EVOLUTION OF THE NORTHERN US CORDILLERA: RESULTS FROM THE IDOR EARTHSCOPE PROJECT (Invited Presentation)

The IDOR EarthScope project acquired and integrated multi-disciplinary data in Idaho and eastern Oregon to investigate tectonic and magmatic modification of the Laurentian cratonic margin. Previous isotope work indicated that the Western Idaho Shear Zone (WISZ) juxtaposes continental and accreted oceanic crust along a steep boundary. Previous structural geology and geochronology showed that the sub-vertical WISZ accommodated significant transpression 103-90 Ma within the active magmatic arc. IDOR geophysical images show that the WISZ cuts vertically through the entire crust, and the Moho has a ~7 km step at the WISZ. Modern thickness of the cratonic crust east of the WISZ plus exhumation indicate that the crust was at least 55 km thick in the late Cretaceous - Paleocene. IDOR petrology and geochemistry data from the voluminous 83-53 Ma portions of the Idaho Batholith east of the WISZ indicate dominant crustal melting with little to no mantle input, except for a small volume at the north end. Therefore, the large post-85 Ma portion of the Idaho Batholith is interpreted to have been created by melting within a thick crustal plateau, not by normal arc processes. IDOR zircon geochronology and geophysics map the regional cratonic terranes across the batholith.

The IDOR data are interpreted in terms of the tectonic evolution of the northern US Cordillerra. Starting ~100 Ma, the Insular Belt, now in Canada, obliquely collided with the western margin of the previously accreted Blue Mountains oceanic terranes. This collision caused large-scale transpression at the WISZ within the active magmatic arc and resultant shortening and steepening of the pre-existing Blue Mountains suture. This collision also shortened and thickened the cratonic crust east of the arc and created an elevated plateau. Transpression at the WISZ stopped at 90 Ma and normal arc magmatism stopped at 85 Ma. Thermal evolution of the over-thickened plateau resulted in intra-crustal melting to form most of the modern Idaho Batholith from 75-60 Ma. The plateau and Idaho Batholith are contemporaneous with compressional tectonics to the east. The end of collision ~55 Ma and resultant reduction in compression enabled Eocene collapse of parts of the over-thickened plateau. This extension resulted in the formation of core complexes and Challis magmatism, and the first mantle-sourced magma since 85 Ma.