ABSTRACT: Application of K-Ar Mapping Method on the Franciscan Complex in Northern California and Redefined Geotectonic Subdivision and Boundaries
MARUYAMA, SHIGENORI, University of Tokyo at Komaba, Tokyo, Japan, YUKIO ISOZAKI,* RYUICHI TAKEUCHI, and NANA TOMINAGA, Yamaguchi University, Yamaguchi, Japan, and HIROYUKI TAKESHITA and TETSUMARU ITAYA, Okayama University of Science, Okayama, Japan
The Franciscan Complex in northern California has been traditionally divided into three distinct units: eastern, central, and coastal belts. These units were based on lithologic assemblage, metamorphism, and fossil ages. In general, the units show a north-south-trending zonal distribution with older and more highly metamorphosed rocks on the continent side. Because these units are loosely defined, however, identifying them and their mutual boundaries is difficult. The terrane concept of early 1980s, an obsession to divide units into as many subunits as possible, obscured our understanding of the Franciscan Complex. New research methods, such as microfossil mapping and K-Ar mapping, based on a concept of oceanic plate stratigraphy, can provide the ultimate criteria to define units and utual boundaries. More than 100 K-Ar ages of metamorphic mica and approximately 30 radiolarian ages were obtained along an east-west transect of the Franciscan Complex, from West Port to Elder Creek via Laytonville, Covelo, and Mendocino Pass, northern California. These data clearly demonstrate that the Franciscan Complex is divided into following four distinct units (tectonically downward from east): 160-120 Ma blueschists; Early Cretaceous accretionary complex having 100 Ma high-P/T metamorphism; Late Cretaceous accretionary complex, and Tertiary accretionary complex. These units are now more clearly defined in terms of the formation age of the accretionary complex and the metamorphic age. The three-dimensional geometry of these units appears to be subhorizontal, suggesting a primary s ructure of the older unit on a high tectonic level.