Examples of Franciscan Complex mélanges in the northernmost California Coast Ranges, a retrospective

Abstract

A variety of mélanges exist within the Franciscan Complex of the northernmost California Coast Ranges. The Cowan mélange classification is employed in their description. Classic turbidite broken formation is Type I mélange that has suffered coaxial 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. Type III mélange (block-in-matrix mixes of greenstone, chert, limestone, keratophyre, tonalite, turbidites, and blueschist) may have emplaced diapirically and/or by sediment gravity-flow. Type IV mélange (block-in-matrix mixes developed along shear zones) with blocks of chert, greenstone, gabbro, sandstone, and conglomerate in metapelitic or serpentinite gouge is associated with major faults. An example of the latter consists of omphacite-bearing tectonic blocks in serpentinite in an area of imbricate thrust faulting of South Fork Mountain Schist over unfoliated turbidites adjacent to the South Fork fault, which separates the Coast Ranges and Klamath Mountains provinces. This serpentinite-schist Type IV mélange has an affinity to the Coast Range ophiolite, which crops out to the south opposite the Great Valley, and perhaps formed during western translation of the Klamath Mountains.