ABSTRACT It is debated whether plate tectonics (horizontal tectonics) or single-lid tectonics (vertical tectonics) dominated the Mesoproterozoic Era. Either rifting of the Nuna/Columbia supercontinent or a localized vertical subsidence and tectonism mechanism within a single tectonic plate is likely recorded in Mesoproterozoic basins. This study summarizes detrital zircon samples from the Mesoproterozoic Belt and Purcell Supergroups and Lemhi subbasin of the western United States and Canada and tests competing rift and intracratonic basin models. Rift models take the observed detrital zircon trends to mean that a non-Laurentian (ca. 1.6–1.5 Ga) detrital zircon component becomes completely absent higher in the section, signifying rifting of the Nuna/Columbia supercontinent at ca. 1.4 Ga. Intracratonic models acknowledge this observed shift in provenance but interpret a long-lived intracratonic setting for the basin following an earlier failed rifting event. The fundamental question is whether the Belt basin represents a failed or successful rift. We used statistical comparison of 72 detrital zircon signatures, reported in the literature and presented in this study, to test the rift model. Samples are not evenly distributed across the basin or its stratigraphy. Non-Laurentian grains are spatially restricted to the northwest part of the basin but are present in all groups, suggesting that the apparent loss of the non-Laurentian population is an artifact of sampling bias. Like stratigraphic boundaries and facies changes, mixing trends are gradual, not sharp or sudden, signifying progressive reworking of Proterozoic zircons and transport from all sides. Archean zircons are localized near the edges of Archean blocks, signifying local down-dropping along cratonic margins. The rift model is therefore rejected in favor of the intracratonic model for the Belt basin on the basis of variable mixing between non-Laurentian and Laurentian sources in both pre–Missoula Group and Missoula Group strata. Far away from plate margins, sediment incrementally filled topographic depressions created by densified and thinned Proterozoic crustal blocks, resulting in vertical down-dropping along preexisting sutures with neighboring Archean blocks. More systematic detrital zircon studies are needed in order to accurately quantify provenance trends in space and time. Continued investigation of the Belt basin may reveal underappreciated or unrecognized vertical tectonic processes that may explain Mesoproterozoic rocks more accurately.

We interpret the Lewis and Clark line of the Montana-Washington Cordillera to represent a rotational shear zone that initiated within the Mesoproterozoic Belt-Purcell basin as northern and southern parts of the basin rotated clockwise about eccentric Euler poles during Early Cretaceous to late Paleocene crustal shortening. The Lewis-Eldorado-Hoadley slab, north of the Lewis and Clark line, rotated about a pole near Helena, Montana, producing the Alberta salient, whereas the Sapphire and Lombard slabs, south of the Lewis and Clark line, rotated about poles located in Idaho, producing the Helena salient. Between the salients, the left-lateral transpressive Lewis and Clark line developed a flower-like structure with steep cleavage and tight folds at deep levels, reverse faults at intermediate levels, and en echelon box-folds at shallow levels. Comparison of older parts of the Lewis and Clark line in the west and younger parts in the east indicate that strain increased with progressive rotation. A topographic ridge was extruded along the Lewis and Clark line and created progressive angular unconformities at the base of early Campanian strata of west-central Montana and influenced sediment dispersion in the foreland basin. Deformation continued along the Lewis and Clark line through Maastrichtian and Paleocene time as the Rocky Mountain fold-thrust belt advanced along the leading edges of the rotating slabs during growth of the Alberta and Helena salients.

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