KINEMATIC AND TEMPORAL CONSTRAINTS FOR TRUNCATION OF THE WESTERN IDAHO SHEAR ZONE

The western Idaho shear zone (WISZ) marks an arc-continent boundary expressed by metamorphic tectonites and steep isotopic gradients. The zone is characterized by a dominantly N-striking sub-vertical foliation and down-dip mineral lineation. At its northern extent, the arc-continent boundary as defined by Sr isopleths, strikes NW. The presence of this apparent structural bend places considerable geometric constraints on strike-slip versus dip-slip displacement histories proposed for the WISZ. Structural analysis suggests that the bend results from overprinting of N-striking structures by NW-striking amphibolite-facies fabrics in a 20 km wide zone that extends well east of the N-S WISZ. The dominant tectonic fabric in the NW-striking shear zone is a moderate-steep NE-dipping mylonitic foliation that is coplanar with decimeter-scale compositional layering, contains a steeply plunging mineral lineation, and variably displays sinistral and top-to-the-southwest shear sense. Fabrics recorded by NW-striking orthogneiss ostensibly post-date most displacement recorded by N-striking structures to the south. Conventional and SHRIMP U/Pb data from NW-striking tonalitic orthogneiss suggests that deformation is younger than 88 Ma.

The apparent westward bend of the arc-continent boundary records kinematically and temporally distinct episodes of deformation along the N-S WISZ and a younger NW-striking shear zone, which truncates it. This truncation limits the age of potentially large magnitude transcurrent WISZ displacement to be pre-88 Ma, although small magnitude displacement along the WISZ coeval with initial displacement along the NW-striking structure cannot be precluded. Truncation is inferred to cause transfer of intra-arc transpressional displacements westward to an unrecognized structure, most likely the southern continuation of the late Cretaceous-Paleocene Coast shear zone. Displacement within the NW-striking shear zone is inferred to produce a sinistral separation of the N-S WISZ. Subsequent widespread early Tertiary extension to the north probably magnified the observed westward step of initial Sr isopleths and inferred northern continuation of the WISZ, since the NW-striking shear zone likely accommodated lateral displacement transfer during Early Tertiary extension.