Lewis and Clark line and the rotational origin of the Alberta and Helena salients, north American Cordillera

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.