THE ROLE OF THE PRECAMBRIAN RIFTED MARGIN ON CRETACEOUS-AGED DEFORMATION

In the Idaho section of the Cordillera, the tectonic evolution of magmatic arcs, continental transforms, and accretionary tectonics are all intricately linked. A mid-Cretaceous magmatic arc existed from southern Idaho (Owyhee Mountains) to at least the Orofino, Idaho, area. The arc was significantly shortened and pervasively deformed by ~91 Ma as a result of the western Idaho shear zone (WISZ), which is everywhere spatially coincident with the sharp ⁸⁷Sr/⁸⁶Sr = 0.706 isopleth. A late syntectonic tonalitic sill (Payette River tonalite) runs the entire length of the ~N-NNE oriented WISZ and also along the EW-oriented part of the Sr isopleth W of Orofino. We recently conducted paleomagnetic and geochronologic analyses on the Payette River tonalite and equivalent-aged rocks. The EW-oriented Julietta section, and the N-NNE oriented section to a lesser degree, show a clear indication of ~30° clockwise rotation since ~85 Ma (the closure temperature of hornblende—as a proxy for the Curie temperature of titanomagnetite—in the tonalite sill).

The implications of the paleomagnetic data are significant and two-fold. First, it indicates that the WISZ did not form in its present orientation, but was oriented at ~330 during deformation and the Julietta section was oriented ~060. In this orientation, the boundaries are parallel to the inferred orientation of the Precambrian rifted margin of the US Cordillera. As such, the Julietta section likely represents a transform fault that was presumably an originally steep lithospheric boundary. In contrast, the present WISZ formed on a rifted margin, which was originally wider but was subsequently steepened as a result of shortening. Second, the paleomagnetic data suggests that ~30° of clockwise rotation must have occurred after 85 Ma. The timing of rotation is poorly constrained, but must have occurred during the Cretaceous-Early Tertiary, during Idaho batholith magmatism and and Sevier thrusting in adjacent Montana. We speculate that deformation of the Lewis and Clark line and/or the trans-Idaho shear zone partly accommodated this rotation. The ultimate cause of both WISZ deformation and the large-scale rotation of the WISZ is likely a terrane collision, presumably of either the Blue Mountain terrain and/or Insular terrain.