The Franciscan Complex :: Essays Papers

The Franciscan Complex IntroductionThe Franciscan Terrane of key California represents an accretionary complex formed by long-term subduction of an pelagic plate under the Western margin of the North American craton. The Franciscan complex is composed of three distinguishable boot ammunitions the eastern belt (Yolla Bolly and Pickett hot flash terranes), the central belt, and the coastal belt. Age and metamorphic grade of the belts decreases to the west (Blake and Jones, 1981). governance 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 delimited by the San Andreas Fault to the east and by the coastal range disfigurement to the west. The coast range fault separates the Franciscan complex with the partly contemporary Great Valley sequence. Debate exists over the tectonic evolution of the Franciscan, touch on around the geographic origin of the Francisc an rock units.Characterization of the cardinal BeltsThe coastal belt of the Franciscan Complex is composed of the youngest and least alter units and makes up the western quarter of all Franciscan rocks. The rocks of the coastal belt ar 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 marine fan systems with twain oceanic and continental provenance. Parts of the belt bear witness 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 come out 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 elder and more metamorphosed units of rock best characterized as a melange. Blocks of graywacke, greenstone, chert, limestone, and blueschists are shorn 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 predominate by pumpellyite which formed within the matrix of graywacke (Hagstrum and Murchey, 1993). The mixing of these units makes a stratigraphic department difficult but analysis of the graywacke slabs indicates that the depositional environment was also deep sea, shape up to the continent. Turbidity currents in this environment deposited much of the sediment in both the coastal and central belts.