ABSTRACT The rifted Precambrian margin of western Laurentia is hypothesized to have consisted of a series of ~330°-oriented rift segments and ~060°-oriented transform segments. One difficulty with this idea is that the 87 Sr/ 86 Sr i = 0.706 isopleth, which is inferred to coincide with the trace of this rifted margin, is oriented approximately N-S along the western edge of the Idaho batholith and E-W in northern Idaho; the transition between the N-S– and E-W–oriented segments occurs near Orofino, Idaho. We present new paleomagnetic and geochronologic evidence that indicates that the area around Orofino, Idaho, has rotated ~30° clockwise since ca. 85 Ma. Consequently, we interpret the current N-S–oriented margin as originally oriented ~330°, consistent with a Precambrian rift segment, and the E-W margin as originally oriented ~060°, consistent with a transform segment. Independent geochemical and seismic evidence corroborates this interpretation of rotation of Blue Mountains terranes and adjacent Laurentian block. Left-lateral motion along the Lewis and Clark zone during Late Cretaceous–Paleogene time likely accommodated this rotation. The clockwise rotation partially explains the presence of the Columbia embayment, as Laurentian lithosphere was located further west. Restoration of the rotation results in a reconstructed Neoproterozoic margin with a distinct promontory and embayment, and it constrains the rifting direction as SW oriented. The rigid Precambrian rift-transform corner created a transpressional syntaxis during middle Cretaceous deformation associated with the western Idaho and Ahsahka shear zones. During the late Miocene to present, the Precambrian rift-transform corner has acted as a fulcrum, with the Blue Mountains terranes as the lever arm. This motion also explains the paired fan-shaped contractional deformation of the Yakima fold-and-thrust belt and fan-shaped extensional deformation in the Hells Canyon extensional province.

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.