equivalents rather than melange, and are arranged in largely upright, parallel imbricate fault slices.
Four major low-angle faults have been clearly recognized. Paleontologic data indicate that at least one fault places older material over younger. Lithic units exhibit various degrees of differences in metamorphic mineral assemblages, textural grade, and amount of internal deformation across the faults, with more intensely deformed and metamorphosed material occupying successively higher structural positions. These contrasts indicate that some of these faults served as surfaces by which deformed rocks were emplaced differentially upward during and/or after metamorphism.
Original bedding and compositional layering in all of the coherent units generally parallel the thrust faults. Two fault-bounded units are each composed of a coherent sequence of basalt at the base overlain by bedded chert, and further overlain by mudstone and graywacke or their metamorphic equivalents. The faults which underlie these two sequences seem to have originated as decollements localized at or near a sedimentary rock – mafic-crust interface, perhaps as discrete underthrust slices. The distinct contrasts in original rock composition among these units plus the aforementioned characteristics seem to indicate that these faults had an early history in the accretionary prism apart from their later use as convenient surfaces for differential upward emplacement of metamorphosed material.