Melanges and related rock bodies have been defined, described, and classified in a variety of ways. Classifications, divisible into eight types, reflect specific definitions of the term melange. Although melanges were originally considered to be tectonic in origin, currently the term is used widely in a descriptive rather than a genetic sense. Melanges are here defined as bodies of rock mappable at a scale of 1:24000 or smaller and characterized both by the lack of internal continuity of contacts or strata and by the inclusion of fragments and blocks of all sizes, both exotic and native, in a fragmented matrix of finer-grained material. Matrix composition and fabric are rejected as definitive criteria. Progressive fragmentation and mixing of original sedimentary, igneous, and/or metamorphic protoliths give rise to broken formations, dismembered formations, and melanges-three classes of unit that, together with formations, represent a continuum of rock bodies ranging from coherent stratified units at one extreme to chaotically mixed masses at the other. Melanges may originate through sedimentary processes, diapiric processes, tectonic processes, or combinations of these processes, and are classified accordingly. 7 on July 29, 2015 specialpapers.gsapubs.org Downloaded from

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Understanding details of accretionary complex architecture is essential to understanding construction of oceanic "outer" sides of orogens. The archi tecture of the Franciscan Complex (California), considered by many to be the "type" accretionary complex, is widely viewed in the context of terranes or belts delimited by reconnaissance mapping that reveals neither regional variations within terranes nor critical details of stratigraphy and structure. The architectural importance of Franciscan mélanges is recognized, but the importance of sandstonematrix mélanges and olistostromal sandstones is not. Largescale mapping in Sonoma and Marin counties, California, shows that Franciscan rocks are deformed, submarinefan units of Facies A-E, plus Facies F olistolithbearing submarine channel sandstones and olistostromal sandstone and shalematrix mélanges. Some mélanges are polygenetic with a sedimentary origin and a tectonic overprint. Glaucophane schists were recy cled into conglomerates and olistostromes. Mappable units constitute mem bers, broken and dismembered formations, and mélanges. Considering the stratigraphy and structure evident at the 1:24,000 scale, accretion via a sub duction channel mechanism is impossible. The SonomaMarin Central belt or Central terrane (mélange) is not a monolithic shalematrix mélange and lacks this characteristic of rocks assigned the same name to the north. Franciscan rocks here structurally underlie thrustfaulted fragments of a regional ultra mafic sheet and, locally, an underlying exotic blockbearing serpentinitema trix mélange. The detailed mapping shows that regional relations among and within Franciscan terranes and belts are poorly understood and suggests that such mapping is needed to clarify accretionary complex architecture and history. The implication for accretionary complex studies, in general, is that, while terrane or belt designations provide a general picture of the collage na ture of accretionary complexes and clarification of regional relationships, only largescale structural and stratigraphic studies can elucidate the architectural details of these orogenic complexes.

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