Jurassic-Cretaceous Paleogeography, Terrane Accretion, and Tectonic Evolution of Western North America, Ronald C. Blakey and Paul J. Umhoefer, Department of Geology, Northern Arizona University, Flagstaff, AZ 86011

Key words: terrane accretion, tectonic evolution, western North America

Reconstruction of Mesozoic Cordilleran accretionary events can be partially constrained by a series of paleogeographic and tectonic maps at consecutive (15-20 m.y.) time intervals. Producing maps with temporal continuity results in a realistic sequence of tectonic events and plate motions, especially when constraining geologic and paleomagnetic data are included. Wrangellia and Guerrero (western Mexico) are considered exotic to North America. We trace the history of several basins and their relations to terranes and tectonic events including the Wrangell Mountains, Tyaughton-Methow, and Great Valley arc-related basins, and the Rocky Mountain foreland basin. The maps are based on a moderate view of terrane translation with respect to 'Baja BC'. We divide Jurassic-Cretaceous events into six time intervals. The following events are shown with moderately detailed paleogeography: 1) 180-160 Ma - major arc and oceanic terranes of diverse and controversial origin were accreted to western North America (eg. Quesnellia, Cache Creek, Stikine, related terranes of Klamaths and Sierra Nevada, southern Wrangellia?); 2) 160-145 Ma - major arc magmatism and sinistral oblique convergence with southerly displacement of outboard terranes; 3) 145-125 Ma - continued sinistral convergence and southward terrane displacement; Guerrero terrane approached Mexico; gaps in foreland basin; 4) 125-105 Ma - Guerrero arc accreted and new arc built on new western margin of Mexico; tectonic escape northward of several terranes; minor arc volcanism and major foreland subsidence; 5) 105-85 Ma - major outboard magmatism on Baja BC with an inner magmatic belt in hinterland of thrust belt; northern Wrangellia accretes and intervening basin closed; Kula-Farallon triple junction formed and Baja BC moved northward from latitude of present S. California; 6) 85-65 Ma - Strong oblique dextral convergence of Kula plate and northward translation of Baja BC; Laramide orogeny initiated along Farallon - North American plate boundary in S. California and NW Mexico; Rocky Mountain foreland basin remained active. When viewed as a whole, the proposed paleogeographic maps form a coherent picture of terrane accretion and dispersal dominated by oblique convergence. One anomaly is the accretion of southern Wrangellia 70 - 80 my before northern Wrangellia.

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