BAJA-BC AT LAST: TERRANE COLLISION IN WESTERN IDAHO AND THE ROLE OF WET MANTLE LITHOSPHERE

The western Idaho shear zone is a mid-crustal exposure of a lithospheric scale, intra-arc shear zone on the western edge of the Idaho Batholith. Fabric analyses suggest dextral transpressional with a significant contractional component to transpression (30-110 km of east-west directed shortening). Strain analysis of isotopic gradients are most consistent with the upper end of these estimates. Therefore, prior to shortening, a Sierra Nevada sized (i.e. 80-100 km wide) magmatic existed at the latitude of Idaho in the Late Cretaceous. Deformation along the western Idaho shear zone resulted in extreme shortening of both the active and inactive portions of the magmatic arc, but not material outside of the magmatic arc. The timing and latitude of the collision correspond exactly to predictions for a Baja-BC collision, from either a fixist or mobilist perspective. The tectonics of the western Idaho shear zone are puzzling for two reasons: 1) The extreme amount of shortening (~90 km of shortening) localized along a very narrow zone (presently <10 km wide); and 2) The deformation shortened the entire magmatic arc, not just the active portion of it. We propose that the answer to this enigma lies in the role of the lithospheric mantle. The lithospheric mantle is considered to be the mechanically strongest layer of the lithosphere, although its strength is drastically reduced by the inclusion of water. The lithospheric mantle below the arc would absorb water driven off of the subducting slab. This water would make lithospheric mantle below the entire arc, both active and inactive, far weaker than the surrounding lithopshere. We propose that the collisional deformation associated was localized into the edge of North America that was underlain by weak, wet lithospheric mantle. This new orogenic style of arc-shortening may be common in the geological record, particularly in regions of oblique terrane collision and may even explain the lack of granites in some collisional orogens. This orogenic style may not have been recognized earlier due to several complications, including: 1) Lineations parallel to vorticity vectors; 2) Difficulty in extracting strain data from middle crustal rocks; and 3) The reactivation of the strong fabrics during subsequent tectonism.