The Franciscan Complex |
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. Structurally, the central belt is dominated by gently to moderately east-dipping faults that again indicate a compression of the belt. Ages for the belt range from late Jurassic to Paleocene (150 - 60 Ma) (Hagstrum and Murchey, 1993).
The eastern belt is commonly subdivided into the Pickett Peak and Jolla Bolly terranes which are juxtaposed along an east dipping, low angle fault. Both contain metamorphosed clastic rocks (quartz-lawsonite-mica schist), metachert (quartz-riebeckite) and metagreenstone along with smaller amounts of septentinite (Isozaki and Blake, 1994). The Pickett Peak terrane is structurally higher and of higher metamorphic grade than the Jolla Bolly terrane. It is the easternmost unit of the Franciscan. The Pickett Peak and Jolla Bolly terranes collectively represent the highest grade of blueschist (high P, low T) metamorphism seen in the Franciscan complex as well as the oldest rocks. Structurally, the belt is dominated by imbricate faulting with large imbricate thrust sheets (nappes) extending from the central belt to the coast ranges fault to the east. Like the other belts, the eastern belt rocks represent the off-scrapings of an oceanic plate as it collided with the North American craton.
Tectonics of the Franciscan Terrane
The tectonics of the Franciscan have been a subject of debate for at least the last 35 years (see Baily, 1964 or Taliferro, 1943) and theories have evolved over time as advances in plate tectonics, paleomagnetism, radiometric dating, etc have spaned new hypotheses and required modification of older interpretation. All recent models recognize plate tectonics as the driving force in the creation and accretion of the Franciscan to the North American craton, but models disagree on geometric interpretations and details of the mechanisms involved.
Traditional tectonic models involve collision of an ocean plate (such as the Farralon plate) with the North American (continental) plate. This collision caused eastward subduction of the Farallon plate under North America. In this model, the Franciscan represents the accretionary wedge, formed in place while the Great Valley sequence represents the forearc (arc-trench gap) basin. No movement other than eastward movement normal to the North American plate is implied in the traditional model (Blake and Jones, 1981). Objections to this model vary in form and content but include some of the following: differences in sandstone petrology and subsea fan faces between the Great valley and some of the coeval coherent Franciscan terranes, variations in sedimentological characteristics of some of the older rocks of the Great valley and correlative strata to the east, and differences in the paleomagnetic signature between the Franciscan and Great Valley rocks (Blake and Jones, 1981) (Hagstrum and Murchey, 1993).
Blake cites differences in sandstone mineralogy, age, and textures as primary reasons to believe that the traditional model is faulty. Blake and others (such as Seiders and Blome, 1987) have analyzed modal compositions of the Franciscan and Great Valley rocks, and noticed differences between the Franciscan and the Great Valley rocks. For example chert, sandstone, and mudstone are more abundant in the Great Valley sequence while felsic volcanic rocks are more abundant in the Franciscan rocks. In some cases differences are slight (ex. chert 65% in G.V., 60% in Fran.) but in other cases marked differences exist (ex. felsic volcanics 1.9% in G.V., 9-15% in Fran.) Blake and others argue that these differences in composition suggest the rocks have different provenance (geographic origin) (Blake and Jones, 1981). The model they suggest involves deposition of the Franciscan rocks away from their present-day location, and later NNE movement of these rocks to their location today.
This model of deposition far removed from the site of actual accretion is supported by several paleomagnetic studies such Hagstrum and Murchey (1993). Using radiolarian cherts found in the central and eastern belts of the Franciscan, Hagstrum and Murchey have taken paleomagnetic measurements to determine paleolatitude. Their findings suggest large-scale movement of the Franciscan rocks from the time of deposition to the time of accretion onto the continent.
Summary and Conclusion
The Franciscan Complex is composed of a lithologically heterogenous assemblage of rocks representing 100 million years of subduction. Broken into three belts, the Franciscan occupies much of western California and can be seen in outcrop throughout the region. The Franciscan complex was deposited mainly in deep sea fan systems with the continent to the east. Later, subduction-related, low P-T metamorphism took place in varying degrees throughout the complex. Debate still exists over the original location of depositon, some say it is near to the present-day location, others claim it was to the south or southwest. Further study may or may not clear up the controversy, however it is clear that workers do not currently agree.
Works Cited