Crustal-Scale Cross-Section of the U.S. Cordillera, California and Beyond, Its Tectonic Significance, and Speculations on the Andean Orogeny

Abstract

A cross-section across northern California from the San Andreas fault to central Nevada exhibits both major east- and west-vergent structures. East-vergent structures include crustal wedging and fault-propagation folds in the Coast Ranges, emplacement of the Great Valley ophiolitic basement over Sierran basement rocks, early east-vergent structures in the latter, displacement along the eastern margin of the Sierra Nevada batholith, and thrust faults in western Nevada. West-vergent structures include faults within the Franciscan complex and "retrocharriage" structures in the Sierra Nevada A model of evolution of the U.S. Pacific margin emphasizes the role of ophiolites, island arc-continental margin collisions, and subduction of a large oceanic plateau. Early Mesozoic subduction along the Pacific margin of North America was modified by a 165-176 Ma collision of a major intra-oceanic arc/ophiolite complex. A complex SW Pacific-like set of small plates and their boundaries at various times may have been present in southern California between 115 and 40 Ma. Subduction of an oceanic plateau about 85-65 Ma (remnants in the Franciscan) produced east-vergent tectonic wedging in the Coast Ranges, possible thrusting along the eastern Sierra Nevada batholith margin, and development of Rocky Mountain Laramide structures. The "Laramide orogeny" is herein redefined to include all late Cretaceous-Early Tertiary (75-45 Ma) fold-thrust structures from the Pacific Coast to the Rocky Mountains. A speculative model for collisional involvement in the Andean orogeny is also presented, based upon timing of the onset of the Andean orogeny, the presence of oceanic terranes along the western margin of the Andes, and the presence along part of the length of the chain of a remnant marginal basin.