REMOVAL OF THE MID-CRETACEOUS IDAHO BATHOLITH BY THE CRUSTAL-SCALE, WESTERN IDAHO SHEAR ZONE

During the mid-Cretaceous, abundant plutonism occurred in the western United States. This plutonism was a result of rapid subduction on the western margin of North America. Voluminous mid-Cretaceous plutonism is found in all coastal batholiths, including the Peninsular Range, the Sierra Nevada, and the Coast Range plutonic complexes, where the mid-Cretaceous sections of these arcs are ~40-60 km wide. In contrast, mid-Cretaceous plutonism in the Idaho batholith occurs in a 10 km wide zone on the batholith's western edge.

Our work suggests that a ~50-60 km wide, mid Cretaceous arc existed in the Idaho Batholith, but has been structurally thinned. Mid-Cretaceous plutons along the western edge of the Idaho batholith are generally deformed by the western Idaho shear zone, which was active during the Late Cretaceous. Fabric in this zone displays sub-vertical, NS-oriented foliation and sub-vertical lineation. Finite strain and vorticity analysis indicates that deformation in the western Idaho shear zone was characterized by dextral transpression, with large amounts of vertical extrusion, which explains the exhumation of ~7 kbar granitic rocks. Deformation within the western Idaho shear zone resulted in a minimum of ~10 km of right-lateral offset and 40-50 km of east-west shortening. Restoration of this contractional deformation yields an initial width of 50-60 km for the zone of mid-Cretaceous plutonism, and the highly elongate plutonic bodies are restored to approximately equant horizontal dimensions. This restoration results in a series of mid-Cretaceous batholiths, of relatively constant width, within the North American Cordillera.