RECONSTRUCTING NORTHERN CORDILLERAN TERRANES ALONG KNOWN CRETACEOUS AND CENOZOIC STRIKE-SLIP FAULTS: IMPLICATIONS FOR THE BAJA BC HYPOTHESIS AND OTHER MODELS

Most tectonic interpretations of the Paleozoic and Mesozoic North American Cordillera are based on the modern distribution of terranes, despite widespread recognition of major dextral offset along Late Cretaceous and Cenozoic strike-slip faults. We present palinspastic restorations of the Canadian and U.S. Cordillera that explicitly account for these offsets. The reconstructions involve several steps. We start by restoring Cenozoic extension in the western U.S. (Basin-and-Range province) and southern Canada (Omineca belt), Cretaceous shortening across the Sevier and Rocky Mountain fold-thrust belts, and Cenozoic offset along the San Andreas fault in order to create a base map. We then restore known or inferred displacements on the major Late Cretaceous to Cenozoic dextral faults of the Canadian Cordillera and Washington. The reconstruction moves the southern end of Insular superterrane (much of Baja BC) to the latitude of the southern Klamath Mountains, the southwest Intermontane superterrane west of the Idaho batholith, and the southern Coast orogen into the intervening region. Total displacements increase from east to west and range from 300 to 800 km. These values are smaller than those determined by paleomagnetic studies (1000-3000 km), but they indicate that discrepancies between geologic and paleomagnetic data sets are not as large as is often assumed. The reconstruction results in a striking juxtaposition of many terranes that have previously been correlated or likely evolved together. For example, it restores the otherwise isolated fragments of Jurassic ophiolite and Cretaceous blueschist in Washington and southern British Columbia to a position next to their virtually identical counterparts in northern California. The restored terranes also in part fill in the region of the Columbia embayment (Washington and Oregon), providing an intriguing new option for what geology originally resided in this enigmatic area.

The model has important implications for the Baja BC controversy and the tectonic evolution of the Cordillera. Most importantly, no interpretations of Late Cretaceous and older geology of the Cordillera should be based on the current location and arrangement of terranes in the northern part of the system, because these terranes were all located much farther south in the Late Cretaceous.