TESTING THE ROLE OF DUCTILE EXTENSION ALONG THE WESTERN IDAHO SHEAR ZONE IN THE SALMON RIVER SUTURE ZONE EAST OF RIGGINS, IDAHO

In the Salmon River Canyon 15 km east of Riggins, Idaho, 10 cross-strike structural traverses were conducted through calc-alkaline metaplutonic and continental metasedimentary rocks along the accreted Blue Mountains Province island arc-North American craton boundary. This amphibolite facies, mid-crustal lithodemic boundary defines the western Idaho shear zone (WISZ), a north-south trending, steeply east-dipping ductile fault zone within the regional-scale contractional Salmon River suture zone.

This study reveals ductile extension structures across the arc-continent boundary, leading to a hypothesis in which late-stage deformation caused extensional displacements along the WISZ. Kinematic analysis normal and parallel to a penetrative stretching lineation confirms that tops-down-east indicators dominate the boundary. These include shear band cleavage, asymmetric sigma- and delta-style mantled porphyroclasts and mica fish, local S-C foliation, and fold vergence distributed across the Kelly Mountain Schist of craton affinity. Extension occurs in discrete high strain zones in the island-arc affinity, tonalitic Van Ridge Gneiss.

While Basin and Range brittle extension is regionally recognized, ductile extension is significant in that this strain style complicates fault development models for the Salmon River suture zone. Ductile extension must overprint earlier contraction and possibly dextral transpression structures. In British Columbia and Alaska, the Coast shear zone share similar structural attributes with the WISZ, including contractional and transpressional faulting, fabric orientation, and plutonism along a steeply-dipping shear zone, as well as east-side-down displacements oriented parallel to a steeply-plunging, down-dip mineral lineation. It has been suggested that similarities among structural fabrics in Alaska and Idaho are consistent with transpression and supportive of large-scale transport along the coast-parallel Baja-B.C. fault system. However, this correlation does not include the role of ductile extension in the formation of fabrics and faults. Based on this study, similar post-accretionary processes exist in Alaska and Idaho such that crustal readjustments and orogenic collapse may be common tectonic processes in the Northwestern Cordillera.