TRANSPRESSIONAL ZONES AND LITHOSPHERIC-SCALE STRAIN LOCALIZATION: AN EXAMPLE FROM THE WESTERN IDAHO SHEAR ZONE

Major transpressional zones occur in a variety of plate tectonic settings, resulting from oblique convergence and/or oblique collision. In the last 30 years, the significance of transpressional orogens (and shear zones) has been the catalyst for characterizing the three-dimensional nature of mountain belts. A number of features that are found in transpressional zones are typified by the western Idaho shear zone (USA). The western Idaho shear zone (WISZ) is the current boundary between oceanic and continental lithosphere. The WISZ runs NS for 400 km, from north of Grangeville (Idaho) 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.

The fabrics within the WISZ – when tilting from normal faulting is removed - are a prominent vertical, NS foliation and a vertical lineation. Shear sense indicators are found on the sub-horizontal plane perpendicular to lineation, not parallel to lineation as expected in a shear zone. Using shape preferred orientations of rigid K-feldspar megacrysts, we were able to independently determine a vorticity vector, which is oriented parallel to the lineation (consistent with the sense of shear markers). Strain estimates from rotated porphyroclasts and tectonic reconstructions suggest that a >60 km wide arc has been shortened to less than 6 km.

Deformation of the WISZ is interpreted to occur in an intra-arc setting due to the collision of the Insular superterrane with North America. The Blue Mountains terranes, immediately adjacent to the west, show no evidence of any deformation at this time. To the east, younger plutons largely obscure the relation, but wallrocks of Precambrian rock do not show pervasive strain consistent with NS-shearing. We attribute this extreme localization of WISZ deformation to a combination of heat associated to the magmatic arc and/or water in the mantle associated with arc magmatism.