CRUSTAL THICKENING CAUSED BY THE CLOCKWISE ROTATION OF THE CENTRAL IDAHO BLOCK: ROLE OF THE LEWIS AND CLARK LINE AND IMPLICATIONS FOR THE TECTONICS OF MONTANA

We present a model of clockwise rotation of a Central Idaho block, with major implications for tectonics of NW conterminous US including aspects of Cretaceous-Paleogene crustal thickening. Paleomagnetic and geochronologic evidence indicate that the area around Orofino, ID, has rotated ~30° clockwise since ~85 Ma. We extrapolate this rotation to all of central Idaho and adjacent eastern Washington, as the reconstruction restores the 87Sr/86Sr = 0.706 isopleth to orientations consistent with the Precambrian rifting: The current NS-oriented margin was originally oriented ~330 – consistent with a Precambrian rift segment – and the EW margin was originally oriented ~060 – consistent with a transform segment. The rotation is hypothesized to have a pivot point near the Idaho-Nevada border. Restoration of the block indicates that continental North America was located further west, in the area known as the Columbia embayment, prior to rotation. Left-lateral motion along the Lewis and Clark zone during the Late Cretaceous-Paleogene likely accommodated this rotation.

There are two consequences for crustal thickening from this clockwise rotation. First, the central Idaho block rotates while the Idaho batholith – with its abundant two-mica granites – intrudes during the rotation. The Idaho batholith is likely not a subduction related magmatic arc, and therefore owes its genesis to crustal thickening. We propose a causal link between the rotation – which causes shortening between two crustal blocks – and the crustal thickening. We also note that the Idaho batholith is in continuity with the two-mica granite belt located further to the south in the U.S. Cordillera (e.g., Nevadaplano). Second, the model suggests differential contraction associated with its pivot point near the Nevada border, with the most displacement occurring to the north. In particular, the maximum amount of displacement, and hence contraction, is predicted to occur just south of the Lewis and Clark zone. In this location, there is major contraction in the Montana disturbed belt and Sapphire block. Consequently, the rotational collision is likely the cause for much of the Montana block uplifts.