TWO HITS, ONE RUN, AND A FEW ERRORS: THE EFFECT OF THE PRECAMBRIAN RIFTED MARGIN ON THE TECTONIC EVOLUTION OF THE NORTHERN US CORDILLERA

The Precambrian rifted margin in western Idaho was subjected to two Mesozoic collisions. The first was the Jurassic suturing of the Blue Mountains terranes to the Laurentian margin at the Salmon River suture zone. The Blue Mountains terranes accreted at a similar time and were likely part of the Intermontane superterrane in British Columbia. The second collision in the middle Cretaceous was caused by the docking of the Insular superterrane. Both fault reconstructions and minimum-displacement paleomagnetic data indicate that the Insular superterrane was located adjacent to the western US at least as far south as the California-Oregon border at ~100 Ma. This oblique collision caused deformation that is spatially coincident with the steep geochemical gradients (Sr, Nd, O) oriented NS in central-southern Idaho and changing orientation 90° near Orofino, ID, to an EW orientation. Recent paleomagnetic work indicates that the entire margin was rotated clockwise 30° after 85 Ma. As such, the Idaho segment restores to a consistent orientation with the rest of the Precambrian margin of western Laurentia: Spreading center segments oriented at 330 and transform segments oriented 060. The implication is that the NS-oriented, transpressional western Idaho shear zone was originally oriented at 330 and, importantly, the Precambrian margin formed the backstop for this major collisional event. Using kinematic vorticity estimates from the western Idaho shear zone, terrane motion was exactly northward. Deformation occurred because the Precambrian margin forms a structural syntaxis during northward translation of accreted terranes (the Precambrian margin is asymmetric – similar to a rip saw – allowing southward motion without collision but not northward motion). This pattern of deformation is strikingly similar in timing and geometry to deformation in southern California, which likely also occurs in a structural syntaxis controlled by the Precambrian rifted margin. The Insular superterrane and parts of the Intermontane superterrane have moved >1000 km to the northwest since mid-Cretaceous: two hits and a run that had major effects on the modern structure of the continent.