The Franciscan Complex

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The Franciscan Complex Introduction The Franciscan Terrane of central California represents an accretionary complex formed by long-term subduction of an oceanic plate under the Western margin of the North American craton. The Franciscan complex is composed of three distinguishable belts: the eastern belt (Yolla Bolly and Pickett Peak terranes), the central belt, and the coastal belt. Age and metamorphic grade of the belts decreases to the west (Blake and Jones, 1981). Formation of the accretionary complex began during the late Jurassic in the eastern belt and has continued into the Miocene along the western coastal belt. The complex trends NNW and is bounded by the San Andreas Fault to the east and by the coastal range fault to the west. The coast range fault separates the Franciscan complex with the partly coeval Great Valley sequence. Debate exists over the tectonic evolution of the Franciscan, centered around the geographic origin of the Franciscan rock units. Characterization of the Three Belts The coastal belt of the Franciscan Complex is composed of the youngest and least deformed units and makes up the western quarter of all Franciscan rocks. The rocks of the coastal belt are composed of arkosic sandstones, andesitic graywackes, and quartzofeldspathic graywackes interbedded with radiolarian chert (turbidite deposits) (Blake and Jones, 1981). These sedimentary rocks suggest a depositional environment of deep-sea fan systems with both oceanic and continental provenance. Parts of the belt show evidence of later metamorphism, principally due to subduction. Low-grade blueschist mineral facies are indicated by the presence of minerals such as laumonite and prehnite-pumpellyite (Blake and Jones, 1981). All rock units show evidence of thrust (imbricate) faulting due to the compressional forces of subduction. Ages of the coastal belt run from as little as 40 Ma (Eocene) to as old as 100 Ma (middle Cretaceous). The central belt of the Franciscan Complex represents older and more metamorphosed units of rock best characterized as a melange. Blocks of graywacke, greenstone, chert, limestone, and blueschists are sheared and thrust upon one another in a choatic mix (Isozaki and Blake, 1994). In contrast to the coastal belt, metamorphism is higher in grade here and dominated by pumpellyite which formed within the matrix of graywacke (Hagstrum and Murchey, 1993). The mixing of these units makes a stratigraphic subdivision difficult but analysis of the graywacke slabs indicates that the depositional environment was also deep sea, near to the continent. Turbidity currents in this environment deposited much of the sediment in both the coastal and central belts.

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