LOCATION OF RIDGE-TRENCH-TRENCH TRIPLE JUNCTIONS ALONG THE NORTHERN CORDILLERAN MARGIN IN THE LATE CRETACEOUS AND EARLY TERTIARY: THE NORTHERN CORDILLERA PERSPECTIVE

Critical to discussions of Late Cretaceous paleogeography of the North American Cordillera are models for the location of the Kula-Farallon-North America triple junction. The models traditionally refer to a northern and southern option, with the northern option in the Pacific Northwest and the southern option somewhere along the Baja California margin. Onshore geology along the margin provides the best record of triple-junction interaction, and current data from Alaska and the Pacific NW indicate the traditional view of the plate geometries in the Late Cretaceous to Eocene may not be accurate. The Late Cretaceous-Early Eocene geologic record at two regions along the northern Cordillera margin is best explained by interaction of a spreading center with a subduction zone. The northernmost region is preserved along the entire length of the southern Alaska accretionary complex, where near-trench plutonism extends for more than 2100 km along strike, from 61 Ma in the southwest to ~ 50 Ma in the southeast. The southern region is along the Oregon to southern Vancouver Island coast, where high-volume basaltic volcanism of the Siletz terrane in the Paleocene-early Eocene may also record a ridge intersection with the continental margin. The Yakutat terrane lay between these two regions and has elements of both. One option is that these two regions record one triple junction, with the Alaskan site being displaced northward from the Vancouver Island region since the Paleocene. Dextral slip does occur in the forearc of the southern Alaska accretionary complex both before and after the ridge passage, but this option requires that oblique convergence was fully partitioned onto forearc faults, leaving no component of oblique convergence to drive the Coast Range shear zone and Tintina fault system, both of which were active at this time. Another option is that a third plate, the Resurrection plate, existed between the Kula and Farallon, and provides a mechanism for two TRT triple-junctions to occur simultaneously along the northern Cordillera margin. Velocity triangles indicate convergence for Resurrection-NA was more dextral oblique than Farallon-NA and provides a satisfying mechanism to explain dextral slip along the Cordilleran margin, at the same time as an eastward migrating triple junction in southern Alaska.