The mid-Cretaceous Peninsular Ranges orogeny: a new slant on Cordilleran tectonics? II: northern United States and Canada

The mid-Cretaceous Peninsular Ranges orogeny occurred in the North American Cordillera and affected rocks from Mexico to Alaska. It formed when a marine trough, open for ~35 Myr, closed by westerly subduction beneath a 140-100 Ma arc complex. In Part I we described the features of the orogen in Mexico and California, west to east: back-arc trough, magmatic arc, 140-100 Ma seaway, post-collisional 99-84 Ma granodioritic-tonalitic plutons emplaced into the orogenic hinterland during exhumation, an east-vergent thrust belt, and farther east, a flexural foredeep. In western Nevada, where the Luning–Fencemaker thrust might be a mid-Cretaceous feature, arc and post-collisional plutons occur in proximity. The orogen continues through the Helena salient and Washington Cascades. In British Columbia, rocks of the 130-100 Ma Gambier arc lie west of the exhumed orogenic hinterland and 99-84 Ma post-collisional plutons to collectively indicate westerly subduction. East-dipping reverse faults near Harrison Lake, active from ~100 Ma until ~90 Ma, shed 99-84 Ma debris westward into the Nanaimo back-arc region. Within Insular Alaska, the Early Cretaceous Gravina basinal arc assemblage was deformed at 100 Ma, and flanked to the east by a high-grade hinterland cut by post-collisional plutons. In mainland Alaska, the 100 Ma collision of Wrangellia and the Yukon-Tanana-Farewell composite terrane occurred above a southward-dipping subduction zone as shown by the 130-100 Ma Chisana arc sitting on Wrangellia and southward-dipping, northerly vergent thrusts in the Lower Cretaceous Kahiltna basin to the north. The outboard back-arc region was filled with post-collisional detritus of the McHugh complex.