THE IDAHO SYNTAXIS: WESTERN IDAHO SHEAR ZONE DEFORMATION THAT HAS GONE ROUND THE OROFINO BEND
Structural work has focused on the kinematics of deformation near Orofino. Foliation is moderate to steeply dipping, and lineations are down-dip. In addition to the reverse motion documented by earlier workers, there is a consistent sinistral component of shear observed when looking perpendicular to the lineation direction. This combination suggests that a combination of transpressive contractional tectonism occurred on the northern section continental margin (denoted by the EW orientation of the Sr 0.706 isopleth). A sinistrally resolved shear sense is consistent with kinematic analyses from the western Idaho shear zone, which indicate a ~60° angle of oblique (dextral) convergence (or 060 in present coordinates). This angle of convergence, when applied to the orientation of the shear zone near Orofino, would result in a sinistral component of deformation.
The activation of a dextral component of shear on the WISZ and a simultaneous sinistral component of shear on the EW-oriented boundary indicates that material movement is being moved into a corner: This is the geometry of a syntaxis. The WISZ deformation occurs at Orofino (it “goes around the Sr bend”), but exhibits an opposite (sinistral) sense of shear at Orofino and westward.