LATE CRETACEOUS AND TERTIARY STRUCTURE OF THE WESTERN IDAHO AND OROFINO SHEAR ZONES, WEST-CENTRAL IDAHO

During the mid-Cretaceous (~90 Ma), the NNE-trending Western Idaho Shear Zone (WISZ) was truncated by the WNW-trending Orofino Shear Zone (OSZ). The inception of the OSZ marked a fundamental change in tectonic morphology of this segment of the North American Cordillera and the structure carried continental rocks southwesterly over an arc assemblage and accreted terranes through the Late Cretaceous and possibly into the early Tertiary. The WISZ coincides with the isotopic boundary between arc assemblage rocks and accreted terranes juxtaposed against continental North America in western Idaho. The WISZ is characterized by intense NNE-trending foliations and mylonitic fabrics in Cretaceous granitoids and older country rocks. The top-to-the-west mylonitic structures within the WISZ are superposed on Jurassic penetrative structures of the Salmon River belt and Wallowa terrane in western Idaho and northeastern Oregon and upon penetrative structures of poorly constrained age in plutonic rocks of the Idaho batholith. From south to north, from the east-west segment of the Salmon River to the Middle Fork of the Clearwater River, a WNW-trending late-stage spaced cleavage, superposed on NNE-trending WISZ structures, progressively increases in development. Near Orofino in west-central Idaho, the WNW-fabric is dominated by a moderate to steep northeasterly dipping mylonitic fabric with a top to the south shear sense. The mylonitic fabric involves rocks dated between 90 and 73 Ma and as the shear zone is approached from the south earlier penetrative fabrics are transposed and ultimately obliterated. The northern and western extent of the OSZ is loosely constrained but based on our local studies it cross-cuts the regional grain and polyphase structures typical of the Rocky Mountain fold-thrust belt in northern Idaho and eastern Washington. The Rocky Mountain belt (RMB) structures consist of a shallowly dipping penetrative fabric related to synmetamorphic isoclinal folds and younger superposed fabrics related to late Mesozoic contraction and Eocene extension. North of the western inferred extent of the OSZ in southeast Washington, RMB structures are well preserved in rocks of the Belt Supergroup and probable Cambrian quartzite.