Sandstone-matrix mélanges, architectural subdivision, and geologic history of accretionary complexes: A sedimentological and structural perspective from the Franciscan Complex of Sonoma and Marin counties, California, USA

Abstract

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 mlanges is recognized, but the importance of sandstonematrix mlanges 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 mlanges. Some mlanges 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 mlanges. 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 (mlange) is not a monolithic shalematrix mlange 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 mlange. 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.