Structure and metamorphism of the Franciscan Complex Mt. Hamilton area, northern California

Truncation of metamorphic isograds and fold axes within coherent terranes o f Franciscan meta graywacke by intervening zones of melange indicate that t he melange is tectonic and formed after the subduction-related metamorphism and folding. These relations are expressed in two terranes of blueschist-f acies rocks of the Franciscan Complex in the Mt. Hamilton area, northern Ca lifornia-the Jurassic Yolla Belly terrane and the structurally underlying C retaceous Burnt Hills terrane. Local preservation in both terranes of basal radiolarian chert and oceanic basalt beneath continent-derived metagraywacke and argillite demonstrates t hrust repetition within the coherent terranes, although these relations are scarce near Mt. Hamilton. The metagraywackes range from albite-pumpellyite blueschists to those containing well-crystallized jadeitic pyroxene, and a jadeite-in isograd can be defined in parts of the area. Primary bedding de fines locally coherent structural orientations and folds within the metagra ywacke units. These units are crosscut by thin zones of tectonic melange co ntaining blocks of high-grade blueschist, serpentinite, and other exotic ro cks, and a broader, but otherwise identical melange zone marks the discorda nt boundary between the two terranes.