Hit-and-run model for Cretaceous–Paleogene tectonism along the western margin of Laurentia

Abstract

ABSTRACT The North American Cordillera experienced major contractional deformation during the Cretaceous–Paleogene, which is commonly attributed to normal subduction transitioning to shallow-slab subduction. We provide details of an alternative hit-and-run model, wherein the Insular superterrane obliquely collided with the North American margin from 100 to 85 Ma (the “hit”), followed by northward translation during continued oblique convergence with North America from 85 to 55 Ma (the “run”). This model assumes that the paleomagnetic evidence from the accreted terranes of the northern North American Cordillera, indicating up to thousands of kilometers of northward movement primarily between ca. 85 and 55 Ma, is correct. The hit-and-run model also incorporates new advances: (1) A worldwide plate reorganization occurred ca. 105–100 Ma; and (2) multiple subducted slabs have characterized subduction systems of the North American Cordillera since ca. 120 Ma. Finally, we explicitly address along-strike variations, such as the role of the preexisting rifted Precambrian margin and Permian–Triassic truncation of North America, in margin-parallel movement along western North America. The 100–85 Ma “hit” phase of the orogeny was characterized by dextral transpressional deformation that occurred simultaneously in the magmatic arcs of Idaho, northern Nevada, eastern California, and the Peninsular Ranges of southern California and northern Mexico. The hit phase also recorded incipient plateau formation, foreland block uplifts in the northern Rocky Mountains, and significant foreland sedimentation in adjacent North America. The transition from “hit” to “run” is hypothesized to have occurred because of the clockwise rotation of a Precambrian promontory in Washington State that was blocking northward translation: This rotation was accommodated by sinistral motion along the Lewis and Clark deformation zone. The 85–55 Ma “run” phase resulted in dextral strike-slip faulting of coastal blocks and significant contractional deformation in adjacent continental North America. The hit-and-run model is consistent with first-order geological and geophysical constraints from the North American Cordillera, and the proposed type of oblique orogeny requires a three-dimensional, time-dependent view of the deformation along an irregular and evolving continental margin.