GEOLOGIC HISTORY OF THE WESTERN IDAHO SHEAR ZONE AND ADJACENT IDAHO BATHOLITH

Western Idaho and easternmost Oregon contain an extremely well exposed and sharp boundary between oceanic and continental lithosphere, which were juxtaposed in the Mesozoic. The 87Sr/86Sr = .706 isopleth marks this boundary, in which Sr isotopic values change from <.702 to >.710 within 30 km. The steepest gradient occurs in ~6 km, which corresponds exactly with the extent of the Late Cretaceous western Idaho shear zone (WISZ) near McCall. The WISZ runs from north of Grangeville (Idaho), where it is cross-cut by the younger Orofino shear zone, southward through the northern Owyhee Mountains. Geochronology suggests that the WISZ was active by ~95 Ma, inactive by ~90 Ma, and cooled to ~350° C by ~80 Ma. Kinematic indicators indicate dextral transpressional deformation during this time period. Relicts of an earlier episode of deformation with unknown kinematics are preserved by older fabrics that are cut by the main fabric of the WISZ. Deformation is interpreted to occur in an intra-arc setting due to the collision of the Insular superterrane (Baja-BC) with cratonic North America.

There are two sharp, ~NS-trending Sr = 0.706 lines regionally: 1) The first recorded in the Cretaceous granitoids and corresponding with the WISZ; and 2) One located ~120 km west that is recorded by Miocene basalts. The trend of the Miocene 0.706 line mirrors the orientation of the fabric in the WISZ. This geology could indicate offset of an originally vertical structure, as proposed by earlier workers, or could reflect the original geometry of the boundary.

The Idaho batholith, defined as all granitoids east of the WISZ, has an enigmatic origin. A major ~90 Ma magmatic pulse may result from delamination. Later magmatism, through the Early Tertiary, may reflect either renewed subduction or crustal melting resulting from collision. At present, there are few constraints on the lithospheric architecture of the Idaho batholith.