WHAT IS FRANCISCAN?: REVISITED

The Franciscan Complex of California is better understood now than it was in 1972, when Berkland et al. defined the Complex and divided into three belts. The three belts first served as major architectural units, but were abandoned by some and subdivided into tectonostratigraphic terranes by others. The Franciscan Complex is still recognized as the folded, faulted, and stratally disrupted rocks that together form the supramantle basement complex of the Northern and Central California Coast Ranges, and their extensions into southern Oregon, exposed west of and structurally below the principal exposures of the Coast Range Fault, Coast Range Ophiolite, and the Great Valley Group. The Complex consists of various wackes and arenites, mudrocks, conglomerates and diamictites, radiolarian cherts and siliceous shale, minor limestone, basic volcanic rocks, minor mafic/ultramafic plutonic rocks, and the zeolite- to blueschist- and eclogite-facies metamorphic equivalents of all of these rocks. It is considered by many to be the archtypical subduction accretionary complex. New data and large-scale maps, historical precedence, and reference to fundamental definitions suggest now (1) that the Belt terminology, nappe designations, and most terrane assignments are inconsistent with existing definitions and our current understandings of Franciscan architecture and character, and should be abandoned; (2) that underthrust-related accretionary units (mélange bodies and underthrust sheets) are the best major architectural units into which the Franciscan Complex should be divided; and (3) that lithostratigraphic and tectonostratigraphic units, such as broken formations and mélanges, mapped at the medium- to large-scale, are the best local units for subdivision of Franciscan architecture. Considerable mapping, detrital zircon dating, and delineation of post-accretion structural deformation are needed for further clarification of Franciscan architecture. Controversies remaining at the forefront include those related to the nature and origins of many Franciscan mélanges, the megathrust-subduction channel controversies, and the chert conundra.