FRANCISCAN COMPLEX: MAPPING OF MÉLANGES AND TERRANES IN THE NORTHERNMOST CALIFORNIA COAST RANGES, A RETROSPECTIVE

Discrimination of mappable rock units reflects lithology, common ancestry and scale among rocks formally designated as Franciscan Complex (a ‘superterrane’?) Franciscan Eastern belt (a composite terrane?) comprises the Pickett Peak terrane (South Fork Mountain Schist, Redwood Creek schist, Patricks Point unit) and Yolla Bolly terrane (Eastern subterrane: chiefly broken formation; Western subterrane: chiefly mélange). Franciscan Central belt (a composite terrane?) consists of NW-trending belts of intercalated mélange and broken formation. In the Cowan (1985) classification, broken formation is Type I mélange that has suffered layer-parallel extension prior to folding. Type II mélange (stratally-disrupted interbedded sequences of radiolarian chert, black argillite, hyaloclastite and pillow basalt) exists as mélange blocks and/or thrust fault-bounded slabs within all Franciscan terranes. Some slabs are extensive enough to be mapped at a 1/12,000 scale. Are they then subterranes? Type III mélange (block mixes of greenstone, chert, limestone, keratophyre, tonalite, turbidites and blueschist that originated as diapirs and/or olistostromes) are common in the Western Yolla Bolly subterrane and Central belt. Type IV mélange (block-in-matrix mixes developed along shear zones) with blocks of chert, greenstone, gabbro, sandstone and conglomerate exist in metapelitic gouge associated with major terrane-bounding faults. Omphacite-bearing tectonic blocks in serpentinite occur in an area of imbricate thrust faulting of South Fork Mountain Schist over Eastern subterrane broken formation, adjacent to the bounding South Fork fault of the Klamath Mountains Western Jurassic belt. This serp.-schist association has an affinity to the Coast Range ophiolite to the south and formed during western translation of the Klamath Mountains. The confusing mix of formal and informal unit nomenclature suggests need for revision of the stratigraphic code for the mapping of exhumed accretionary prisms.