ABSTRACT: New Geotectonic Subdivision of the Franciscan-Klamath Region Redefined by Formation Age of Accretionary Complexes
ISOZAKI, YUKIO, Yamaguchi University, Yamaguchi, Japan, and SHIGENORI MARUYAMA, University of Tokyo at Komaba, Tokyo, Japan
The geotectonics of northern California is confusing because of the many (>20) allochthonous terranes defined by individual terrane stratigraphy. Most of the terranes, in fact, do not represent geologic entities that existed upon oceanic plates before accretion, but accretionary complexes that formed in situ along the North American margin. Terrane stratigraphy hardly describes a primary coherent sedimentary succession but instead describes an apparent superposition of rocks secondarily mixed by subduction-related tectono-sedimentary processes. Based on the new concept of oceanic plate stratigraphy of accretionary complexes, all the units in the Franciscan-Klamath region in northern California are reclassified into the following nine units (tectonically downward from east): 570 Ma ontinental margin Trinity ophiolite; 450-440 Ma blueschists, Devonian accretionary complexes; 230-210 Ma blueschists, Middle-Late Jurassic accretionary complexes; 160-130 Ma blueschists; 100-90 Ma blueschists, Late Cretaceous accretionary complexes; and Tertiary accretionary complexes. These units all occur as subhorizontal tectonic alternations of high-P/T units and low-P accretionary complexes. The most important orogenic feature in northern California is the episodic zoned growth of accretionary complexes, including high-P/T parts, with tectonically downward-younging polarity. This structure in northern California is a primary configuration of the entire Cordilleran orogen in western North America because of its potential lateral extent of more than 3000 km, from southern Alaska to so thern California. Strike-slip tectonics is not responsible for this primary orogenic structure, but for minor second-order modification.
AAPG Search and Discovery Article #91016©1992 AAPG-SEPM-SEG-EMD Pacific Section Meeting, Sacramento, California, April 27-May 1, 1992 (2009)