Franciscan Mélanges as a model for eugeosynclinal sedimentation and underthrusting tectonics
The traditional view representing eugeosynclines as elongate troughs having active volcanism is challenged. Eugeosynclinal rocks are commonly tectonic melanges, representing mixtures of rocks derived from more than one realm of deposition. The Franciscan melange includes ophiolites emplaced at a mid-ocean ridge, radiolarites deposited on an abyssal plain, and flysch turbidites poured into a deep-sea trench. The Franciscan melange was pervasively sheared when the North American plate rode over a Cretaceous Neo-Franciscan plate. The plate junction is not a single overthrust surface. The densely spaced shear surfaces within the melange signify a westward migration of the plate junction as the Franciscan was being underthrust. The Franciscan glaucophane schists, metamorphosed under very high pressure 130–150 m.y. ago, record a pre-Cretaceous Franciscan history. Geological evidence suggests that those once deeply buried rocks were squeezed out during a Late Jurassic alpine type of deformation (Nevadan orogeny), when the North American continent collided with a micro-continent (Salinia). The Cretaceous underthrusting along a consuming plate margin resulted in melange deformation and in lawsonite-schist metamorphism of the neo-Franciscan rocks, which are coeval with the Great Valley sequence.